by Shannon Omari Liburd
Bachelor of Science in Aerospace Engineering
MIT, 2008
Submitted to the Engineering Systems Division
in Partial Fulfillment of the Requirements for the Degree of
Master of Science in Technology and Policy
at the
Massachusetts Institute of Technology
June 2010
© 2010 Massachusetts Institute of Technology. All rights reserved.
Signature of Author…………………………………………………………………………………………………
Technology and Policy Program, Engineering Systems Division
May 7, 2010
Certified by…………………………………………………………………………………………………………
John H. Lienhard V
Collins Professor of Mechanical Engineering
Thesis Supervisor
Accepted by………………………………………………………………………………………………………...
Dava J. Newman
Professor
of
Aeronautics
and
Astronautics
and
Engineering
Systems
Director,
Technology
and
Policy
Program

by Shannon O. Liburd
Submitted to the Engineering Systems Division on May 7, 2010 in Partial Fulfillment of the
Requirements for the Degree of Master of Science in Technology and Policy
Abstract
Worldwide water scarcity, especially in the developing world, provides the impetus for utilizing inexpensive desalination technologies on a wider scale to contribute to freshwater supply. Small-scale desalination technologies, such as solar-driven humidification dehumidification (HDH), are needed to help provide clean drinking water to people living in coastal areas. This thesis explores the question of whether the fills used in the humidifier of the HDH system, which allow for increased contact area between the water and air streams, can be made of locally available materials such as charcoal, bamboo, and louffa found in Haiti. It also addresses how the institutional, economic, social and technological barriers to successful deployment of renewable energy (RE) desalination technologies such as HDH can be overcome.
Charcoal, louffa and bamboo custom fills were experimentally tested in a benchtop cooling tower to determine their suitability for use in the humidifier of a HDH system.
The fills’ transfer characteristics and pressure drop data were obtained and analyzed to determine the overall fill performance in terms of fan power consumption. The lower the fan power consumption required by the fill, the better the fill performance.
The performances of the custom fills were compared with each other and with two commercial thin film fills. The louffa fill performed the best among the custom fills, having power consumption 2.9 and 4.4 times less than the charcoal and bamboo fills, respectively. The louffa fill is therefore recommended for use in the humidifier.
To help overcome the barriers facing RE desalination policy and implementation, several strategies are recommended: a decentralized regulatory system for water supply, public-private financial arrangements and supporting policies; market analysis of prospective RE desalination systems, targeted R&D to make improved system components and a community platform for the various stakeholders to work together. Most importantly, the general public must be engaged throughout the entire process to foster transparency, community trust and public acceptance of the desalination technology.
Thesis Supervisor: John H. Lienhard V
Title: Collins Professor of Mechanical Engineering
Project Supervisor: Amy B. Smith
Title: Senior Lecturer of Mechanical Engineering
3

1 Introduction 8
1.1 Project Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
1.2 The Central Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
1.3 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
2 Desalination Background 10
2.1 Worldwide Water Scarcity . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
2.1.1 Water Issues in Haiti . . . . . . . . . . . . . . . . . . . . . . . . . .
13
2.2 Why Desalination? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
2.3 Desalination Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
2.3.1 Conventional Desalination Technologies . . . . . . . . . . . . . . . . .
17
2.3.2 Limitations of Conventional Technologies . . . . . . . . . . . . . . . .
24
2.3.3 Renewable Energy Desalination . . . . . . . . . . . . . . . . . . . . .
25
2.3.4 Humidification Dehumidification (HDH) Desalination . . . . . . . . .
32
2.3.5 Types of HDH Systems . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.3.6 Possible Improvements to the HDH Cycle . . . . . . . . . . . . . . .
38
3 Application of Solar HDH Desalination in the Developing World 39
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.2 Cooling Tower Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.3 Fills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3.4 Cooling Tower Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.5 Experimental Cooling Tower Test . . . . . . . . . . . . . . . . . . . . . . . .
50
3.5.1 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
3.5.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
3.5.3 Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . .
54
3.5.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
3.5.5 Error Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
3.5.6 Implications for Fills Made of Local Materials . . . . . . . . . . . . .
72
4 Issues Relevant to HDH Desalination 73
4.1 Technical Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
4.1.1 Technological Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
4.2 Economics of desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
4.2.1 Economical Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
4.2.2 Economics of HDH System . . . . . . . . . . . . . . . . . . . . . . . .
75
4.2.3 Water Price in Haiti . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
4.3 Socio-economic Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
4.3.1 Social Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
4.3.2 Water Quality and Public Perception . . . . . . . . . . . . . . . . . .
87
4.4 Environmental Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
4.4.1 Possible Environmental Effects . . . . . . . . . . . . . . . . . . . . .
90
4.4.2 Brine Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
4

4.4.3 Concentrate Disposal Methods and Mitigation . . . . . . . . . . . . .
94
4.5 Political Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
4.5.1 Institutional Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
4.5.2 Water Regulatory Framework in Haiti . . . . . . . . . . . . . . . . .
97
4.5.3 Desalination Regulatory Framework in the U.S. . . . . . . . . . . . .
99
4.5.4 Policy Gaps, Links and Recommendations for Increasing Desalination 100
5 The Effect of Stakeholders on Seawater Desalination Policy and Implementation 102
5.1 Role of Market in Overcoming Barriers . . . . . . . . . . . . . . . . . . . . . 102
5.2 Role of R&D in Overcoming Barriers . . . . . . . . . . . . . . . . . . . . . . 103
5.3 Stakeholder Activities for Desalination Awareness and Growth . . . . . . . . 104
6 Summary and Conclusions
References
A Appendix
106
108
112
5

1 Global water stress indicator map [4] . . . . . . . . . . . . . . . . . . . . . .
11
2 Worldwide freshwater stress in 1995 and 2025 [5] . . . . . . . . . . . . . . .
11
3 Daily per capita water use by country [7] . . . . . . . . . . . . . . . . . . . .
12
4 Worldwide use of improved drinking-water sources in 2008 [9] . . . . . . . .
13
5 Haitian girl collecting water from an open water source [18] . . . . . . . . .
15
6 Map of Haiti [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
7 The world’s water supply [22] . . . . . . . . . . . . . . . . . . . . . . . . . .
16
8 U.S. and global desalination sources and process distribution [25] . . . . . .
18
9 Schematic diagram of multi-stage flash desalination process [24] . . . . . . .
18
10 Schematic diagram of multi-effect evaporator desalination process (horizontal tube-parallel feed configuration) [24] . . . . . . . . . . . . . . . . . . . . . .
20
11 Schematic diagram of single stage mechanical vapor compression desalination process [24] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
12 Block diagram of reverse osmosis operations (optimal pressure recovery devices not depicted) [24] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
13 Schematic diagram of electrodialysis desalination process [24] . . . . . . . . .
23
14 Characteristics of the two main thermal desalination technologies and the two main mechanical desalination technology options [30] . . . . . . . . . . . . .
25
15 Possible combinations of renewable energy systems with desalination technologies [29] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
16 Renewable energy-driven desalination processes and energy sources [29] . . .
27
17 Recommended renewable-energy desalination combinations [28] . . . . . . .
28
18 Water cycle [26] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
19 Solar still [26] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
20 HDH desalination (air heated, open cycle) [26] . . . . . . . . . . . . . . . .
35
21 HDH cycles [26] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
22 HDH unit with closed water cycle/open-air [34] . . . . . . . . . . . . . . . .
37
23 HDH unit with closed-air/open-water cycle [26] . . . . . . . . . . . . . . . .
38
24 Counter-flow cooling tower [40] . . . . . . . . . . . . . . . . . . . . . . . . .
41
25 Cross-flow cooling tower [40] . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
26 CF-1200 fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
27 CF-1900 fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
28 Louffa fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
29 Charcoal fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
30 Bamboo fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
31 Cooling tower process heat balance [41] . . . . . . . . . . . . . . . . . . . . .
49
32 HC891 benchtop forced-draft cooling tower unit . . . . . . . . . . . . . . . .
51
33 Actual benchtop cooling tower apparatus . . . . . . . . . . . . . . . . . . .
53
34 Me comparison CF-1200 (H=0.152 m) . . . . . . . . . . . . . . . . . . . . .
58
35 Me comparison CF-1200 (H=0.305 m) . . . . . . . . . . . . . . . . . . . . .
59
36 Me comparison CF-1200 (H=0.457 m) . . . . . . . . . . . . . . . . . . . . .
59
37 Me comparison CF-1900 (H=0.152 m) . . . . . . . . . . . . . . . . . . . . .
59
38 Me comparison CF-1900 (H=0.305 m) . . . . . . . . . . . . . . . . . . . . .
60
6

39 Me comparison CF-1900 (H=0.457 m) . . . . . . . . . . . . . . . . . . . . .
60
40 CF-1200 transfer characteristic plots . . . . . . . . . . . . . . . . . . . . . .
62
41 CF-1900 transfer characteristic plots . . . . . . . . . . . . . . . . . . . . . .
63
42 Custom fill transfer characteristic plots (H=0.152 m) . . . . . . . . . . . . .
64
43 Custom fill transfer characteristic plots (H=0.305 m) . . . . . . . . . . . . .
65
44 Custom fill transfer characteristic plots (H=0.457 m) . . . . . . . . . . . . .
66
45 CF-1200 fill pressure drop (H=0.457 m) . . . . . . . . . . . . . . . . . . . .
66
46 CF-1900 fill pressure drop (H=0.457 m) . . . . . . . . . . . . . . . . . . . .
67
47 Louffa fill pressure drop (H=0.457 m) . . . . . . . . . . . . . . . . . . . . .
68
48 Charcoal fill pressure drop (H=0.457m) . . . . . . . . . . . . . . . . . . . .
68
49 Bamboo fill pressure drop (H=0.457 m) . . . . . . . . . . . . . . . . . . . .
69
50 Fan power consumption for tested custom fills . . . . . . . . . . . . . . . . .
70
51 Fan power consumption for tested fills (H=0.457 m) . . . . . . . . . . . . .
71
52 General principles for cost of water [54] . . . . . . . . . . . . . . . . . . . .
82
53 General principles for value of water [54] . . . . . . . . . . . . . . . . . . . .
83
54 Surface water disposal problems and mitigation [63] . . . . . . . . . . . . . .
95
1 GOR of the main thermal desalination technologies [27] . . . . . . . . . . . .
19
2 Benchtop cooling tower temperature measurements . . . . . . . . . . . . . .
51
3 Cooling tower test measurements . . . . . . . . . . . . . . . . . . . . . . . .
55
4 CF-1200 L/G test conditions for H=0.152 m, 0.305 m, 0.457 m . . . . . . . .
56
5 Transfer characteristic correlations according to Merkel approach (H=0.152) 61
6 Transfer characteristic correlations according to Merkel approach (H=0.305
m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
7 Transfer characteristic correlations according to Merkel approach (H=0.457
m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
8 Fill Pressure Drop Correlations (H=0.457 m) . . . . . . . . . . . . . . . . .
63
9 Uncertainties of measured variables . . . . . . . . . . . . . . . . . . . . . . .
71
10 Uncertainty contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
11 Distribution of costs for conventional (RO and MSF) and renewable energy driven plants [50] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
12 Comparison of HDH process (with waste heat) with other processes [50] . . .
78
13 Cost comparison for small-scale desalination methods [52] . . . . . . . . . .
79
14 Four solutions used to remineralize desalinated water [62] . . . . . . . . . .
91
15 Water remineralization process comparison [62] . . . . . . . . . . . . . . . .
91
16 Concentrate characteristics for various desalination technologies [63] . . . .
94
7

Water scarcity is a serious problem that will increase in the coming decades and disproportionately impact those in developing countries. While functional desalination technologies exist, currently there are no cost-effective, high efficiency desalination systems for people living on less than $2/day. Solar-driven humidification dehumidification (HDH) desalination has the potential to be an appropriate technology for this market. It has the flexibility to provide decentralized clean water, it uses a renewable energy source, it has moderate installation and operating costs and it does not require skilled operators to maintain it. The purpose of this study is to examine the feasibility of the implementation of small-scale (100 m 3 /day
<), solar-driven HDH desalination in the developing world, particularly in conjunction with an ongoing project in Pestel, Haiti. A technical, policy, environmental and socio-economic assessment of the application of such technology will be made in the context of impoverished, remote and potentially off-grid areas.
The central questions that will be examined in this thesis are:
1. Are small-scale (100 m 3 /day <) solar-driven HDH desalination systems made from lowcost, locally available materials in Pestel, Haiti technically and economically feasible?
2. What mechanisms are needed to sustainably implement small-scale remote HDH desalination systems in Pestel, Haiti?
3. What are the barriers with respect to small-scale, renewable energy desalination implementation in coastal areas and how can they be addressed for successful technology deployment?
These questions will be addressed through technical experimentation and thorough literature review.
8

The thesis is divided into five parts that address different components of the central questions. Section two gives background on worldwide water scarcity, discusses how desalination plays an important role in reducing water scarcity and offers an overview of different desalination technologies with an emphasis on HDH desalination. Section three discusses the potential for HDH application in developing countries by using local materials to reduce the cost of system components. It provides an overview of cooling tower theory, which is important for understanding how the humidifier in the HDH system works. It also provides the results of the experimental investigation into the use of different locally available fill materials in Haiti for use in the system’s humidifier. Section four examines the technical, economic, social, environmental and policy issues relevant to the successful implementation of HDH desalination in developing countries. It also outlines recommendations on how to overcome the existing barriers. Section five discusses the roles that stakeholders, the market, research and development (R&D) and the public play concerning seawater desalination policy and implementation. The summary, conclusion and recommendations for future work are provided in section six.
9

Water scarcity means that the annual water supply of a region is below 1,000 m 3 /person.
Water stress is defined as between 1,000 m 3 and 1,700 m 3 /person [1]. 1,000 m 3 is the annual amount of water deemed necessary to satisfy basic human needs [2]. Figure 1 is an indicator map of worldwide water scarcity in 2003. According to the 1997 Population Action
International estimate, in 2050 the world’s relative freshwater sufficiency will be 58% while water stress and scarcity will account for 24% and 18% respectively [3]. Figure 2 illustrates global water stress in 1995 and in 2025.
People use four thousand cubic kilometers of water each year around the world, for domestic, agricultural and other industrial purposes. This water use does not include consumptive uses such as energy generation, mining and recreation [2]. However, there is great disparity between water consumption in developed and developing regions. For example, in 2004 the average water use per capita in the U.S. (2,026 m 3 /p/yr) was approximately three times higher than that in India (641 m 3 /p/yr) [6]. Figure 3 shows the great disparity in average water use per person per day in 2006 between developed and developing countries. From
Figure 3 it appears that the average water use per person per day in the U.S. is approximately 30 times greater than that of a person in Haiti. In Haiti in the year 2000 the total freshwater withdrawal was 0.99 km 3 /yr with the industrial, domestic and agricultural sectors accounting for 1%, 5% and 94% respectively. (1 km 3 of water is 1 billion m 3 or 264 billion gallons of water.) The global per capita freshwater withdrawal in the year 2000 was 116 m 3 /yr (30,644 gal/yr) [8]. World water demand, approximately 4,200 km 3 in 2000, has more than tripled over the past half century and is estimated to be about 30% of the world’s total accessible fresh water supply. That fraction may reach 70% by 2025 [2].
In addition to the overall scarcity of freshwater in the world, there is the added problem
10

Figure 1: Global water stress indicator map [4]
Figure 2: Worldwide freshwater stress in 1995 and 2025 [5]
11

Figure 3: Daily per capita water use by country [7] of the lack of clean drinking-water. UNICEF/WHO estimate that globally 884 million people do not use improved sources of drinking-water, almost all of them in developing regions [9].
Sub-Saharan Africa accounts for a third of that number with only 60% of the population using improved sources of drinking-water [9]. Improved sources of drinking water include a household connection, a public standpipe, a borehole, a protected spring, a protected dug well and rainwater [10]. Figure 4 shows the worldwide use of improved drinking water in
2008.
Population growth and climate change will bring new water supply challenges. By 2050 the world population is projected to grow to at least 9.4 billion and the great majority of the people will live in developing countries [2]. Climate change will cause places that were once habitable to be uninhabitable. This phenomenon will result in mass migration of refugees to neighboring locations, placing strains on the available water supply and causing conflict.
It is estimated that more than 2.7 billion people will face severe water shortages by the year
2025 if the world continues consuming water at the same rate per capita, and if the real
12

Figure 4: Worldwide use of improved drinking-water sources in 2008 [9] population growth fits the forecasted trend [11].
2.1.1 Water Issues in Haiti
More than 60% percent of Haiti’s total population of approximately nine million people does not have access to clean water [12]. In the Western Hemisphere the country is ranked last on the International Water Poverty Index. The country’s continued political instability and the
7.0 magnitude earthquake that struck 25 km west of Port-Au-Prince, the capital of Haiti, on
January 12, 2010 have only worsened the water situation [13]. An estimated three million people were affected by the earthquake, and Haiti’s main infrastructure was demolished [14].
Prime Minister Jean-Max Bellerive estimated that 250,000 residences and 30,000 commercial buildings had collapsed or were severely damaged [15]. Before the earthquake nearly a third of the population resided in urban areas [12]. Since the earthquake, around 600,000 people have fled the capital for cities like Cap Haitien, in the north, and Hinche, in the central plateau. The population of Gonaïves, a port city on the west coast roughly midway between the country’s two major fault lines, has swollen to 300,000 from 200,000 in less than three
13

months [16]. There has also been reverse migration from hard hit cities and towns to rural areas. This shift is putting further pressure on rural households, affecting the socio-economic stability in areas already grappling with meager resources. Haiti’s Ministry of Agriculture estimates the number of people leaving cities for rural areas could reach 1.5 million [17].
Even when a public water system is available, getting water is a daily struggle. Many
Haitians have to travel long distances to collect water for drinking, washing, cooking, cleaning, and bathing and it still has to be purified prior to drinking. In addition, potable water is not free. For the 80% of Haitians who live in poverty, the cost of clean drinking water can be a significant challenge [19]. This fact may force them to consume water from unclean sources. See Figure 5.
Depletion and contamination of resources supplying water is a major issue. Haiti’s aquifers are being depleted. Aquifers are replenished by the absorption of rainwater. As a result of soil erosion due to deforestation, there is limited topsoil to absorb sufficient amounts of rainwater and much of the rainwater runs off into the sea. The aquifer in the capital of
Haiti, the Port-au-Prince aquifer, currently is so low that a lack of pressure has begun to allow saltwater to seep in. In four to nine years Port-au-Prince will have to tap into another aquifer farther away from the city [12]. Additionally, nearly every water source in Haiti is contaminated by human waste: there are no public sewage treatment or disposal systems anywhere in the country [20]. The lack of clean drinking water contributes to the highest infant and child mortality rate in the Western Hemisphere. In developing countries like Haiti, up to 90% of diarrheal illness, a leading cause of death, can be attributed to unsafe water and poor sanitation [19]. Therefore, there is a clear need for decentralized, cost-effective water technologies for providing clean drinking-water. Haiti is the western one-third of the island Hispaniola and is surrounded by the Caribbean Sea and the North Atlantic Ocean.
See Figure 6. It has a coastline of 1,771 km [8]. Thus, appropriate desalination technologies can be used to help alleviate water stress in Haiti.
14

Figure 5: Haitian girl collecting water from an open water source [18]
Figure 6: Map of Haiti [8]
15

Three-quarters of the earth is covered by water. The oceans represent the earth’s largest water reservoir, accounting for 97.5%. Only 2.5% of the earth’s water is freshwater and less than 1% of the freshwater is available for use since the rest is frozen in ice caps and glaciers
[21]. Figure 7 illustrates the world’s water supply graphically.
The abundance of saltwater presents great potential for seawater desalination to help increase the world’s potable freshwater supply. Worldwide, only 1% of drinking water is produced by desalination, supplied by more than 12,500 plants in more than 120 countries.
Considering that almost one quarter of the world’s population lives less than 25 km from the coast, seawater could become one of the main sources of freshwater in the near future.
Additionally, conventional seawater desalination technologies produce relatively inexpensive freshwater that costs between $0.5/m 3 -$1.0/m 3 . In terms of the geographical breakdown of the desalination market, the regions of the Middle East clearly dominate the demand with over 50% of the market share, followed by Asia-Pacific, America and Europe, which each share about 10% of the market [23].
Figure 7: The world’s water supply [22]
16

2.3.1 Conventional Desalination Technologies
There are three main processes for the desalination of seawater or brackish water: thermal distillation, use of a semi-permeable membrane to separate fresh water from concentrate and chemical approaches to desalination. The first approach involves using thermal means to effect a phase change of the water (i.e. to vapor) and to separate the new phase from the remaining salt solution. The heat source may be obtained from a conventional fossil-fuel or from a renewable energy source (RES). These thermal distillation processes account for a large portion of the world’s desalination capacity [24], as shown in Figure 8. In the membrane processes, electricity is used either for driving high pressure pumps or for establishing electric fields to separate ions [26]. Chemical approaches to desalination are more varied than the other two and include processes such as ion exchange, liquid-liquid extraction, and gas hydrate or other precipitation schemes. Ion exchange is used to soften brackish water.
With the exception of ion-exchange, it is generally found that chemical processes are too expensive to produce fresh water. Even ion exchange is impractical for treating water with higher levels of total dissolved solids (TDS) [24].
The most important commercial thermal distillation processes are multi-stage flash (MSF),
Multi-Effect Evaporation (MEE) and Vapor Compression (Thermal and Mechanical). MSF is widely used in the Middle East and accounts for over 40% of the world’s seawater desalination capacity. MSF is a distillation (thermal) process that involves evaporation and condensation of water. The evaporation and condensation steps are coupled in MSF so that the latent heat of evaporation is recovered for reuse by preheating the incoming water. Each stage of an MSF unit operates at a successively lower pressure in order to maximize water recovery. Another key design feature of MSF systems is bulk liquid boiling, which alleviates problems with scale formation on heat transfer tubes [24]. See Figure 9.
The gained output ratio (GOR) is a performance ratio often applied to thermal desali-
17

Figure 8: U.S. and global desalination sources and process distribution [25]
Figure 9: Schematic diagram of multi-stage flash desalination process [24]
18

nation processes. GOR is essentially the effectiveness of water production and an index of the amount of heat recovery effected in the system [26]. The higher the GOR, the better the thermal desalination technology. GOR is defined as the ratio of the latent heat of evaporation ( h f g
) of the pure water produced ( m pw
) to the heat input (
·
Q in
) to the cycle [27], as shown in Equation 1.
GOR = m pw h f g
·
Q in
Table 1 shows the range of GOR for typical thermal desalination systems.
(1)
Multi-Effect Evaporation (MEE) or Multi-Effect Distillation (MED) is a distillation process related to MSF. MEE is not widely used because of problems with scaling on the heat transfer tubes, but it has gained attention due to its better thermal performance compared to MSF. In MEE, vapor from each stage is condensed in the next successive stage, giving up its heat to drive more evaporation. To increase MEE performance, each stage is run at a successively lower pressure. This technique gives the plant the flexibility to be configured for high or low temperature operation. The low temperature setting allows the use of lowgrade waste heat and helps reduce corrosion and scaling [24]. See Figure 10. The difference between MEE and MSF is that in MEE the evaporation of salt water occurs by boiling, causing scale to form on the heat exchanger surface. In MSF, the saline feed is first heated in tubes without being allowed to boil with little precipitation of scale on the inside of the tubes; it is then made to evaporate in chambers by successive flashing to lower pressures
[21].
Table 1: GOR of the main thermal desalination technologies [27]
Thermal Desalination System GOR
Solar Still
Existing HDH
MSF
MED
< 0.5
0.2 - 4.5
8 -12
12 -16
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Figure 10: Schematic diagram of multi-effect evaporator desalination process (horizontal tube-parallel feed configuration) [24]
Vapor compression (VC) processes rely on reduced pressure operation to drive evaporation. The heat for the evaporation is supplied by the compression of the vapor, either with a mechanical compressor (mechanical vapor compression, MVC, Figure 11) or a steam ejector (thermal vapor compression, TVC). After the vapor is compressed, it is condensed to generate potable water.
Vapor compression processes are particularly useful for small to medium installations.
MVC systems generally only have a single stage, while TVC systems usually have several stages. The thermal efficiency of TVC systems is increased by adding additional stages but in MVC adding stages or effects increases only capacity [24].
Membrane processes are the second important class of industrial desalination technology.
These are primarily reverse osmosis (RO) and electrodialysis (ED). RO works by applying pressure to force a solution through a membrane, retaining the solute on one side and allowing the pure solvent to pass to the other side. See Figure 12.
This process is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of
20

Figure 11: Schematic diagram of single stage mechanical vapor compression desalination process [24]
Figure 12: Block diagram of reverse osmosis operations (optimal pressure recovery devices not depicted) [24]
21

high solute concentration when no external pressure is applied. Pressurizing the saline water accounts for most of the energy consumed by RO. The higher the salinity of the solution, the larger the pressure required to perform the separation and the greater the amount of energy consumed. Consequently, RO is often the method of choice for brackish water, where only low to intermediate pressures are required. The operating pressure for brackish water systems ranges from 15-25 bar and for seawater systems from 54-80 bar (the osmotic pressure of seawater is about 25 bar). The water recovery of seawater RO systems tends to be low, typically 40%, since the pressure required to recover additional water increases as the brine stream increases. RO membranes are sensitive to pH, oxidizers, organics, particulates and other foulants. Pretreatment of all the feed water, even the 60% that will eventually be discharged, is required before it enters the membrane. Pretreatment of feed water is an important system consideration since it can have a significant impact on the cost of RO [24].
Electrodialysis (ED) is used to transport salt ions from one solution, through ion-exchange membranes, to another solution under the influence of an applied electric potential difference.
ED utilizes a direct current source and a number of flow channels separated by alternating anion (negatively charged) and cation (positively charged) selective membranes to achieve the separation of water and dissolved salts (Figure 13) [24].
This process is done in a configuration called an electrodialysis cell. The cell consists of a feed (diluate) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. Typically, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells. Since the driving force for the separation is an electric field, ED is capable of removing only ionic components from solution, unlike RO or distillation. Electrodialysis processes are unique compared to distillation techniques and other membrane based processes, such as reverse osmosis, in that dissolved species are moved away from the feed stream rather than the reverse. The energy required to separate the ions from solution increases with con-
22

Figure 13: Schematic diagram of electrodialysis desalination process [24] centration and so ED is generally limited to brackish water. The ED membrane units are subject to fouling and some pretreatment of the feed water is usually necessary. The electrodialysis reversal (EDR) process was developed to help eliminate membrane fouling. In the
EDR process, the membrane polarity is reversed several times an hour. This technique has the effect of switching the brine channels to freshwater channels, and the freshwater channels to brine channels, thus breaking up and flushing out deposits [24].
With respect to process selection between RO and ED, the choice of the most relevant technology mostly depends on the feed water quality, level of technical infrastructure (availability of skilled operators and of chemical and membrane supplies) and user requirements.
For example, both RO and ED can be used for brackish water, but RO is better for seawater desalination given its higher energy efficiency at feed water salinity higher than 2,000 parts per million (ppm). ED is preferable for brackish water desalination given its relatively higher efficiency and robustness. Pretreatment is usually more strict in the case of RO since RO membranes are very susceptible to fouling. On the other hand, since ED removes only ions from water, additional measures like disinfection or removal of particles may be required
23

[28].
2.3.2 Limitations of Conventional Technologies
Conventional desalination units are centralized due to their large size and required energy input. They are driven with waste heat or conventional generated power and are mainly operated in or near urban centers at relatively huge capacities. These complex and mostly standardized technologies are difficult to downscale to sizes smaller than 500 m ³ /day or to adapt for use within decentralized applications like rural villages that are desperately in need of clean, potable water [29]. Decentralized water production is important for remote regions, which have neither the infrastructure nor the economic resources to run conventional desalination plants [26]. Generally, water must be transported via pipes or trucks to remote regions that need it. Water transport can be prohibitively expensive, limiting the quantity of freshwater that reaches the people in rural areas. It is important to have a decentralized water supply to meet the immediate water needs of people living in those areas.
Another limitation of conventional desalination technologies is that they require expertise to operate and maintain. People in remote, rural areas would have to be trained in operating and maintaining any conventional desalination technology in order to mitigate scaling and increase system life. Trained labor for technical support of the technology is a serious issue that needs to be adequately addressed for sustainable use of small-scale desalination applications in less developed areas.
Water and energy are intrinsically connected because conventional desalination processes consume large amounts of energy to produce potable water. Figure 14 shows the energy use estimates for the saltwater desalination technologies described above. The range shown for MED/TVC covers simple MED as well as combined MED/TVC plants. It should be noted that power consumption does not include power losses induced by cogneration due to increasing outlet temperature at the turbine. Furthermore, plant cost increases with product water quality and energy efficiency [30].
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Figure 14: Characteristics of the two main thermal desalination technologies and the two main mechanical desalination technology options [30]
In addition to the issue of massive energy consumption, most desalination plants using these technologies are fossil-fuel driven. This results in green house gas emissions and a large carbon footprint, which have detrimental effects on the environment. Fossil-fuel powered desalination plants are also subject to the price volatility and availability of oil, which can make energy use quite expensive. Desalination technologies based on renewable energy are desirable to reduce the scarcity of energy, high cost and environmental impact issues associated with conventional desalination technologies.
2.3.3 Renewable Energy Desalination
Renewable energy desalination is increasingly being considered for wider scale implementation, especially in developing regions, given the limitations of conventional desalination technologies. Remote regions like inland rural villages, coastal areas and little islands tend to have available renewable energy sources (RES). Since conventional energy supply is not always possible or easily implemented in these isolated regions, RES represent the best energy supply option for autonomous desalination systems. Renewable energy (RE) driven desali-
25

nation promotes self-sufficiency. The operation and maintenance (O&M) of these systems in remote areas is often easier than that of conventional desalination units. Furthermore, the implementation of RE-desalination systems enforces sustainable socioeconomic development by utilizing local resources. Given the fact that seawater desalination processes are highly energy-consuming, RE provides an unlimited source of energy, allowing for diversification of energy resources. This situation helps to avoid dependence on external energy supply.
Over the last two decades, numerous desalination systems utilizing RE have been constructed [29]. Most of these plants have been installed as small capacity research or demonstration projects. Renewable energies and desalination technologies can be combined in various configurations. Figure 15 shows some possible combinations where SD stands for direct solar distillation, MEH is Multi-Effect Humidification and MD are membrane distillation systems.
Figure 16 presents the installations of several desalination processes in conjunction with renewables, regarding small-scale systems up to 50 m 3 /day. As seen in Figure 16, the most popular combination is Photovoltaic (PV) with Reverse Osmosis followed by wind. However, all RE-desalination systems combined represent less than 1% of the total desalination capacity [29]. Figure 17 shows recommended RE-desalination systems based on the system size, the RE source available and the product water.
Photovoltaic (PV) Desalination
Since RO and ED require electricity to power their membrane processes, PV is typically the most applicable RE source. PV as well as RO and ED are mature and commercially available technologies. The feasibility of PV-powered RO or ED systems, as valid options for desalination at remote sites, has been proven and there are commercially-available, standalone, PV-powered desalination systems. The main problems of these technologies are the high system cost, availability of PV cells in remote regions and membrane fouling [28].
Accelerated membrane fouling may occur due to variable operation of the desalination system based on the intermittency of the RES. Fouling can result in frequent membrane cleaning
26

Figure 15: Possible combinations of renewable energy systems with desalination technologies
[29]
Figure 16: Renewable energy-driven desalination processes and energy sources [29]
27

Figure 17: Recommended renewable-energy desalination combinations [28] and increased membrane replacement. The repetitive starting due to RES intermittency can also cause other problems like inverter overload and the need to flush the membranes after each stop of the RO [31].
Given solar intermittency, energy storage is an important issue for PV desalination. Batteries are used for energy storage for PV cells because they provide backup power when there is no available solar resource, thereby prolonging operation of the desalination system. However, non-sealed deep cycle batteries require careful maintenance and consequently higher skill levels for sustainable operation. These higher skill levels do not tend to be locally available in remote areas and so technology training is needed. Battery replacement can also be expensive: while they must be replaced every five to seven years or longer if properly maintained, without proper maintenance they will only last one year. Technically feasible alternatives are batteryless, energy recovery PV desalination systems, and there has been successful operation of batteryless PV desalination systems. With energy recovery, a tested batteryless PV desalination system was able to use its energy very efficiently, making up for the absence of batteries [31].
Wind Desalination
The electrical or mechanical power generated by a wind turbine can be used to power desalination plants. Like PV, wind turbines are mature, commercially available technologies.
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Wind turbines have great potential for seawater desalination along coastal areas that have a high availability of wind energy resources. Wind turbines may be coupled with RO and
ED desalination units [28]. A number of units have been designed and tested with regard to the coupling of wind turbines and RO [31]. Most of them have been installed at the
Canary Islands, Spain: a 200 m 3 /day wind–RO plant for brackish water desalination, a 56 m 3 /day hybrid diesel–wind–RO plant providing fresh water and electricity for local people, a battery-less wind–RO and a wind–ED experimental plant [28]. There have also been a few applications of mechanical VC installations powered by wind turbines. For example, a wind MVC plant was installed in Gran Canaria in 1999. The main conclusion from this installation was that the start-up process took too long and that conventional MVC was not compatible with intermittent operation, as a hard layer of scale developed within a few weeks time [31].
Hybrid Desalination
Desalination hybrid units have also been implemented. A hybrid desalination system can be a combination of fossil-fuel and RES or of multiple RES. For example, in Loughborough,
UK in 2003, a 0.5 m 3 /hour PV-wind turbine-RO system was commissioned without batteries.
It was demonstrated that this system is technically possible and that high energy efficiency can be achieved [31]. In a recent paper, Carta et al. presented a fully autonomous, batteryless system, which consists of a wind farm supplying a group of eight RO modules and in the
Canary Islands, Spain there is a hybrid diesel-wind RO plant providing potable water. Some hybrid wind-solar desalination systems are used to capture the two forms of renewable energy based on the fact that for some locations, according to metrological data simulations, the wind and solar time profiles do not coincide [28].
Nuclear Desalination
Nuclear desalination can be employed for producing pure water. Cogeneration nuclear plants that produce thermal energy, electricity and pure water can help to reduce the cost of desalinated water. Recent studies have shown that only RO and MED are compatible for
29

combination with nuclear desalination. The modular high temperature gas cooled reactor
(MHTGR) and the liquid metal fast breeder reactor (LMFBR) can be employed in the new design of water and energy cogeneration plants because of their small size and compatibility with desalination applications. The average annual cost of water production consists of annual production charges, operation and maintenance cost and fuel cost. Nuclear plants are very capital intensive. It was reported that the annual capital charges represent more than 70% of the total annual average production cost. Despite the cost, nuclear energy is used for desalination purposes in the city of Skeichenko in Russia. The complex is a large multi-purpose plant and it has been supplying the city with fresh water, electricity and thermal energy. It provides 0.14 million m 3 of fresh water per day and generates 150MW of electric power [32].
Ocean Power Desalination
Wave energy and wave-powered desalination technologies are in the prototype stage and are not yet commercially available. Wave-power desalination has high potential because the two main ingredients of (wave) energy and (sea) water are both available in abundance and at the same location. The wave-powered desalination plant prototypes built have all used reverse osmosis for the desalination process. The RO plants have either been powered by electricity generated by a wave energy plant or directly by using sea-water pressurized by the action of the waves. Current wave-powered desalination technologies are large-scale and typically with unit capacities in the range of 500–5,000 m 3 /day. Smaller desalination units
(less than 500 m 3 /day) are technically feasible but the development effort for this size of unit is currently small. Wave-powered desalination can be done in three ways: 1) direct pressurization of sea-water (avoiding the generation of electricity) that is then fed into a RO desalination plant to produce fresh water 2) creating a temperature difference between the water surface and deep sea-layers through a process called Ocean Thermal Energy Conversion
(OTEC), which provides low grade thermal energy suitable for distillation processes and 3) tidal energy, which can be extracted using tidal barrages or tidal turbines to provide energy
30

as a rotating shaft similar to wind turbine desalination [29].
Geothermal Desalination
Geothermal energy is a mature technology that can be used to provide energy for desalination at a competitive cost. Geothermal desalination may be appropriate at sites where drinking water is scarce and geothermal resources with temperatures of 80-100 ° C can be developed at acceptable costs ( < $11.2/GJ and < $3.3/m 3 respectively). For reservoirs with higher temperatures there is also the option to generate geothermal power for use in a desalination plant [29].
Geothermal energy sources may be classified in terms of the measured temperature as low
( < 100 ° C), medium (100 ° C - 150 ° C) and high temperature (> 150 ° C). The main advantage of geothermal energy relative to other renewable energy technologies is that thermal storage is unnecessary since it is both continuous and predictable. Geothermal energy can be utilized in a variety of different ways for desalination. Geothermal energy can be directly used in combination with thermal desalination technologies like MED, MEH, TVC and MD (low temperature) or with MSF (medium temperature). Moreover, the thermal energy of high temperature geothermal fluids can be converted into electricity or shaft power, permitting the coupling with other desalination systems like RO, ED and MVC [29].
It is recognized that there is significant potential to improve desalination systems based on geothermal energy. In the 1970’s, the first geothermal energy powered desalination plants were installed in the U.S., testing various potential configurations for the desalination technologies, including MSF and ED [28]. During the 1990’s a research project on Milos Island in
Greece demonstrated that it is technically feasible to utilize low enthalpy geothermal energy for electricity generation and seawater desalination [29].
Solar thermal Desalination
Megawatt scale solar power generation using Concentrating Solar Power (CSP) technology can be achieved by using any one of the three main types of concentrating solar power systems: linear concentrator, dish/engine and power tower systems. All of these configu-
31

rations are based on glass mirrors that continuously track the position of the sun, using the reflected sunlight to heat a fluid flowing through the tubes. The hot fluid then is used to boil water, producing high-pressure, high-temperature steam, for use in a conventional steam-turbine generator to produce electricity [33].
Several configurations are possible for CSP-desalination plants: (i) MSF distillation units operating with steam extracted from steam turbines or supplied directly from boilers; (ii) low-temperature MED using steam extracted from a turbine and; (iii) seawater RO desalting units supplied with electricity from a steam power plant or from a combined gas/steam power cycle. Currently, no commercial or even demonstration installations of CSP combined with desalination exist [29].
2.3.4 Humidification Dehumidification (HDH) Desalination
Nature uses solar energy to desalinate ocean water through the water cycle, as shown in
Figure 18. In the water cycle, the sun’s solar irradiation evaporates a portion of the ocean’s surface water and the water vapor rises humidifying the surrounding air which acts as a carrier gas. The humidified air rises, convects and condenses forming clouds. The clouds then “dehumidify” in the form of rain. The manufactured version of this natural process is known as the humidification dehumidification (HDH) desalination cycle.
Figure 18: Water cycle [26]
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The predecessor of the HDH cycle is the simple solar still. The solar still is similar to a greenhouse system in the manner in which it captures the solar energy. The incident solar radiation is transmitted through the glass cover or similar transparent material having the property of transmitting incident short-wave solar radiation and it is absorbed as heat by a black surface in contact with the salt water in the basin of the still. Some of the water evaporates and the water vapor condenses on the surface of the solar still, which is at a lower temperature because it is in contact with the ambient air, and is collected for use. See Figure
19. Well-designed units can produce 2.5 - 4 L/m 2 per day [29]. Solar stills have the advantage of ease of construction and maintenance. However, they have several disadvantages. The most prohibitive drawback of a solar still is low efficiency. For a solar still the GOR is less than 0.5. Thus, large areas of land are required to produce relatively small amounts of water.
Another disadvantage of the solar still is that the various functional processes (solar absorption, evaporation, condensation and heat recovery) all occur within a single component, reducing its thermal efficiency. A major improvement in solar still design is possible through the multiple use of the latent heat of condensation in the still [34]. Some multi-effect still designs recover and reuse the heat of condensation, increasing the still efficiency but the overall performance is still relatively low [26]. By separating these functions into distinct components, thermal inefficiencies may be reduced and overall performance can be increased.
The HDH process is the most promising recent development in solar desalination. The
Figure 19: Solar still [26]
33

HDH process is based on the fact that air can be mixed with large quantities of water vapor. The vapor carrying capability of air increases with temperature. The HDH technique is especially suited for seawater desalination when the demand for water is decentralized.
Several advantages of this technique include flexibility in capacity, moderate installation and operating costs, the possibility of using low-grade thermal energy (solar, geothermal, recovered energy or cogeneration) and simplicity [34]. The process is easy to operate and it does not require skilled operators. Another advantage is that the recovery ratio, the amount of water produced per kilogram (kg) of seawater feed, tends to be lower for HDH than conventional systems. This feature reduces the need for brine pre-treatment or brine disposal processes. Some pre-treatment or bleeding of the water leaving the humidifier in closed water cycles is needed, however, to prevent accumulation of salt and fouling of the heat exchangers in the HDH unit. The HDH process consists of three subsystems: (a) the air and/or water heater, which can use various sources of heat like solar thermal, (b) the humidifier or the evaporator and (c) the dehumidifier or condenser [26]. In this process, air is heated and humidified by hot water received from a solar collector. It is then dehumidified in a large surface condenser using relatively cold saline feed. Most of the latent heat of condensation is used for preheating the feed. The simplest form of HDH is illustrated in
Figure 20.
2.3.5 Types of HDH Systems
HDH systems are classified into three broad categories. One category is based on the source of energy used, such as renewable energy. For example, solar thermal HDH or hybrid HDH.
The second classification is based on the cycle configuration. There are two main types of
HDH cycles: closed-water, open-air (CWOA) cycle and closed-air, open-water (CAOW). An open-water, open-air cycle is also possible but since it has a lower thermal efficiency than the other two cycles, it will not be discussed. The air in these systems is circulated by natural or forced convection (fans). Figure 21 illustrates these two types of cycles.
34

Figure 20: HDH desalination (air heated, open cycle) [26]
In a CWOA cycle (Figure 22) the closed-water circulation is in contact with a continuous flow of cold outside air in the evaporation chamber. The air is heated and loaded with moisture as it passes upwards through the falling hot water in the evaporation chamber.
After passing through a condenser cooled with cold seawater, the partially dehumidified air leaves the unit, while the condensate (distillate) is collected. The water is recycled or recirculated. Incoming cold air provides a cooling source for the circulating water before it re-enters the condenser. The productivity of units working on this principle is high, but the power required for air circulation is also very high [34]. One disadvantage of the CWOA cycle is that when the humidification process does not cool the water sufficiently, the water temperature to the inlet of the condenser is higher, resulting in lower air dehumidification and lower water production [26]. However, in the case where efficient humidifiers are used, cooling the water as low as possible up to the limit of the ambient wet-bulb temperature, the closed water system yields more water than the open-water system.
35

Figure 21: HDH cycles [26]
In a CAOW cycle (Figure 23) the humidifier is irrigated with hot water and the air stream is heated and humidified using the energy from the hot water stream. The humidified air is cooled in a heat exchanger using seawater as the coolant. The seawater gets preheated in the process and is further heated by a heat source before it returns to the humidifier. The dehumidified air stream from the condenser is then circulated back to the humidifier. Experimental evidence has shown that for the closed-air, water-heated cycle, natural circulation of air yields better efficiency than forced circulation of air [26].
36

Figure 22: HDH unit with closed water cycle/open-air [34]
It is of critical importance to understand the relative technical advantages of each of these cycles and to choose the one that best meets the user specifications in terms of thermal efficiency and water production cost.
The third classification of HDH system is based on whether the air or the water is heated.
The performance of the system heavily depends on whether the air or water is heated. There is extensive knowledge of solar water heating devices but relatively little work has been done on air heating solar collectors. Typically, air-heated systems have higher energy consumption than water-heated systems because in the air-heated cycle the air heats up the water in the humidifier and this energy is not subsequently recovered from the water [26]. On the other hand, in the water-heated cycle, the water stream is cooled in the humidifier and the energy is transferred or recovered in the air stream. Enhanced latent heat recovery is needed to minimize the energy consumption and the resulting cost of these cycles. Figure 21 shows the different HDH cycle combinations discussed above.
37

Figure 23: HDH unit with closed-air/open-water cycle [26]
2.3.6 Possible Improvements to the HDH Cycle
A couple of methods can be used to improve the solar HDH desalination technology. These methods include sub-atmospheric pressure operations through the use of a vacuum and using thermal storage for sustained system operation even when the solar or RES is unavailable.
Operating the HDH unit at pressures below atmospheric increases the humidity ratio, resulting in an increase in water production [26]. Adding thermal energy storage to the HDH system can result in an improvement of the overall system efficiency by enabling 24-hour operation of the unit [35]. Installing thermal storage equipment such as hot water tanks is one way to improve the system efficiency. A major factor prompting 24-hour per day operation of these HDH units is the realization that the major capital cost of these units is due to the humidifier and condenser [34]. Continuous (24 hour/day) operation and distillate production of these HDH units helps reduce the cost of water, making HDH an economically feasible option relative to small-scale (5-100 m 3 /day) RO desalination systems.
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Solar-driven HDH has potential to help meet the water needs of people in remote, coastal areas without sufficient access to freshwater. Concerning the application of solar HDH in the developing world, the challenge for the near future seems to be the development of small, autonomous, modular, flexible and reliable units, offering O&M at reasonable cost, in order to serve the segment of isolated users [28]. Although more research is needed on HDH and its system costs, once the technology is further developed and proven on a larger scale, it can play a significant role in increasing freshwater supply in coastal areas. Sections 3.2 and
3.3 discuss cooling towers and cooling tower fills respectively.
A cooling tower is a heat rejection device, which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature [36]. The type of heat rejection in a cooling tower is termed "evaporative" in that water is evaporated into a moving air stream with the objective of cooling the water stream [37]. The heat from the water stream transferred to the air stream raises the air’s temperature and its relative humidity (usually to 100%), and this air is discharged to the atmosphere. Evaporative heat rejection devices such as cooling towers are commonly used to provide significantly lower water temperatures than are achievable with non-direct contact heat rejection devices like conventional heat exchangers [36].
Common applications for cooling towers are providing cooled water for refrigeration, airconditioning, industrial processes and electric power generation. The smallest cooling towers are designed to handle water streams of only a few gallons of water per minute, while the largest cool hundreds of thousands of gallons per minute for large power plants [36].
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There are two principal types of cooling towers: counter-flow and cross-flow. Each has the same fundamental components, but the configuration of these components differs to accommodate the difference in the air stream direction. In a counter-flow cooling tower,
Figure 24, air travels upward through the fill or tube bundles, opposite to the downward motion of the hot water sprayed from above [36]. Heat and mass are transferred and the water enthalpy decreases while that of air increases [38].
In a cross-flow cooling tower, Figure 25, air enters through the side of the fill and leaves from the top as the water moves downward [36]. Crossflow towers have a smaller footprint than counter-flow towers of the same capacity. This feature can be an advantage for sites where space is limited [39].
Counter-flow towers are the more common tower type and have the advantage of lower pumping costs (because the water is generally pumped to a lower elevation than in crossflow towers of similar size) [39]. Thermodynamically, the counter-flow arrangement is more efficient, since the air-water enthalpy potential difference is held approximately constant throughout the process, resulting in a higher thermodynamic efficiency. Ultimately, the economic choice between a counter-flow and cross-flow cooling tower is determined by the effectiveness of the fill, design conditions and the costs of tower manufacture [41].
Cooling towers are also characterized by the means by which air is moved. Mechanicaldraft towers rely on power-driven fans to draw or force the air through the tower [36]. The two types of mechanical-draft towers are forced-draft and induced-draft. In the induced-draft tower, the fan is located internally, at the top of the tower and air is drawn or induced from the bottom of the tower. In the forced-draft tower, the fan is mounted at the base and air is forced in at the bottom and discharged at low velocity through the top. This arrangement has the advantage of locating the fan and drive outside the tower. However, because of the low exit-air velocity, the forced-draft tower is often subjected to excessive recirculation of the humid exhaust vapors back into the air intake, reducing tower performance. The induceddraft tower is better than the forced-draft tower because the induced draft tower eliminates
40

Figure 24: Counter-flow cooling tower [40]
Figure 25: Cross-flow cooling tower [40]
41

the poor air distribution that occurs from the high velocity fan discharge into the base of the tower. On the other hand, the induced-draft tower has the problem of the hot, humid exit air corroding the fan. The induced-draft tower is the most common type used in the
United States [41].
Natural-draft cooling towers use the buoyancy of the exhaust air rising in a tall chimney to provide the draft. Natural-draft towers can be either counter-flow or cross-flow types and operate using those same principles. The heat removed from the water and transferred to the air causes the warm, moist air, leaving the top of the fill to rise naturally (induces a draft), creating a continual air stream upward through the tower [39]. Since the air inlet temperature is usually lower than that of the water inlet temperature, the water is cooled both by evaporation and sensible heat loss or heat that is removed without phase change
[37]. Natural-draft towers have extremely high construction costs but low operating cost, since there is no mechanical equipment needed to move the air. This high initial cost makes them practical only for applications having very large water volumes, such as large power plants [39]. A fan-assisted natural-draft cooling tower employs mechanical draft to augment the buoyancy effect [36].
The set of experiments reported in this thesis use a counter-flow cooling tower and so counter-flow cooling towers will be the focus of the remainder of this section. As mentioned above, the counter-flow tower cools water by spraying hot water from above into an air stream from below. The heat-transfer process involves (1) latent heat transfer owing to vaporization of a small portion of the water and (2) sensible heat transfer due to the difference in temperature of water and air. Approximately 80% of the heat transfer in a standard cooling tower is due to latent heat and 20 percent to sensible heat [41].
An indication of the moisture of the air is its wet-bulb temperature. Practically, the cold water or exit water temperature approaches but does not equal the ambient air wet-bulb temperature in a cooling tower. This is because it is impossible to contact all the water with fresh air as the water drops through the wetted fill surface to the basin. The approach of the
42

cooling tower is the difference between the cold water temperature and the ambient or inlet wet-bulb temperature. In practice, cooling towers are seldom designed for approaches less than 2.8
° C. Important cooling tower performance factors include air-to-water contact time or water retention time, amount of fill surface, fill height, air and water mass flow rates and breakup of water into droplets [41].
In order to increase the cooling rate, the interface area between air and water is increased by providing packed beds or baffles which are also known as fills. Cooling tower fills play an important role in increasing the effective surface contact area between air and water to promote better heat and mass transfer. Fills are discussed in greater detail in section 3.3.
There are two general categories of cooling tower fills: structured or systematically-arranged and random or dumped. Splash fills and film fills fall under the category of structured packings. Random packings are elements with a given form dumped randomly in the column over its supporting grid. The advantages of random packings are easy production and easy dumping. Their main disadvantages are poor distribution of the phases over the crosssection of the apparatus and often higher pressure drop relative to structured packings [42].
Structured packings are packings with regular shape and are used when it is important to have a low gas-flow pressure drop. They are usually crimped layers of corrugated sheets or wire mesh and sections of these packings are stacked in the column [41].
The most commonly used cooling tower fills are film fills. They form a thin layer of water over the fill surface and drive cooling performance by having a large surface area of water film in contact with the cooling tower air. This arrangement reduces the problem of carryover of water droplets into the atmosphere and allows higher air velocities to be used
[43]. Film fills also typically have a lower air-side pressure drop as compared to splash fills, where a large water surface area is achieved by forming droplets [44]. However, water quality
43

must be good for film fills to be used; otherwise, fill clogging and fouling will result.
Splash fills typically are used where water quality is poor and where fill fouling occurs.
Splash fills work by breaking up the hot circulating water into small droplets that create an increased surface area, which allows for both convective and evaporative cooling. Splash fill designs can be grouped into two categories: profile designs and grid packs [44]. Grids are systematically arranged packings with an open-lattice structure [41]. Typical splash fills have about half the thermal performance of film fills. The lower thermal performance is due to the splash fill’s inability to equal the surface area of film fills coupled with the higher air-side pressure drop of splash fills [44].
The objective of any packing is to maximize efficiency for a given capacity, at an economic cost. To achieve these goals, packings are shaped to maximize the specific surface area ( i.e. surface area/unit volume), spread the surface area uniformly, maximize the porosity per unit column volume, minimize friction and pressure drop, and minimize cost. An additional packing functional requirement for the particular experiment in this thesis is ease of packing construction from locally available materials in Haiti. A tradeoff exists when determining the ideal packing size because maximizing packing efficiency (specific area) and maximizing capacity (void fraction) are in direct conflict [41]. Thus, the selection of the dimensions of the packing can be made only through an optimization procedure. The geometrical characteristics that must be measured for the packing are the packing nominal size ( D p
), the specific surface area ( a ) in m 2 /m 3 and the void fraction ( E ) in m 3 /m 3 [42].
The polyvinyl chloride (PVC) CF-1200 and CF-1900 Brentwood film fills and louffa, charcoal and bamboo custom fills that were tested using the benchtop cooling tower are shown in Figures 26, 27, 28, 29 and 30 respectively.
The void fraction of the CF-1200 and CF-1900 fills is approximately 97%. The specific surface area of the CF-1200 and CF-1900 fills is 226 m 2 /m 3 and 157.5 m 2 /m 3 respectively.
The individual louffa and bamboo pieces both had heights of 0.152 m. The approximate diameter of the louffa and bamboo pieces was 0.07 m and 0.013 m respectively. The car-
44

Figure 26: CF-1200 fill
Figure 27: CF-1900 fill
45

Figure 28: Louffa fill
Figure 29: Charcoal fill
46

Figure 30: Bamboo fill bonized corn cob pieces had an upper limit of length and an upper limit of diameter of 0.068
m and 0.011 m respectively.
Merkel [45] developed a method for the performance evaluation of cooling towers in the 1920s.
The Merkel method of the cooling tower heat transfer process is the most generally accepted and its employment is recommended by international standards [46]. Merkel analysis is based on enthalpy potential difference. Each particle of water is assumed to be surrounded by a film of air, and the enthalpy difference between the film and the surrounding air provides the driving force for the cooling process [41]. Merkel analysis relies on several critical assumptions to simplify the solution of the complex process of heat and mass transfer in wet-cooling towers: namely, that the Lewis number ( Le f
) is equal to unity, the exiting air is saturated and the reduction of the water flow rate due to evaporation is neglected in the energy balance
[46]. Lewis number is a dimensionless number defined as the ratio of thermal diffusivity to mass diffusivity. It is used to characterize fluid flows where there is simultaneous heat and
47

mass transfer by convection. Kröger [43] gives a detailed derivation from first principles of what is referred to as Merkel’s number for a counter-flow configuration. Merkel’s equation is given by Equation 2:
M e =
KaV
L
=
T
ˆ wi c pw dT w
( h !
− h )
T wo
(2) where K = mass transfer coefficient, kg/m 2 s; a =surface area per unit volume m − 1 ; V =active cooling volume, m 3 ; h !
= enthalpy of saturated air at bulk water temperature, J/kg; h = enthalpy of air-water mixture at wet-bulb temperature, J/kg; and T wi and T wo are the entering and leaving water temperatures respectively, ° C; L = water flow rate, kg/s; and
Me is the Merkel number, or transfer characteristic according to the Merkel method. The right-hand side of Equation 2 is expressed entirely in terms of air and water properties and is independent of tower dimensions [41]. It can be solved if the water inlet and outlet temperatures, air inlet dry bulb and wet bulb temperatures, air outlet dry and wet bulb temperatures, water mass flow rate, and air mass flow rate are known [46]. Equation 2 was solved using the 4-point Chebyshev numerical integration method as shown in Equation 3:
KaV
L
=
T
ˆ wi cp w dT w
( h !
− h )
T wo
!
T wi
− T wo
(
4
1
∆ h
1 where
∆ h
1
= value of ( h !
− h ) at T
∆
∆
∆ h h h
2
3
4
= value of ( h !
− h ) at T
= value of ( h !
− h ) at T
= value of ( h !
− h ) at T wo wo wi wi
+ 0 .
1( T wi
− T wo
)
+ 0 .
4( T wi
− T wo
)
− 0 .
4( T wi
− T wo
)
− 0 .
1( T wi
− T wo
) [41].
1
+
∆ h
2
1
+
∆ h
3
+
1
∆ h
4
) (3)
It is important to note that if the transfer characteristic of a wet-cooling tower fill is determined by a particular method, like the Merkel method, the same method must be used in the subsequent wet-cooling tower design calculations [46].
Figure 31 provides a graphical representation of the water and air relationships and the driving potential which exist in a wet, counter-flow cooling tower process. The water operating line or enthalpy of saturated air at a given water temperature is shown by line AB
48

Figure 31: Cooling tower process heat balance [41] and is fixed by the inlet and outlet tower water temperatures.
The air operating line or enthalpy of air stream begins at C, vertically below B and at a point having an enthalpy corresponding to that of the entering wet bulb temperature.
Line BC represents the initial driving force ( h !
− h ) . The coordinates refer directly to the temperature and enthalpy of any point on the water operating line but refer directly only to the enthalpy of a point on the air operating line. The liquid-gas mass flow rate ratio
L/G is the slope of the operating line. The air leaving the tower is represented by point D.
The cooling range is the projected length of line CD on the temperature scale and is the difference between the hot-water temperature entering the tower and cold-water temperature leaving the tower. The cooling tower approach is shown on the diagram as the difference between the cold-water temperature and the ambient wet bulb temperature. The integral in
Equation 2 is represented by the area ABCD in Figure 31 [41]. This value is known as the tower characteristic, varying with the L/G ratio.
49

3.5.1 Objective
The objective of the experiments was to determine the packing performance characteristics, heat transfer and pressure drop of several packings made using local materials (bamboo, carbonized corn cobs or charcoal and louffa) available in Haiti. The performance of the fills was assessed using the Merkel method of analysis and the following correlations were determined:
• The fill transfer characteristic (i.e. Me) correlations as a function of water-air mass flow ratio ( L/G )
• The fill pressure drop empirical correlations as a function of air velocity and water mass flux ( L/A ) .
3.5.2 Experimental Setup
Apparatus Overview
Experiments were carried out to determine the thermal performance characteristics of custom-made packings under steady state conditions. The apparatus can be considered in terms of the column, water circuit, air circuit and the measuring devices. The air circuit consists of the fan, the air distribution chamber and the orifice. The water circuit consists of the water distribution system, the droplet arrester or drift eliminator and the basin. Refer to Figure 32 for the schematic of the benchtop cooling tower. The cooling tower column dimensions are 150 mm x 150 mm x 600 mm high. To facilitate observation of the bed, the column was constructed with transparent PVC.
For Figure 32 the temperature measurement descriptions are given in Table 2.
A picture of the apparatus is provided in Figure 33.
50

Figure 32: HC891 benchtop forced-draft cooling tower unit
Table 2: Benchtop cooling tower temperature measurements
Temperature
T ai
T wbi
T ao
T wbo
T wi
T wo
Description
Inlet air dry bulb temperature
Inlet air wet bulb temperature
Exit air dry bulb temperature
Exit air wet bulb temperature
Inlet water temperature
Exit water temperature
51

Refer to Appendix A for the experimental apparatus limitations, specifications and the desired experimental parameter ranges.
Water Circuit
The water circuit is an open water loop, taking warm water from the sink, to the water flow meter or control valve, to the column cap where its temperature is measured before it is sprayed over the packing through the water nozzle. The water is uniformly distributed over the top of the packing and as it spreads over the packing, a large thin film of water is exposed to the air stream. During its downward passage through the packing, the water is cooled, largely by the evaporation of a small portion of the total flow. The cooled water falls from beneath the packing into the basin, from where it flows past a thermocouple and into the load tank, where it exits to the drain. A Dwyer rotameter is used to measure the water flow rate.
Air Circuit
Air from the atmosphere enters the fan at a rate that is controlled by the intake damper setting. The ambient air conditions have a significant effect on the cooling tower performance.
The fan forces the air into the distribution chamber and the air passes wet and dry bulb thermocouples before it enters the packed column. As the air stream flows through the fill, its moisture content increases and the water is cooled. On leaving the top of the column, the air passes through the drift eliminator, which traps most of the entrained droplets and returns them to the fill. The air is then discharged to the atmosphere via the air-measuring orifice at the tower outlet. The air passes wet and dry bulb thermocouples before it exits the tower. The air mass flow rate (G) is measured by a pressure differential across the air exit orifice, connected to an inclined manometer. The air flow rate may be calculated according to Equation (4), where Y is a constant equal to 0.00438 m 2 and ρ
G is the density of moist air (kg moist air/m 3 ):
G = Y
!
ρ
G
∆ P orif ice
.
(4)
Measurements
52

Figure 33: Actual benchtop cooling tower apparatus
The quantities measured in the experiments are given in Table 3. The outlet orifice pressure drop is measured in order to calculate the air mass flow rate, the pressure drop across the fill is measured using an inclined manometer and the water flow rate is based on the Dwyer rotameter reading. The air dry and wet bulb temperatures at the fan inlet
(base of column) and outlet (exit from column), in addition to the temperatures of the water at inlet and outlet, are measured with a digital temperature indicator with a thermocouple selector switch.
Instrumentation
Temperatures: Two pairs of wet and dry-bulb Omega type “T” thermocouples for air entry and exit from the tower respectively. Two type “T” Omega thermocouples for water entry and exit to tower. Thermocouples have an ambient reference temperature connected to the interface directly. Refer to Figure 32 for thermocouple locations.
Air flow measurement: Sharp-edged orifice with pressure tapping at tower outlet connected to an inclined manometer to determine the orifice pressure drop. Refer to Figure 32 for the location of the connection of the orifice differential pressure tap.
Fill pressure drop: Pressure tap above and below the fill to determine the fill pressure
53

drop using an inclined manometer. Refer to Figure 32 for the locations of the fill static pressure taps.
Water flow rate: The Dwyer rotameter indicates and controls the water flow rate.
3.5.3 Experimental Procedure
Inlet Water Temperature
The inlet water temperature was held fixed at 39 ° C, which was the temperature of the water exiting the sink. Brentwood Industries, a manufacturer of commercial fills, tests at
37.8
° C since it has been shown by many experiments that the Merkel analysis overestimates the heat transfer at higher hot water temperatures [47].
Air Wet Bulb and Dry Bulb Temperatures
The experimental lab’s ambient air wet bulb and dry bulb temperatures were used. The temperature variations were held within ± 2 ° C.
Data Collection
All measurements were conducted in steady state. The system took between 5-10 minutes to reach steady state. A data set for nine water-air loading (L/G) conditions at a given fill height (H) were manually recorded. At least one data point from the set was randomly picked and repeated to determine the repeatability of the results. A complete test on a fill was completed when it was tested for three distinct fill heights of 152.4 mm, 304.8 mm and
457.2 mm respectively. These fill heights were chosen in order for the fills to fit well inside the cooling tower column and for ease of cutting the fills. Data collection was done quickly once the system was stable. Experimental measurements were taken for the parameters in
Table 3.
Test Point Matrix
For a full fill test for a given fill height, nine L/G water-air loadings were tested. The nine water-air loads for the proposed experiment were experimentally determined. For uniform
54

Table 3: Cooling tower test measurements
Parameter Name
Water mass flow rate
Air mass flow rate
Inlet water temperature
Water outlet temperature
Parameter Symbol Units
L
T
T
G wi
T wo ai
Air inlet dry bulb temperature
Air inlet wet bulb temperature
Air inlet wet bulb temperature
Air outlet dry bulb temperature
Air outlet wet bulb temperature
Fill differential pressure
Ambient barometric pressure
Orifice differential pressure ∆
T wbi
T wbi
T ao
T wbo
∆
P
P
P f ill a orif ice
° C
° C
° C
° C kg/s kg/s
° C
° C
° C
Pa
Pa
Pa water distribution it was important for the water flux or water loading ( L/A ) to be between
0.8 and 4.2 kg/m 2 s [40]. Brentwood noted that it gets incomplete wetting of packings with water loadings less than 2.7 kg/m 2 s. Since the test was done in steady state, the air-water mass flow rate ratio was never high enough to flood the fills. These water-air loading set points were in the range 0.25 < L/G < 5. The variation of the mass flow rate ratio ( L/G ) was obtained by appropriately varying the air and water mass flow rates respectively. The measurements were taken starting from the lowest L/G ratio to the highest. The test was repeated for the given packing three times at the three different fill heights. Table 4 displays the chosen L/G test conditions for the CF-1200 fill for the three different fill heights as an example of how the L/G measurements were recorded for each fill.
Transfer Characteristic Dependencies
The transfer characteristic (Me) correlations for wet, countercurrent cooling tower fills are functions of L/G and the fill height. Inlet air dry bulb and wet bulb temperatures and inlet water temperature do not have a significant effect on the cooling tower fill loss coefficient once corrected for density. Therefore, the loss coefficient was determined by measuring the pressure drop across the fill [48].
Data Analysis
55

CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
CF-1200
Table 4: CF-1200 L/G test conditions for H=0.152 m, 0.305 m, 0.457 m
Fill
CF-1200
CF-1200
CF-1200
CF-1200
Height (m) L (kg/s) G (kg/s) L/G
0.152
0.152
0.152
0.152
0.152
0.152
0.152
0.152
0.152
0.305
0.305
0.305
0.305
0.305
0.305
0.305
0.305
0.305
0.457
0.457
0.457
0.457
0.457
0.457
0.457
0.457
0.457
0.038
0.057
0.132
0.132
0.132
0.132
0.132
0.132
0.132
0.038
0.057
0.126
0.126
0.126
0.126
0.126
0.126
0.126
0.038
0.057
0.126
0.126
0.126
0.126
0.126
0.126
0.126
0.025
0.069
0.072
0.067
0.052
0.044
0.038
0.032
0.072
0.067
0.070
0.052
0.043
0.037
0.032
0.028
0.029
0.026
0.071
0.071
0.069
0.052
0.044
0.038
0.032
0.029
0.026
1.8
2.4
2.9
3.4
4.3
4.8
0.5
0.8
3.9
4.3
4.8
2.4
2.9
3.3
3.9
5.2
0.5
0.8
1.9
3.1
3.6
4.1
4.7
0.5
0.8
1.9
2.6
56

From the collected data for a given fill, the Merkel number (Me) was calculated for each of the nine L/G ratios and for each of the three fill heights. From this data the transfer characteristic correlation for the fill was determined. From the pressure drop measurements
( ∆ P f i
) empirical pressure drop correlations were determined for the fill height of 457.2 mm.
Pressure drop data for the other two fill heights was not obtained and so correlations were not made for those heights.
System Issues
It is important to manage the wall water or water diverted to the wall to mitigate the issues associated with scaling up the size of the cooling tower. The same water spray system was employed in the fill test and subsequent cooling tower application of the fill to eliminate the effects of droplet size and distribution on the transfer coefficient [46].
3.5.4 Experimental Results
Validation of Cooling Tower Results
Before the custom (bamboo, louffa, and charcoal) fills were tested using the benchtop cooling tower, the cooling tower apparatus had to be validated. Two methods were used to ensure data validity using the apparatus:
1. The Omega T-type thermocouples were tested to ensure that they provided consistent, reliable data.
2. Data was taken for two Brentwood cooling tower fills, the CF-1900 and CF-1200, using the benchtop cooling tower. The transfer characteristic for each fill was compared with the transfer characteristic data that Brentwood published for those fills.
The experimental fill transfer characteristic obtained using the CF-1200 and CF-1900 fills in the benchtop cooling tower varied by at most 32.6% from the transfer characteristic data that Brentwood published. For the CF-1200 fill, the water to air mass flow ratio ranged between 1 and 3 (1 < L/G < 3). Brentwood tested the CF-1200 fill for heights of 0.61 m,
0.91 m and 1.2 m respectively. Brentwood’s CF-1200 transfer characteristic correlation was
57

KaV /L = 0 .
967(
L
G
− 0 .
779
) · H 0 .
632 .
(5)
Figures 34, 35 and 36 compare the plots for the CF-1200 transfer characteristic versus L/G for the Brentwood correlation and benchtop cooling tower experimental results for fill heights of 0.152 m, 0.305 m and 0.457 m respectively.
For the CF-1900 fill, the water to air mass flow ratio ranged between 0.5 and 3 (0.5 <
L/G < 3). Brentwood tested the CF-1900 fill for heights of 0.61 m, 0.91 m, 1.2 m, 1.5 m and 1.8 m respectively. Brentwood’s CF-1900 transfer characteristic correlation was
KaV /L = 0 .
696(
L
G
− 0 .
707
) · H 0 .
714 .
(6)
Figures 37, 38 and 39 compare the plots for the CF-1900 transfer characteristic versus L/G for the Brentwood correlation and the benchtop cooling tower experimental results for fill heights of 0.152 m, 0.305 m and 0.457 m respectively.
Fill Transfer Characteristics
A summary of the fill transfer characteristic correlations, according to the Merkel approach, is shown in Table 5 for a fill height of 0.152 m, in Table 6 for a fill height of 0.305
m and in Table 7 for a fill height of 0.457 m. The plots of the Brentwood and custom fill transfer characteristic correlations for the fill heights of 0.152 m, 0.305 m, and 0.457 m are
Figure 34: Me comparison CF-1200 (H=0.152 m)
58

Figure 35: Me comparison CF-1200 (H=0.305 m)
Figure 36: Me comparison CF-1200 (H=0.457 m)
Figure 37: Me comparison CF-1900 (H=0.152 m)
59

Figure 38: Me comparison CF-1900 (H=0.305 m)
Figure 39: Me comparison CF-1900 (H=0.457 m)
60

shown in Figures 40, 41, 42, 43 and 44.
Fill Pressure Drop
Pressure drop is an important fill characteristic. The lower the fill pressure drop, the better because this results in a lower pumping power requirement and thus less power consumption. The pressure drop for each fill was measured in Pascals at an air density of 1.1
kg/m 3 with an inclined manometer while the fan air velocity and water mass flow rate were varied. The fan air velocity was gradually increased between 0.5 m/s and 3.0 m/s and the water mass flow rate was changed to three different set points of 0 kg/s, 0.091 kg/s and
0.126 kg/s. These water mass flow rates correspond to water fluxes of 0 kg/m 2 s, 4.1 kg/m 2 s and 5.6 kg/m 2 s respectively. The pressure drop data was recorded for each setting and for each fill at a height of 0.457 m. Figures 45, 46, 47, 48 and 49 plot the pressure drop for the CF-1200, CF-1900, louffa, charcoal and bamboo fills respectively. Table 8 displays the pressure drop correlations for each fill in the form:
∆ P = ( B + C · v D air
) + L/A ( E + F · v I air
) (7) where B, C, D, E, F, and I are constants, v air is the air velocity and L/A is the water flux.
For the water flux of 5.6 kg/m 2 s the fill pressure drop in order from highest to lowest was the bamboo, charcoal, louffa, CF-1900 and CF-1200 fill. For the water flux of 4.1 kg/m 2 s in order of highest fill pressure drop to lowest: bamboo and charcoal were tied for the highest pressure drop, followed by louffa, CF-1900 and CF-1200. For the water flux of 0 kg/m 2 s the
Table 5: Transfer characteristic correlations according to Merkel approach (H=0.152)
Fill
CF-1200
CF-1900
Charcoal
Louffa
Bamboo
Merkel Empirical Relation Correlation
Coefficient
Me=0.9327
( L/G ) − 0 .
914
Me=0.5126
( L/G ) − 0 .
625
Me=0.8466
( L/G ) −
Me=1.1481
( L/G ) −
Me=0.7311
( L/G ) −
0 .
594
0 .
966
0 .
687
(R 2 )
0.9801
0.9912
0.9421
0.9948
0.9779
Correlation
Average %
Uncertainty
9.4
3.3
5.8
5.0
4.0
Correlation
Max %
Uncertainty
13.3
5.2
23.8
8.3
6.6
61

Table 6: Transfer characteristic correlations according to Merkel approach (H=0.305 m)
Fill
CF-1200
CF-1900
Charcoal
Louffa
Bamboo
Merkel Empirical Relation Correlation
Coefficient
Me=1.0723
Me=0.6377
Me=1.623
Me=1.4202
(
(
(
(
L/G
L/G
L/G
L/G
)
)
)
)
−
−
−
Me=1.2363
( L/G ) −
0
0
1
.
.
.
885
633
0 .
889
−
1 .
012
000
(R 2 )
0.9928
0.9945
0.9806
0.9933
0.9694
Correlation
Average %
Uncertainty
3.7
2.6
8.2
9.64
4.4
Correlation
Max %
Uncertainty
7.4
6.4
31.0
22.5
8.5
Table 7: Transfer characteristic correlations according to Merkel approach (H=0.457 m)
Fill
CF-1200
CF-1900
Charcoal
Louffa
Bamboo
Merkel Empirical Relation Correlation
Coefficient
Me=1.0347
( L/G ) −
Me=0.8067
( L/G ) −
Me=3.9702
Me=1.2354
(
(
L/G
L/G
)
)
−
−
Me=1.4426
( L/G ) −
0 .
848
0 .
677
1 .
723
0 .
854
0 .
724
(R 2 )
0.9659
0.9953
0.9824
0.9934
0.9000
Correlation
Average %
Uncertainty
4.1
3.1
5.6
4.8
4.7
Correlation
Max %
Uncertainty
8.3
5.0
13.9
8.8
9.9
Figure 40: CF-1200 transfer characteristic plots
62

Figure 41: CF-1900 transfer characteristic plots fill pressure drop in order from highest to lowest was charcoal, bamboo, louffa, CF-1200 and
CF-1900.
Fill Comparison
To compare the overall performance of the fills, their transfer characteristic and pressure drop had to be combined into the metric of fan power consumption. The data from the expressions for Me vs L/G and ∆ P vs air velocity with water loading (flux) as a parameter
Constant
B
C
D
E
F
I
Table 8: Fill Pressure Drop Correlations (H=0.457 m)
Equation:
CF-1200
(R 2 =0.9790)
0.3992
3.2498
2.1851
1.4459
1.7931
0.1070
∆ P = ( B
CF-1900
(R 2 =0.9107)
-222.7662
230.2587
-0.0679
1.2088
0.1074
3.3738
+ C · v D air
(R 2
) + L/A
Charcoal
=0.9687)
3.0298
81.2022
1.9405
-4.6872
17.4330
5.1459
( E +
(R
F
2
· v I air
Louffa
=0.9614)
-20.7525
26.0532
1.5815
0.4448
5.5351
1.2926
)
Bamboo
(R 2 =0.9431)
-1.3329
25.2751
1.8033
8.74027
34.9581
1.9898
63

Figure 42: Custom fill transfer characteristic plots (H=0.152 m)
64

Figure 43: Custom fill transfer characteristic plots (H=0.305 m)
65

Figure 44: Custom fill transfer characteristic plots (H=0.457 m)
Figure 45: CF-1200 fill pressure drop (H=0.457 m)
66

Figure 46: CF-1900 fill pressure drop (H=0.457 m) were entered into the CTI 3.1 Toolkit software package created by the Cooling Tower Institute
(CTI) to facilitate cooling tower design [49]. The Toolkit analysis program enabled the calculation of the power required for each fill to meet a cooling tower approach of 4 ° K.
In order to complete this analysis in Toolkit, a cooling tower range of 5 ° C, a wet bulb temperature of 26 ° C, a relative humidity of 100% and the intercept ( c ) and the slope ( m ) of the Merkel number correlation ( M e = c ( L/G ) m ) for a fill of a given height were input. The
L/G value was adjusted until the desired approach of 4 ° K was attained. From this L/G , the air mass flow rate ( G ) was calculated for a fixed water mass flow rate of 0.126 kg/s.
The centrifugal fan power, in units of Watts, was then calculated according to Equation 8 where V = specific volume of air (m 3 /kg dry air), G = air mass flow rate (kg dry air/s),
η = centrifugal fan efficiency (30%) and ∆ P f ill
= the fill pressure drop ( P a ) at a given air velocity.
P =
V · G · ∆ P f ill
η
67
(8)

Figure 47: Louffa fill pressure drop (H=0.457 m)
Figure 48: Charcoal fill pressure drop (H=0.457m)
68

Figure 49: Bamboo fill pressure drop (H=0.457 m)
In this way the effects of the fill thermal performance (Me) and pressure drop ( ∆ P ) were combined. The lower the required fan power, the better the fill performance. As shown in Figure 50 the louffa was the best custom packing because it had the lowest fan power consumption for all three fill heights. The absolute uncertainty or error displayed on the bar graphs of Figure 50 is 2.477 Watts (W). For fill heights of 0.305 m and 0.457 m the charcoal fill performed better than the bamboo fill. The fan power required for the charcoal fill was
8% and 50% less power than the bamboo fill for heights of 0.305 m and 0.457 m respectively.
However, for the fill height of 0.152 m the bamboo fill consumed 24% less power than the charcoal fill. Figure 51 displays the fan power consumption for all five of the fills that were tested for a fill height of 0.457 m. As a base for comparison, for a fill height of 0.457 m, the fan power consumption for the CF-1900 and CF-1200 was 7.8 W and 9.6 W respectively.
For the same fill height the louffa fan power consumption was 20.8 W, or approximately
2.7 times the fan power consumption of the CF-1900 fill, which had the lowest fan power consumption of all the fills tested. In order to potentially lower the pressure drop of the
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Figure 50: Fan power consumption for tested custom fills louffa fill, it can be cut vertically in half and the halves stacked front to back in the cooling tower column.
3.5.5 Error Analysis
The errors, in terms of absolute uncertainty, associated with each of the observed measurements are tabulated in Table 9. These uncertainties represent the manufacturer error associated with the actual instrument (thermocouples, inclined manometer, flow meter, barometer).
These errors were used to calculate the overall uncertainty in the Merkel number for each trial. The majority of the uncertainty is due to the T wo and the T wi thermocouple readings since the Merkel number is most sensitive to these parameters. The percent uncertainty
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Figure 51: Fan power consumption for tested fills (H=0.457 m)
Table 9: Uncertainties of measured variables
Variable Units Absolute Uncertainty
∆ P orif ice
Pa
Pa ∆ P f ill
T
L ai
T ao
T wi
T wo
T wbi
T wbo kg/s
° C
° C
° C
° C
° C
° C
3.74
4.98
0.0028
0.50
0.50
0.50
0.50
0.50
0.50
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associated with each of the measurements is in Table 10. As a result of these uncertainties, the overall uncertainty in the Merkel number was approximately 10.3%.
3.5.6 Implications for Fills Made of Local Materials
As expected, fills made of local materials such as bamboo, charcoal and louffa have a lower performance than high-efficiency, industrial PVC packings (CF-1200, CF-1900). However, of the three custom fills tested, louffa had the best overall (combined thermal and pressure drop) performance. Although its performance, measured in terms of fan power consumption, was 2.7 times worse than the Brentwood CF-1900 fill, it still has the most potential as a locally available, inexpensive fill for use in the humidifier of the HDH system. For a fill height of 0.457 m, the charcoal and bamboo fills performed 2.9 and 4.4 times worse than the louffa fill respectively. As backup, if louffa is not available, the charcoal and then the bamboo fill could be used in the humidifier.
Table 10: Uncertainty contributions
Partial Derivative % Uncertainty
∂M e/∂ ∆ P orif ice
∂M e/∂L
3.57%
0.53%
∂M e/∂T
∂M e/∂T
∂M e/∂T
∂M e/∂T
∂M e/∂T
∂M e/∂T ai ao wi wo wbi wbo
0.00%
0.00%
32.84%
62.46%
0.59%
0.00%
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In this chapter the obstacles related to the development of RE-desalination generally and
HDH desalination in particular are identified and categorized along technical, economic, social, environmental and political lines. Where relevant, this chapter also addresses these issues in the specific context of Haiti.
4.1.1 Technological Barrier
Conventional desalination technology is considered mature, although there are still significant margins that can be gained regarding efficiency increase, as well as volume and costs decrease.
Concerning RE-desalination generally, and HDH in particular, there tends to be a lack of suitable design guidelines and tools to implement these technologies on a large-scale.
Worldwide, there are only a few specialists in solar desalination installations [29]. More people must be trained in this technology. There is also a demand for standardization and performance validation in order to make these RE-desalination systems comparable.
The installation of solar thermal desalination systems needs to be as easy as possible. Some possible ideas to achieve ease of operation are self-explanatory modular components, low-tech components that can be easily and economically replaced (i.e. fills made from local materials in the humidifier of the HDH system) and having experienced water plant operators available to help with training locals in the operation and maintenance of the technology.
For many desalination technologies, the focus has been on the development of relatively large plants, so there is a lack of technologies appropriate for small-scale applications. These undeveloped technologies include small capacity pumps and control systems for decentralized desalination systems, suitable pre- and post-treatments of the water, suitable energy recovery technologies, energy storage and methods for safe and efficient disposal of the brine in inland plants [29]. Furthermore, complementary components such as energy storage equipment,
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anti-fouling materials and control systems are needed for autonomous operation of the smallscale desalination plants.
The variability of energy supply from RES is a major technical issue for RE-desalination.
There is typically a mismatch between the energy supply and demand. The designs of conventional desalination plants are based on a constant supply of energy. The HDH technology is appropriate for small-scale, decentralized use but it also has the problem of intermittent operation due to the variability of solar radiation intensity. The fluctuation of the energy supply can have negative effects on the HDH system, including biological fouling that can cause contaminated product water, increased maintenance requirements and under-utilization. Under-utilization typically leads to higher specific costs of water, which results in the plant being less commercially attractive. Energy storage can be used to increase the HDH plant operation time and to stabilize water production. However, storage options are limited. Typically for thermal energy storage, the main option today is storing it in a tank of a thermal fluid (i.e. water, oil) depending on the operation temperature.
Additionally, specific control software to guarantee stable, autonomous operation during the common oscillations of solar supply is also lacking [29].
4.2.1 Economical Barrier
Market and risk uncertainty are the primary obstacles to wide-scale deployment of REdesalination. RE-desalination is a relatively new area and little is known about its market potential. There is a lack of comprehensive market analysis as to the size, locations and segments of the market. Without such analysis, it is difficult to determine where and how to enter the market and how long it may take to receive a return on investments. Given the uncertainty associated with the return on the investment, the magnitude of risk associated with investment in the technology is unclear. Consequently, investors are hesitant to invest
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and if they do decide to invest, they require high rates of return on their investment [29].
Development of a clear and reliable market and risk analysis is needed to help correct this issue.
RE-desalination small-to-medium enterprises (SME) lack the financial resources and local knowledge needed to promote their technologies and penetrate the most promising niche markets. RE-desalination has the greatest potential in developing countries, where there is a lack of infrastructure and need for decentralized, off-grid potable water supply. Unfortunately, these markets are typically relatively far away with difficult access, inadequate currency, political risks and different cultural norms concerning business and utility services.
It is also crucial that the water produced from the technology be affordable because the customers in this market typically live off on less than $2/day. However, if the technology is affordable enough and impacts numerous people, then with economies of scale the SME can make a profit. The SME should also utilize the resources available from NGO water support programs, microfinance and financial aid (i.e. World Bank) available for installations in developing countries. All of these issues make it important for SME with appropriate desalination technologies to collaborate with local companies where the technology is to be deployed. It is also important for the SME to collect and disseminate relevant experiences and information obtained in the region to the RE-desalination community [29].
4.2.2 Economics of HDH System
The principal components of the HDH system are the solar collector, the humidifier and the dehumidifier. The solar collector is the main component of a solar desalination unit and any improvement in its efficiency will have a direct bearing on the water production rate and the product cost. The solar collector is a crucial component in the HDH system because it is the majority of the system’s capital cost: 40-45% for air heated systems and 20-35% for water heated systems. Therefore, it is important to ensure the reliability of the solar collector and to have a high specific water production. Specific water production is the amount of
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water produced per m 2 of solar collector area per day and is an indicator of the solar energy efficiency of the HDH cycle. At present there are no commercial systems that utilize a solar air heater for solar desalination. Representative values for air temperature rise and solar irradiation are 50 ° C and 1 kW/m 2 respectively. The best performing air-heating collector under these conditions has an efficiency of only 32%, where solar collector efficiency is defined as the heat gained by air divided by solar incident radiation [26]. Currently, the prices of the collectors are relatively high. Only a reduction in these prices through increased collector efficiency and cost-effective materials will enable the cost of the desalinated water to drop to economical levels, allowing solar–thermal units to compete with conventional desalination techniques [50].
The share of the fill in the total cost of the cooling tower or humidifier is normally only
10-20% depending on the type and size of the cooling tower. There is a direct relationship between the efficiency of the fill and the size of the tower. A low efficiency fill will increase the cross-sectional area of the tower and the required pumping head. To compensate, a larger fill height is required to bring the cross-sectional area of the tower to an acceptable figure. The problem is that this increases the cooling tower height, the amount of material needed for construction and the resulting cooling tower cost. A high efficiency fill, such as the film fill, will considerably reduce the required cross-section and the first year cost of a counter-flow cooling tower [51]. However, as mentioned in section 3.3, the problem with film fills is that they are more prone to fouling compared to lower efficiency splash type fills.
Energy costs can determine whether a desalination system is successful or not. If the energy costs are too high, especially in developing countries with limited energy infrastructure, the desalination system may not be feasible. For HDH systems, the energy costs associated with the condensers and pump operation, as well as the energy savings associated with the fill choice and coupling the system to waste heat energy sources, may end up being crucial in developing a commercially viable system.
For desalination, the balance between investment and operational costs is one of the main
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aspects for fundamental decisions regarding two possible strategies to minimize lifetime water costs: low investment or low operational costs [52]. There is evidence that the HDH process can economically compete with conventional desalination processes. The HDH system costs are site-specific based on local material costs and the availability of solar insolation. However, there is not sufficient data to estimate the costs of HDH given that no commercial HDH system has been analyzed for cost data in a specific region. The data in Table 11 is illustrative for displaying comparative cost trends between solar desalination and conventional RO and
MSF systems. The comparison shows that investment costs are the highest, but energy costs are the lowest in the case of RE-desalination because the energy is “ free ” and renewable.
The “ free ” energy is partially offset by increased amortization costs [50]. It is worth noting that the data in Table 11 is almost 20 years old and might no longer be valid, but the cost trends are still valid. As solar RE technology improves, the investment costs will be driven down to comparable levels to the investment costs of conventional desalination systems.
Table 12 shows a comparison of equipment capital cost for the HDH process with other conventional techniques made by El-Dessouky in a study utilizing waste heat from a gasturbine powered HDH desalination unit [50]. The main take-away from Table 12 is that the
HDH process powered by waste heat has the lowest specific cost compared to other conventional processes based on waste heat. Thus, the HDH process appears to be economically attractive, making it a suitable replacement for all other forms of solar desalination in the smaller capacity range.
For small-scale installations in the range from 500 up to 5,000 m 3 /day, water costs in
Table 11: Distribution of costs for conventional (RO and MSF) and renewable energy driven plants [50]
Type of Process Investment costs (%) Operational costs (%) Energy costs (%)
Conventional (RO)
Conventional (MSF)
Renewable
22-27
25-30
30-90
14-15
38-40
10-30
59-63
33-35
0-10
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Table 12: Comparison of HDH process (with waste heat) with other processes [50]
Parameter Process
HDH
Single MSF
Dual MSF
Single RO without energy recovery
Single RO with energy recovery
Dual RO without energy recovery
Dual RO with energy recovery
Specific
Cost
($/m 3 d)
287
1451.18
3647.4
1022.8
1127.4
1906.3
2225.8
Specific
Power
(kJ/kg)
61.26
294.21
96.519
50.420
33.619
36.20
27.20
Water to Power
Ratio (kg/kWh)
3.9772
-
23.919
-
-
36.819
49.19
the range of $1.0/m 3 - $4.6/m 3 are reported. Very small installations of capacities from 5-
100 m 3 /day are currently mainly served by small, brackish RO installations. Operation and maintenance costs are high for installations of this size. Regarding seawater RO, maintenance companies report maintenance and energy costs between $1.3/m 3 - $4.2/m 3 including labor costs. These costs result in water prices ranging from $2.1/m 3 up to $6.3/m 3 . The main advantage of HDH relative to small-scale RO is its low maintenance demand and no need of chemical pretreatment. Additionally, the use of low temperature heat as a main energy source for HDH allows the application of waste heat from small diesel or gas electrical generators
(combined heat and power, CHP) for supplying relatively inexpensive heat where the grid is nonexistent or unreliable. This development makes HDH appropriate for remote villages.
Savings in thermal to mechanical conversion losses allow HDH to compete economically with the RO process for such decentralized applications [52].
The main conclusion drawn from Table 13 is that RO has light cost advantages at smallscale operations over 5 m 3 /day in installations where standard electrical grid connection is available and electricity prices are at or below $0.22/kWh. In all other cases when comparing costs and ease of operation, the use of thermal desalination units such as HDH should be considered [52].
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Table 13: Cost comparison for small-scale desalination methods [52]
Costs in $/m
(operation/total)
HDH waste heat
HDH solar thermal
HDH autonomous
RO grid connected
RO-genset
RO-PV
3 Heat
Source
CHP solar collector solar collector
-
-
-
Electricity
Source
CHP
Grid
PV
Grid
Generator
PV
1 m 3
$4.5/$9.25
$10.77/$13.23
$13.12/$15.21 $8.86/$10.11
-
-
-
/day 5 m 3 /day
$4.27/$5.88
$7.71/$9.32
$1.3/$4.18
$1.49/$4.03
$1.04/$3.88
10 m 3 /day
$3.58/$4.98
$7.11/$8.52
$7.65/$8.55
$2.09/$6.26
$4.18/$8.35
$6.26/$10.44
4.2.3 Water Price in Haiti
Drinking water is considered a basic human right. Given this fact, water price structures are such that the cost of water production is not represented by the price that consumers pay for their water. In many cases the price paid is much less than the cost of the water production, with subsidies effectively provided by central government or local authorities.
(The costs of water provided by traditional systems is approximately $1/m 3 .) This situation is further complicated because the costs of water distribution are also generally difficult to isolate [29]. These factors make it hard for commercial RE-desalination technologies to compete because, relative to the subsidized public resources, the water from RE-desalination plants is too expensive. Therefore, there needs to be coordination between the local water authority and the SME to work out a water allocation and distribution plan for villagers along the coast that cannot be reached by the centralized public water system. This kind of coordination will provide the SME with a long-term market and the country’s government with another option to supply its’ population’s drinking water needs.
In Haiti water sachets or packets containing about seven ounces of water typically sell for about one gourde or approximately three cents. Water trucks have become one of the main water distributors in Haiti. A truck of water can cost anywhere from $30 to well over $100 for the consumer, depending on location within the city. Those that have cisterns buy the water from the water trucks, then resell it by the bucket at eight gourdes, about 25 cents,
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for a big bucket and four gourdes for a small bucket. These prices are expensive relative to the cost of government supplied water, which costs about a gourde per bucket [12].
The community that this project focuses on is the town of Pestel, which has a population of approximately 3,000. People are most likely to buy water during the dry season from November to January. Most people in the village earn between $1-$2/day, which is approximately 40 gourdes. They spend five gourdes per five gallon bucket of water and use between four to ten buckets per day. (This water may not be clean.) Usage depends on the size of the household and on average ranges between two and five gallons per person per day.
There are also 20 fluid ounce water bottles that cost twenty gourdes but most villagers do not purchase these because they are too expensive [53].
Given the low price of water in Haiti and in many other developing countries, for a HDH water plant provider to make a significant profit, the water produced will have to be low cost and will have to be demanded by thousands of people having large economies of scale. The
HDH water plant distributor should also target schools, hospitals, central water districts and other large institutions or populated areas that may be better able than individual consumers to pay higher prices for clean water. Alternatively, if the price of the water produced is to be subsidized, the water plant operator will need to establish a payment plan with the government or local NGOs for providing a valued public service. This situation leads to the question of the value of water and how to supply it while taking the concepts of equity, efficiency and sustainability into consideration.
Water is both a social and an economic good. Although access to clean drinking water is a basic human right, it should not be freely distributed. In the past, most cities and utilities in the world have provided water to their customers almost free of charge because water was a relatively cheap and abundant resource and because it is considered a basic necessity. Now with much larger communities requiring water service due to increased population size and decreased freshwater availability, the only way to ensure that everyone has access to water is to ration it in some way.
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One way to promote equity, efficiency and sustainability in the water sector is through water pricing. Studies have shown that if water resources are managed in an integrated fashion where the economics, legal and environmental aspects complement each other, increased water prices do improve equity, efficiency and sustainability. It is basic economic theory that an increase in price reduces demand and increases supply. However, the less obvious benefits of increased water price policy include improved managerial efficiency due to increased revenues, water conservation and environmental sustainability and an extension of water services. Low-priced water encourages excessive consumption by those connected to the supply system, which limits the water utility’s coverage. The poor are left to purchase higher priced water from vendors. Consequently, the poor are able to afford only small quantities of water enough for bare necessities but not hygienic needs. Higher water rates encourage water conservation and allow utilities to extend improved water services to those currently not served and forced to purchase water from vendors at very high prices. This results in more equitable water distribution and a reduction in the per unit cost of water to poor people. Additionally, when the price of water reflects its true cost, the resource will be put to its most valuable uses, where value depends on individual preference [54].
In order to adequately price water, its full cost and value must be determined. The full cost includes O&M costs, capital costs, opportunity costs and economic and environmental externalities. The full value of water includes benefits to users, benefits from returned flows, indirect benefits and intrinsic values. Figure 52 and Figure 53 provide a visual breakdown of the full water cost and full water value respectively and an illustration of how the components relate to each other. Rogers et al. (1998) in “Water as a Social and Economic Good: How to Put the Principle into Practice” gives detailed definitions of each of these water cost and value components. For economic equilibrium, the value of water should equal the full cost of water. From the full cost and value of water, the price can be set by the relevant political and social stakeholders to ensure cost recovery, equity and sustainability. Water prices must also reflect supply characteristics like water quality, reliability and frequency of supply. The
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Figure 52: General principles for cost of water [54] price may or may not include subsidies. It is important to note that the prices for water are not determined solely by cost. Using pricing policies still requires significant government intervention to ensure that equity and public goods issues are sufficiently addressed [54].
Several water pricing strategies can be considered for providing quality drinking water to villagers in Pestel, Haiti from a solar-driven HDH system. Since most of the villagers in Pestel live on $1-$2/day, an appropriate tariff structure is needed to meet the different social, political and economic goals of supplying clean drinking water during the dry season.
Consumers want quality water at an affordable and stable price. Suppliers prefer to cover all costs and to have a stable revenue base. A two-part tariff system for water use in Pestel is one option to meet these objectives. The two-part tariff structure has fixed and variable elements.
The fixed element varies according to some characteristic of the user and the variable part charges the consumer according to consumption level and encourages conservation [54].
The two-part tariff will provide the supplier with a stabilized revenue base. The fixed
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Figure 53: General principles for value of water [54]
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element will protect the water supplier from demand fluctuations and reduces financial risks.
For Pestel, the variable element could operate using an increasing block tariff (IBT) system.
IBT is a progressive tariff that provides different prices for two or more pre-specified blocks of water. The price rises with each successive block. When designing the IBT structure the utility must decide on the number of blocks, volume of water use associated with each block and the price to be charged for each block. This system allows the utility to provide a lifeline to the poor at below-cost rate and to charge higher prices beyond this minimum volume.
Under this system the poorer households get access to low-rate water since they possess fewer water consuming appliances. The system also allows for rich-to-poor and industrialto-domestic subsidies. If the two-tariff structure is insufficient for providing water to the villagers in Pestel, then tariff structures such as lifeline rates, IBT’s or lump sum credits can be used to equitably supply water [54].
It is important to note that governments have many rationales for providing subsidized water beyond notions of human rights. In recent decades there has been increased debate around the status of water as a “human right,” a resource which cannot be owned and which all should have access to for their own use and survival. Serious problems can result if the water supply to a given poor population is fully privatized by an unscrupulous water company that has no regard for equitable water distribution. Governments may provide subsidized water to obviate these fears.
At the heart of colonial theory is the idea of the core being served by the periphery, whereby a resource rich area lacking the technology or capital to exploit its own resources is thus exploited “for its own benefit” by another group with the power and means to do so.
In the case of water, this is called a “hydraulic empire” or a water monopoly. In a “hydraulic civilization” an entity maintains power over a population through exclusive control of access to water. For example, control over water emerged as a major issue in Latin America in the
1990’s following World Bank loans to Bolivia to modernize and later privatize the municipal water systems of La Paz-El Alto and Cochabamba. The Bolivian government auctioned
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the public utilities in charge of water and sold them to Aguas del Tunari, a subsidiary of
Bechtel Corporation, the largest engineering company in the U.S. The terms of this contract stipulated that control over all water in Cochabamba was the property of Aguas del Tunari.
This became a major clashing point, as almost 40% of the city was receiving its water from informal systems not linked to the city’s water supply [55]. Water control by Aguas del Tunari effectively signaled the end of local control of water and meant that the new corporate owners would have the right to place Bolivian water on the international market.
This situation effectively led to hydraulic control over the cities involved [56]. In these type of situations, government intervention and regulation is needed to ensure that water access equity is enforced.
4.3.1 Social Barrier
Typically there has been a negative perception of desalination by the population and sometimes opposition of local communities to installation of desalination plants. A major misperception is that desalinated water is not suitable for drinking, either because of individual prejudice or cultural issues. Other negative perceptions of desalination technologies are that they are uneconomic, unreliable, environmentally damaging and/or aesthetically unpleasing. Some of these negative perceptions will have arisen because of failures of prototype renewable energy or desalination technologies and some due to a misunderstanding of the technologies. For example, people in developing countries might have a negative perception of RE-desalination technologies because of intermittent water supply based on the availability of the RE resource. They may also have concerns that the technology will fail quickly because it is complex and will not be properly maintained. These negative perceptions will result in limited community support where the system is installed and a lack of popular support from institutions and politicians because of perceived rather than actual deficiencies
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[29].
Public acceptance is crucial for application of a RE-desalination technology and widescale implementation. The public’s attitude towards the technology must be monitored throughout the life of its operation. Often it is not about the engineering, although this factor is important, but about building community trust and ownership of the project. There is often a cultural gap between project developers and the end-users. This gap may result in the project failing for non-technical reasons such as the installation of the desalination system being viewed as something foreign or there is conflict about who controls the system
[29]. To build community trust and ownership, it is important that the local community is involved with the implementation of the technology and is trained to maintain the system.
Establishment of a community water-board to maintain the system and to monitor proper distribution of the water produced will help ensure that the community takes shared ownership of the consequent outcomes of the system. The local water board must take sufficient efforts to understand the public’s underlying beliefs or perceptions behind their response to the desalination technology in order to address their concerns through an education program or alternative method.
The objective of an education program about RE-desalination is to enlighten and persuade the general public about the safety and positive benefits associated with the adoption of RE-desalination technology. The information campaign should aim to convince the community that the proposed use of desalination technology for producing potable water will
1) not threaten the health of those consuming it, 2) will produce economic benefits to the community, 3) is favored by people in the community, and 4) will combat future or present water supply shortage [57]. The best way to communicate that the desalinated water will not threaten the health of those consuming it is to learn what people already believe, tailor the communication to this knowledge and to the decisions people face and then subject the resulting message to careful empirical evaluation [58]. Once the community feels a sense of ownership of the RE-desalination technology and is well-informed about how it functions
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and how to maintain it, it will have a larger positive impact on meeting the community’s water demands.
4.3.2 Water Quality and Public Perception
Desalinated waters or highly soft waters produced by desalination plants cannot be directly used as they are unpalatable, unhealthy and corrosive [59]. Desalinated or demineralized water is water that is almost or completely free of dissolved minerals as a result of distillation, deionization, membrane filtration, electrodialysis or other technology. The TDS in such water varies but it can be as low as 1 mg/L. Desalinated or demineralized water without further mineral enrichment is inappropriate for consumption for three reasons:
1. Demineralized water is highly corrosive and, if untreated, it will attack the water distribution piping and storage tanks leaching metals and other materials from the pipes.
2. There are health risks from consumption of demineralized or low-mineral water as a result of dietary deficiency.
3. Distilled and low mineral content water (TDS < 50 mg/L) can have poor taste characteristics and the water is reported to be less thirst quenching [60].
Sufficient experimental evidence confirms the health risks from drinking low-mineral water.
Results from both animal and human volunteer studies were in agreement and showed that low-mineral water markedly: 1) increased diuresis (almost by 20% on average), body water volume and serum sodium concentrations, 2) decreased serum potassium concentration, and
3) increased the elimination of sodium, potassium, chloride, calcium and magnesium ions from the body [60]. The body needs an adequate intake of electrolytes and ingestion of distilled water leads to the dilution of the electrolytes dissolved in the body’s water.
There is a low level of essential elements in low-mineral water. The modern diet of many people, especially in developing countries, may not be an adequate source of minerals and microelements. Although drinking water is not the major source of essential elements for
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humans, in the case of people with mineral deficient diets, even the relatively low intake of some essential elements may play a relevant protective role. This is because elements are usually present in water as free ions and are more readily absorbed from water compared to food, where they are mostly bound to other substances. Additionally, when used for cooking, soft water was found to cause substantial losses of all essential elements from food
(vegetables, meats, cereals). These losses may reach up to 60% for magnesium and calcium or even more for some other microelements [60]. In contrast, when hard water is used for cooking, the loss of these elements is much lower.
There is practically zero calcium and magnesium intake from low mineral water. Calcium and magnesium are both essential elements. Numerous international studies have reported that soft water (i.e. water low in calcium and magnesium) and water low in magnesium is associated with increased morbidity and mortality from cardiovascular disease (CVD) compared to hard water and water high in magnesium. It has also been suggested that intake of low-magnesium water may be associated with a higher risk of bone fracture in children, motor neuronal disease, pregnancy disorders such as pre-term birth and low weight at birth and some types of cancers. [60].
The corrosive nature of demineralized water and potential health risks related to the distribution and consumption of low TDS water have led to recommendations for the minimum and optimum mineral content in drinking water. Based on the currently available data, various researchers have recommended the following levels of calcium, magnesium, specific ion content and water hardness in drinking water:
• For magnesium, a minimum of 10 mg/L and an optimum of about 20-30 mg/L
• For calcium, a minimum of 20 mg/L and an optimum of about 50 (40-80) mg/L
• For total water hardness, the sum of calcium and magnesium should be 2 to 4 mmol/L
• For TDS an optimum of 200-400 mg/L
• For Na an optimum of 0-100 mg/L
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• For Cl an optimum of 30-150 mg/L
• For sulfate an optimum of 0-200 mg/L
• The bicarbonate recommendation is to have a concentration equivalent to the hardness content [59].
Remineralization is used to overcome these deficiencies. A commonly used operation in the remineralization process is to place CO
2 acidified desalinated water in contact with a bed of domestic limestone or calcium carbonate. Limestone dissolution is a slow ratecontrolling step that adds two essential ingredients to the water, bicarbonate alkalinity and calcium content: CaCO
3
+ CO
2
+ H
2
O = Ca 2+ 2 HCO −
3
[59]. Alternatively, blending the desalinated water with small volumes of more mineral-rich waters to improve its taste and reduce its corrosiveness to the distribution network is also suitable. The procedure for adding minerals to water is not complex. A remineralization solution can be prepared in a clean reservoir under constant stirring using the same water that will be in the product. The remineralization solution can also be pasteurized. A pump can be used to inject a portion of the remineralization solution either directly or to a feed tank maintained under agitation to avoid precipitation of salts [61]. Table 14 shows four solutions that are widely used to remineralize desalinated water and Table 15 qualitatively compares these processes.
When desalinated water needs to be remineralized, the key considerations in supplementing minerals are:
• potential health benefits;
• taste;
• product stability;
• quality of the salts;
• industrial procedures; and
• cost [61].
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As discussed previously the health benefits of remineralizing desalinated water are clear but acceptable water taste is a more subtle issue. Consumer taste preferences are crucial because upon consumption the consumer may immediately accept or reject the water. If the taste of the desalinated remineralized water is unacceptable, the consumers may opt for a different source of drinking water that tastes good but is unclean, despite the higher water quality of the remineralized water. Therefore, it is crucial that the water producer seeks feedback from the consumers about the acceptability of the water attributes (i.e. taste, smell and appearance or color) and responds appropriately.
Concerning product stability and quality of the salts, the salt concentrations that can be added without exceeding the solubility of the salts in the water at 20 ° C must be calculated to prevent precipitation. Solubility can be improved if the water is carbonated since lower pH usually enhances solubility. With respect to cost, to add 20 mg of calcium to water, the cost would rise by U.S. $0.00222/L of product (U.S. $2.2 per 1000 liters) if prepared from calcium sulfate and magnesium chloride or by U.S. $0.00198/L of product (U.S. $1.98 per
1000 liters) if prepared from calcium chloride and magnesium sulfate. These costs exclude the costs of electricity and mixers/pasteurizers [61].
4.4.1 Possible Environmental Effects
Environmental issues related to desalination are a major factor in the design and implementation of desalination technologies. Some major environmental concerns include issues related to location of desalination plants and water intake structures, and concentrate management and disposal [63]. The desalination plant location is important for several reasons: proximity to the population center, distance from the saltwater source to the plant and availability of the needed infrastructure.
If the proposed desalination plant is being constructed next to a population center, land use and noise pollution from the construction must be considered. If planners place a desali-
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Table 14: Four solutions used to remineralize desalinated water [62]
Remineralization Description Minerals
Process
1
2
Blend with 1% clarified seawater
+pH neutralization
15 mg/L Mg + 5 mg/L
Ca +125mg/L Na + 220 mg/L Cl + 25 mg/L
SO
4
, pH 7-7.5
80 mg/L CaCO
7-7.5
3
, pH
3
4
CO
2 addition+Calcite
Limestone (CaCo
Na
2
CO
+ Na
2
3
CO
2 addition
+Dolomite
,
) percolation
CO
NaHCO
3
3
Addition of CaCl
3
3
,
MgO) percolation +
Limestone (CaCO
MgCO
3
2
+
80 mg/L CaCO
7-7.5
100 mg/L CaCO pH 7-7.5
3
3
, pH
, 100 mg/L Na + 50 mg/L Cl,
Table 15: Water remineralization process comparison [62]
Process Investment Operation Water Quality
1 Very Low Low
Ease of
Operation
Easy
2
3
4
High
High
Very Low
High
High
Low
Medium water quality- high sodium chloride content
Good water quality
(very small sodium increase)
Very good water quality (very small sodium increase, more magnesium)
Medium water quality- high sodium chloride content
Easy
Easy
Time consuming
(chemical dissolution)
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nation plant in densely populated areas, it may impact the residential environment. Some desalination plants generate noise due to the use of high-pressure pumps. Noise pollution can be mitigated by using canopies or acoustical planning. Since construction can be timeconsuming, inconvenient, noisy and disruptive to the environment, it is ideal to have as little construction as necessary. If the fuel resources, electricity connection and water connections are near the proposed plant site, then there will be less construction. After construction begins, planners should develop an environmental monitoring plan to ensure the project meets established environmental guidelines. Management plans are also important during the plant’s operation to ensure consistent environmental acceptability [63].
Construction of water intake structures and pipelines to carry feedwater and concentrate discharge may cause disturbances to environmentally sensitive areas. The water intake structures, unless properly designed, may kill fish. There is also a risk of polluting the groundwater from the drilling process when installing feedwater pumps. Leakage from pipes that carry feedwater into the desalination plant and concentrated brine out of the plant can cause damage to groundwater aquifers. To help prevent this damage, plants should have sensors to detect this leakage and workers to notify plant operators if leaks develop in the pipes [63].
Concerning infrastructure, if the plant is obtaining the needed electricity from the grid or other fossil fuel sources, the gas emissions and air pollution must be addressed. The burning of fossil fuels for energy use will lead to increased air pollution, which can have a serious effect on public health. There is also concern regarding the quantity of chemicals stored at the plants. Chemical spill risks require storing the chemicals away from residential areas [63]. It should be noted, however, that in developing countries, emissions and other environmental issues are secondary to economic development and survival.
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4.4.2 Brine Discharge
Desalination plants generate two products: fresh water and concentrate (reject or residual stream) byproduct. Brine concentrate is high in salinity (i.e. contains a TDS concentration
> 36,000 mg/L) and may contain low concentrations of certain chemicals used during pretreatment and post-treatment cleaning processes as well as elevated temperatures. These concentrate properties can lead to problems for the marine habitats and the receiving water environments. Critical concentrate parameters are TDS, temperature, and specific weight
(density). The amount of concentrate produced from a desalination plant is a factor of the desalination process’ recovery rate (product water/feedwater). Generally, membrane plants have a higher recovery rate than distillation plants, resulting in a higher amount of salt in the concentrate. This trend is observed in Table 16, which shows concentrate characteristics for RO and MSF/MED desalination technologies. The solar-driven HDH desalination plant typically has a low recovery rate of 5% so brine disposal is not a significant issue. Distillation desalination plants usually can reduce the concentrate density by diluting it with cooling water before it is discharged into receiving water. Table 16 also shows that concentrate from distillation processes is generally warmer than concentrate from membrane processes. Many desalination plants dispose of brine in surface water. Compared to freshwater, concentrate has a higher density due to the higher salt concentration [63]. The concentrate tends to sink in water with lower salinity (lower density). This tendency for the concentrate to sink in the receiving water has negative effects on the marine environment.
The disposal of desalination concentrate is often a leading factor in determining the costeffectiveness of a project. If brine disposal regulations are in place and the desalination plant has difficulty satisfying them, then fees can accrue against the plant. The U.S. Environmental Protection Agency (EPA) has initiated the classification of membrane concentrate as an industrial wastewater. This regulatory classification has made it essential that the desalination plants in the U.S. be able to properly dispose of the concentrate that they produce.
Regulation of concentrate disposal will tend to keep the application of desalination technolo-
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Table 16: Concentrate characteristics for various desalination technologies [63]
Process
Feedwater
Recovery Rate
Temperature
Concentrate
Blending
Final
Concentration
Factor
RO
Brackish
60-85%
Ambient
Possible, not typical
2.5-6.7
RO
Seawater
30-50%
Ambient
Possible, not typical
1.25-2.0
MSF/MED
Seawater
15-50%
10-15 ° F above ambient
Typical, with cooling water
<1.15
gies near the coastlines where saline or brackish environments are located which have the greatest feasibility for use in disposal options [64].
4.4.3 Concentrate Disposal Methods and Mitigation
Concentrate disposal methods include surface disposal (surface water and submerged disposal), deep well injection, land application, evaporation ponds, brine concentrators and zero liquid discharge (ZLD) technologies. Since surface disposal is the most common method of concentrate disposal it, will be the focus here. Surface water disposal includes disposal into freshwater, tidal rivers and streams; coastal waters such as oceans, estuaries and bays; and freshwater lakes or ponds [63].
Surface water disposal takes place immediately at the coastline and its appropriateness depends on the surroundings and properties of the receiving water. If the area is highly populated, disposal may be a problem because of the interference of the mixing zone with recreation on the beach. In this case submerged disposal, disposing the concentrate underwater using long pipes stretched out into the ocean, may be more appropriate. However, this method places benthic marine organisms living at the sea bottom at risk. As concentrate enters the receiving water, it creates a high salinity plume which either sinks, floats or stabilizes in the seawater based on its relative density. The radius of the plume impact varies; without proper dilution, the plume may extend for hundreds of meters, beyond the mixing
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zone, harming the ecosystem along the way. The type of dispersion and natural dilution of the concentrate plume that may occur depends on the discharge pipe’s location. Factors such as waves, tides, currents and water depth are all important aspects that determine natural dilution and the amount of mixing that may occur at the concentrate mixing point [63].
If natural dilution is not enough to properly diffuse the concentrate, then desalination plants use artificial dilution methods including efficient blending and diffusers or utilize mixing zones prior to surface disposal. Mixing zones are quantified limits within the receiving waters where the law allows surface water to exceed water quality standards due to the existence of point source disposal. Blending involves mixing the concentrate with cooling water, feedwater or other low TDS waters before disposal. Diffusers are jets that dilute the concentrate at the concentrate disposal outlet for maximum mixing. Mitigation efforts related to chemical use in the desalination plant include using non-toxic additives and de-chlorination techniques which limit the toxic chemical concentrations that enter the environment. Using materials in the desalination process that are less likely to corrode can limit the occurrence of corrosion products in the water [63]. Figure 54 shows the main concerns with surface water disposal, as well as mitigation methods to reduce those concerns.
Figure 54: Surface water disposal problems and mitigation [63]
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4.5.1 Institutional Barrier
In general, water authorities have been found to be reluctant to commission RE-desalination technologies because of their confidence with conventional water technologies and a culture of risk avoidance [29]. Water authorities prefer to install technology that they are familiar with and that is well understood. RE-desalination technologies like HDH are perceived as risky since they have not been commercially proven and because of a relative lack of knowledge and experience with the technologies.
In addition to the perceived risk of the newness of RE-desalination technology, given the technologies decentralized nature, water authorities may also perceive them as a political risk. The provision of water supplies has typically been provided using a centralized approach, where water supply and quality can most easily be controlled. Many RE-desalination technologies are generally small-scale and suitable for community-led water provision. This situation might result in a perceived loss of control by the water providers, making it unlikely for them to adopt RE-desalination technologies. This institutional aversion is compounded by the fact that local rural communities might not trust water supplies powered by RE and would in some cases prefer to rely on traditional fresh water supplied even in cases where the supply is of low quality [29].
The legal structures required to ensure specific water quality standards typically favor the centralized approach of water provision. Consequently, the legal structures are often highly bureaucratic, not tailored for small-scale independent water production, and require a large investment of time and effort for each source of water supplied. Many RE-desalination plants are developed by independent water suppliers and have only a small capacity. Thus, the legal overhead is relatively large, making the installation potentially uneconomic. The issue is further complicated by the fact that in many countries the management of energy is totally separated from the management of water, so the coordinated organization and provision of these two services is difficult. The separation of the management of energy and
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water means that the benefits of RE-desalination are not always fully recognized because decision-makers focus on either the supply of water or the supply of energy [29].
The institutional barrier of lack of training and infrastructure can also be problematic.
Many developing countries lack the materials and infrastructure needed to construct and sustain the required RE-desalination technology. As a result, the materials must be imported at a high price. If a component in the system fails, unless the technology was built appropriately using locally available materials, the replacement parts will need to be imported, reducing the overall system sustainability. These factors result in reduced plant availability, reluctance to offer service contracts and reluctance to purchase the system without service contracts [29]. It is important that the RE-desalination system is an appropriate technology built using local materials, expertise and input to ensure system practicality and sustainability. It is also essential that a group of people in the village be trained to maintain the system regardless of whether the system is purchased with a service contract.
4.5.2 Water Regulatory Framework in Haiti
In terms of both water supply and sanitation Haiti’s coverage levels in urban and rural areas are the lowest in the Western Hemisphere [65]. The quality is also inadequate. In rural areas, systems have often fallen into disrepair, either not providing any service water at all or providing service only to those close to the source. In almost all urban areas, water supply is intermittent. Sewer systems and wastewater treatment are nonexistent and there is no legislation concerning desalination. Foreign and Haitian NGOs play an important role in the sector given the weakness of the public institutions.
The main public sectors in the Haitian water sector are two state owned enterprises:
CAMEP (Centrale Autonome Métropolitaine d’Eau Potable), responsible for providing water in the Port-au-Prince metropolitan area, and SNEP (Service National d’Eau Potable), responsible for providing water nationally (secondary cities and, in theory, for rural areas).
The absence of management, regulations and funding has crippled the two government-owned
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water services, leaving the country’s water resources polluted and severely depleted. Neither agency has been able to maintain or update their equipment and water lines, adapt to changes in population or respond to the country’s growing environmental crisis. Estimates on the percentage of metropolitan Port-Au-Prince that is being serviced by CAMEP vary from 20 percent to 30 percent. However, these figures are uncertain because CAMEP’s service is intermittent, their metering system is inconstant and people often break their pipes and steal the water to sell for a profit. According to the Haitian Institute of Statistics and
Information, SNEP is servicing only 16 percent to 24 percent of the population [12].
There is no institutional responsibility for sanitation in Haiti, since the mandates of
CAMEP and SNEP currently do not include sanitation. Both entities theoretically operate under the authority of boards, including representatives of several ministries. Since these boards have not met for more than a decade, both entities are de facto under the sole control of the Ministry of Public Works, Transport and Communications (MTPTC). MTPTC currently does not have a water and sanitation directorate, although its creation is foreseen
[65]. MTPTC now envisages creating a water and sanitation directorate as part of a draft framework law for the sector.
A fundamental problem for Haiti is that there is no Water Ministry. The responsibilities for ensuring delivery of safe water are spread throughout government agencies, including those related to agriculture, public works and public health. “It is very difficult to control because there are so many people involved and nobody is in control exactly,” said Benoit
Frantz, the general secretary of CAMEP [12]. There are hundreds of water committees, called CAEPs (Comités d’Aprovisionnement en Eau Potable) or simply Comités d’Eau, in charge of water systems in rural areas and small towns. Their degree of formalization and effectiveness varies considerably. The best water committees meet regularly, closely interact with the community, regularly collect revenues, hire a plumber who performs routine repairs, have a bank account and are registered and approved by SNEP. However, many water committees fall short of these expectations. There is no national or regional registry
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of water committees or water systems and there are no associations of water committees at the municipal, departmental or national level. The Ministry of Public Works now refers to these committees officially as water and sanitation committees (Comité d’Approvisionnement en Eau Potable et Assainissement—CAEPA) to reflect the broader role the committees are expected to play in the future [65].
4.5.3 Desalination Regulatory Framework in the U.S.
The legal and institutional structure of the U.S. has ensured that the states and localities have the main burden of regulation and decision-making on desalination. Although under the constitution, U.S. federalism has decisive power over local governments at the state level, federal constitutional provisions have remained almost entirely in the background. States usually oversee special districts concerned with water issues and the regulation of private and public utilities. It is at the level of localities and sub-state regions that most provision of infrastructure like water and electricity gets decided. The local politics of desalination is often the most decisive for approval even when the processes take place at higher levels. In some states, like California, the requirements that states impose in coastal areas can decisively influence the prospects and character of desalination processes. This circumstance is largely due to the fact that companies or local governments seeking to develop desalination plants are responsible for obtaining the numerous required overlapping permits for implementation
[66]. Limited cooperation between the energy and water authorities often results in small producers having to go to many different organizations that deal with water, energy and the environment for securing all the permits needed to construct the desalination plant [29]. For example, in California up to 24 separate permits from an array of agencies, at multiple levels of government, may be required. Consequently, much of the expertise on these projects goes into filing permit applications [66].
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4.5.4 Policy Gaps, Links and Recommendations for Increasing Desalination
Despite a more centralized context of desalination policy-making in developing countries relative to the U.S., the U.S. desalination regulatory framework is likely to have far-reaching consequences for other regions of the world. Several lessons that can be learned from the
U.S. regulatory experience with desalination are the importance of fostering public-private financial desalination arrangements, creating decentralized water producers that can abide by the local regulations of the water authority and reducing the bureaucracy of the legal structures.
Public-private financial arrangements and policy sources of support are crucial for the success of a desalination project. Even before the cost of desalination has decreased enough to make itself marketable, it can be widely cost-effective with a combination of private investment and public subsidy. Private investment by itself may be too unreliable to provide the basis for investment in desalination. Local or national regulation may ultimately be necessary for market investments and the reliability of desalination projects. Private investment is needed for desalination to be carried out in developing countries. To make privatized arrangements accountable, protections through regulation at multiple levels, including local review, are critical. RE has been largely supported in the U.S. through support policies like feed in tariffs, quota schemes, tax incentives, investment grants and cap and trade [29].
An investigation into how these policies can be applied and enforced for RE-desalination in a given country must be conducted. In the U.S. the placement of desalination plants has proceeded according to the logic of private investment rather than policy guidance. Investors tend to focus on communities with greater ability to pay for investments in plants and infrastructure for desalination technology. Only in heavily subsidized states like California has the implementation of desalination plants followed patterns of public investment. From an equity standpoint and not in terms of economic efficiency, in order to equitably supply and distribute desalinated water, it is important to have the proper balance of public and private interest, support and investment.
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Decentralized regulation for water supply is important for RE-desalination. As observed in the U.S., having a centralized desalination regulation is problematic for states. While there can be centralized general oversight and an overaching desalination regulatory framework, decentralized local water authorities are the most effective at regulating local desalination plants and holding them accountable. However, there are several drawbacks to the localized desalination regulatory approach. One problem is that the localized nature of regulation means that little attention is given uniformly to social and environmental equity among different places. Additionally, there are concerns of a fragmented regulatory desalination framework that will be difficult for water plant operators and desalination investors alike to navigate. Non-uniformity also presents the issue of investors going to local areas that have a regulatory climate favorable to their business interests which might have negative social and/or environmental impacts for the local community [29]. Thus, an overaching centralized framework is needed to help ensure social equity and justice to help prevent business abuse and exploitation from happening due to a lack of adequate local standards.
The extensive regulatory and procedural requirements that surround desalination offer numerous opportunities for debate over the strengths and weaknesses of a desalination project and are a significant barrier to the successful development of desalination plants [66].
The knowledge debate over desalination projects is good because it allows for constructive criticism that makes the proposed desalination project stronger and more environmentally responsible. To address the issue of extensive regulatory and procedural requirements there needs to be more coordination between the energy and water authorities. In every country the RE-desalination community must lobby for greater cooperation between the power and water sectors in governmental and non-governmental institutions and work with local authorities to identify the bottlenecks in licensing and eliminate them. This cooperation will dramatically reduce the legal bureaucracy. It will enable decentralized supply of desalinated water similar to how the U.S. legal framework has allowed for decentralized electricity production and subsequent sale at the central market level via the national grid [29].
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The development of reliable and detailed market analysis for RE-systems is one of the most important but also most challenging tasks. The main requirements of this type of market analysis include identifying and analyzing in detail the main target groups for each kind of different available RE-desalination combination and quantifying the demand by these groups for the water desalination technology. RE-desalination product developers who decided to enter a market outside their home country must collaborate with the appropriate local companies in order to identify niche markets where the users are willing and able to pay for the technology. In order for a SME to overcome barriers associated with the local legal system, currency and political developments, it needs to get as much external support as possible. The SME will need a good network to establish a local presence with sales, marketing and technical staff. If the SME works together with local companies, agencies and/or missions and obtains support from international development organizations, it can develop this network. Additionally, the RE-desalination community can get organized, collect relevant information and make it available to its members to help them expand to new markets
[29].
The desalination industries need to lobby the water authorities in countries to develop a suitable water pricing structure. Introducing water pricing that accounts for full cost recovery of the “real cost” of water is crucial. The real cost of water is based on the cost of water supply, maintenance and new infrastructure, environmental and resource costs, and the volume of water used. Successful water pricing will require a good understanding of the relationship between price and use for each sector and needs to account for local conditions.
The challenge is to define water pricing that reflects the costs but allows equitable access to safe water. Traditionally public subsidies have been used to accomplish this aim, especially in places where real water costs are much higher than the income of the local people. However,
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the structure and mechanism of the subsidies has to be incorporated in a pricing system that still lets the market choose the most efficient water supply solution, while encouraging efficiency in the use of the water [29].
Targeted R&D is needed to substantially increase the market for and worldwide application of RE-desalination. R&D priorities include focused efforts on developing the components necessary for the smooth and efficient coupling of the existing desalination and RE technologies and development of the elements that will make RE-desalination robust for standalone operation in harsh environments. Some issues that RE-desalination developers have to address and that need more R&D are adaptation of pumps and energy recovery systems for efficient operation in small-scale plants, suitable small-scale electric and thermal energy storage, use of seawater resistant materials, automated and environmentally friendly pre and post-treatment technologies and control systems that optimize the system performance and minimize maintenance requirements. With respect to the latter issue, doing R&D on how to minimize the impacts on the desalination plant due to energy variability is one possibility. The desalination process and its components should be reassessed and designed towards a new desalination technology able to operate under variable energy supply. Research is needed on improved control algorithms that result in control software that can ensure the best use of the available energy and that protect the system from energy supply fluctuations
[29].
Other areas of research that must be conducted are R&D that supports the development of hybrid systems with more than one source of energy and cogeneration plants that produce water and power. Hybridization with the electricity grid, together with a tailor-made control system, can guarantee continuous plant operation. Cogeneration of electricity with the desalination of water will enable optimal utilization of the desalination plant and make
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the desalination process diverse and more economically attractive. The management of the available electricity and water produced would depend on the needs and on the tariff structures of both commodities in the area of operation [29].
Wide-scale growth of RE-desalination requires a worldwide RE-desalination community with an effective communication platform. Companies that produce and sell RE-desalination plants, academics who research desalination technology, country-specific relevant water regulatory bodies and the general public should be members of this community. The barriers and problems associated with RE-desalination could be discussed and addressed by this community, making it easier for the RE-desalination producers and sellers to gain market share. More specifically, the community can work to convince manufacturers to produce equipment tailored to the RE-desalination industry, complete a thorough market analysis and remove bureaucratic barriers with respect to plant installation. The community can also convince water authorities and policy makers to adjust the water pricing system to an acceptable metric. It can also share experiences or best methods on what has worked and failed concerning overcoming social barriers or public resistance [29]. Transparency is key to further increasing the general public’s understanding of and trust in RE-desalination.
To foster greater transparency and understanding, university classes concerning the topic should be expanded and continued, more workshops for professionals should be developed and local community water boards should be established to clarify misunderstandings and remove misconceptions about the technology.
This RE-desalination community and communication platform will not develop on its own but must be established. Either the European Union (E.U.), the U.S., the International
Energy Agency (IEA), a large RE-desalination company or some other entity must initiate it. All interested companies have to be found and integrated. A website has to be created
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where all the information and questions can be stored and exchanged. To ensure that the
RE-desalination community can function for a long time, funding must be guaranteed either from E.U. or U.S. projects and collecting member fees. A model for this community could be
SolarPaces, which has facilitated the market entry of CSP for over 30 years. It is managed under the umbrella of the IEA to help find solutions to the worldwide energy problems [29].
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RE-desalination generally, and HDH in particular, have a critical role to play in meeting the world’s rapidly increasing water demands. For RE-desalination to become widely spread, it must overcome technological, social, economic, political and environmental barriers. Key factors to surmount these barriers are RE-desalination R&D, market signals that reduce the perceived risks associated with RE-desalination and effective action by the appropriate stakeholders for raising RE-desalination awareness and growth.
Since a large portion of the future water need will be in developing countries, appropriate desalination technologies that can increase water are critical. Solar-driven HDH desalination has a crucial role to play in producing potable water for people in coastal regions of developing countries. Solar-driven HDH is an appropriate technology for coastal regions of developing countries because it has the potential to be made using local materials, it uses a renewable energy source, it does not require skilled labor for O&M and it is a source of decentralized water production. Making the necessary trade-offs to reduce the overall system cost, such as using local manual labor and materials, in the HDH system will make it more affordable for people living on $1-$2/day. By experimentally determining the thermal performance and pressure drop of custom cooling tower fills such as bamboo, louffa and charcoal, it was concluded that the louffa fill had the highest overall performance. The tested custom fills are locally available in Haiti and can be easily replaced at no cost. Using a louffa fill in the humidifier of the HDH desalination unit is one way to reduce the system cost while still maintaining adequate thermal performance relative to PVC commercial fills.
The next steps to further the successful implementation of a HDH desalination system in a developing country such as Haiti are to design several prototype HDH systems that can meet both user requirements and the price target for the clean water produced. To minimize
HDH system cost, it is important to identify and design around each of the key contributors to cost such as the condenser, the solar heater, the water pump and the fan. These design trade-offs can be satisfied by finding acceptable substitutes from locally available materials
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as well as from local input and expertise in building the prototype. Then the prototype performance must be tested in the field in order to acquire data and user feedback that can be used to improve the system. In addition to the technical next steps there are also political issues that must be resolved. The RE-desalination community and platform must be created in order to raise RE-desalination awareness and to increase the growth of this field through political lobbying and knowledge sharing. With an effective RE-desalination platform the barriers associated with RE-desalination can be targeted and overcome. With successful implementation of HDH, developing countries will have another water supply option to meet their rapidly increasing water demand.
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Apparatus Limitations
The following restrictions apply:
Maximum water stream temperature: 40 ° C
Maximum water mass flow rate: 0.139 kg/s
Maximum airflow rate: 119 L/s
Maximum fan speed: 4,000 rpm
Desired Experimental Parameter Ranges
Experiments are to be conducted in steady state for the following conditions.
L/G ratio (mass flow rate water/mass flow rate air): 0.25-5
Inlet water temperature ( T wi
): 39 ° C
Three fill heights ( H ): 152.4 mm (0.5 feet), 304.8 mm (1.0 foot), and 457.2 mm (1.5 feet) respectively
112

No. of packings tested: 5 1) louffa 2) charcoal 3) bamboo 4) CF-1200 5) CF-1900
Specifications
All components are mounted on a robust base plate. The cooling tower components include:
(i) Air distribution chamber.
(ii) A centrifugal fan with intake damper.
(iii) A 3/8 MP125N 316 stainless steel BETE spray nozzle.
(iv) A water-collecting basin.
(v) A column cap which fits on top of the column and includes a 80 mm diameter sharp edged orifice and pressure tapping, and a droplet arrester.
(vi) A Dwyer (0.2-2.2 GPM) rotameter catalogue no. RMC-142-SSV
113