Thermally Driven Sea Water Desalination using the Multi Effect
Humidification Dehumidification Method
H. Müller-Holst and W. Schölkopf*
TiNOX GmbH
Gesellschaft für Energieforschung und Entwicklung mbH
Schwere-Reiter-Str.35/2B, D - 80797 München
Germany
Email: soldesal@gmx.net
ZAE Bayern e.V.
Bavarian Centre for Applied Energy Research
Div. 4: Solar Thermal and Biomass
Walther-Meißner-Str. 6, D-85748 Garching, Germany
Abstract
The Multi-Effect-Humidification and Dehumidification (MEH)-Process represents the heart of the desalination
systems that are presented in this paper. The thermally driven distillation method was developed and improved
in the last years by research & development of our partner ZAE Bayern in close cooperation with different
companies in Germany. The long term reliability of the MEH-units and the low need for maintenance during
operation have been proven in several projects over more than seven years now. Referring on this experience,
the system presents itself ideally adapted for the use in remote areas.
The implementation of the MEH-Distillation unit into a well designed system comprising a suitable source of
process heat was performed in several pilot installations. For remote areas, the supply can be fulfilled by solar
thermal collectors in combination with a thermal storage tank or by the utilisation of waste heat.
Theoretical investigations on the heat and mass transfer of the unit helped to show the way to an optimized
design of the heat and mass transfer units inside the distillation chamber. This gives reason for the prospect that
the performance of the unit can still be improved by a factor of two to three in terms of thermal energy demand
of the desalination process. It will then be in the range of 40 to 60 kWh/m³ (620-930 BTU / Brit. gallon).
Continuing from those investigations, TiNOX will make the MEH-desalination unit an economically competitive
product in the near future - designed for a typical daily production of 1 000 to 10 000 litres - which is ready for
market introduction in the middle of the year 2002.
1. INTRODUCTION
Drinking water of acceptable quality has become a scarce commodity in many places of the world. More and
more often only brackish water or polluted water is available. The intrusion of salt water into former fresh water
coastal wells is a demanding problem. This brings about an increasing interest in new desalination technologies
suitable also for decentralised locations. Established methods as e.g. Multi-Stage-Flash-Evaporation and MultiEffect-Evaporation, Vapour Compression and Reverse Osmosis can be applied at capacity ranges of some 100
up to several 100 000 m³ per day fresh water production. They cannot be economically used in regions with low
infrastructure nor for the small scale supply of decentralised regions due to their permanent need of qualified
maintenance and electricity supply. Here the use of small-scale, decentralised desalination systems is desirable
and makes economic sense. The present desalination system fills this gap.
In the last years the efficiency of the components of the MEH-unit was improved at the laboratory of our
research partner ZAE Bayern in Munich, Germany and implemented and tested at pilot installations. They were
installed and measured from October 1992 to March 1997 in an installation on the west coast of Fuerteventura
(Canaries, Spain), in Gran Canaria (2000-2001) and in the Sultanate of Oman (2000-2001). The task of this
campaign was to gain information about the long-time reliability of the MEH-Units. The GOR-factor (Heat
recovery factor, ≈ PR) was improved up to a value of 5 in the pilot plant and up to 8 in the laboratory.
The present MEH-Distillation module delivers now up to 40 litres per hour. This corresponds to a daily fresh
water production capacity of nearly 1 m³ when the module can be operated 24 hours. In case of solar thermally
driven operation this implements that a thermal storage tank needs to be included. Another convincing alternative is the combination of a MEH-distillation unit with the decentralised production of electricity by gas-,
Diesel- or biomass driven generator and the use of it's waste heat for the desalination process (CHP).
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By modular increase of the units, the production of fresh water can be adapted to the individual need of a
determined customer in the range of between 0.5 to 50 m³ (110 to 11000 gpd) of high quality drinking water per
day.
From theoretical investigations of the heat and mass transfer inside the distillation chamber, a significant optimization potential has been discovered. The decrease of the distance between the heat and mass exchange surfaces
enables improved transfer coefficients. The reason is that due to higher flow speed of the humid air in narrow
ducts the Nusselt and Sherwood numbers are enhanced and the concentration gradients between flow and
surface conditions of state are increased. The use of materials with enhanced heat transfer coefficients enables
further improvements in the specific energy consumption of the process.
2. THE MEH-DISTILLATION METHOD
2.1.
Description of the Process
Figure 1 illustrates the operation of the MEH-unit. The principle of this distillation process is based on the
evaporation of water and the condensation of steam to and from humid air. The humid air flows in a circuit
driven by natural convection between condenser and evaporator (clockwise in Figure 1). Evaporator and
condenser are located in the same thermally insulated box.
In the evaporator the hot (80-85°C) seawater, being heated e.g. in a solar collector, is distributed onto vertically
hanging fleece sheets made of polypropylene (PP) and is slowly trickling downwards. The air moves in countercurrent flow to the brine through the evaporator and becomes saturated with humidity. Partial evaporation cools
the brine which leaves the evaporation unit concentrated with higher salt content and a temperature of approx.
45°C.
Heat Source ( 75...85°C)
Condenser
Preheated
Sea Water
ur
nat
al
Con
vec
t io
n
Evaporat or
Hot Sea Water
Cold Raw Water
Brine Reflux
Distillate
Figure 1: Illustration of the distillation system
The condenser unit is located opposite to the evaporator. Here the saturated air condenses on a flat plate heat
exchanger made of Polypropylene (double webbed slabs). The distillate runs down the surfaces and is trickling
into a collecting basin. The heat of condensation is mainly transferred to the cold sea water flowing upwards
inside the flat plate heat exchanger. Thus the temperature of the brine in the condenser rises from 40 °C at the
inlet to approx. 75 °C at outlet.
In the next step the brine is heated up to the evaporator inlet temperature, which is between 80 to 85 °C. This
heat can be performed e.g. by highly efficient solar collectors, from a thermal storage tank or by waste heat.
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2.2.
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Natural Convection
The humid air inside the distillation box is transported by a natural convection air loop from the evaporator to
the condenser. No van or blower is needed and no additional energy is consumed here. Another outstanding
advantage is that the circulation is self-adjusting to the optimum operating state regarding to the pre-set parameters and the formation of exchange surfaces [2]. By changing the inlet parameters - evaporator inlet temperature
and raw water volume flow - it is possible to influence and optimise the thermal efficiency of the system for a
determined solar power input. The optimum mode of operation depends on both technical and economic factors
and has to be appropriate for the materials involved as well. It is a compromise between the highest possible
evaporator inlet temperature in favour of optimum heat- and mass transfer inside the distillation module and the
limitations of this temperature in favour to gain acceptable collector efficiency and to minimise the over all
thermal losses.
2.3.
Main Advantages of the MEH-System
The outstanding advantages of the MEH-desalination system are:
•
•
•
•
•
•
•
•
•
Low specific process heat consumption of 40-60 kWhthermal per meter cube of fresh water
Low temperature process heat at 85 °C can be supplied by solar collectors, waste heat or other sources of
low temperature heat ( e.g. geothermal)
Raw water of any salinity can be used (brackish, sea, even brine of other processes)
No chemical pretreatment needed
Low auxiliary energy demand (pumps) can be provided by photovoltaic
Product water is distillate (best quality product water)
Modular set up - capacities of up to 50 m³ per day are considerable
Simple and robust construction, easy maintenance
No corrosion because all components in contact with brine are made of specially treated materials
3. PROTOTYPE PLANTS
3.1.
Solar Desalination systems with thermal storage tank
To enable solar thermally driven desalination systems to run 24 hours a day, thermal storage needs to be
implemented into the system described above. In order to overcome periods without solar irradiation (night,
clouds), the collector area has to be increased by a factor 4 to 5 in order to load the storage tank during sunshine.
Because the distillation unit is the most costly part of the system, the distillate costs finally decrease significantly
if the system can be operated 24 hours per day [5].
Two different strategies are considerable to operate a solar desalination system with storage tank
implementation. One is to run the whole system with salty raw water, the other possibility is to separate heat
supply and distillation unit by a thermal heat exchanger. Both possibilities are investigated in parallel in two
projects in co-operation with different partners.
In preparation of pilot installations of those two systems, detailed simulation calculations have been performed
at ZAE Bayern to find the proper design for each system [6]. The performance of the distillation module has
been investigated in the laboratory at ZAE Bayern, the collector parameters were delivered by the manufacturers.
The task of the calculations was to find optimised component sizes (collectors, storage tank).
3.2.
SODESA-System: Sea water resistant collectors and storage tank (Gran Canaria)
The configuration favoured in the SODESA project comprises an ambient pressure thermal storage tank (6.3 m³)
and a collector field (47 m²) being flown through directly by the seawater. The system yields a high conversion
efficiency due to elimination of heat exchangers, but higher demands are to be made on the materials used. In
the scope of SODESA, sea water proof collectors have been developed by the Fraunhofer Institute for Solar
Energy Systems, ISE, Freiburg, Germany. The system is illustrated in Figure 2.
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Figure 2: The SODESA-System comprising sea water resistant solar collectors
Compared to the system described in chapter 3.3, this configuration is more convincing from the physical
engineering point of view. The solar heat input from the collectors can be used directly in the evaporation unit of
the distillation module. No transfer loss regarding the exergy of the collectors is to be coped with.
On the other hand all components, valves and controllers need to be sea water resistant up to 90 °C in this
configuration. This is the main disadvantage of this design. The installation component costs rise enormously
because no standard control devices and piping equipment can be used. The design parameters of this system are
shown in Chapter 3.4.
3.3.
Pressurised storage, EFPC Collector, Titanium heat exchanger (Muscat, Oman)
The pilot system being realized in the Sultanate of Oman is investigated in co-operation with the Sultan Qaboos
University of Muscat, Faculty of Mechanical Engineering. Standard solar collectors in connection with a
pressurised storage tank are used. The storage tank was manufactured in Oman. The heat supply part of the
system is filled with fresh water in a closed loop. The sea water fed side of the system is separated from the heat
supply by a titanium heat exchanger. This material offers best performance facing the very aggressive hot sea
water.
The separated circuits are the main advantage of this system design. This enables standard components to be
used on the heat supply side. The storage tank is made from mild steel and all piping is standard copper installation. The whole circuit is at 2.5 bars of pressure and is designed for a temperature of up to 120 °C. The storage
tank in this system can be smaller (3.2 m³), the temperature of the load flow to the heat exchanger (and then to
the distillation unit) is mixed together from hot water from the storage tank and reflux water from the heat
exchanger. Controllers are standard parts from solar domestic hot water installations. The system design is
shown in Figure 3.
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Figure 3: MEH-Desalination system applying separated heat supply and distillation
4. COGENERATION OF WATER AND ELECTRICITY
A very promising approach is the supplementation of the MEH-distillation unit with a small diesel or gas generator using it's waste heat for the distillation process (CHP). The two necessities for electricity and drinking
water coincide very naturally in many locations (Remote villages, camping sites, decentralised hotels and
apartment villages).
The matching of electricity requirement and correlated amount of waste heat on the one hand with the water
need of a small community on the other hand appears to fit quite well, as can be seen from this example:
Example:
Small community, 50 Persons.
Assumed fresh water need: 2.5 m³ per day.
Electricity demand per person and day: 2 kWh; 100 kWh per day for the community. Diesel driven electrical
generator: Power: 15 kW electrical (peak), 30 kW thermal;
Operation time of generator: 8 hours per day; achieved waste heat is 240 kWh per day.
Possible yield from the MEH-Unit: about 2.2 m³ of distillate.
Blending the water with sea water for non-food application delivers requested amount of water.
The relationship is illustrated in Figure 5.
Waste Heat Utilization - Example
Small Community
50 People
Water supply:
2.4 m³/day
Demand:
Water: 2.5 m³/day
Electricity: 100 kWh/day
Distillation
Unit(s)
100 kWh/day
Diesel
Generator
15 kW el.
30 kW thermal
thermal
input:
Thermal
Storage Tank
160 kWh
30 kW
10 kW
100 kWh/
m³
Figure 4: Example of fresh water and electricity supply for a small community
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5. OPTIMIZATION POTENTIAL
A physical model of the desalination module describes the unit as a parallel connection of elementary finite
elements. The model considerations were implemented into a mathematical model which is solved with the help
of a finite element modeling environment (EES). The influence of temperature dependant coefficients as heat
conductivity, diffusion coefficient, viscosity of humid air flow, Nusselt and Sherwood numbers as well as the
mass transfer coefficient were considered. The local conditions in a finite element calculation separating the
desalination module into 110 slices was applied to achieve more information about the influence of viscous
friction in the natural convection loop on the process.
The performance of the process is influenced by the enhancement of heat and mass transfer due to increased
humidity concentration gradients and improved Sherwood numbers at higher flow speeds. This enhancements
are straight respectively quadratic proportional with the distance of mass transfer surfaces d. The mean flow
speed of the humid air increases linear with smaller spacing due to the assumed self-adjustment of capacity
rates. Thus the friction force is quadratic inversely proportional with the distance of the mass exchange surfaces
d.
This implicates the existence of an optimum distance dOpt. which is determined by the thermal properties of the
condenser plates and the inlet parameters temperature and mass flow. Figure 5 shows the optimum state
calculation varying the distance d for two different condenser plate materials (Polypropylene, k = 230 W/m²K
and metal, k =2300 W/m²K). In this configuration the optimum balance between increasing friction force due to
narrow channeling and enhanced mass transfer is defined.
Distance of mass transfer surfaces in mm
1
2
3
4
6
7
8
9
Distillate production
Metall-Condeneser
PP-Condenser
130
Spezific Energy Demand in kWh/m³
5
120
10
44
43
110
42
100
90
Spez. Energy Demand
Metall-Condenser
PP-Condenser
80
70
41
40
60
50
39
40
30
0
1
2
3
4
5
6
7
8
9
Distillate production in l/h
140
0
38
10
Distance of mass transfer surfaces in mm
best distance
Figure 5: Calculation of optimum mass transfer surface distance for the given configuration
The calculation is performed under the assumption of 85 °C evaporator inlet temperature, a constant condenser
inlet temperature of 25 °C and a volume flow of raw water of 500 l/h.
It can be seen that the main effect of the optimization is related to the specific energy demand of the process
which is nearly halved from the state of the art configuration. This becomes clear when looking at Figure 6a). In
contrast to figure 5, herein the specific energy consumption of the process within the typical operation range of
the Multi-Effect-Desalination module is shown. The volume flow of the unit can be varied over a wide range
resulting in a varying spectrum of specific energy demands. The typical operation modes are in the range of 500700 l/h raw water load flow which cause a distillate production rate of between 40 and 55 l/h.
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St at e of t he Art
Enhanced Mat erial
Enhanced Geomet ry
Enhanced Mat erial & Geomet ry
180
Specific Energy Dem and in kW h/m ³
Müller-Holst
160
St at e of t he art
140
120
pot ent ial for
developm ent in
energy efficiency
100
80
60
im proved configurat ion
40
Typical operat ion range
20
300
400
500
600
700
800
Volume flow raw water in l/h
St at e of t he Art
Enhanced m at erial
Enhanced geom et ry
Enhanced m at erial and geom et ry
Dist illat e Product ion in l/ h
80
70
60
50
40
30
20
300
400
500
600
700
800
Raw w at er volum e flow in l/h
Figure 6: Calculation of specific energy demand and distillate production rate for different operation
modes and configurations
6. SUMMARY AND OUTLOOK
The operation reliability and the applicability of the Multi Effect Humidification Process for remote areas at
stand alone operation has been demonstrated in different pilot projects under the leadership of the Bavarian
Centre for applied Energy research (ZAE Bayern) in Munich, Germany. The theoretical optimization potential
was derived on the basis of many years experimental experience using a specially designed simulation tool
which was thoroughly evaluated with the experimental experience.
TiNOX GmbH in Munich has the experience with materials useful to enhance the performance of the distillation
process in the way which is shown by the theoretical investigations. The market introduction of the improved
solar desalination systems "MiniSal" and "ProfiSal" , capable of producing 5 and 10 m³ of fresh water per day, is
envisaged for the second half of the year 2002.
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ACKNOWLEDGEMENTS
Special thanks goes to the following institutions:
•
•
The Middle East Desalination Research Center (MEDRC) for financal support of the project in Oman
(Project No. 98-AS-024B)
The European Commission for financial support of the SODESA-Project (Contract No. JO-CT98-0229)
REFERENCES
(1) Baumgartner T., Jung D., Kössinger F., Schrag H. and Sizmann R. (1992): Solar–thermische
Trinkwasserbereitung, Proceedings 8. Internationales Sonnenforum, June 23-July 2, Berlin DGS (German
Section of the ISES), München, pp 432-437
(2) Spirkl W., Ries H. Optimal finite-time endoreversible processes, Physical Review E, Volume 52, Number 4
(1995), pp. 3485-3489
(3) Jung D., Kössinger F., Schölkopf, W. (1994): Betriebserfahrungen mit kleinen, solarthermisch betriebenen
Entsalzungsanlagen, Proceedings 9. Internationales Sonnenforum, June 28-July 1, Stuttgart, DGS (German
Section of the ISES), München, pp 1491-1498
(4) Müller H., Engelhardt M., Hauer A., Schölkopf W. (1996): Solarthermal Seawater Desalination using a
Multi Effect Humidification System, Proceedings FAO-SREN Workshop on Decentralized Rural Energy
Sources, March 18-21, Freising, Germany
(5) Müller-Holst,H., Engelhardt M., Herve M., Schölkopf W. (1998): Solarthermal Seawater Desalination
Systems for Decentralised Use, Proceedings of the Sixth Arab International Solar Energy Conference,
AISEC-6 "Bringing Solar Energy into the Day Light", 29 March - 1 April 1998, Muscat, Sultanate of
Oman
(6) Müller-Holst H., Engelhardt M., Schölkopf W.: Small-scale thermal seawater desalination simulation and
optimization of system design, Desalination 122 (1999) pp. 255-262, Elsevier New York, Amsterdam,
Tokyo, Singapore, Rio de Janeiro Jan. 1999
(7) Rommel M., Hermann M., Koschikowski J.: The SODESA Project: Development of solar collectors with
corrosion-free absorbers and first results of the desalination pilot plant, Proceedings of the Mediterranean
Conference on Policies and Strategies for Desalination and Renewable Energies, 21-23 June 2000,
Santorini Island, Greece
(8) Müller-Holst H., Schölkopf W.: Multi Effect Humidification Sea Water Desalination using Solar Energy or
Waste Heat -Various implementations of a new technology. Proceedings of the 7th Arab International
Solar Energy Conference Sharjah, UAE, February 2001
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