Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
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PV AND THERMALLY DRIVEN SMALL-SCALE, STAND-ALONE
SOLAR DESALINATION SYSTEM WITH VERY LOW
MAINTENANCE NEEDS
Hassan E. S. Fath*, Samy M. Elsherbiny and Alaa A. Hassan
Mech. Eng. Dept., Alexandria Univ., Alexandria, Egypt
E-mail h_elbanna_f@yahoo.com
Matthias Rommel, Marcel Wieghaus and Joachim Koschikowski
Fraunhofer Institute for Solar Energy Systems ISE, Germany
Mostafa Vatansever
Fentec, Turkey
ABSTRACT
This paper presents the thermal performance of a membrane distillation (MD) solar
desalination unit located in the Mechanical Engineering Department site, Alexandria
University, Alexandria, Egypt. The unit is designed and installed as a part of a
partially funded European Commission (EC) project named; “PV and thermally driven
small-scale, stand-alone desalination system with very low maintenance needs
(SMADES)”.
The basic innovative MD principle and module is highlighted. The Alexandria MD
unit is described and the unit performance is presented for the 6-months of its
operation period. The unit performance covers; the transient changes in the unit
productivity, unit feed water, brine and product water temperatures and conductivities,
unit salt rejection, solar collector and MD process efficiencies. The unit performance,
in clear and cloudy days, for typical summer and winter months, is presented.
For a sunny day, of 7.25 kWh/day say, the results indicate that the unit produces about
11.2 liter/day for every square meter of collector area. The relatively high productivity,
above that of the conventional solar still, is due to the partial recovery of the
condensation energy. The overall unit productivity has been correlated against the
solar irradiation as:
Daily production(lit/m2 .day) = 1.666 × Daily radiation (kWh/m2 .day) - 1.67
Keywords: Desalination, Membrane Distillation, Solar Energy
INTRODUCTION
The international rapid developments, the industrial growth, and population explosion
all over the world have resulted in a large escalation of demand for fresh water. On the
other hand, the surface water (rivers and lakes) pollution caused by industrial and
agricultural wastes and the large amount of sewage, limit the suitability of many fresh
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water availability resources. By the beginning of this century, fresh water shortages and
quality became an international problem confronting human groups and countries. The
problem is more apparent in the Arabian and Middle Eastern/North African (MENA)
countries due to the very limited natural resources of fresh water.
Desalination (desalting) has been presented for decades as a suitable alternative for the
partial solution of the world fresh water crisis. Desalination of brackish and sea water
can provide the need of drinking water without any serious impact on the environment.
As a result, there has been a dramatic world wide increase in the number and capacity
of desalination plants, Wangnick [1]. Although it seems more expensive in areas with
surface or ground water availability, it is not so in areas 300 – 500 km far from fresh
water resources. For large water production, conventional desalination technologies as
MSF, MEE, RO, VC,…etc. have been proven to be technically and economically
suitable. However, for (i) small communities of limited fresh water demand (up to 10
m3/day), (ii) areas far from the sources of water and energy (fuel & electricity), and
(iii) communities of small technical capabilities, solar desalination is more applicable.
Solar desalination in many respects might be an ideal solution to small communities in
many MENA countries, as shown by Malik et al. [2] and Fath [3], due to the following
facts:
i-
iiiii-
Many of these countries enjoy an abundant solar intensity (Annual daily average
is between 200 – 300 W/m2) and large annual sun hours (3000 – 5000 hrs/year),
and therefore incident energy of about (5 – 8 kWhr/m2 day)
The diurnal and seasonal fluctuations in solar desalination productivity are
intrinsically linked to the fluctuating water demand,
Solar energy is almost available in every location and, in addition, is an
environment friendly energy resource (with no CO2 emission)
Solar desalination can be divided into direct and indirect technologies. In the direct
methods, the solar energy collector and desalination component are an integral unit,
e.g. solar stills, see references [4 to 10]. A general rule of thumb for simple solar stills
is that a solar collection area of about 1 m2 is needed to produce 3-5 liters of water per
day. Thus, for a 1 and 10 m3/day facility, a land area of about 250 and 2500 m2 would
be required, respectively. These areas may even be doubled to allow for spacing. Thus,
a large solar collection area is required with the resultant high capital costs. To
improve the performance of solar stills, multi-effect solar stills were used. In the
indirect method, the solar energy is first collected, converted to usable heat or
electricity then it is used as an energy source for the different desalination
technologies. Many researchers have investigated and developed different indirect
solar desalination systems, PV-RO, and Solar thermal-MEE units, etc., [11 to 15].
Each of these indirect solar desalination systems has its positive and negative features
and none of them has proved to be the best either technically or economically.
Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
251
SMADES PROJECT
SMADES is an EC partially funded project in solar desalination. The overall objective
of the SMADES project is the development of stand-alone desalination systems for
arid and semi-arid remote regions with a lack of drinkable water but a high solar
irradiation. The modular system set-up is based on the highly innovative membrane
distillation (MD) technology. The system integrates both solar thermal and solar PV
energies. The desalination energy is supplied entirely by solar thermal collectors and
the electrical auxiliary energy is supplied by PV.
To gather a detailed present state of technology and problems on small scale standalone desalination plants, a technical study was carried out in the beginning of the
project. A second study, after reasonable developments have been carried out to
discover the need and the social impact for potential users and to focus on the legal
frame works and economic influences.
The technical focus of the project is on the improvement of desalination modules
based on Membrane Distillation (MD) for stand-alone systems. The MD technique is
particularly suitable for maintenance-free and solar supplied systems. The used
membranes have a high resistance against different water composites and operate in a
temperature range of 60 to 90°C in which collectors achieve a high efficiency and
good performance. The developed technique was modular and covered a wide range of
capacities (150 Lit/day up to 10 m3/day) so that systems can be adapted to any
particular demand.
Two different types of MD modules as well as one type of heat exchanger were
developed based on the same spiral-winding technology used for the MD modules. For
very small and compact applications, the MD module was equipped with internal heat
recovery. The aim is to achieve Gained Output Ratios (GOR’s) of 6 to 8 and low
module pressure losses (0.2 to 0.6 bar) to reduce pump energy. For larger applications
(larger than 1 m3/day) MD modules without internal heat recovery but with an external
heat exchanger were used. The aim was to increase the module output (the output of
modules without internal heat recovery can be two to three times higher), to decrease
production costs and to increase the energy efficiency of the whole system (GOR’s >
10) by connecting several MD modules to a single high efficiency heat exchanger.
MEMBRANE DISTILLATION
Membrane Distillation (MD) is an innovative membrane technology, [16 to 21].
Contrary to membranes for RO, MD membranes are hydrophobic. This means that, up
to a certain limiting pressure, the membrane can not be wetted by liquid water. In
Figure (1), the principal set-up for MD module with internal heat recovery is sketched.
For MD system, on one side of the membrane there is a lower temperature, for
example 60 °C, then there exists a water partial pressure difference between the two
sides of the membrane and thus water evaporates through the membrane. The water
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vapor condenses on the other side of the membrane and the distillate is formed. Some
experience was gathered in recent experimental investigations with spiral wound MD
modules with a basic set-up as sketched in Figure (2). The experiments show
promising results.
Heat
Exchanger
Condenser
Heat
Source
Feedwater
Distillate
Evaporator
Brine
Membrane
Figure (1) Membrane Distillation Principles
Figure (2) Spiral Wound MD Module
•
•
The advantages of the MD systems for stand-alone units were found to be:
The process works at low temperatures (60 to 90°C) which is important for a
high efficiency of solar thermal collectors. (It is also possible to utilize waste
heat from diesel engines or heat from co-generation plants.)
Contrary to RO-systems, no chemical pre-treatment of the feed water is
necessary. Simple pre-filtration is sufficient.
Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
•
•
•
•
•
•
253
There are no problems in the intermittent operation mode. Also long times of
dry-out situations are not important because the membrane is hydrophobic.
The membranes are non-sensitive against algae, etc.
The modules work at atmospheric pressure. It is not a vacuum technology as in
MSF/MEE or pressurized technology as in RO.
The salinity of the feed water has almost no influence on the efficiency of the
process.
Membrane distillation produces very pure distillate. The electrical conductivity
is in the range of 2-10 µS/cm.
Due to the modular set-up systems, a high range of water capacity units can be
built.
In the present SMADES project, MD technology was further developed to be applied
and adopted to the conditions for solar thermally driven desalination systems. A
screening of suitable materials was carried out to achieve long time resistant systems.
Control strategies for long term maintenance-free operation was developed. Lessons
learned from earlier operating experience of solar thermal driven pilot plants with
different distillation devices (for example Multi Effect Humidification) showed that
more emphasis has to be given to a well developed total system design. To the present
knowledge of the proposed project, the MD technology holds more development
potential than other thermally driven desalination techniques for small stand-alone
systems.
LAYOUT OF ALEXANDRIA COMPACT SYSTEM
Figures (3) and (4) show the photo and the flow diagram of Alexandria compact
system, respectively. This system was installed and mounted on the roof of the thermal
laboratory, mechanical engineering department of Alexandria University, Alexandria,
Egypt, in June 2005 and has been working properly since this time till now. The
objective of installing this system is to carry-out experimental investigations with long
term maintenance free operation under real weather conditions.
The system is supplied by water from a 500 litre feed storage tank mounted at the end
of the top section of the solar collector. From that storage tank the cold feed water is
pumped to the condenser section of the MD module. The feed water is preheated using
the latent heat of condensation of the distillate. The preheated feed water leaves the
condenser and then enters the bottom section of the solar collector. The heat absorbed
by the solar collector during the day time is transferred to the feed water (which may
be sea water or brackish water). The heated feed water leaves the collector area on the
top, passes a degasser bottle to free it from gases and then enters the evaporator side of
the MD module. Part of the heated feed water is evaporated through the membrane
while the concentrated brine is recirculated back to the feed storage tank. The
distillate after being condensated is either collected in a distillate tank for real use or
fed back to the raw water tank to form a closed loop of experiments.
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Due to the separation of distillate, the water level in the storage tank decreases and the
salt concentration in this tank increases as the brine comes back to this tank from the
membrane module. To overcome the increase in salt concentration, the storage tank is
refilled with fresh feed water from a well or another source of raw water. When the
liquid level in the feed water storage tank drops to half the tank height, a switch
activates the refilling pump and the storage tank is refilled with fresh feed water. The
refilling process continues until the water level reaches about 90% of the storage tank
height at which level another actuator switches the refilling pump off. The system is
self-running since the pumps and all other electrical devices are supplied by the PV.
The operation is only possible during day time because there is no other source of
electricity used.
The solar collector consists of three rectangular similar sections connected in series.
The dimensions of each section are 2020 mm × 1020 mm × 80 mm with a total area of
2.06 m2 and an aperture area of 1.91 m2. Each collector section consists of 10 absorber
finned tubes made from aluminium and connected in parallel to the header tubes. Each
absorber fin has a width of 100 mm with a thickness of 2.0 mm. The aluminium finned
tubing is equipped with Cu-Ni10 tubes to resist the corrosion if sea water is used. This
material withstands hot salt water and is often used in large scale desalination plants.
The outer diameter of the riser tubes is 10 mm and the diameter of the header tubes is
20 mm.
Figure (3) A photo of MD Alexandria Compact System
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Figure (4) Flow diagram of MD Alexandria Compact System
The riser tubes are joined to the header tubes by brazing. In order to minimize the
thermal contact resistance between the aluminium absorber profile and the Cu-Ni10
riser tube, the riser tube is pushed into the open cylindrical back site of the aluminium
profile, and then the tubular aluminium profile and the riser tube are grouted together.
The sketch in Figure 5 shows a cross section of the absorber profile. The four ends of
the header tubes are passed through the collector casing to achieve a high flexibility
with respect to collector field design and collector field interconnection.
Aluminum fin profile
Cu-Ni10 tube
Figure (5) Absorber fin profile with grouted Cu-Ni10 tube
In order to achieve the operation temperatures of the membrane distillation (MD)
system of 80 to 90°C, the solar thermal flat plate collector has to have a selectively
coated absorber to reduce the heat loss by radiation. The absorber plate is made from
aluminium. Its selective surface is applied by an electro-chemical process. The
achieved characteristic values for the selective surface are 0.93 absorpitivity and 0.18
emissivity.
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The glass cover has a low iron content to reduce losses due to the absorption of
radiation by iron particles. A low-iron glass cover is used to increase transmission to
the absorber. The transmission of the solar radiation for glass with low iron is about
10% higher than for standard window glass with high iron content. The transmissivity
of the glass used is 0.89.
The back side of the collectors as well as the walls are insulated with mineral wool
with thickness 60 mm on the back side and 30 mm on the walls. The collector casing is
made of anodised aluminium. This material is resistant against salty air and strong
weather conditions as sandstorm.
The collector efficiency, η, curve shown in Figure (6) was determined according to the
code EN12975-2 at the test centre for solar thermal collectors at Fraunhofer ISE in
Germany.
Figure (6) Collector Efficiency Curve of the FENTEK Cu-Ni10 collector
The measuring instrumentation and the data acquisition system were adapted to the
different requirements of the compact system. So, the system which focuses on
experimental investigations is highly equipped with sensors and with an electronically
data acquisition system. Figure (7) provides a list of all sensors and their locations in
the hydraulic loop. The sketch represents the highly equipped system for experimental
investigations.
Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
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Figure (7) Sensors list and Layout Plan
UNIT PERFORMANCE RESULTS & DISCUSSION
The unit is continuously running since its complete installation and collection of full
measurements started in July 2005. The data are taken every 10 seconds for all
temperatures, flow rates, electric conductivities and global solar radiation as indicated
in Figure (7). To show the performance, data for a clear day (17 Aug, 2005) and for a
cloudy day (30 Oct., 2005) are presented. Figure (8) shows the collected data for the
clear day from sunrise to sunset. The maximum solar intensity during this day, Iglob
was 995 W/m2 and the working evaporator inlet temperature was above 60°C. The
electric conductivities for feed and distillate water are 526 and 3 µS/cm respectively.
The daily production is 64 Lit/day (11.2 Lit/m2day) while the accumulated solar
energy is 41.6 kWh/day (7.25 kWh/m2day).
For the cloudy day, Figure (9) shows a maximum solar intensity of 1231 W/m2.
However, the total solar energy is 29.5 kWh/day (5.15 kWh/m2day) and the daily
production is 23.6 Lit/day (4.11 Lit/m2day). The electric conductivities for feed and
distillate water are 672 and 2 µS/cm respectively. The relation between the daily
radiation and daily production is shown in Figure 10. It is clear that the distillate daily
production is directly proportional to the daily total radiation. A linear correlation over
a 6-months operation period is given as:
Daily production(lit/m2 .day) = 1.666 × Daily radiation (kWh/m2 .day) - 1.67
(1)
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The scatter about the correlation is mainly due to solar radiation at early morning or
close to sunset where there is no production since evaporator inlet temperature is too
low to operate the MD.
The maximum and minimum condenser and evaporator temperatures are shown in
Figure (11). The values are almost constant over the operating period with an average
temperature difference in evaporator of 43 °C and 18 °C in the condenser.
The solar collector and MD process efficiencies are defined as:
ηc =
∑ m feed × Cp (Tco − Tci )
∑ I glob × A
η process =
(2)
∑ m dist × Latent Heat
∑ I glob × A
(3)
where:
A
= collector area (m2)
mfeed = feed water flow rate (kg/s)
Tci
= collector inlet temperature
mdist
Iglob
Tco
= distillate flow rate (kg/s)
= Global solar irradiation (W/m2)
= collector outlet temperature
Figure (12) shows an average collector efficiency of 50% and a process efficiency of
90% which indicates only 10% heat losses from the MD. The percentage of salt
removal from the feed water is defined as:
% Salt Removal = (1 −
distillate conductivity
) × 100
Feed conductivity
The percentage of salt removal is found to be about 99.5%.
(4)
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Compact System Alex. 17th Aug.
Tcond_in
Tcond_out
Tevap_in
Tevap_out
V_dist
Cond_feed
V_feed
Iglob
Cond_dist
100
1000
60
600
40
400
l/h mikroS/cm
800
C
l/h mikroS/cm
80
20
200
0
0
0
0
0
0
:00
20
:00
19
:00
18
:00
17
:00
16
:00
15
:00
14
:00
13
:00
12
:00
11
:00
10
9 :0
8 :0
7 :0
6 :0
Figure (8) Data for a Clear Day
Compact System Alex. 30th Oct.
Tcond_in
Tcond_out
Tevap_in
Tevap_out
V_dist
Cond_feed
V_feed
Iglob
100
20
800
60
600
40
400
200
0
0
:00
19
:00
18
:00
17
:00
16
:00
15
:00
14
:00
13
:00
12
:00
11
:00
10
0
9:0
0
8:0
0
7:0
Figure (9) Data for a Cloudy Day
l/h mikroS/cm
l/h mikroS/cm
C
1000
80
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14
Daily Production (l/m^2.day)
12
y = 1.6656x - 1.6697
10
8
6
4
2
0
0
1
2
3
4
5
6
7
8
Daily Radiation (kwh/m^2.day)
Figure (10) Effect of Daily Radiation on Daily Production
90.0
80.0
Min Tcon
70.0
Max Tcon
60.0
Min Tev
Temp (C)
Max Tev
50.0
40.0
30.0
20.0
10.0
0.0
1
5
9
13
17
21
25
29
33
37
41
45
49
53
57
61
65
69
73
Day No.
Figure (11) Temperature Distributions
77
81
85
89
93
97
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100
90
80
Efficiency %
70
60
50
40
30
Collector Eff.
20
Process Eff.
10
0
0
10
20
30
40
50
Day No.
60
70
80
90
100
Figure (12) Solar Collector and MD Process Efficiencies
CONCLUDING REMARKS
1- A full description of a desalination unit based on membrane distillation is
given. The unit is self operating using a PV panel to run the feed pump and
solar collectors to heat up the feed water. Condensation energy is recovered
in the condenser channel of the membrane to preheat the feed water.
2- A sample of the measurements for a clear day and a cloudy day is presented
and indicates a high productivity of 11.2 Lit/m2.day for a total solar energy
of 7.25 kWh/m2.day.
3- A correlation between the unit productivity and total solar radiation is given.
4- The unit shows a high salt rejection performance as it reduced the electric
conductivity of the feed water from 670 to about 3 µS/cm, for the product.
This gives a salt rejection percentage of about 99.5%.
5- The MD process efficiency is about 90% and the solar collector efficiency is
about 50%.
6- The system is very suitable and promising for arid areas in the Arabian and
north African regions.
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ACKNOWLEDGEMENT
The authors would like to acknowledge the EC for partially funding the project “PV
and Thermally Driven Small-Scale, Stand-Alone Solar Desalination System with very
Low Maintenance Needs – SMADES”, contract No: ICA3-CT-2002-10025.
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