3.16

Solar Desalination

E Tzen, Centre for Renewable Energy Sources and Saving (CRES), Pikermi, Attica, Greece
G Zaragoza and D-C Alarcón Padilla, Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT),
Plataforma Solar de Almeria, Almeria, Spain
© 2012 Elsevier Ltd. All rights reserved.

3.16.1
3.16.2
3.16.3
3.16.4
3.16.5
3.16.6
3.16.7
3.16.8
3.16.8.1
3.16.8.2
3.16.8.3
3.16.8.4
3.16.8.5
3.16.8.6
3.16.8.7
3.16.9
3.16.10
3.16.11
3.16.12
References

Introduction

Solar Thermal Desalination Systems
Photovoltaics-Driven Desalination Systems
Solar Stills
Solar Humidification–Dehumidification
Solar Membrane Distillation
Technologies Selection Guidelines
Solar Desalination Applications
Solar Thermal MES Plant for Seawater Desalination, Abu Dhabi, UAE
Solar Thermal MED Plant for Seawater Desalination, Almeria, Spain
PV–RO Plant for Seawater Desalination, Lampedusa Island, Italy
PV–RO Plant for Brackish Water Desalination, Ceara, Brazil
PV–RO Plant for Seawater Desalination, Pozo Izquierdo, Gran Canaria Island
PV Water Pumping RO for Brackish Water Desalination, Saudi Arabia
PV–RO Brackish Water Desalination, Aqaba, Jordan
Lessons Learned
Economics
Market
Conclusions

Glossary
Brackish water Saline water with a salt concentration
ranging from 1000 mg/l to about 25 000 mg/l total
dissolved solids (TDS).
Desalination Process of removing salts from water
sources.
Distillation A method of desalting water that uses heat to
vaporize water and collect the condensed water.
Electrodialysis (ED) A process by which ions are
transferred through membranes to a more concentrated
solution as a result of using a direct current electrical

potential.
Electrodialysis reversal (EDR) A variation of ED in which
polarity and cell function change periodically to maintain
efficient performance.
Energy recovery Possible energy saving in reverse osmosis
in which the concentrate stream, under pressure, is used to
drive a turbine that provides part of the feed requirement.
kWh Kilowatthours A measure of electrical usage.
Membrane In desalting, used to describe a
semipermeable film. Membranes used in electrodialysis

529

530

536

541

542

543

546

547

547

549


553

556

558

558

560

563

564

564

564

565


are permeable to ions of either positive or negative charge.
Reverse osmosis membranes ideally allow the passage of
pure water and block the passage of salts.
Performance ratio (PR) A performance rating associated
with the distillation desalting process. It is defined as the
number of pounds of distillate produced for each 1000
Btu of heat input, or as kg/MJ in metric.
Post-treatment The processes, such as pH adjustment and

chlorination, that may be employed on the product water
from a desalting unit.
Pretreatment The processes such as chlorination,
clarification, coagulation, scale inhibition, acidification,
and deaeration that may be employed on the feed water to
a desalting unit to minimize algae growth, scaling, and
corrosion.
Recovery ratio The ratio of the product flow rate to the
feed water flow rate.
Reverse osmosis (RO) Method of desalination which
uses pressure to move water from a concentrated solution
to a dilute solution through a membrane separating two
solutions.

3.16.1 Introduction
Converting seawater or brackish water into freshwater is a promising approach to overcome the insufficiency in water supply caused
by population increase, agricultural and irrigation needs, industrial needs, etc. Production of freshwater using desalination
technologies driven by renewable energy sources (RESs) is thought to be a viable solution to the water scarcity in remote areas
characterized by lack of potable water and lack of an electricity grid. RES–desalination matching is mainly categorized as distillation

Comprehensive Renewable Energy, Volume 3

doi:10.1016/B978-0-08-087872-0.00316-4

529


530

Applications


desalination technologies driven by heat produced by RESs, and membrane and distillation desalination technologies driven by
electricity or mechanical energy produced by RESs.
Indirect use of solar energy by means of solar thermal systems and photovoltaics (PVs) in tandem with desalination seems to be the
most applicable technology. Direct use of solar energy for desalination, such as the use of solar stills, is the oldest, simplest, and most used
method. Figure 1 presents the possible combinations of solar energy technologies with desalination. The selection of the most
appropriate combination is mainly site specific. Several parameters that affect the final decision are discussed in the following paragraphs.
Many applications of relatively small scale exist around the world; some of the most known are examined in this chapter. The
majority of the solar desalination systems involve the use of photovoltaic-driven reverse osmosis (PV–RO) units for brackish and
seawater desalination. Most of the already existing applications have been built within National or European projects and are pilot
demonstration units [1].
Some of the installed units cover basic needs of the region where they have been installed, while some are no longer in operation.
Nevertheless, the lessons learned from their operation have been passed on and are the guidelines for the new installations.
This chapter focuses on the state of the art of the solar desalination technologies, their current applications, the lessons learned,
and the economics and market for these technologies.

3.16.2 Solar Thermal Desalination Systems
Solar energy refers to applications of solar thermal conversion and PV conversion. Solar thermal systems are usually classified
according to the temperature level reached by the thermal fluid in the collector (Figures 2 and 3). The thermal effects produced by

Electric energy

Solar energy
Direct use

Solar
stills

Indirect use
T

h
e
r
m
a
l
e
n
e
r
g
y

Photovoltaics
Reverse osmosis
Electrodialysis

Multieffect distillation
Solar
thermal
collectors

Multistage flash
Membrane distillation
Humidification−
Dehumidification

Figure 1 Solar energy–desalination matching.

Figure 2 Parabolic-trough collectors, CIEMAT PSA, Almeria, Spain.



Solar Desalination

531

Figure 3 Fresnel Technology, CIEMAT PSA, Almeria, Spain.

solar radiation enables Man to take direct advantage of them by using devices that collect, concentrate, and intensify the heat and
transfer the thermal energy to other fluids by heating them. Depending on the design of the collector, it can provide heat for
domestic applications, industrial processes, and electricity production.
There are basically two types of solar collectors:
1. Stationary or nonconcentrating
2. Concentrating.
A nonconcentrating collector has the same area for intercepting and absorbing solar radiation, whereas a sun-tracking, concentrating
solar collector usually has concave reflecting surfaces to intercept and focus the sun’s radiation to a smaller receiving area, thereby
increasing the radiation flux. Table 1 presents the different types of available solar collectors and their main characteristics [2].
A solar thermal energy system mainly consists of a solar collector array, a storage tank, and necessary controls (Figure 4). The
solar collector system provides the desalination unit with the required hot steam.
Analytically, solar thermal distillation plants include a field of solar collectors, where a thermal fluid is heated. This hot fluid is
used, by means of a heat exchanger, to warm up the feedwater circulating through the distillation plant. The collectors must be able
to heat the thermal fluid up to medium temperatures so that after appropriate heat transfer, the water fed to the evaporator reaches
temperatures between 70 and 120 °C. Temperature limits protect the distillation plant from scaling and corrosion problems.
The best known solar thermal distillation combinations are the solar multistage flash (MSF) and solar multieffect distillation
(MED). Both processes are classified as phase-change or thermal processes. Distillation units routinely use designs that convert as
much thermal energy as possible by interchanging the heat of condensation and heat of vaporization within the units. The major
energy requirement in the distillation process is, thus, providing the heat of vaporization to the feedwater.
MSF and MED processes consist of a number of stages or effects at successively decreasing temperatures and pressures, and
generally operate on the principle of reducing the vapor pressure of water within the unit to permit boiling to occur at lower
temperatures, without any extra heat.


Table 1
Main types of solar collectors Reproduced from: Kalogirou S (2003) The potential of solar industrial process heat applications.
Applied Energy 76(4): 337–361.

Motion

Collector type

Absorber type

Concentration ratio

Indicative temperature range
(°C)

Stationary

Flat-plate collector (FPC)
Evacuated tube collector (ETC)

Flat
Flat

Single-axis tracking

Compound parabolic collector (CPC)
Linear Fresnel reflector (LFR)
Parabolic-trough collector (PTC)
Cylindrical-trough collector (CTC)

Parabolic-dish reflector (PDR)
Heliostat field collector (HFC)

Tubular
Tubular
Tubular
Tubular
Point
Point

1
1
1–5
5–15
10–40
15–45
10–50
100–1000
100–1500

30–80
50–200
60–240
60–300
60–250
60–300
60–300
100–500
150–2000


Two-axes tracking

Note: Concentration ratio is defined as the aperture area divided by the receiver/absorber area of the collector.


532

Applications

Three-way valve

Solar collector
array
Conventional
boiler

Hot water
storage tank

Desalination
unit

Solar pump

Figure 4 Typical configuration of a solar thermal desalination plant [2]. Adapted from Kalogirou S (2003) The potential of solar industrial process heat
applications. Applied Energy 76(4): 337–361.

The MSF process is based on the generation of vapor from seawater or brine caused by a sudden pressure reduction when
seawater enters an evacuated chamber [3]. The process is repeated stage by stage at successively decreasing pressures. This process
requires an external steam supply, normally at a temperature between 100 and 110 °C. The maximum temperature is limited by the

salt concentration to avoid scaling, and this limits the performance of the process.
There are two process arrangements for the MSF process: once-through and brine recirculation. In the brine recirculation MSF,
the system is divided into the heat-rejection, the heat-recovery, and the heating sections (see Figure 5). Seawater is fed through the
heat-rejection section that rejects thermal energy from the plant and discharges the product and brine at the lowest possible
temperature [3]. The feed is then mixed with a large mass of water, which is recirculated around the plant. This water then passes
through a series of heat exchangers to raise its temperature. The water then enters the solar collector array or a conventional brine
heater to raise its temperature to nearly the saturation temperature at the maximum system pressure. The water then enters the first
stage through an orifice and in doing so has its pressure reduced. Since the water was at the saturation temperature at high pressure,
it becomes superheated and flashes into steam. The vapor produced passes through a wire mesh (demister) to remove any entrained
brine droplets and then into the heat exchanger, where it condenses and drips into a distillate tray. Generally, only a small
percentage of this water is converted to steam (water vapor), depending on the pressure maintained in this stage, since boiling will
continue only until the water cools [5]. This process is repeated through the plant as both brine and distillate streams flash as they
enter subsequent stages, which are at successively lower pressures.

To atmosphere
Steam jet ejector

HP steam supply
LP steam supply

Cooling water return
Concen­
trate
heater

Sea water supply
Distillate

Tube bundle section


Deaerator
Flash chamber section
Decarbonator

Make-up

Stage 0

Heat
rejection
section

Stage 2 Stage 1

Heat recovery section
Condensate
pump

Blowdown
pump
Concentrate recycle

Figure 5 MSF brine recirculation schematic. Adapted from Watson IC, Morin OJ, Jr., and Henthorne L (2003) Desalting Handbook for Planners, 3rd edn.
USA: U.S. Department of the Interior Bureau of Reclamation [4].


Solar Desalination

533


In MSF, the number of stages is not tied rigidly to the performance ratio (PR), which is the ratio of the distillate produced to the
heat input (steam consumed, kilograms per megajoule) required from the plant. In practice, the minimum temperature must be
slightly greater than the PR, while the maximum is determined by the boiling-point elevation. The minimum interstage temperature
drop must exceed the boiling-point elevation for flashing to occur at a finite rate. This is advantageous because as the number of
stages is increased, the terminal temperature difference over the heat exchangers increases and hence a smaller heat transfer area is
required with obvious savings in plant capital cost.
MSF is the most widely used desalination process in terms of capacity. MSF plants are generally built-in units of about
4000–55 000 m3 day−1. Current commercial installations are designed with 10–30 stages (2 °C temperature drop per stage) [3].
The MED process takes place in a series of vessels (effects) and uses the principles of condensation and evaporation at
reduced ambient pressure in the various effects. This permits the seawater feed to boil without the need for supply of
additional heat beyond the first effect. In general, an effect consists of a vessel, a heat exchanger, and devices for transporting
the various fluids between the effects. As with the MSF plant, the incoming brine in the MED process also passes through a
series of heaters, but after passing through the last of these, instead of entering the brine heater, the feed enters the top effect,
where the heating steam raises its temperature to the saturation temperature for the effect pressure. Further amounts of steam,
either from a solar collector system or from a conventional boiler, are used to produce evaporation in this effect. The vapor
then goes, in part, to heat the incoming feed and, in part, to provide the heat supply for the second effect, which is at a lower
pressure and receives its feed from the brine of the first effect (see Figure 6). This process is repeated all the way through
(down) the plant. The distillate also passes down the plant. Unlike the MSF plant, the PR for an MED plant is more rigidly
linked to and cannot exceed a limit set by the number of effects in the plant. For instance, a plant with 13 effects might
typically have a PR of 10. However, an MSF plant with a PR of 10 could have 13–35 stages depending on the design. MED
plants commonly have PRs as high as 12–14 [3].
The choice and optimization of the PR and the number of effects in an MED plant is a matter of balancing the high energy costs
associated with a setup with a low PR and a small number of effects against the high capital costs of a setup with a large number of
effects, and large transfer surfaces in both the effects and the feed heaters. Three main arrangements have evolved for MED processes.
They are based primarily on the arrangement of the heat exchanger tubing which can be as follows [4]:
• Horizontal tube arrangement
• Vertical tube arrangement
• Vertically stacked tube bundles.
MED plants are typically built-in units of 2000–20 000 m3 day−1. Some of the more recent plants have been built to operate with a
top temperature (in the first effect) of about 70 °C, which reduces the potential for scaling of seawater within the plant [5].


Steam jet ejector

HP steam supply
Feed water supply
Main
condenser

Steam supply

Tube
bundle
section

Tube
bundle
section

Tube
bundle
section

Tube
bundle
section

Tube
bundle
section


Condensate
return

Cooling
water
return
(2)

(2)

Effect pumps

(1) Degassifier deaerator

(2)

(2)

Effect pumps

(2) Concentrate chamber

(2)

(1)

Make-up
pump

Product

to
storage

Blowdown

Figure 6 MED schematic [4]. Adapted from Watson IC, Morin OJ, Jr., and Henthorne L (2003) Desalting Handbook for Planners, 3rd edn. USA: U.S.
Department of the Interior Bureau of Reclamation.


534

Applications

As can be concluded from the above, thermal distillation technologies are best fitted to large capacities; however, research
has been done in small capacities also. Several solar thermal plants have been installed and examined around the world (see
Table 2) [1].
An example is the MSF plant installed in 1987 in El Paso, Texas. The combination was somewhat unusual involving a 3355 m2
solar pond and a cogeneration system, producing electricity in a Rankine cycle and water in a 24-stage MSF evaporator capable of
producing 19 m3 day−1.
More reference cases can be found at San Luis de la Paz, Mexico, where a double solar field (194 m2 flat-plate collectors plus
160 m2 concentrating collectors) provides heat for a 10 m3 day−1 MSF unit, with 10 stages. The plant was commissioned in 1980.
One more example is found in Lampedusa Island in Italy. The plant was commissioned in 1983. The MSF plant had a capacity of
7.2 m3 day−1 driven by 408 m2 solar collectors. Another solar MSF system was installed in Safat, Kuwait, in 1981. The 12-stage MSF
plant had a capacity of 10 m3 day−1 driven by 220 m2 parabolic-trough solar collectors.
Concerning solar MED applications in 1981, a solar MED plant of 10 m3 day−1 capacity was installed in Takeshima Island in
Japan, and in the same year a 2 m3 day−1 solar MED plant started its operation at the Black Sea, Bulgaria.
A very famous solar MED plant is the one in Abu Dhabi, United Arab Emirates. The system consists of 1862 m2 evacuated-tube
collectors (ETCs) and an MED plant of around 120 m3 day−1 distillate water capacity. The plant has many years of satisfactory
operation. The plant is discussed analytically in the following paragraphs.
Another well-known example of a solar MED plant of 3 m3 h−1 water capacity and 500 m2 compound parabolic concentrators

(CPC) is located at the Plataforma Solar de Almeria (PSA), Spain, as part of the AQUASOL project (Figure 7). The plant is described
analytically in the following paragraphs [6].
A small solar thermal MED plant was commissioned in 2002 in Muscat in the Sultanate of Oman (owned by the M/S Power
System International) and was operational for a year (see Figure 8). The pilot MED plant was designed to produce 1 m3 day−1 in 9 h

Table 2

Solar thermal desalination plants

Location

Desalination unit

Solar collector

Year of installation

Hazeg, Tunisia
San Luis de la Paz, Mexico
Lampedusa Island, Italy
Safat, Kuwait
Takami Island, Japan
Abu Dhabi, UAE
Almeria, Spain
Almeria, Spain
Almeria, Spain, AQUASOL project

0.1–0.35 m3 h−1 distillation
10 m3 day−1 MSF
7.2 m3 day−1 MSF

10 m3 day−1 MSF
16 m3 day−1 MED
80–120 m3 day−1 MED
72 m3 day−1 MED
24 m3 day−1 MED
30–40 l day−1

1980
1980
1983
1984
–
1984
1993
1988/1990
1998

Al Azhar, PSA
Pozo Izquierdo, Gran Canaria,
Spain, SODESA project
Oman

0.2 m3 day−1 MSF
0.6 m3 day−1

–
352 m2 FPC + PTC
408 m2 low-concentration solar collectors
220 m2 low-concentration solar collectors
FPC

ETC
PCP
Parabolic concentrating
6 m2
Vacuum-tube solar collectors
FPC + PVs
50 m2 solar collectors + PVs

1 m3 day−1 MED

5.34 m2 VTC

2002

Figure 7 Compound parabolic collectors (CPC), CIEMAT PSA, Almeria, Spain.

1998/2000
2000


Solar Desalination

535

Figure 8 View of the solar thermal MED unit in Oman. Adapted from Report on the Status of Autonomous Desalination Units Based on Renewable
Energy Systems (2005) INCO-CT-2004-509093, Co-ordination Action for Autonomous Desalination Units Based on Renewable Energy Systems,
ADU-RES Project. www.medrc.org.om

of operation during the day, using solar energy. The innovative techniques and methods used in the plant included
high-temperature tubular solar collectors, scale-preventing coating, and a device for water softening. The desalination system

operated at a top brine temperature of 100 °C. Water recovery reached 80–85%. The MED unit included 12 effects and 6 preheaters.
The thermal energy consumption was 64 kWh m−3, while the electric energy consumption was 1.4 kWh m−3. The plant was tested
with water salinity of 30 000–35 000 ppm total dissolved solids (TDS). The salinity of the water produced by the plant was
80–120 ppm TDS.
The solar thermal system consisted of high-temperature vacuum tubular solar collectors with mirror concentrators, a separator
for steam and water separation, and a solar tracking system. The solar power plant included 16 collector panels, with an effective
collector area of 5.34 m2. The circulation flow rate through the whole collector system was 460 l h−1 and the maximum pressure of
operation was 1.05 bar. The system’s electricity demand was satisfied by 2 m2 PV cells [6]. The project was cofinanced by the Middle
East Desalination Research Center (MEDRC) in Oman.
Also, a number of small distillation plants have been installed in Tunisia: a 40–50 l h−1 single-effect evaporation process for
brackish water desalination in Hazeg, Sfax (1986), an MED of 150–200 l day−1 for seawater desalination in Béni Khiar, Nabeul
(2003), and a solar multiple condensation evaporation cycle (SMCEC) system of 12–30 l h−1 for brackish water desalination in the
University of Sfax, Sfax [7].
Finally, a solar MED plant of small scale has been recently installed in Paphos, Cyprus, to cover the water needs of a public
swimming pool (see Figures 9–11).
The one-effect MED unit desalinates seawater and has a capacity of 1 m3 day−1. The solar thermal plant consists of 110 m2
high-efficiency selective flat-plate collectors. The MED plant operates at a temperature around 75 °C. The plant was developed
within the ADIRA project [8].

Heat
exchanger

Solar
collectors
(100 m2)

Condenser

Evaporator


Thermal
storage
tank
3
(5m )

Distillate
Posttreatment
Potable water

Seawater

Saline water

Figure 9 Schematic diagram of the solar MED plant.


536

Applications

Figure 10 The MED seawater desalination plant, Paphos, Cyprus.

Figure 11 The solar collector area, Paphos, Cyprus.

3.16.3 Photovoltaics-Driven Desalination Systems
PVs are specially designed semiconductor devices that convert sunlight directly into electricity. They are modular devices having
long life and characterized by low maintenance requirements. The basic component of a PV system is the solar cell. Groups of
cells are mounted on a glass plate and wired in series to form a PV module. Groups of modules electrically connected together
form a PV array (Figure 12). The nominal voltage and current of an array depend on the number of modules connected in series

and in parallel. PV arrays can be mounted on fixed or on sun-tracking structures to maximize the incident solar radiation on the
solar cell surface. The power production capacity of a PV array is expressed in watt peak (Wp) units. A solar cell is said to be of
1 Wp power if it produces 1 W of electric power when exposed to ‘peak’ solar irradiance (1000 W m−2) at a solar cell temperature
of 25 °C [9].

Figure 12 The 4 kWp photovoltaic system, CRES, Greece.


Solar Desalination

537

There are various types of PV cells. The main ones are monocrystalline, polycrystalline, amorphous silicon, and other thin films.
PVs can be used directly with the load, such as in water pumping and grid connected or stand-alone.
The energy production unit consists of a number of PV modules, which convert solar into direct current (DC) electricity. The
most suitable desalination processes for this combination should use electricity in some form. Therefore, reverse osmosis (RO) and
electrodialysis (ED) appear as the most suitable choices for coupling with PV systems.
RO is a membrane separation process in which the water from a pressurized saline solution is separated from the solutes (the
dissolved material) by making it flow through a membrane. The amount of desalinated water that can be obtained (recovery ratio)
ranges between 30% and 75% of the volume of the input water, depending on the initial water quality, the quality of the product
needed, and the technology and membranes involved.
No heating or phase change is necessary for this separation. The major energy required for desalting is for pressurizing the
feedwater (the saline feedwater is pumped into a closed vessel where it is pressurized against the membrane) (see Figure 13).
Theoretically, the only energy requirement is to pump the feedwater at a pressure above the osmotic pressure. In practice,
higher pressures must be used, typically 14–25 bar for brackish water and from 55–80 bar for seawater desalination, in order to
have a sufficient amount of water pass through a unit area of membrane. Figure 13 shows an illustration of the feed being
pressurized by a high-pressure pump and which is made to flow across the membrane surface. Part of this feed passes through
the membrane, where most of the dissolved solids are removed. The remainder, together with the remaining salts, is rejected at
high pressure. In large plants, it is economically viable to recover the rejected brine energy with a suitable brine turbine [3].
Such systems are called energy recovery devices. The fraction of power, recovered by the power recovery device, depends on the

type and efficiency of the power recovery equipment used. In general, two recent developments have helped to reduce the
operating cost of RO plants in the past decade: the use of energy recovery devices and the development of more efficient
membranes (operational at lower pressures). The main advantages of RO process are the modularity/compactness and the
sufficient performance and reliability in all scales.
ED is an electrochemical process and a low-cost method for the desalination of brackish water. Due to the dependency of the
energy consumption on the feedwater salt concentration, the ED process is not economically attractive for the desalination of
seawater.
In the ED process, ions are transported through a membrane by an electric field applied across the membrane. The process
utilizes a DC electric field to remove salt ions in the brackish water [3]. Saline feedwater contains dissolved salts separated into
positively charged sodium and negatively charged chlorine ions. These ions will move toward an oppositely charged electrode
immersed in the solution, that is, positive ions (cations) will move toward the negative electrode (cathode) and negative ions
(anions) toward the positive electrode (anode). If special membranes, alternatively cation-permeable and anion-permeable,
separate the electrodes, the gap between these membranes will be depleted of salts [3]. In an actual process, a large number of
alternating cation and anion membranes are stacked together, separated by plastic flow spacers that allow the passage of water
(Figure 14). The streams through alternating flow-spacers are a sequence of diluted and concentrated water which flow parallel to
each other. To prevent scaling, inverters are used to reverse the polarity of the electric field in about every 20 min. As the energy
requirements of the system are proportional to the water’s salinity, ED is economically attractive for low concentration brackish
water with TDS equal to or less than 3500 ppm.
A typical stand-alone system consists of the PV modules, the charge controller(s), the battery bank, and the inverters(s). The
main advantages in the coupling of PVs with desalination units are the ability to develop small-size desalination plants, the limited
maintenance cost of PVs, as well as easy transportation and installation.

High pressure
pump

RO plant

Steriliser
Cartridge
filter


Seawater

Antiscalant

Dual
media
filter

Acid

Membranes
modules

Decarbonator

Pretreatment
Brine to
waste
Energy recovery
turbine (if fitted)
Figure 13 RO schematic [2].

Posttreatment

Lime
Product


538


Applications

Feed in
Concentrate in
Electrode feed

Top end plate

Electrode waste

(−) Cathode
Cation transfer membrane
Demineralized flow spacer
Anlon transfer membrane
Concentrate flow spacer

(+) Anode
Electrode feed

Bottom end plate
Electrode waste
Product
Concentrate out

Figure 14 ED stack assembly. Adapted from Watson IC, Morin OJ, Jr., and Henthorne L (2003) Desalting Handbook for Planners, 3rd edn. USA: U.S.
Department of the Interior Bureau of Reclamation.

RO usually uses alternating current (AC) for the pumps, which means that DC/AC inverters have to be used. Energy storage is again a
matter of concern, and batteries are used for PV output power smoothing or for sustaining system operation when sufficient solar

energy is not available. The typical PV–RO applications are of stand-alone type, and there exist some interesting examples. Table 3
presents the data on the PV–RO plants that have been installed within the last two decades for seawater and brackish water desalination.
Several plants have been built during the 1980s. A brackish water desalination application was installed in 1982 in Perth,
Australia. The plant consisted of 1.2 kWp PV to drive a 0.1 m3 h−1 RO unit. Another such plant was installed in 1984 in Vancouver,
Canada: a seawater RO unit of 1 m3 day−1 product water capacity with a 4.8 kWp PV system. In 1984, a 11.2 kWp PV to drive a
5.7 m3 day−1 seawater RO unit was installed in Doha, Qatar.
Another RO plant, set up in 1986, is located at El Hamrawein, at the edge of Red Sea. The PV array is rated at 19.84 kWp, delivering
voltage of 104 V for the pumps as main consumption plus a secondary array rated 0.64 kWp at 24 V for instruments and control. The
battery storage unit has a capacity of 208 kWh and is designed for 3 days of autonomy. The RO plant has a capacity of 10 m3 h−1,
operating at a pressure of 13 bar. The feedwater is brackish water having a salinity of 4400 mg l−1 TDS. The unit energy consumption is
0.89 kWh m−3.
During 1990–2000, with technical improvements in both technologies, bigger RES desalination plants were installed to cater to
water needs [11]. For RO, the development of efficient energy recovery devices and the operation of the membranes at lower
pressures significantly reduced the energy requirements and obviously the power requirements of the RES power supply plants. On
the other hand, the cost of PV fell dramatically.
During this period, a lot of work was done by the Instituto Tecnológico de Canarias (ITC) in Spain [12]. Several combinations of
RES desalination systems such as photovoltaic–electrodialysis reversal (PV–EDR), wind–mechanical vapor compression (MVC),
and PV–RO were installed and examined; one of them is presented in this chapter.
Furthermore, in order to reduce the cost and maintenance requirements, the direct coupling of PV to RO unit is examined. In most
cases, the power variability from the solar source reduces the lifetime of the membranes. The Centre for Renewable Energy Systems
Technology, CREST, UK, installed a 1.54 kWp PV-powered seawater RO unit without batteries. The system operates at variable flow,
enabling it to make efficient use of the naturally varying solar resource, without the need of batteries. The same RO unit has also been
coupled and tested with a wind turbine without any battery bank. Frequent replacement of the RO membranes is mentioned.
The electricity from PVs for desalination applications can be used for electromechanical devices such as pumps, or in a DC device
for ED. ED uses DC for the electrodes at the cell stack, and hence it can use the energy supply from the PV panels without major


Solar Desalination

Table 3


539

PV–RO applications for seawater (SW) and brackish water (BW) desalination

Plant location

Water type

RO capacity

PV installed (Power)

Commissioning year

El Hamrawein, Egypt
Hassi-Khebi, Algeria
University of Almeria, Spain
Lampedusa Island, Italy
Lipari Island, Italy
Sadous Riyadh Region, Saudi Arabia
St. Lucie, Florida
Gillen Bore, Australia
Maagan Michael, Israel

BW
BW
BW
SW
SW

BW
SW
BW
BW

10 m3 h−1
0.95 m3 h−1
2.5 m3 h−1
3 + 2 m3 h−1
2 m3 h−1
600 l h−1
0.6 m3 day−1
1.2 m3 day−1
0.4 m3 h−1

1986
1988
1988
1990
1991
1994
1995
1996
1997

Pozo Izquierdo, Gran Canaria Island, DESSOL
Sadous Village, Saudi Arabia
CREST, UK
CRES, Greece


SW
BW
SW
SW

3–4 m3 day−1
600 l h−1
3 m3 day−1
130 l h−1

White Cliffs, New South Wales, Australia
Aqaba, Jordan
Ksar Ghilane, Tunisia
ADIRA project
Benhssaine Morocco, ADIRA Project (Figure 15) [10]
Msaim, Morocco, ADIRA project
(Figures 16 and 17) [10]

BW
BW
BW

500 l day−1
3.4 m3 h−1
2.1 m3 h−1

∼20 kWp
2.6 kWp
23.5 kWp
100 kWp

63 kWp
10.89 kWp
2.7 kWp
4.16 kWp
3.5 kWp PV,
0.6 kW W/T
+3 kW diesel
4.8 kWp
10.08 kWp
2.4 kWp
4 kWp PV,
1 kW W/T
150 Wp
16.8 kWp
10.5 kWp

BW
BW

1 m3 h−1
1 m3 h−1

4.8 kWp
3.9 kWp

2006–07
2006–07

1998–2000
2001

2001–02
2002
2003
2004
2006

Figure 15 The PV–RO unit in Benhssaine, Morroco. Adapted from Mokhlisse A (2008) A new experience of fresh water supply in two rural villages in
Morocco, ADIRA Workshop on Desalination Powered by Renewable Energy, Athens.

modifications. Yet, some kind of power conditioning is required in this case as well for a typical photovoltaic–electrodialysis
(PV–ED) system design. Batteries are used for PV output power smoothing or for sustaining system operation when sufficient solar
energy is not available. Furthermore, energy storage is again a matter of concern.
ED is an electrochemical process and a low-cost method for the desalination of brackish water. Due to the dependency of the
energy consumption on the feedwater salt concentration, the ED process is not economically attractive for the desalination of
seawater.
A small number of PV–ED plants have been installed exclusively for brackish water desalination. In the city of Tanot, in Thar
Desert, Rajasthan, India, a small plant was commissioned in 1986, featuring a PV system capable of providing 450 Wp in 42 cell
pairs. The ED unit includes three stages, producing 1 m3 day−1 water from brackish water of 5000 mg l−1 TDS. The unit energy
consumption is 1 kWh kg−1 salt removed.
Another application for seawater desalination has been reported from Japan. A 25 kWp PV system was used to drive a
10 m3 day−1 ED plant. The system is located at Oshima Island, Nagasaki, and has been operating since 1986. The quality of the
water produced is reported to be below 400 ppm TDS.
In 1988, another ED plant was developed in Fukue City in Japan. The ED units desalinate low-concentration brackish water of
700 ppm TDS, resulting in very low energy demand. A 65 kWp PV array supplies enough energy to produce an average of 200 m3 day−1


540

Applications


Figure 16 The PV–RO unit in Msaim, Morocco. Adapted from Mokhlisse A (2008) A new experience of fresh water supply in two rural villages in
Morocco, ADIRA Workshop on Desalination Powered by Renewable Energy, Athens.

Figure 17 The 1 m3 h−1 RO unit, Morocco. Adapted from Mokhlisse A (2008) A new experience of fresh water supply in two rural villages in Morocco, ADIRA
Workshop on Desalination Powered by Renewable Energy, Athens.

Figure 18 PV–EDR plant, Spencer Valley, New Mexico.

of potable water. Battery storage of 1200 Ah provides constant power. A 30 kVA inverter supplies AC power to the pumps, while the
electrodes are powered by a DC bus. Due to natural fluctuations in feedwater salinity and temperature, the water production rate and
energy requirements fluctuate between 130 and 370 m3 day−1 and 0.6 and 1.0 kWh m−3, respectively [13].
A small experimental unit was reported in Spencer Valley, New Mexico, by the Bureau of Reclamation, USA. Two separate PV
arrays are used: two-tracking flat arrays, 1000 Wp power, 120 V, with DC/AC inverters for pumps, plus three fixed arrays, 2.3 kWp,
50 V for EDR supply (see Figures 18 and 19). The EDR design calls for 2.8 m3 day−1 product water from a feed around 1000 mg l−1
TDS. Units of power consumption was 0.82 kWh m−3 and the reported cost was 11 065 ECU (US$ 13 500).


Solar Desalination

2 PV arrays with passive
trackers
(1.0 kw)

−1

32 l

541

m


120 Vdc

Feed pump

Three-phase
AC voltage

DC/AC
inverter

Submersible
well
pump

Brackish water
storage tank

Ion
exchange
unit

24 Vdc
50 Vdc

3 Fixed PV arrays,
(2.3 kw)

−1


3.8 l

m

50 Vdc
Voltage
controller

Electrodialysis stack

1.9 l

−1

−1

1.9 l

m

Battery
storage

m

Evaporation
pond
Potable water

Figure 19 Schematic of installation at the Spencer Valley. Source: NREL.


3.16.4 Solar Stills
Solar distillation is a process in which the energy of the sun is directly used to evaporate freshwater from sea or brackish water. The
process has been used for many years, usually for small-scale applications.
Solar distillation systems are classified into two groups [14] in terms of energy supply: (1) passive or conventional and (2) active
solar stills. The passive solar stills use solar energy as the only source of thermal energy. In active solar stills, extra thermal energy from a
solar collector or any available waste heat is directed to the solar still for faster evaporation. Humidification–dehumidification (HD)
process, described in the next section, is an active solar technique.
Solar stills are the simplest devices that are used to obtain freshwater using solar energy as the sole energy supply. The basic
principle of solar water distillation is simple as distillation replicates the way nature makes rain. In such systems, energy is required
only to power the circulating water pumps.
A solar still consists of a shallow basin with a transparent cover designed to act as a condenser (Figures 20 and 21). Water in
the basin is heated by the sun to produce vapor. The vapor produced by the evaporation of seawater is condensed on the cool
surface of the roofs of the stills, and the condensate is collected as the product water. Well-designed units can produce around
2.5 l m−2 day−1 with a thermal efficiency of 50%. Parameters that affect the efficiency of solar stills and the amount of water

Glass
n

tio

sa

n
de

on

C


Evaporation

Seawater
Fresh water

Seawater basin

Figure 20 A simplified schematic diagram of a solar still. Source: CRES. Adapted from Karen TE (1997) Desalination of Village Scale, Renewable Energy
Powered Desalination, NREL, USA [13].


542

Applications

Figure 21 Solar distillation plant in Kastelorizo, Greece.

desalinated are the solar potential of the specific site, the area of the still, the ambient air temperature, the feedwater temperature,
insulation of the still, slope of the cover, and the depth of the water in the still.
The main advantage of the solar stills is the simplicity of their construction from locally available and low-cost materials.
Maintenance of the stills involves periodic cleaning of debris and dust from the cover and checking for leakages, or removal of salt
deposits.
The main development of this technology was in the 1960s and 1970s, concerning the improvement of the solar stills efficiency
and the reduction of construction cost. Over this period, many distillation plants were constructed around the world. Passive solar
distillation is usually suitable for small-scale applications. In remote areas where low-cost land is available and solar radiation is
high, passive solar distillation can be viable.
A great number of installations have been constructed around the world in Australia, Haiti, Spain, India, United States, Mexico,
Greece, etc. However, most of them are not in operation today. Several of the old plants were affected by storm damage, structure
failure, sealing problems, and leakage.
One of the biggest plants was installed in 1967 in Patmos Island in Greece. The solar still had an area of 8640 m2 and a product

water capacity of 26 m3 day−1 from seawater.
Until today, a significant number of variations aiming on the improvement of solar stills have been examined and several new
plants have been installed.
Many studies discuss in detail the effect of different factors, such as solar input, ambient temperature, water depth, and wind
velocity, on the performance of the still. For most cases, even under optimized operating conditions, the reported efficiency of the
single-basin solar still was in the range of 30–45%, with <5 l m−2 daily freshwater production. This low efficiency is mainly due to
the complete loss of latent heat of condensation of water vapor on the glass cover of the solar still [15].
Efforts have been focused for some time on recovering the latent heat of condensation. Preheating of the feedwater by passing it
over the glass cover allowed only partial utilization of the latent heat, resulting in a limited increase in still production. More
significant improvements in solar still design were achieved through the multiple use of the latent heat of condensation within the
still. In a multiple-effect unit consisting of several cells, heat is supplied only to the first cell (effect). Water is evaporated in
the second effect as it trickles over a metallic surface heated by the condensation of the vapor from the first effect. This allows the
utilization of the latent heat of condensation at different levels. In the literature, an efficiency of 57% for a double-effect still has
been reported while other reports indicate a production ranging up to 13 l m−2 day−1. This increased productivity was achieved by
using mirrors as solar reflectors to increase the solar energy received so that the operating temperature could reach 85 °C in the
multieffect still. However, if the reflector area is included in the calculation, productivity is still not better than 6 l m−2day−1.
Multieffect solar stills may be used for efficient production of desalinated water but only with small capacities because the
condenser is an integral part of the still. The low heat and mass transfer coefficients in this type of still require operation at
relatively high temperatures, and thus the use of large, expensive, metallic surfaces for evaporation and condensation [15].

3.16.5 Solar Humidification–Dehumidification
A recent modification of the solar stills is the so-called humidification–dehumidification (HD) process. The concept of HD is the
collection of potable water by first humidifying an air current by contact with warm seawater and then dehumidifying by cooling. By
this method, the defect (operation of distillation plants at high temperatures, around 115 °C, generates scaling problems) of high
temperature used in other distillation processes is avoided and the desalination performance is an improvement over what is
realized in the conventional solar stills [16].
More analytically, in the HD process, a dry airstream is enriched with vapor in a humidification unit and then the vapor is
recondensed in a dehumidification unit where freshwater is collected.



Solar Desalination

543

These processes seem to be highly promising, first for the high efficiency, and second, for the possibility of using renewable
energies to power them, such as the use of direct solar energy for desalting saline water.
Several aspects of HD have been examined in order to improve their efficiency and the amount of produced water [17, 18]. The
multieffect humidification–dehumidification (MEH) process is the most efficient HD process, and extensive research has been
carried out by different institutions, especially in Germany, to develop an efficient means of utilizing solar energy for water
desalination [19]. Small plants based on MEH were constructed and tested in different countries. The research proves that the
solar HD process has much room for improvement.

3.16.6 Solar Membrane Distillation
Membrane distillation (MD) is the youngest of the membrane separation processes that is being used to desalinate water. It was
introduced commercially on small scale in the 1980s. The process combines both the use of distillation and membranes. In the
process, saline water is warmed to enhance vapor production, and this vapor is exposed to a hydrophobic membrane that allows the
passage of vapor, but not water (see Figures 22 and 23). After the vapor passes through the membrane, it is condensed on a cooler
surface to produce freshwater. In the liquid form, the freshwater cannot pass back through the membrane, so it is trapped and
collected as the output of the plant.
Compared to the more commercially successful processes, MD requires more space and may use a considerably large pumping
energy per unit of production.
The main advantages of MD are in its simplicity and the requirement for only small temperature differentials to operate the
process. Like any distillation process, its energy requirement and product water quality are independent of the feedwater quality.
Because it operates at lower temperatures (50–80 °C), the feedwater can be heated by solar thermal collectors, while the electric
energy required for pumping could be provided by PVs.

Heat
exchanger
Condenser


Feedwater

Heat
source

Distillate
Evaporator

Brine

Membrane

Figure 22 Membrane distillation principle. Adapted from Fath H, Elsherbiny SM, Alaa A, et al. (2008) PV and thermally driven small-scale, stand-alone
solar desalination systems with very low maintenance needs. Desalination 225: 58–69.

Condenser canal
Evaporator canal
Condensation foil
4
Distillate canal
5
1

3

2
Hydrophotic membrane

1 Condenser inlet
2 Condenser outlet

3 Evaporator inlet
4 Evaporator (brine) outlet
5 Distlilate outlet
Figure 23 Spiral wound MD module. Adapted from Fath H, Elsherbiny SM, Alaa A, et al. (2008) PV and thermally driven small-scale, stand-alone solar
desalination systems with very low maintenance needs. Desalination 225: 58–69.


544

Applications

Typical characteristics of the process are as follows:
•
•
•
•
•
•
•
•

Efficient and compact spiral-wound MD modules
Recovery of the heat of condensation integrated in the module design
No feedwater chemical pretreatment required
Low system pressure
Insensitivity to dry-running and fouling
Negligible scaling problems due to process temperatures around 80 °C
Production of pure distillate water
Modularity.


Many pilot plants have been built and are being operated with encouraging results. During 1990s, at least two solar MD demonstration
projects were built [13]. A 0.05 m3 day−1 system using 3 m2 solar collectors was built at the University of New South Wales in Australia.
A calculated efficiency of 17 l day−1 m−2 of collector area is mentioned. In 1994, a solar MD plant was developed in Water Re-use
Promotion Center in Tokyo, Japan. The system produces 40 l h−1. Automatic controls start up the desalination system whenever
sufficient sunlight is present to provide hot water from the solar collectors and electricity for pumping from the PV panels.
Furthermore, in the last decade, several solar MD systems have been installed and examined, mainly within EU projects
(SODESA, MEMDIS, and SMADES). The Fraunhofer Institut, Germany, has done a lot of work on the development of MD modules
and on the provision of compact solar MD units. Some of the characteristics of the MD modules developed by Fraunhofer are as
follows:
•
•
•
•
•

Distillate output: 10–30 l h−1 (80 °C evaporator inlet, 300 l h−1 feed flow)
Operation temperature range: 50–85 °C
Thermal energy demand: 90–200 kWh m−3
High water quality of the produced water: 5–50 μS cm−1
No pretreatment of the feedwater required.

The first compact solar MD system by the Fraunhofer Institut was developed in 2004. The system consists of 6 m2 2AR-collectors
(AR, antireflective) having an operating collector temperature up to 85 °C, 80 Wp PV module, and one MD module with a daily
water capacity of 80–150 l.
In the period 2003–06, several other solar MD plants were installed in Jordan, Gran Canaria, Egypt, and Morocco by Fraunhofer
within EU projects for the desalination of seawater and tap and brackish water. The systems integrate solar, thermal, and PV energy.
The desalination energy is supplied entirely by solar thermal collectors and the electrical auxiliary energy (feedwater pumping) is
supplied by a PV system.
Figures 24 and 25 present the stand-alone solar MD system installed at Alexandria University, Alexandria, Egypt, in June 2005
within SMADES project [20].

The solar collector consists of three similar rectangular 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. The electric conductivities for feed

Figure 24 MD compact system (SMADES), Alexandria, Egypt. Adapted from Fath H, Elsherbiny SM, Alaa A, et al. (2008) PV and thermally driven
small-scale, stand-alone solar desalination systems with very low maintenance needs. Desalination 225: 58–69.


Solar Desalination

DFM
FFM
FP
RP
TS
V

S1
Degaser
S2

545

Distillate flow meter
Feed flow meter
Feed Pump
Refiling Pump
Thermostat
Valve

S3

Pyranometer
Brine

Photovoltaic cell

Feed
Raw water

Solar collector

Overflow

V3

60°C
TS1

Max
level

V4

Pressure gage

Degaser
V2

CondF

P


DFM

FP

MD module

Min
level

Filter V1
FFM

45°C
TS2
RP

CondP
Cond

Evap

Figure 25 Flow diagram of MD compact system, Alexandria (SMADES). Adapted from Fath H, Elsherbiny SM, Alaa A, et al. (2008) PV and thermally
driven small-scale, stand-alone solar desalination systems with very low maintenance needs. Desalination 225: 58–69.

Distillate
Solar
irradiation

Heater

exchanger

Brine
disposal

Storage
tank
MD modules

Collector field
Feed pump
Control unit
Manual
switches

PV
AC
Battery
Battery
charger

DC

PV
PV array

Figure 26 Flow sheet of the MD unit in Aqaba, Jordan. Adapted from Banat F, Jwaied N, Rommel M et al. (2007) Performance evaluation of the ‘large
SMADES’ autonomous desalination solar-driven membrane distillation plant in Aqaba, Jordan. Desalination 217: 17–28 [21].

and distillate water are 526 and 3 μS cm−1, respectively. The daily production is 64 l day−1 (11.2 l m−2day−1), while the accumulated

solar energy is 41.6 kWh day−1 (7.25 kWh m−2day−1). The process works at a lower temperature of 60–80 °C [21].
At the system installed in the Marine Science Station in Aqaba, Jordan, within SMADES, the energy for the desalination process
is also supplied entirely by solar thermal collectors in the form of heat at temperatures in the range of 60–80 °C (see Figures 26
and 27). The desalination units are improved MD modules with internal heat recovery function. The electrical auxiliary energy,


546

Applications

Figure 27 View of the main components of the large SMADES system in Aqaba, Jordan. Adapted from Banat F, Jwaied N, Rommel M et al. (2007) Performance
evaluation of the ‘large SMADES’ autonomous desalination solar-driven membrane distillation plant in Aqaba, Jordan. Desalination 217: 17–28 [21].

Figure 28 Collector field of the SPMD system in Aqaba. Adapted from Banat F, Jwaied N, Rommel M et al. (2007) Performance evaluation of the ‘large
SMADES’ autonomous desalination solar-driven membrane distillation plant in Aqaba, Jordan. Desalination 217: 17–28 [21].

which is required to drive the pumps and valves for the automatically operated systems, is supplied by PV panels. The feedwater is
real seawater from the Red Sea. The seawater desalination system’s primary power supply is generated by PV panels. The system
supplies and stores energy according to demand. The unit is operated with untreated seawater.
The collector area is 72 m2, the hydraulic loop of the collector field comprises a solar heat storage tank of 3 m3 and the collector
loop is separated from the seawater loop of the MD desalination modules. The effective membrane area in each MD module is
10 m2. The system consists of four MD modules operated in parallel.
The system provides an output in the range of 2–11 l day−1 m−2 with specific energy consumption in the range of
200–300 kWh m−3. The distillate is water of low TDS with conductivity in the range of 20–250 μS cm−1.
Figure 28 shows the solar collectors and the PV panels on the top of the housing building, three separated collector racks each of
12 collectors and four PV modules on top.
Automatic controls start up the desalination unit whenever sufficient sunlight is present to provide hot water and electricity for
pumping from the solar collectors and PV panels.

3.16.7 Technologies Selection Guidelines

Autonomous systems are developed in remote areas where no electricity grid is available. Due to the dispersed population that characterizes
the South Mediterranean and Gulf areas, relatively small systems are used to cover the potable water needs in remote villages. The main
desirable features for such systems are low-cost, low-maintenance requirements, simplicity of operation, as well as high reliability [9].
The decision to use an autonomous solar desalination system, as well as the selection of the most appropriate combination,
depends on several parameters and is mostly site specific.
For solar desalination systems, the most important parameter concerning the power supply system is sufficient solar potential in
the selected region. Other factors that should be taken into consideration and that affect the final unit cost of water and the life of the
plant are the availability of land, the land cost, and the availability of technical staff and local market.


Solar Desalination

547

Regarding desalination, a number of basic parameters should be investigated. The first is the evaluation of the overall water
resources. This should be done both in terms of quality and quantity (for brackish water resources). In inland sites, brackish water
may be the only option. In these cases, care should be taken on the brine disposal output and rejecting them without polluting the
local water resources.
On a coastal site, seawater is normally available. Specific guidelines are followed, in this case, during disposal of the brine
disposal into the sea.
Distillation processes are used mainly for seawater desalination, while membrane processes are used for a wide range of salinity
from brackish water to seawater.
Application of the ED process is preferred for brackish water desalination.
The determination of the quality of the water produced depends on the purpose of the plant, for instance, whether the plant is used
for potable, agricultural, or industrial needs. Distillation processes are used for the production of distillate water, while membrane
processes are used for the production of potable water. However, with an extra treatment of the distillate water, it is possible to produce
potable water as well; with the use of a dual-purpose RO plant, it is possible to produce water of very low conductivity.
From the energy point of view, the main supply to the desalination plant is a large thermal input. Like all thermal processes,
distillation demands a high-energy input (due to the energy required for change of phase). Besides, some auxiliary electricity is also
required for pumping (electricity could be produced via PVs).

On the other hand, the solar thermal systems are so much dependent on the radiation (day/night) that some heat storage is
always required. The accumulator may thus become the main subsystem of the plant, and the adopted control strategies become of
particular importance.
For MSF evaporators, the PR increases with temperature so that high temperatures (up to 120 °C) are preferred. This in turn
increases the risks of scaling and corrosion. MED evaporators, nowadays, operate at lower temperatures (around 70 °C) and those
hazards are reduced.
Finally, the control of such evaporators must be very accurate, particularly the flash equilibration in MSF. The system is unstable
in small sizes. This leads to the use of medium and large size evaporators (thousands of cubic meters per day capacity), which do not
quite fit with the sizes and capacities usually applied with renewable energies, unless a huge solar field could be built, which in turn
implies large ground surfaces. Therefore, the combination, solar–thermal distillation seems best suited for medium and high
production capacities.
On the other hand, RO is available in a large range of sizes and water salinities. The use of energy recovery devices can
considerably reduce the energy requirements. The stability of the power supply is important for the membrane’s life.
ED is available also in small sizes; however, this process is mainly used for the desalination of brackish water due to the
dependency of the energy consumption on the feedwater concentration.
Simplicity of operation and maintenance, as well as system automation, is a major concern.
Commercial maturity in solar desalination units is not easy to be found; however, a sufficient number of plants exist that can be
validated by their examination.

3.16.8 Solar Desalination Applications
3.16.8.1

Solar Thermal MES Plant for Seawater Desalination, Abu Dhabi, UAE

The Abu Dhabi solar desalination plant is situated at the Umm Al Nar Power and Desalination Station about 20 miles east of Abu
Dhabi city. The plant was commissioned in 1984. The plant was designed as a demonstration unit aimed at evaluating the technical
and economic feasibility of using this type of technology to provide the remote coastal communities in UAE with freshwater. The
seawater has a salinity of 55 000 ppm TDS.
A bank of evacuated-tube and flat-plate collectors with a total absorber area of 1862 m2 is used to provide the thermal energy
required by a multiple-effect stack-type evaporator having a rated capacity of 80–120 m3 day−1. The maximum daily distillate

production corresponding to the optimum operating parameters was found to be 120 m3 day−1 (full load) for 8 months of the year,
that is, February, March, April, May, June, August, September, and October. The production level during the other months is
somewhat lower, reaching its lowest value of 90 m3 day−1 in December (see Figure 29) [22].
In order to ensure that the evaporator can run 24 h per day during sunny days, a thermally stratified heat accumulator is
incorporated in the design to provide the thermal energy required during the nighttime.
The electrical energy required by the different pumps is provided from the main grid.
The bank of evacuated-tube solar collectors, whose orientation with respect to the sun has been optimized to collect the
maximum amount of solar radiation, is used to heat the collector fluid (water) to a maximum temperature of about 95 °C. The
effective collector–absorber area of the bank is 1862 m2. The collector bank, which is divided into six blocks, consists of 1064
collector units, each having an effective absorber area of 1.75 m2. Twenty-eight collector units are combined to form a single group of
collectors with its own support structure. Thirty-eight such groups are arranged in U-shape to form the whole collector bank. All the
groups are connected and each is provided with two isolating valves – at the group inlet and exit headers, a drain valve and an air
vent. The collector efficiency ranges from 0.5 to 0.7, whereas the heat efficiency ranges from 41.4% in winter to 47.4% in summer.
A bypass line is installed between the heat collector bank and the heat accumulator tanks to allow the heat collecting water to
recirculate through the collector bank if the water temperature at the discharge of the collector bank is below a set point, which can


548

Applications

Distillate production m−1 day

Monthly average daily distillate
production of the solar plant
140
120
100
80
60

40
20
0

Jan

Feb Mar

Apr May Jun

Jul

Aug Sep Oct Nov Dec

Month (1985)

Figure 29 Monthly average water production [22].

Heat accumulators

Solar collector field

Heating water
pump

MES seawater evaporator
Effect #1
Effect #2
Effect #3


Intel valve M
Tank #1

Tank #2

Preheaters

Tank #3

Bypass valve M
Effect #18

Feedwater pump

Condenser
Heat collecting pump
Seawater
Distillate

Seawater pump

Brine blowdown

Figure 30 Solar thermal multistack evaporator plant in Abu Dhabi [23].

be adjusted manually. The heat collecting water from the collector bank is introduced at the top of the heat accumulator system
(Figure 30).
The heat accumulator system is designed to provide thermal energy to the evaporator during its 24 h daily operation – autonomy
period about 16 h. During extended overcast periods or hazy days, when sandstorms prevail, the plant may shut down due to
insufficient thermal energy storage. The heat accumulator consists of three heavily insulated carbon steel tanks filled with hot water,

and having a total capacity of 300 m3. The evaporator is of the horizontal-tube, thin-film type and is designed for a rated distillate
output of 120 m3 day−1 from seawater. The multieffect stack-type evaporator (MES) has 18 effects stacked one on top of the other.
The PR is 12.4, where the heating fluid transfers from 95 to 99 °C.
The plant has been in operation for over 16 years. After 16 years, the performance of the collector field and evaporator
subsystems has not declined to any appreciable degree.
No problem has so far been encountered with any of the tube bundles; distillate conductivity has been in the range of
10–20 mS cm−1. This is an indication that a correct choice of tube and tube sheet materials has been made by the manufacturer.
This desalination technology has proved its reliability and flexibility for variable load operation and is worth serious consideration
as a provider of freshwater in remote communities.
Table 4 provides the technical characteristics of the system [9].
Capital and operating costs, as well as the cost of the water produced by the Abu Dhabi solar desalination plant, have been
calculated to assess the economic performance of the system. The capital cost figures were determined from actual prices provided
by the plant manufacturer (Sasakura and Sanyo of Japan). The calculations of the water cost are based on the following economic
assumptions [23]:
• Evaporator lifetime: 20 years
• Heat accumulator lifetime: 20 years
• Solar collector’s lifetime: 20 years


Solar Desalination

Table 4

549

Technical characteristics of the Abu Dhabi plant

Site characteristics
Annual mean solar radiation
Annual mean ambient temperature

Average wind speed

5000 kcal (m2 day)
30 °C
5 m s−1

MED plant
Product water capacity
Design raw water salinity
Product water salinity
Performance ratio
Number of effects
Number of preheaters
Evaporator
Design seawater temperature
Heating water temperature
Brine blowdown temperature
Brine blowdown salinity
Specific heat consumption
Hours of operation

80–120 m3 day−1
55 000 ppm TDS
50 ppm TDS
12.4
18
17
Horizontal tube, thin-film type
35 °C
95–99 °C

43 °C
1.4 Times the raw water salinity
43.8 kcal kg−1 distillate
24 h day−1

Solar collector field
Solar collector type
Total absorber area
Absorber area per single panel
Number of collector panels
System type (stand-alone grid)
Selective coating
Absorber area
Circulation flow rate
Maximum operating pressure

Evacuated tube, flat plate
1862 m2
1.75 m2
1064
Stand-alone + diesel generator
Absorptivity α ≥ 0.91,Emissivity ε ≤ 0.12
1.75 m2
700–1800 l h−1
6 bar

Heat accumulator
Type
Total capacity
Number of tanks

Accumulator fluid
Tank pressure
Insulation type
Diesel generator set

Thermally stratified, sensible heat
300 m3
Three cylindrical tanks in series
Water
Atmospheric pressure
10 cm fiber glass
50 KVA

• Interest rate: 8%
• Plant availability: 85% (7446 h yr−1).
The capital costs are as follows:
• Evaporator: $299 180
• Heat accumulator: $91 304
• Solar collectors: $1 098 580.
Local fabrication and installation costs were estimated at 30% of the sum of the above three capital cost components, which results
in an amount of $446 719. Marine transportation costs amounted to $102 000. Thus, the installed capital cost of the plant was
$2 037 783.
The operation and maintenance (O&M) costs consist of the following main cost components: cost of chemicals (antiscalant for
seawater treatment, corrosion inhibitor for collector fluid, and seawater disinfectant to inhibit bacterial growth); spare parts costs;
cost of operation and maintenance personnel; and cost for electrical power consumption. Based on an average daily water
production of 100 m3 day−1, the unit water cost is estimated at around $7 m−3 of distillate.
The Abu Dhabi solar thermal MES plant has had a successful operation, proving that with an improved sizing, it is feasible to
produce water of sufficient quality at a reasonable cost.

3.16.8.2


Solar Thermal MED Plant for Seawater Desalination, Almeria, Spain

The Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain, and Deutsche Forschungsanstalt für
Luft und Raumfahrt (DLR), Germany, decided in 1987 to develop an advanced solar thermal desalination system, thus initiating the


550

Applications

so-called Solar Thermal Desalination (STD) project carried out at the PSA until 1994 [24]. The following two project phases were
scheduled and executed during this period aiming to achieve specific project objectives:
• Phase I: To study the reliability and technical feasibility of solar–thermal technology application to seawater desalination.
• Phase II: To develop an optimized solar desalination system by implementing specific improvements in the system initially
installed at the PSA, which could make it more competitive against conventional desalination systems.
Phase I was launched in 1988 and its evaluation was completed in 1990. During this phase, a solar desalination system was
implemented at the PSA. This desalination system was composed of
1. a 14-effect MED plant,
2. a solar parabolic-trough collector field, and
3. a thermocline thermal energy storage tank.
These subsystems were interconnected as shown in Figure 31. The system operates with synthetic oil that is heated as it
circulates through the solar collectors. The solar energy is thus converted into thermal energy in the form of sensible heat of
the oil, and is then stored in the thermal storage tank. Hot oil from the storage system provides the MED plant with the
required thermal energy. The desalination plant installed at PSA uses sprayed horizontal tube bundles for seawater
evaporation, which must be limited to around 70 °C to reduce scale formation. The MED plant is composed of 14 cells
or effects at successively decreasing temperatures and pressures from cell (1) to cell (14). Table 5 provides the technical data
of the system.
The seawater is preheated from cell to cell in the 13 preheaters. From cell (1), the seawater passes on from one cell to another by
gravity before being extracted from cell (14) by the brine pump. Part of the seawater used to cool the condenser is rejected and the

rest is used for the feedwater required to spray the cell (1) tube bundle. The fresh distilled water is extracted from the condenser by
means of a distilled water pump (Figure 32).
The plant can also be fed with steam at 16–26 bar. High-pressure steam is produced in the PSA’s electricity generation system
to drive a small power plant. A small fraction of this steam can be used to feed the desalination plant, where it is sent to
thermo-compressors, and mixed with steam produced in the fourteenth effect. This mixture is then ejected into the evaporator
of the first cell to restart the desalination process. In this case, the MED plant consumption is lower. A vacuum system,
composed of two ejectors (not shown in Figure 31) driven by seawater at 3 bar, is used to evacuate the air from the unit at

Solar
collectors

Low
pressure
boiler
70 °C
1

0.35
bar

Thermocompressors
2

High−pressure steam
(16−26 bar)

Heat
transfer
loop


Thermal
storage
tank
(Thermocline)

3

Electricity
generation
system
14

Feedwater
(8 m3 h–1)
Oil circuit
Seawater
Distillate
Brine

Final
condenser

Brine
3
(6 m h–1)
Vacuum

Figure 31 Schematic diagram of the solar MED system installed at PSA at Phase I of STD project.

Reject

3
(12 m h–1)
Distillate
(3 m3 h–1)

Seawater
(20 m3 h–1,25 °C)


Solar Desalination

Table 5

551

Technical characteristics of the Almeria plant

Nominal distillate production
Heat source energy consumption
Performance ratio (kilogram distillate/2300 KJ heat input)
Output salinity
Seawater flow
At 10 °C
At 25 °C
Feedwater flow
Brine reject
Number of cells
Vacuum system

3 m3 h−1

190 kW
>9
50 ppm TDS
8 m3 h−1
5 m3 h−1
8 m3 h−1
5 m3 h−1
14
Hydroejectors (seawater at 3 bar)

Figure 32 View of the solar MED plant at PSA/CIEMAT.

start-up and to compensate for the small amounts of air and gases released from feedwater and from small leaks through the
gaskets.
The most outstanding evaluation results obtained during Phase I were the following:
• High reliability of the system, as no major problem was observed in the coupling of the solar collector field with the MED plant.
• Low thermal inertia: it usually took 35 min to reach the nominal distillate production.
• Specific electricity consumption in the range of 3.3–5 kWh m−3 of distillate.
• The plant showed a PR (e.g., number of kilograms of distillate produced by 2300 kJ heat input) within the range of 9.4–10.4
when operating with low-pressure steam. PR increases up to the range of 12–14 if high-pressure steam is used to feed the
plant.
From the results obtained during Phase I, it was possible to identify potential relevant improvements that could be implemented in
the MED solar system to increase its efficiency and competitiveness.


552

Applications

This analysis concluded that

• the plant electrical demand could be reduced by replacing the initial hydroejector-based vacuum system with a steam ejector
system, and
• the plant thermal demand is 50% reduced by incorporating a double-effect absorption heat pump coupled to the MED
plant.
Since these improvements would considerably reduce the specific cost of distillate produced by the optimized solar MED
desalination system, it was decided that the Phase II of the STD project be carried out. A schematic diagram of the improved
desalination system in which an absorption heat pump was coupled to the MED plant has been shown in Figure 33. The heat pump
delivers around 200 kW of thermal energy at �65 °C to the MED plant. The desalination process in the plant evaporator body uses
only 90 kW of the 200 kW, while the remaining 110 kW is recovered by the heat pump evaporator at 35 °C and pumped to usable
temperature of 65 °C. For this, the heat pump needs 90 kW of thermal power at 180 °C. The energy consumption of the
desalination system was thus reduced from 200 to 90 kW.
The improvements implemented in the desalination system (i.e., absorption heat pump and steam-ejector-based vacuum
system) reduced the thermal energy consumption of the desalination system by 44%, that is, from 63 to 36 kWh m−3, and electricity
consumption by 12%, from 3.3 to 2.9 kWh m−3.
A new R&D European project, named AQUASOL, was initiated in 2002, trying to improve the existing system at PSA. AQUASOL
project objective was the development of a low-cost and more energy-efficient seawater desalination technology based on MED
process with zero brine discharge. Specific proposed technological developments (new design of CPC solar collectors and a new
prototype of absorption heat pump, hybridization with natural gas, and recovering of salt) were implemented to both improve the
energy efficiency of the process and for process economy. The expected result was an enhanced MED technology with market
possibilities and suitable to be applied in the Mediterranean area and similar locations around the world. If a fuel cost (i.e., natural
gas) of € 4.5 GJ−1 is considered, the needed cost of solar system (considering a solar contribution of 50% to the overall system) for
the achievement of the same economic competitiveness as conventional MED plant is equivalent to around € 150 m−2 of solar
collector.

Steam

Solar
collectors

Low

pressure
boiler
70 °C
0.35
bar

Vapor

Steam
generator
130 °C

Vapor

10
bar

Condensate

1

Vapor

2

7

Heat
transfer
loop


Steam
electron

Thermal
storage
tank
(Thermocline)

13

65 °C
0.25 bar
180 °C
10 bar

35 °C
0.067 bar

Double effect
absorption
heat pump

14

Feed water
3 –1
(8 m h )

Oil circuit

Seawater
Distillate
Brine
Brine
3 –1
(3 m h )

Figure 33 Improved solar MED system (Phase II of STD project).

Seawater Distilate
3 –1
3 –1
(8 m h ) (3 m h )


Solar Desalination

3.16.8.3

553

PV–RO Plant for Seawater Desalination, Lampedusa Island, Italy

The matching of PVs with RO has a large number of applications due to the modularity, efficiency, and simplicity of the
combination. The largest and most known stand-alone PV–RO seawater plant was installed in 1990 in Lampedusa Island in Italy
(Figure 34). The plant is characterized by successful operation providing freshwater at a reasonable cost. The RO unit consists of two
units with a total water production capacity of 5 m3 h−1. The power supply system consists of 100 kWp PV arrays, 2 � 2000 Ah at
220 V (DC) – 880 kWh batteries and inverters [9].
The system was sized to provide 5 m3 h−1 of desalinated water for 3 days of 8 h operation on three consecutive
nonsunny days (Figure 35). The original plant was powered by a 100 kW PV system. Having run as a demonstration

plant for 5 years and shown that the unit can perform satisfactorily as an autonomous system, it was decided in 1995
to modify the system and incorporate it into the island grid. This allows the RO plant to be run continuously at
full output, which makes better use of this capital resource. The PV system is then used to reduce the consumption of
diesel fuel.
The pretreatment of the desalination system consists of addition of chemicals to prevent colloidal and alkaline scaling and
passing of the feedwater through cartridge filters (5, 20 μm) before entering the high-pressure pump (Figure 36).
The high-pressure pump is a piston pump, which includes an energy recovery system, recovering 15–20% of the consumed
energy. The energy requirements of the RO plant (including the energy recovery system) are of the order of 5.5 kWh m−3 of produced
water.
Both units have similar layout, with freshwater flow of 3 m3 h−1 for the first unit (three pumps, three permeators) and 2 m3 h−1
for the second unit (two pumps, two permeators). The module arrangement of each unit is of one stage, operating with seawater.
Spiral wound permeators are used.
The salt content of the produced water is <500 ppm, in compliance with World Health Organization (WHO) specifications for
drinking water. In Table 6, the technical data of the RO unit, as well as of the power supply system, are presented.
The power supply system consists of a 100 kW PV array, a battery bank with storage capacity of 2 � 2000 Ah at 220 V (DC), and
two sinusoidal inverters in order to convert the DC from the battery bank to AC for the desalination unit (Figure 37). The inverters
have been sized to allow easy starting of the 22 kW motors.
The cost of the system is analyzed as follows:

Equipment

Cost

PV array
Batteries
RO unit

10 000
125
19 000


ECU (kWp)
ECU (kWh)
ECU (m3h)

Staff (one employee)
Energy
Chemicals
Membrane replacement
Spares

20 000
0.7
0.1
0.25
0.05

ECU (year)
ECU (m3)
ECU (m3)
ECU (m3)
ECU (m3)

Electricity production cost

0.7

ECU (kWh)

Total water cost


6.5

ECU (m3)

O&M cost

Figure 34 The PV plant in Lampedusa, Italy. Adapted from Morris R (1999) Renewable Energy Powered Desalination Systems in the Mediterranean
Region. France: United Nations Educational, Scientific and Cultural Organization [25].