Desalination by Using Alternative Energy: Review and State-of-the-Art

Abstract

Energy is a critical parameter for economic and of vital importance in social and industrial development, as it is also quality water. Numerous low-density population areas lack not only fresh water availability, but in most of the cases electrical grid connection or any other energy source as well, except for renewable energy sources, mostly referring to solar radiation. For these regions desalination is a moderate solution for their needs. In using RE desalination there are two separate and different technologies involved: energy conversion and desalination systems. The real problem in these technologies is the optimum economic design and evaluation of the combined plants in order to be economically viable for remote or arid regions. Conversion of renewable energies, including solar, requires high investment cost and though the intensive R&D effort technology is not yet enough mature to be exploited through large-scale applications. This paper presents a review of the highlights that have been achieved during the recent years and the state-of-the-art for most important efforts in the field of desalination by renewable energies, with emphasis on solar energy applications.

Full text

Desalination 203 (2007) 346–365
Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation
between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the
European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.
*Corresponding author.
Desalination by using alternative energy:
Review and state-of-the-art
E. Mathioulakis*, V. Belessiotis, E. Delyannis
National Center for Scientific Research (NCSR) “Demokritos”, Aghia Paraskevi, 153-10, Athens, Greece
Tel. +30 (210) 6503817; Fax +30 (210) 6544592; email: sollab@ipta.demokritos.gr
Received 6 February 2006; accepted 15 March 2006
Abstract
Energy is a critical parameter for economic and of vital importance in social and industrial development, as it is
also quality water. Numerous low-density population areas lack not only fresh water availability, but in most of the
cases electrical grid connection or any other energy source as well, except for renewable energy sources, mostly
referring to solar radiation. For these regions desalination is a moderate solution for their needs. In using RE
desalination there are two separate and different technologies involved: energy conversion and desalination systems.
The real problem in these technologies is the optimum economic design and evaluation of the combined plants in
order to be economically viable for remote or arid regions. Conversion of renewable energies, including solar,
requires high investment cost and though the intensive R&D effort technology is not yet enough mature to be
exploited through large-scale applications. This paper presents a review of the highlights that have been achieved
during the recent years and the state-of-the-art for most important efforts in the field of desalination by renewable
energies, with emphasis on solar energy applications.
Keywords: Desalination; Alternative energy; Review
1. Introduction
Water and energy are two of the most important
topics on the international environment and de-
velopment agenda. The social and economic
health of the modern world depends on sustainable
supply of both energy and water. These two critical
resources are inextricably and reciprocally linked:
the production of energy requires large volumes
doi:10.1016/j.desal.2006.03.531
0011-9164/07/$– See front matter © 2007 P ublished by Elsevier B.V.

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 347
of water while the treatment and distribution of
water is equally dependent upon readily available,
low-cost energy. The ability of nations to provide
both clean, affordable energy and water is being
seriously challenged by a number of emerging
issues.
Through the natural water cycle, one may have
the wrong impression that fresh water is a renew-
able item, and in some way it is, even though the
availability of quality fresh water resources is
decreasing dramatically due to the population in-
crease, the irrational waste and especially to severe
contamination of the existing resources. There is
a world-wide crisis concerning availability of
good quality water, and this goes to local, regional
and national levels, despite the huge amount of
water covering the earth’s surface. This situation
leads to the application of cleaning methods or to
desalination of brackish and seawater.
In most of the arid, semi-arid and remote re-
gions fresh water is very scarce. This is especially
true for the Mediterranean basin and the Middle
East regions where many big cities and small
villages suffer from lack of quality fresh water
but, at the same time, they are blessed with abun-
dant salt-water sources. In seawater desalination
the trend refers mainly to large centralized or dual-
purpose desalination plants, as being more econo-
mical and suitable for large density population
areas, ignoring through this practice small poor
communities. However, numerous low-density
population areas lack not only fresh water but, in
most the cases, electrical power grid connections
as well. For these regions renewable energy desali-
nation is the only solution.
Since ancient times humans have used renew-
able energy sources (RES), but the remarkable
development of renewable systems has taken place
after the petroleum crisis of 1973. At present, the
increasing preoccupation for the environment has
consequently increased the interest for the use of
renewable energies. Naturally, one has to consider
the existing main limitations, related to the tem-
poral and space-dependent character of these
resources, the high land requirements and invest-
ment costs of renewable energy facilities.
Apart from the referred reasonable arguments
for the use of RES towards the emerging and
stressing energy problems, there is a number of
reasons related to the more specific issue, the one
of the suitability of RES for seawater desalination:
• Arid regions are often remote, coastal areas or
little islands where renewable energy sources
are available and conventional energy supply
is not always possible or at least easy to imple-
ment. In these cases RES represent the best
energy supply option for autonomous desalina-
tion systems.
• Climatic reasons lead to remarkable agreement
on a time-basis, between the availability of
RES, especially when referring to solar energy,
and the intensive demand of water. Further-
more, often freshwater demand increases due
to tourism, which is normally concentrated at
times when the renewable energy availability
is high.
• Both renewable energy systems and desalina-
tion refer to self-sufficiency and local support.
The operation and maintenance of related sys-
tems in remote areas are often easier than con-
ventional energy ones. Furthermore, the imple-
mentation of RES-driven desalination systems
enforces sustainable socioeconomic develop-
ment by using local resources.
• Renewable energies allow diversification of
energy resources and help to avoid external
dependence on energy supply. This has to be
seen through the prospect of the least develop-
ed countries, where major water shortage
problems exist, considering as well the fact that
seawater desalination processes are highly
energy-consuming methods.
However, present situation does not reflect the
obvious advantages of RES and desalination con-
junction. More specifically, RES-driven desalina-
tion systems are scarce, presenting usually limited
capacity. Accordingly, they only represent about

348 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
0.02% of the total desalination capacity [1]. The
reasons for this are related to various, often corre-
lated, aspects:
• Technology: The use in desalination systems
of alternative energy sources imposes the con-
junction of two separate and different techno-
logies: the energy conversion and the desalina-
tion systems. Both are considered mature to a
lesser or greater degree, even though there are
still significant margins regarding the effici-
ency increase, as well as volume and costs de-
crease. A real challenge for these technologies
would be the optimum technological design
of combined plants through a system-oriented
approach.
• Cost: Exploitation of RES and development
of desalination plants represent capital-inten-
sive installations. By this time, renewable ener-
gy technologies are not considered totally ma-
ture and the various system components are
still expensive. Even though prices decrease
continuously, still in many cases they are pro-
hibiting for commercialization.
• Availability: Renewable energies are unlimit-
ed, being though transient, thus presenting
intermittent character, leading to limitations
concerning the maximum exploitation capaci-
ties per time unit. Furthermore, the geographi-
cal distribution of RES potential does not al-
ways comply with the water stress intensity at
a local level.
• Sustainability: In most of the cases, the matu-
rity of the associated technologies does not
match the low level of infrastructures which
often characterizes places with severe water
stress. Experience has shown that several
attempts to integrate advanced desalination
solutions in isolated areas failed due to lack of
reliable technical support.
Through the above mentioned framework, the
following discussion concentrates on the main
RES-driven desalination concepts, aiming to light
up their prospective characteristics and trace the
problems arising. The criteria of analysis refer
amongst others to RES availability, technological
maturity, simplicity, availability of local resources
for handling and maintenance, possibility to en-
sure a given level of fresh water production, suit-
ability of the system to the characteristics of the
location, efficiency, etc.
2. Matching renewable energies with desalina-
tion units
Renewable energies and desalination plants are
two different technologies, which can be com-
bined in various ways. The interface between the
renewable energy system and the desalination sys-
tem is met at the place/subsystem where the energy
generated by the RE system is promoted to the
desalination plant. This energy can be in different
forms such as thermal energy, electricity or shaft
power. Fig. 1 shows the possible combinations.
One has to note that in this figure, there have been
introduced some changes regarding relevant
references [2]. These changes concern the inser-
tion of direct solar distillation (SD), humidifica-
tion–dehumidification (HD) and membrane dis-
tillation (MD) systems.
Renewable energy-driven desalination systems
fall into two main categories: thermal processes
and electromechanical processes. As regards the
energy source, a desalination plant powered by
renewable energy is likely to be a stand-alone sys-
tem at a location which has no electricity grid.
Stand-alone systems are often hybrid systems,
combining more than one type of renewable ener-
gy sources, for instance, wind and solar energy or
including a diesel generator. In order to ensure
continuous or semi-continuous operation inde-
pendent of weather conditions, stand-alone sys-
tems usually include a storage device.
In recent years, due to intensive R&D efforts
and to operation experience gained, advances in
conventional desalination plants, steam or elec-
trically driven, refer to a significant efficiency
increase and reduction of cost [3,4]. On the other

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 349
hand, the situation with renewable energy-driven
desalination systems is quite different. At present
time, these systems evolve through the R&D stage,
or they are implemented as pilot plant size appli-
cations, presenting, in general, capacities from a
few m3 up to 100 m3. There have also been some
demonstration plants of medium size, mainly
solar-powered, but a minority of those has pre-
sented successful operation characteristics [4,5].
Not all the combinations of RES-driven desa-
lination systems are considered to be suitable for
practical applications; many of these possible
combinations may not be viable under certain cir-
cumstances. The optimum or just simple specific
technology combination must be studied in con-
nection to various local parameters as geogra-
phical conditions, topography of the site, capacity
and type of energy available in low cost, avail-
ability of local infrastructures (including electri-
city grid), plant size and feed water salinity. Gene-
ral selection criteria may include robustness, sim-
plicity of operation, low maintenance, compact
size, easy transportation to site, simple pre-treat-
ment and intake system to ensure proper operation
and endurance of a plant at the often difficult con-
ditions of the remote areas.
Fig. 1. Possible technological combinations of the main renewable energies and desalination methods.
Geot
h
e
rm
a
l Solar Win
d
Electricity
Heat PV Solar Thermal Electricity Shaft
ED
RO
MVC
MED
MSF
TVC
MD
ED
RO
MVC
Solar
Collectors
Electricity
MED
MSF
TVC
MD
RO
MVC
SD
HD
RE
Direct
Process
Fig. 2 shows that the most popular combination
is the use of PV with reverse osmosis [6]. Table 1
gives an overview of recommended combinations
depending on several input parameters, noting
though that other, additional combinations are also
possible [4]. Indeed, PV is considered a proper
solution for small applications in sunny areas. For
larger units, wind energy may be more attractive
as it does not require anything like much ground.
This is often the case on islands where there is a
good wind regime and often very limited flat
ground.
The general tendency is to combine thermal
energy technologies (solar thermal and geothermal
energy) with thermal desalination processes and
electromechanical energy technologies with
desalination processes requiring mechanical or
electrical power. Therefore, the following combi-
nations are the options most commonly used when
desalination units are powered by renewable ene-
rgy [4]:
• PV or wind-powered reverse osmosis, electro-
dialysis or vapour compression;
• Solar thermal or geothermal energy and dis-
tillation processes.

350 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
Solar PV;
43%
Wind;
20%
Hybrid;
10%
Solar
Thermal;
27%
MED;
14%
Other;
4%
MSF;
10%
ED; 5%
RO;
62% VC; 5%
Fig. 2. Renewable energy-driven desalination processes and energy sources.
Table 1
Recommended renewable energy–desalination combinations
System size Feed water
quality Product
water RE resource
available Small
(1–50 m3d–1) Medium
(1–50 m3d–1) Large
(1–50 m3d–1)
Suitable combination
Distillate Solar * Solar distillation
Potable Solar * PV–RO
Potable Solar * PV–ED
Potable Wind * * Wind–RO
Brackish
water
Potable Wind * * Wind–ED
Distillate Solar * Solar distillation
Distillate Solar * * Solar thermal–MED
Distillate Solar * Solar thermal–MED
Potable Solar * PV–RO
Potable Solar * PV–ED
Potable Wind * * Wind–RO
Potable Wind * * Wind–ED
Potable Wind * * Wind–MVC
Potable Geothermal * * Geothermal–MED
Seawater
Potable Geothermal * Geothermal–MED
Although there do not exist any extensive
references concerning the real cost of water pro-
duced by these installations, prices that have been
theoretically calculated [7–9] for large capacities
are higher than these from conventional desalina-
tion plants. In any case one has to wonder whether
this is the main problem for RES-driven desalina-
tion installations being scarce, as a glass of drink-
ing water in remote and arid regions is actually
precious and cost can be considered a matter of
minor weight.
The following analysis presents, in more detail,
the potential RES-driven desalination systems.
Emphasis is paid to the systems with the higher

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 351
prospects on a research and technological basis.
Evidently, these systems comprise the most
promising practical solutions within the medium-
term time period.
3. Thermal solar desalination
3.1. Desalination powered by thermal solar energy
Thermal solar energy is considered to be one
of the most promising applications of renewable
energies to seawater desalination, as it is suitable
for arid and sunny regions. A thermal solar distilla-
tion system usually consists of two main parts,
the collecting device and the distiller. Solar ther-
mal desalination processes are characterized as
direct processes when all parts are integrated into
one system, while the case of indirect processes
refers to the heat coming from a separate solar
collecting device, usually solar collectors or solar
ponds.
Solar stills belong to the case of direct pro-
cesses, and due to the interest they present, they
will be discussed thoroughly below. The low
efficiency of the still, mainly due to the high heat
loss from its glass cover, has led many researchers
through the survey of design concepts that would
decrease the loss of latent heat of condensation at
the glass cover or furthermore would partly re-
cover this energy. Thus, the idea of utilizing latent
heat of condensation via multi-effect solar stills
has come out. The basic principle imposes the use
of condensation heat of the vapour from the n-th
effect, for the evaporation of water at the n + 1st
effect. Actually, one should talk of direct processes
utilizing humidification–dehumidification tech-
niques through a broad area of design solutions
[10–14], leading eventually to significantly im-
proved performance, compared to a simple solar
still.
The indirect-type stills are based on the fact
that heat is provided only at the first stage of such
a multi-effect unit, thus the use of external heat
source is possible. Conventional solar thermal col-
lectors, corrosion-free collectors developed for the
specific application [15,16] or even evacuated tube
collectors [17] have been used as the external heat
source.
Within the category of indirect processes
installations based on conventional thermal
desalination technology, as MED and MSF, are
also included. For reasons related to the com-
plexity and the cost of desalination units, these
plants are usually of greater size. Even though
during last years the development of such instal-
lations has been rather abandoned, several MED
and MSF pilot plants have been designed and
tested during the past, especially in late 90s. These
installations have been driven by a flat plate, para-
bolic trough or vacuum solar collectors [17–19].
The evaluation of these plants has shown that
MED has greater potential than MSF for designs
with high performance ratio and, moreover, the
MED processes appear to be less sensitive to cor-
rosion and scaling than the MSF processes [4].
Solar thermal energy can be used, in principle,
for the production of electricity or mechanical
energy. Evidently, the process of thermal energy
conversion is accompanied with a decreased effi-
ciency. During the past, only single attempts are
reported, as the case of a solar-assisted freezing
plant powered by a point-focusing solar collector
field [20], a cogeneration hybrid MSF–RO system
driven by a dual-purpose solar plant [21], and an
RO plant powered by flat-plate collectors with
freon as the working fluid [22].
Finally, special reference has to be given to
the solar pond-powered desalination plants, select-
ed by some authors as amongst the most cost-ef-
fective systems [23,24]. With relevance to this
concept, different plants were implemented coupl-
ing a solar pond to an MSF process [21,25–27].
3.2. Solar stills
In solar distillation there exists a huge amount
of bibliography describing all possible configura-
tions of solar stills, including theories, models and
experimental results. Usually they are limited to
each special still design, while some of them

352 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
propose comprehensive reviews and cost analysis
[8,10,28]. Most of the studies try to increase the
efficiency by using latent heat of condensation or
coils to preheat the feed water and increase vapour
condensation [29], separated evaporation and con-
densing zones [30,31], capillary film techniques
[32]. Others try to increase feeding water temp-
erature by various techniques, as connection with
solar collectors [33], use of intermediate storage
in connection to collectors [34], or integration of
solar still in a multi-source, multi-use environment
[35].
Conventional simple greenhouse-type solar
distillation plants present specific disadvantages
as low efficiency, high initial capital cost (counter-
balanced in part by lower operational cost), large
installation surface areas, vulnerability to extreme
weather conditions (especially plastic covered
stills), risk of formation of algae and scale on the
black surface and need for special care to avoid
problems related to dust deposition on the trans-
parent cover.
At present time, only a few small solar distilla-
tion plants exist world-wide, referring amongst
others to the case of India. Despite the consider-
able number of plants developed in the past,
including as well plants still existing, there are
not available, at least published, analytical infor-
mation about real operation conditions, cost of
produced water, cost of installation, operation and
maintenance or any other problem raised, i.e. in-
formation potentially important for the optimum
design of new installations.
Solar distillation plants are not commercialized
yet, except for a few individual units. In fact, in
many cases, one may use the term of semi-em-
pirical devices, thus characterizing the design
level. Constructions are made with locally avail-
able materials resulting in economic figures varia-
tion from region to region. On the other hand, the
overall economics and operation conditions are
of major importance to be known.
Considering the above mentioned status of
solar stills, the main question/problem that arises
could be formulated as follows: What must be
done for further development, in order solar dis-
tillation plants to become viable and reliable in-
stallations for small communities?
Through this approach, the following priority
actions arise:
• To formulate a well-designed prototype still,
and investigate the applicability of optimum
design and mathematical models that are wide-
ly applied in solar stills. Some authors have
developed simple models to predict daily out-
put depending on geometry of construction and
operational parameters of the still, being appli-
cable though to a wide range of single solar
still configurations [34,36]. This has to be ex-
tended for the case of multiple solar stills, as
well.
• To find the best materials of construction. The
term best should refer to materials that combine
long life of the stills, resistant behaviour (e.g.,
no corrosion in fresh and salt water) with the
lower cost possible. This can be achieved only
if global information is available.
• To study and find the optimum operational
conditions in order to avoid scale formation
and algae deposits into the basin and onto the
black surface, thus improving absorptivity of
solar radiation.
• To construct small plants in poor small com-
munities and collect all possible information
about construction, operation and mainte-
nance, providing at the same time water to the
people of dry regions.
• To elaborate a reliable economic evaluation of
a solar distillation plant. This should lead to
an optimum economical characterization of the
system, comprising criteria of location, nece-
ssary equipment size, labour cost and energy
analysis. This type of analysis is considered
to be complex and time consuming, mainly due
to the detailed meteorological data need.
Despite the problems and priority actions
referred, even through an existing semi-empirical

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 353
design level, solar distillation plants may be the
only solution for some small communities where
the only item that counts is fresh water production.
3.3. Humidification–dehumidification
As it has already been mentioned, the humidi-
fication–dehumidification (HD) principle has
been developed while trying to solve the major
problem of solar stills, that is the energy loss in
the form of latent heat of condensation. Solar
desalination based on the HD principle results in
an increase in the overall efficiency of the desa-
lination plant and therefore appears to be the best
method of water desalination with solar energy.
At present time, the HD desalination process is
considered a promising technique for small-
capacity, solar-driven desalination plants. Through
this approach, one has to note the relevant signi-
ficant bibliography, as well as the fact of various
experimental units based on the principle of solar
powered HD being constructed in different parts
of the world (for an comprehensive review see
[38,39]).
The process presents, indeed, several attractive
features, including operation at low temperature,
ability to combine with renewable energy sources
(i.e. solar, geothermal), modest level of technology
employed, simplicity of design and ability to be
manufactured locally. Furthermore, it has the ad-
vantage of separating the heating surface from the
evaporation zone, therefore, the heating surface
is relatively protected from corrosion or scale de-
posits [11].
The basic operation principle of the HD pro-
cess is the evaporation of seawater and con-
densation of water vapour from the humid air
taking place inside the unit at ambient pressure,
while there is continuous humid air flow from the
evaporator to the condenser including recovery
of latent heat of condensation for the preheating
of the feed water (Fig. 3). More thoroughly, when
circulating air comes in contact with hot saline
water in the evaporator, a certain quantity of va-
pour is extracted by the air. Part of the vapour
Fig. 3. General layout of an HD unit:1 evaporation area,
2 condensation area, 3 heat source, 4 air circulation,
5 feed saline water, 6 distillate, 7 brine, 8 brine recircu-
lation reflux.
mixed with air may be recovered as condensate,
by bringing the humid air in contact with a cooling
surface in another exchanger, in which saline feed
water is preheated by the latent heat of conden-
sation. On the basis of this principle, there have
been proposed several configurations, differing
through the following points: the overall arrange-
ment of the main unit (open-air closed-water cycle
and open-water closed-air cycle), the circulation
of the humid air between the evaporator and the
condenser (natural or forced), the packing material
placed in the evaporator (Raschig rings, tissues,
fleece, stems) and the way to bring the heat to the
unit (heating of the feed water or heating of the
circulating air) [38,39].
The first specific bibliographic reference to the
HD desalination is set by the year 1966 and con-
cerns a pilot plant of 5000 US gallons (14.3 m3)
per day. It was called “Humidification Cycle Dis-
tillation”. The plant consisted of a condenser tow-
er, an evaporator tower with Raschig rings and
long, plastic solar collectors [40]. A second refer-
ence, in 1968, presents the parametric analysis of
two experimental units, of 3 L.d–1 and 196 L.d–1
capacities, equipped with surface condenser and
c
e
d
g
f
f
i h j

354 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
with Raschig rings as the packing material in the
evaporator. Nevertheless, by the early 1960’s, the
multi-effect humidity approach (MEH) has been
introduced, referring to the distillation by natural
or forced circulation of an air loop saturated with
water vapour [41]. It has to be noted that the term
“multiple effect” does not necessary refer to well
distinguished stages, but to the fact that evapora-
tion and condensation happen continuously over
the whole temperature range between the con-
denser inlet and evaporator outlet.
Since its introduction, the MEH process has
been evolved and various configurations have
been investigated in different countries. Veza and
Ruiz [42] suggested the use of an external cooler
in a direct, forced circulation process without
latent heat recovery. Investigation of an integrated
collector, evaporator, and condenser [43] has
shown that the efficiency of these desalination
units can be significantly higher than that of a
single-basin-still, while other studies refer to ty-
pical water production not more than 6 L.m–2d–1
[44], accompanied though to gain output ratio
(GOR) values higher than 3 [45]. One may also
note the existence of studies proving the positive
influence of the increase of the water temperature
at the inlet to the evaporator of the MEH units, as
well as of air-circulation, on the desalination pro-
ductivity [46]. A more consistent and systematic
investigation of forced or natural circulation units
has been attempted by Farid et al., reporting as
well the development of a rigorous detailed model
that can be used to describe the performance of
the desalination units working on the humidifica-
tion–dehumidification principle [47–49].
Important can be considered the work referring
to the investigation of long-term performance and
the optimization of a compact, natural circulation,
thermal solar-driven HD plant, constructed on the
basis of the design originally developed by the
University of Munich [50]. Two prototype plants
were installed and measured from October 1992
to March 1997 in the Canary Islands [51]. The
performance of the units has been improved over
the years and an average daily production of
11.8 L.m–2.d–1 has been obtained from the systems
without thermal storage, on an average gain output
ratio (GOR) between 3 and 4.5. From this work
an improved concept has come out, including a
conventional heat storage tank and heat exchanger
between the collector and the distillation circuit,
so as to enable 24-h operation of the distillation
module [52]. Within the framework of the same
project (SODESA), a new collector with corro-
sion-free absorbers was developed [15].
The solar multiple condensation evaporation
cycle (SMCEC) concept has been proposed as
well suited for developing countries with extended
rural areas, because of its simplified design, low
maintenance, extended life time and low capital
cost [53]. The three main parts of the SMCEC
desalination unit (solar collector, condensation
tower and evaporation tower filled with thorn
trees), were optimized through a detailed modell-
ing, simulation and experimental validation.
Other relevant works present systems of closed
water circulation in contact with a continuous flow
of cold air in the evaporation chamber [54], open-
air cycle systems [55] characterized by a high
power requirement for air circulation, as well as a
solar operated HD desalination system driven by
a solar pond [56]. Recently, Nafey et al. presented
an experimental and numerical investigation
concluding that the productivity of HD systems
is strongly affected by the saline water temperature
at the inlet the humidifier, the dehumidifier cool-
ing water flow rate and the recirculation air flow
rate [57]. In addition, Ettouney presented a critical
analysis of the four main configurations of the
dehumidification process, concluding to the need
to fully optimize these configurations in order to
define the best design and operating conditions
[58].
3.4. Membrane distillation
Membrane distillation (MD) is a thermally
driven, membrane-based process. It first appeared

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 355
at the end of 60s, and it constitutes the most recent
development in the field of thermal desalination
processes. The process takes advantage of the
temperature difference between a supply solution,
coming in contact with the surface, on one side,
of the readily selected micro-porous membrane,
and the space, on the other side of the membrane
(Fig. 4). This temperature difference results in a
vapor pressure difference, leading to the transfer
of the produced vapor, through the membrane, to
the condensation surface. The overall process is
based on the use of hydrophobic membranes, per-
meable by vapor only, thus excluding transition
of liquid phase and potential dissolved particles
[59,60].
In a typical MD system, the feed water (e.g.,
25°C inlet temperature) passes through the con-
denser channel, from its inlet to its outlet, while
warming up (e.g., 65°C outlet temperature). The
hot feed solution (e.g., Thigh = 80°C inlet tempera-
ture) is directed along this membrane, passing the
evaporator channel from its inlet to its outlet, while
cooling down (e.g., Tlow = 40°C evaporator outlet
Fig. 4. Principle of the MD process (left) and schematic diagram of a typical MD system (right).
Coolant
Hot feed
solution
Membrane
Condensation
plate
Condensation
Channel
temperature). The input heat, necessary to achieve
the required temperature gradient between the two
channels (e.g., 80 – 65°C = 15°C), is introduced
into the system between the condenser outlet and
the evaporator inlet, by the heat medium.
The basic requirements for the membrane refer
to the negligible degree of liquid phase permeance,
while the opposite is required for the air phase, as
well as to low thermal conductivity, sufficient but
not excessive thickness, long-time resistance to
the seawater contact, low water absorbency, and
adequate level of porosity (70–80%). Materials
that have been used on a research level are PTFE,
PVDF, PE and PP. Typical sizes for the porosity
are 0.06–0.85, for the pore sizes 0.2–1.0 µm and
for thickness 0.06–0.25 mm.
Despite the fact that the process has been
known for a considerable number of years, and
the existing interest for commercial exploitation
in desalination applications, the high cost and the
problems associated with the use of membranes
have prohibited the development of commercial
products. On the other hand, there has been imple-

356 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
mented considerable research activity, as depicted
through the existing relevant bibliography. Sug-
gestively, studies [61–71] are referred, as well as
a remarkable evaluation of the results obtained
up to now [72]. The relevant studies conclude that
the feed water salt concentration has a very small
effect on the permeate flux and thermal efficiency.
On the contrary, increasing the inlet temperature
of the hot solution has a major effect on the per-
meate flux. For instance an increase from 40 to
80°C would increase the flux by nearly an order
of magnitude and the thermal efficiency by 12%.
The selectivity of membrane desalination is higher
than this of any other membrane desalination pro-
cess, and the produced water, as measured through
the experiments, is proven to be purer even than
the water produced by MSF.
Within the field of desalination processes, one
has to point out the MD process taking place on
atmospheric pressure, and on temperatures not
exceeding 80°C, raising energy demand to that
of thermal energy only. For this reason, MD is a
process with several advantages regarding the
integration into a solar, thermally-driven, desalina-
tion system. In addition, integrating membrane
distillation with other distillation processes seems
to be promising, for example by using MD as a
bottoming process for MSF or ME, so that the
hot reject brine from MSF or ME could operate
as the feed solution for the MD plant [73].
In bibliography specific reports to solar, ther-
mally driven, MD systems are detected. Bier and
Plantikow present the results for a prototype of
an autonomous solar MD system, powered by a
51 m2 collector field, and being installed on 1993
on the Island of Ibiza, Spain. A distillate produc-
tion of 40–85 l/h is reported for feed water flows
of 0.8–1.7 m3/h [74]. Koschikowski et al. discuss
the design and development of a stand-alone MD
system powered by 5.9 m2 of corrosion-tie, sea-
water-resistant, thermal collectors. The maximum
of distillate gain during the test period of summer
2002 was about 130 l/d under the meteorological
conditions of Freiburg (Germany) [75]. Recently,
the same authors presented the first results from
the on-going tests of a new compact, thermal solar-
powered MD unit, installed in the Canary Island,
showing that distillate production rates of about
25 l m–2 d–1 are realistic [76].
A modified air gap MD process (Memstill pro-
cess) aiming at presenting a low-cost alternative
solution to both large- and small-scale RO, MSF
and MED practices, was developed by a con-
sortium of nine parties. According to the author,
MEMSTILL® houses a continuum of membrane
evaporation stages in an almost ideal counter cur-
rent flow process, making it possible to achieve a
very high recovery of evaporation heat [77].
Kullab et al. present results from the investigation
of a stand-alone air-gap MD unit integrated with
non-concentrating solar collectors, aiming to the
development of a commercial prototype for mass
production. Scaling-up of the MD unit was accom-
plished on the basis of experimental data obtained
from the prototype and a simulation was perform-
ed for a specific case study [78].
In summary, MD desalination seems to be a
highly promising process, especially for situations
where low-temperature solar, waste or other heat
is available. When operating between the same
top and bottom temperatures as MSF plants, MD
with heat recovery can operate at performance
ratios about the same as the commercial MSF
plants, but with much lower pumping power
consumption. MD systems are very compact,
similar to RO ones, and at least 40-fold more com-
pact than other distillation desalination systems
such as MSF. It is obvious though that a more in-
tensive research and development effort is needed,
both in experimentation and modelling, focused
on key issues such as long-term liquid/vapour
selectivity, membrane aging and fouling, feed-
water contamination and heat recovery optimiza-
tion. Scale-up studies and realistic assessment of
the basic working parameters on real pilot plants,
including cost and long-term stability are also
considered to be necessary.

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 357
4. Electromechanical processes
4.1. PV-driven RO and ED processes
There are mainly two PV driven membrane
processes, reverse osmosis (RO) and electrodia-
lysis (ED). The first one (RO) is a pressure-driven
membrane process where a feed stream flows
under pressure through a semi-permeable mem-
brane, separating two aqueous streams, one rich
in salt and the other poor in salt. Water will pass
through the membrane, when the applied pressure
is higher than the osmotic pressure, while salt is
retained. As a result, a low salt concentration per-
meate stream is obtained and a concentrated brine
remains at the feed side. ED is an electricity-driven
process where an ionic solution is pumped through
anion- and cation-exchange membranes arranged
in an alternating pattern between an anode and a
cathode. When an electrical potential is applied
between the two electrodes, the cations migrate
towards the cathode but are retained by positively
charged anion-exchange membranes, while the
anions migrate towards the anode and are retained
by cation-exchange membranes. The described
phenomenon results in ion concentration in alter-
nate compartments (concentrate or brine compart-
ment), while other compartments simultaneously
become depleted (dilute compartment). Com-
paring RO and ED, one could say that in reverse
osmosis the water is transported through the mem-
brane and the electrolytes are retained, while in
electrodialysis the electrolytes are transported
through the membrane and water is more or less
retained [79,80].
From a technical point of view, PV as well as
RO and ED are mature and commercially avail-
able technologies at present time. Besides that,
RO represents 42% of world-wide desalination
capacity and more than 88% of membrane pro-
cesses production [81]. The feasibility of PV-pow-
ered RO or ED systems, as valid options for desali-
nation at remote sites, has also been proven [82].
Indeed, there are commercially available stand-
alone, PV powered desalination systems [83]. The
main problem of these technologies is the high
cost and, for the time being, the availability of
PV cells.
With regard to the process selection, the choice
of the most relevant technology mostly depends
on the feed water quality, level of technical infra-
structures (availability of skilled operators and of
chemical and membrane supplies) and user re-
quirement:
• Both RO and ED can be used for brackish wa-
ter desalination, but RO constitutes a more rea-
listic choice for seawater desalination, since it
presents higher energy efficiency than ED
when feed water salinity is higher than, let us
say, 2000 ppm. ED is preferable for brackish
water desalination, due to its relatively higher
efficiency and robustness.
• Considering the feed water quality, pre-treat-
ment is often more strict in the case of RO,
since RO membranes are very susceptible to
fouling. On the other hand, as ED only removes
ions from the water, additional measures may
be required (disinfection, removal of particles,
etc.) [84,85].
• Considering the energy supply, RO presents
lower energy consumption but ED shows
better behaviour considering intermittent or
fluctuating electrical power, as a consequence
of changes in solar resource intensity.
Several RO or ED desalination systems driven
by PV have been installed throughout the world
in the last decades, most of them being built as
experimental or demonstration plants. An over-
view of technology options with reference to exist-
ing systems is given in [19]. Some authors present
experimental or simulation results [86–91], while
some others concentrate on cost analysis [92–94].
The challenge for the near future seems to be
the development of small, autonomous, modular,
flexible and reliable units, offering operation and
maintenance at reasonable cost, in order to serve
the segment of isolated users [80]. On that level,
the development of battery-less systems, as well

358 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
as the use of recovery devices, is of special im-
portance.
Obviously, batteries increase the overall
productivity of the PV system in an intermittent
electrical power context induced by fluctuating
solar radiation. However, they require careful
maintenance and, hence, higher skills for sustain-
able operation, conditions which are proved to be
difficult at remote sites. For this reason, inter-
mittent operation of direct connected PV–RO and
PV–ED plant may be a promising option. This
requires modification of common design rules for
the electronics and the water processing part of
the plant.
The battery-less option has been discussed by
some authors in PV desalination applications
[95,96]. Recently, Richards et al. presented results
from field performance test of a 1000 l/d PV
powered hybrid ultrafiltration–nanofiltration or
RO system. The system operates without batteries,
and is designed to desalinate groundwater of
marginal and brackish water [97]. Advantages of
battery-less systems with electronic power con-
verters are simplified configuration, compact de-
sign, improved robustness and long life of all com-
ponents of the power supply sub-system. Disad-
vantages are higher cost and possible availability
problems of power electronics, longer periods in
‘stand by’ mode with related risks of membrane
fouling, critical importance of optimized sizing
of sub-systems.
Another promising option is that of the energy
recovery devices. Pelton wheel turbines and pres-
sure exchangers are commonly used in RO
desalination to recover part of the feed pressure,
but both systems have been available for large
plants only. However, some authors have recently
reported applications in which hydraulic motors
and a Spectra Clark Pump were used to recover
energy for smaller plants [96,98]. According to
their conclusions, these systems may ensure
higher efficiency over a wide range of flow rates
and look very promising for PV–RO plants.
Nevertheless, conclusions from long-term opera-
tion in real conditions still have to be reported, in
particular concerning fouling and scaling pheno-
mena.
4.2. Wind power and hybrid wind power–PV
The electrical or mechanical power generated
by a wind turbine can be used to power desalina-
tion plants. Like PV, wind turbines represent a
mature, commercially available technology for
power production. A comprehensive review of
wind technology has been proposed by Acker-
mann et al. [99]. Wind power is an interesting
option for seawater desalination, especially for
coastal areas presenting a high availability of wind
energy resources. Wind turbines may, for instance,
be coupled with RO and ED desalination units.
According to some authors, wind powered RO
plants appear to be one of the most promising alter-
natives of renewable energy desalination [100].
Several simulation studies show the feasibility
of wind powered desalination technologies,
through the analysis of different types of mem-
branes and feed water quality levels [101–103].
There are also several installations powered by
wind turbines, either connected to a utility network
or operating in a stand-alone mode. Most of them
have been installed at Canary Islands, Spain: a
200 m3/d wind–RO plant for brackish water de-
salination [104], a 56 m3/d hybrid diesel–wind–
RO plant providing fresh water and electricity for
local people [19], a battery-less wind–RO [105]
and a wind–ED experimental plant [106]. Based
on the long-term experience accumulated at
Canary Islands, the Canary Islands Technological
Institute developed the concept AEROGEDESA,
referring to a compact, stand-alone wind–RO sys-
tem with capacities between 5 and 50 m3/d. Fur-
thermore, experimental investigation of a shaft
power-driven wind–RO system has been reported
by the same Institute [19].
Besides that, several other wind–RO desalina-
tion plants have been reported in the literature
[107–112] as well as some mechanical VC instal-

E. Mathioulakis et al. / Desalination 203 (2007) 346–365 359
lations powered by wind turbines [22,113,114].
In a recent paper, Carta et al. have presented a
fully autonomous, battery-less system which
consists of a wind farm supplying the energy needs
of a group of eight RO modules [115]. The main
innovation of this system refers to the implemen-
tation of an automatic operation strategy, con-
trolling the number of RO modules that have to
be connected in order to match the variable wind
energy supply.
The idea to use hybrid wind–solar systems for
desalinization is based on the fact that, for some
locations, wind and solar time profiles do not coin-
cide. Obviously, the opportunity to install such a
hybrid system has to be carefully investigated by
means of simulation [116], using typical metro-
logical data. Finally, it is to be noted that some
small-scale experimental pilot plants of this type
are reported in the literature [88,99,117,118].
5. Desalination powered by geothermal energy
Even though geothermal energy is not as com-
mon in use as solar (PV or solar thermal collectors)
or wind energy, it presents a mature technology
which can be used to provide energy for desalina-
tion at a competitive cost. Furthermore, and com-
paratively to other RE technologies, the main ad-
vantage of geothermal energy is that the thermal
storage is unnecessary, since it is both continuous
and predictable [119]. A high-pressure geothermal
source allows the direct use of shaft power on
mechanically driven desalination, while high tem-
perature geothermal fluids can be used to power
electricity-driven RO or ED plants. However, the
most interesting option seems to be the direct use
of geothermal fluid of sufficiently high tempera-
ture in connection to thermal desalination techno-
logies [4].
The first geothermal energy-powered desalina-
tion plants have been installed in the United States
by the 70s [120,121], through the testing of vari-
ous potential options for the desalination techno-
logy, including MSF and ED. Ophir presents an
economic analysis showing that high-temperature
geothermal desalination plants could be a viable
option [122]. Karytsas proposed a technical and
economic analysis of an MED plant, with a capa-
city of 80 m3/d, powered by low-temperature geo-
thermal source and installed in Kimolos, Greece
[123]. Bourouni et al presented results from an
experimental investigation of two polypropylene
made HD plants powered by geothermal energy
[124]. Recently, Bouchekima discussed the per-
formances of a hybrid system, consisting of a solar
still in which the feed water is brackish under-
ground geothermal water [125].
Finally, the availability and/or suitability of
geothermal energy, and other RE resources, for
desalination, are given by Belessiotis and Delya-
nnis [126].
6. Conclusions
The use of renewable energies for desalination
appears nowadays as a reasonable and technically
mature option towards the emerging and stressing
energy and water problems. However, and despite
intensive research world-wide, the actual penetra-
tion of RES-powered desalination installations is
too low.
During the recent past, there has been a rather
intense attempt to develop effective large-scale
desalination plants, mainly powered by renewable
sources. Through this activity, considerable expe-
rience has been gained, even if this option appears
to have entered a phase of relative stagnation.
Yet, numerous low-density population areas
lack not only fresh water but, in most of the cases,
electrical power grid connections as well. For
these regions renewable energy desalination is
often the only solution. Through this situation
there is a growing interest for the development of
small-scale autonomous solutions, also confirmed
by the respective bibliography.
Thus, what is important now is to move to-
wards the development of integrated solutions,
ensuring reliability, robustness, sustainability in

360 E. Mathioulakis et al. / Desalination 203 (2007) 346–365
terms of local resources and effective performance
at acceptable cost. From this point of view, there
have been detected some promising technological
options that have to be studied more thoroughly
aiming to achieve optimum operation and over-
come specific construction and operational prob-
lems. Amongst the issues to be investigated, one
may mention the following:
• The humidification–dehumidification process
presents several attractive features, including
the ability to combine with low temperature
renewable energy sources, modest level of
technology employed, simplicity of design
and, most of all, relatively high efficiency
compared to other thermal processes. Thus, it
is of great importance to work further on prob-
lems related to material optimization and on
the establishment of realistic cost figures in
actual operation conditions.
• MD desalination seems to be a highly pro-
mising process, especially for situations where
low-temperature solar, waste or other heat is
available. It is obvious though that a more in-
tensive research and development effort is
needed, both in experimentation and modell-
ing, focused on key issues such as long-term
liquid/vapour selectivity, membrane aging and
fouling, feed water contamination and heat
recovery optimization. Scale-up studies and re-
alistic assessment of the basic working parame-
ters on real pilot plants, including cost and
long-term stability are also considered to be
necessary.
• PV and wind powered desalination techno-
logies appear to be a rather mature option from
a technological point of view. The challenge
here would be to propose more efficient and
sustainable solutions though system inte-
gration and subsystem packaging. Upon this,
the issue of storage becomes of critical impor-
tance, having to cope with the intermittent and
fluctuating character of the renewable source.
Concerning storage, three main options are
discussed: improved or new storage technolo-
gies (potentially including hydrogen), optimal
management of modular type desalination
units according to the energy availability, and
implementation of new control strategies,
supported by appropriate electronics, allowing
battery-less operation. Concerning efficiency,
a promising option appears to be the use of
recovery devices, designed specifically for
small scale applications.
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