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Solar thermal desalination technologies
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Abstract
The use of solar energy in thermal desalination processes is one of the most promising applications of the
renewable energies. Solar desalination can either be direct; use solar energy to produce distillate directly in the
solar collector, or indirect; combining conventional desalination techniques, such as multistage flash desalination
(MSF), vapor compression (VC), reverse osmosis (RO), membrane distillation (MD) and electrodialysis, with
solar collectors for heat generation. Direct solar desalination compared with the indirect technologies requires
large land areas and has a relatively low productivity. It is however competitive to the indirect desalination plants
in small-scale production due to its relatively low cost and simplicity. This paper describes several desalination
technologies in commercial and pilot stages of development. The primary focus is on those technologies suitable
for use in remote areas, especially those which could be integrated into solar thermal energy systems.
Keywords: Solar energy; Desalination; Solar stills
1. Introduction
The lack of potable water poses a big problem
in arid regions of the world where freshwater is
becoming very scarce and expensive. Clean drinking
water is one of the most important international
health issues today. The areas with the severest
water shortages are the warm arid countries in
the Middle East and North Africa (MENA) region.
These areas are characterized by the increase in
ground water salinity and infrequent rainfall. The
increasing world population growth together
with the increasing industrial and agricultural
activities all over the world contributes to the
depletion and pollution of freshwater resources.
Desalination is one of mankind’s earliest forms
of water treatment, and it is still a popular treatment
solution throughout the world today. In nature,
solar desalination produces rain when solar radiation
is absorbed by the sea and causes water to
evaporate. The evaporated water rises above the
surface and is moved by the wind. Once this vapor
*Corresponding author. cools down to its dew point, condensation occurs,
634 H.M. Qiblawey, F. Banat / Desalination 220 (2008) 633–644
and the freshwater comes down as rain. This basic
process is responsible for the hydrologic cycle.
This same principle is used in all man-made distillation
systems using alternative sources of heating
and cooling.
Desalination uses a large amount of energy to
remove a portion of pure water from a salt water
source. Salt water (feed water) is fed into the
process, and the result is one output stream of pure
water and another of wastewater with a high salt
concentration.
It has been estimated by Kalogirou [23] that
the production of 1000 m3 per day of freshwater
requires 10,000 tons of oil per year. This is highly
significant as it involves a recurrent energy
expense which few of the water-short areas of the
world can afford. Large commercial desalination
plants using fossil fuel are in use in a number of
oil-rich countries to supplement the traditional
sources of water supply. People in many other
areas of the world have neither the money nor oil
resources to allow them to develop on a similar
manner. Problems relevant to the use of fossil
fuels, in part, could be resolved by considering
possible utilization of renewable resources such
as solar, biomass, wind, or geothermal energy. It
often happens that the geographical areas where
water is needed are well gifted with renewable
energy sources (RES). Thus, the obvious way
is to combine those renewable energy sources to
a desalination plant, in order to provide water
resources as required. In fact, most developing
countries, with vast areas but having no access to
electric grid, appear to be well versed in renewable
energies. Such sources, able to be used directly
even at far remote and isolated areas, could be
exploited to power low to medium scale desalination
plants. The World Health Organization estimates
that over a billion people lack access to
purified drinking water and the vast majority of
these people are living in rural areas where the
low population density and remote locations make
it very difficult to install the traditional clean
water solutions.
Recently, considerable attention has been
given to the use of renewable energy as sources
for desalination, especially in remote areas and
islands, because of the high costs of fossil fuels,
difficulties in obtaining it, attempts to conserve
fossil fuels, interest in reducing air pollution, and
the lack of electrical power in remote areas. It is,
however, to be noted that in spite of the aforesaid
favorable characteristics, the renewable energy
contribution to cover energy demand worldwide,
though increasing, is still marginal. Aside from the
hydroelectric energy, the other principal resources
(solar, wind, geothermal) cover together little more
than 1% of the energy production worldwide [9].
Owing to the diffuse nature of solar energy,
the main problems with the use of solar thermal
energy in large-scale desalination plants are the
relatively low productivity rate, the low thermal
efficiency and the considerable land area required.
However, since solar desalination plants are characterized
by free energy and insignificant operation
cost, this technology is, on the other hand,
suitable for small-scale production, especially in
remote arid areas and islands, where the supply
of conventional energy is scarce [28]. Apart from
the cost implications, there are environmental
concerns with regard to the burning of fossil fuels.
The coupling of renewable energy sources with
desalination processes is seen by some as having
the potential to offer a sustainable route for
increasing the supplies of potable water.
Solar energy can directly or indirectly be harnessed
for desalination. Collection systems that
use solar energy to produce distillate directly in the
solar collector are called direct collection systems
whereas systems that combine solar energy collection
systems with conventional desalination
systems are called indirect systems. In indirect
systems, solar energy is used either to generate the
heat required for desalination and/or to generate
electricity that is used to provide the required
electric power for conventional desalination plants
such as multi-effect (ME), multi-stage flash (MSF)
or reverse osmosis (RO) systems [16].
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2. Solar technologies
Different solar energy collectors may be used
in order to convert solar energy to thermal energy.
In most of them, a fluid is heated by the solar
radiation as it circulates along the solar collector
through an absorber pipe. This heat transfer fluid
is usually water or synthetic oil. The fluid heated
at the solar collector field may be either stored at
an insulated tank or used to heat another thermal
storage medium.
The solar collector may be a static or suntracking
device. The second ones may have one or
two axes of sun tracking. Otherwise, with respect
to solar concentration, solar collectors are already
commercially available; nevertheless, many collector
improvements and advanced solar technologies
are being developed. The main solar
collectors suitable for seawater distillation are as
follow.
2.1. Salinity-gradient solar ponds
This is a shallow pond with a vertical saltwater
gradient, so that the denser saltier water
stays at the bottom of the pond and does not mix
with the upper layer of fresher water. Consequently,
the lower salty layer gets very hot (70–85°C).
This heat can be used to make electricity (with
additional heating from traditional sources),
provide energy for desalination, and to supply
energy space heating in buildings. A solar pond
(SP) is a thermal solar collector that includes its
own storage system. A solar pond collects solar
energy by absorbing direct and diffuse sunlight.
It consists of three layers of saline water with
different salt concentrations. Salt-gradient solar
ponds have a high concentration of salt near the
bottom, a non-convecting salt gradient middle
layer (with salt concentration increasing with
depth), and a surface convecting layer with low
salt concentration. Sunlight strikes the pond surface
and is trapped in the bottom layer because
of its high salt concentration. The highly saline
water, heated by the solar energy absorbed in the
pond floor, cannot rise owing to its great density.
It simply sits at the pond bottom heating up until
it almost boils (while the surface layers of water
stay relatively cool)! The bottom layer in the
solar pond, also called the storage zone, is very
dense and is heated up to 100°C [22]. This hot
brine can then be used as a day or night heat
source from which a special organic-fluid turbine
can generate electricity. The middle gradient
layer in solar pond acts as an insulator, preventing
convection and heat loss to the surface. Temperature
differences between the bottom and surface
layers are sufficient to drive a generator. A transfer
fluid piped through the bottom layer carries heat
away for direct end-use application. The heat
may also be part of a closed-loop Rankine cycle
system that turns a turbine to generate electricity.
The annual collection efficiency for useful heat
for desalination is in the order of 10–15%. Larger
ponds tend to be more efficient than smaller ones
due to losses at the pond edge.
Solar ponds produce relatively low grade, less
than 100°C, thermal energy and are therefore generally
considered well suited for supplying direct
heat for thermal distillation processes. Due to their
ability to store energy, however, solar ponds are
also used to produce electricity. Solar ponds are
particularly well suited to association with desalination
plants as waste brine from desalination
can be used as the salt source for the solar pond
density gradient. Using desalination brine for
solar ponds not only provides a preferable alternative
to environmental disposal, but also a convenient
and inexpensive source of solar pond
salinity.
2.2. Flat-plate collector
Flat-plate collectors (FPCs) are used as heat
transfer fluid, which circulates through absorber
pipes made of either metal or plastic. The absorber
pipes are assembled on a flat plate and they usually
have a transparent protective surface in order to
minimize heat losses. They may have different
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selective coatings to reduce heat losses and to
increase radiation absorption. Thus the thermal
efficiency increases although the collector cost
also increase.
A typical flat-plate collector is an insulated
metal box with a glass or plastic cover and a darkcolored
absorber plate. The flow tubes can be
routed in parallel or in a serpentine pattern. Flat
plate collectors have not been found as a useful
technology for desalination [5,18]. Although they
have been used for relatively small desalinated
water production volumes, production of large
volumes of water would require an additional
energy source, for example, a desalination facility
in Mexico derives energy from flat plate collectors
and parabolic troughs.
2.3. Evacuated tube collector
Heat losses are minimized in evacuated tube
collectors (ETCs) by an evacuated cover of the
receiver. This cover is tubular and made of glass.
In addition, a selective coating of the receiver
minimizes the losses due to infrared radiation.
There are two different technologies of evacuated
tubes: (1) Dewar tubes two coaxial tubes made
of glass, which are sealed each other at both ends;
and (2) ETC with a metallic receiver, which
requires a glass to metal seal. There are different
designs depending on the shape of the receiver.
ETCs are set in conjunction with reflective surfaces:
a flat-plate or a low-concentrate reflective
surface as a compound parabolic one. Usually a
number of evacuated tubes are assembled together
to form a collector. Evacuated tube collectors
require more sophisticated manufacturing facilities
than flat-plate collectors. With evacuated tube
collectors, higher temperatures can be reached
and efficiencies tend also to be higher.
For the most part, however, evacuated tube
collectors are preferred to flat plate collectors.
Although the evacuated tube collectors are
more expensive, $300–$550/m2 as opposed to
$80–$250/m2 for flat plate collectors, less of them
and less land area would be needed for the same
level of energy production. Also, since evacuated
tube collectors produce temperatures of up to
200°C, they are particularly suited as an energy
source for high temperature distillation [5]. An
evacuated-tube collector generally consists of a
fluid-filled absorber tube surrounded by a vacuum.
2.4. Parabolic trough collector
A parabolic trough is a linear collector with a
parabolic cross-section. Its reflective surface concentrates
sunlight onto a receiver tube located
along the trough’s focal line, heating the heat
transfer fluid in the tube. Parabolic troughs typically
have concentration ratios of 10 to 100, leading
to operating temperatures of 100–400°C.
Parabolic trough collectors (PTCs) require sun
tracking along one axis only. In this way, the
receiver tube can achieve a much higher temperature
than flat-plate or evacuated-tube collectors.
The parabolic trough collector systems usually
include a mechanical control system that keeps the
trough reflector pointed at the sun throughout the
day. Parabolic-trough concentrating systems can
provide hot water and steam, and are generally used
in commercial and industrial applications [39].
Still, among solar thermal technologies, solar
ponds and parabolic troughs are the most frequently
used for desalination [40]. Due to the
high temperatures parabolic troughs are capable
of producing high-grade thermal energy that is
generally used for electricity generation [5]. Parabolic
troughs could be a suitable energy supply
for most desalination methods, but in practice,
have mainly been used for thermal distillation as
these methods can take advantage of both the heat
and electricity troughs produce. Other methods
of desalination would receive little or no benefit
from the heat produced. The unit cost of these
solar thermal energy production methods directly
increases with the temperatures they can yield.
As such, flat plate collectors and solar ponds are
the least expensive of these on a unit basis and
H.M. Qiblawey, F. Banat / Desalination 220 (2008) 633–644 637
parabolic troughs are the most expensive. Where
land is inexpensive then, solar ponds are preferred
due to their low cost and their ability to store
energy. This is why it is sometimes economical
to even produce electricity from solar ponds when
thermal energy cannot be used. Where land prices
are high or electricity or high temperatures are
needed, parabolic troughs are generally the preferred
source of solar thermal energy. Absolute
preferred methods, however, can be expected to
be highly site specific.
3. Direct solar desalination
The method of direct solar desalination is
mainly suited for small production systems, such
as solar stills, in regions where the freshwater
demand is less than 200 m3/day [17]. This low
production rate is explained by the low operating
temperature and pressure of the steam. Numerous
attempts have been made by many investigators
in order to produce freshwater by means of solar
energy. The simple solar still of the basin type is
the oldest method and improvements in its design
have been made to increase its efficiency [28].
3.1. Single-effect solar still
A solar still is a simple device which can be
used to convert saline, brackish water into drinking
water. Solar stills use exactly the same processes
which in nature generate rainfall, namely evaporation
and condensation. Its function is very
simple; basically a transparent cover encloses a
pan of saline water. The latter traps solar energy
within the enclosure. This heats up the water causing
evaporation and condensation on the inner
face of the sloping transparent cover. This distilled
water is generally potable; the quality of the
distillate is very high because all the salts, inorganic
and organic components and microbes are
left behind in the bath. Under reasonable conditions
of sunlight the temperature of the water
will rise sufficiently to kill all pathogenic bacteria
anyway. A film or layer of sludge is likely to
develop in the bottom of the tank and this should
be flushed out as often as necessary.
In order to evaporate 1 kg of water at a temperature
of 30°C about 2.4 × 106 J is required.
Assuming an insolation of 250 W/m2, averaged
over 24 h, this energy could evaporate a maximum
of 9 L/m2/day. In practice heat losses will occur
and the average daily yield which might be
expected from a solar still is 4–5 L/m2/day.
Today’s state-of-the-art single-effect solar stills
have an efficiency of about 30–40% [25].
Material selection for solar stills is very
important. The cover can be either glass or plastic.
Glass is considered to be best for most long-term
applications, whereas a plastic (such as polyethylene)
can be used for short-term use. Single-basin
stills have been much studied and their behavior
is well understood. Fath [14] has done a valuable
review of the latest development on this topic.
The daily amount of drinking water needed
by humans varies between 2 and 8 L per person
[34]. The typical requirement for distilled water
is 5 L per person per day. Therefore 2 m2 of still
are needed for each person served. The singlebasin
still is the only design proven in the field.
One of the main setbacks for this type of
desalination plant is the low thermal efficiency
and productivity. This could be improved by
various passive and active methods. The solar still
integrated with a heater or solar concentrator
panel is generally referred to as an active solar
distillation while others are referred to as passive
stills. Passive solar distillation is an attractive
process for saline water desalination in that the
process can be self-operating, of simple construction
and relatively maintenance free. These
advantages of simple passive solar stills however,
are offset by the low amounts of freshwater
produced, approximately 2 L/m2 for the simple
basin type solar still [41] and for the need for
regular flushing of accumulated salts [24]. Modifications
using passive methods include basin
stills, wick stills, diffusion stills, stills integrated
638 H.M. Qiblawey, F. Banat / Desalination 220 (2008) 633–644
with greenhouse, and other configurations. These
modifications will be briefly presented.
4. Modifications using passive methods
4.1. Basin stills
The operating performance of a simple basin
type passive still can be augmented by several
techniques such as
• Single slope vs. double slope basin stills: Single
slope still gave better performance than a
double slope still under cold climatic conditions
while the opposite is true under summer climatic
conditions [24].
• Still with cover cooling: Increasing the temperature
difference between the basin (heat source)
and the cover (heat sink) lead to increase the
water evaporation rate [20]. In stills with cover
cooling, cooling water or saline solution is fed
in the gap of a double glass cover to maximize
the temperature difference. The cost, as such,
is increased.
• Still with additional condenser: Fath [14] found
that adding a passive condenser in the shaded
region of a single slopped still increases the still
efficiency by 45%.
• Still with black dye: Injecting black dye in the
seawater increases the distillate yield [14].
4.2. Wick stills
In a wick still, the feed water flows slowly
through a porous, radiation-absorbing pad (the
wick). Two advantages are claimed over basin
stills. First, the wick can be tilted so that the feed
water presents a better angle to the sun (reducing
reflection and presenting a large effective area).
Second, less feed water is in the still at any time
and so the water is heated more quickly and to a
higher temperature. Tanaka et al. [36] have proven
the superiority of the tilted wick type solar still
and confirmed an increase in productivity by
20–50%. Simple wick stills are more efficient
than basin stills and some designs are claimed to
cost less than a basin still of the same output. A
simple multiple wick solar still made of a frame
of aluminum, a glass cover and a water reservoir
made of galvanized iron was designed by Sodha
et al. [32]. Foam insulation was supported beneath
the aluminum bottom by a net of nylon ribbon.
The authors claimed the present design to offer
several advantages including lightweight and low
cost of the still and a significant output.