Solar desalination is a technique to desalinate water using solar
energy. There are two basic methods of achieving desalination using
this technique; direct and indirect.
3 Types of solar desalination
Multi-stage flash distillation
Multi-stage flash distillation (MSF)
4.1 Towered desalination plant built in Pakistan
4.2 Solar humidification–dehumidification
5 Problems with thermal systems
6 Solutions for thermal systems
7 See also
9 External links
In the direct method, a solar collector is coupled with a distilling
mechanism and the process is carried out in one simple cycle. Solar
stills of this type are described in survival guides, provided in
marine survival kits, and employed in many small desalination and
distillation plants. Water production by direct method solar
distillation is proportional to the area of the solar surface and
incidence angle and has an average estimated value of 3-4L/m2/day.
Because of this proportionality and the relatively high cost of
property and material for construction direct method distillation
tends to favor plants with production capacities less than
Indirect solar desalination employs two separate systems; a solar
collection array, consisting of photovoltaic and/or fluid based
thermal collectors, and a separate conventional desalination plant.
Production by indirect method is dependent on the efficiency of the
plant and the cost per unit produced is generally reduced by an
increase in scale. Many different plant arrangements have been
theoretically analyzed, experimentally tested and in some cases
installed. They include but are not limited to multiple-effect
humidification (MEH), multi-stage flash distillation (MSF),
multiple-effect distillation (MED), multiple-effect boiling (MEB),
humidification–dehumidification (HDH), reverse osmosis (RO), and
Indirect solar desalination systems using photovoltaic (PV) panels and
reverse osmosis (RO) have been commercially available and in use since
2009. Output by 2013 is up to 1,600 litres (420 US gal) per
hour per system, and 200 litres/day per square metre of PV
panel. Municipal-scale systems are planned.
Utirik Atoll in
the Pacific Ocean has been supplied with fresh water this way since
Indirect solar desalination by a form of
humidification/dehumidification is in use in the Seawater Greenhouse.
Methods of solar distillation have been employed by humankind for
thousands of years. From early Greek mariners to Persian alchemists,
this basic technology has been utilized to produce both freshwater and
medicinal distillates. Solar stills were in fact the first method used
on a large scale to process contaminated water and convert it to a
In 1870 the first US patent was granted for a solar distillation
device to Norman Wheeler and Walton Evans. Two years later in Las
Salinas, Chile, Charles Wilson, a Swedish engineer, began building a
direct method solar powered distillation plant to supply freshwater to
workers at a saltpeter and silver mine. It operated continuously for
40 years and produced an average of 22.7 m3 of distilled water a day
using the effluent from mining operations as its feed water.
Solar desalination of seawater and brackish groundwater in the modern
United States extends back to the early 1950s when Congress passed the
Conversion of Saline Water Act, which led to the establishment of the
Office of Saline Water (OSW) in 1955. The OSW’s main function was to
administer funds for research and development of desalination
projects. One of the five demonstration plants constructed was
located in Daytona Beach, Florida and devoted to exploring methods of
solar distillation. Many of the projects were aimed at solving water
scarcity issues in remote desert and coastal communities. In the
1960s and 70’s several modern solar distillations plants were
constructed on the Greek isles with capacities ranging from 2000 to
8500 m3/day. In 1984 a MED plant was constructed in Abu-Dhabi with
a capacity of 120 m3/day and is still in operation. In Italy, an
Open source design called "the Eliodomestico" by Gabriele Diamanti was
developed for personal use at the building materials price of $50.
Of the estimated 22 million m3 of freshwater being produced a day
through desalination processes worldwide, less than 1% is made using
solar energy. The prevailing methods of desalination, MSF and RO,
are energy intensive and rely heavily on fossil fuels. Because of
inexpensive methods of freshwater delivery and abundant low cost
energy resources, solar distillation has, up to this point, been
viewed as cost prohibitive and impractical. It is estimated that
desalination plants powered by conventional fuels consume the
equivalent of 203 million tons of fuel a year. With the approach
(or passage) of peak oil production, fossil fuel prices will continue
to increase as those resources decline; as a result solar energy will
become a more attractive alternative for achieving the world’s
Types of solar desalination
There are two primary means of achieving desalination using solar
energy, through a phase change by thermal input, or in a single phase
through mechanical separation. Phase change (or multi-phase) can
be accomplished by either direct or indirect solar distillation.
Single phase is predominantly accomplished by the use of photovoltaic
cells to produce electricity to drive pumps although there are
experimental methods being researched using solar thermal collection
to provide this mechanical energy.
Multi-stage flash distillation
Multi-stage flash distillation (MSF)
Multi-stage flash distillation
Multi-stage flash distillation is one of the predominant conventional
phase-change methods of achieving desalination. It accounts for
roughly 45% of the total world desalination capacity and 93% of all
Solar derivatives have been studied and in some cases implemented in
small and medium scale plants around the world. In Margarita de
Italy there is a 50–60 m3/day MSF plant with a salinity
gradient solar pond providing its thermal energy and storage capacity.
In El Paso, Texas there is a similar project in operation that
produces 19 m3/day. In Kuwait a MSF facility has been built using
parabolic trough collectors to provide the necessary solar thermal
energy to produce 100 m3 of fresh water a day. And in Northern
China there is an experimental, automatic, unmanned operation that
uses 80 m2 of vacuum tube solar collectors coupled with a 1 kW
wind turbine (to drive several small pumps) to produce 0.8 m3/day.
Production data shows that MSF solar distillation has an output
capacity of 6-60 L/m2/day versus the 3-4 L/m2/day standard output of a
solar still. MSF experience very poor efficiency during start up or
low energy periods. In order to achieve the highest efficiency MSF
requires carefully controlled pressure drops across each stage and a
steady energy input. As a result, solar applications require some form
of thermal energy storage to deal with cloud interference, varying
solar patterns, night time operation, and seasonal changes in ambient
air temperature. As thermal energy storage capacity increases a more
continuous process can be achieved and production rates approach
Towered desalination plant built in Pakistan
In 1993 a desalination plant was invented in Pakistan, producing 4
liters of water per square meter per day, which is at least ten times
more productive than a conventional horizontal solar desalination
plant. The structure is a raised tower made of concrete, with a
tank at the top. The whole plant is covered with glass of the same
shape, but slightly larger, allowing for a gap between the cement
tower and the glass.
The tank is filled with saline water and water from an outside tank,
drop by drop water enters the inner tank. The excessive water from the
inner tank drips out onto the cement walls of the tower, from top to
bottom. By solar radiation, the water on the wet surface and in the
tank evaporate and condense on the inner surface of the glass cylinder
and flow down onto the collecting drain channel. Meanwhile, the
concentrated saline water drains out through a saline drain.[citation
In this process fresh saline water is continuously added to the walls
from the top of the tower. After evaporation, the remaining saline
water falls down and drains out continuously. The movement of water
also increases the energy of molecules and increases the evaporation
process. The increase in the tower’s height also increases the
Whereas in the conventional system water that is filled remains at a
standstill for several days, a condenser is provided at the top in an
isolated space, allowing cold water to pass through the condenser. The
condensed hot vapors and hot water from the condenser are also thrown
on the cement wall.
This plant’s base is 3.5 by 1.5 by 10 feet (1.07 m
× 0.46 m × 3.05 m) high, and gives about 12
litres (3.2 US gal) of water per day. Built horizontally, a
structured plant receives solar radiation at noon only. But Zuberi’s
plant is a vertical tower and receives solar energy from sunrise till
sunset. From early morning, it receives perpendicular radiation on one
side of the plant, while at noon its top gets radiation equivalent to
the horizontal plant. From noon till sunset, the other side receives
maximum radiation.
By increasing the height, the tower plant receives more solar energy
and the inner temperature increases as height increases. Ultimately
this increases the water yield.
Different successive plants were constructed during the 1960s. A
number of experiments have been conducted and a much more productive
plant has been developed, with further work continuing.
This project can be implemented anywhere there is ground water, brine
or sea water available with suitable sun. During different experiments
a plant 6 feet (1.8 m) high can attain a temperature of
60 °C (140 °F), while a plant of 10 feet (3.0 m) high
can reach a temperature of up to 86 °C (187 °F).
Main article: Solar humidification
The solar humidification–dehumidification (HDH) process (also called
the multiple-effect humidification–dehumidification process, solar
multistage condensation evaporation cycle (SMCEC) or multiple-effect
humidification (MEH), is a technique that mimics the natural water
cycle on a shorter time frame by evaporating and condensing water to
separate it from other substances. The driving force in this process
is thermal solar energy to produce water vapor which is later
condensed in a separate chamber. In sophisticated systems, waste heat
is minimized by collecting the heat from the condensing water vapor
and pre-heating the incoming water source. This system is effective
for small- to mid- scale desalination systems in remote locations
because of the relative inexpensiveness of solar thermal collectors.
Problems with thermal systems
There are two inherent design problems facing any thermal solar
desalination project. Firstly, the system's efficiency is governed by
preferably high heat and mass transfer during evaporation and
condensation. The surfaces have to be properly designed within the
contradictory objectives of heat transfer efficiency, economy, and
Secondly, the heat of condensation is valuable because it takes large
amounts of solar energy to evaporate water and generate saturated,
vapor-laden hot air. This energy is, by definition, transferred to the
condenser's surface during condensation. With most forms of solar
stills, this heat of condensation is ejected from the system as waste
heat. The challenge still existing in the field today, is to achieve
the optimum temperature difference between the solar-generated vapor
and the seawater-cooled condenser, maximal reuse of the energy of
condensation, and minimizing the asset investment.
Solutions for thermal systems
One solution to the barrier presented by the high level of solar
energy required in solar desalination efforts is to reduce the
pressure within the reservoir. This can be accomplished using a vacuum
pump, and significantly decreases the temperature of heat energy
required for desalination. For example, water at a pressure of 0.1
atmospheres boils at 50 °C (122 °F) rather than
100 °C (212 °F). . However, the realization of a domed
reservoir as proposed in the article in ref is impossible. The
atmospheric pressure creates a force of 9 metric tons/square meter (
the difference between 1 atm outside and 0.1 atm inside) and so the
total vertical force applied by the dome on the concrete wall (without
any weight of the dome itself!) should be about 7.2 million tons on
this wall, that means 2300 tons per meter of the concrete wall. More:
it is impossible to build a dome able to resist.
Solar Energy Vacuum Seawater Desalination.
^ a b García-Rodríguez, Lourdes; Palmero-Marrero, Ana I.;
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^ "Trunk size solar desalination unit"
^ "Container size solar desalination unit"
^ "Al-Khafji plant"Arab News item 2013
^ "Utrik RO unit a big success"Marshall Islands Journal Jan 17th 2014
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^ National ENERGY GLOBE Award Pakistan
^ The MEH-method (in German with english abstract): Solar Desalination
using the MEH method, Diss. Technical University of Munich
^ "Archived copy". Archived from the original on 2008-12-21. Retrieved
2008-11-05. Large scale Solar
Desalination using Multi Effect
Autonomous desalination in the Mediterranean: ADIRA
European Solar Thermal Technology Platform, ESTTP. ESTTP
Network on renewable energy based desalination: Coordination Action -
SEA Panel — manufacturer of personal solar desalination systems
European project supporting the use of renewable energy for powering
SPX Global manufacturer of solar powered water system