Solar Desalination
Encyclopedia
Solar desalination is a technique to desalinate
water
using solar energy. Solar desalination in the modern era extends back to the early 1950s when simple solar still
s were studied for remote desert
and coast
al communities. However, because of inexpensive water pumps and pipelines and declining energy costs in the 20th century, solar stills have become less of a viable solution for these community-scale projects. This trend is currently reversed again, because all of the fossil resources required for 20th century desalination are becoming scarce. The future of desalination has to rely on the use of solar-energy only. Many desalination plants have begun retrofitting and new sustainable technologies, which can also serve as decentralized utilities are being developed by many specialized companies.
-driven process that forces the separation of fresh water from other constituents through a semipermeable membrane
. This is the preferred method in large-scale desalination implementations where electricity is cheaply available.
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 collector
s.
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.
Desalination
Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...
water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
using solar energy. Solar desalination in the modern era extends back to the early 1950s when simple solar still
Solar still
A solar still is a low-tech way of distilling water, powered by the heat of the sun . Two basic types of solar stills are box and pit stills. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic...
s were studied for remote desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
and coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...
al communities. However, because of inexpensive water pumps and pipelines and declining energy costs in the 20th century, solar stills have become less of a viable solution for these community-scale projects. This trend is currently reversed again, because all of the fossil resources required for 20th century desalination are becoming scarce. The future of desalination has to rely on the use of solar-energy only. Many desalination plants have begun retrofitting and new sustainable technologies, which can also serve as decentralized utilities are being developed by many specialized companies.
Types of solar desalination
Solar power can be used directly to desalinate water thermally or it can be converted into electricity. Electrically and mechanically driven systems utilize reverse osmosis.Reverse Osmosis
Reverse osmosis is a pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
-driven process that forces the separation of fresh water from other constituents through a semipermeable membrane
Semipermeable membrane
A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...
. This is the preferred method in large-scale desalination implementations where electricity is cheaply available.
Solar Humidification-Dehumidification
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 cycleWater cycle
The water cycle, also known as the hydrologic cycle or H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Water can change states among liquid, vapor, and solid at various places in the water cycle...
on a shorter time frame by evaporating
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
and condensing
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
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
Waste heat
Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature 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 collector
Solar collector
-See also:*Solar thermal collector*Solar water heating*Solar air heating*Photovoltaic module*Renewable heat*Concentrating solar power...
s.
Problems
There are two inherent design problems facing any 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 reliability.Secondly, the heat of condensation
Standard enthalpy change of vaporization
The enthalpy of vaporization, , also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas at a given pressure .It is often measured at the normal boiling point of a substance; although tabulated values are usually...
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
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 amount of energy required for desalination. For example, water at a pressure of 0.1 atmospheres boils at 50°C rather than 100°C.See also
- DesalinationDesalinationDesalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...
- Point Paterson Desalination PlantPoint Paterson Desalination PlantThe Point Paterson Desalination Plant is a planned municipal-scale solar-powered desalination plant with land-based brine disposal just outside Port Augusta, South Australia. The Point Paterson Project utilises a salt flat owned by a salt company but which has not been in use for solar salt...
- Solar Powered Desalination UnitSolar Powered Desalination UnitA Solar Powered Desalination Unit designed for remote communities has been tested in the Northern Territory of Australia.The Reverse Osmosis Solar Installation uses membrane filtration to provide a reliable and clean drinking water stream from sources such as brackish groundwater...
- Solar stillSolar stillA solar still is a low-tech way of distilling water, powered by the heat of the sun . Two basic types of solar stills are box and pit stills. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic...
- Seawater GreenhouseSeawater GreenhouseThe Seawater Greenhouse is a technology that enables the growth of crops in arid regions, using a greenhouse structure, seawater and solar energy...
External links
- Autonomous desalination in the Mediterranean: ADIRA
- European Solar Thermal Technology Platform, ESTTP. ESTTP
- Optimized solar thermal desalination system
- Network on renewable energy based desalination: Coordination Action - ADU-RES
- Solar Thermal Desalination SolarSpring
- SEA Panel — manufacturer of personal solar desalination systems
- European project supporting the use of renewable energy for powering desalination: ProDes
- SPX Global manufacturer of solar powered water systems for remote areas