Solar Desalination

Solar desalination is a method that is used to remove salt and minerals from water, using solar energy.  The purpose is to make the water more suitable for human consumption. Only 3% of the earth’s water supply is fresh water. It is projected that by the year 2025, 3.5 billion people will be affected by water shortage (Walker, 2013).  Concentrated solar power (CSP) is the technology that is used by solar desalination plants. In this technology, reflectors are used to focus sunlight from a large area onto a smaller area in order to drive heat engines. Some concentrators focus sunlight on photovoltaic panels. Water evaporates due to the application of solar energy, condenses in the air, and then drips back into a collecting unit.

Two methods are used in solar desalinization, Reverse Osmosis and Solar Humidification-Dehumidification (HDH).  In the Reverse Osmosis method, water is driven across a semi-permeable membrane. Large facilities that produce both electricity and water use this method more. In the HDH method, the process involved is similar to the natural water cycle. Water is first evaporated, and then condensed in a separate unit.  Heat released from the condensation process is collected in some HDH systems, and can be re-used. This method is cost-effect for medium and small operations because solar collectors are not expensive.

Comparative review of desalination methods

Distillation and filtering are in use in many parts of the world. A problem with this method is scaling.  Dissolved solids in the water precipitate when water is heated over 160 degrees Fahrenheit. This precipitate forms a scale on the surface of the machinery. The scale in turn interferes with heat transfer and reduces the efficiency of the equipment.     

Multistage flash distillation is used by about 80% of desalination plants that are land based. In this procedure, the pressure is lowered to allow water to boil at a temperature below 160 degrees Fahrenheit. This prevents scaling. The process is cost effective in terms of energy use. It has a low investment cost, but has a very high cost of maintenance.

Reverse Osmosis, Forward Osmosis and Molecular osmosis are not as commonly used as the Multistage Flash distillation.  These methods use the process of osmosis, using either difference in electric potential or pressure, to effect desalination.

Dual purpose power plants produce both water and electricity. They have a disadvantage of creating a trade-off between water production and power production. The price of fuel affects the price of the products. Dual purpose power plants have a very high overhead.

Electrodialysis is a fairly new method still. Desalination is achieved using direct current, magnetism or alternating current.  Its advantage is that it leads to a simple separation and no change of the matter is effected.  But it is too expensive, especially for developing countries. It also uses a large amount of electricity.

Chemical desalination is a process that uses chemicals that have an affinity for the salts that are already dissolved in the water, to effect desalination. This method is even less common. It is used in emergencies and for just small quantities of water.

Solar evaporation panels (SEA) are somewhat similar to solar desalination plants. They have an advantage of being able to be configured so that they collect salt crystals, and do not give off toxic brine solutions. They are also cheaper than solar desalination units because they are made from plastic and are powered 100% by sun energy.  They do not have to be used with electrical infrastructures. This gives them a long term advantage of low costs over other methods.

Solar desalination plants have a different cost distribution from other methods.  They have a high initial investment cost.  The other disadvantage of solar desalination plants is that they occupy a lot of land and there is no economy of scale. After they are set up, however, the energy is free and the maintenance cost is very low

Environmental considerations in desalination

General environmental impacts of desalination include the fact that the ecology is affected when water is removed from a body of water.  The ecology is again affected when brine run-offs are returned to the ocean. Brine causes increased salinity. This can lead to habitat alteration and death of the indigenous aquatic species. It is especially damaging when it stays concentrated for a while in the ocean floor. Desalination facilities give off greenhouse emissions. Their location is also important because they occupy habitats and affect indigenous populations. With regards to solar desalination, most of these impacts are reduced or eliminated entirely. Solar desalination is a zero carbon emission process.  Toxic chemical run-offs are eliminated with solar desalination. Solar desalination requires a large amount of heat which can be a problem, as well as, excess heat is problematic to dispel into the atmosphere. This necessitates considerations for more efficient heat transfer surfaces in the plant.

Short and long term viability of solar desalination

Currently solar desalination plants are growing in use in different countries. A concluding observation is that solar desalination plants are a viable option for meeting water needs for the future.  They may be implemented either as stand-alone projects or, in conjunction with other methods that are sustainable, towards meeting current and future water needs. 

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