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Articles of Interest
Solar desalination

Solar Desalination
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.

Methods
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/d.. 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 200m3/d.

Indirect solar desalination employs two separate systems; a solar collection array, consisting of either photovoltaic or fluid based collectors, and a separate conventional desalination plant. Production by indirect method is dependent on the thermal 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), Multiple Stage Flash Distillation (MSF), Multiple Effect Distillation (MED), Multiple Effect Boiling (MEB), Humidification Dehumidification (HDH), Reverse Osmosis (RO), and Freeze effect distillation.

History
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 potable form.

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, Carlos 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.[1] In the 1960’s 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.

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 desalination needs.

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.

Multiple Stage Flash Distillation (MSF)
Multiple 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 thermal methods.

Solar derivatives have been studied and in some cases implemented in small and medium scale plants around the world. In Margarita de Savoya, Italy there is a 50-60 m3/d 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/d. 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/d.

Production data shows that MSF solar distillation has an output capacity of 6-60 L/m2/d versus the 3-4 L/m2/d 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 maximum efficiency.