Solar desalination is a desalination technique powered by solar energy. The two common methods are direct (thermal) and indirect (photovoltaic).
History
Solar distillation has been used for thousands of years. Early Greek mariners and Persian alchemists produced both freshwater and medicinal distillates. Solar stills were the first method used on a large scale to convert contaminated water into 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, Swedish engineer Charles Wilson began building a solar distillation plant to supply freshwater to workers at a saltpeter and silver mine. It operated continuously for 40 years and distilled an average of 22.7 m3 of water a day using the effluent from mining operations as its feed water.
Solar desalination in the United States began in 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. OSW's main function was to administer funds for desalination research and development projects. One of five demonstration plants was located in Daytona Beach, Florida. Many of the projects were aimed at solving water scarcity issues in remote desert and coastal communities. In the 1960s and 1970s several distillation plants were constructed on the Greek isles with capacities ranging from 2000 to 8500 m3/day. In 1984 a plant was constructed in Abu-Dhabi with a capacity of 120 m3/day that is still in operation. In Italy, an open source design called "the Eliodomestico" by Gabriele Diamanti was developed for personal costing $50.
Of the estimated 22 million m3 daily freshwater produced through desalination worldwide, less than 1% uses 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 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.
Methods
In the direct (distillation) method, a solar collector is coupled with a distilling mechanism. 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 is proportional to the area of the solar surface and solar incidence angle and has an average estimated value of . Because of this proportionality and the relatively high cost of property and material for construction, distillation tends to favor plants with production capacities less than .
Indirect desalination employs a solar collection array, consisting of photovoltaic and/or fluid-based thermal collectors, and a separate conventional desalination plant. Many arrangements have been analyzed, experimentally tested and deployed. Categories include multiple-effect humidification (MEH), multi-stage flash distillation (MSF), multiple-effect distillation (MED), multiple-effect boiling (MEB), humidification–dehumidification (HDH), reverse osmosis (RO), and freeze-effect distillation.
Indirect solar desalination systems using photovoltaic (PV) panels and reverse osmosis (RO) have been in use since 2009. Output by 2013 reached per hour per system, and per day per square metre of PV panel. Utirik Atoll in the Pacific Ocean has been supplied with fresh water this way since 2010.
Indirect solar desalination by a form of humidification/dehumidification is in use in the seawater greenhouse.
Indirect
Large solar desalination plants typically use indirect methods. Indirect solar desalination processes are categorized into single-phase processes (membrane based) and phase change processes (non-membrane based). Single-phase desalination use photovoltaics to produce electricity that drive pumps. Phase-change (or multi-phase) solar desalination is not membrane-based.