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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. Sunlight may provide heat for evaporative desalination processes, or for some indirect methods, convert to electricity to power a membrane process.
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 200m3/day.
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 freeze-effect distillation.
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 2010.
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 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, 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 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.
Multi-stage flash distillation (MSF)
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 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/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 maximum efficiency.
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.
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 production.
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).
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 rates 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 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
Efficient desalination systems use heat recovery to allow the same heat input to provide several times the water than a simple evaporative process such as solar stills.
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).
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