Solar desalination

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Solar_desalination

Solar desalination is a technique to produce water with a low salt concentration from sea-water or brine using solar energy. There are two common methods of solar desalination. Either using the direct heat from the sun or using electricity generated by solar cells to power a membrane process.

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 3–4 litres per square metre (0.074–0.098 US gal/sq ft). 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 200 m³/d (53,000 US gal/d).

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.^[2] 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 (53 US gal) per day per square metre of PV panel.^[5][6] 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. 

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, 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 1970s several modern solar distillations plants were constructed on the Greek isles with capacities ranging from 2000 to 8500 m³/day.^[3] 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 desalination is predominantly accomplished in a solar-powered desalination unit, which uses photovoltaic cells that 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.

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).

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 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. 

Single-phase solar desalination

In indirect, or single phase, solar-powered desalination, two different technological systems are combined: a solar energy collection system (e.g. through the use of photovoltaic panels) and a proven desalination system such as reverse osmosis, are combined. Single phase solar desalination is predominantly accomplished by the use of photovoltaic cells that produce electricity to drive pumps used for reverse osmosis desalination. However, alternative experimental methods are being researched, which use solar thermal collection to provide mechanical energy to drive the reverse osmosis process. 

Solar-powered reverse osmosis

In reverse osmosis desalination systems, seawater pressure is raised above the natural osmotic pressure, forcing pure water through membrane pores to the fresh water side. Reverse osmosis (RO) is the most common desalination process in terms of installed capacity due to its superior energy efficiency compared to thermal desalination systems, despite requiring extensive water pre-treatment. Furthermore, part of the consumed mechanical energy can be reclaimed from the concentrated brine effluent with an energy recovery device.

Solar-powered RO desalination is common in demonstration plants due to the modularity and scalability of both photovoltaic (PV) and RO systems. A detailed economic analysis and a thorough optimisation strategy of PV powered RO desalination were carried out with favorable results reported. Economic and reliability considerations are the main challenges to improving PV powered RO desalination systems. However, the quickly dropping PV panel costs are making solar-powered desalination ever more feasible.

While the intermittent nature of sunlight and its variable intensity throughout the day makes PV efficiency prediction difficult and desalination during night time challenging, several solutions exist. For example, batteries, which provide the energy required for desalination in non-sunlight hours can be used to store solar energy in daytime. Apart from the use of conventional batteries, alternative methods for solar energy storage exist. For example, thermal energy storage systems solve this storage problem and ensure constant performance even during non-sunlight hours and cloudy days, improving overall efficiency.