-Solar desalination is a desalination technique powered by solar energy. The two common methods are direct (thermal) and indirect (photovoltaic).
Method
Solar desalination is a technique that harnesses solar energy to convert saline water into fresh water, making it suitable for human consumption and irrigation. The process can be categorized based on the type of solar energy source utilized. In direct solar desalination, saline water absorbs solar energy and evaporates, leaving behind salt and other impurities. An example of this is solar stills, where an enclosed environment allows for the collection and condensation of pure water vapor. On the other hand, indirect solar desalination involves the use of solar collectors that capture and transfer solar energy to saline water. This energy is then used to power desalination processes such as Humidification-Dehumidification (HDH) and diffusion-driven methods.
Direct
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 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, distillation tends to favor plants with production capacities less than 200 m3/d (53,000 US gal/d).
Single-effect
This uses the same process as rainfall. A transparent cover encloses a pan where saline water is placed. The latter traps solar energy, evaporating the seawater. The vapor condenses on the inner face of a sloping transparent cover, leaving behind salts, inorganic and organic components and microbes.
The direct method achieves values of 4-5 L/m2/day and efficiency of 30-40%. Efficiency can be improved to 45% by using a double slope or an additional condenser.
Indirect
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.
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.
Inherent design problems face thermal solar desalination projects. First, the system's efficiency is governed by competing heat and mass transfer rates during evaporation and condensation.
Second, 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 solar stills, this heat is emitted as waste heat.
Solutions
Heat recovery allows the same heat input to be reused, providing several times the water.
One solution is to reduce the pressure within the reservoir. This can be accomplished using a vacuum pump, and significantly decreases the required heat energy.
For example, water at a pressure of 0.1 atmospheres boils at 50 °C (122 °F) rather than 100 °C (212 °F).
Single-phase solar desalination
In indirect, or single phase, solar-powered desalination, two systems are combined: a solar energy collection system and a desalination system such as reverse osmosis (RO). The main single-phase processes, generally membrane processes, consist of RO and electrodialysis (ED). Single phase desalination is predominantly accomplished with photovoltaics that produce electricity to drive RO pumps. Over 15,000 desalination plants operate around the world. Nearly 70% use RO, yielding 44% of desalination. Alternative methods that use solar thermal collection to provide mechanical energy to drive RO are in development.
Reverse osmosis
RO is the most common desalination process due to its efficiency compared to thermal desalination systems, despite the need for water pre-treatment. Economic and reliability considerations are the main challenges to improving PV powered RO desalination systems. However, plummeting PV panel costs make solar-powered desalination more feasible.
Solar-powered RO desalination is common in demonstration plants due to the modularity and scalability of both PV and RO systems. An economic analysis that explored an optimisation strategy of PV-powered RO reported favorable results.
PV converts solar radiation into direct-current (DC) electricity, which powers the RO unit. The intermittent nature of sunlight and its variable intensity throughout the day complicates PV efficiency prediction and limits night-time desalination. Batteries can store solar energy for later use. Similarly, thermal energy storage systems ensure constant performance after sunset and on cloudy days.
Batteries allow continuous operation. Studies have indicated that intermittent operations can increase biofouling.
Batteries remain expensive and require ongoing maintenance. Also, storing and retrieving energy from the battery lowers efficiency.
Reported average cost of RO desalination is US$0.56/m3. Using renewable energy, that cost could increase up to US$16/m3. Although renewable energy costs are greater, their use is increasing.
