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Types of Indirect solar desalination
Large solar desalination plants are usually driven by indirect methods. Indirect solar desalination processes to extract water is categorized into single-phase processes (membrane based) and phase change processes (non-membrane based). Single-phase desalination uses photovoltaic cells from the solar desalination unit to produce electricity that drive pumps. Phase-change (or multi-phase) solar desalination is a non-membrane based indirect desalination system.

Single-phase desalination processes include reverse osmosis and membrane distillation, where membranes are required for water to flow through. Reverse osmosis (RO) makes up about 52% of all indirect solar desalination methods. Pumps push salt water through RO modules, selected depending on salt water concentrations, at high pressure. RO systems operate based on pressure differences. The higher the operating pressure, more energy is required to pump water. To filter out seawater, 55-65 bar pressure is required. An average of 5 kWh/m3 of energy is typically required to run a large-scale RO plant. Membrane distillation (MD) utilizes pressure difference from two sides of a microporous hydrophobic membrane. Fresh water can be extracted through four MD methods: Direct Contact (DCMD), Air Gap (AGMD), Sweeping Gas (SGMD) and Vacuum (VMD). An estimated water cost of $15/m3 and $18/m3 is generated for medium-scale solar-MD plants. Energy consumption using MD methods range from 200-300 kWh/m3. Experimental methods using solar thermal collection to provide this mechanical energy is currently being researched.

Phase-change (or multi-phase) solar desalination is a thermal, non-membrane based indirect desalination system. This includes multi-stage flash, multi-effect distillation (MED), and thermal vapor compression (VC). It is accomplished by using phase change materials (PCMs) to maximize latent heat storage and high temperatures. MSF phase change temperature range between 80°C - 120°C, 40°C - 100°C for VC and 50°C - 90°C for the MED method. Multi-stage flash (MSF) method is the second largest desalination contributor after RO. MSF processes require seawater to travel through a series of vacuumed reactors held at successively lower pressures. Additional heat is added to seawater to capture latent heat of vapor. As seawater flow through the different reactors, steam is collected which is then condensed to produce fresh water. In Multi-effect distillation (MED), seawater flows through successively low pressure vessels and reuses latent heat to vaporize seawater for condensation. MED desalination requires lower energy consumption compared to MSF due to higher efficiency in thermodynamic transfer rates.