Page - 68 - in Clean Water Using Solar and Wind - Outside the Power Grid

68 Clean Water Using Solar and Wind: Outside the Power Grid Distillation requires a substantial amount of heat energy and must be coupled with other heat-producing applications. Distillation has mainly been developed in oil- and gas-producing countries, where it can be linked to thermal power facilities and thus use the heat they emit to produce evaporation. The advantage of this technique is that it does not require special pre-treatment of the water before its evaporation. Today distillation is done more elaborately, and the most common method is multistage flash distillation (MSF) where the water is heated and the pressure decreased so that the water “flashes” into steam. MSF  requires large amounts of energy to produce fresh water (typically 12–15 kWh and sometimes as much as 25 kWh per  m3). Two  types  of  energy are required for the operation of a thermal desalination plant: • Low-temperature heat, which is the main portion of energy input, • Electricity, which is used to drive the system’s pumps. Solar PV or wind power can be used to power the pumps. This may require 3–5 kWh per m3. The other thermal desalination process is multiple effect distillation (MED). Here the water passes through several evaporators in series. Vapour from one series is subsequently used to evaporate water in the next. The MSF and MED technologies are industrial processes suitable for large-scale operations. They are very expensive for small-scale operations. MED is more efficient than MSF. Thermal desalination is still the dominating technology in the Gulf countries and North Africa, but globally membrane-based methods are now the most common ways to desalinate seawater. Distillation plants generate less waste (called brine) than membrane- based methods like reverse osmosis (RO), and there are no filters or membranes to get clogged. The brine issue is further examined below. The energy used by desalination systems may be in the form of either work (such as electricity) or heat (normally as low-temperature steam). These forms of energy are distinct and cannot simply be added to find a “total” energy requirement. Because of the second law of thermodynamics electric energy can be converted to heat, but low- temperature heat cannot be converted to electric power. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user