California recently got relief from a multi-year drought. At present, the cities of Cape Town, South Africa and Sao Paulo, Brazil are coping with the effects of reduced rainfall. Ocean coastal cities such as Cape Town and indirectly Sao Paulo which is located near the coastal city of Santos, have the option of using seawater desalination to provide potable water for their populations. It is possible to combine compresed-air-energy-storage with reverse-osmosis desalination. A recent development involves placing ballasted giant plastic bags at greater depths of the seafloor, located offshore and with air-pressure pipelines connecting to the coastal compressor and generating installations.
During the overnight hours when demand for power is low, off-peak electric power drives the air compressors to recharge the submerged air bags. The heat-of-compression would push air temperature to well above the boiling point of water at atmospheric pressure . . . . . saturated water held under high pressure is an excellent high-temperature refrigerant that may be used as the working fluid to transfer the heat-of-compression into heat-of-fusion thermal energy storage. Compressed air held above a preset pressure can operate a reverse-osmosis (R-O) seawater desalination operation. If a coastal mountain is available as is the case at Cape Town, seawater may be pumped uphill during the overnight hours and force of gravity should provide sufficient pressure to operate R-O seawater desalination.
Researchers into grid-scale thermal energy storage have explored deep-level aquifer thermal energy storage, transferring heat from thermal power stations such as nuclear (and future radiation-free nuclear conversion involving fusion technology). Mega-scale (seasonal) underground thermal energy could sustain seasonal thermal power generation with the exhaust heat sustaining thermal seawater desalination.
When drought impacts inland locations, seawater desalination ceases to be a viable option. Adversely affected regions that have high humidity and zero rainfall are possible candidates for water-from-air extraction technology. At some locations, fog fences installed across mountain valleys can extract up to 6-quarts of potable water per square metre per hour, courtesy of prevailing winds. Off peak overnight electric power can operate refrigeration-cycle based water-from-air extraction technology, if overnight air temperatures are high. However, while some desert regions experience extreme daytime heat, night time temperatures can drop to near the freezing point of water. Off-peak electric power can be applied to use the low overnight temperatures to condense humidity from the air while simultaneously recharging a low-temperature thermal storage system.
The availability of low-cost, low-temperature thermal energy storage could allow for efficient operation of water-from-air extraction technology during the hours of peak humidity, which usually occurs after sunset and before air temperature plunges down to near the freezing point of water. Perhaps there might be opportunity, at some locations, for banks of liquid-cooled water-from-air extraction radiators that source the low temperature from cool-thermal-storage, to be installed across mountain valleys in order to funnel the humid air to the water extraction points, perhaps assisted by mega-scale fans. However, at drought stricken inland locations where energy-efficient, water-from-air extraction technology could operate, the price of water will be high.