# Use of Phase Change Materials for Food Applications--State of the Art in 2022
## 2 Food Applications of PCM
### Food Production
#### 2.3.3 Solar Water Purification  
Water is crucial for us. We need some 2 L or more per day of water at high purity, just for drinking. And we need much more water for many other things, in the food area for cooking, dish washing, and of course for animals to drink, and for the irrigation of plants, the latter not requiring too high purity. Water is found in nature in different sources, directly from precipitation as rain, collected in rivers, temporarily stored in lakes and reservoirs, or for longer as groundwater, and last but not least the oceans. Depending on the source, the water can contain solid particles, dissolved chemicals like salts or gases, and biological matter like bacteria etc. Removing undesired substances from water to get water for consumption, safe to drink or for other purposes, is called water purification. Today, safe drinking water is still not sufficiently available in many developing countries, and during severe droughs can even be rare in developed countries. Thus, water purification is a topic of great and even increasing importance. A wide range of methods for water purification is available [95], for example physical methods like sedimentation, filtration through sand or membranes, and distillation, the addition of chemicals like in flocculation and chlorination, or the use of electromagnetic radiation, specifically UV. In a particular case, the best method depends on the purity required, impurities in the available water, then the suitability of purification methods for that case, and finally the availability and the economics. Important on a large scale is regarding impurities removing salts, specifically for water from the oceans, called desalination, and for drinking water removing harmful bacteria etc., called disinfection. Currently two technologies dominate: thermal desalination, where evaporation and later condensation are used for desalination, and reverse osmosis, where saltwater is pressed under high pressure through membranes which keep the salt back. The most common method of thermal desalination is using waste heat from power plants that run on fossil fuels. Because of that, there is a strong trend to use the reverse osmosis, preferably with electricity from solar or wind. However, reverse osmosis has problems with additional waste from chemicals needed to keep the installations working. Therefore, thermal desalination using solar heat comes in focus. According [96], in fossil driven thermal power plants the waste heat used is typically at temperatures between 65 \({}^{\circ}\)C and 115 \({}^{\circ}\)C. Solar thermal technologies could thus use cheap and efficient thermal energy storage, compared to electrical energy storage. Thus, water desalination plants working on the principle of solar thermal evaporation and later condensation are discussed and investigated, but still at a rather low level. Fossil fuel prices just increased recently. Literature research gave just one existing plant and a few research activities, and nothing on refurbishing fossil driven thermal desalination with a solar thermal heat source, as is discussed for coal power plants. Aalborg CSP [97] together with Alfa Laval developed a solar desalination system, combining their experience in solar power and desalination. A concentrated solar power (CSP) field produces hot water, which by a hot water storage tank allows to run the desalination unit 24 h a day. In the desalination unit, the hot water is used to evaporate seawater and then condensate the vapor, thereby producing freshwater. A system was installed in Australia in 2016, by Sundrop Farms, and produces according the website of Aalborg CSP 250,000 m\({}^{3}\) of fresh water per year. According an article published on the SolarPACES website [98], the US company Katz Water Technologies, founded 2016, developed an efficient thermal desalination unit, which won the first two stages of the DOE's stepped series of awards to develop a concentrated solar thermal (CST) technology.