# 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
According information on the DOE website [99], the intention of the funding program is to develop solar thermal desalination technology, including addressing storage technology. The situation for very small installations, just one to several m\({}^{2}\) in size, is different, so discussed separate. Their design can be extremely simple, in a single setup and operating completely passively. A container is covered by a transparent window where the sunlight enters. The sunlight is absorbed at a dark surface, which gets heated thereby, and gives the heat to the water to be evaporated; often this is simply a basin. The water evaporated, which is then clean water, fills the container and is then condensed at the window, which is typically the coldest surface of the container interior. An inclination of the window makes the droplets of condensed clean water move along the window downward, to a spot where it is collected. The design can be varied with regard to the materials used, the shape and dimensions, and other things. Solar stills with the simplest possible design are commercially available from several manufacturers. Regarding R&D, to improve their efficiency and reduce the cost, a number of reviews are available. Refs. [100; 101; 102] give water yields up to 10 L/m\({}^{2}\) per day; reasonable, as the heat of evaporation of water is about 0.7 kWh/kg and for solar input of 1 kW for 12h. This shows a main weakness of the simple designs: recovery of the condensation heat to use it again. Multiple evaporator/condenser surfaces and heat regain, as in commercial plants, reach better yields. Ref. [103] developed a simple, yet multistage configuration, and reached in a small prototype with 10 stages a solar-to-vapor conversion efficiency of 385% with a production rate of 5.78 L/m\({}^{2}\) per hour. Another crucial weakness of solar thermal desalination is that solar heat is only available at sunshine. However, storage of solar heat to drive desalination at night can be done by thermal energy storage. Ref. [104] reviewed R&D on the use of PCM to improve the daily productivity of solar stills. They collected dozens of publications, tabulated the type of solar still, the PCM used, and main results. These comprise passive (all in one) as well as active (external source/collector... ) solar stills, with the PCM improving the yield in a wide range, varying with PCM used, integration approach, and still type. Ref. [105] made another review, covering an even bigger variety of systems. For simple stills, the maximum yield found was 8.1 L/m\({}^{2}\) per day. For active solar stills, yields up to 11 L/m\({}^{2}\) were found, in systems using evacuated tube collectors and external condensers. These values are still below or around the expected maximum of 10 L/m\({}^{2}\) per day, not surprising as the limiting factor is the solar energy absorbed per day. Except for one, all systems discussed had no multiple stages, were without heat regain, which seems to be crucial for further, significant yield gains. Worth mentioning is that already with the existing systems, estimated cost for freshwater were found to be in the order of 0.02$ per liter.