# Use of Phase Change Materials for Food Applications--State of the Art in 2022
## 2 Food Applications of PCM
### Food Production
#### 2.3.2 Greenhouses
They report reductions in temperature swing by up to 6 \({}^{\circ}\)C, fuel savings up to 48%, and payback periods reported are between 3 months and 6 years. Another recent review [87] discusses the integration of PCM in passive systems, by integration of PCM in wallboards on greenhouse walls or storage containers, and active heating systems, by combining PCM with heaters, heat pumps, and solar thermal collectors. Commonly, TES is by sensible heat, using rock beds, water reservoirs, or underground pipes. The paper has an extensive literature on TES with PCM used in greenhouses reviewed. It lists a few publications dealing with the combination with heaters and heat pumps, showing significant reductions in energy consumption, and payback period between 4 months and 8 years. For the integration with solar thermal collectors, the analyzed publications show a significant increase in nighttime greenhouse temperature by the use of solar energy stored in the PCM during daytime, by up to 6 \({}^{\circ}\)C. Regarding the use of PCM wallboards, increased storage capacity and a wide range of nighttime temperature increase from 0.8 \({}^{\circ}\)C up to 12 \({}^{\circ}\)C was found. If there is no wall, PCM can be stored in containers distributed in the greenhouse, even in the root zone of the plants. For the use of PCM in storage containers, temperature reduction in daytime and increase in nighttime were up to 7 \({}^{\circ}\)C. The authors conclude that selection of the PCM, its melting temperature, quantity, encapsulation, and type of integration, and that appropriate design parameters (like greenhouse size, shape, orientation, heat transfer coefficient of the envelope...) should be selected, taking into account climatic conditions and control, to achieve significant saving of energy and cost. Ref. [88] performed such a study, with eight different locations which have dry climate and mainly a cooling load for most time of the year, using an HVAC system which is solar driven absorption refrigeration system, and varying the PCM temperature between 21 \({}^{\circ}\)C and 27 \({}^{\circ}\)C.  
While there is a large number of R&D publications, there seems to be yet rather little economic activity. Ref. [89] describes experimental testing in small greenhouse in Asheville, North Carolina, USA. Initially, the greenhouse was experiencing very high wall temperatures, much higher than the ambient. To reduce the excessively high wall temperature, a 22 \({}^{\circ}\)C PCM was installed at the walls, which resulted in a reduction of the afternoon peak temperature from 47 \({}^{\circ}\)C to 35.5 \({}^{\circ}\)C. It is interesting that the second test of commercial products also was in an attempt for limiting/reducing extreme temperatures, but instead for overheating it was with respect to the opposite of overheating, meaning protection from frost. Frost damage by freezing in plant tissues at temperatures below 0 \({}^{\circ}\)C can be severe as a single occurrence can destroy a harvest, or plants in general. But it is not only a problem in unprotected agricultural areas, but generally a problem in nature. It is therefore not surprising that nature throughout evolution has found countermeasures. Basically, these are lowering the freezing point as well as avoiding nucleation of water in plant tissues, or more straightforward, avoiding cooling to subzero temperatures. And here, some plants are using the latent heat of water outside the plant tissue which when freezing releases its latent heat and thereby protects the plant itself from freezing [90; 91]. Amongmany other methods, the use of the latent heat of water when freezing is also an effect used by humans. As described by [92] (CHAPTER 7--ACTIVE PROTECTION METHODS) in detail. Its commercial use by sprinklers is e.g., described in [93]. The use of commercial, encapsulated PCM from RGEES, for frost prevention, describes [94]. Experiments were done on a farm, using pouches hung in a tree and also flat containers on the ground.