# 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  
Large parts of the world have distinct seasons. Often, for a part of the year growing plants for food production in the natural ambient is not possible, or limited to a selection of plants. This limitation can be avoided by growing plants in an artificial environment, e.g., in a house. This way, sometimes even plants are grown which have no suitable season in a region at all. Early forms of an artificial environment were ordinary brick or wooden houses with a larger area of glass windows for solar irradiation of the plants, and some additional means of heating. Nowadays, common is the use of simple metal frames, and transparent plastic sheets are often used instead of glass windows. Sometimes even the sunlight is replaced by lamps. Thus, the term \"greenhouse\" nowadays generally refers to a constructed enclosing which creates an artificial environment, designed for growing of out-of-season or out-of-region plants. It assures the right climatic or better growing conditions (temperature, humidity, solar irradiation,... ), artificially, for growing the plants in an otherwise unsuitable or at least not optimum environment. Common is the use of greenhouses in cold, or in dry climates. Their size ranges from small private ones for self-consumption to large ones for industrial production. To get an idea of the scale of the use of greenhouse constructions, in the Netherlands, in 2017 greenhouses occupied nearly 5000 ha = 50 km\({}^{2}\)[83], and in 2018 their surface area in Andalucia, a region of Spain, covered even 35.489 hectaresa [84]. To assure optimum conditions, besides temperature, humidity, and irradiation, also ventilation is needed. Ventilation supplies fresh air for photosynthesis, can remove plant pathogens, and allows the access of pollinators like bees to the plants. Means for supplying heat or cold, keeping humidity, for solar irradiation, and for ventilation, in many ways affect each other. For example, solar irradiation also supplies heat, ventilation can supply heat or cold depending on the outside air conditions... And plants vary a lot in their optimum growing conditions. This makes the design and operation of greenhouses a complex topic, with obviously no single solution. This is also reflected in the R&D publications. Already in 1978, a book described the technology of solar greenhouses in detail [85], including the storage of heat by sensible heat in the ground, walls, or barrels filled with water, and as an upcoming technology paraffins and salthydrates as PCM. They can be used completely passive, or with some active air motion, and just buffer temperature swings. Especially in cold climates, heating is often crucial and commonly done actively by burning fossil fuels which causes considerable operation costs. Thus, heating by free solar heat is desirable, which requires thermal energy storage to shift excess heat from days with sunshine to days without as well as nights (instead of ventilating excess heat away). Also, if using waste heat from elsewhere, or if using a heat pump, thermal energy storage can help. Ref. [86] reviewed the use of PCM in greenhouses, specifically their choice and way of application. They reviewed a large list of R&D publications, regarding the PCMs used, their properties and stability, and how the PCMs are applied in greenhouses, the performance and parametric sensitivity, from experiments as well as numerical models. The types of PCMs are mainly paraffins and fatty acids, and the salhydrates CaCl2-6H2O and Na2 SO4-10H2O, which cover melting points between 2 \({}^{\circ}\)C and 70 \({}^{\circ}\)C. They found several types of applications: heating/cooling with the PCM integrated into the north wall, integrated in a heat exchanger, or directly in the solar thermal system. Generally, the performance in completely passive systems is with regard to temperature swings, while in active heating systems there is usually a setpoint and performance is as energy savings.