## 2 Literature Review
### LHTES Systems in Greenhouses
#### 2.1.1 LHTES in Opaque Wall of Greenhouses
* _Reflective surfaces_: Some greenhouse designs use reflective materials on the north wall to bounce available sunlight back into the greenhouse, indirectly increasing the overall light and heat levels. * _Efficient heating systems_: In colder climates, supplemental heating systems may be used to provide additional heat during extremely cold periods. These systems are typically placed strategically to distribute heat evenly throughout the greenhouse. * _Proper ventilation_: Adequate ventilation is essential to prevent excessive heat buildup on warmer days. Ventilation systems, such as ridge vents and side vents, can help regulate the temperature and avoid overheating. * _Location and orientation_: Choosing the right place and orientation for the greenhouse can also influence its exposure to sunlight and, consequently, its heat management. Advanced greenhouse technologies, such as active solar heating, geothermal systems, or heat exchangers, might also be used to optimize energy efficiency and heat distribution. The following is a review of previous works on the applications of LHTES in opaque greenhouse walls. The thermal performance of a north wall constructed with a PCM as a storage medium in an east-west-oriented greenhouse was examined and addressed by [PERSON] et al. (2019). As a PCM, CaCl\({}_{2.6}\)H\({}_{2}\)O was employed. To explore the influence of the PCM on greenhouse temperature and humidity, a numerical thermal model was built that considered the greenhouse's different components (cover, plants, inside air, and north wall PCM) and was based on the greenhouse heat and mass balance. Calculations were performed for the average decade climate of January in Morocco. The outcomes demonstrate that with 32.4 kg of PCM per square meter of the greenhouse's ground surface area, the plants' and inside air temperatures were found to be 6-12 \({}^{\circ}\)C higher at night in the winter with fewer  
Figure 7: The employed techniques for preventing heat loss through the north wall of the greenhouse (2019, 2020, 2021). oscillations. At night, the relative humidity was determined to be generally 10% to 15% lower. The components' energy balance in the greenhouse equipped with a PCM, presented by [PERSON] et al. [24], is illustrated in Figure 8.  
Because of a thermally stable layer inside the wall caused by the limited heat transfer properties of building materials, the solar energy collected by the wall in passive solar greenhouses is insufficient to heat the interior environment. [PERSON] et al. [25] proposed and introduced a novel system that combines an active-passive triple-phase change material wall (APTPCMW) with solar concentrators to address this issue. An experiment was devised and carried out to explore the active heat storage capability of the APTPCMW. The experiment showed how much the newly proposed system may improve the intermediate layer of the APTPCMW's heat storage capacity. The APTPCMW's optimal operating settings were 0.4 m between air tunnels, downward flow direction for heated air within the tunnel, 0.26 m/s supply air velocity, and 60 \({}^{\circ}\)C supply air temperature. The experimental setup's schematic and pictures suggested by [PERSON] et al. [25] are demonstrated in Figure 9.  
[PERSON] et al. [26] conducted a study using both experimental and computational methods to assess the impact of PCMs on the indoor thermal environment of greenhouses under various weather conditions and over an extended period in the heating season. The study was carried out in a typical greenhouse in Beijing, China, and significant parameters, such as the indoor air temperature, outdoor air temperature, solar radiation, surface temperature of greenhouse envelopes, and soil temperature, were regularly recorded for 61 days. The effectiveness of PCMs in greenhouses was assessed using several indicators, including the operative temperature, daily effective accumulative temperature, irradiated surface temperature of the north wall, average temperature of PCMs, and daily heat storage and release.