## 2 Literature Review
### LHTES Systems in Greenhouses
#### 2.1.1 LHTES in Opaque Wall of Greenhouses
GH-20 PCMs are shape-stabilized solid-liquid PCMs constructed from high-density polyethylene, which supports the PCM while improving heat conductivity, and paraffin wax, which acts as a dispersed phase transition material. In greenhouse applications, the performance of PCMs is significantly influenced by the weather. In contrast to a cloudy day, a bright day will aid in promoting their effectiveness. The experimental setup's schematic and pictures, considered by [PERSON] et al. [26], are presented in Figure 10.  
Figure 8: Greenhouse components’ energy balance equipped with PCM [24]. Figure 10: The suggested experimental setup’s schematic by [PERSON] et al. [26]. Figure 9: The suggested experimental setup’s schematic by [PERSON] et al. [25]. It has been claimed that one effective way to improve the internal thermal climate of solar greenhouses is to apply a PCM to their north wall. Nevertheless, these kinds of walls always have thermally stable layers, significantly limiting their ability to store heat. [PERSON] et al. [27] proposed an active-passive ventilation wall with a PCM to address this problem, and a comparative analysis utilizing both experimental and numerical methods was carried out to show its advantages over conventional walls. The effectiveness of the wall's middle layer, the inner temperature environment, and plant development were all shown to be positively impacted by the recently suggested strategy, according to estimates or observations of several crucial qualities. The results demonstrated the proposed wall's high efficacy in raising the temperature of its irradiated surface and middle layer. The newly presented wall had no thermally stable layer, leading to a minimum 1.34 \({}^{\circ}\)C temperature rise. Additionally, the suggested solution increased the wall's capacity to store heat by 35.27-47.89% and release heat by 49.93-60.21%. As a result, the average increase in indoor air temperature, daily effective accumulative temperature, and soil temperature were 1.58-4.16 \({}^{\circ}\)C, 33.33-55.06%, and 0.53-1.09 \({}^{\circ}\)C, respectively. The fruit yield, stem diameter, and plant height increased by 28%, 25%, and 30%, respectively. The experimental setup's schematics suggested by [PERSON] et al. [27] are demonstrated in Figure 11.  
The wall's thermal characteristics significantly influence the climate inside a solar greenhouse, and crops grown inside can avoid freezing damage by enhancing the material's ability to retain heat. [PERSON] et al. [28] presented a unique greenhouse wall using PCMs and micro-heat pipe arrays (MHPAs) to increase the temperature of the internal temperature stabilization layer and the wall's heat storage and heat release capabilities. As a result, a normal greenhouse and an experimental greenhouse with the suggested wall were built. By combining MHPAs with a PCM, the heat transfer bottleneck caused by the PCM's low thermal conductivity and the traditional wall temperature stabilization layer is effectively resolved, increasing the amount of heat stored in the wall. This ensures crop development  
Figure 6:0 The suggested experimental setup’s schematic by [PERSON] et al. [27]. at night. The average daily total heat storage for the proposed wall was 18.89 MJ\(\cdot\)m\({}^{-3}\), compared to 9.67 MJ\(\cdot\)m\({}^{-3}\) for the conventional wall (95.35% growth). Moreover, the average heat release was 17.58 MJ\(\cdot\)m\({}^{-3}\), compared to 8.95 MJ\(\cdot\)m\({}^{-3}\) for a typical wall (96.42% growth), raising the nighttime air temperature and creating an environment favorable for crop development. The suggested system's schematics and pictures by [PERSON] et al. [28] are demonstrated in Figure 6:1