## 2 Literature Review
### LHTES Systems in Greenhouses
#### 2.1.2 LHTES in Greenhouse-Solar Collector Hybrid System  
In some greenhouses, solar collectors are used in the vicinity of the greenhouse or inside the greenhouse, and the excess heat of the greenhouse, which is supplied by the sun's radiation, is absorbed by these collectors. Also, PCM storage is used inside these collectors, which stores the absorbed heat by phase change. At night, when the sun's radiation is unavailable, the heat stored by this material in the collector returns to the greenhouse environment. In the following, the previous works in the field of the applications of LHTES in greenhouse-solar collector hybrid systems are reviewed. [PERSON] [29] aimed to heat a greenhouse with a 180 m\({}^{2}\) floor area utilizing a seasonal TES system using paraffin wax as a PCM and the latent heat storage approach. Five primary components made up the system: (i) flat plate solar air collectors (as a heat collection unit),  
Figure 12: The suggested system’s schematics and pictures by [PERSON] et al. [28]. (ii) a latent heat storage (LHS) unit, (iii) an experimental greenhouse, (iv) a heat transfer unit, and (v) a data gathering unit. The outdoor heat collection unit comprised 27 m\({}^{2}\) of south-facing solar air heaters set at a 55-degree tilt angle. The steel tank utilized as the latent heat storage unit had a diameter and a total volume of 11.6 and 1.7 m, respectively. In total, 6000 kg of paraffin was used to fill the LHS unit or 33.33 kg of PCM per square meter of ground surface area in the greenhouse. The system efficiency was assessed using energy and exergy analysis. The LHS unit's heat transfer rate varied from 1.22 to 2.63 kW, while its rate of heat storage varied from 0.65 to 2.1 kW. The LHS unit transferred and stored thermal exergy at an average daily rate of 111.2 W and 79.9 W, respectively. The average net energy and exergy efficiencies over the trial period were determined to be 40.4% and 4.2%, respectively. During the charging phase, the impact of the heat transfer fluid's temperature differential at the LHS unit's input and output on the calculated energy and exergy efficiency values was assessed. The suggested experimental setup's schematics by [PERSON] [29] are demonstrated in Figure 13.  
[PERSON] and [PERSON] [30] examined the thermal performance of a PCM storage unit integrated into a greenhouse. The storage unit was part of a heating system of ten separate solar air collectors built for a greenhouse's interior heating and PCM charging. CaCl\({}_{2}\).6H\({}_{2}\)O, with a melting point of 29 \({}^{\circ}\)C, was employed as a PCM. Hot air from a ten-piece solar air collector was routed through the PCM to charge the storage unit. The outside air was heated using the stored heat before entering a greenhouse. The PCM used to examine the transient thermal behaviour of the storage unit during the charge and discharge periods produced experimental data that served as the foundation for this work. Comparing the suggested size of integrated PCM collectors to the traditional heating device, the recommended size of collectors delivered around 18-23% of the greenhouse's daily thermal energy consumption for 3-4 h. Moreover, the greenhouse's interior and outside temperatures differed by 6 to 9 \({}^{\circ}\)C due to the integration of solar air collectors and PCM systems for the greenhouse. The schematic for the experimental setup suggested by [PERSON] and [PERSON] [30] is illustrated in Figure 14.  
Figure 13: The suggested experimental setup’s schematic by [PERSON] [29]. Two comparable greenhouses with nocturnal shutters were created and placed in the CRTEn (Research and Technologies Centre of Energy) in Tunisia to minimize energy usage in agricultural greenhouses at night by [PERSON] et al. (2018) and [PERSON] et al. (2018). The first had a heating system installed. A collector for solar air heaters that retain latent heat is made up of the sun's heating system. A heating system was included with the first. Thermal solar energy was conserved throughout the day but replenished at night.