## 2 Literature Review
### SHTES Systems in Greenhouses
#### 2.2.3 Rock Bed as SHTES System  
Using a rock bed as the TES system involves capturing heat, frequently from solar or other renewable sources, and transferring it to rocks via a heat transfer fluid. Because of their adequate thermal capacity, rocks store this energy as perceptible heat. The stored heat is extracted and used to heat the area or water on demand. This method has advantages like long-term storage and durability but drawbacks such as delayed heat release and sufficient insulation. For effective implementation, site-specific factors and technological improvements must be carefully considered. Rock beds may be used efficiently in greenhouses to increase temperature control and energy efficiency. The granite bed acts as a thermal mass in this application, storing extra heat during periods of solar gain and gradually releasing it during cooler periods. The pebbles in the bed absorb heat throughout the day when the greenhouse receives enough sunshine. As temperatures fall in the evenings or during gloomy times, the stored heat is gradually released, assisting in maintaining a more steady and appropriate temperature for plant development. This thermal buffering effect can help to provide a more stable and regulated climate within the greenhouse, which reduces the need for extra heating or cooling equipment. Previous efforts on the use of rock beds, particularly in greenhouses, will be covered in the following sections. [PERSON] et al. [48], in 1996, presented an experimental investigation of sensible energy storage in a rock bed in horizontally arranged cylindrical ducts beneath the soil in a tunnel greenhouse. The study showed that the rock bed is practical by analyzing many recorded data, namely the rock bed's input and outflow temperatures and sun radiation. Compared to a conventional greenhouse (without TES), the greenhouse's temperature was raised due to the heat returned to it by the rock bed as the TES system. Plants may be sufficiently protected from the detrimental low temperatures that may occur in the Algiers region during the winter months by the heat supplied by the storage, which was stated as a temperature elevation of up to 7 \({}^{\circ}\)C over the outside temperature. The schematics of the proposed system by [PERSON] et al. [48] are demonstrated in Figure 28. In this figure, different parts of the proposed system are shown separately. It is worth mentioning that a blower is connected to the pipes installed in the rock bed to transfer the inside air of the greenhouse to the rock bed environment for heating or cooling purposes. Figure 27: The schematics of the proposed system by [PERSON] et al. [47]. [PERSON] et al. [49] researched the possibility of storing solar energy for greenhouse heating in an underground rock bed. Two identical polyethylene tunnel greenhouses, each with a 15 m\({}^{2}\) ground area, were used for the experiments. One of the greenhouses had two ditches dug and insulated in its soil; the ditches were filled with rocks. The greenhouse air was driven through the rock bed by a centrifugal fan with an airflow rate of 1100 m\({}^{3}\)/h. The fan was controlled by two thermostats that released or stored energy as needed. The greenhouses were not used to grow crops, and the vents were kept covered unless extremely humid conditions existed. This study showed that, even after a clear day, the rock bed system maintained the inside air temperature above the outside air temperature at night. The system's energy recovery or release efficiency was higher than 80%, despite its 34% solar energy collection efficiency. A numerical mathematical model accurately reflected real facts. An economic analysis indicates that the rock bed system is more economical than the fuel-burning heating systems commonly used in Turkish greenhouses, which burn LPG or petroleum. The rock bed system caused a 6-9 \({}^{\circ}\)C temperature difference between the air within and outside. The schematics of the proposed system by [PERSON] et al.