## 2 Literature Review
### SHTES Systems in Greenhouses
#### 2.2.2 Underground Thermal Energy Storage (UTES) System as SHTES System  
The UTES technique includes storing and recovering thermal energy from underground geological formations. This method enhances energy efficiency and sustainability by enabling the storage of extra energy for later use. UTES is classified into three primary categories [42]:  
* _Aquifer Thermal Energy Storage (ATES)_: This involves injecting surplus thermal energy, usually from sources like solar collectors or industrial processes, into an underground aquifer during periods of low energy demand. The stored heat is then extracted for space heating or other applications when needed, typically during colder periods. * _Borelole Thermal Energy Storage (BTES)_: BTES systems use a network of boreholes drilled deep into the ground to store and retrieve thermal energy. Like ATES, excess heat is stored in the ground during warm periods and extracted during colder periods. Heat exchange fluid circulating through the boreholes facilitates the transfer of thermal energy. * _Seasonal Thermal Energy Storage (STES)_: STES involves storing thermal energy for more extended periods, typically from one season to another. This can be achieved using  
Figure 23: The schematics of the proposed system by [PERSON] et al. [41] (red line: hot water–blue line: cold water). Figure 22: The schematics of the proposed system by [PERSON] et al. [40]. **(A)** 3D view, **(B)** 2D view (front), and **(C)** 2D view (side). underground structures, such as large rock or water reservoirs, where heat is stored during warmer seasons and extracted during colder periods. BTES proves highly beneficial in greenhouses by efficiently managing seasonal heating and cooling demands. The technology allows for storing excess thermal energy during periods of surplus, such as hot summer days, and its retrieval during colder periods, offering a sustainable and reliable heating source. By reducing the reliance on conventional energy sources, BTES helps minimize operational costs and contributes to environmental sustainability. It enables consistent temperature regulation for optimal plant growth, improves crop yields, and can be integrated with various renewable energy sources, aligning with greenhouse operators' goals to enhance energy efficiency and reduce environmental impact. Additionally, BTES is valuable for off-grid applications, providing a self-sufficient energy solution for remote greenhouse locations [43]. Horizontal Borehole Thermal Energy Storage (HBTES)  
[PERSON] et al. [44] emphasized the usage of heat pumps connected to solar collectors and HBTES heat exchangers in both computational and experimental studies to maximize the use of renewable energy sources for greenhouse heating. Their research illustrated how well a heat pump system performed in Tunisia's climate with help from solar and geothermal energy. They presented that solar and geothermal energy-assisted heat pumps can save conventional energy and compete with traditional heating systems in heating mode. Also, because the drops in ground temperature during the experimental tests did not surpass 1 \({}^{\circ}\)C, the HBTES heat exchanger may be utilized successfully for heating greenhouses. The schematics of the proposed system by [PERSON] et al. [44] are depicted in Figure 24.  
Figure 24: The schematics of the proposed system by [PERSON] et al. [44]. Over the last few decades, solar and geothermal energy have been routinely employed to heat greenhouses. However, solar energy was inaccessible at night, and geothermal energy was frequently provided at a significant capital cost. [PERSON] et al. [45] suggested a unique Geothermal Battery Energy Storage (GBES) system that combines solar heat storage with geothermal energy to heat a greenhouse. The system was evaluated in three different modes of operation. The GBES-based greenhouse had a temperature that was 3.7 \({}^{\circ}\)C higher than a traditional greenhouse in Mode A.