## 2 Literature Review
### LHTES Systems in Greenhouses  
#### 2.1.1 LHTES in Opaque Wall of Greenhouses  
Typically, greenhouse walls are made of materials that allow sunlight to pass through, such as glass or specialized greenhouse plastic. The orientation of the walls and the greenhouse can affect how much sunlight the plants receive throughout the day. North-facing walls generally receive less direct sunlight, while south-facing walls receive the most. Figure 6 illustrates the energy transfer processes within greenhouses [17]. During the daytime, solar radiation penetrates through the EVC film on the front roof and is absorbed by the inner surface of the north wall and the soil inside the greenhouse. Solar energy is then collected and stored within the interior of the north wall and soil. Figure 5: The graphical abstract of the present review paper to illustrate the applications of thermal batteries in greenhouses. As night falls, a thermal insulation curtain covers the front roof. The thermal energy stored in the north wall and soil is now utilized to heat the indoor air on the corresponding surfaces through natural convection. Simultaneously, the surfaces of the other walls are heated through radiation, which helps compensate for any thermal losses to the external environment. This process aids in maintaining a more stable and regulated temperature within the greenhouse during colder periods [18; 19; 20]. The orientation of a greenhouse is an essential consideration in its design and placement. The north wall of a greenhouse typically receives the least amount of direct sunlight throughout the day. This is because the sun is positioned in the southern sky for locations in the Northern Hemisphere. As a result, the north-facing wall of the greenhouse has the most minor exposure to direct sunlight compared to the other walls. The north wall of a greenhouse is often designed with specific features in mind [21; 22; 23]:  
* _Insulation_: since the north wall receives less sunlight, it may need to be better insulated to prevent heat loss and maintain a more stable temperature inside the greenhouse, especially during colder seasons. * _Fever openings_: to reduce heat loss, the north wall may have more irregular openings, such as doors and vents, than the other walls. * _Shade-tolerant plants_: due to reduced sunlight, shade-tolerant plants are often placed near the north wall of the greenhouse. * _Reflective surfaces_: some greenhouse designs incorporate reflective surfaces on the north wall to bounce available sunlight back into the greenhouse and improve overall light distribution. It is important to note that greenhouse design and construction can vary widely depending on the specific climate, plant types, and the structure's intended use. Therefore, while the north wall is a general reference point for orientation, actual greenhouse design considerations will depend on various factors specific to each situation. The researchers suggested and evaluated some methods to solve the problem of the north wall wasting the absorbed energy by the greenhouse. Some are applicable in storing energy inside the greenhouse and solving the greenhouse's waste behaviour. In a traditional greenhouse, the primary concern is capturing and retaining heat inside the greenhouse to maintain a suitable temperature for plant growth, especially during colder periods. Various methods are employed to manage heat distribution effectively and prevent excessive heat loss through the north-facing wall, shown in Figure 7 and explained as follows [21; 22; 23]:  
Figure 6: Energy transfer processes in a greenhouse [17]. * _Insulation_: The north wall is usually well insulated to reduce heat loss. Proper insulation helps to minimize temperature fluctuations and stabilize the greenhouse environment. * _Thermal mass_: Incorporating thermal mass materials, such as water barrels or stone walls (rock), inside the greenhouse can help store excess heat during the day and release it slowly during colder nights, helping to maintain a more stable temperature.