## 3 Thermal Aspects for Improving Energy Performance
### Maintenance of the Greenhouse Environment
#### 3.1.3 Passive Heating and Cooling Systems  
Passive heating and cooling systems in greenhouses are efficient and sustainable methods for controlling the temperature within these structures, taking advantage of natural resources and physical phenomena, without the need for active external energy sources such as electricity or fossil fuels. These systems are designed to maximize the use of solar energy, wind, and the thermal properties of the soil to create an ideal growing environment. The north wall, seen in (Figure 7b) acts as a passive form of greenhouse heating technology, absorbing solar heat during the day due to high thermal capacity materials such as concrete, stone or barrels of water. This heat is gradually released at night, maintaining a more stable temperature. Its strategic location maximizes solar exposure in other areas of the greenhouse, reducing the need for external energy sources and contributing to sustainability. Furthermore, it promotes thermal stability, benefiting plant growth. This method, together with openings for natural ventilation, (Figure 7a), demonstrates how design strategies can be used to maximize thermal efficiency and promote sustainability in greenhouses, reducing dependence on external energy sources and optimizing plant growth. The use of the north wall in agricultural greenhouses faces several limitations that directly impact its thermal and energy performance. Studies identify that the internal configuration of the wall, such as concavo-convex structures, significantly influences the heat storage and release capacity, with honeycomb walls presenting better thermal insulation and heat storage properties compared to flat walls [123; 124]. Furthermore, the choice of materials, including the use of phase change materials (PCM), can improve energy efficiency, although the presence of thermally stable layers can reduce the heat storage capacity of the north wall [125; 126]. Adequate thermal insulation of the north wall is also crucial to reduce heating demand, potentially reducing the need for external energy by up to 31.7% [87]. Therefore, optimizing the design and materials of the north wall is critical to maximizing the thermal efficiency and sustainability of agricultural greenhouses. In addition to the strategic use of the north wall for passive heating, other complementary techniques are essential for optimizing passive heating and cooling systems in greenhouses. For example, the use of glass roofs or translucent materials allows sunlight to pass through, which is converted into heat inside the greenhouse, a principle known as the greenhouse effect. During the colder months, this helps keep the greenhouse warm without the use of active heating systems. To prevent overheating during the warmer months, automated or manual shading systems can be implemented. Figure 7: Greenhouse with gable roof (Even-span). (**a**) Greenhouse with ventilation opening; (**b**) Greenhouse with north wall. Shading in agricultural greenhouses is a technique used to control the intensity of solar radiation that reaches plants grown inside the greenhouse. This can be conducted using different materials [91], such as shading screens as we can see in (Figure 8) [127] or special paints applied to greenhouse covers [128]. Shading may be necessary to reduce excessive solar radiation and, consequently, the temperature inside the greenhouse, especially in places with a hot climate. Furthermore, shading can help control plant transpiration, increasing relative air humidity and reducing water stress [129]. However, it is important to remember that shading can also negatively affect plant growth and development if the light intensity is reduced excessively [130; 131]. Therefore, it is necessary to adjust the shading rate according to the specific needs of the cultivated plants and the climatic conditions of the region. The field of shading in agricultural greenhouses is addressed by several studies, each exploring different methods and their implications for agriculture in regions of high solar radiation.