## 3 Thermal Aspects for Improving Energy Performance
### Maintenance of the Greenhouse Environment
#### 3.1.3 Passive Heating and Cooling Systems
The concave-convex configuration of the wall can vary significantly in thermal performance, affecting heat storage capacity [123]. Honeycomb walls show better thermal insulation properties compared to flat ones, but the implementation of phase change materials (PCM) in the north wall, despite improving the thermal environment, faces the challenge of thermally stable layers that can reduce the thermal insulation capacity [126]. Shading in agricultural greenhouses, although effective in mitigating excessive heat in hot, sunny regions, has several limitations that include the significant reduction in solar radiation required for photosynthesis, negatively impacting crop development. Inadequate application of shading can lead to increased relative humidity, increasing the risk of fungal diseases, and uneven distribution of light within the greenhouse, affecting the uniformity of plant growth. Furthermore, shading strategies, such as the use of nets or reflective paints, can result in variations in internal temperature which, if not adequately managed with effective ventilation systems, may not provide the desired thermal relief or even increase thermal load. The correct selection and implementation of shading techniques, therefore, requires careful consideration of specific crop needs, local climatic conditions, and integration with other greenhouse microclimate management practices to ensure a balance between protection from excessive heat and the maximization of agricultural production [137; 138; 139]. On the other hand, ventilation systems play a crucial role in both heating and cooling systems in greenhouses, acting both passively and actively. Ventilation systems in greenhouses are essential for climate control, acting on both cooling and heating to maintain the ideal temperature for plant growth. In summer, they facilitate the expulsion of hot air and the entry of cooler air, while in winter, they help in the uniform distribution of hot air generated by heating systems. In addition to regulating temperature, ventilation is essential to remove excessive humidity, prevent plant diseases, and renew \(CO_{2}\), a vital nutrient for photosynthesis. Implementing ventilation, whether passive [140] or forced [141], is crucial to dissipate excessive heat, remove accumulated moisture, reduce the risk of fungal and bacterial diseases, and provide \(CO_{2}\) necessary for photosynthesis. Passive ventilation, also known as natural ventilation, as we can see in Figure 9a, takes advantage of wind forces and thermal differences to move air through the greenhouse, constituting a sustainable and economical option that does not require the use of electrical energy. Greenhouses designed for this type of ventilation have openings in the roof and sides, which can be adjusted manually or through automated systems to control the airflow [142; 143]. Alternatively, active ventilation also called forced ventilation, seen in Figure 9b, uses fans and other electrical devices to control the internal conditions of the greenhouse, offering more rigorous control of environmental variables. These systems are often integrated with thermostats and hygrometers, allowing automatic adjustment of temperature and humidity to maintain optimal growing conditions [141]. When designing ventilation systems for greenhouses, it is essential to consider the local climate, the type of plants grown, and the orientation and construction of the greenhouse. Ventilation must be evenly distributed to avoid areas of air stagnation, adjustable to meet the specific needs of plants at various stages of growth and in different climatic conditions [144]. The article by [PERSON] et al. [145] reviews advances in greenhouse technology, focusing on environmental control and the effectiveness of natural ventilation. It discusses various models for evaluating ventilation in greenhouses, and factors influencing ventilation effectiveness, and suggests areas for future research to develop dynamic ventilation models for solar and tunnel greenhouses in China.