## 3 Thermal Aspects for Improving Energy Performance
### Maintenance of the Greenhouse Environment
#### 3.1.4 Active Heating and Cooling Systems  
As for active systems, they are characterized by the use of equipment and technologies that require external energy to operate, playing a crucial role in maintaining ideal conditions inside the greenhouse, regardless of external climate variations. Different active heating methods are adopted, each with its own particularities and efficiencies, including combustion, electric, hot water pipe systems, geothermal and solar heating. Combustion heating involves the use of gas, oil or biomass as fuels. These are burned in a boiler, generating heat which is then distributed evenly throughout the greenhouse  
Figure 9: Greenhouses with different forms of ventilation. (**a**) Greenhouse with natural ventilation system [147]; (**b**) Greenhouse with natural ventilation system [147]. via ducts or radiators. This method allows for rapid adjustment of internal temperature, being effective in maintaining ideal growing conditions even in cold climates, as detailed by [152; 153]. The electric heating system operates through resistances that convert electrical energy into heat. The advantage of this system lies in its ability to precisely control and uniformity of temperature distribution, facilitating the maintenance of a stable cultivation environment as pointed out by [154]. However, the operating cost can be high due to the price of electricity. Heating through hot water pipes works by circulating heated water inside pipes that run through the greenhouse. This system disperses heat efficiently and evenly, ensuring an ideal temperature distribution for plant development. The water is heated externally and its circulation through the tubes allows direct heat transfer to the greenhouse's internal environment [155]. The use of geothermal energy to heat or cool greenhouses takes advantage of the constant temperature below the Earth's surface. For heating, geothermal systems capture heat stored underground to increase the temperature inside the greenhouse during the colder months. Using heat pumps and circulation systems, it is possible to extract this heat and use it to heat the interior of the greenhouse. This technology offers a sustainable solution with low operational costs, as it uses a renewable and constantly available energy source [156; 10; 11; 157]. Solar heating, on the other hand, captures the sun's energy through solar panels. These convert solar energy into heat, which is then distributed within the greenhouse. This system not only reduces dependence on fossil fuels but also minimizes the environmental impact of greenhouse cultivation [158; 159]. It is a sustainable solution that takes advantage of a renewable and abundant resource to create an optimized growing environment, as illustrated in Figure 10a. Figure 10b highlights the practical application and effectiveness of geothermal technology in greenhouses, reinforcing its role as a viable and efficient alternative for sustainable greenhouse heating. [PERSON] et al. [10] investigated the impact of using ground-to-air heat exchanger (EAHE) systems in agricultural greenhouses, highlighting significant improvements in thermal performance and reductions in environmental impact. They experimentally found that EAHE can cut heating energy consumption by more than 40% and lower \(CO_{2}\) emissions by more than 100 g/h. EAHE efficiency fluctuates with the operating mode (heating or cooling) and specific conditions, such as the difference between the target temperature and the air temperature emitted by the EAHE, directly influencing both energy consumption and \(CO_{2}\) emissions. As for cooling, there are ventilation systems to regulate the temperature, some accompanied by exhaust fans to improve air circulation, using the temperature difference between the inside and outside to create an airflow that reduces internal heat. Adjustable thermal curtains help control the entry of light and heat, using materials that reflect solar radiation or thermally insulate, preventing excessive heat from penetrating the environment [110].