## 3 Thermal Aspects for Improving Energy Performance
### Maintenance of the Greenhouse Environment
#### 3.1.3 Passive Heating and Cooling Systems
[PERSON] et al. [132] investigated the application of dye-sensitized solar cells (DSSC) as shading in greenhouses, offering new perspectives for the manipulation of solar radiation. This study addresses the potential and effectiveness of DSSC as shading in greenhouses, evaluating its unique characteristics and advantages, especially in manipulating solar radiation through the optimal choice of photosensizers. Additionally, the document consolidates materials used in manufacturing the DSSC for greenhouse shading, detailing photo-sensitization and light collection within the PAR wavelength for sustainable plant growth. The study by [PERSON] et al. [134] addresses the impact of shading by photovoltaic panels in greenhouses oriented from north to south on tomato productivity in southeastern Spain. Evaluating three levels of shading (15%, 30%, 50%) and control without shading (0%), radiation, temperature, pH, substrate electrical conductivity, productivity and fruit quality were monitored. It was revealed that increased shading reduces available radiation, negatively affecting fruit productivity and quality, highlighting the need to optimize shading to balance agricultural production and energy generation. The study by [PERSON] and [PERSON] [135] explores a prototype photovoltaic greenhouse with variable shading, designed to optimize agricultural production and energy generation. By rotating the solar panels, internal shading is adapted according to climatic conditions, maintaining internal radiation at ideal levels for cultivation. Analyzing solar radiation throughout the year, with clear and partially cloudy skies, the study demonstrates how  
Figure 8: Greenhouses equipped with photovoltaic panels. (**a**) Greenhouse structure model equipped with normal photovoltaic panels; (**b**) Prototype greenhouse structure equipped with smart photovoltaic panels [133]. adjusting shading can efficiently regulate the indoor microclimate, satisfying the needs of cultivated plants. [PERSON] et al. [133] carry out a literature review on smart and solar covers on greenhouse roofs, focusing on smart photovoltaic (PV) systems, optimization of power conversion efficiency (PCE), and integration of the Internet of Things (IoT) to improve sustainability and reduce operating costs of commercial greenhouses. The research addresses advances in IoT systems and artificial neural networks for automation, the use of quantum dots and semi-transparent organic solar cells to improve energy efficiency, and the potential for energy cost savings of between 40% and 60% through the replacement of conventional sources with solar energy. The limited commercialization of emerging innovations such as transparent and semi-transparent PVs is emphasized, but the significant role of smart materials and smart energy harvesting in improving thermal regulation, preventing frost, and managing pests and diseases is highlighted, culminating in lower post-harvest losses and better agricultural yields. [PERSON] et al. [136] examine the integration of inorganic and organic photovoltaic (PV) technologies in greenhouses, highlighting the methodology of literature review, simulation studies and experimental work to evaluate energy, economic and environmental performances, in addition to the effects on crop growth. Explores the potential of organic solar cells, sensitized by dyes and perovskites, due to their semi-transparency and flexibility, allowing easy integration with existing or new greenhouse structures. The results highlight the great potential of these new PV technologies to promote sustainable, self-sufficient and intelligent greenhouses, with an emphasis on tuning the spectral characteristics of cells with the needs of plants, optimizing the use of solar energy and creating an environment conducive to crop growth, especially in hot and tropical regions. The main limitations in the use of the north wall in agricultural greenhouses focus on aspects such as the effectiveness of heat storage and release, influenced by the internal structure and material of the wall.