Theoretical development and experimental validation of a thermal model comparing different greenhouse covering materials

Abstract

This study compared greenhouse covering materials for small to mid-scale greenhouse producers in cold regions. Small gothic greenhouses commonly use polyethylene, resulting in significant plastic waste due to the need for replacement every 3 to 5 years. To address this issue while minimizing heating loads, new covering materials with improved durability and energy efficiency created from recycled products (e.g., polymethyl methacrylate) are being developed. Potential energy savings should be assessed since their spectral and thermal properties may positively impact both solar gains and heat transfer. A comparison between conventional (e.g., polyethylene) and alternative covering materials (e.g., polycarbonate and polymethyl methacrylate) was then carried out through numerical modeling written in Python This model takes detailed parameters into account: crops, construction and covering materials, greenhouse configurations, and localization. It uses hourly weather data including temperature, humidity, atmospheric pressure, cloud cover, wind speed, and solar irradiance. The model calculates heat losses and gains through the roof, walls, perimeter, and ground, considering longwave and shortwave radiation, conduction, convection, infiltration, and energy sinks and sources induced by plant evapotranspiration or environmental control systems. Results indicated that the model effectively predicts the heating of a double polyethylene-covered greenhouse located in the province of Quebec, Canada. The simulation of the same greenhouse covered with a polymethyl methacrylate revealed that heat loads can be reduced by 8.5 %. The thermal analysis also showed that, heat used in ventilation for dehumidification could represent 29 % of all energy consumption. This study enlightens several ways to improve sustainability of the greenhouse industry regarding energy consumption and plastic waste.