Simplified heat-transfer calculation for the trapezoidal cross-section wall in Gobi solar greenhouses and greenhouse thermal-load prediction

Gobi solar greenhouses (GSGs) with trapezoidal cross-section walls, through the passive utilisation of solar energy, provide appropriate environmental conditions for sustainable crop growth in otherwise non-arable lands. The Hexi Corridor in China is known for its serious desertification and cold climate putting pressure on the regional GSG production; there, auxiliary heating strategies according to the greenhouse thermal load are required for maintaining plant thermal comfort. However, the highly transient heat transfer of the trapezoidal cross-section wall makes the GSG thermal-load evaluation challenging. This study proposes a new, simple, and accurate approach for calculating the dynamic heat transfer of the trapezoidal cross-section wall. The heat-transfer resistances quantifying the steady-state thermal behaviour of the wall and the unit frequency-response functions describing the transient one were obtained by using discrete Fourier series and a twodimensional frequency-domain finite-element model. These predetermined factors allowed the calculation of the hourly space-heat gain through the trapezoidal cross-section wall in a spreadsheet under arbitrary boundary conditions. The new approach was experimentally validated under different weather conditions. Furthermore, a GSG thermal load prediction model was developed. The results showed that, in a typical sunny phase, the heating load occurred around 8:00 and lasted four hours every day, averaging 0.56 GJ per day; in the coldest phase, the heating load occurred during the entire daytime and part of the night-time, averaging 1.42 GJ per day. This approach should be insightful regarding the heating-system planning during GSG design and environment-regulating judgement during GSG operation. (c) 2023 IAgrE. Published by Elsevier Ltd. All rights reserved.