The optimal solar capture surface for building heating: method, parameters and scenarios

Abstract

This study investigates the sizing of solar thermal systems used for space heating and domestic hot water production by introducing a key optimization parameter: the ratio between the building’s thermal loss coefficient and the installed solar collector area (H/SC). Using this indicator, the performance of a solar thermal installation serving a multi‑apartment building located in Climate Zone II (Bucharest) is evaluated. The system employs flat-plate collectors with kC = 3 W/m²K and operates under a low‑temperature regime (50/30°C). The analysis includes the determination of thermal transfer modules, monthly efficiency assessment, calculation of the solar coverage factor, and evaluation of the seasonal energy balance. Results show high solar contribution during transition months (March–April, September–October), often exceeding 100%, while winter performance remains limited (15–30%). Three sizing scenarios (H/SC = 1.3, 11.3, and 52.2) are compared, highlighting the trade‑offs between investment cost, energy yield, and risk of seasonal overproduction. The study concludes that optimal sizing is achieved when the system is dimensioned for transition months (October/April), whereas sizing for the critical winter month (December) becomes economically inefficient without long‑term thermal storage or auxiliary energy sources