Using solar-assisted biogas for space heating in rural China: An optimization case study

•Solar -assisted biogas for space heating was investigated.•An optimization method based on matching supply and demand is proposed.•Balancing supply and demand by adjusting hydraulic retention time was studied.•1/3 of the heating cost can be saved through optimization. While biomass feedstock is readily available in rural areas of China, offering considerable potential for biogas utilization, the predominant use of biogas systems is limited to cooking rather than heating. This limitation arises from the mismatch between the high demand for heating in winter and the relatively low biogas production rates, creating a supply–demand disparity. The primary objective of this study is to enhance the heating reliability of the biogas system while simultaneously reducing the overall heating system cost. To achieve this goal, we propose an optimized operational strategy and design method centred around the concept of supply–demand matching. Our approach involves implementing a passive solar greenhouse-assisted centralized biogas production and decentralized heat supply system. The size of the biogas plant is tailored to meet specific user demands, with user demand regulation achieved through biogas storage capacity. Optimization techniques include feed time and insulation quilt control, leveraging waste heat recovery, as well as biogas storage optimization through adjustments in hydraulic residence time. The optimized design of the representative project demonstrates a significant reduction in the construction cost of the digester and storage tanks, which was 1/3 less compared to the pre-optimization period. Considering government subsidies, the price of biogas was only 54% of the local natural gas price, substantially reducing the overall cost of heating.