Multi-mode solar photovoltaic energy utilization system for Plateau buildings in rural areas

•A novel photovoltaic-based off-grid energy supply system is designed;•Optimal control strategy is proposed to minimize grid electricity consumption;•Indoor temperature is maintained above 16.3 °C during 75 % time;•The maximum comprehensive conversion efficiency of the PV system reaches 84.1%;•The photovoltaic self-consumption ratio reaches as high as 93.2% in March. Traditional solar thermal systems with water as the heat transfer medium generally encounter the freezing and overheating problems, which significantly increases the operational and management challenges of the energy systems, especially for remote rural households under extremely cold climates in Qinghai-Tibet Plateau. In the present study, a novel photovoltaic-based off-grid energy supply system is proposed to meet the lighting, heating and hot water demands for remote and dispersed rural households. The electric heating films convert the power generated by photovoltaic arrays into heat and further store in phase change material to meet the heating demand of the building throughout the day. While the surplus electricity is consumed by domestic hot water and lighting loads, and the energy stored in batteries can meet the lighting requirements during the nighttime. Field experimental results show that, the system achieves a 100 % heat supply from solar energy during extremely cold seasons. In case of no grid electricity consumption, the indoor average temperature in the heating room is 18.3 °C and maintained above 16.3 °C during 75 % of the time. Under the specified heat flux of the electric heating films, the phase change material can completely change the state within 4 h to significantly improve the thermal storage efficiency. The system flexibly releases the heat at night to control the indoor temperature fluctuations within 6.3 °C to effectively guarantee the thermal comfort. Under the heating conditions, the photovoltaic arrays adjust the power output in response to solar radiation, thus maintaining the minimum power generation efficiency and the maximum system conversion efficiency at 16 % and 84.1 %, respectively. During periods of weak solar radiation, the photovoltaic power is used for energy storage, or domestic hot water and lighting. The solar contribution to domestic hot water and lighting energy usage increases from 20 % during severe cold season to 65 % during non-heating season. It can be concluded that, the designed system can reduce building energy costs and significantly im