Performance investigation of a solar heating system with underground seasonal energy storage for greenhouse application

This study reports the performance of a demonstrated 2304 m2 solar-heated greenhouse equipped with a seasonal thermal energy storage system in Shanghai, east China. This energy storage system utilises 4970 m3 of underground soil to store the heat captured by a 500 m2 solar collector in non-heating seasons through U-tube heat exchangers. During heating seasons, thermal energy is delivered by the heat exchange tubes placed on the plants shelves and the bare soil. The system can operate without a heat pump, which can save electricity consumption and further enhance the solar fraction. It was found that in the first operation year, 331.9 GJ was charged, and 208.9 GJ was later extracted for greenhouse space heating. No auxiliary heating equipment was installed so that solar energy covered all the heating loads directly or indirectly. It was demonstrated that this system was capable of maintaining an interior air temperature that was 13 °C higher than the ambient value when the latter temperature was −2 °C at night. The ECOP (electrical coefficient of performance) of the first operation year was approximately 8.7, indicating a better performance than the common heat pump heating system.