Heat storage and release characteristics of the combined heat storage system for prefabricated solar greenhouses with flexible material wall

The integration of the active heat storage system utilizing multiple heat storage and release media for prefabricated solar greenhouses with flexible material wall (PGFMW) can solve the problem of "good thermal insulation and weak heat storage" in PGFMW. However, it is still unclear on the heat storage and exothermic properties of the multiple media in the combined heat storage system. This study aims to perform on a PGFMW that equipped with a combined "air source ground heat exchange-back wall water circulation (AGHE-WWC)" heat storage system. A systematic investigation was also made on the individual and combined heat storage and release properties. A series of experimental tests were conducted to calculate the energy transfer. The results indicated that the minimum temperature of indoor air at night time was maintained above 10℃, and the maximum difference in temperature between indoors and outdoors was 26.5 °C in winter. The primary heat storage medium was identified as the 0-50 cm soil layer of the cultivation area in the PGFMW. Furthermore, the heat storage capacity of the soil layer reached 706.30 MJ on sunny days, accounting for 63.5% of the total heat storage. The difference in temperature between the inlet and outlet of the underground heat exchange was up to 10.8 °C, which increased by 54.4% in the heat storage capacity of the soil layer. The water circulation was used to store the heat energy of 424.04 MJ on the day, which accounted for up to 45.1% of the total heat storage. Specifically, after cloudy days, the temperature of 8 m³ of water in the storage tank increased from 11 to 35℃ by water circulation over two sunny days. In order to evaluate the rate of heat storage and release for the system in combination and individually, an evaluation index called the heat storage and release efficiency ratio (HRE) was inducted to assess system performance. It was found that the maximum absolute value of the HRE for water circulation system was 1.62, which was 1.8 times than the soil HRE on the same day. It proved that the water circulation system releases heat more rapidly compared to the soil on cloudy days. In terms of the total heat, the soil was remained the primary source of indoor heat release on the consecutive cloudy days. In addition, the "AGHE-WWC" heat storage system was realized to fullfill the heat demand for a total of three consecutive cloudy days. Meanwhile, the average heating coefficient of performance (COP) and the uderground heat exchange COP were 9.16 and 6.82, respectively. The combined COP of the "AGHE-WWC" heat storage system was 8.85, while the energy saving rate was as high as 60.26%, compared with the heat pump. The finding can provide a strong reference for heat storage and release in combined heat storage systems.