Experimental investigation on heating performance of long- and short-term PCM storage in Chinese solar greenhouse

Phase change material (PCM) board on north wall of the greenhouses can absorb and store solar heat during the daytime, then releases it at night to provide the thermal environment for crop growth. However, its capability to regulate the greenhouse temperatures in severe winter is limited, necessitating an additional heat source. The heat gap within the greenhouse can be replenished flexibly and promptly by controllable triggering of supercooled salt hydrate for long-term heat storage. This study proposed a synergetic energy storage and release system, combining the short-term storage and passive release of the PCM wallboard with the long-term storage and active triggering solidification of the supercooled salt hydrate in separate storage unit. The temperature regulation effect of this synergetic system was experimentally investigated in a test Chinese greenhouse. The results showed that the combination of the PCM north wallboard with the supercooled salt hydrate storage unit reduced the daytime average temperature by 2.0 degrees C similar to 3.8 degrees C compared with the control greenhouse. In sunny days, the PCM north wallboard could meet the nighttime heat demand of the greenhouse with the average temperature rise of 5.2 degrees C similar to 6.7 degrees C. In severe weather, by triggering supercooled sodium acetate trihydrate (SAT, CH3COONa center dot 3H(2)O, T-m = 58.0 degrees C) at late night, the temperature rose by 5.3 degrees C in 1 h, effectively heating for 5 h. While supercooled sodium thiosulfate pentahydrate (STP, Na2S2O3 center dot 5H(2)O, T-m = 48.5 degrees C) was triggered, the temperature rose by 3.3 degrees C in 1 h, effectively heating for 4 h. Furthermore, the synergetic system was still effective for greenhouses under ventilation and irrigation conditions during the day. This study provides guidance for the development of synergetic PCM systems with long- and short-term solar thermal storage and release for greenhouse heating.