Experimental study on shallow soil assisted heat release-storage system with water-water heat pump in solar greenhouse

During cold winter night, the air temperatures inside solar greenhouses are low for crop growth, especially during the latter half of the night, due to the limited heat storage capacity of the greenhouses. Thus, in the present experiment, a solar and shallow soil assisted heat release-storage system with water-water heat pump was designed. During the daytime, the solar energy obtained from the greenhouse back wall was absorbed and released to the shallow soil using circulated water with a water pump. Before midnight, the heat energy stored in the soil automatically was released to the air inside the greenhouse duo to the temperature differences between the soil and air. When air temperature inside the greenhouse was lower than its set-point, the heat pump was used to exact the heat energy from the soil for heating the greenhouse air. The results showed that the coefficient of performance of the system was more than 3 even on a cloudy day and about 42% energy had been saved compared with conventional coal-fired hot water heating system. After covering the insulation layer, the average air and soil temperature was 3.2 and 3.3℃ higher inside the experimental greenhouse than that in the control greenhouse respectively due to the higher heat storage. After turning on the heat pump, the average air and soil temperature inside the experimental greenhouse was 5.7 and 2.9℃ higher than that in the control greenhouse, respectively.