An Active Heat Storage-Release Unit Assisted with a Water-Water Heat Pump for Heating in a Chinese Solar Greenhouse

During cold winters, the air temperature after midnight in a Chinese Solar Greenhouse is usually very low and cannot meet the need of crop growth. To increase the night-time air temperature, an active heat storage-release unit assisted with water-water heat pump system (AHSHP) was studied in a Chinese solar greenhouse (G(hp)). This system consisted of a solar collector, heat-storage device, a water pump and a heat pump. The solar collector was fixed on the North wall and the heat storage device was fixed at the depth of 0.6 m from the soil of the G(hp). During the daytime, the water circulating pump was turned on, and the solar collector absorbed the solar radiation, with the circuit transferring the heat to the shallow soil. At night, when the air temperature in the greenhouse was lower than 15 degrees C, the heat pump was used for transferring the heat from the soil to the air in the greenhouse. The results showed that the average daily COP was 3.9 when the air temperature inside the greenhouse ranged from 14.7 degrees C to 16.1 degrees C and the outside temperature ranged from -5 degrees C to -11 degrees C during 2:00 am similar to 8: 00 am. The average air temperature in the G(hp) was 5.5 degrees C higher than that in the control greenhouse (G(c)) and the soil temperature at the depth of 0.6 m decreased from 15.0 degrees C to 13.3 degrees C, while that above the depth of 0.3 m was almost unaffected by the heat pump during that time. The above results indicated that the AHSHP is an effective method to increase the night-time air temperature in Chinese Solar Greenhouses.