Application of heating system with active heat storage-release and heat pump in solar greenhouse

Abstract: The Chinese solar greenhouse has a unique greenhouse structure that regards solar energy as the main energy source, and has characteristics such as high efficiency, energy saving, and low cost. During a cold winter night, air temperature inside a solar greenhouse is low for crop growth, which would affect crop yield and quality, due to the heat-transfer characteristics and heat capacity limit of the north wall. In recent years, in trying to promote the heat storage capacity of the solar greenhouse, the thought of active heat storage-release came forward. Solar energy is a kind of clean renewable energy, but has intermittent and unstable performance when used for greenhouse heating. Meanwhile, the heat collecting efficiency of the solar thermal collector decreases with an increase in operating temperature. Thus, an active heat storage-release system (AHSRS) is difficult to use to ensure an appropriate temperature for a solar greenhouse in a frigid region or when it encounters weather conditions with weak solar radiation. As an efficient means of raising low-grade energy, the heat pump has been more and more applied to greenhouse heating which can reduce the operating temperature of the AHSRS when used in combination. In order to promote heating performance and stability of the AHSRS and improve air temperature inside a solar greenhouse at night, based on the concept of active heat storage-release, an active heat storage-release associated with heat pump heating system (AHSRHPS) applicable to solar greenhouse heating was designed in the present study. During the day, the solar energy reaching the north wall surface was absorbed by the circulating water and stored in reservoirs when the AHSRS was running. Running the heat pump unit was intended to promote low-grade heat energy and reduce the circulating water temperature which contributes to increasing the heat collecting efficiency of the AHSRS and maximum water temperature of the reservoir. When air temperature inside the solar greenhouse was low at night, the heat energy was released through the AHSRS. Tests for the AHSRHPS were carried on from 5 Dec. 2012 to 5 Feb. 2013. The results showed that when there was a sunny and cloudy day in winter, the air temperature inside the experimental greenhouse with the AHSRHPS was higher than that in comparative greenhouse both in the day and at night and the air temperature difference ranged from 5.26 to 6.64℃. In addition, the heating effect was more obvious when solar radiation was stronger during the day and the outdoor air temperature was lower at night. The coefficient of performance of the heat pump unit reached 4.38～5.17. The heat source temperature of the heat pump unit was ideal because of the sufficient heat supplied by the AHSRS and the outlet water temperature of the evaporator became the dominant factor influencing the COPHp of the heat pump unit, meanwhile, the COPHp value decreased with an increase in outlet water temperature of the evaporator. Under the specific thermal environment of the solar greenhouse, with running the heat pump unit for 1.5～3 hours per day, the heat collecting efficiency of the AHSRS increased to 72.32%～83.62%, and the heat collecting power was 156.26～258.05 W/m2. The COPSys of the overall system reached 5.59, and the energy-saving effect was obvious. Made from cheap materials, the active heat storage-release devices were much cheaper than traditional solar energy collectors. Compared with ground and water source heat pump units, the AHSRHPS doesn't need fan coil units or other heat dissipation facilities. Meanwhile, deep wells or buried pipes used as heat sources were never needed either. The high performance and low cost make AHSRHPS present a good application prospect.