Performance testing on warming effect of heat storage-release metal film in Chinese solar greenhouse

Abstract: The Chinese Solar Greenhouse (CSG) that is widely used in North China is characterized by a lean-to south-facing roof, a removable insulating blanket and a solid north wall. The south facing roof structure and removable insulating blanket maximize the exposure to short-wave radiation during the day and minimize heat loss at night, respectively. To increase the year-round greenhouse production in North China, a sustainable heating method needs to be developed to increase the night air temperature during the winter in CSGs. Solar heating is an inexpensive and effective way to heat greenhouses, and has been investigated by several previous studies. For the present study, a heat storage-release metal film system that was attached to the north wall was developed for CSG night temperature improvement. Two experimental greenhouses were located in Beijing, China, with a floor area of 304 m2 each. Environmental parameters (temperature, humidity, heat flux) inside and outside the greenhouse were investigated, including the average solar collection efficiency of the heating system and the energy saving rates. The results showed that the average solar collection efficiency of the system was 83%, 1.6 times greater than the reported value of a heat storage-release metal film system installed in a small CSG. The energy collection efficiency during the daytime decreased sharply with declining plate-air temperature differences. To have high energy collection efficiencies, plate-air temperature differences must be kept high and this can be achieved by applying a heat pump to reduce the circulating water temperature and transfer the energy to another water tank. The effective collector absorptivity was 0.81 and heat transfer was by natural convection. During the relatively cold nights of December 23 and 24 with the lowest outdoor air temperature of approximately -18℃, the inside air temperature of the experimental CSG also was 3.7℃ higher than in the reference CSG after starting operation of the heat storage-release metal film system. The night air temperature in the experimental CSG was increased by 2.4℃ on average compared to the reference CSG. The performance of the heat storage-release metal film system can be analyzed via the collected and released heat. The variations of the total heat collected and released by the heat storage-release metal film system during the day/night periods investigated were presented together with the radiation sum at the back wall over the total area of the heat storage-release metal film system. The system collected much more energy during sunny days than during cloudy days. The utilization ratio of the collected heat of the heat storage-release metal film system was calculated and it was between 64% and 71%. So the collected heat was not utilised completely during the night as some heat was lost during transport and storage. The heat collecting efficiency for these days was calculated as 86%, 82% and 82%, respectively which was nearly constant. So the use of the heat storage-release metal film system for heating the greenhouse at night during the winter can improve the environmental conditions inside Chinese solar greenhouses for crop production, achieving high energy collection efficiency and a reduction in energy consumption.