Performance of a water-circulating solar heat collection and release system for greenhouse heating using an indoor collector constructed of hollow polycarbonate sheets

A heating system is part and parcel of a greenhouse to maintain an interior air temperature proper for plant growth in cold winter. Various solar heating systems have been developed for greenhouse application, in which installing the solar collector indoors has become a common practice to capture indoor excess solar heat during daytime and release that heat at night for heating; thus, the greenhouse thermal comfort for day and night can be improved. To explore and achieve better solar energy utilization in greenhouses, we proposed a water-circulating solar heat collection and release system with an indoor collector constructed of hollow polycarbonate sheets. The collector also functions as a heating radiator during nighttime, thereby saving capital investment and simplifying system structure. Field tests were performed to investigate system's thermal and energy-saving performance by analyzing several core parameters. The daily average heat collection ratio was 72.1%. The minimum nighttime air temperature heated by the system was increased by 3.9 degrees C on average during six continuous winter days of severely adverse weather in Beijing. The heating coefficient of system performance for a Chinese solar greenhouse was 5.9 over a period of 34 days. The novel technology proposed in this study using superior solar collector construction and water-flow organization scheme enables a larger collector's effective heat-absorbing area than that of other water-circulating solar heat utilization systems of the same type, thereby producing increased thermal and energy-saving performance. (C) 2019 Elsevier Ltd. All rights reserved.