Abstract:A plastic tunnel covered with thermal blanket (hereafter referred to as “PTET”) is type of plastic tunnel with large span. This facility has the advantages of low construction cost, high land utilization efficiency and large space, compared with Chinese solar greenhouse, which was widely applied in Northern China for vegetable production over winter. Hence, PTET has been employed as a substitute for Chinese solar greenhouse in recent years. However, there are no passive heat storage walls in PTET. It is necessary to supply auxiliary heating in PTET to maintain high indoor air temperature (Tin) and meeting the requirements of the indoor vegetables. To solve the problem, a thermal collecting and releasing system developed with fancoil units (TSFU) was employed to avoid high cost and air pollution caused by the application of conventional heating methods. The objective was to analyze the thermal performances of TSFU in PTET. In this experiment, TSFU was composed of 15 fancoil units hanging beneath the ridge, a water tank, a water pump and several pipes. By circulating water within the system, TSFU collected the surplus air thermal energy in daytime and released them in nighttime to heat the PTET. According to the test, the thermal energy released at nighttime (Qr) in solar days and cloudy days were (433.0±48.6)MJ and (199.3±0.1)MJ, respectively. Qr in the cloudy day was just 46.0% of that in the solar days, due to low Tin in the daytime. As a result, Tin in the nighttime of solar days and cloudy days were elevated by (2.5±0.4)℃ and (1.1±0.3)℃, respectively. The coefficient of performance (COP) of TSFU in solar days and cloudy days were 2.9 and 3.1, respectively. Compared with the air heater using natural gas as the fuel, the heating cost of PTET can be decreased by 40.2% by employing the TSFU. Besides, a dynamical model for simulating the temperature of water in the water tank was developed and used to analyze the factors affecting the thermal performances of TSFU. With the simulation results, it was found that, by increasing the overall heat exchange efficiency (η) and numbers (n) of fancoil units to 0.44 and 25, respectively, Qr in the solar days can be increased by (67.4±14.9)% and (76.1±14.7)%, respectively, due to enhanced heat exchange rate of TSFU. Under the above conditions, Qr in the cloudy days was increased by (14.9±7.6)% and (17.0±4.5)%, respectively, as well. COP was raised over 36±06. On the other hands, by increasing the water volume in the tank (V) to 52m3,Qr in the cloudy days was increased by (317±203)%, while COP was increased by 1.0±0.7. However, Qr and COP in the solar days was not improved. Conversely, by decreasing V to 13m3, Qr and COP were not impacted in the solar days but decreased in the cloudy days. In conclusion，it was feasible to employ TSFU to improve Tin of PTET during winter.
