Performance of Solar Utilization System Developed with Capillary Tube Mats to Collect and Release Heat in Chinese Solar Greenhouses

Abstract

HighlightsA solar utilization system was developed with capillary tube mats (SUSCTM) to heat Chinese solar greenhouses (CSGs) actively during winter nights.In the solar day, SUSCTM collected 28~33% of the accumulated solar energy on the north-wall and released 40.3~48.6% of the collected heat in night under the experimental conditions.The thermal performances of SUSCMT can be further improved by improving the heat insulation of the water tank, doubling and elongating the capillary tubes.Abstract.A solar utilization system composed of capillary tube mats, a water tank, recirculating pipes, and a pump (SUSCTM) was developed to improve the temperature of Chinese solar greenhouses (CSGs) during winter nights. The capillary tube mats were mounted on the north-wall to intercept solar radiation. By circulating water within the SUSCTM during the daytime, the solar energy can be absorbed and stored in the water tank. In the same way, the heat stored in the water tank can be recovered to increase the indoor air temperature (Tin) in the night. During the daytime hours of sunny days, SUSCMT stored the heat of 102.4~135.4 MJ under the test conditions, which accounts for 28~33% of the accumulated solar energy on the north wall. The heat released by the SUSCTM during the nighttime hours of sunny days accounted for 40.3~48.6% of the collected heat. As a result, the minimum Tin in the night was elevated by 1.6°C~1.9°C, with the coefficient of performance (COP) of 1.3~1.8. On the other hand, the SUSCMT only operated during the night hours of cloudy day and elevated the minimum Tin by 1.5°C. The COP was 1.2 as a result. In addition, the amount and flux of heat released by the SUSCMT per the north wall area matched the conventional north wall of CSGs. Based on the proposed model, the thermal performances of SUSCMT can be further improved by improving the heat insulation of the water tank, doubling and elongating the capillary tubes. Thus, the SUSCMT has the potential to increase the Tin of CSG during winter nights. Keywords: Capillary tube mats, Chinese solar greenhouse, Indoor air temperature, Solar utilization system, Water circulation.