High-efficiency and energy-saving control strategy for the water-circulating solar energy system in the greenhouse

Abstract: A water-circulating solar energy system has been widely used in the field of greenhouse heating. But, real-time heat harvesting is still lacking in the arrangement of time points or indoor air temperature. It is a high demand to consider the thermal condition of the collector surface in the current operation control system. This study aims to explore the high-efficiency and energy-saving operating system. An intelligent control device was first developed for solar heat collection and release. A simulation was then performed on the appearance and condition of the water-circulating solar collector (also as the heating radiator) without the impact of water flow. A new control strategy was finally proposed using the simulating device. Specifically, the difference between the surface sol-air temperature and the tank water temperature was utilized to control the daytime heat collection, whereas, the surface sol-air temperature was to control the nighttime heat release. A theoretical analysis was also implemented to verify the control strategy. The surface sol-air temperature of the device surface in the daytime was used to reveal the collectible excess solar heat on the collector surface. As a result, the balance was achieved between the solar radiant heat absorbed by the surface and the heat exchanged between the surface and the internal environment, and between the surface and the greenhouse environment. Thereby, the control strategy accurately enables heat collection at the right time. The sol-air surface temperature at night was closely related to the indoor air temperature. Correspondingly, heat-releasing control was essential using indoor air temperature. The field tests were carried out to investigate the solar heat collection and release effect of the control strategy applied to the water-circulating solar energy system with an indoor collector (as a heating radiator during nighttime) constructed of hollow polycarbonate sheets. And a comparison was also made with the existing control strategy capability. During daytime, weather conditions had significant influence on the surface sol-air temperature. The maximum temperature reached 59.9 ℃ on a sunny day, much higher than those on cloudy and overcast days (47.2 and 35.0 ℃, respectively). The heat collection on a sunny day (404.1 MJ) was also much higher than those on cloudy and overcast days (225.9 and 62.7 MJ, respectively). Obviously, the setting time points led to some issues for the heat collection control, such as less heat collection (1.4 h) or ineffective operation (1.7 h) on a sunny day, and long-term ineffective operation on cloudy and overcast days. The control system of indoor air temperature also missed some heat collection opportunities, due to the low air temperature. Particularly, the heat (31.8 MJ) needed to be collected for a significant energy saving (coefficient of performance: 20.2) in the early stage of heat collection with the strong solar radiation. Besides, the short-term ineffective operation often occurred (0.7 and 2.4 h on cloudy and overcast days, respectively). By contrast, the new control strategy of heat collection was achieved in the higher heat collection with the lower energy consumption. The heat release control also performed better to reduce the ineffective operation time, due to the rapid response of surface sol-air temperature to exchange in solar thermal energy. The control strategy was also applied in the water circulation systems, in order to tap the harnessing potential of solar energy and saving energy. Besides, the heat collection control strategy can be expected to apply in forced-circulation solar water heating systems. The control strategy can be further optimized for the more ideal heat collection and release. The finding can provide the technical reference to improve the structure and installation of the simulating device in the temperature change management of heat release.