Design of the water circulation temperature control system for solar greenhouses and its cooling performance in summer

Production capacity of the summer vegetables is often required for the solar greenhouses in northern regions. In this study, the temperature regulation was proposed and then validated to mitigate the severe high-temperature stress on the crops. A water circulation temperature-control system (WCTCS) was also utilized, the water as the primary medium of the heat exchange. Differential pressure and gravitational potential energy were then designed for the self-circulation. A better balance was achieved in the low-cost and high-efficiency temperature control under the greenhouse environment. There were several key components in the WCTCS. Among them, the water bags and heat exchangers were strategically placed inside and outside the greenhouse, in order to serve as the thermal storage and release units, respectively. The high- and low-level water tanks were integrated with the circulation pumps into a closed-loop network for heat management. A systematic investigation was conducted to evaluate the system’s performance and energy balance after exergy analysis. Furthermore, the comparison was also made on an experimental greenhouse that was equipped with the WCTCS and a control greenhouse with conventional natural ventilation. The results demonstrated that there was the remarkable efficacy of the WCTCS. Firstly, the internal air temperature of the experimental greenhouse was significantly lowered during peak summer high-temperature periods. The average air temperature was consistently 1.5–5.1 ℃ lower than that of the control under standard natural ventilation, with the maximum difference of the average temperature reaching 8.2 ℃. Since a slight gradient of the horizontal temperature was observed, there was a uniform vertical distribution of the air temperature, thus promoting a consistent microclimate over the crop canopy. Secondly, by analyzing the variation characteristics of the light and thermal environment inside the solar greenhouse, the maximum temperature difference between the experimental greenhouse with the water circulation temperature control system and the control greenhouse on sunny days reached 7.1 ℃. Compared with the control greenhouse, the water circulation temperature control system extended the duration within the most suitable temperature range of 22–25 ℃ by 170 min, prolonged the period within the normal growth temperature range of 20–30 ℃ by 120 min, effectively shortened the duration of high-temperature stress, and provided longer suitable temperature periods for tomatoes. Thirdly, the water circulation temperature control system operated stably, with a daily cooling capacity of 34141.4 to 49230.8 kJ provided by the gravitational potential water circulation, demonstrating good cooling buffer performance. Fourthly, the experimental greenhouse covered an area of 1536 m². Based on the analysis, it was deduced that the optimal heat dissipation effect was achieved when the number of finned-tube radiators was increased to 15 sets, the total water volume of the system was raised to approximately 25 m³, and the number of water bags was increased to 18. Finally,the water circulation during the cooling process was a closed loop, with only minimal water evaporation occurring locally in the pipeline near the water tank. As a result, the water resource recycling rate reached 95%, and the system saved 53.8% more energy compared to the wet curtain fan cooling system. In conclusion, the effective and low-cost control system of the supplementary temperature was successfully developed to provide the critical technical support for the safe production of vegetables in solar greenhouses during summer. This work can hold substantial practical and theoretical significance to extend the summer production cycle for the overall sustainability and productivity of the facility agriculture.