Equipping multi-span greenhouses with a positive-pressure ventilation and cooling system

Multi-span greenhouses have been widely used in recent years. Conventional negative pressure fan-pad cooling has suffered from the low uniformity of indoor air temperature and the integrity of the cultivation area. Existing positive pressure ventilation with the air-handling corridor is also limited to low integration and difficult to control. In this study, a packaged positive pressure ventilation and cooling system was designed for highly efficient and quality crop production in the multi-span greenhouses during warm seasons. This system consisted of an equipment room, a combined air conditioning unit with three-sided air intake, ventilation ducts, and a control module. It can be further extended with heat sources and a water recirculating system, enabling integrated greenhouse climate conditioning based on positive pressure ventilation. The procedure of the air flows during cooling was as follows: The outdoor air entered the equipment room via outer vents, evaporative cooling in the air conditioning unit, and the air was conveyed into the greenhouse via underground ducts, while warm air was exhausted under roof vents. Field tests were conducted in Shouguang, Shandong Province, China. The results showed that the cooling system with the external shading screen maintained the daily mean air temperature between 28.4 and 32.5 ℃, which was 0.8 to 3.8 ℃ lower than the outdoor temperature during the peak temperature hours (10:00-16:00) in summer. The vapor pressure deficit of the indoor air averaged 0.87-1.33 kPa during operation. The daily mean relative humidity ranged from 62% to 80%, 17-29 percentage points higher than outdoors. The air was uniformly distributed over the cultivation area at the terminal air outlets. The airflow was also delivered at the velocities of 7.7-13.3 m/s, with the uniformity (standard deviation) of 1.9 m/s. In horizontal, the uniformity of air temperature reached 0.4 ℃ inside the greenhouse under the supply air condition. Vertically, the air temperature increased with height, with a temperature gradient of 0.76 ℃/m, and a 3.1 ℃ difference between the tomato canopy and the greenhouse roof. The high-pressure fogging system was installed inside the greenhouse. A three-dimensional cooling performance was achieved to reduce the vertical temperature gradient to 0.5 ℃/m. The designed specific ventilation rate of the greenhouse was 0.028 m/s for cooling purposes. The actual ventilation rate and system power consumption were 0.014 m/s and 15.2  W/m², respectively, during the test. The good performance was achieved in the average cooling capacity of 144.2 W/m² in the greenhouse, an energy efficiency ratio of 9.5, and an average indoor–outdoor air temperature difference of 2.1 ℃ (08:30–17:30). The overall cooling efficiency reached 95.9%. The daily average water consumption rate ranged from 0.033 to 0.065 g/(m²·s) for evaporative cooling. Compared with the negative pressure fan-pad cooling system, the proposed packaged positive pressure ventilation and cooling system requires a lower specific ventilation rate to achieve the same greenhouse cooling amplitude, while exhibiting superior cooling uniformity and efficiency. In comparison with the air-handling corridor based positive pressure ventilation system, the proposed system offers a longer air supply distance, though with relatively higher energy consumption. This finding can provide an efficient mechanical ventilation solution for cooling multi-span greenhouses and support the design of semi-closed greenhouses.