Performance of Household Heat Pumps for Nighttime Cooling of a Tomato Greenhouse during the Summer

To produce high-quality greenhouse crops year round, a cooling method to decrease the air temperature is required in summer. In the present experiment, multiple small-capacity (2.8 kW) household heat pumps were used for cooling a greenhouse in Chiba, Japan, during summer nights. The environmental conditions inside the greenhouse, average coefficient of performance (COP) of eight heat pumps, and amount of water drained from the heat pumps were investigated. The results showed that when the heat pumps were used in the greenhouse, the air temperature, CO₂ concentration, and water vapor pressure deficit were more suitable for crop growth than when the heat pumps were not used. Average COP for cooling ranged from 5.8 to 10.7, which was 1.3 to 2.4 times greater than the value obtained under Japanese Industrial Standard conditions, due to the smaller difference in air temperature between inside and outside the greenhouse during the nighttime(20:00-05:00) from20June 20 July 2010. The amount of water drained from the heat pumps accounted for 15% to 25% of the amount of irrigated water. These results indicate that the environmental conditions inside a greenhouse can be improved by using heat pumps during summer nights achieving high COP and low net water consumption.