Comparison of energy consumption and tomato yield and quality from greenhouses heated by an oil heater or an air-source heat pump

Air-source heat pumps (HP) are known to have higher energy efficiency than oil fan heaters, and have recently replaced them in Japanese greenhouses. However, HPs require a defrost period when operating under conditions of low temperature and high humidity, which consumes more energy. Therefore, the practical advantages of HPs over oil fan heaters remains unknown. In particular, there is little information about the quality and yield of vegetables cultivated using HPs. In this study, we compared the energy usage of an HP with that of an oil heater, and assessed tomato yield and quality in two identical greenhouses to elucidate whether using HPs in protected horticulture is effective not only in terms of energy conservation but also for crop productivity. Energy consumption of the oil fan heater was 3.5NDASH3.6 times greater than that of the HP in winter. Energy consumption of the HP and the oil fan heater were strongly correlated. The two conditions did not differ in the first flowering day, yield, weight distribution of the fruits, or in sugar and ascorbic acid content. These results indicate that the change from oil fan heaters to HPs in greenhouses results in substantial reductions in energy consumption without negative effects on vegetable products.