Saving Energy with a Hybrid Heating System using an Air-to-air Heat Pump and Oil Heater in a Rose Greenhouse

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We compared the total cost of greenhouse heating between a conventional oil-fired air heater and a hybrid system comprising an electric heat pump (rated at 49.2 W m-2) and an oil-fired air heater (rated at 199.4 W m-2) in identical rose greenhouses in Iwata city, Shizuoka, Japan. The greenhouses had a heat load coefficient of 3.7 W m-2 K-1. The average nighttime temperature in the hybrid-system greenhouse during August and September was 2.2°C lower than that in the oil-heater greenhouse owing to nighttime cooling by the heat pump. To maintain a minimum temperature of 18°C required an annual consumption of heavy oil for heating of 12.61 L m-2 by the hybrid system and 33.49 L m-2 by the oil heater. The annual consumption of electricity was 14.91 MJ m-2 by the hybrid system and 1.13 MJ m-2 by the oil heater. The total energy input for heating was 43.77 MJ m-2 for the hybrid system and 81.62 MJ m-2 for the oil heater. Using 3.79 as the coefficient of performance of the heat pump gave an estimated energy use of 78.19 MJ m-2 for heating by the hybrid system, lower than the calculated value of 81.62 MJ m-2; that is, the rated COP was exceeded. At an oil price of 65.8 yen L-1, the annual running cost was 1645 yen m-2 in the hybrid-system greenhouse and 2162 yen m-2 in the oil-heater greenhouse. The annual cut rose production was 712.7 g per plant in the hybrid-system greenhouse and 643.2 g per plant in the oil-heater greenhouse. In particular, production in the hybrid system was 53% higher in August and September, when nighttime cooling was operated. Overall, the hybrid system resulted in savings of 46% of input energy and about 24% of energy costs.

Saving Energy with a Hybrid Heating System using an Air-to-air Heat Pump and Oil Heater in a Rose Greenhouse

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