Abstract
Experimients were carried out to analyze the heating efficiency and coefficient of performance (COP) of an air type heat pump with a 7.5kW compressor used in a greenhouse 352m2 in area along with the development of methods for efficient and economic operation of the pump.
The air temperature in the greenhouse was maintained at 15 or 13°C, when the ambient air temperature was not lower than -1--2.5°C. The maximum difference between the temperature inside the greenhouse and the ambient air temperature was 17°C.
When the ambient air temperature decreased markedly, the heat pump was operated frequently to remove the frost covering the outer part of the instrument. When the defrosting operations were carried out too frequently, the air temperature in the greenhouse could not be maintained at 15°C.
When the set point of the night temperature was varied 16→13→10°C the air temperature in the greenhouse could be easily maintained at the set point. Therefore it was considered that the modification of the set point of the night temperature was the most suitable method to control the temperature by the air to air type heat pump.
The values of the COP of the heat pump ranged from 2.4 to 3.2, and there was a high correlation between the values of the ambient air temperature and the COP.
Assuming that the electricity charge is 25 yen per kW and heavy oil charge is 65-85 yen per liter the electricity charge for heat pump is equivalent to the heavy oil charge required for heating the air.
