Development of low-cost evaluation method for coefficient of performance of heat pump for heating greenhouses

Heat pumps (HPs) combined with groundwater have been used for heating greenhouses. Visualizing the coefficient of performance (COP) of an HP system through a display panel in the greenhouse or a remote display on a device such as a mobile phone could help farmers avoid wasting power and overpumping groundwater and enable them to use HPs in an energy-efficient manner. Simple COP estimation is important for reducing the number of temperature sensors attached to HPs; doing so could make HPs systems including visualization affordable for farmers. In this study, the COP was measured using an HP converted from an affordable air conditioner into a liquid-to-liquid heat exchanger unit. The COP equation was used with the temperatures of condensers and evaporators, which can be measured easily. The water temperature was simulated to verify this COP equation based on the energy balance equation incorporated with variables such as tank water temperature, water injection rate, and temperature of tank water inflow and outflow. The results obtained using equations with two and three coefficients showed good agreement with the measured values, particularly when the tank water temperature was 5–40 °C.