Development of low-cost evaluation method for coefficient of performance of heat pump for heating greenhouses
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Development of low‑cost evaluation method for coefficient of performance of heat pump for heating greenhouses Moritani Shigeoki1 · Sasaki Kazuya2 · Itaka Kenji3 Received: 24 October 2018 / Accepted: 11 November 2019 © Springer Nature B.V. 2020
Abstract 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. Keywords Energy balance equation · Tank water temperature · Condenser · Evaporator List of symbols α1, β1, γ1 Model-fitting parameters 𝛼2′ , 𝛽2′ Model-fitting parameters t Time (s) ∆t Time step (s) m Number of time with ∆t step (dimensionless) tm Time at mth number (s) Cw Water specific heat (kJ kg−1 K−1) * Moritani Shigeoki moritani@hirosaki‑u.ac.jp 1
Faculty of Agriculture and Life Science, Hirosaki University, 3 Bunkyo, Hirosaki, Aomori 0368561, Japan
2
Faculty of Science and Technology, Hirosaki University, 3 Bunkyo, Hirosaki, Aomori 0368561, Japan
3
Institute of Regional Innovation, Hirosaki University, 2‑1‑3 Matsubara, Aomori, Aomori 0300813, Japan
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Mw Mass of tank water (kg) qw Water discharge rate (kg s−1) Pcomp Power input by compressor (kW) Tw Temperature of tank water (K) Tw,m Temperature of tank water at time tm (K) Ti Inlet water temperature (K) To Outlet water temperature (K) Tw,con Water temperature of condenser (K) Tw,eva Water temperature of evaporator (K) Tcon Condensing temperature (K) Teva Evaporation temperature (K) Q Heat transfer rate (kW) Qcon Heat transfer rate of condenser (kW) Qeva Heat transfer rate of evaporator (kW) Abbreviations COP Coefficient of performance COPest Estimated COP COPideal Ideal maximum COP
1 Introduction Heaters are an essential in greenhouses for improving the quality and quantity of vegetables and fruits, particularly in cold regions and during the winter. In most cases (> 90%), greenhouses are heated by burning fossil fuels such as oil (Kawasaki 2015). Hea
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