Experimental data and modelling of a dual source reversible heat pump equipped with a minichannels evaporator

Abstract

• Experimental investigation on a novel dual source (air/ground) reversible heat pump. • A minichannel heat exchanger working as evaporator and condenser is presented. • A physical model of the heat pump has been developed and validated. • R454B and R452B are assessed as possible low-GWP substitutes for R410A. • The model has been used to develop a switching strategy between the thermal sources. This paper presents an experimental investigation and a numerical analysis of an innovative dual source heat pump. The combined use of air or ground as heat source/sink can be a strategy to improve the efficiency of the heat pump at lower costs because the length of the borehole heat exchangers can be reduced. The present heat pump combines several solutions to go beyond the state of the art: it can work with air or ground as heat source/sink and it can operate with a traditional finned coil heat exchanger or with a minichannels heat exchanger. Experimental tests have been conducted in a climatic chamber to study the performance of the heat pump at full and partial loads when operating in summer and winter modes. The minichannels heat exchanger works both as the condenser and as the evaporator. In the winter configuration, the problem of frost formation in the minichannel heat exchanger is avoided because the heat pump operates in ground mode at low air temperature. In the summer mode, in the minichannels condenser the temperature approach between the air and the refrigerant is lower compared to the finned coil heat exchanger (on average 1.1 K lower) and the minichannels technology allows to reduce the refrigerant charge in the system. A mathematical model of the heat pump has been developed and validated against experimental data. The model provides accurate predictions of the heat pump performance with deviations below 10 %. The heat exchanger modelling is based on a physical approach that allows to calculate the heat transfer coefficients and pressure drop in the heat exchangers. This approach requires more computational effort, but it allows to predict the performance of the heat pump when considering different refrigerants as the low-GWP alternatives R454B and R452B. The model shows that when the heat pump works with R454B and R452B, the cooling and heating capacity are about 10 % lower compared to those obtained with R32, while the differences in terms of performance coefficients are negligible. The model is then used to implement a control strategy for switching between the two heat sources to achieve always the highest performance depending on the operative conditions. In the present case, the results of the model show when the air source should be preferred: in the winter season when the air temperature is by 5 K higher than the water temperature returning from the borehole heat exchanger, in the summer season when the air temperature is by 10 K lower than the water temperature from the ground.