Abstract
The configuration of a recently installed CO2 heat pump system with a cooling capacity of 33.5 kW and heating capacity of 35.8 kW used on a dairy farm is presented. This system has been studied in order to determine the optimal performance when used for combined water heating and refrigeration. Furthermore, the effect of the interstage pressure on the CO2 heat pump system has been investigated. This required the development of a thermodynamic cycle based on the configuration of the actual constructed system and data acquired from monitoring. A compressor model with a compression ratio over three was developed and validated against experimental data with an acceptable agreement. The numerical model of the counter-flow heat exchanger between water and sCO2 has been developed assuming that the water outlet temperature is variable (just over the set water temperature), which is more consistent with actual practice. In the desktop study, the optimal COPs for heating, cooling and combined performance have been simulated considering the interstage pressure, discharge pressure and gas cooler outlet temperature. It was found that the optimal combined COPs generally decrease from 3.84 to 3.68 with the interstage pressure from 29.65 bar to 53.60 bar. Through optimisation of discharge pressure, it was found that the optimal combined COP of 4.27 occurs at the same pressure where the optimal heating COP of 2.63 is achieved (at a discharge pressure of 94 bar), while the optimal cooling COP of 1.66 occurs when the discharge pressure is around 88 bar. A further optimisation study was conducted based on a fixed cooling capacity, and it was found that the optimal cooling COP of 1.78 can be achieved when the discharge pressure is 80 bar, while the optimal heating and combined COPs are still achieved at the same discharge pressure of 93 bar (with the heating COP of 2.59 and combined COP of 4.18). Based on the simulation results, it can be summarised that the overall system thermal performance designed for heating and refrigeration is dominated by the heating performance.
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