A simplified method to evaluate the energy performance of CO2 heat pump units

The prediction of the performances of CO2 transcritical heat pumps demands accurate calculation methods, where a particular effort is devoted to the gas cooler modelling, as the correlation between high pressure and gas cooler outlet temperature strongly affects the cycle performance. The above-mentioned methods require a large amount of input data and calculation power. As a consequence they are often useless for the full characterisation of heat pumps which are sold on the market. A simplified numerical method for the performance prediction of vapour compression heat pumps working in a transcritical cycle is presented, based only on performance data at the nominal rating conditions. The proposed procedurewas validated against experimental data of two different tapwater heat pumps. For the considered units, simulation results are in good agreement with the experimental ones. The deviations range from -6.4% to +1.7% and from -3.8% to +5.8% for the COPH of the air/water heat pump and the water/water heat pump, respectively. The heating capacity deviations stayed within -5.5% and +1.7% range and within -5.0% and +7.9% range for the same units. The proposed mathematical model appears to be a reliable tool to be used by the refrigeration industry or to be implemented into dynamic building-plant energy simulation codes. Finally, it represents a useful instrument for the definition of tailored approximated optimal high pressure curve considering the operating characteristics of the specific CO2 transcritical unit. It could also be implemented on board of a real unit control system where it could be used as model coupled to computational intelligence algorithms for pressure optimisation.