Analysis of the optimal gas cooler pressure of a CO2 heat pump with gas bypass for hot water generation

Abstract

This paper presents an experimental and numerical model-based approach for the estimation and control of the optimal heat rejection pressure in a one-stage transcritical CO2 heat pump equipped with an internal heat exchanger (IHX) and a liquid receiver. Once mass flow rate and compressor power consumption has been experimentally characterized, the correlations obtained have been introduced in a numerical model used to study the influence that evaporation temperature, superheating degree, gas cooler outlet temperature, IHX efficiency, liquid receiver pressure and compressor characteristics have on the optimal pressure. The results obtained show that only the gas cooler outlet temperature has a decisive influence on the optimal pressure, so it is possible to obtain a correlation for the optimal pressure based only on that temperature. However, that correlation does not limit the compressor discharge temperature, which can reach values near to 170 °C, leading to lubricating oil degradation. In order to limit that temperature to 140 °C, an alternative correlation for the optimal gas cooler pressure is proposed. This correlation takes into account not only the gas cooler outlet temperature but also the evaporation temperature and the compressor suction temperature.