Transcritical-transcritical cascade CO2 heat pump cycles for high-temperature heating: A numerical evaluation

Industrial heat pumps are becoming increasingly popular as a low-emissions technology due to the ability to leverage renewable electricity at high efficiencies. While heat pumps are already well-established for process temperatures below 100 degrees C, high-temperature heat pumps that can produce greater temperatures are limited by high pressure and temperature requirements that surpass the performance of conventional heat pump compressor technologies. Alongside this, new heat pump cycles which are being developed need to be compatible with low GWP and low ODP refrigerants. This paper presents a numerical evaluation on the thermodynamic performance of high-temperature heat pumps for supplying 200 degrees C sink temperatures with large temperature glides. Four new transcritical-transcritical cascade CO2 heat pump systems are modelled in this study which aim to limit the pressure requirements required to achieve high sink temperatures. An air heating case study for milk powder spray drying is presented. To achieve a COP of 3, all cycles had a similar maximum air outlet temperature between 110 degrees C and 113 degrees C. The transcritical-transcritical cascade cycle with internal heat exchangers on both the top and the bottom cycles achieved the lowest compressor discharge pressure (13.47 MPa maximum) for this case. The same cycle also achieved the highest COP of 2.22 when air outlet temperature was fixed at 200 degrees C (i.e., complete replacement of external heating). For such a heat pump to be economically feasible for heating air to 200 degrees C, an electricity-to-reference price ratio equal to or below 2.06 is required.