Impact of an internal heat exchanger on a transcritical CO2 heat pump under optimal pressure conditions: Optimal-pressure performance of CO2 heat pump with IHX

Abstract

Transcritical R744 (CO2) heat pumps are a low-GWP high-efficiency alternative for domestic hot water generation and space heating. For high set-point temperatures, however, the COP drops and the optimum operating pressure increases. For this reason, the effect on system performance achieved by the inclusion of an Intermediate Heat Exchanger (IHX) is analyzed in depth. This is carried out by means of a numerical-experimental approach in which the effectiveness of the IHX and other characteristic parameters of the heat pump cycle are studied as a function of the IHX heat transfer area. The numerical modeling of the heat pump components is implemented for transient simulations of system heating-up process. Experimental tests with and without IHX are carried out with a water-to-water heat pump that feeds an accumulator tank, to assess its influence on the system’s COP and the electrical consumption. Simulations reveal that improving the efficiency of the IHX, by increasing the exchange area, improves the system’s performance. For the cases with higher IHX exchange area, the optimum pressure in the system is lower. For the heating experiments, the resulting thermodynamic COP is 7.55 % higher in the case with IHX, while the effective COP increases by 4.26 %. In consequence, the use of the IHX is recommended for the conditions analyzed.