Experimental study of the performance of a water-to-water heat pump working with R744/R290 mixtures as refrigerant for low-to-medium ΔT applications

This study experimentally investigates a water-to-water heat pump operating with pure R744 and R744/R290 mixtures (0-20% propane) for low-to-medium Δ T applications, corresponding to water temperature increases of 5–20 K at the gas cooler. COP, heating capacity, and optimal gas cooler pressure are evaluated with and without internal heat exchanger (IHX). Three approaches are adopted: in the first, the refrigerant charge is fixed at the R744 reference value with the IHX active; in the second, the same charge is used with the IHX bypassed, which limits tests to propane fractions ≤ 10% due to superheat constraints; in the third, the charge is optimized for the 85/15% and 80/20% w mixtures to match the superheat of the pure R744 baseline. With the IHX active, high R290 fractions (15-20%) increase COP by up to 21.6% compared to pure R744. Without IHX and with R290 up to 10%, COP gains reach 7%, while under optimized charge the 85/15% mixture provides no significant improvement, whereas the 80/20% mixture improves COP. COP is consistently higher with the IHX, confirming its beneficial role. Heating capacity decreases with increasing propane content, by up to 12% at fixed charge and up to 26% when the charge is adjusted to maintain comparable superheat. In transcritical conditions the optimal gas cooler pressure decreases with increasing R290 fraction: up to 10% it follows a nearly linear trend related to the reduction in mixture critical pressure, whereas at higher fractions it approaches the mixture critical pressure, allowing subcritical or near-critical operation.