Performance analysis of heat pumps with zeotropic mixtures at different load conditions

Abstract

Heat pump cycles with zeotropic mixtures as working fluids and temperature glides on both heat sink and heat source sides are analyzed. Five binary zeotropic mixtures, selected based on previous studies in the literature, are examined. The cycles are initially optimized for each binary mixture at three different heat source temperature glides (10–30 K) and fixed heat sink temperature glide (30 K). The variation in the heating COP of the optimized designs is then computed and compared for five different part load scenarios down to 40% of the design point heating load. The design point optimization is performed for different single and double-compression cycle layouts, based on which the standard vapor compression cycle with a suction gas heat exchanger is adopted as the most economical option for the rest of the study. The part load cycle simulations demonstrate that, when the heat source temperature glide is large, two of the studied part load scenarios can lead to increased COP; these are when the reduction in the heating load is due to (1) an increase in both the heat sink (e.g. district heating water) inlet and heat source (e.g. process water) outlet temperatures, and (2) a decrease in both heat sink and source flow rates. Moreover, it is observed that different mixtures show different levels of sensitivity to a decrease in heating load. Among the studied mixtures, DME/Iso-Pentane while outperforming others in all studied cases, has the most unfavorable trend at part load, hence losing its benefit margin over other candidates.