Thermo-economic evaluation of low-GWP CO2-based zeotropic mixtures in space heating heat pumps with and without internal heat exchanger

Abstract

Transcritical CO₂ heat pumps are commonly used for tap water heating in buildings; however, their performance is often limited in space heating applications. Low-GWP CO₂-based mixtures can enhance subcritical operation and reduce irreversibility during transcritical processes. Promoting this technology requires a comprehensive understanding of its thermo-economic performance; yet, there is a lack of relevant studies. This paper develops thermodynamic models using energy, exergy, and exergoeconomic analyses to evaluate the performance of four CO₂-binary mixtures (CO₂/R41, CO₂/R1234yf, CO₂/R290, and CO₂/R1270) in space heating heat pumps. These mixtures are compared against pure CO₂ in two cycle configurations: with and without an internal heat exchanger (IHX). The evaluation is conducted in accordance with the EN 14511–2 standard across various heating temperatures. Results indicate that the CO₂/R41 blend achieves significant COP improvements, exceeding pure CO₂ by up to 17.1% at 30/35 °$C$ and 8.3% at 47/55 °$C$. CO₂-based mixtures also significantly lower optimal discharge pressures, with reductions ranging from 24.4% for CO₂/R41 to 52.9% for CO₂/R1270. The inclusion of an IHX has a notable effect on COP, particularly for CO₂/R41, where performance improves when the CO₂ mass fraction exceeds 70% at lower temperatures. Exergy analysis demonstrates that the CO₂/R41 mixture achieves the highest exergy efficiency up to and including a CO₂ mass fraction of 80%, outperforming pure CO₂ by up to 16.3%. Furthermore, CO₂/R41 exhibits 16.3%–19.8% lower total exergy destruction cost rates compared to pure CO₂, with significant cost reductions in the throttling valve (25.5%–42.5%). These findings highlight the potential of CO₂/R41 as a highly effective and economically viable option for space heating heat pumps, offering superior performance and reduced operational costs compared to pure CO₂ and other mixtures.