Feasibility analysis of thermal storage defrosting method for air source heat pump: From energetic and economic viewpoints

Abstract

Frosting issue of air source heat pump (ASHP) is a continuing concern. The conventional reverse cycle defrosting (RCD) method absorbs heat from indoor heat exchanger, thus greatly impacts the indoor thermal comfort. To solve the problem of insufficient defrosting heat supply for RCD method, thermal energy storage (TES) based defrosting method has gained increasing attention over the past years. Yet existing studies have only focused on specific prototype tests. There lacks general analysis on system characteristics and design considerations. To better address application concerns of TES-based defrosting method, this study carried out feasibility analysis for the TES-based ASHP system from both energetic and economic aspects. The energetic analysis reveals that the stable operation of TES-based ASHP relies on proper system design and control. Feasibility conditions concerned with key operating parameters are defined and deduced. Impact factors on average heating efficiency are also analyzed. Theoretical and numerical results indicate that energy saving potential of TES-based ASHP over conventional ASHP is conditional on the defrosting discount factor of conventional RCD method. TES-based ASHP enhances heating efficiency of conventional ASHP by 3.0% to 19.5% at ambient from −2°C to 2 °C due to relatively large defrosting discount factor of conventional RCD method. With respect to the economic aspects, the capital cost of TES-based ASHP is 26% higher than that of the conventional ASHP. But the total life cycle cost can be reduced by higher heating efficiency. The condensing heat storage ASHP exhibits cost advantages to the conventional ASHP when defrosting discount factor of conventional ASHP is higher than 1.13. Further prototype test with the TES-based ASHP could shed more light on the feasibility characteristics as revealed by the theoretical analysis.