Thermodynamic performance analysis of air cycle heat pump for extremely cold regions applications

Heating in extremely cold regions requires efficient and environmentally friendly solutions. This paper presents a fresh air cycle heat pump (ACHP) system aimed at improving energy efficiency and air quality. Four ACHP configurations are compared, and corresponding mathematical models are developed. A low-pressure fresh air ACHP experimental setup is constructed to validate the models. Off-design performance analysis is conducted, including changes in outdoor and indoor temperatures and relative humidities. The results reveal that each cycle has an optimal pressure ratio for maximum coefficient of performance (COP). The recuperative cycle achieves the highest COP, with its optimal pressure ratio significantly lower than the other cycles. The two fresh air cycles (c and d) perform slightly better than the basic cycle (a), maintaining comparable or higher COP at lower pressure ratios. Under off-design conditions, as outdoor temperature decreases, although the COP of all cycles reduces, the fresh air cycles decrease more slowly. Furthermore, as indoor temperature increases, the COP of all cycles decreases, but fresh air cycles show better performance in terms of temperature rise and heating capacity. Specifically, cycle d achieves 1.8 times the heating capacity of cycle a at 30 °C. The indoor relative humidity significantly impacts the fresh air cycles, particularly water vapor phase-change mass, with cycle c showing 32 % of vapor change in the expander, compared to <5 % in cycle d. Fresh air cycles are suitable for dry, extreme cold regions, thus, water removal strategies are needed in humid climates.