A comparison of heating performance for vapor injection and vapor bypass air source heat pumps under frosting-defrosting conditions

Vapor injection (VI) can effectively improvement the heating performance of air source heat pumps (ASHPs) in cold environment and has been widely adopted. However, its dedicated compressor structure is complex, and the associated cost increase cannot be overlooked. Vapor bypass (VB) technology can also improve the performance of ASHPs and offers a lower cost increment, demonstrating considerable application potential. However, under the conditions of low-temperature frosting-defrosting, the comparison of heating characteristics between the two technical schemes is not clear. To evaluate the application value of VB technology, using an experimental approach, this work compares the heating and defrosting capabilities of VI and VB systems under four low-temperature outdoor conditions (2/1 °C, −7/-8 °C, −12/-13 °C, −15/-16 °C) and analyzes the performance characteristics and application scenarios of the two technical approaches. Employing a power-matching principle for comparability, the study evaluates indexes including capacity, coefficient of performance (COP), suction/discharge pressures and temperatures, and defrost dynamics. Results indicate that VI system exhibits higher peak heating capacity and faster defrost speed. However, VB system demonstrates superior average heating capacity, operational stability, and longer heating cycles, owing to its frost-suppression capability. Especially under the low-temperature and high-humidity condition (2/1 °C), COP of VB system improve by 9.6% compared to VI system. Under other low-temperature conditions (−7/-8 °C, −12/-13 °C, −15/-16 °C), the performance of VB system is either comparable to or slightly lower than that of VI system, with COP variation rates of −3.4%, 1.3%, and 0.4%, respectively. Additionally, a cost analysis reveals that VB system has a lower incremental manufacturing cost. Under the conditions of frosting-defrosting, VB technology can achieve comparable performance improvements at a lower incremental cost compared to VI technology. These findings proven that VB technology is an excellent alternative solution for ASHP and provide a comprehensive experimental guidance for the selection of low-temperature ASHP technologies in scenarios where costs are limited.