A Critical Comparative Review of Finned-Tube and Microchannel Heat Exchangers for Heat Pump Water Heating Applications

Abstract

Heat pump water heaters (HPWHs) are becoming ever more relevant as an effective approach to electricity reduction and greenhouse gas emissions that accompany conventional resistance water heaters. The design of the heat exchanger significantly affects the heating rate, temperature lift, pressure drop, refrigerant charge, size, and coefficient of performance (COP) of the system. This review paper makes a comparative assessment of finned-tube heat exchangers (FTHEs) and microchannel heat exchangers (MCHEs) for HPWHs with emphasis on the relationship between the thermal-hydraulic performance of components and energy efficiency of the system. The analyzed literature shows heat transfer coefficients on the refrigerant side as approximately 1,000–3,000 W/(m²·K) for finned-tube heat exchangers and 2,000–8,000 W/(m²·K) for microchannel heat exchangers, while the latter have significantly higher area density and around 30–45% lower refrigerant charge. Additionally, the former can significantly reduce the volume and mass of heat exchangers under the same heat load conditions. Nevertheless, all these improvements come at the expense of higher pressure drop, sensitivity to two-phase flow distribution and blockage, manufacturing tolerance requirements, cost of low-volume production and lower serviceability. From nominal operating conditions, the literature in the source manuscript demonstrates an overall system-level improvement of the COP of MCHE-based HPWH of about 2–6%, while it can be non-existent or less pronounced in cases of low ambient temperature and off-design operation. Therefore, from the review performed, it is not possible to conclude that one technology outperforms another. The MCHEs show promise when the issues of compactness, low refrigerant charge, and high efficiency of heat transfer are important, while the FTHEs demonstrate their advantages in the case of low cost and reliability.