A large-scale air heat collector in an air-source heat pump: experiment-based computational analysis of heat transfer duty throughout a frosting cycle with various fin pitches
Air-source heat pumps play an increasingly central role in low-carbon heating systems. However, their performance under subzero outdoor conditions is significantly constrained by frost accumulation on finned heat exchanger surfaces. Frost build-up not only impedes airflow and increases air-side pressure drop but also degrades thermal conductance, thereby reducing overall system efficiency. Despite its practical importance, experimental data on the relationship between key design parameters, such as fin pitch, and frosting behavior in large-scale heat collectors operating under realistic conditions remain scarce. To address this gap, the present study investigates the influence of two commonly used fin pitches (4 mm and 5 mm) on the thermal performance of air heat collectors. A computational analysis is conducted to evaluate the degradation of heat transfer capacity over time owing to frost formation. The calculation methods for the influence of frost are based on earlier experimental data measured with similar heat exchanger coils in laboratory scale (595 mm x 420 mm x 182 mm). The results indicate that, at 0 ?C temperature and 80% or 90% relative humidity, the average heating capacity over a complete operating cycle decreases by 16-26% due to frost accumulation. Consequently, a comparable capacity margin should be incorporated into system design to ensure the desired output. Furthermore, the comparative analysis suggests that a 4 mm fin pitch offers higher heat collection duty than 5 mm throughout the operation cycle, resulting in approximately 5 % higher average duty even under severe frosting conditions. The findings also help identify the optimal timing for defrosting cycles. Overall, this study provides new insights into frost-induced performance degradation and optimal fin design, contributing to more accurate performance prediction and improved design of air heat collectors, particularly relevant to the accelerating electrification of heat production.