superhydrophobic, icephobic, ice adhesion, corrosion testing, coating
Mitigating frost on heat exchanger coils is key for developing high-efficiency heat pumps and enabling the widespread adoption of cold-climate heat pumps. Frost reduces heat exchanger (HX) performance by impeding airflow and adding thermal resistance, therefore taxing the system to consume more energy to satisfy temperature setpoints. Accordingly, heat pump systems have defrost cycles, which typically involve electrical heaters or hot-gas bypass systems, that consume extra energy to melt away the impeding frost/ice layer on coils. As presented in prior literature, enhanced HX surfaces (such as louvered fins or increased fin density) can accelerate frost development and thus have faster performance degradation through increased pressure drop across the coils. Thus, non-enhanced fin surfaces (such as wavy fins) with low fin densities, are typically employed in HVAC systems to minimize frosting impacts, however resulting in less compact units with lower performance under dry conditions. An alternative solution could be the use of durable superhydrophobic/icephobic coatings. This paper presents a systematic approach for testing various coatings for their viability to mitigate frost on Tube-Fin HXs. The tests shown in this paper were used as preliminary screening tests to identify coatings for a more comprehensive frost development assessment. Aluminum fin stock samples were coated by several coating vendors for understanding their hydrophobicity, icephobicity, and durability. This involved (a) an ice adhesion test to measure the maximum amount of shear force required to remove ice from the surface; (b) cyclic corrosion testing (CCT-4 standard) while qualitatively monitoring wear; (c) adhesion testing (ASTM D3359 standard) to further understand the coating-substrate bond strength; and (d) post-corrosion ice adhesion tests to characterize durability and potential performance of coatings over time in real-world environments. While most coatings maintained their wettability state after being placed in the corrosion chamber for over 1000 hours, qualitative wear and performance was shown to vary between different coatings of different chemical compositions. Variances in additives and base chemistries were shown to impact the long-term performance of the coatings. Selected coatings were then identified for a more comprehensive frost development assessment in a temperature and humidity-controlled wind-tunnel.