Feasible study of chemical oxidized SS304 sheets as solar selective absorbers for flat plate collectors & performance validation in solar water heater systems

Solar water heater (SWH) systems play a crucial role in utilizing renewable solar energy for domestic hot water generation, offering a sustainable alternative to conventional heating methods. Flat plate collectors (FPCs), a widely used configuration in SWH systems, rely heavily on the efficiency of the absorber surface to maximize solar energy capture and minimize thermal losses. In this study, a cost-effective and scalable solution was developed using chemically oxidised stainless steel 304 (SS 304) sheets as the absorber material for flat plate collectors. The chemically treated SS 304 surfaces exhibited a nanoporous oxide morphology, which significantly enhanced light trapping and solar radiation absorption. Optimised samples demonstrated high solar absorptance (α) in the range of 0.89–0.91 and low thermal emittance (ε) between 0.20 and 0.30, indicating a favorable balance for efficient solar thermal conversion. To ensure practical applicability, the chemical oxidation process was scaled up to coat 2-m-long SS 304 sheets, which were used to build a 1 × 2-m flat plate collector integrated into a thermosiphon-based solar water heater. Repeatable field tests showed an average thermal efficiency of 37.3 %, with low uncertainty less than 2 %. Multiple real field testing under different solar radiation conditions confirmed the consistency and durability of the coating performance. The findings underscore the potential of this low-cost, scalable approach for large-scale solar thermal applications, particularly in regions where affordability and ease of fabrication are critical.