Comparative Evaluation of S-CO2 with Antifreeze Heat Transfer Fluids as an Application in Solar Flat Plate Collectors

In developing countries like India, where access to reliable electricity is limited, solar water heaters that run on the sun’s free energy are an appealing alternative. Here, the effectiveness of a solar water heater is evaluated in the cold climatic region of Ladakh, India. The high altitude, low humidity, and ample sunshine in Ladakh make it an excellent choice for solar water heating systems. Due to very cold climatic conditions, it is a big challenge to prevent water freezing in solar water heating systems (SWHS). Hence, in this study, the analysis of a flat plate collector (FPC) based SWHS at ISO 9806:2017 conditions under three sky models is done. The effectiveness of an FPC is compared by considering supercritical carbon dioxide (s-CO2) and antifreeze heat transfer fluids (HTF) like ethylene glycol water mixtures (EGW 40%). It was observed that using s-CO2 in FPC provides non-freezing, higher useful heat gain resulting in higher efficiency, lower viscosity resulting in higher Reynolds number, lower pressure drop and pumping power requirement compared to EGW mixtures. s-CO2-based solar collectors comprise high working pressures, phase-changing possibilities with surrounding temperature, and significant property fluctuations near its critical point, making SWHS design challenging. Therefore, these topics need thorough research.