Biradar, M.K.Parmar, D.N.Yadav, A.K.2026-02-042022Renewable Energy, 2022, 189, , pp. 865-8809601481https://doi.org/10.1016/j.renene.2022.03.056https://idr.nitk.ac.in/handle/123456789/22611Solar water heating system is inefficient during winter due to the chances of water freezing and higher viscosity at low temperatures. Several investigations are being done to increase the efficiency of the solar water heater using various secondary fluids for different climatic conditions. This paper emphasises on the study of heat transfer and fluid flow behavior of CO<inf>2</inf> based naturally circulated indirect solar water heating system. Subcritical (liquid and vapour) and supercritical CO<inf>2</inf> are considered as loop fluid, and the results are compared with water based system. Three-dimensional computational fluid dynamics simulations are carried out for two different weather conditions i.e., winter (278 K) and summer (305 K). Results are obtained for 33° collector inclination angle from horizontal at various operating pressures 50–70 bar for subcritical and 80–100 bar for supercritical CO<inf>2</inf>. The CO<inf>2</inf> based system yields very high Reynolds number (subcritical liquid: ∼160 times; subcritical vapour: ∼204 times; supercritical vapour: ∼260 times) and very high Nusselt number (subcritical liquid: ∼14 times; subcritical vapour: ∼19.5 times; supercritical vapour: ∼48 times) compared to water based system. Supercritical CO<inf>2</inf> based system exhibits 12% higher energy efficiency compared to water. Whereas, subcritical vapour based system exhibits 140% higher exergy efficiency relative to water based system. © 2022 Elsevier LtdComputational fluid dynamicsEnergy efficiencyExergyFlow of fluidsLiquidsReynolds numberSolar heatingSolar water heatersSupercritical fluid extractionCFD analysisExergy AnalysisNatural-Circulation LoopSolar thermal collectorSolar water heating systemsSubcritical supercriticalSupercriticalSupercritical carbondioxidesSupercritical CO 2Water-based systemsCarbon dioxidecarbon dioxidecomputational fluid dynamicsefficiency measurementenergy efficiencyheat transferheatinginstrumentationsummerviscosity