This study presents a thermal-hydraulic assessment of an indirect solar water heating system employing carbon dioxide (CO2) as the heat-transfer fluid in a flat-plate collector. The work compares superheated and supercritical operating regimes under identical geometric, flow, and standardized outdoor conditions. A distributed one-dimensional MATLAB model is formulated by coupling the Hottel-Whillier energy balance with real-gas CO2 thermophysical properties obtained by integrating REFPROP. The collector tube is discretized axially to capture the strong variations in specific heat, density, viscosity, thermal conductivity, Nusselt number, pressure drop, and local heat transfer coefficient near the critical and pseudo-critical regions. The model is evaluated for inlet temperatures of 300–340 K and operating pressures of 50–100 bar under blue-sky, hazy-sky, and grey-sky Standard Reporting Conditions of ISO 9806:2017. The analysis shows that operating near 80 bar provides the most favorable thermal-hydraulic outcomes. The heat transfer improves due to enhanced real-gas properties, while hydraulic loss and pumping power decrease because of the higher density and lower flow velocity of pressurized CO2. Within the investigated range, supercritical operation improves collector efficiency by approximately 15–20% compared with superheated operation at comparable boundary conditions. The results provide design guidance for CO2-based solar thermal collectors and highlight that high-pressure material selection, sealing, safety, and techno-economic assessment are necessary before practical deployment.
- Open Access
- Article
Thermal-Hydraulic Performance of a Solar Flat Plate Collector Using CO2 under Superheated and Supercritical Regimes
- Wasim Ashraf 1,*,
- Maddali Ramgopal 2,
- Mahendra V. Reddy 2
Author Information
Received: 04 Feb 2026 | Revised: 28 Jul 2026 | Accepted: 21 Aug 2026 | Published: 31 Aug 2026
Abstract
Graphical Abstract
Keywords
indirect solar water heating system | flat plate collector | thermophysical properties of CO2 | superheated and supercritical-CO2 | thermal hydraulic model
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