Towards a general design framework for external tubular solar receivers using pressurised gaseous or supercritical fluids

This work presents a comprehensive analysis of the thermo-fluid parameters governing the design of external tubular solar receivers operating with pressurised gaseous and supercritical fluids. Contrary to the state of the art regarding the use of these fluids in central receivers, this study identifies an optimum threshold operating pressure of 40–50 bar, corresponding to energy efficiencies of approximately 82 %. Beyond this range, efficiency gains become marginal and do not offset the higher investment cost of a high-pressure installation. Although the thermal performance of solar receivers is generally influenced by both pressure loss and total heat loss, the analysis shows that pressure loss has only a limited impact on external tubular receivers, whereas heat losses are the dominant factor. These losses primarily depend on the receiver's cooling effectiveness and external surface area. Cooling effectiveness improves with a higher overall heat transfer coefficient and a lower mean operating temperature. The primary parameters that reduce the external surface area are higher fluid pressure, a greater temperature rise between the receiver inlet and outlet and a larger tube diameter, provided that the maximum temperature remains below the material's allowable limit. This study also demonstrates that the total annualised cost of these pressurised receivers mainly depends on the investment cost, which is proportional to receiver dimensions. Consequently, all parameters that reduce the external surface area also reduce cost. Finally, different working fluids were compared. Among them, CO 2 provides superior thermal performance and lower cost, primarily due to its high density at intermediate pressures, which decreases both pumping power requirements and receiver surface area. • There is a threshold operating pressure at 40–50 bar for external tubular receivers. • Key factors in receiver thermal performance: cooling efficiency and external area. • Cooling improves with higher heat transfer coefficient and lower fluid temperature. • Receiver size decreases with higher pressure, larger ΔT and greater tube diameter. • CO 2 provides better performance and lower cost, due to its high density at 40–50 bar.