Experimental thermal and exergy performance of a flat spiral external receiver for a Scheffler dish concentrator

This study experimentally evaluates the thermo-energetic and exergy performance of a novel flat spiral copper tube external receiver incorporated into a 16 m2 Scheffler dish–based solar concentration system operating under realistic outdoor conditions in Dehradun, India. The receiver consists of a 12.5 m-long copper tube (12 mm diameter) arranged in a planar spiral within a 0.5 m aperture to enhance uniformity of heat transfer and reduce wall–fluid temperature gradients under non-uniform solar flux. Experiments were conducted at beam irradiance levels ranging from 420 to 680 W m−2, with water circulated in a closed-loop circuit at a mass flow rate of 0.048 kg/s as the working fluid. The concentrator delivered up to 5.45 kW of optical power to the receiver from an incident solar input of 6.58 kW, corresponding to an optical transfer efficiency of 75.5%. The receiver produced an average useful thermal power of 2.96-3.36 kW, with a peak output of 4.68 kW, achieving average thermal efficiencies of 54.0-61.0% and a maximum instantaneous thermal efficiency of 73.21% and a daily-peak thermal efficiency of 67.93%. Exergy efficiency ranged from 0.15% to 2.18%, reaching a maximum of 6.59%, indicating receiver-level irreversibility as the dominant thermodynamic loss mechanism. Thermodynamic analysis reveals that solar absorption irreversibility accounts for approximately 74% of total exergy destruction, with wall-to-fluid heat transfer gradients and external losses contributing 14% and 12%, respectively. Hydraulic analysis confirms a pressure drop of approximately 33.2 kPa across the spiral, with pumping power of only 2.67 W, representing 0.071% of mean thermal output.