Thermodynamic optimisation of the integrated design of a small‐scale solar thermal Brayton cycle

摘要

The Brayton cycle’s heat source does not need to be from combustion but can be extracted from solar energy. When ablack cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and convertedinto a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise thegeometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainlydue to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermalBrayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver andcounterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. Amodified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18m are considered. Twocavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a functionof the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively largewhen compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net poweroutput of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a systemoperating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small.For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in sucha way that the internal irreversibility rate is almost three times the external irreversibility rate.