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

The Brayton cycle’s heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating 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 such a way that the internal irreversibility rate is almost three times the external irreversibility rate.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1099-114X

Performance evaluation of tube-in-tube heat exchangers with heat transfer enhancement in the annulus

Different techniques as angled spiraling tape inserts, a round tube in side a twisted square tube and spiraled tube in side the annulus have been used to enhance heat transfer in the annulus of tube-in-tube heat exchangers. The heat transfer enhancement in the shell can be supplemented by heat transfer augmentation in tubes using twisted tape inserts or micro-finned tubes. The effect of the thermal resistance of the condensing refrigerant could also be taken into consideration. To assess the benefit of using these techniques extended performance evaluation criteria have been implemented at different constraints. The decrease of the entropy generation can be combined with the relative in crease of the heat transfer rate or the relative reduction of the heat transfer area to find out the geometrical parameters of the tubes for optimal thermodynamics performance. The results show that in most of the cases considered, the angled spiraling tube insert technique is the most efficient