Optimum operating conditions of the small-scale open and direct solar thermal Brayton cycle at various steady-state conditions

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

A heat source can be considered as the Brayton cycle’s life support. This heat source can be extracted from solar energy. The small-scale open and direct solar thermal Brayton cycle with recuperator has several advantages, including lower cost, low operation and maintenance costs and it is highly recommended. The main disadvantages of this cycle are the pressure losses in the recuperator and receiver, turbo-machine efficiencies and recuperator effectiveness, which limit the net power output of such a system. The irreversibilities of the solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. Thermodynamic optimization can be applied to address these disadvantages to optimize the receiver and recuperator and to maximize the net power output of the system at any steady-state condition. The dynamic trajectory optimization method is applied to maximize the net power output of the system by optimizing the geometries of the receiver and recuperator limited to various constraints. Standard micro-turbines and parabolic dish concentrator diameters of 6 to 18 meters are considered. An optimum system geometry and maximum net power output is generated for each operating condition of each micro-turbine and concentrator combination. Results show the optimum operating conditions as a function of system mass flow rate. The optimum operating point of a specific micro-turbine is at a point where the internal irreversibilities are approximately three times the external irreversibilities. For a specific environment and parameters there exists an optimum receiver and recuperator geometry so that the system produces maximum net power output.