Operating conditions of an open and direct solar thermal Brayton cycle with optimised cavity receiver and recuperator

Abstract

The small-scale open and direct solar thermal Brayton cycle with recuperator has several advantages, including low 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, turbomachine 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. In this paper, thermodynamic optimisation is applied to concentrate on these disadvantages in order to optimise the receiver and recuperator and to maximise the net power output of the system at various steady-state conditions, limited to various constraints. The effects of wind, receiver inclination, rim angle, atmospheric temperature and pressure, recuperator height, solar irradiance and concentration ratio on the optimum geometries and performance were investigated. The dynamic trajectory optimisation method was applied. Operating points of a standard micro-turbine operating at its highest compressor efficiency and a parabolic dish concentrator diameter of 16 metres were considered. The optimum geometries, minimum irreversibility rates and maximum receiver surface temperatures of the optimised systems are shown. For an environment with specific conditions and constraints, there exists an optimum receiver and recuperator geometry so that the system produces maximum net power output.

Minimum entropy generation due to heat transfer and fluid friction in a parabolic trough receiver with non-uniform heat flux at different rim angles and concentration ratios

“…Syltherm 800 [38] was used as the absorber tube heat transfer fluid, its properties were entered as temperature dependent polynomials for specific heat capacity (c p ) , density (ρ) and thermal conductivity (λ) and piece-wise temperature dependent polynomial for viscosity (μ) as determined by curve fitting from the manufacturer's data sheets [38] and given by Eqs. (22)(23)(24)(25)(26). Sample thermal properties of Syltherm 800 at T inlet = 400 K, 550 K and 650 K are shown in Table 2.0.…”

“…This method has been termed 6 the entropy generation minimisation method [21]. Several researchers have applied the entropy generation minimisation method to the analysis and optimisation of engineering systems [8,22,23] as well as to heat transfer and fluid flow problems [24][25][26][27]. Moreover, the entropy generation minimisation method has been shown to be applicable to a wide range of engineering systems such as small-scale wood fired circulating fluidised bed adiabatic combustor as demonstrated in the study by Baloyi et al [28], analysis of exergy recovery from the exhaust cooling in a DI diesel engine as demonstrated by Ghazikhani et al [29] and in characterising the effects of fuel additives on exergy parameters of engines [30] and many others.…”

Heat transfer and thermodynamic performance of a parabolic trough receiver with centrally placed perforated plate inserts

“…For such a configuration, the destruction of available work or exergy loss is less when compared with a configuration with higher entropy generation rates [26]. As such, the entropy generation minimization method has been used by several researchers for optimization of thermodynamic systems [36,37] while others have applied it to analysis heat transfer problems [38][39][40]. In heat transfer enhancement, the entropy generation in the enhanced device should be lower than entropy generation in a non-enhanced device for better thermodynamic performance [26].…”

Numerical investigation of entropy generation in a parabolic trough receiver at different concentration ratios

“…Entropy generation minimisation combines the fundamental principles of fluid mechanics, heat transfer and thermodynamics in establishing the irreversibility in system components. Several researchers have studied entropy generation in fluid flow and heat transfer for various system configurations and boundary conditions [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. For solar collectors, Bejan [23] …”