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The first law and second law efficiencies are determined for a stainless steel closed-tube open rectangular cavity solar receiver. It is to be used in a small-scale solar thermal Brayton cycle using a micro-turbine with low compressor pressure ratios. There are many different variables at play to model the air temperature increase of the air running through such a receiver. These variables include concentrator shape, concentrator diameter, concentrator rim angle, concentrator reflectivity, concentrator optical error, solar tracking error, receiver aperture area, receiver material, effect of wind, receiver tube diameter, inlet temperature and mass flow rate through the receiver. All these variables are considered in this paper. The Brayton cycle requires very high receiver surface temperatures in order to be successful.These high temperatures, however, have many disadvantages in terms of heat loss from the receiver, especially radiation heat loss. With the help of ray-tracing software, SolTrace, and receiver modelling techniques, an optimum receiver-to-concentrator-area ratio of A' ≈ 0.0035 was found for a concentrator with 45° rim angle, 10 mrad optical error and 1° tracking error. A method to determine the temperature profile and net heat transfer rate along the length of the receiver tube is presented. Receiver efficiencies are shown in terms of mass flow rate, receiver tube diameter, pressure drop, maximum receiver surface temperature and inlet temperature of the working fluid. For a 4.8 m diameter parabolic dish, the larger the receiver tube diameter and the smaller the mass flow rate through the receiver, the higher the receiver surface temperature and the less efficient the collector becomes. However, the smaller the receiver tube diameter, the higher the pressure drop through the tube and the smaller the second law efficiency. It was found that the 2 receiver with larger tube diameter would perform better in a solar thermal Brayton cycle. An overall solar-to-heat efficiency of between 45% and 70% is attainable for the solar collector using the open-cavity receiver.Keywords: solar, receiver, cavity, tracking, Brayton, efficiency 1.Introduction and background The solar thermal Brayton cycleThe closed Brayton cycle was developed in the 1930s for power applications [ The open Brayton cycle uses air as working fluid, which makes this cycle very attractive for use in water-scarcecountries. The open and direct solar thermal Brayton cycle is shown in Fig. 1 [2]. The parabolic dish concentrator is used to reflect and concentrate the sun's rays onto the receiver aperture so that the solar heat can be absorbed by the inner walls of the receiver. The heat is then transferred to the working fluid (air). The compressor increases the air pressure before the air is heated in the receiver. The compressed and heated air expands in the turbine, which produces rotational power for the compressor and the electric load.In the recuperator, hot exhaust air preheats the colder air before it enters the receiver. For the so...

SUMMARYThe 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 six to 18 metres 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.

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Highlights • A solar dish collector with spiral absorber is investigated experimentally. • A thermal model developed in EES is validated with experimental results. • Water, thermal oil and air are examined at various mass flow rates and temperatures. • Maximum exergetic efficiency is 7.58% for thermal oil at inlet temperature of 155 °C. • System is feasible where solar potential is 1600 kW h/m 2 and heating cost 0.15 €/kW h.