Abstract:
The small-scale dish-mounted open solar thermal Brayton cycle (1-20 kW) with recuperator has an advantage in terms of cost and mobility and can offer an off-grid electricity solution to the people of the water-scarce southern Africa. South Africa has an advantage in terms of solar resource, but this solar resource is not used extensively due to high-cost and low-efficiency solar-to-electricity systems. The dish-mounted solar thermal Brayton cycle with recuperator offers a solution. However, heat losses and pressure losses in the cycle components can decrease the net power output of the system tremendously. In addition, the costs due to solar tracking and perfect dish optics can be high. The purpose of the study was to develop the small-scale (1-20 kW) dish-mounted open solar thermal Brayton cycle by optimising an open-cavity tubular solar receiver and counterflow plate-type recuperator with the method of total entropy generation minimisation. The optimised receiver was also tested in an experimental dish collector set-up. Modelling methods to predict the performance of the cycle and to optimise the solar receiver and recuperator were developed and tested so that the small-scale open solar thermal Brayton cycle could be developed further. SolTrace was used as ray-tracing method to determine the effects of inaccurate dish optics. An optimum concentration ratio of 0.0035 was identified for a collector with a maximum tracking error of 1° and an optical error of 10 mrad. It was shown that the open-cavity tubular solar receiver surface temperature and net heat transfer rate for heating air depended on the receiver size, mass flow rate through the receiver, receiver tube diameter, receiver inlet temperature and dish errors. Receiver efficiencies of between 43% and 70% were found for a receiver with mass flow rates of between 0.06 kg/s and 0.08 kg/s, tube diameters of between 0.05 m and 0.0833 m, air inlet temperatures of between 900 K and 1 070 K operating on a dish with 10 mrad optical error and maximum solar tracking error of 1°. With the use of Matlab and Flownex, it was shown that the small-scale open solar thermal Brayton cycle could generate a positive net power output with solar-to-mechanical efficiencies in the range of 10-20% with much room for improvement. The maximum receiver surface temperature was restricted to 1 200 K and the recuperator weight was restricted to 500 kg. An experimental set-up with a 4.8 m diameter parabolic dish with rim angle of 45° on a two-axis tracking system was constructed to test the receiver. An optimised open-cavity stainless steel tubular receiver with tube diameter of 88.9 mm was tested in the experiment. The experimental results showed the challenges regarding the design and construction of a solar thermal Brayton cycle collector. It was found that the insulation arrangement around the large receiver tube diameter influenced the heat loss due to convection and conduction. Results showed that with further research, the small-scale open solar thermal Brayton cycle could be a competitive small-scale solar energy solution to the people of South Africa.
