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Supercritical carbon dioxide (sCO2) Brayton cycle is an emerging technology to be used as a power block with concentrated solar power (CSP) systems, tower type, sCO2 Brayton cycle has the potential to be competitive with traditional Rankine steam cycle. Most of the studies have been focused on the steady state analysis of this technology. This research has developed numerical models for five configurations of sCO2 Brayton cycles operating under quasi steady state conditions. The studied cycles are connected directly to the solar central receiver tower, which is used to provide heat input to the cycles; consequently, the heat addition is changing over time as a function of solar radiation. During the off load operation, the mass flow rate of the cycle is changing with the goal of keeping the turbine inlet temperature at 700°C. The compressor and turbine use a partial load model to adjust velocities according to the new mass flow rate. Also, the heat exchangers effectiveness are adjusted as they present off-design operation. In the recompression cycle, the model permits to explore the relationship between recompression fraction and the ambient temperature. It is demonstrated that the power generated by the cycle may be improved more than 6 % if the recompression fraction is continuously changed and controlled as a function of the ambient temperature.

Bladed structures offer an approach to improve the efficiency of conventional concentrating solar power (CSP) cylindrical receivers, due to improved light-trapping via the cavity effect, and by allowing more tubes to be compressed into a smaller aperture, enabling the flux on the aperture to be increased without exceeding the peak flux limitation on individual tubes. In this paper, we present an optical model of a hypothetical bladed receiver mounted on the tower of the Sandia National Solar Thermal Test Facility (NSTTF). We examine the impact of receiver geometric parameters including receiver width, receiver height, number of blades, blade depth and blade angle, through analysis using ‘Tracer’, an open-source Python-based Monte Carlo ray tracing library. Validation of Tracer is provided, through comparison with results from other tools. At the optimal configuration, 15 blades with a depth of 4.5 m and angle of 63.9° from the vertical are spaced vertically over a 9.6×9.6 m back wall. In this configuration, the peak flux occurs on the back plane and is considerably lower than a corresponding flat receiver. The design-point receiver optical efficiency increases from 93.8% for a flat receiver to 98.5% for the bladed configuration, and is shown to be robust to sun position changes.

Convective air flows are a significant source of thermal loss from tubular cavity receivers in concentrating solar-thermal power (CSP) applications. Reduction in these losses is traditionally achieved by tailoring the cavity geometry, but the potential of this method is limited by the aperture size. The use of active airflow control, in the form of an air curtain, is an established practice to prevent infiltration of cold air through building doorways. Its application in reducing solar receiver convective heat loss is new. In this study, computational fluid dynamics (CFD) simulations are presented for the zero wind case, demonstrating that an optimised air curtain can readily reduce convective losses by more than 45%. A parametric investigation of jet direction and speed indicates that two distinct optimal air curtain flow structures exist. In the first, the jet reduces the size of the convective zone within the cavity by partially sealing the aperture. The optimum velocity range for this case occurs with a low strength jet. At higher jet speeds, the losses are generally set by the flow induced in the cavity and entrainment into the jet. However, a second optimal configuration is discovered for a narrow range of jet parameters, where the entrainment is reduced due to a shift in the stack neutral pressure level, allowing the jet to fully seal the cavity. A physical model is developed, based on the fluid physics of a jet and the ‘deflection modulus’ concept typically used to characterise air curtains in building heating and ventilation applications. The model has been applied to the solar thermal cavity case, and shows good agreement with the computational results.Copyright © 2015 by ASME

We investigate losses and costs associated with direct steam generation via parabolidal dish concentrators and steam transport to a central steam Rankine power cycle for electricity generation. This study presents a power plant model that accounts for the effects of shading, steam transport, energy conversion at the power block, and capital costs of land and pipework. The pipe network topology used was optimised using a genetic algorithm based on evolution of minimal spanning trees connecting all dishes to a central power block. Optimal pipe sizing of the network is determined by considering the trade-off of frictional losses against thermal losses and material costs. Weather data provides input for the solar resource, and shading is calculated using an established numerical model. The plant model is used to determine the collector layout for which the effective annual revenue is maximised. Results show that the optimal rectangular layout is closely spaced in the North-south direction, along which most of the pipe links run, while East-west spacing is less important. The annual thermal performance of the optimised dish field on a per-unit-area basis is then compared to simulation of a parabolic trough employed for the same purpose. A detailed breakdown of the thermal analysis used forms the basis of comparison between the collector types, giving the overall advantage and a comparison of various sources of loss. We demonstrate that dish fields can collect approximately 49% more thermal energy annually per unit collector area than a trough system employed for the same purpose.Copyright © 2015 by ASME