Exergetic analysis of supercritical CO2 Brayton cycles integrated with solar central receivers

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Vasquez Padilla, R, Too, YCS, Benito, R & Stein, W 2015, 'Exergetic analysis of supercritical CO2 Brayton cycles integrated with solar central receivers', Applied Energy, vol. 148, pp. 348-365.

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Concentrated Solar Power (CSP) is a promising alternative for generating renewable energy. One of the most prominent CSP technologies is the central receiver tower with heliostat field, which has received attention in the last decade due to its high achievable temperatures and potential cost reduction. In order to make this technology economically viable, research has been focused on the solar field, solar receiver, energy storage and power block. The power block is one of the important components since improving system efficiency leads to reductions in the storage, solar field and receiver sizes and costs. Recently, supercritical CO2 Brayton cycles have emerged as an alternative for power block with central receiver tower systems due to higher thermal efficiencies and compactness compared to traditional steam Rankine cycles. In this paper, detailed energy and exergy analysis of four different supercritical CO2 Brayton cycle configurations (Simple Brayton cycle, Recompression Brayton cycle, Partial cooling with recompression and Recompression with main compression intercooling) were performed with and without reheat. Prior the compressor inlet, dry air cooler is used for all supercritical Brayton cycles studied in this paper. A solar receiver, replacing the heater and reheater for conventional Brayton cycles, is also used to provide heat input to the cycles. The simulations were carried out for Alice Springs (Australia) solar conditions and optimum operating conditions of the supercritical cycle were obtained by optimising the first law thermodynamic efficiency. The effect of turbine inlet temperature and the cycle configuration on the thermal performance and exergy destruction was analysed. The results showed that the thermal efficiency of the supercritical CO2 Brayton cycle increases monotonically with the temperature of the cycle. The recompression cycle with main compression intercooling achieved the best thermal performance (ηI=55.2% at 850 °C). The detailed exergy analysis also showed that the solar receiver has the highest exergy destruction (>68%) while turbines and compressors have minimal contribution (less than 3%). Furthermore, it is noted that the exergy efficiency has a bell shaped curve, reaching at maximum value between 700–750 °C depending on the cycle configuration.