A trade-off between maxima in efficiency and specific work output of super- and trans-critical CO2 Brayton cycles

• Super- and trans-critical CO2 cycles for concentrated solar and nuclear power.
• Using real gas EOS and accounting for cycle irreversibilities.
• Thermodynamics of maximum efficiency and maximum specific work output.
• Generation of operational domain and procedures for regulation.
• Empirical relations between temperature, pressure and efficiency.

Several operational aspects for thermal power plants in general are non-intuitive and involve simultaneous optimization of a number of operational parameters. In the case of solar operated power plants, it is even more difficult due to varying heat source temperatures induced by variability in insolation levels. This paper introduces a quantitative methodology for load regulation of a CO2 based Brayton cycle power plant using the ‘thermal efficiency and specific work output’ coordinate system. The analysis shows that a transcritical CO2 cycle offers more flexibility under part load performance than the supercritical cycle in case of non-solar power plants. However, for concentrated solar power, where efficiency is important, supercritical CO2 cycle fares better than transcritical CO2 cycle. A number of empirical equations relating heat source temperature, high side pressure with efficiency and specific work output are proposed which could assist in generating control algorithms.

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