Modeling Off-Design Operation of a Supercritical Carbon Dioxide Brayton Cycle

by John J. Dyreby, Sanford A. Klein, Gregory F. Nellis, & Douglas T. Reindl
University of Wisconsin - Madison, Solar Energy Laboratory

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In the search for increased efficiency of utility-scale electricity generation, Brayton cycles operating with carbon dioxide have found considerable interest. Due to the unique properties of carbon dioxide, high design-point efficiencies can be realized by operating the compressor near the critical point (7.4 MPa and 31°C). In this paper, we show that the thermal efficiency and power production of a cycle using fixed turbomachinery designed to provide optimal performance under these conditions will decrease as the compressor inlet temperature increases. Conversely, turbomachinery designed to provide optimal performance at a higher compressor inlet temperature (e.g., 60°C), which results in lower efficiency at the design point, exhibits an increase in both efficiency and power production under off-design conditions as the inlet temperature is reduced.

The findings of this research are significant in that they suggest the optimal design for the turbomachinery in a supercritical carbon dioxide (S-CO2) Brayton power cycle, considering its overall performance, may not coincide with the optimal design suggested by a simple on-design thermodynamic analysis. Initial results suggest that designing for a higher compressor inlet temperature will not significantly degrade plant efficiency and it can yield better off-design power production, possibly increasing the overall power plant performance evaluated on an annual basis. These results are particularly relevant to renewable energy applications that are inherently transient, such as concentrating solar power (CSP) systems.