Development of Tools for Supercritical CO2 Cycle Analysis at MIT

bby A. R. Ludington, P. Hejzlar, & M. J. Driscoll
Massachusetts Institute of Technology

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The supercritical carbon dioxide cycle has been studied at MIT in the form of the recompression Brayton cycle and an in-house code, CYCLES, has been developed to model the steady state performance of the cycle for non-ideal components and piping system losses. Enhancements to this steady state code now allow the user to investigate the effect of gaseous impurities on the efficiency of the cycle, as the critical point of the working fluid is shifted by the minor components. This is of interest because helium additions may be desired in order to maintain a leak detection capability. Repeated cycle runs show that helium mole fractions above 0.005 result in total cycle efficiency losses greater than 1.0%. Another effort has been focused on the production of a supercritical carbon dioxide radial compressor code based on a mean-line analysis. This code sizes the compressors and produces off-design performance maps for supercritical fluids. The use of these compressor curves allows more accurate modeling in transient analysis codes also being developed at MIT. The new compressor code produced a single-stage main compressor design and a two-stage radial compressor design for 207 MWe power conversion system, achieving total-to-static compressor efficiencies of 90.4% and 91.2%, respectively. Both designs achieve sufficiently wide operating ranges between stall and choke, down to low speeds (70% of the design speed of 3600 RPM). Improvements to the steady-state code CYCLES and the development of compressor models have both contributed to the further understanding of steady-state and transient operation of supercritical carbon dioxide cycles.