Supercritical CO2 Regeneration Bypass Cycle - Comparison to Traditional Layouts

by Vaclav Dostal & Jan Dostal
Czech Technical University in Prague

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This paper describes a new layout of supercritical CO2 cycle - a supercritical CO2 regeneration by pass cycle. This cycle is simpler then the commonly used recompression cycle. It does split the recuperator into two parts, the high temperature and the low temperature recuperator, but the split of flow does not take place before the precooler. Instead the flow is split after the main compressor; therefore the recompressing compressor is not necessary. A new supercritical CO2 cycle analysis code has been developed for this analysis and it is still under development in order to extend it for different cycle layout analysis, including the heat exchanger design and optimization. In the analysis it is show that in the pinch-point region this cycle reaches higher efficiencies than the simple Brayton cycle. The pinch-point region is at lower pressure ratios which simplifies the compressor design, but makes the specific work of the cycle lower. The recompression cycle achieves higher efficiencies, but the difference is not as significant as for the simple Brayton cycle. Given the simpler design of the regeneration bypass cycle it may be of interest for some applications of the supercritical CO2 cycle. In addition, the variation of the flow split is much simpler in this cycle as it does not affect the compressor performance. By proper variation of the flow split an almost constant dependence of the efficiency on the pressure ration is achieved. Therefore, a better off design point performance is expected. This work is a part of the Czech research program on the supercritical CO2 cycle, which currently involves constructions of two experimental supercritical CO2 loops for the performance confirmation of different supercritical CO2 cycle layouts as well as for the investigation of some basic phenomena necessary for the successful application of the supercritical CO2 cycle. The first experimental loop is a small one with the power of about 500 kW. It has been designed and is now in preparation for construction, with some parts already put in place. The larger experimental loop power will around 10 MW. This power should ensure demonstration of the supercritical CO2 cycle feasibility as it will use the same technology and engineering concepts (especially in the turbomachinery area) that a large 300 MWe loop would use.