Research on the Supercritical Carbon Dioxide Cycles in the Czech Republic

by Vaclav Dostal & Martin Kulhanek
Czech Technical University in Prague

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Research on the supercritical CO2 cycles is quite wide in the Czech Republic. There are at least 4 institutions which participate in this research and cover most of the issues necessary to the future successful deployment of the supercritical CO2 cycle. The Czech Technical University in Prague and the Nuclear Research Institute Řež coordinates the research program in the field of the supercritical CO2 cycle in the Czech Republic. The research can be divided into two main parts, an experimental part and the analysis and simulation part. Two experimental facilities for the supercritical CO2 cycle are currently envisioned. One will be located in the Nuclear Research Institute Řež and is currently under construction. It is scheduled to become operational in the year 2009 or 2010. The power of this experimental facility will be around 500 kW and will consist of some parts that were used in the previous research on the supercritical CO2 cycle in the research institute in Běchovice, Czech Republic. The new parts will allow raising the temperature to 650°C. The loop will be highly variable to allow for different types of tests such as materials and corrosion, heat transfer, component testing, compression near critical point phenomenon investigation as well as the whole conversion cycle tests. There is also a plan to construct a supercritical CO2 cycle test loop in Pilsen, Czech Republic (the loop will be operated by the Nuclear Research Institute Řež). Having the electric power of 2 to 3 MW, this loop should be capable of proving the cycle concept. As far as the analysis and simulations are concerned, at the Czech Technical University in Prague master and doctoral degree students are developing a code capable of optimizing the cycle and simulating the operational transients and control schemes. Technical University in Brno focuses on sodium to supercritical CO 2 heat exchangers for the SFR program. The cycle code from the Czech Technical University in Prague will be capable of analyzing many different cycle layouts. Currently, the code is capable of steady state analysis only. Simple Brayton cycle, pre-compression cycle, re-compression cycle, re-compression cycle with split expansion, partial cooling cycle and partial cooling cycle with improved regeneration were investigated. The component characteristics, such as machinery efficiencies or heat exchanger effectiveness, were assumed at this point, but their effect and precise values will be investigated in the future. Pressure drops are currently neglected. The following conclusions can be drawn from the current analysis. Operating close to the critical point does not improve the simple Brayton cycle thermal efficiency significantly. Only for compressor outlet pressure of about 25MPa an attractive efficiency is achieved, but very close to the pinch-point. The pre-compression cycle can reach high efficiency for compressor outlet pressure 10MPa. The efficiency of the pre-compression cycle mainly depends on the pre-compressor inlet temperature and the ratio of pressures ratios. The re-compression cycle achieves the highest efficiency at higher pressures (more than 20 MPa). The re-compression cycle with split expansion cycle behaves similarly to the re-compression cycle, but the efficiency is lower. However, it is still high enough to be attractive in the case when lower pressure in the heat source is required. Partial cooling cycle reaches highest efficiencies among all the cycles at 10 and 15 MPa and about the same efficiency as the recompression cycle at 20 MPa. The performance of the partial cooling cycle with improved regeneration is difficult to assess with the current code, since the assumption on the heat exchanger effectiveness are difficult to satisfy. Analysis with the detail design of the cycle components is necessary.