Thermodynamic Aspects of Cycles with Supercritical Fluids

by Petr Hájek
Research centre Rez ltd.

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The supercritical power cycles are taking advantage of real gas behaviour in order to achieve high thermal efficiency. There are two main types of supercritical cycles, one uses water and the other carbon dioxide. The supercritical water cycle enhances thermal efficiency by rising turbine inlet temperature, while the supercritical carbon dioxide (S-CO2) takes advantage of reduction of compressor input work due to properties change close to the critical point (30.98°C, 7.38MPa).

Thermodynamic analysis and comparison of different modifications of the S-CO2 cycle from the point of cycle thermal efficiency have been performed. Analyzed cycles were: simple Brayton cycle, pre-compression cycle, recompression cycle, split expansion cycle, partial cooling cycle and partial cooling cycle with improved regeneration. Different computer codes were developed for each cycle to evaluate all thermodynamic states. For optimizing the structure of the cycles, compressor inlet and turbine inlet temperatures were held constant (32°C and 550°C) and other parameters such as compressor outlet pressure, turbine pressure ratio were varying. Also some analysis of using S-CO2 in combination with steam cycle was made. For standard PWR is this combined cycle very promising. An experimental S-CO2 loop was built in 1999 in the Czech Republic. The main objective was to obtain experimental data for comparison with previous theoretical studies. This facility was the first of its kind in the world. Its operation and performed measurements have provided many interesting data and thus brought valuable operational experience as well as new objectives for future research and development of S-CO2 cycles.