Book Description

The supercritical carbon dioxide (S-CO2) Brayton cycle is under development at Argonne National Laboratory as an advanced power conversion technology for Sodium-Cooled Fast Reactors (SFRs) as well as other Generation IV advanced reactors as an alternative to the traditional Rankine steam cycle. For SFRs, the S-CO2 Brayton cycle eliminates the need to consider sodium-water reactions in the licensing and safety evaluation, reduces the capital cost of the SFR plant, and increases the SFR plant efficiency. Even though the S-CO2 cycle has been under development for some time and optimal sets of operating parameters have been determined, those earlier development and optimization studies have largely been directed at applications to other systems such as gas-cooled reactors which have higher operating temperatures than SFRs. In addition, little analysis has been carried out to investigate cycle configurations deviating from the selected 'recompression' S-CO2 cycle configuration. In this work, several possible ways to improve S-CO2 cycle performance for SFR applications have been identified and analyzed. One set of options incorporates optimization approaches investigated previously, such as variations in the maximum and minimum cycle pressure and minimum cycle temperature, as well as a tradeoff between the component sizes and the cycle performance. In addition, the present investigation also covers options which have received little or no attention in the previous studies. Specific options include a 'multiple-recompression' cycle configuration, intercooling and reheating, as well as liquid-phase CO2 compression (pumping) either by CO2 condensation or by a direct transition from the supercritical to the liquid phase. Some of the options considered did not improve the cycle efficiency as could be anticipated beforehand. Those options include: a double recompression cycle, intercooling between the compressor stages, and reheating between the turbine stages. Analyses carried out as part of the current investigation confirm the possibilities of improving the cycle efficiency that have been identified in previous investigations. The options in this group include: increasing the heat exchanger and turbomachinery sizes, raising of the cycle high end pressure (although the improvement potential of this option is very limited), and optimization of the low end temperature and/or pressure to operate as close to the (pseudo) critical point as possible. Analyses carried out for the present investigation show that significant cycle performance improvement can sometimes be realized if the cycle operates below the critical temperature at its low end. Such operation, however, requires the availability of a heat sink with a temperature lower than 30 C for which applicability of this configuration is dependent upon the climate conditions where the plant is constructed (i.e., potential performance improvements are site specific). Overall, it is shown that the S-CO2 Brayton cycle efficiency can potentially be increased to 45 %, if a low temperature heat sink is available and incorporation of larger components (e.g., heat exchangers or turbomachinery) having greater component efficiencies does not significantly increase the overall plant cost.