Supercritical carbon dioxide (S-CO2) Brayton cycle represents significant advantages in solar tower application. Various configurations of S-CO2 Brayton cycle employing recuperation, recompression, intercooling and reheating have been investigated. The thermodynamic performance of each cycle configuration is optimized by using Genetic Algorithm in which the maximum cycle efficiency is defined as the objective function. The optimization process is comprehensive, i.e., the decision variables such as temperature and pressure of turbines, compressors, re-heaters, inter-coolers, and the pinch point temperature difference are optimized simultaneously. The recompression inlet temperature and mass flow fraction are also optimized along with other decision variables where that is the case. The main limiting factors in the optimization process are maximum cycle temperature, minimum heat rejection temperature, and pinch point temperature difference. The maximum cycle pressure is also a limiting factor in all studied cases except the simple recuperated cycle. The optimized cycle efficiency can vary from 55.77% to 62.02% where the highest value is obtained for the recompression recuperated cycle with reheating and intercooling. The optimization is based on thermodynamic analysis only, even though decision making for practical systems should be based on thermo-economic optimization.

In this paper, two configurations of the S-CO2 Brayton cycles (i.e., the single-recuperated and recompression cycles) are thermodynamically modeled and optimized through a multi-objective approach. Two semi-conflicting objectives, i.e., cycle efficiency (ηc) and cycle specific power (Φsp) are maximized simultaneously to achieve Pareto optimal fronts. The objective of maximum cycle efficiency is to have a smaller and less expensive solar field, and a lower fuel cost in case of a hybrid scheme. On the other hand, the objective of maximum specific power provides a smaller power block, and a lower capital cost associated with recuperators and coolers. The multi-objective optimization is carried out by means of a genetic algorithm which is a robust method for multidimensional, nonlinear system optimization. The optimization process is comprehensive, i.e., all the decision variables including the inlet temperatures and pressures of turbines and compressors, the pinch point temperature differences, and the mass flow fraction of the main compressor are optimized simultaneously. The presented Pareto optimal fronts provide two optimum trade-off curves enabling decision makers to choose their desired compromise between the objectives, and to avoid naive solution points obtained from a single-objective optimization approach. Moreover, the comparison of the Pareto optimal fronts associated with the studied configurations reveals the optimum operational region of the recompression configuration where it presents superior performance over the single-recuperated cycle.

Solar thermal power plants have been constructed over the past two decades to reduce harmful emissions and provide a long-term solution for oil independent electricity generation. Of the solar power plant solutions, Rankine cycle based machines have most widespread uses. This study focuses on the modeling of a solar retrofit to a typical combined cycle power plant. The goal is to operate the plant 17 hours per day, making use of thermal storage capability so that the plant may operate even during a portion of the night time. The plant will be located in Orlando, Florida to take advantage of the abundance of sun in that geographic location. On the cycle side, the amount of solar collectors, the working fluid, and the turbine are considered. The thermal storage system, on the other hand, must be designed based upon a balance between cost and storage density. A decision will be made from existing sensible heat solid storage materials. The storage material evens out the energy supplied to the turbine working fluid between the peak solar radiation of the day time and the absence of solar radiation at night. This plant can be implemented in two ways: as a completely newly constructed power plant or as an addition to a HRSG (Heat Recovery Steam Generation) configuration, which can be retrofitted to an existing combined cycle power plant to increase its overall efficiency. In this study, the addition of a solar air collection system with a storage unit to a HRSG combined cycle power plant is proposed. The HRSG will be designed using a series of energy balances for each component. This proposed plant will then be compared with a similar solar plant to examine its feasibility in terms of land area. The storage unit devised comprises 1377 m3 and stores approximately 3900 GJ of thermal energy, which equates to 8 hours of run time when solar radiation is not available. The benefit of this addition to the plant is that the storage reduces the gas turbine run time necessary to provide hot gas to the HRSG. The total cost of the storage medium is approximately $8 million.

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