The demand for load following operation capabilities of the nuclear power plants is increased in accordance with the recently increased renewable energy utilization. In order to cope with hourly load change due to the intermittence problem of the renewable energy, the concept of thermal energy storage with $CO_2$ power conversion system is suggested as an ad hoc system to the existing nuclear power plant for this purpose in this thesis.
For the power conversion of the stored thermal energy, supercritical $CO_2$ based power conversion system is studied due to the compactness and high performance while having simple system configuration. In this thesis, the $CO_2$ based mixture power cycles are studied to improve the disadvantages of the existing pure carbon dioxide power system. Among various substances, a multi-variable comparison method was utilized to distinguish the suitable mixing additives by maintaining the advantage of the pure $CO_2$ system. It was confirmed that moving the critical point of $CO_2$ is effective to improve the power system’s performance by mixing heavy molecular weight substances such as organic refrigerants and solvents. Through the thermodynamic analysis, it was confirmed that the mixture power cycle can achieve higher efficiency than the pure $CO_2$ system by lowering the minimum pressure and increasing recuperation while avoiding thermodynamic limitations near the critical point. In order to show the feasibility of the suggested mixture power cycle, lab-scale compressor and heat exchanger performance tests were conducted. Through the experimental results of turbomachinery and heat exchanger, the feasibility of the suggested mixture power cycle is demonstrated while reducing the uncertainties of mixture properties.
From the experiments and thermodynamic analysis, it is concluded that the suggested thermal energy storage system integrated nuclear power plant with $CO_2$ mixture power cycle will enable the nuclear power plant operation for daily load following operation without changing the nuclear reactor core thermal power. It is further expected that the mixture power cycle can be utilized for general heat source with low temperature especially in marine propulsion systems or space applications due to the compactness and high efficiency.