Experimental approach to develop the transient PCHE model for near CO2 critical point operation

Abstract

The demand for renewable energy is rapidly increasing worldwide to reach the carbon net-zero goal. However, the intermittency of renewable energy requires a base power source that can replace fossil fuels and provide fast load-following operation. Small modular nuclear power plants using supercritical carbon dioxide (sCO2) power cycles satisfy these requirements. However, the sCO2 power cycle has a significant property change near the critical point, and the pre-cooler outlet operating near the critical point significantly impacts the compressor inlet property. The accurate analysis of the pre-cooler under the transient conditions is imperative for maintaining off-design performance and operating with the sufficient stability margin of the compressor. Therefore, performing an accurate transient analysis of the pre-cooler is important. The 1-D heat exchanger model for a system transient analysis code with only the heat exchanger design information is accurate in a steady state. However, based on the literature review and from the author's experience, the difference between the actual conditions and model predicted conditions occurs under a transient. Therefore, in this thesis, the author uses an experimental approach to develop a transient 1-D heat exchanger modeling procedure that can reduce the difference between the real conditions and the model predicted conditions. To develop the transient 1-D heat exchanger modeling procedure, the transient 1-D heat exchanger model analysis and literature review were carried out to determine the variables with high uncertainty, such as effective wall thermal inertia and transient heat transfer coefficient, and set up an experimental scenario to evaluate these selected variables. By performing a sensitivity analysis on highly uncertain variables, a method is proposed to calculate the optimal effective wall thermal inertia and transient heat transfer coefficients that can reduce the difference between the real and model conditions. The proposed procedure is applied to a PCHE-type pre-cooler of the ABC test loop, and eight mass flow rate alternating experiments on the CO2 and water sides, respectively, were performed for the regions where the real gas effect is either strong or weak. The sensitivity analysis results showed that the difference between the real and model conditions can be reduced by calculating the optimal values within the physical range of the variables with high uncertainty of the PCHE-type heat exchanger. To evaluate the scalability of the transient 1-D heat exchanger modeling procedure, the conceptual design of the pre-cooler for 100 kWth, 1 MWth, 10 MWth, and 30 MWth systems was performed to evaluate the feasibility of the modeling procedure. As a result, the 1-D heat exchanger modeling procedure proposed in this thesis can reduce the difference between the real heat exchanger conditions and the model predicted conditions in the pre-cooler of a nuclear reactor system.