Four different control methods for ramping down the power output from a Supercritical Carbon Dioxide (sCO2) simple cycle were studied to support the development of 10 MWe Pilot Plant Test Facility, funded by the US Department of Energy. These detailed transient models are written using NPSS (Numerical Propulsion System Simulation). The main components of the NPSS models include a compressor, turbine, High-Temperature Recuperative heat exchanger (HTR), cooler, heater, pipes, and valves. In the transient models, the thermal mass and CO2 fluid volume for each main component are based on representative data or proven design practices for the corresponding component. The steady-state performance of each main component has been validated with representative data while the transient performance of the HTR has been validated with published experimental data. The models have been used to study the methods to ramp down the power output. The methods include extracting the CO2 from the inventory, reducing the opening of turbine inlet throttle valve, and increasing the temperature of the cooling water entering the cooler. These methods, along with a hybrid method of combining the first two methods, were evaluated for the rate of turndown in the power output, operability of the compressor, and cycle efficiency. The preliminary results suggest that inventory extraction is the most efficient but has a slow turndown rate while turbine throttle control is less efficient but results in a faster turndown rate. In addition, the inventory extraction reduces the margin of the compressor choke line but the turbine throttle control increases the margin of the choke line.

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