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(749h) Dynamic Modeling for Improved Operation and Control of a Supercritical CO2 Brayton Power Cycle

Authors: 
Zitney, S. E., National Energy Technology Laboratory
Liese, E. A., National Energy Technology Laboratory
Mahapatra, P., National Energy Technology Laboratory
Albright, J., National Energy Technology Laboratory
Supercritical carbon dioxide (sCO2)Brayton power cycles are an attractive alternative to conventional sub/supercritical steam Rankine power plants. The high density of the sCO2 working fluid allows for highly compact equipment and a power cycle with potentially higher thermal efficiencies and lower overall cost of electricity. Due to these advantages over traditional systems, the U.S. Department of Energy, under its Supercritical Transformational Electric Power (STEP) Program, is working with industry and research partners to design, build, and operate a 10 MWe (net) sCO2 pilot plant test facility that uses a natural gas-fired furnace as the indirect heat source. The new facility will support the demonstration of systems integration, performance, operability, and controls for the future commercialization of utility-scale sCO2 Brayton power cycles. In this presentation, a pressure-driven dynamic process model of an indirect-fired 10MWe sCO2 recompression Brayton cycle is used to develop and evaluate operational and control strategies for improving cycle performance during transient, off-design conditions.

Considering Carnot's rule, the sCO2 cycle efficiency is maximized by keeping the ratio between the hot turbine inlet temperature (TIT) and the cold main compressor inlet temperature (MCIT) as high as possible. To maintain the TIT at its maximum design point during cycle turndown and ramp-up operations, this study uses inventory control wherein sCO2 is removed and added, respectively, from the closed cycle. The impacts of inventory tank capacity and initial pressure on maximum sCO2 cycle turndown are investigated. An advanced controller logic is developed that regulates the inventory inlet and outlet valves in a fast and effective manner to achieve the TIT control objectives while satisfying process constraints. Similarly, cooling water flow is used to maintain the MCIT at its minimum design point slightly above the critical temperature of CO2, thereby avoiding transition to the two-phase region. In addition, a controller is designed that aims to maintain an optimal flow split between the main compressor and bypass compressor streams during transient operations. The presentation will include results for various cycle turndown and ramp-up scenarios, as well as analysis of trade-offs between systems efficiency and control system performance.