(170h) Load-Following Control for a 10 MWe Supercritical CO2 Recompression Brayton Power Cycle

Supercritical carbon dioxide (sCO₂) Brayton power cycles have recently gained attention as an attractive alternative to the more conventional Brayton and Rankine cycles. The high density of the sCO₂ working fluid allows for equipment with a more compact design 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 Program, is currently in the process of building a nominal 10 MWe (net) sCO₂ Brayton cycle test facility to accelerate scale-up, development, and commercial deployment. In this work, a previously designed dynamic process model of a 10MWe indirect recuperated sCO₂ recompression Braytoncycle is used to develop traditional as well as advanced controllers for efficient load-following performance.

Temperature control at various strategic locations is critical for achieving high efficiency in the sCO₂ cycle while ensuring stable operation during load-following. Critical locations where temperature must be controlled within a narrow window at the face of disturbances include: turbine, main compressor, main and bypass fluid mixing point, and the sCO₂ cooler. Temperature control at the turbine, main compressor, and mixing point is integral to efficient cycle operation, while the main compressor inlet temperature is also critical to avoid transition to the sub-critical phase. Since the main compressor inlet temperature remains very close to the critical point, only a narrow temperature margin is affordable to avoid phase transition. Since the cooling water temperature in the sCO₂ cooler at the main compressor inlet should not exceed certain value to avoid scale formation and the compressor surge must be avoided under all circumstances, controlling the main compressor inlet temperature becomes challenging. Furthermore, critical temperatures mentioned earlier are tightly coupled leading to a highly interacting system. Configurational changes along with novel control structures are developed to address this control problem.

Performance of the control system is evaluated under nominal and multiple off-design scenarios under various load ramp rates. For the turbine inlet temperature control, two approaches are considered. The first approach uses inventory control, whereby CO₂ is removed from the closed cycle to maintain the turbine inlet temperature during cycle heat input disturbances. The second approach uses heat input adjustments (natural gas and/or air flow to an external combustor) to maintain this temperature. Studies on load following performance will also be presented when using the first control approach for variations in cycle heat input and the second control approach for fluctuations in inventory. For the main compressor inlet temperature control, configurational changes include recycling a portion of the cooling water from the outlet of the main compressor inlet cooler. The presentation will include critical analysis of trade-offs between systems efficiency and control system performance under the constraints of process operating window at the face of various load ramp rates.