Control of a Supercritical CO2 Recompression Brayton Cycle Demonstration Loop

Abstract

The U.S. Department of Energy is currently focused on the development of nextgeneration nuclear power reactors, with an eye towards improved efficiency and reduced capital cost. To this end, reactors using a closedBrayton power conversion cycle have been proposed as an attractive alternative to steam turbines. The supercriticalCO2 recompression cycle has been identified as a leading candidate for this application since it can achieve high efficiency at relatively low operating temperatures with extremely compact turbomachinery. Sandia National Laboratories has been a leader in hardware and component development for the supercriticalCO2 cycle. With contractor BarberNichols Inc., Sandia has constructed a megawattclass SCO2 cycle testloop to investigate the key areas of technological uncertainty for this power cycle and to confirm model estimates of advantageous thermodynamic performance. Until recently, much of the work has centered on the simple SCO2 cycle—a recuperated Brayton loop with a single turbine and compressor. However, work has recently progressed to a recompression cycle with splitshaft turboalternatorcompressors, unlocking the potential for much greater efficiency power conversion, but introducing greater complexity in control operations. The following sections use testing experience to frame control actions made by test loop operators in bringing the recompression cycle from cold startup conditions through transition to power generation on both turbines, to the desired test conditions, and finally to a safe shutdown. During this process, considerations regarding the turbocompressor thrust state, CO2 thermodynamic state at the compressor inlet, compressor surge and stall, turbine u/c ratio, and numerous other factors must be taken into account. The development of these procedures on the Sandia test facility has greatly reduced the risk to industry in commercial development of the SCO2 power cycle.