Control strategies for supercritical carbon dioxide power conversion systems

Abstract

The supercritical carbon dioxide (S-C02) recompression cycle is a promising advanced power conversion cycle which couples well to numerous advanced nuclear reactor designs. This thesis investigates the dynamic simulation of, control strategies for, and selected transient results for an indirect S-CO2 recompression cycle. The cycle analyzed is a 600 MWth, highly recuperated, single shaft recompression power conversion cycle with a turbine inlet temperature of 6500C. The cycle features relatively high net efficiency (-47%) at relatively low heat addition temperatures, primarily due to efficient compression. The bottom of this cycle approaches (but in the steady state does not cross) carbon dioxide's critical point, where high fluid densities (-600 kg/m 3) allow efficient compression. Dynamic simulation of this cycle is complicated by its key features: single-shaft constant-speed turbomachinery, main and recompression compressor in parallel, operation of the main compressor inlet very close to the critical point, and rapid fluid property changes surrounding the critical point. A dynamic simulation and control code for gas-cooled Brayton Cycle reactor power conversion systems (PCS) has been significantly modified and enhanced to use supercritical carbon dioxide as the working fluid. These modifications include the incorporation of accurate yet fast fluid properties, more detailed modeling of turbomachinery performance, and rapid yet accurate calculation of heat exchange in printed circuit heat exchangers, even with rapid fluid property changes. Of particular significance are the methods devised to overcome convergence problems caused by compression near the critical point of C02, and the attendant large variations in properties in the main compressor, precooler and low temperature recuperator.

Coding innovations have made faster than real time simulation possible (on today's off the shelf hardware), which makes plant simulator and control applications feasible. This code was used to devise and investigate some of the major control strategies required to operate the cycle: high and low temperature control, three variations of turbine bypass, and inventory control. Using these strategies various transients were investigated including part-load operation, loss-of-load, loss of heat sink, over-power, and startup/shutdown.