Turbine Bypass, Mass Inventory, and Mixed-Mode Generator Power Control of S-CO₂ Recompression Cycle

The supercritical carbon dioxide (S-CO₂) recompression cycle is a power conversion cycle compatible with intermediate-temperature nuclear reactors. The main advantage of the S-CO₂ cycle is relatively high efficiency (∼47% at the turbine inlet temperature of 650°C). The dynamic characteristics and control of this cycle remain areas of active research because of the cycle's unique features, in particular, large fluid property changes near the critical point. This paper reports the conceptual development of a dynamic S-CO₂ recompression cycle controller designed to efficiently respond to a demand for reduction of generator electric power. The S-CO₂ cycle generator electric power production can be controlled using either turbine bypass (TB) or mass inventory (MI) controllers. Turbine bypass is a fast response controller, which reduces generator power by opening the TB valve. Mass inventory is a slow response controller, with a time constant an order of magnitude larger than that of the TB controller. The MI controller reduces generator electric power through decreasing the inventory of CO₂ gas in the cycle by pumping some of the gas into a storage reservoir. Both TB and MI controllers operate in conjunction with a precooler temperature controller, which maintains compressor inlet conditions near the critical point. Although using a TB controller allows for quick reduction of the generator electric power, S-CO₂ cycle thermal efficiency is reduced during the steady-state operation. Cycle efficiency can be improved if cycle control is transitioned from the TB to the MI controller. However, directly switching from the TB to the MI controller would result in a spike in generator power because of the large discrepancy between the time constants of the two cycle control modes. To address this deficiency, we have designed a mixed-mode (MM) controller to transfer cycle control to MI mode after steady state has been reached in TB mode. In the MM controller, both TB and MI controllers operate simultaneously, thus maintaining nearly constant generator electric power during S-CO₂ cycle control transitioning. Design of an MM controller for the S-CO₂ cycle does not appear to have been previously reported in literature. To test our controller design, we have performed proof-of-concept numerical experiments. All controllers in this study were implemented as proportional-integral controllers using the System Control Module (SCM) language. Gain coefficients for all controllers were determined via numerical experiments, in which response of the S-CO₂ cycle was calculated with the GPASS (General Plant Analyzer and System Simulator) software package. Gain coefficients and cycle timescales were calculated under idealized conditions of instantaneous measurement response.