Inventory control assessment for small scale sCO2 heat to power conversion systems

The control of the main cycle parameters in supercritical CO2 (sCO(2)) systems during off-design and transient operation is crucial for advancing their technological readiness level. In smaller scale power units (<0.5-5 MW), costs and complexity constraints limit the number of auxiliary components in the power loop, making the design of the control system even more challenging. Among the possible strategies, the regulation of system inventory, which consists in varying the CO2 fluid mass in the power loop to achieve a given control target, represents a promising alternative. Such technique however poses several technical challenges that are still to be fully understood. To fill this gap, this work presents a comprehensive steady-state and transient analysis of inventory control systems, referring in particular to a 50 kW sCO(2) test facility being commissioned at Brunel University. Stability implications (e.g. pressure gradients in the loop) and the effects of variable inventory tank size are discussed. Tank volumes 3 times higher than the one of the power loop (including the receiver) can lead to a higher controllability range (+/- 30% of the nominal turbine inlet temperature) and an extended availability of the control action (slower tank discharge). A PI controller is also designed to regulate the turbine inlet temperature around the target of 465 degrees C in response to waste heat variations.