Dynamics of SCO2 heat to power units equipped with dual tank inventory control system

A key aspect in upscaling the technology readiness level of supercritical CO2 (sCO2) power generation systems is the control of the main cycle parameters (i.e. temperature at the turbine or compressor inlet) at off-design conditions and during transient operation. A further challenge in small scale (<0.5MWe) systems is the limited number of control variables due to the streamlined configuration of the power units. Among the possible control strategies, is the regulation of the system inventory, which consists of the variation of the CO2 fluid mass (or charge) in the power loop to achieve a given control target. Such strategy, which relies on different storage tanks for injections/withdrawals of the working fluid into/from the system, poses several technical challenges that are still not fully understood. To fill the gap, this work presents an analysis of inventory control systems. The impact of this control approach is investigated using a high-fidelity one-dimensional simulation platform calibrated on a 50 kW simple regenerative High Temperature Heat to Power sCO2 test facility being commissioned at Brunel University London. Transient simulations are carried out to assess the dynamics of the main thermodynamic variables in the power loop and the inventory tanks. Stability implications (e.g. pressure gradients in the loop) as well as the effects of size of the inventory tanks are discussed. Inventory tanks with a volume 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).