Dynamic Performance and Control Analysis of a Supercritical CO2 Recuperated Cycle

Abstract

Supercritical CO2 (sCO2) power cycles represent a promising technology for driving the energy transition. In fact, various research projects around the world are currently studying the possible applications of this technology, which is characterized by high efficiency, competitive costs, compact machinery, and enhanced flexibility with respect to competing systems, such as steam-based power cycles. Within this context, the European Union (EU)-funded SOLARSCO2OL project aims to build a MW-scale sCO2 pilot facility coupled with a concentrated solar power (CSP) plant. A transient model of the demonstration plant was previously developed in the transeo simulation tool by the Thermochemical Power Group (TPG) of University of Genoa to study the operational envelope of the cycle. In the present work, the model is upgraded to take into account all the relevant fluid-dynamic and thermodynamic phenomena affecting the transient behavior of the plant. In particular, a detailed crossflow sCO2–air cooler model is now included, which is crucial for assessing the compressor inlet temperature (CIT) behavior and controllability. The system has to comply with several constraints, such as compressor surge margin, turbomachinery inlet temperatures, and compressor inlet pressure (CIP). The desired net power output should also be guaranteed. The dynamic responses of the system to step variations in various input variables were recorded and used to design and tune the main operational controls. The input variables considered include: (1) compressor rotational speed, (2) anti-surge valve (ASV) fractional opening, (3) mass flowrate of air through the cooler, (4) mass flowrate of the molten salts through the heater, and (5) CO2 inventory for injection and extraction of working fluid. The implemented control structure includes proportional–integral–derivative (PID) controllers, feedforward action, and their combinations. The controllers are tuned using a mix of established methods, such as Cohen–Coon response-based PID tuning and adjustments from feedforward controls. The feedforward controls were designed taking into account the steady-state values from off-design simulations, as well as the interactions between each controller and the other controlled variables. The final control setup is tested on various power ramps to assess the capability of the prototype cycle in load following and disturbance rejection, showing very good performance in set-point tracking.