Dynamic modelling and control of supercritical CO₂-power cycles

The way in which energy is demanded and supplied is undergoing a significant change. The transition from fossil fuels, which can be consumed on request, to renewable energy sources, which are only available at certain times and for a limited time period, is imperative. In order to utilise these sources, it is necessary to identify flexible methods of managing their variable availability.

Firstly, this necessitates the development of storage technologies. Secondly, the stored energy must then be converted to electricity when required. For this purpose, supercritical CO₂ (sCO₂) power cycles, which employ highly compressed CO₂ as the heat transfer medium, can be implemented. The facilities of these cycles are compact in size, which allows their installation in a wide variety of locations due to minimal spatial requirements, and are highly efficient.

Overview of an sCO2 cycle with variable power output and various renewable energy sources.

Moreover, given that they operate above the critical temperature of CO₂ at 31 °C, any renewable heat source -including stored energy- that exceeds this temperature can be utilised. The objective identified is to operate these systems in regards to the demand of electricity. In instances where renewable energy sources are not available, stored energy is retrieved, with the sCO₂ power cycle converting the energy. Conversely, if the renewable energy is available, the cycle is halted. This signifies that the system must be capable of rapid start-ups and shut-downs, as well as operations in off-design cases, all while maintaining a high efficiency.

To address this, a dynamic model of an sCO₂ Brayton cycle is developed in MATLAB Simulink to explore different operational strategies. The system is composed of three heat exchangers (heater, cooler, recuperator) and a turbine and compressor. The institute’s own experimental sCO₂ facility CARBOSOLA, is being used to validate the model.

Example of a control mechanism with bypass and inventory mass control for dynamic operation.	Model of a discretised Printed Circuit Heat Exchanger, typically used for sCO2 applications.