This work describes the one-dimensional, thermo-hydraulic model of the sCO₂ cycle Sofia which was made within the Efekt project to investigate optimal control methods and behaviour of the cycle during its operation. This dynamic model contains all devices like turbomachinery, heat exchangers or valves and piping including heat loss, according to the concept of the 1 MWe sCO₂ cycle, to be realised in the site of a fossil power plant in the Czech Republic.

Model assembly and calculations are performed with the commercial Modelica-based library ClaRaPlus using the simulation environment Dymola and in combination with another Modelica-based library, UserInteraction; the real-time simulations, with some parameter changes during the calculation, are made and described in this paper.

Nominal parameters were achieved during the steady-state simulation, except the lower mass flow of sCO₂. Transient simulation of power turbine start-up from standby state and results are also presented in this paper. The nominal state is
achieved with the semi-automatic procedure in approx. 3 hours. The simulation results allow more detailed analyses of control methods and a better understanding of real device control and behaviour during start-up, shutdown, or other transients.

Careful manipulation with turbine valves in cooperation with the pressuriser operation was identified as crucial for optimal control of the system. Also, the initial amount of CO₂ in the pressuriser affects its behaviour during transients.