Improving the load flexibility of coal-fired power plants by the integration of a thermal energy storage

Improving the flexibility of conventional power plants is one key challenge for the transformation of the energy system towards a high share of renewable energies in electricity generation. Flexible and dispatchable power plants will contribute to this ongoing transformation process as they compensate the fluctuating electricity generation from renewable energy sources such as wind and photovoltaics. In this context, dynamic simulation models offer an efficient tool to evaluate flexibility measures and the resulting highly transient power plant operation. In this paper, the buildup of a dynamic power plant model using the modeling language Modelica in the simulation environment Dymola is presented. The detailed dynamic power plant model is validated successfully against measurement data from the underlying coal-fired reference power plant. The paper then focuses on the integration of a steam accumulator - also known as a Ruths storage - into the power plant process in order to increase its flexibility. The results of the dynamic simulations show that charging the steam accumulator leads to a reduction of the net power up to 7.0%. By discharging the Ruths storage an additional net power of 4.3% can be activated very quickly. Thus, the storage integration leads to an improved load flexibility with regard to a temporary reduction of the minimum load as well as to the possibility of performing a load change at a constant firing rate (e.g. to participate on the quarter-hourly intraday markets). Furthermore, the integrated energy storage enhances the provision of primary control reserve significantly by +/- 2.8%.