Development of Integration Methods for Thermal Energy Storages Into Power Plant Processes

Abstract

Germany’s current energy policy is focused on the replacement of the conventional powered electrical energy supply system by renewable sources. This leads to increasing requirements on the flexibility for the conventional thermal power plants. Larger differences between energy supply from renewable energy sources and energy demand in the grid lead to high dynamic requirements with respect to the load change transients. Furthermore, a reduction of the required minimum load of existing thermal power plants is necessary. The existing power plants are indispensable for securing the network stability of the power grid. Accordingly, activities to improve the flexibility of existing power plants are required. By the use of thermal energy storage (TES) it is possible to increase the load change transient. Furthermore, it is possible to temporarily provide an increased generator power and reduce the minimum technical load of the unit. Currently, there is no closed methodical approach for the load profile-dependent and location-based dimensioning and integration of TES into thermal power plants. The aim is to generate contributions for the development of a universal design method. This requires the provision of characteristics for dimensioning and integration of TES into thermal processes. For this purpose, it is necessary to derive quantifiable information on the required capacity, performance and stationary and dynamic operating conditions. Starting from analyzing the anticipated, site-specific load profiles the derivation of concepts for technical implementation, feedback on the process and cost of the thermal storage unit takes place. In order to investigate the technical feasibility, the implementation of storage and the associated control concepts as well as to validate the developed design models, the test facility THERESA has been built at the University of Applied Sciences in Zittau (Germany). The acronym THERESA is the abbreviation for thermal energy storage facility. This test facility includes a reconstructed thermal water-steam process, similar to a power plant with integrated TES. The test facility is unique in Germany and enables the delivery of saturated steam up to 160 bars at 347 ° and superheated steam up to 60 bars at 350 °C with an overall thermal power of 640 kW. The design, planning and construction of the facility took 3 years and required an investment volume of 3 mill. Euro. The facility includes two preheater stages, steam generator, super heater, direct TES with mixing preheater and a heat sink. The TES with a volume of 600 L as well as the mixing preheater are prototypes which developed for the special requirements of the facility. Based on this facility, it is possible to investigate methods for the flexibilization of thermal power plants with TES under realistic parameters. Furthermore, the test facility allows the development of control and regulatory approaches as well as the validation of simulation models for process expansion of thermal power plants. Initial investigations show the impact of a simulated load reduction at the heat sink on the system behavior. Here, the load reduction takes place from the heat sink in the storage without changing the steam production. The development and construction of the test facility were funded by the Free State of Saxony and the European Union. The further work on the development of the integration methods are funded by the European Social Fund ESF.