For conventional power plants, the integration of thermal energy storage opens up a promising opportunity to meet future technical requirements in terms of flexibility while at the same time improving cost-effectiveness. In the FLEXI- TES joint project, the flexibilization of coal-fired steam power plants by integrating thermal energy storage (TES) into the power plant process is being investigated. In the concept phase at the beginning of the research project, various storage integration concepts were developed and evaluated. Finally, three lead concepts with different storage technologies and integration points in the power plant were identified. By means of stationary system simulations, the changes of net power output during charging and discharging as well as different storage efficiencies were calculated. Depending on the concept and the operating strategy, a reduction of the minimum load by up to 4% of the net capacity during charging and a load increase by up to 5% of the net capacity during discharging are possible. Storage efficiencies of up to 80% can be achieved.

Thermal energy storage (TES) systems are key components for concentrated solar power plants to improve their dispatchability and for shifting the energy production efficiently to high revenue periods. The commercial state of the art is the molten salt two tank storage technology. However, this TES confronts some issues like freezing and decomposition, which require continued technical attention. Furthermore, the molten salt itself is very expensive compared to other storage materials. A TES option that possesses a high cost reduction potential and the ability to increase the whole power plant efficiency is the regenerator-type energy storage. Here, a packed bed inventory of waste metallurgical slag from electric arc furnace (EAF) can achieve further cost reduction. Despite previous studies regarding the use of steelmaking slag as an inventory material for thermal energy storages, there are still basic questions to be answered. This work presents experimental thermal performance and thermo-structural stability studies of slag-based TES, obtained during the European project REslag. The EAF slag and different insulation options were tested for their thermomechanical strength in a uniaxial compression test rig. The thermal cyclic behavior was investigated in a pilot TES plant with temperatures up to 700 °C. The experimental results confirm the suitability of steelmaking slag as thermal energy storage inventory material. Furthermore, a comparison of experimental and simulation model results shows an agreement of over 90%.

Grid-compliant integration of renewable energies will in future require considerable increases in flexibility in the operation of conventional power plants. The integration of thermal energy storage systems (TES) into the power plant process can create considerable improvements, for example, in the speed of load change and partial load behavior. In the case of existing plants, there are thus good prospects of upgrading for more flexible operation, which promises improvements in the energy system that can be achieved in the relatively short term. The aim of this publication is, therefore, to identify integration options for TES in coal-fired power plants which would enable the desired high flexibility potentials and, at the same time, include cost-efficient solutions. By means of an iterative approach between future scenarios of the energy market, the power plant process, and the TES component, favored configurations were developed from a wide range of integration concepts. For this purpose, thermodynamic simulation studies were performed, operating concepts were developed, economic assessments were made, design calculations were performed, and experimental investigations on different TES options were realized. The results obtained can serve as a basis for the demonstration of a promising TES technology in an existing hard coal-fired power plant.

By using metallurgical slag from an electric arc furnace that is otherwise not recycled but deposited as an inventory material in thermal energy storage for concentrated solar power plants, it is possible to make a significant step forward in two transformation processes: energy and raw materials. As this type of slag has not been considered as an inventory material for this purpose, it is important to clarify fundamental questions about this low-cost material and its storage design. In this paper, design studies of slag-based thermal energy storage are carried out. Different slag-specific design concepts are developed, calculated and evaluated by a method based on established management tools. Finally, concepts for further investigations are defined. The highest aptitude value and the lowest risk value are achieved by the vertical storage design with axial flow direction. Therefore, it is taken as the lead concept and will be considered in complete detail in further research. Also, a closer look, but not as detailed as the lead concept, is taken at the horizontal storage with axial flow and the vertical storage with radial flow direction.

Solar power towers with thermal energy storage based on direct-flow regenerators have the potential to generate cost-effective base-load electricity. An inventory option that opens up further cost-saving potential but has not yet been extensively investigated for this application is slag from electric arc furnace. This use has not only economic advantages, but also serves environmental protection, since a large proportion of this type of slag is currently not used any further, but is disposed of in landfills. In the completed EU project REslag, various subsequent uses of the slag were investigated, including the possibility presented here of using sintered slag pebbles as an inventory for regenerators in solar power towers with air as the heat transfer fluid. The main results from the different phases of the project are presented, with a focus on the investigations not yet published. In addition to results from thermal simulations on different designs and on the partial load and off-design behavior of the storage lead concept “Axial flow—standing”, these are mainly results from fluid mechanical calculations on the distributor design of the storage and from material investigations on the slag. In summary, it can be stated that the sintered slag pebbles are thermally, mechanically and chemically competitive with conventional inventory materials and the principle feasibility of a slag-based storage was confirmed by the results of these investigations. The defined storage lead concept was elaborated in detail and the performance of the design was confirmed by simulations and experiments.

This study presents a systematic analysis of Brayton batteries using Ebsilon Professional^® simulations. Over 200,000 concept configurations were evaluated, with less than 1% proving physically feasible. The research aimed to assess electricity generation; coupled generation of electricity and heat; coupled generation of electricity and cooling; and coupled generation of electricity, heat, and cooling, all with or without waste heat integration. Efficiency ranged from 20% to 50% for electricity generation alone, with higher efficiencies at a compressor discharge temperature of 625 °C compared to 450 °C. Co-generation improved the overall efficiency, although at the expense of power efficiency. Notably, simultaneous electricity, heat, and cooling generation solutions were absent within the study’s parameters. Lead concepts, predominantly air-based systems with or without charging line recuperators and heat exchange at various stages, were identified. These will undergo detailed dynamic system simulations, focusing on thermal energy storage. Comparison with the existing literature was limited due to differing parameters and topologies, highlighting the value of this systematic analysis in identifying optimal solutions.