For steam-driven industrial processes, Latent Heat-Thermal Energy Storages are a promising option to increase their flexibility regarding fluctuating renewable energies. This work provides the foundation for the quantitative analysis of control strategies for such phase change material steam generators and their practical system integration in once-through operation. Experiments are conducted in a self-built, one-kilowatt-scale single-tube test rig with extensive measurement equipment and precise fluid control, employing water/steam as the heat transfer fluid and PLUSICE A133 as the phase change material. Tests at different mass flow rates and pressures of the HTF show the strong correlation between the mass flow rate of the HTF and the storage power in practice. In addition, it is found that a reduction in the mass flow rate significantly increases the effective capacity during charging and discharging: By halving the mass flow rate during charging, it can be increased from 62 % to 77 % at a pressure level of 6.7 bar and from 3 % to 21 % at a pressure level of 3.0 bar. An increased pressure level can further increase the usable capacity of the storage material. The experimental results thus prove the effectiveness and relevance of considering heat transfer fluid control aspects already in the storage design process. Furthermore, the validation of a numerical storage design model shows that the peak power during charging can be accurately predicted by the model with deviations <3 %. With sufficiently long charging processes, also the simulated transient power profiles and outlet temperatures of the heat transfer fluid can be used reliably for storage design.

This chapter provides an overview of the section on mechanical energy storage. While pumped storage is presented in a separate section, this section focuses on the other concepts based on either kinetic or potential energy storage. Despite this common basis, the storage concepts described here form a rather heterogeneous group, in which each technology presents specific challenges and there are major differences in terms of development status and application areas. A classification of mechanical storage concepts is presented, the basic concepts and development status of the various technologies are described, and an overview of the chapters in this section is given.

Thermo-mechanical energy storage concepts use a combination of thermal storage and thermodynamic cycles to store and release electrical energy. This introductory chapter discusses the motivation for this approach and describes the general characteristics of thermo-mechanical energy storage. Many combinations of thermal energy storage and thermodynamic cycles have been proposed for technical implementation; this chapter presents a classification of the basic concepts and variants. An overview of the chapters in the section on thermo-mechanical energy storage is given.

Pumped thermal energy storage (PTES) is a thermo mechanical storage concept in which a heat pump cycle is used to charge a thermal storage, which delivers heat to operate a power cycle during discharge. The first pumped thermal energy storage concepts were based on steam cycles combined with steam accumulators. More recent configurations apply combinations of latent heat storage and sensible heat storage to increase efficiency. This chapter present the thermodynamics of current PTES concepts based on Rankine cycles, an overview of various PTES systems based on Rankine cycles is provided. These variants differ in complexity and maturity, for some of these variants the integration of low temperature waste heat during the charging process is an option. The relevance of the efficiencies of key components for the roundtrip efficiency and for the required specific capacities of engines and storages is shown. An outlook on the advancement of PTES concepts based on Rankine cycle by the development of components adapted to the specific boundary conditions is presented.

In PTES the main focus is usually on electrical energy storage. However, during charging and discharging, large quantities of thermal energy, multiples of the electrical energy, have to be transferred to and from the system. Thermal integration at the cold end of the system could increase the over all efficiency of PTES. Simultaneously, a higher entropy production could be tolerated, allowing for a simpler design of the cycle and the used components, such as heat exchangers. The chapter gives a theoretical introduction into the basic thermodynamics of thermally integrated PTES. To characterize a thermally integrated PTES key parameters are defined. Finally, various application fields and cases are presented.

The share of electricity generated by intermittent renewable energy sources is increasing (now at 26% of global electricity generation) and the requirements of affordable, reliable and secure energy supply designate grid-scale storage as an imperative component of most energy transition pathways. The most widely deployed bulk energy storage solution is pumped-hydro energy storage (PHES), however, this technology is geographically constrained. Alternatively, flow batteries are location independent and have higher energy densities than PHES, but remain associated with high costs and short lifetimes, which highlights the importance of developing and utilizing additional larger-scale, longer-duration and long-lifetime energy storage alternatives. In this paper, we review a class of promising bulk energy storage technologies based on thermo-mechanical principles, which includes: compressed-air energy storage, liquid-air energy storage and pumped-thermal electricity storage. The thermodynamic principles upon which these thermo-mechanical energy storage (TMES) technologies are based are discussed and a synopsis of recent progress in their development is presented, assessing their ability to provide reliable and cost-effective solutions. The current performance and future prospects of TMES systems are examined within a unified framework and a thermo-economic analysis is conducted to explore their competitiveness relative to each other as well as when compared to PHES and battery systems. This includes carefully selected thermodynamic and economic methodologies for estimating the component costs of each configuration in order to provide a detailed and fair comparison at various system sizes. The analysis reveals that the technical and economic characteristics of TMES systems are such that, especially at higher discharge power ratings and longer discharge durations, they can offer promising performance (round-trip efficiencies higher than 60%) along with long lifetimes (>30 years), low specific costs (often below 100 $ kWh −1 ), low ecological footprints and unique sector-coupling features compared to other storage options. TMES systems have significant potential for further progress and the thermo-economic comparisons in this paper can be used as a benchmark for their future evolution.

The storage system investigated in this work, namely the CellFlux system, consists of a regenerator type thermal energy storage volume which is coupled to a heat exchanger by a circulating intermediate working fluid. The numerical simulations in this work are based on experiments conducted with a large scale pilot plant having a bed length of more than 10 m. The storage volume is of a novel design with horizontal flow direction and is filled with hollow bricks as sensible heat storage material. So far, most publications focus on packed bed storage systems, often with molten salt or oil, only few consider regularly shaped channels with gaseous flow. For the investigation in this part of the publication, a one-dimensional dispersion concentric model for channel flow is implemented in MATLAB/Simulink. The analysis in part I of this publication has revealed a radial flow maldistribution, which can be predicted by a correction function correlated beforehand. The commonly used assumption of plug flow, however, will be afflicted with an error. Thus, the aim of this work is to determine the accuracy of the model both under the plug flow assumption and the application of aforementioned correction function. Three single blow experiments, where the storage is charged or discharged from a uniform initial temperature, are compared to the numerical model. The average temperature deviation between experiment and simulation reaches up to 10% without the correction function, but improves significantly through its application and then ranges between 0.4% and 4.8%. Moreover, studies based on larger experiments have either not observed or generally not addressed the effect of a flow maldistribution. This appears to be an issue for small scale experiments where typically more sophisticated models are applied. If it was possible to model the entire storage volume with the present 1D model, the flow maldistribution effects could average out. Also, computing effort would be low and allow simulations on system level. This approach was taken only by few authors so far, particularly for packed bed systems and was also not experimentally verified. In order to correctly depict the experimental set up, the model considers the different composition of the interior, such as flow distributors and inlet/outlet cones as well as the thermal heat capacity of the walls and losses to the environment. As a result the model shows good agreement with the experiments: the mean temperature difference of the exit temperature between experiment and simulation during cyclic operation remained below 5%.

Within the thermal energy storage initiative, National Demonstrator for IseNtropic Energy (NADINE) storage, three projects are carried out focusing on thermal energy storage at different temperature levels. Thermal storage units are key components of Carnot batteries, which are based on the intermediate conversion of electric energy into heat. Pumped thermal energy storage (PTES) is an emerging Carnot battery concept variant for the flexible management of supply and demand of electricity, heat, and cold. A counterclockwise thermodynamic cycle operated by surplus electricity is used to charge a thermal storage, which delivers heat to operate a power cycle during discharge. The absence of geographic constraints, a theoretical roundtrip efficiency of 100%, and a small environmental footprint are promising features of PTES. PTES is also able to provide low‐cost backup capacity in case of shortages in available renewable energy. Various options are proposed for the technical implementation of PTES, using various combinations of engines, thermal storage units, and working fluids. The resulting systems differ in efficiency, costs, maturity, and complexity. Herein, a thermodynamic analysis of five different PTES variants is presented. The results should help to identify priorities in the further development of the PTES variants.