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.

Today's heat demand of industrial processes is mainly supplied by the combustion of fossil fuels. Within this paper, a thermal energy storage system using the Rotating Drum Heat Exchanger is proposed for a carbon-neutral steam generation and additional co-generation of electricity. At the Rotating Drum Heat Exchanger, a phase change material (PCM) solidifies on the outer surface of a drum, which is partially immersed into liquid PCM, while water evaporates on the inner surface of the drum. With this design, the storage density of the proposed nitrate salts as storage material can be increased up to 330 kW.m(-3) by utilizing the energy stored within the phase change and the energy stored due to the temperature change of the liquid and solid storage material. The storage system is sized for the generation of 20 000 kg.h(-1) of saturated steam at 2.5 bar, 8 bar, 20 bars and 75 bar of steam pressure. During the discharge process, a surface-specific heat transfer of above 300 kW.m(-2) and a share of electricity generation of up to 24 % can be achieved, which shows the high potential of the Rotating Drum Heat Exchanger. The thermal energy storage system can either be charged by fluctuating renewable energy or can be used to decouple the steam and electricity production of today's cogeneration plants. The presented storage system can thus make a decisive contribution to decarbonization and flexibilization of the industrial process steam supply.

With the concept of the rotating drum heat exchanger, latent heat can be released with a high and constant surface specific heat flux. Phase change material (PCM) solidifies on the outer surface of a drum, which is steadily removed by a fixed scraper during rotation. Two novel calculation approaches for determining the heat transfer and the layer thickness for a rotating drum heat exchanger are developed and validated with existing experimental data. This includes the identification of correlations for the thickness of the adhering liquid layer after the surface emerges from the liquid PCM and the local surface coefficient of heat transfer on the outside of a partially immersed rotating drum. While a calculation approach based on the quasistationary simplification underestimates the experimentally measured heat transfer for rotational speeds above 4 min(-1) by 31% on average, a detailed transient numerical simulation based on a time-varying finite difference scheme reproduces the experimentally measured heat transfer with an accuracy of 8% on average. By applying the transient numerical simulation to a rotating drum heat exchanger using sodium nitrate as the PCM, a surface specific heat transfer based on the entire drum surface of up to 400 kW.m(-2) can be assumed, showing the high potential of the rotating drum heat exchanger for industrial and power plant applications.

With latent heat thermal energy storages, thermal energy can be stored at a constant temperature level with high storage density using the enthalpy of the solid-liquid phase change of a material. During the discharge process of a latent heat thermal energy storage, phase change material (PCM) solidifies at the heat transfer surface and increases the thermal resistance. This decreases the transferred thermal power with time and the state of charge. The rotating drum heat exchanger, experimentally investigated in detail for the first time in this paper, overcomes this limitation by removing the PCM layer. While a heat transfer fluid passes through the inside of the rotating drum, which is partially immersed in liquid PCM, the PCM solidifies at the outer side. The solidified layer is removed at each rotation by a fixed scraper. Thus, the layer thickness and the thermal power are kept constant over time. The solidified PCM can be stored separately from the liquid phase, resulting in a complete independence of thermal power and storage capacity. A commissioned experimental test rig using a low temperature PCM is used for the investigation of the heat transfer potential, the layer thicknesses, the mechanical energy needed for the removal of the solidified layer and as a proof of concept. The experimental data show a consistent heat transfer which is increasing for higher rotational speeds. With the presented test rig, the heat transfer density is up to 6.8 kW.m(-2) based on the total drums shell surface of the rotating drum at a temperature difference of 5 K between the melting point of the PCM and the temperature of the heat transfer. Adhering liquid PCM increases the total heat transfer by up to 60% as the liquid PCM solidifies also after the surface left the liquid PCM. While the measured solidified layer decreases to below 0.05 mm with higher rotational speeds, the adhering layer is slightly increasing. The results show the high potential of the rotating drum heat exchanger concept for the generation of steam out of a high temperature PCM.

This chapter describes the basic concepts of modeling aquifer thermal energy storage (ATES) and borehole thermal energy storage (BTES) systems. Their fundamental equations are solved by using the finite element method. The numerical analysis of BTES systems requires specific attention due to the extreme slenderness of the borehole heat exchangers (BHEs). The numerical approaches are implemented in the finite element simulator FEFLOW, which also incorporates specific tools to input and monitor the simulations for single BHEs and arrays of BHEs. Applications are presented to verify the model results against theoretical solutions and measured field data as well as to discuss the role of the groundwater influence on seasonal storage efficiency of BTES.

Thermal energy can be stored in latent heat thermal energy storages with high exergetic efficiency at a constant temperature level during the phase change of a storage material from solid to liquid. While discharging, Phase Change Material (PCM) solidifies at the heat transfer surface limiting the heat transfer due to the low thermal conductivity of commercially available PCMs. The presented rotating drum latent heat exchanger keeps the solid PCM layer at a constant thickness below 0.1 mm to maximize the heat flux. Here, a tempered rotating drum is partially immersed into liquid PCM which solidifies at the drums outer surface and is scraped off every rotation. An experimental test rig using decanoic acid as PCM is used to validate available theoretical data of a quasi-stationary analytical approach and a 1-D numerical scheme. Results are showing a heat flux density of up to 25 kW/m2 at a temperature difference of 10 K related to the active heat transfer surface. Doubling the temperature difference increases the heat transfer by a factor of 1.5 - 1.8 due to nonlinear effects. In addition, liquid PCM is adhering at the surface released out of the liquid PCM. Thus, the active heat transfer surface can be enlarged to the entire drums surface depending on the rotational speed. The storage density can be enlarged by overheating the liquid storage material to use the sensible heat as well. Furthermore, the heat flux is constant and fully controllable by adjusting the rotational speed and the solidified PCM can be stored separately from the liquid phase, resulting in a complete independence of thermal power and storage capacity. The presentation will show the latest experimental and theoretical results of the working test rig

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.

Thermal energy storage combined with thermal cycles is an alternative option for storage in electrical power grids. Intermediate storage of electric energy as heat offers advantages such as free choice of site, small environmental footprint, life expectancies of 20-30 years and optional low-cost backup capacity. The key element in pumped thermal energy storage (PTES) concepts is the application of a left running thermal cycle to transform low temperature heat into high temperature heat, which is stored in the thermal storage during charging. PTES allows higher storage efficiencies than a direct electric heating of the thermal storage unit. The optional combination of electricity and heat during charging and discharging makes PTES a promising tool for the management of various types of energy in systems with high shares of renewable energy. CHEST (Compressed Heat Energy STorage) is a specific PTES variant based on Rankine cycles using either water or organic media as the working fluid in combination with latent heat storage units. This paper focuses on the application of CHEST for the management of heat and electricity. Different options for the implementation of CHEST will be presented, for these variants, characteristic values such as operating parameters and power ratio are given and the required components described. The focus is on the technological possibility of using pumped thermal energy storage as a sector-coupling technology for heat and electricity through low temperature heat integration. In addition, new findings of an in-depth numerical simulation of a fully heat-integrated, subcritical PTES using butene as the working fluid are presented. (C) 2019 Elsevier Ltd. All rights reserved.

A latent heat storage system for the production of superheated steam at >21 bar and 300 degrees C with a capacity of over 1.5 MWh has been developed, designed and is in build. The storage unit concept uses extended finned tubes with a high packing factor and sodium nitrate as the storage material, which changes phase from liquid to solid during charging. The storage unit is an upscaling from smaller designs, and the upper and lower headers were adapted for the operating conditions and upscaling. The storage contains 852 finned tubes, each ca. 6 m in tube length, so that a semi-automated assembly method was necessary for build. This large-scale storage system is being integrated into an operational process, from which real integration and operation experience can be gained. The integration is in a cogeneration plant in Saarland, Germany and the latent heat storage system serves as a backup to a gas turbine for steam generation. The storage design and build is discussed. Topics such as the manufacturing of finned-tubes, upscaling of headers as well as relevant permitting processes are critical for the development of large-scale latent heat storages for industrial as well as CSP and solar process heat applications.

In future energy systems, storage technologies for electrical energy are considered to be a key component for increasing the share of renewable energy use. Pumped thermal energy storage technologies represent a promising approach to complement established storage technologies such as pumped-hydro power storages without their geological restrictions. Assuming an ideal, reversible and adiabatic energy conversion process, the stored electrical energy can be entirely recovered. However, the efficiency of real processes is limited by irreversibilities. These exergy losses can be compensated by the integration of low temperature heat. The exergetic efficiency can be further increased by using thermal energy provided during discharging. In this paper, a fully heat-integrated, subcritical PTES using butene as the working fluid is presented. The results of a detailed numerical simulation of the cycle regarding exergy losses during heat transfer, efficiencies of machinery and parasitic energy consumption are shown. A maximum net electrical power ratio between charging and discharging of 125% is obtained, while the maximum exergetic efficiency is 59%. The conducted numerical simulation includes pressure losses and pinch points, allowing for a more in-depth understanding and for a pre-optimization of the hydraulic design.

This contribution deals with recently realized as well as planned small solar district heating systems with seasonal thermal energy stores in Europe. It focuses on systems with less than 1 000 m2 of solar collector area and less than 1 000 m3 volume of seasonal thermal energy store. Different technical characteristics of systems in Poland, Spain and Germany are shown. As high storage efficiency in small systems is difficult to achieve, particular attention is given to the design of the seasonal thermal energy stores and other components such as high temperature heat pumps that are necessary to reach an efficient operation of the plant.

The effective thermal conductivity of fine-grained and coarse-grained expanded perlites, fumed silica and a mixture of coarse-grained expanded perlite and fumed silica at different vacuum pressures is experimentally investigated in this paper. For the investigations two different test rigs – a guarded cylinder apparatus and a guarded hot plate apparatus – were used. The lowest effective thermal conductivities were performed with fumed silica at ambient pressure, with the mixture at pressures between 1 and 10 mbar and with fine-grained expanded perlite at 0.1 mbar and lower. Mixtures of fumed silica and expanded perlites appear to be a great chance to achieve low effective thermal conductivities and lower prices compared to pure fumed silica at vacuum pressures that can be easily achieved for large-volume thermal energy stores.

This contribution introduces an innovative, sustainable heat supply concept. This concept, which is based on solar thermal and geothermal heat generation and advanced heat storage technologies, will be realized for the first time as a heat supply system for new apartment buildings. The key feature of the system is the fact, that on an annual basis only 1 kWh of electricity will be required to generate 10 kWh of heat. This contribution is focused on the presentation of the above described, newly developed system concept as well as first results of the design and simulation studies for a pilot heat supply system that is planned to be realized in a complex of three multi-family houses within a total living area of over 3 500 m(2) in the German city Crailsheim, which is located 80 km north-east of Stuttgart.The development of the concept is part of the research project '1to10 - Development, testing and demonstration of a sustainable, standardized solar-geothermal heat supply concept'. In the conceptual design phase of the project, theoretical work for the realization of such systems is performed. (C) 2016 The Authors. Published by Elsevier Ltd.