Solid Media Thermal Energy Storage System for Heating Electric Vehicles: Advanced Concept for Highest Thermal Storage Densities

The integration of thermal energy storage systems enables improvements in efficiency and flexibility for numerous applications in power plants and industrial processes. By transferring such technologies to the transport sector, existing potentials can be used for thermal management concepts and new ways of providing heat can be developed. For this purpose, technology developments for solid media high-temperature thermal energy storage systems are taking place for battery-electric vehicles as part of the DLR Next Generation Car (NGC) project. The idea of such concepts is to generate heat electrically, to store it efficiently and to discharge it through a bypass concept at a defined temperature level. The decisive criterion when using such solutions are high systemic storage densities which can be achieved by storing heat at a high temperature level. However, when storing high temperature heat increasing dimensions for thermal insulation are required, leading to limitations in the achievable systemic storage density. To overcome such limitations, an alternative thermal insulation concept is presented. Up to now, conventional thermal insulations are based on sheathing the storage containment with efficient thermal insulation materials, whereby the thickness results from safety restrictions with regard to the permitted maximum surface temperature. In contrast, the alternative concept enables through the integration of the external bypass into the thermal insulation systemic advantages during the charging and discharging period. During discharging, previously unused amounts of heat or heat losses within the thermal insulation can be integrated into the bypass path and the insulation thickness can be reduced during loading through active cooling. Using detailed models for both the reference and the alternative thermal insulation concept, systematic simulation studies were conducted on the relevant influencing variables and on the basis of defined specifications. The results confirm that the alternative thermal insulation concept achieves significant improvements in systemic storage densities compared to previous solutions and high potentials to overcome existing limitations

Ceramic high temperature plate‐fin heat exchanger: A novel methodology for thermomechanical design investigation

The basic methodology of a novel, time-saving approach for critical thermomechanical design studies of ceramic high temperature plate fin heat exchanger is presented. This approach allows the determination of local displacements, by applying the outer heat exchanger boundary conditions on a substitute model. These displacements are then used for detailed calculation of local stresses. The methodology is based on the effective Young’s modulus, effective shear modulus and effective Poisson ratio. Simulation models have been developed to determine these effective substitute properties. A model verification has been performed with a compression test rig. The simulation predicts the experimental results with deviations below 3%, which proves the feasibility and reliability of the effective material models. In order to reduce the parametric effort of the substitute simulation model, information about the material behavior is important. Here, the results indicate an orthotropic material behavior of the fin structure. This reduces the independent substitute material properties required for the characterization of the substitute model, which also reduces the overall simulation time

High-Performance Solid Medium Thermal Energy Storage System for Heat Supply in Battery Electric Vehicles: Proof of Concept and Experimental Testing

The reduction of global CO2 emissions requires cross-sectoral measures to reduce fossil energy consumptions and to strengthen the expansion of renewable energy sources. One element for this purpose are thermal energy storage systems. They enable, due to their time-decoupled operation, increases in systemic efficiency and flexibility in various industrial and power plant processes. In the electricity and heat sector such solutions are already commercially available for large-scale applications or are focused in diverse R&D projects, but are largely new in the transport sector. By transferring existing concepts specifically to the requirements for the heat supply of battery electric vehicles, efficiency improvements can also be achieved in the transport sector. The idea is to provide the required heat for the interior during cold seasons via a previously electrical heated thermal energy storage system. Thus, battery capacities can be saved, and the effective range of the vehicle can be increased. Basic prerequisites for this concept are high systemic storage densities and high performances, which must be justified to commercial battery powered PTC-elements. Compared to large-scale applications, this results in new challenges and design solutions needing finally a proof of concept and experimental tests under vehicle typical specifications. For the first time, a novel thermal energy storage system based on ceramic honeycombs with integrated heating wires and a double-walled, thermally insulated storage containment was developed and constructively realized. This storage system meets all the requirements for the heat supply, reaches high systemic storage and power densities and allows due to its high flexibility a bifunctional operation use: a cyclic storage and a conventional heating mode. In the focused storage operation, high-temperature heat is generated electrically through heating wires during the charging period and transferred efficiently via thermal radiation to the ceramic honeycombs. During the discharging period (driving) the stored thermal energy is used for heating the interior by a bypass control system at defined temperatures with high thermal output. The systematic measurement campaigns and successful model validations confirm high electrical heating powers of 6.8 kW during the charging period and a heat supply with a thermal output of 5 kW over more than 30 min during the discharging period. Despite current infrastructure and test rig restrictions, high systemic storage densities of 155 Wh/kg with constant discharging outlet temperatures are reached. Compared to battery powered heating systems, the experimental results for the developed thermal energy storage system confirm an excellent level of competitiveness due to its high performance, operational flexibility and low-cost materials

THERMO-MECHANICAL INVESTIGATION OF PACKED BEDS FOR THE LARGE-SCALE STORAGE OF HIGH TEMPERATURE HEAT

ABSTRACT Thermal storage systems are central elements of various types of power plants operated from renewable and conventional energy sources. Where gaseous heat transfer media are used, a regenerator-type heat storage based on a packed bed inventory is a particularly cost-effective solution. However, suitable design tools that cover the thermo-mechanical aspects of such a design are still missing today. As a basis for such a tool, this contribution presents a novel approach to investigate the thermo-mechanical behaviour of such a storage under thermocyclic operation. The relevant relations are formulated on the basis of the discrete element method (DEM). Results of simulation runs determine the temporal and spatial displacements and acting forces for the individual bodies. Coupling the equations to a simplified thermal model allows to investigate the thermo-mechanical behaviour. Initial results for a thermocyclic operation using simplified assumptions are presented. BACKGROUND Thermal energy storages for the high temperature range are central components for power plants driven from renewable energy: Heat storage allows solar thermal power plants to continuously operate beyond sunshine duration. In fossil CHP power plants they increase the operational flexibility and thus improve the revenue situation. Industrial waste heat use and electricity storage based on Adiabatic Compressed Air Energy Storages (ACAES) are further examples. An increasing interest in these technologies calls for large-scale storage solutions in a temperature range between 500-1000°C with storage capacities up to 3GWh for discharge durations between 4 and 12h. In many applications the heat is transferred by gaseous heat transfer media, such as air or flue gas. Here, a direct contact between the heat transfer fluid and storage inventory is a particularly cost-effective design solution. Installations of these socalled regenerator-type heat storages have been used in the steel and glass industry for many decades. The storage inventory is stacked from ceramic bricks. To reach the cost targets for power plant applications, regenerators based on a packed bed inventory are a promising option. They offer a large specific heat transfer area and high heat transfer rates, as well as the potential to reduced investment costs, especially for natural stones as an inventory material

The Heat-Storing Micro Gas Turbine – Process Analysis and Experimental Investigation of Effects on Combustion

Renewable energy sources such as wind turbines and photovoltaics are the key to an environmentally friendly energy supply. However, their volatile power output is challenging in regard to supply security. Therefore, flexible energy systems with storage capabilities are crucial for the expansion of renewable energy sources since they allow storing off-demand produced power and reconverting and supplying it on-demand. For this purpose, a novel power plant concept is presented where high-temperature energy storage (HTES) is integrated between the recuperator and the combustor of a conventional micro gas turbine (MGT). It is used to store renewable energy in times of oversupply, which is later used to reduce fuel demand during MGT operation. Hereby, pollutant emissions are reduced significantly, while the power grid is stabilized. This paper presents a numerical process simulation study, aiming to examine the influence of different storage temperatures and load profiles of HTES on the MGT performance (e.g., fuel consumption, efficiency). Furthermore, relevant operating points and their process parameters such as pressures, temperatures, and mass-flow rates are derived. As operation conditions for the combustor are strongly influenced by the HTES, the paper contains a detailed theoretical analysis of the impact on combustor operability and includes an experimental investigation of the first combustor design adapted for the compound and tested under higher inlet temperatures conditions

Technical Development and Economic Evaluation of the Integration of Thermal Energy Storage in Steam Power Plants

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