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Thermal Storage
BMWi
Chemical heat storage 12.4.2017
Experiments on the flow properties of lime hydrate. At the Chair of Energy and Environmental Process Engineering at the University of Siegen, the transportation of a granulate is being investigated. The so-called “fill angle” provides information about the flow properties of the material. The more acute the angle is, the more difficult the movement is.
© DLR

More efficiency through bulk movement

Chemical heat storage with Calziumoxid is being tested by Scientists

of the DLR in the project BERTI. As a special feature, the laboratory

unit of Stuttgart researchers has a wandering reaction bed. As a result

the heat capacity of the storage and its heat output can be designed

independently of one another.

Project status Model of particle movement
Temperature type High temperature
Storage/Charging Direct or indirect
Charging temperature 400-600 °C
Discharge temperature 400-600 °C
Storage capacity adjustable
Energy storage density 250 kWh/m³, 370 kWh/t
Project duration Ocotber 2013 until December 2017

The possibility to store energy, especially the storage of thermal energy, plays a key role for a cost- as well as resource-efficient future energy supply. However, the realisation of economic solutions for thermal energy storage mainly depend on the storage density and the storage material. Thermochemical storages, where the heat of a reversible chemical reaction is used, presents a promising technology especially due to their high storage density, which can also be implemented with cost-effective materials, e.g like hydrated quick lime.

  • Unmoved thermochemical heat storage system on laboratory scale: In the framework of the CWS project (regarding chemical heat storage) a thermochemical heat storage system was built at laboratory scale at the German Aerospace Center in Cologne. As in the BERTI project (regarding reaction bed with moving thermochemical storage material), Ca(OH)2/CaO was also investigated as a reference system in the CWS project. The reactor has a thermal output of up to 10 kW and a capacity of 8 kW of chemically stored energy (corresponds to approx. 22 kg Ca(OH)2). This capacity shall now be expanded in the framework of the BERTI project by developing a reactor through which the storage material is transported. However, the BERTI reactor will be a laboratory reactor with low output because the concept must first be developed. © DLR
  • Fluid bed heat exchanger © Bühler AG
  • Vertical moving bed reactor: Examples of moving bed reactors for filling material on an industrial scale as have been manufactured by our project partner Bühler AG. A thermochemical storage system with moving bed reactor may look like this in the future. © Bühler AG
  • SEM image of Ca(OH)2 in clay: In order to allow the movement of the storage material, the starting material must be modified. The approach followed in the BERTI project is to produce stable particles with a particle size of > 0.2 mm because smaller particles do not move well due to the strong attraction (cohesion) between the individual particles. Positioning the storage material in a porous clay matrix is one way to product large particles. The calcium carbonate and clay are mixed together like a dough and then burnt out. The burned clay with calcium oxide remains. The particle size of this material can then be adjusted on a dish granulator. © BWC, Universität Siegen
  • Cold test for the movement of the filling: Studies are being carried out at the German Aerospace Center (DLR) regarding the movement of the storage material. The movement of the filling presents a challenge because the starting material is a very fine powder. This problem is further increased because the distance to the thermal transfer area must be small due to the material’s heat conductivity. In the image, you can see a cold model of a plate heat exchanger with a plate distance of 2 cm. 2 cm is the maximum distance for unchanged material for which the thermal conductivity in and out of the reaction material is still adequate. The powder is transported from above through an auger conveyor into the reactor, through which it then travels propelled by gravity and is again transported away below through a second auger conveyor. © DLR
  • Change in volume during the reaction of CaO with Ca(OH)2: At the Institute of Buildings and Material Chemistry at the University of Siegen work is continuing on the production of larger, more stable particles. The change in volume of the storage material due to the chemical reaction presents a great challenge. For example, when clay is used as the carrier matrix, it slightly ruptures during the reaction because the CaO crystals grow irregularly and in doing so a mechanical load is applied to the clay. © DLR
  • Granulate before and after fluidisation: The movement of the storage material likewise represents a mechanical load. The image shows a clay CaO granulate before fluidisation in a fluidised bed. The forceful movement leads to abrasion and therefore to a great reduction of the particle sizes. © DLR
  • Granulate before and after fluidisation: The movement of the storage material likewise represents a mechanical load. The image shows a clay CaO granulate after fluidisation in a fluidised bed. The forceful movement leads to abrasion and therefore to a great reduction of the particle sizes. © DLR
  • Experiments on the flow properties of lime hydrate: It can also be tested whether the material can be pneumatically conveyed through a tube. The flow properties of the storage material have a great influence on the possible geometries of the reactor as well as on the transport and storage of the storage material. © DLR
  • Experiments on the flow properties of lime hydrate. At the Chair of Energy and Environmental Process Engineering at the University of Siegen, the transportation of a granulate is being investigated. The so-called “fill angle” provides information about the flow properties of the material. The more acute the angle is, the more difficult the movement is. © DLR

In the context of the project "BERTI - Reaction Bed with moving Thermochemical Storage Material" a lead concept for thermochemical heat storage with moving thermochemical storage material is developed and subsequently demonstrated in laboratory scale. The movement of the reaction bed allows the combination of a very high storage density - which represents a distinct advantage of chemical reactions - with the separation of storage capacity from storage performance (thermal power).

Wärmeleistung von der Speichergröße entkoppeln

BERTI tries to demonstrate two main advantages of thermochemical storage by use of gas-solid reactions:

  • separated reactants and thereby lossless storage possibilty
  • the detachment of storage capacity from thermal perfomance can be realised by a separate storage of the solid material and a controlled supply to the reactor as needed.

However, a key role play the possibilty to move the solid bulk material.

In order to enable efficient operation of a thermochemical storage with moving bulk material, a close collaboration between materials and engineering science is necessary. In the beginning of the project a reactor design is specified, which provides the basis for the subsequent course of the project. Hereafter, the three main issues concerning a moving reaction bed for thermochemical heat storage are addressed simultaneously: material development, movement of the bulk material and reaction control. The final goal of the project is a verification of the general functionality in lab-scale.

Storing without losses

Contrary to alternative technologies, thermochemical heat storage offers the possibility to store thermal energy in principle without losses. The stored thermal energy can even be upgraded thermally by variation of reaction conditions. Both points provide additional degrees of freedom especially for efficiency enhancement. The main focus of the project lies on the movement of a storage material for thermochemical heat storages in combinatino with an efficient reaction control. Both parts present novel issues scientifically as well as technologically.

Storage opens up new fields of application

BERTI addresses the core of development of innovative thermal energy storage and opens up new fields of application from heat storage with integrated heat transformation via concentration of waste heat flows and their concentrated discharge through to lossless transport of thermal energy. However, a key role play the possibilty to move the solid bulk material. In this this project engineering and materials science are combinded with the objective to develop an advanced solution for thermochemical heat storage.

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Supported by: The Federal Government on the basis of a decision by the German Bundestag

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Research funding

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