Deutsche Version  ACT
Thermal Storage
Sorption heat storage 12.4.2017

Activated carbon (right) is produced from walnut shells.
© Fraunhofer ISE

Sorptive heat storage using activated carbon

Activated carbon can be produced from a large variety of carbonaceous

source materials. Different methods have also been established for the

process steps of carbonization and activation for use in commercial

applications. In the project HyAcitv researchers optimize activated

carbons and in a further step, molded articles are produced for use in

open sorption systems. The application of these moldings is demonstrated

in a laboratory storage system.

Project status Near completion
Type of storage Adsorption/Absorption
Temperature type low temperature
Storage/Charging direct
Storage time long (seasonal)
Number of cycles 30-50, depending on application
Charging temperature 70-120°C
Discharge temperature 25-40°C
Storage capacity 3600 kJ
Energy storage density >360 MJ/m³
Project duration December 2013 until May 2017

The main focus of the development project is the targeted adaptation of the sorption properties of activated carbon to specific application environments, such as the adsorption of water in an open sorption heat storage system. In this instance, the optimization criteria include all of the important parameters, such as production costs and the expense for primary energy, uptake or storage capacity, discharging power, and material stability during the typical service life of the product.

  • The figure shows molded articles made of activated carbon with cell densities between 300 and 600 cpsi. © Dinex GmbH
  • Uptake characteristics of modified activated carbon in comparison with reference materials. © Fraunhofer ISE
  • Installation of a sample in a measuring instrument for determination of pore size distribution (Hg porosimetry) © Fraunhofer ISE
  • Pore size distribution of untreated and modified activated carbons © Fraunhofer ISE
  • Dr. Stefan Henninger is seen filling the rotary tube furnace with the source material (walnut shells). © Fraunhofer ISE
  • Lateral view into the quartz tube of the rotary tube furnace during the carbonization process © Fraunhofer ISE
  • Tubular furnace in the open position, with gases from pyrolysis igniting and forming flames © Fraunhofer ISE
  • Activated carbon (right) is produced from walnut shells. © Fraunhofer ISE

Due to its hydrophobic properties, the use of activated carbon for the adsorption of water, such as for seasonal sorption heat storage systems, was previously not feasible. The approach of hydrophilizing activated carbon makes it possible to develop this class of materials for the adsorbate of water, which is interesting both in terms of economic aspects and low consumption of primary energy in the production step. It is possible to obtain optimally adapted adsorption characteristics for the adsorbate of water by focused selection and optimization of the parameters source material, carbonization/activation, and post-preparative modification. In a further step, molded articles are produced from the optimized activated carbons for use in open sorption systems; the application of these moldings is demonstrated in a laboratory storage system.

Project plan

The fundamentals for the following experimental work packages are developed during the first phase of the project. This includes bibliographical research on source materials and implementation methods as well as the evaluation of commercially available activated carbons. After approximately 6 months, the next step is the initiation of work on optimizing the activated carbons and the shaping process, accompanied by an ongoing classification of the material properties. Once these tasks have been completed successfully, the last six months of the project involve demonstrating the developed materials and molded articles in a laboratory-scale storage system.

The most important milestones (after approximately 1.5 and 2 years) will include documenting hydrothermal stability of the optimized material and demonstrating a substantial increase in the water adsorption capacity.

Economic and ecological advantages of activated carbons

Through experience from previous research projects and fundamental calculations for (seasonal) heat storage by adsorption, the resulting project focus is on producing a storage system design and storage material as cost-effectively as possible from an economic and primary energy perspective. In this context and with respect to required raw materials and processing, activated carbons have distinct advantages when compared with materials, such as zeolite, which are produced by solvothermal synthesis. By the subsequent modification of the activated carbon inner surface, it can also be utilized for the adsorption of water, which is the least critical of all adsorbates and refrigerants, and is at the same time ideal for thermal storage due to its high evaporation enthalpy.

Using an oxidation treatment, different functional groups can be produced and fixed on the activated carbon inner surface. As preferred adsorption sites, these groups in turn have a material impact on the adsorption characteristics. By adapting the type of treatment and other parameters, it is possible to precisely tailor the properties to various desired application environments. The combination of a cost-effective source material and adapted modifications produces a sorption material that can compete in terms of sorption characteristics with materials that are significantly more expensive to produce. Particularly for adsorption-based thermal energy storage applications, this is an important step towards economic and primary energy amortization of capital expenditures for the storage unit.

The materials have to retain their sorption characteristics under normal conditions of use throughout the entire service life of a storage system. This is a critical factor, particularly for modified materials and high humidity and temperature conditions. These factors will therefore be monitored by performing appropriate aging tests throughout the course of the project to ensure that the necessary stability exists. Here it is possible to draw upon experience with hydrothermal stability tests over several years in a number of facilities, including both closed and open sorption systems.

Activated carbon optimization
This key part of the project addresses both post-preparative modification of the activated carbon and activated carbon production, particularly physical and chemical activation. The objective of this modification is to produce the desired specific sorption characteristics; the suitability of specific source materials for each specific modification can be evaluated through experiments and analysis.

Efficient processes that combine adequate mass and heat transfer require optimized components in terms of fluid dynamics and high thermal conductivity. Our partner in this project, Dinex GmbH, will produce molded articles from the optimized activated carbon. These moldings will combine the properties of mechanical stability, low pressure loss, good thermal conductivity, and rapid adsorption kinetics.

To optimize activated carbon and molded articles, the material properties are studied on an ongoing basis with the results then used as an important input for further refinement of the modification and shaping processes. The analyses will include gas and water sorption analysis, surface activation analysis, mercury intrusion analysis, thermal conductivity analysis, and cyclical hydrothermal and mechanical stability tests.

Laboratory storage system demonstration unit
The last module of this project is the construction of a laboratory storage system demonstration unit with an approximate volumetric capacity of 5-10 liters. For this purpose, the associated honeycombs will be produced and then bonded into an overall adsorber, prior to installation in an enclosure. The proposed application will be an open system with finely structured honeycomb units. The laboratory storage system can be charged by means of air circulation and a heating unit; discharge occurs by the circulation of moisturized air and extraction of heat with an air/water heat exchanger.

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


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Research funding

The information system EnArgus provides information on research funding, including on this project (German only).