Deutsche Version  ACT
Thermal Storage
Latent heat storage 12.4.2017

Copper coated porous steel hollow sphere
© Fraunhofer IFAM Dresden

Pourable PCM for high temperatures

Dresden researchers develop small pourable and pumpable heat storage capsules for the temperature range from 130 to 350 ° C. The hollow spheres are made from metal alloys. Depending on the application, the diameters vary from 2 to 6 mm. The capsules contain salt mixtures for latent heat storage.

Project status Selction of appropriate PCM
Temperature type high temperature
Storage/Charging indirect
Energy storage density 250 MJ/m³
Number of cycles Very high (> 10,000)
Charging temperature 130-300°C
Discharge temperature 130-300°
Project duration December 2013 until November 2016

The specific research topic is the development of small, pour- and pumpable heat storage capsules for various applications. The PCM-filled hollow spheres shall for instance: act as a mechanically stable, pumpable thermal capacity increasing the heat capacity of heat transfer oils, enable a very good heat transfer between gaseous media and the PCM serve as a catalyst carrier allowing a steady operating temperature by buffering temperature inhomogeneities.

  • Depending on the temperature ranges different PCM materials are used © Fraunhofer IFAM Dresden
  • The porous metal hollow spheres are infiltrated with PCM © Fraunhofer IFAM Dresden
  • PCM - filled hollow spheres float in thermal oil, providing additional heat capacity © Fraunhofer IFAM Dresden
  • PCM - filled hollow spheres serve as a heat exchanger © Fraunhofer IFAM Dresden
  • Copper coated porous steel hollow sphere © Fraunhofer IFAM Dresden

Good heat transfer opens up possibilities

Common latent heat storage is typically characterised by relatively poor heat transfer between heat transfer medium and the PCM. In recent years requirement a demand was repeatedly made for bulk PCM, that is able to provide good perfusion by fluids and high heat transfer rates. Potential applications are the storage of waste heat in industrial processes and the improvement of catalysis efficieny by a more regular operating temperature. Another approach is the appreciable increase of the typically low heat capacity of heat transfer media, for example thermal oil, therefore enabling more effective heat transfer in the temperature range between 130 and 350 °C. In a period of three years the associates hollomet, Fraunhofer IFAM Dresden, Institute of Air Handling and Refrigeration Dresden (ILK) and Remetall Drochow develop the necessary technologies for the meso-scale PCM-capsules right up to a demonstrator in pilot-plant scale.

Starting points for the optimisation are the selection of appropriate PCM (salts and mixtures) considering phase change temperature, melting enthalpy und especially the corrosion interaction with the porous shell of the metallic capsules. Good adhesion of the catalytic coating is another aim of the optimisation. The application of the PCM-filled capsules as pumpable heat capacities requires improved strength of the sintered spheres and stability of the sealing.

Three steps to the goal

The project is divided into three main tasks: (1) Investigation, experimental characterisation and selection of suitable materials and material combinations for the phase change material (PCM), heat transfer media, capsules (hollow spheres) and sealing - milestone after 13 months: selection of appropriate material combinations is finished (2) Development of the manufacturing process for the capsules together with the infiltration and sealing technology in a lab scale - milestone after 24 months: PCM-filled capsules are available. (3) Design of the sealing technology in pilot-plant scale, testing of coated capsules as catalyst in beds through-flowed by gas and investigation of the heat capacity-increase of heat transfer media, Development of a demonstrator.

Processes and equipment

The aim of the sub-project of hollomet is the development of qualified processes and manufacturing modules for PCM filled metal hollow sphere production at a pilot plant scale. A slurry to coat green spheres will be developed and a thermal threatment regime for existing equipment will be selected. To fill the spheres a dedicated manufacturing module will be developed and built. A method to close surfaces of filled spheres will be turned into a pilot plant. Proper basic parameters for this processes will be identified and finally spheres will be coated to buildup demonstration samples.

Development of hollow metal spheres

Aim of sub-project 2 is the development of PCM-filled metallic hollow spheres (diameter a few mm) as high performance thermal energy storage capsules for temperatures between 130 °C and 350 °C . Metallic hollow spheres are manufactured by a specific powder metallurgic method, that has to be optimised and adjusted in respect of the sphere material used. Testing will be realised in lab scale. Furthermore part of work is the development of the PCM-infiltration and the sealing of the capsules. The sub-project will be completed by investigations about heat transfer and flow in a static bed of PCM-filled hollow spheres using gaseous and liquid media.

Selection of phase change materials

ILK Dresden will select and characterize the phase change materials and thermal fluids and will test the cycle stability and lifetime of the PCMs and of the filled  capsules. Furthermore ILK will perform end evaluate corrosion tests of the selected capsule materials with PCM. In co-operation with AEL 4 different test rigs for characterizing the dynamic behaviour of the PCM filled spheres at elevated temperatures.

Hollow spheres as a catalyst carrier

Remetall will test the PCM filled spheres as a catalyst carrier. Therefore the filled and sealed spheres will be coated with a washcoat and a catalytic active layer to examine the total oxidation of VOC pollutants. Because of the heat storage behaviour of the capsules a better performance during the start phase of the catalysts is expected.

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


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