High Performance Heat Storage

Thermal energy storage systems (heat/cold storage) with high performance are gaining more and more importance, especially for increasing the efficiency of cyclical thermal processes. At Fraunhofer IFAM Dresden, intensive research is carried out in the field of latent and sorptive heat storage systems.

Latent heat accumulators utilize the fusion heat of a Phase Change Material (PCM) and thus store heat with a high storage density at moderate costs and almost constant temperature. Sorptive heat accumulators use the binding heat of an adsorption or a chemical reaction (thermochemical accumulators).

Our Energy and Thermal Management business area offers many years of expertise along the entire development chain of innovative thermal storage technologies using in-house material know-how (e.g. cellular metals):

1. Concept – Before any storage development, it is important to analyze the thermal system in which the storage is to be integrated. Then suitable technological approaches can be selected.
   
2. Design – In order to minimize development costs, a wide range of mathematical simulations are carried out to coordinate the storage parameters and system boundary conditions, and to derive specifications for material development.
   
3. Development –
   Together with our in-house materials scientists, for example, metal-PCM composite materials are developed and characterized in the thermo-technical laboratory, joining technologies are optimized and prototype storage elements are manufactured.
4. Validation – As a final step, the experimental validation of prototypical storage devices in our laboratory and the upscaling to the application is performed.
   

The most important factor limiting the performance of a latent heat storage device is the poor thermal conductivity of PCMs. Metallic heat-conducting structures in a PCM significantly increase performance with only a slight loss of storage capacity. Metallic fibre structures, as developed at Fraunhofer IFAM Dresden, are optimally suited due to

• the wide range of processable metals (aluminium, copper, iron),
• high flexibility regarding porosity (70 to 95 %),
• high thermal conductivity in fibre direction,
• mall cavity size (mm) for fixing the PCM and
• recisely fitting fabrication, which allows the insertion and mechanical pressing of classic heat transfer tubes.    

The picture shows the mechanical connection of aluminium fibre rings (1) with an aluminium tube and a prototype storage element developed on the same basis (2) with melted PCM (paraffin). The technology can be easily transferred to larger tube bundle latent heat storage systems (3). The compatibility of PCM and metal structure must be verified (characterization of PCM).

Both prototypical storage elements and finished demonstrators can be loaded and unloaded in a fully automated way with water as a heat carrier in a cyclical process in the thermo-technical laboratory and thus be tested (4). The metal-fibre paraffin storage unit shown in figure (3) achieved a power density of 375 kW/m³ in its completed form during the test.

At higher storage temperatures, nitrate salts and their eutectic mixtures are typically used as PCMs in conjunction with 3D metal wire structures, as these are better suited for combination with salt PCM because of

- a broad selection of metallic materials,
- a significantly more corrosion-resistant surface structure and
- more flexible possibilities for the application of corrosion protection layers.

The 3D wire structures are developed and manufactured by external partners.

With the help of suitable mathematical simulations, attention is paid in particular to the ideal positioning of the heat transfer tubes in the structure and their effective thermal contact with the wire structure (without material joining).

In our thermo-technical laboratory, storage demonstrators can be experimentally validated in a thermo-oil-based high-temperature cycling line with temperatures of up to 350 °C. Thermal cycling (loading/unloading) takes place in a narrow temperature range around the melting point of a PCM and all thermal and fluidic parameters are measured.

As an alternative to using heat-conducting structures in PCMs, encapsulation in small structures - in this case in the mm range (meso encapsulation) - can also be used to achieve high thermal performance, thus enabling the realization of small heat transport paths.

For this purpose, metallic hollow spheres developed and manufactured at Fraunhofer IFAM Dresden are ideal. They are available with diameters between 3 and 8 mm. As shown in the picture

• a polystyrene sphere is coated with a metal powder binder suspension and then dried (green) and sintered,
• the hollow metal ball is infiltrated through its porous shell with liquid PCM, cleaned and galvanically sealed.

The technology is available for paraffins with melting temperatures above 40 °C, for other organic PCMs corrosion tests are required. The loading/unloading time is in the single-digit minute range.

The PCM-filled spheres can be used either stationary as a bulk through which the heat transfer medium flows or dynamically as floating heat capacities in a heat transfer medium. All these application scenarios can be tested in our thermo-technical laboratory.