HiPel – metal-coated sorption pellets for thermal storage and adsorption applications

Adsorbents for dehumidification and heat storage

Sorption pellets made of NaY zeolite in various sizes and shapes.

The principle of adsorption is used in many technical applications, for instance to dehumidify air or gases or to separate mixtures of materials. A further application is sorptive thermal storage, which can be used to increase energy efficiency in industrial processes and thermal technology applications. Due to ease of handling and manufacture, pelleted adsorbents (e.g. zeolites, activated carbon, silica gel etc.) in spherical or cylindrical forms are generally used. However, these often have restricted mechanical stability and extremely limited thermal conductivity. This is a disadvantage for the performance of many applications and thermal storage in particular.

Project information

Project title

MEF HIPEL – High performance composite pellets

 

Project duration

September 31, 2011 – August 31, 2013

 

Coordinated by

  • Fraunhofer IGB

 

Project partners

  • Fraunhofer IKTS, Dresden
  • Fraunhofer IWU, Chemnitz

Improved sorption pellets – increased stability and thermal conductivity

Within a project funded by the Fraunhofer-Gesellschaft, the Fraunhofer Institutes IGB, IKTS and IWU are developing metal-coated pellets that, when used as packing provide distinctly increased thermal conductivity, while retaining the same adsorption capacity and are mechanically stable. When used in heat storage devices or heat pumps, increased heat conduction enables significantly increased power density through faster loading and unloading. This should make the temperature regulation of the sorption bed in chemical reaction technology simpler and more effective. An additional goal is to minimize abrasion losses in the transport of materials and filling of the reactor due to increased mechanical stability. This leads to longer lifetimes in sorbent packing and enables new reactors (flow reactor) and mobile applications. In order to achieve the best possible adsorption characteristics the pellets should have a high internal porosity. The manufacturing method should be suitable for the production of large quantities.

Manufacturing process and material tests

Aluminum and copper coated sorption pellets

The project involves the manufacture of cylindrical pellets coated with copper, aluminum or other metals, in which sorbent granules are filled into thin-walled metal tubes. The initial step was the design of a suitable manufacturing process, which consists of the four partial steps of granulation, filling the metal tubes, flow-rolling and separation of the pellets. It is a particular challenge here to carry out the thickening of the material so that the sorbent remains in the metal casing but still retains sufficient porosity to maintain the adsorption characteristics. Model experiments were carried out at the Fraunhofer IKTS using NaY zeolite and various binders. The output pellets are pressed into cylindrical pellets and their strength and specific surface are determined.

Based on these previous experiments, the Fraunhofer IWU designed and constructed a test facility for the manufacture of 5–10 kg of adsorption pellets. The facility will shortly make larger quantities of coated pellets available, in order to test these in the model thermal storage unit at the Fraunhofer IGB.

Examination of the adsorption characteristics

In parallel, reference measurements were carried out using uncoated pellets at the Fraunhofer IGB. Two qualities of zeolite spheres (diameter 1.6–2.5 mm and 2.5–5 mm), as well as uncoated cylinders (5.25x10 mm) with an alternative binder, which is also used in the coated pellets, were tested with regard to their performance in a closed adsorption storage unit. It was shown that size and alternative binders had little effect on the sorption characteristics, such as adsorption capacity and mass-specific heat storage density, in the cylindrical pellets compared to spheres.

Measurement of the thermal conductivity of packing

In addition, the Fraunhofer IGB has designed and constructed an apparatus for the measurement of the thermal conductivity of packed beds under variable air or gas pressure in the reactor. The measuring principle is based on the hot wire method, which enables fast and accurate measurements. Measurements are taken in a high vacuum tight container that can be equipped with differently sized sample containers, depending on the quantities of adsorber material. This enables fast reference measurements of the thermal conductivity of sorption pellets under various process conditions.

Outlook

Once sufficient quantities of coated pellets are available, these will be examined in the adsorption thermal storage unit (standard 5–10 L storage capacity) at the Fraunhofer IGB under various operating conditions and the results will be compared with the reference data. Investigations of the mechanical stability of the pellets are carried out in parallel. It is then possible to determine the application-relevant use of the novel pellets and the increase in performance achieved. In regard to thermal storage, a significant increase in the power density achieved and therefore greatly reduced loading and unloading times are expected. Finally, a cost-benefit calculation for industrial production will be created.

Funding

We would like to thank the Fraunhofer-Gesellschaft for funding the project “HiPel”, within the scope of its SME-oriented internal research program (MEF).