Adsorptive Hydrogen Storage: Experimental investigations on thermal conductivity in porous media
The objective of this work was to install and verify the Hot Disk TPS measurement setup for thermal conductivity measurements, and to carry out experiments on various porous materials. A literature survey on gas/solid porous media, with emphasis on the transport mechanisms and predictive models, was conducted. Special interest was taken in the widely-used Zehner/Bauer/Schlünder (ZBS) model for effective stagnant thermal conductivity of packed beds. Great care was shown in the determination of bed-properties such as porosity, because of the large effect it has on the effective thermal conductivity.
The porous materials investigated were the Metal Organic Framework (MOF) hydrogen adsorbents Cu-btc (HKUST-1) and Fe-btc-xerogel. Large (⌀1.395mm) and smaller (⌀0.38mm) glass beads served as a reference material for preliminary tests and setup validations. In a later stage the Cu-btc and Fe-btc was experimentally investigated. Thermal conductivity measurements were conducted on a packed bed with air, nitrogen (N2) or helium (He) as fluid, in temperatures ranging from 243K<T<423K at an absolute pressure of zero to 0.5 bar.
The smaller glass beads (⌀0.38mm) were also tested together with an open-cell, high-porosity aluminum foam. The purpose of the metal foam in adsorption hydrogen storage is to increases the effective thermal conductivity of the bed. Experiments showed that applying the aluminum foam increased the magnitude of the effective thermal conductivity of a bed consisting of glass beads and air by a factor of 17 from 0.22 W/m×K to 3.7W/m×K at room temperature.
The preliminary experiments revealed a calibration error in the Hot Disk software, creating a discontinuity in the effective thermal conductivity in the range of 273K<T<283K. Outside that range, the Hot Disk measurement setup provides accurate measurements of the effective thermal conductivity of porous materials.
Hot Disk gives a measurement uncertainty of 5%. In addition to this comes the uncertainty of the theoretical model, due to the input of measured parameters such as porosity. An uncertainty analysis on the ZBS model gave an uncertainty of approximately ±10% for the glass beads and ±5% for the MOF, respectively. Adding the uncertainty of the ZBS model to the uncertainty of the experiments gives a total uncertainty of 15% for the glass beads experiments and 10% for the MOF.
Through a least square procedure, the solid conductivity of the MOF materials were fitted to the values of the ZBS model, determining temperature dependent functions for the solid conductivity yielding for each of the MOF?s. The ZBS model proved to be a reliable estimate for the effective thermal conductivity in a packed bed, differing from the measurements with less than 10%.
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2013. , 206 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-22781Local ID: ntnudaim:9942OAI: oai:DiVA.org:ntnu-22781DiVA: diva2:652823
Næss, Erling, ProfessorSchlemminger, Christian