Pitt study reveals heat transfer mechanisms in MOF-based adsorbed natural gas storage; advancing the technology
22 January 2016
Researchers at the University of Pittsburgh’s Swanson School of Engineering are working with metal-organic frameworks (MOFs) to develop a new type of natural gas storage system that would adsorb the gas like a sponge and allow for more energy-efficient storage and use. In the journal Physical Review Letters, the team reports on their study of heat transfer mechanisms in such systems, work that may advance the technology.
Traditional CNG tanks are empty structures that require the gas to be stored at high pressure, which affects design and the weight of the vehicle. Dr. Christopher Wilmer and his lab are instead focused on porous crystal/gas systems, specifically MOFs, which possess structures with extremely high surface areas. However, Dr. Wilmer notes, one of the biggest challenges in developing such an adsorbed natural gas (ANG) storage system is that the process generates significant heat which limits how quickly the tank can be filled.
Unfortunately, not a lot is known about how to make adsorbents dissipate heat quickly. This study illuminates some of the fundamental mechanisms involved.
—Dr. Wilmer
The study showed that the thermal conductance of the system (crystal and gas) is dominated by lattice thermal conductivity in the crystal, and that conductance is reduced as the concentration of gas in the pores increases.
The researchers observed this mechanism from classical molecular simulations of a monatomic gas in an idealized porous crystal structure.
They showed that the decreased conductivity associated with increased gas concentration is due to phonon scattering in the crystal due to interactions with gas molecules. Calculations of scattering rates for two phonon modes revealed that scattering of the lowest frequency mode scales linearly with gas density. The researchers concluded that this result suggests that the probability of a phonon-gas collision is simply proportional to the number of gas molecules in the pore.
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| Idealized porous crystal structure (blue spheres) containing adsorbed gas molecules (orange spheres). Gas adsorption into nanoporous crystals (e.g., metal-organic frameworks) reduces the system’s thermal conductance due to phonon scattering in the crystal due to interactions with gas molecules. Click to enlarge. |
According to Dr. Wilmer, gases have a $500-billion impact on the global economy, but storing, separating, and transporting gas requires energy-intensive compression. His research into MOFs is an extension of his start-up company, NuMat Technologies, which develops MOF-based solutions for the gas storage industry.
By gaining a better understanding of heat transfer mechanisms at the atomic scale in porous materials, we could develop a more efficient material that would be thermally conductive rather than thermally insulating. Beyond natural gas, these insights could help us design better hydrogen gas storage systems as well. Any industrial process where a gas interacts with a porous material, where heat is an important factor, could potentially benefit from this research.
—Dr. Wilmer
Resources
Hasan Babaei and Christopher E. Wilmer (2016) “Mechanisms of Heat Transfer in Porous Crystals Containing Adsorbed Gases: Applications to Metal-Organic Frameworks” Physical Review Letters doi: 10.1103/PhysRevLett.116.025902
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