Améliorer le froid à adsorption grâce aux "MOF"

Les réseaux métallo-organiques (MOF) possèdent des caractéristiques d'adsorption remarquables permettant une meilleure efficacité et une utilisation sur une plus large échelle.
Adsorption cooling systems offer substantial energy and environmental advantages as they can be activated by low-grade thermal energy and, depending on application, and can use water as a refrigerant. However, the systems tend to be large and heavy, relying on common sorbents such as silica gel and zeolite with mediocre water-uptake properties.

But better sorbents such as metal-organic framework (MOF) compounds are about to change this: these porous crystalline materials composed of metal ions or metal clusters bridged by organic linking groups have outstanding adsorption properties, allowing for better efficiency and use on a larger scale.

For example HKUST-1, a copper benzene tricarboxylate-based MOF showed a 93% enhancement, while MIL-101, a MOF made from chromium and benzene dicarboxylate linkers can be loaded with roughly 1.4 g of water per gram of sorbent (140% water per weight) more than four times the loading of conventional sorbents. Customizing the organic linkers and metal clusters also enable tuning MOF hydrophilicity. A recent study showed that with the right tuning, certain MOFs could soak up water even when ambient humidity was as low as 30%.

The drawback is that upon repeated cycling, the water-uptake capacity of all commercially available MOFs decreases and longer tests in an industrial rig are needed to learn more about the long-term stability of MOFs. US Pacific Northwest National Laboratory (PNNL) conducted a 3-month test in 2013 involving “many tens of thousands of cycles” on an industrial-sized unit with an as-yet undisclosed MOF which maintained its water-uptake and cooling capacities throughout the trial run.

The team is now working on a prototype naval waste-heat recovery unit for the Navy and studying MOF interactions with fluorocarbon refrigerants.