ShareImproving adsorption cooling thanks to MOFs
Metal-organic framework (MOF) compounds are porous crystalline materials composed of metal ions or metal clusters bridged by organic linking groups; they have outstanding adsorption properties, allowing for better efficiency and use on a larger scale.
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.
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.