Thermal systems use heat to produce cold, and vice versa. To do so, a material is needed that can dissipate water vapor particularly well and quickly. A new method simply applies this property as a layer onto the components.
Refrigerators have the human body as an example: When we perspire, water evaporates on our skin and cools it. The lower the atmospheric pressure, the easier this is. If the process is transferred to a vacuum, water already evaporates at a few millibar and a temperature of 10 degrees. So that the devices continuously cool, the vapor must be removed. This is achieved, for example, by an electric compressor, just as the water vapor in our refrigerators is removed from the gas phase and then re-liquefied. An alternative is the thermal compressor, a porous material that can absorb water vapor. In this variant, the operating power is not electrical, but thermal. Heat pumps or chillers operated in this manner produce cold from heat, and vice versa. So far, however, these have not been able to prevail entirely over their electricity-powered counterparts; their power density is too low. What is lacking are materials and components that are capable of sufficiently discharging the water vapor in a shorter time.
Materials must absorb water vapor
Researchers at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg have now closed this gap. Their metal organic frameworks (MOFs) are particularly well suited to absorb water vapor. In this process, a metallic cluster and organic linkers together form a three-dimensional porous structure. “MOFs can be put together arbitrarily like Lego bricks and outperform every previously known class of material in terms of flexibility. The materials are porous and have interior surfaces which can add up to 4,000 square meters per gram. This is sufficient space for the water vapor to be able to adsorb and accumulate,” explains Dr. Stefan Henninger, Head of the Sorption Materials Group at the ISE.
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The Latest on: Metal organic frameworks
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The Latest on: Metal organic frameworks
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