Thermal systems use heat to produce cold and vice versa. To do so, a material is needed that can dissipate water vapor well and quickly. One new method simply applies this property as a layer onto components of a heating or cooling system.
Researchers at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany, found a way to lower the atmospheric pressure to accelerate evaporation and produce the evaporative cooling effect.
The researchers found that when evaporative cooling processes take place in a vacuum, water evaporates at a few millibars and a temperature of only 50°F (10°C). Yet in order for devices to continuously cool, the vapor must be removed. This is achieved, for example, by an electric compressor. One familiar example is the water vapor in household refrigerators, which is removed from the gas phase and then re-liquefied.
An alternative is a thermal compressor, which uses a porous material that can absorb water vapor. In this variant, the operating power is not electrical but thermal. Heat pumps or chillers operating in this manner produce cold from heat and vice versa. These devices have not taken the place of their electricity-powered counterparts because their power density is too low. What these devices lack, say the researchers, are materials and components that are capable of sufficiently discharging the water vapor in a shorter time.
Researchers at the Fraunhofer Institute say they have now closed this gap. Their metal organic frameworks (MOFs) are well suited to absorb water vapor. In this process, a metallic cluster and organic linkers 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,” says Dr. Stefan Henninger, head of the Sorption Materials Group at the ISE. “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.”
Together with his colleagues, Henninger has investigated a large number of MOFs and identified those that are particularly stable with respect to water vapor. These materials can absorb up to 1.4 grams of water per gram of material, as opposed to the previous absorption of 0.4 grams. Yet he notes that MOFs are available only in powder form, making them difficult to incorporate into the relevant device structures such as heat exchangers. As loose granules, only selective contact exists between the adsorbent material and the component, limiting the material or heat transfer.
In order to achieve the greatest possible surface, it is better to apply the MOFs as a layer, which the researchers have accomplished. Layers can be applied directly without the necessity of auxiliary layers in between. The resulting products reach the cooling and heating thicknesses of 50 to 150 µm that are useful for cooling and heating.
In prototype form, the MOFs are directly crystallized onto metals. For other materials such as ceramics, binder-based coatings are used. In both methods, the components of the device are simply immersed in a fluid containing all the essential components of the material.
“The MOF layer grows directly on the component at a rate of up to 50 micrometers per hour. This is significantly faster than before,” Henninger says. The researchers have so far coated components of up to 5.9 by 15.8” (15 by 40 cm) with the new procedure.
The technology is not limited to cooling and heating equipment, say the researchers. “Due to the enormous flexibility of MOFs and our manufacturing process, a variety of applications is possible. We can apply the desired structure quickly to almost any component. In principle, our technology could be beneficial for every process in which material or heat transfer plays a role,” says Henninger. An example would be processes in the chemical industry where gases are separated and heating is thereby created or needed.
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