Building material with switchable heat emission

The reversible formation of a solid layer is the secret of the thermoregulating material. © University of Chicago PME – Image courtesy of Hsu Group

Cooling is required in summer, while thermal insulation is required in winter. Researchers have now developed an enveloping material for buildings that can be adjusted to these different needs using a sophisticated mechanism: the material can be reversibly switched from a state with low to one with intensive heat radiation, resulting in a warming or cooling effect. The effect is based on the formation or dissolution of a solid layer in the material. According to the developers, the system has considerable potential for saving energy for temperature regulation in buildings.

The importance is immense: The temperature regulation setting in the buildings of the world consumes enormous amounts of energy, because mostly air conditioning and heating systems are used. "For a long time, people took for granted controlling the indoor temperature without considering how much energy it takes," says Chun Hsu of the University of Chicago. "But as we move towards a climate-friendly future, we need to consider different ways to favorably influence building temperatures in more energy-efficient ways," said the senior author of the current study.

Adaptability is required

To give buildings a basic ability to stay cool, the principle of radiant cooling can be used. Materials are used that emit particularly intense infrared radiation - the invisible heat radiation that also emanates from people and warm objects. Conversely, materials with very low emissivity values ​​can be used to reduce heat loss from buildings in cold climates. But in many regions, depending on the season, one or the other is in demand. Few climates require year-round heating or cooling by air conditioning. According to forecasts, the changeability of the weather could also increase significantly as a result of climate change.

In order to give building surfaces more adaptability in terms of heat radiation, Hsu and his colleagues have now used a process based on the principle of so-called electrochromism. This technology is based on changing optical material properties through an external electric field or current flow. So far, however, this technology has been largely limited to the visible wavelength range. In addition, there was a problem in that previously customary electrolyte substances are easily combustible and are therefore not suitable for use in flat coverings of buildings. The researchers say that the new electrochromic material can now also score points in this respect.

From metal to liquid and back

The non-combustible material is a clever layering system: the upper unit is formed from a combination of polyethylene, a subtle gold mesh and a touch of graphene. This layer functions as an electrode and is transparent. Underneath is a liquid electrolyte based on perchlorate in which copper is dissolved. The bottom layer is formed by a copper foil and serves as the second electrode of the system. If a comparatively low voltage is now applied, the copper dissolved in the electrolyte forms a solid metal layer under the upper electrode. If the electrical polarity is reversed, this layer then dissolves again.

In both states, the material has very different heat radiation properties, the researchers explain: The copper layer leads to low heat emission. Without them, however, the electrolyte radiates intensely in the infrared range – around a factor of ten more. "We ended up finding an energy-saving way to treat a building like a person: add a layer when you're cold and take a layer away when you're warm," says Hsu. Tests of the material showed that it can switch quickly and reversibly between the metallic and the liquid state - and also with endurance: the system is still working efficiently after 1800 cycles.

Energy saving potential

"So with this kind of smart material, you could control the temperature in a building without using a lot of energy," Hsu says. "Because once you've switched between states, you don't have to expend any more energy to stay in either state." Using model calculations based on their results, the researchers were also able to show that the use of the material in buildings in regions with seasonally fluctuating temperatures could lead to significant energy savings.

However, some development work is still needed before the system can be used, the researchers point out. Because so far they have only produced pieces of the material with a diameter of about six centimeters. But according to them, the concept can be scaled up. For example, it could make sense to assemble many pieces of such units to form larger plates. Researchers are also currently working on optimizing manufacturing and material costs to make the concept workable. “We have now first shown that radiation control can play a role in controlling building temperatures at different times of the year. Now we are working with other experts and the construction sector to see how this can contribute to a more sustainable future,” says Hsu.

Source: University of Chicago, professional article: Nature Sustainability, doi: 10.1038/s41893-022-01023-2

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