Passive solar windows heat up in cold weather

October 24, 2017 by Lisa Zyga, Phys.org feature
(a) Artistic depiction of a nanoantenna surface. (b) Scanning electron microscope micrograph of the nanoantennas. (c) Photographs of the new surface held against a window. Credit: Jӧnsson et al. ©2017 American Chemical Society

Researchers have developed a way to transform ordinary windows into solar-powered heaters that use the sun's energy to increase the window temperature by up to 8 K (nearly 15 °F) in cold weather. The researchers expect that the new solar thermal surfaces will lead to significant energy savings through reduced heating costs.

The scientists, led by Alexandre Dmitriev at the University of Gothenburg, along with coauthors from universities in Sweden, China, Iran, and the US, have published a paper on the solar thermal surfaces in a recent issue of Nano Letters.

"We've developed a surprisingly simple, cheap, and effective way to transform regular glass windows into solar-powered heat-screens that could significantly change the thermal balance of living and working spaces, especially if one thinks of the ever increasing amount of huge glass surfaces used in modern architecture," Dmitriev told Phys.org.

The new surfaces are yet another application of nanotechnology, as the main functional components are plasmonic nanoantennas. The tiny antennas are made of nickel-aluminum oxide sandwiches, shaped like nanoellipses, and patterned as an array onto glass. With the help of electron oscillations, or surface plasmons, on the surfaces of these materials, the nanoantennas strongly absorb light, which heats the entire surface.

In the new study the researchers demonstrated that, when sunlight shines on the surface, light is absorbed more efficiently from the front side (with the antennas) than the back side (the substrate). This directionality in light absorption makes the surfaces attractive for window applications, as sunlight can be absorbed most efficiently from the outside of the window. In addition, the surfaces are highly transparent, appear colorless, and almost completely preserve the color spectrum of sunlight.

As the researchers explain, cold windows have a larger impact on heating a building than might be expected. This is because, when a person sits next to a cold window, the person radiates their body heat toward the window and the window acts like a "heat sink." To compensate for this loss of heat, the indoor temperature needs to be increased to maintain a comfortable environment. As the new window surface can increase the temperature of the by several degrees, it has the potential to offer a large energy savings.

The researchers expect that the new surface may have other applications beyond windows. The nanoantennas are highly modular, can be "painted" onto any surface or, to preserve the directionality in absorption, can be directly transferred onto virtually any . They can also be manufactured out of a wide variety of materials, as well as tuned to absorb light of different wavelengths, which changes their color.

"All of these advantages may lead to applications where transparency, directionality, and thermal properties are simultaneously important, with possibilities including radiative cooling, solar-powered thermal isolation, and others," Dmitriev said. "These antennas might also potentially be coupled to molecular systems that are able to store sunlight as heat and release it on demand."

In the future, the researchers plan to work on achieving even larger temperature increases by enabling the surfaces to absorb ultraviolet and near-infrared radiation, which constitute a significant portion of solar radiation.

Explore further: Transparent solar technology represents 'wave of the future'

More information: Gustav Jӧnsson et al. "Solar Transparent Radiators by Optical Nanoantennas." Nano Letters. DOI: 10.1021/acs.nanolett.7b02962

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8 comments

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IronhorseA
5 / 5 (3) Oct 24, 2017
Do we switch them out during the summer?
Whydening Gyre
5 / 5 (1) Oct 24, 2017
Do we switch them out during the summer?

My understanding is that they are electronically tunable...
AlmostClever
not rated yet Oct 24, 2017
I wonder if expansion/contraction cycles in "high volume manufactured" window glass would be of concern.
Eikka
2 / 5 (4) Oct 24, 2017
Why heat the glass? It's only going to conduct the heat out.

Regular IR reflective coating would make the glass appear warm without being warm, and has the added bonus of keeping the heat out in the summer.
Caliban
4 / 5 (4) Oct 24, 2017
Why heat the glass? It's only going to conduct the heat out.

Regular IR reflective coating would make the glass appear warm without being warm, and has the added bonus of keeping the heat out in the summer.


Prolly use the interior of double and or triple paned insulated windows. Add to that the new Transparent solar pv layer, and you've got a rather potent power generating/heating combination, especially in the glass and steel construction that is so ubiquitous in modern buldings.
Zzzzzzzz
3 / 5 (4) Oct 24, 2017
Why heat the glass? It's only going to conduct the heat out.

Regular IR reflective coating would make the glass appear warm without being warm, and has the added bonus of keeping the heat out in the summer.


No energy is expended to heat the glass. The fact that it is heated by the sun reduces the energy requirement to heat the building. Your bias is shining thru once again.....
Eikka
5 / 5 (1) Oct 27, 2017
The fact that it is heated by the sun reduces the energy requirement to heat the building.


But the heat from the sun would be collected to use anyways as it heats the interior of the room. Now it gets stopped in the window, and a greater part of it would be conducted out.

In contrast, an IR reflective glass would reflect the heat captured by the room back into the room and less would conduct out through the glass.

Your bias is shining thru once again


It seems far easier to just shoot the messenger than consider what they have to say. The proof here is in the pudding though. Make two boxes, one covered in IR absorbing glass and the other in IR reflective glass. See which one heats up more under sunlight.

The reflective glass works better, because the sunlight coming in is re-transmitted at a lower frequency IR, which gets trapped by the glass. The IR absorbing glass warms up itself, and part of the heat is lost through the other side.
Eikka
5 / 5 (1) Oct 27, 2017
To explain it better; consider a room with a window but no glass.

The sun shines in. Sunlight is heavy in the visible wavelenghts and near-IR because it's a high temperature black body radiator. The room absorbs the light, and re-radiates it out in the far-IR wavelenghts because the room is a low temperature black body radiator.

Now we put a regular pane of glass in the window. It reflects some far-IR back into the room without absorbing it, thus retaining the heat. We add an IR-reflective layer, and the effect is enhanced. The window is cool, but feels warm to stand by because it is returning the heat back to the sender.

Now we change it to an IR-absorbing glass: the window no longer reflects the heat back into the room, but becomes warm itself, and being warm it also radiates in the far-IR band. It radiates on both sides. The window feels warm to stand by, because it is radiating heat at you, but at the same time half of the heat goes out the other side.

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