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Cool! New rooftop materials can beam heat directly into space


Researchers Linxiao Zhu, Prof Shanhui Fan and Aaswath Raman. Credit: Norbert von der Groeben/Stanford


Scott Sutherland
Meteorologist/Science Writer

Thursday, November 27, 2014, 2:22 PM - Looking for a way to lower your electricity bill in the summer? How about just beaming the excess heat from your house straight through the atmosphere, into the depths of space?

It's not exactly a new concept, but what we choose to put on our rooftops - colours, materials, etc - can make a big difference to how much heat our homes and office buildings accumulate. This passes on to how much cooling these buildings need and that translates into how expensive it is to maintain the building.

Current cooling systems just dump excess heat outside, where it combines with all the heat radiating off concrete and asphalt to contribute to the Urban Heat Island. Furthermore, the infrared waves radiating out from this hot air, as well as from the building roof itself, are readily absorbed by greenhouse gases, like carbon dioxide, thus contributing to global warming and climate change.

CLICK BELOW TO WATCH: Meteorologist Jaclyn Whittal discusses how the Urban Heat Island works.

However, researchers with Stanford University have come up with a new material - called a photonic radiative cooler - that can solve these problems in two ways, and using one of the coldest 'heat sinks' we have available to us.

"We've created something that's a radiator that also happens to be an excellent mirror," Stanford research associate Aaswath Raman said in a press release.

"Every object that produces heat has to dump that heat into a heat sink," Stanford electrical engineering professor Shanhui Fan added. "What we've done is to create a way that should allow us to use the coldness of the universe as a heat sink during the day."

More than just a high-tech mirror


An illustration of the photonic radiative cooler in action.
Credit: Fan Lab/Stanford University

The thin, mirrored material that Fan, Raman and their colleagues have developed uses layers of silicon dioxide and hafnium oxide, laid out in a two-part, alternating pattern. The bottom four layers are optimized to reflect as much incoming solar radiation as possible, and in tests they achieved a 97 per cent reflection rate. The top three layers act as the radiator, but rather than just indiscriminately radiating away that heat into the atmosphere, the layers focus the radiation into a specific range of wavelengths - in the 'thermal imaging' range of infrared, between 8 and 15 micrometres. In this range, the atmosphere is transparent, meaning that the radiation can completely escape into space. Even carbon dioxide and water vapour are mostly, if not completely, transparent at those wavelengths.

"Think about it like having a window into space," Fan said in the statement.

In tests, the research team found that the small radiator was cooler than the air around it by about 5 degrees C. If this was applied on a large scale, that amount of cooling could substantially reduce the need for air conditioning. Also, given that this is a passive system, thus requiring no electricity to run, it can translate into real savings on a power bill.

Savings, but not without costs

While this sounds like the answer to all our problems of building cooling, there are a few downsides.

First, in order to radiate the heat from the building away, there needs to be an effective, and efficient way of moving that heat from the building to those top layers of the material. Second, covering a building with this material will need panels much larger than the tiny plate of it they have for testing. Depending on how expensive these processes are, how useful the radiator is could come down to how much it costs compared to the savings it generates.

Hidden benefits?

In addition to lowering our cooling costs, these radiators could help us to breathe a little easier as well.

Studies have been showing that planting more vegetation and using solar-reflective materials on roofs can significantly reduce the urban heat island, and thus the number of ozone days or smog events that residents have to endure. Moving from darker to lighter materials in cities - especially replacing dark asphalt with lighter concrete - can have a big impact as well, by reducing the amount of light absorbed by urban surfaces, and thus the amount of heat emitted into the urban airmass.

There are some instances where ground-level ozone concentrations can spike over a city or region due to a high-albedo surface on the ground. Chemicals or pollutants in the air that are precursors to ozone can receive a 'double-blast' of sunlight - once as the light shines down from space, and again as it is directly reflected back towards space by the highly-reflective surface, like a fresh layer of ice or snow (or even concrete) - thus producing more ozone than would have formed without that surface being there. So, adding mirrors like this to buildings could produce a similar effect on some days. These instances are fairly uncommon, though, mainly occurring in winter, when ground-level ozone values are typically fairly low. So, efforts to introduce more green-space and more reflective, high-albedo materials into urban areas can have a beneficial effect on air quality, both by lowering both energy consumption and temperatures in the urban heat island.

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