How mirrored roofs will augment data center air cooling

A new roofing material could slash data center energy costs, if it can be made to work right.

mirrored lead image
Credit: Stanford

Data center managers looking to reduce costs might want to take a look at a new roofing material that’s being developed by some Stanford University egg-heads.

The scientists reckon that their super-reflective, mirror-like substance could be a cheaper and more ecologically sound add-on to air conditioning. It works by disseminating heat out of the area it's trying to cool.

Essentially, the cold universe acts as a heat sink.

What is it?

A thin-film coating on a roof-installed product reflects light from the sun. Direct sunlight is one cause of building heat-up.

The tech should also channel heat away from within the structure, according to Chris Cesare, writing about the technology in Stanford News.

Reflected heat

Heard it all before, you might say? What about Mylar, that DuPont-marketed silvery polyester foil that we see in car windshields on a hot day (or don’t see stuffed in attics) as a radiant barrier form of insulation?

Mylar and its foil variants, like space blankets, reflect heat. In the space blankets case, it reflects the body’s heat back towards the wearer.

Well, this is a similar idea, but a bit more high-tech.

Passive cooling

Photonic radiative cooling, as Shanhui Fan, one of the electrical engineering professor inventors, and his cohorts call it, uses a special, tuned frequency created by the heated material. It sends infrared light containing the heat through a transparent window in the atmosphere and on to the universe.

That window exists because there’s nothing there to absorb that particular frequency. In other words, the atmosphere is transparent to that infrared wavelength.

This atmospheric-window channeling keeps heat away from buildings and from interfering with the atmosphere. Air around the building is kept cool, and maybe global warming is reduced as well.

Passive cooling also saves energy because air conditioning use is minimized.

Dumping the heat

It sounds reasonably simple—just reflect the created heat. But it’s actually quite difficult to achieve, because, in order to cool, you need to be able to reach a temperature that’s below that of the ambient air.

That’s easy-ish to do at night, because the sun isn’t creating heat, and loads tend to be smaller—people are asleep.

During the day, it’s trickier. In fact, it requires a technological breakthrough. One big problem, according to Fan, is that you need the great window in the sky to achieve radiative cooling, yet direct sunlight puts the kibosh on the result.

Fan, in a letter written in Nature, says his and his buddies' thermal photonic concept, using their special silver, silicon dioxide, and hafnium oxide material, achieved radiative cooling of almost 5 degrees Celsius during the day. That's considered a good result.

He says they reflected 97% of sunlight through the atmospheric window with the special film. 

Issues

The only slight problem, though, is that they haven’t yet quite figured out a good way to conduct the heat from within the building to that outside film coating and beyond. One option might be to pump water around the building, and then cool the water with the material, Fan says.

Production issues

Which leads me on to our Stanford researchers’ second problem, which is one of production.

Its experimental model is 1.8 microns thick, thinner than the aluminum foil you get at an average dollar store and certainly too thin to wrap a turkey, or indeed create a roof with. In my home-improvement weekend-warrior’s opinion—it’s going to need a structural backing, which conceivably could obstruct heat emanating from within the building. Heat does rise, though.

And with 15% of the power used in buildings in the U.S. operating air conditioning systems, according to Cesare in his Stanford article, this photonic approach could make a lot of tree huggers and data center owners very happy if it can be made to work right.

Data centers have the large roof expanse that will be likely required for an efficient implementation.

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