A 21st century twist on an ancient method of making ice could revolutionise ways of staying cool on our ever-hotter planet.
Everyone knows there’s no way to beat the laws of physics. Drop a metal bar in water, and it sinks. Throw a stick in the air, and it’ll come back to earth. Put something cold in the noon-day sun, and it’ll warm up.
But some of mankind’s biggest scientific breakthroughs have come from spotting loopholes in these iron-clad laws.
Hammer the metal bar into a tin can, and it’ll float. Carve the wood into an aerofoil, and it will glide.
Now scientists have found a way of seemingly defying thermodynamics, and its insistence that you can’t make objects cooler than their surroundings without using energy.
A team from the University of Colorado, Boulder, have created a thin sheet of material made from a polymer coated in silver and embedded with tiny glass balls.
Known as a metamaterial, it’s a combination that subtly alters the thermal properties of whatever it covers, allowing it to cool even in the searing heat of the day – and without using any energy.
It pulls off this trick by exploiting a phenomenon used by ancient desert civilisations to make ice during the summer, thousands of years before the invention of the refrigerator.
After sunset, water would be poured into shallow trenches exposed to the clear night sky. As the hours passed, ice would start to form in the trenches – despite the temperature still being well above freezing.
On the face of it, this defies the laws of thermodynamics, according to which heat flows from hot to cold bodies, unless energy is used to reverse the flow.
And while summer nights are cooler than the days, they’re still typically pretty warm - so why does the cooling keep going?
The answer is literally out of this world.
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Space is bitterly cold, and it’s only the famous “greenhouse effect” of our atmosphere that stops temperatures plummeting far below zero after sunset.
Put simply, the molecules of gases and water vapour that make up the atmosphere block most of the heat radiation from seeping out into the bitter-cold cosmos.
Most - but not all. There’s a “hole” in our planet’s greenhouse: a range of radiation wavelengths which the atmospheric gases allow through.
Water exposed to a clear night sky loses much of its warmth through this “hole” and into the icy cold of deep space.
As a result, the water can become colder than its surroundings – while keeping the laws of physics intact.
Known as radiative cooling, it’s a phenomenon with the potential to do more than make ice on clear summer nights.
In principle, it could provide a zero-energy source of cooling for buildings – ideal for future cities in an ever-hotter planet.
But to make the most of it, some way of channelling as much heat as possible through the “hole” is needed – and making it work all day long, not just at night.
That’s the challenge, and it seems the University of Colorado team have pretty much cracked it.
The mirror-like sheen of their metamaterial foil acts as an incredibly efficient mirror, bouncing back 96 per cent of the sunlight striking it. That alone helps keep whatever is under the foil cool.
But the real secret of the material lies in the tiny glass spheres embedded in it. These channel heat radiation into those wavelengths that can pass through the atmosphere and into deep space.
As a result, even during the day time, anything covered by the foil behaves as if it’s night – and cools to temperatures lower than its surroundings, without requiring any energy.
In real-life tests reported in the monthly international energy research journal Joule, the team reports that water covered by the foil stayed around 10°C cooler than its surroundings, even in intense summer sunlight.
The foil itself is said to be cheap and simple to mass-produce, with just 10 - 20 square metres of it being enough to keep a small house cool in summer.
But the team believes the foil will also have industrial applications, such as for use in power plants and data centres, where shedding heat cheaply is a priority.
They also see applications in boosting the efficiency and lifetime of solar panels.
Constant exposure to sunlight can cause these panels to overheat, reducing performance. According to the team, applying the foil to the surface of solar panels could boost their efficiency by 1 to 2 per cent – which adds up to a lot of power over the whole area of an array.
Energy-free cooling is just the latest breakthrough made possible by the seemingly magical properties of metamaterials.
In recent months, researchers have unveiled a slew of such materials. They include a composite metamaterial that bends sound waves around small objects until they levitate, and a stretchy film made from aluminium and plastic that makes objects invisible to heat detectors – ideal for military camouflage.
Some researchers think it may one day be possible to make objects disappear completely, using a metamaterial “invisibility cloak”. Already, Chinese scientists have been reported to be experimenting with metamaterials that bend microwaves around aircraft, making them invisible to radar.
Arthur C Clarke, the futurist and author of 2001: A Space Odyssey, once said that "any sufficiently advanced technology is indistinguishable from magic". So far, the promise of metamaterials looks nothing short of sorcery.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK
