Radiation

How to chill an object by sending its heat into space

solar panels
solar panels

When a refrigerator cools your food, it takes the heat away and dumps it into your kitchen. That adds to your home’s cooling bills. Likewise, when your air conditioner cools your home, it sends that heat outdoors. It also makes things warmer for everyone else in your neighborhood. The farther away you can send heat, the better. And there’s not much farther you can send it than outer space. Now, researchers have built a device to do just that. It cools down an object by radiating its heat directly into space.

For now, the device isn’t too practical. But its designers say that such cooling methods, combined with other techniques, might one day help people get rid of unwanted heat. The device would be especially well suited for arid regions, they add.

Radiation is the means by which electromagnetic waves carry energy from one place to another. This energy might be starlight traveling through space. Or it could be the heat of a campfire warming your hands.

The bigger the temperature difference between two objects, the quicker that heat energy can radiate between them. And not many things are colder than outer space, notes Zhen Chen. He’s a mechanical engineer at Stanford University in Palo Alto, Calif.

Outside of the envelope of gases surrounding Earth — our atmosphere — the average temperature of space is about –270° Celsius (–454° Fahrenheit). Chen and his team wondered if they could take advantage of this big temperature difference between Earth’s surface and outer space to cool an object on Earth, using radiation.

For an object on Earth to shed energy to space, radiation must travel through the atmosphere. The atmosphere doesn’t let all wavelengths of radiation through, Chen points out. But certain energy wavelengths can escape with little resistance.

One of the atmosphere’s clearest “windows” is for wavelengths between 8 and 13 micrometers. (At these wavelengths, electromagnetic radiation is invisible to the human eye. Because their energy is lower than that of red light, these wavelengths are called infrared.) Fortunately, says Chen, objects at about 27 °C (80.6 °F) radiate much of their energy in just that window.

Building a heat-emitting device

To study the new concept, Chen’s team built an object they would try to cool. They used mostly silicon. The basic ingredient in beach sand, silicon is both cheap and sturdy. It’s also the material computer chips are made from. That meant Chen’s team could use the same techniques used in making computer chips.

selective emitter
In a new cooling device, a shiny layer of aluminum (bright layer at bottom) and a coating of silicon nitride (top surface) help radiate heat from a layer of silicon (middle) into space.

The base of their object was a super-thin disk of silicon, about twice the thickness of a human hair. That layer was for structural support. To that, they added a thin layer of aluminum. It reflected light waves like the shiny layer on the back of a glass mirror. The aluminum layer would send the object’s heat upward, toward space.

Next, the researchers added the layer of material they wanted to cool. It, too, was made of silicon, but was much thinner than the base layer. It was just 700 nanometers — billionths of…

5 Strange Facts About the Planet Earth

You know what it’s like: You live somewhere all your life but never realize just how great it is until someone comes to visit. While it’s just a shame we don’t get any visitors to marvel at all the peculiarities of our home planet, here are five facts you might still appreciate.

1. EARTH’S A GIANT DYNAMO.

The core of the Earth is a solid lump of nickel and iron, rotating in a sea of molten iron and nickel. This rotation functions the same way winding up a hand-held generator does, giving Earth an enormous magnetic field that extends up to 50,000 kilometers out into space. This magnetic field is crucial for life on Earth, as without it we would be exposed to the full force of the Sun’s radiation. As well as causing cancers and other radiation-aggravated conditions, the radiation’s sheer force would blow our atmosphere into space, as happened with Mercury, and to a lesser extent, Mars. Instead, charged particles are (mostly) harmlessly deflected away, giving rise to the auroras.

It’s not all good though: Any particles that hit the Earth head-on tend to get trapped in the field and can’t get out. These so-called Van Allen Radiation Belts can pose a hazard for astronauts who leave low Earth orbit.

2. IT’S THE DENSEST PLANET IN THE SOLAR SYSTEM.

While Earth may not be the biggest planet in the system, it is the biggest rocky planet in the solar system, and also the densest. Therefore, Earth has by far the highest surface gravity of any terrestrial object in the solar system. This is both a blessing and a curse.

The reason for the high density is the large deposits of heavy elements in the Earth’s makeup. Elements such as lead and uranium are much rarer on other worlds, which gives us a huge advantage in the amount and variety of construction materials available here on Earth. The high gravity has also demanded that humans develop the reflexes and endurance necessary to cope with such gravity, meaning we are far more durable than the potential delicately boned, sloth-like creature we could be had we evolved in low gravity.

Unfortunately, that high gravity makes Earth the worst place…