Air conditioning could be replaced with mirrors

Scientists have developed a mirror that could see air conditioning units becoming obsolete. Placed on the roof of buildings, the mirrors are designed to reflect light and heat back into the subzero temperatures of space.

The researchers believe that the mirrors could dramatically reduce the amount of commercial energy used by business and shopping centres to keep their buildings cool.

In the US, around 15% of energy used in commercial buildings goes towards air conditioning, but the scientists are optimistic that in some cases the mirrors could completely remove the need for additional cooling.

Comparative research carried out in Stanford, California found that a black surface on a building’s roof reached 60C above the average air temperature when in direct sunlight, while an aluminium surface reached 40C. The mirrored surface was actually 5C cooler than the surrounding air temperature however.

Shanhui Fan, an expert in photonics at Stanford University who led the development of the mirror, said: “If you cover significant parts of the roof with this mirror, you can see how much power it can save.”

Buildings warm up in a variety of ways however. In warmer climates hot air rushes in through open windows and doors, which the use of things like water heaters bleeds warm air into the building interior. Then there is also infra-red and visible light from the sun.

The Stanford mirror was designed in a specific manner. It was made to reflect 97% of the visible light that falls on it back into the atmosphere. More importantly though, when the mirror heats up it emits the heat as infra-red radiation of a specific wavelength that easily passes back through the Earth’s atmosphere and safely into the cold depths of space.

To cool any object you need something called a “heat sink”. This is something that is cooler than the object you’re trying to reduce in temperature and serves as a dump for the heat, drawing it from the object and warming itself. With the Stanford mirror the heat sink is the universe itself.

The mirror is constructed from several wafer thin materials. The bottom layer is reflective silver. The next layers alternate between silicon dioxide and hafnium oxide, designed to improve the reflective nature of the silver but also turn the mirror into a thermal radiator. When the silicon dioxide heats up, it releases the heat as infra-red light at a wavelength of 10 micrometres. This wavelength is not absorbed by the atmosphere so the light passes straight out into space. The total thickness of all the mirror’s layers comes to around two micrometres, or two thousandths of a millimetre.

During testing the mirror was found to have a cooling power of 40 watts per square metre, an annual saving of 100MWh per year on a three storey building. The mirrors cost between $30 and $70 per metre to install.

However, the mirror’s developers say that it does not provide a solution for climate change. “Roof space accounts for only a small portion of the Earth’s surface, so at this point we don’t think this would be a geoengineering solution. Rather, our contribution on the green house gas emission issue is simply to reduce electricity consumption,” Shanhui Fan said.