Intermediate 
A mirror that sends heat into cold space has been designed by scientists to replace air-conditioning units that keep buildings cool on Earth.
Researchers believe the mirror could slash the amount of energy used to control air temperatures in business premises and shopping centres because they wont need to use cooling systems.
Around 15% of the energy used by buildings in the USgoesonairconditioningbuttheresearchers say that, in some cases, the mirror could completely offset the need for extra cooling.
In a rooftop comparison in Stanford, California, scientists found that a roof that had been painted black reached 60C more than the air temperature in sunlight and aluminium reached 40C more. However, the mirror was up to 5C cooler than the surrounding air temperature.
If you cover signi cant parts of the roof with this mirror, you can see how much power it can save. You can signi cantly offset the electricity used for air conditioning, said Shanhui Fan, an expert in the study of light at Stanford University, who led the development of the mirror. In some situations, you will be able to completely offset the air conditioning.
Buildings warm up in a number of different ways. Hot-water boilers and cooking areas release heat into their immediate surroundings. In hot countries, warm air comes in through doors and windows. Then, there is visible light and infrared radiation from the sun, which also heat up buildings.
The Stanford mirror was designed to re ect 97% of the visible light that falls on it. But, more importantly, it works as a thermal radiator. When the mirror is warmed up, it releases heat at a speci c wavelength of infrared light that passes easily through the atmosphere and out into space.
To make anything cool, you need what engineers call a heat sink: somewhere to put unwanted heat. The heat sink has to be cooler than the object that needs cooling or it will not do its job. For example, a bucket of ice will cool a bottle of wine because it becomes a sink for heat in the liquid. The Stanford mirror relies on the best heat sink: the universe itself.
The mirror is built from several layers of very thin materials. The  rst layer is re ective silver. On top of this are layers of silicon dioxide and hafnium oxide. These layers turn the mirror into a thermal radiator. When silicon dioxide heats up, it radiates the heat as infrared light at a wavelength of around ten micrometres and the heat passes straight out to space. The total thickness of the mirror is around two micrometres or two thousandths of a millimetre.
The cold darkness of the universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day, the scientists write in Nature. In tests, the mirror had a cooling power of 40 watts per square metre at outside temperatures.
 Writinginthejournal,Fansaysthatthecostofthe mirrors is between $20 and $70 per square metre. He calculates an annual electricity saving of 100MWh on a three-storey building.
Fan said that the mirror could cool buildings but he said that the mirrors would not slow down global warming. Roof space is only a small portion of the Earths surface so the mirror is not a solution to the problem of global warming. But our mirror will help limit greenhouse gas emissions by reducing electricity consumption, he said.
Im really excited by the potential it has, said Marin Solja__i__, a physicist at the Massachusetts Institute of Technology. You could use this on buildings and spend much less money on air conditioning or maybe you wouldnt need it at all. You could put it on top of shopping malls. Im really excited by the potential it has, said Marin Solja__i__, a physicist at the Massachusetts Institute of Technology. You could use this on buildings and spend much less money on air conditioning or maybe you wouldnt need it at all. You could put it on top of shopping malls. Im really excited by the potential it has, said Marin Solja__i__, a physicist at the Massachusetts Institute of Technology. You could use this on buildings and spend much less money on air conditioning or maybe you wouldnt need it at all. You could put it on top of shopping malls.