'Heat-proof' glass could save on air-con

The benefits will be most felt in the buildings of the world's hottest cities

A new coating for glass that can keep the heat from the sun out, saving on air-conditioning costs, has been developed by scientists in London.

The glass has a microscopic coating that allows glazing to keep heat out of buildings while allowing light to pass through.

The key is a thin coating of vanadium dioxide, around the thickness of a human hair, that is placed on normal window glass.

"This coating can actually be used for modifying the properties of the glass," said Ivan Parkin, of the team at University College London that has developed the coating.

"When put onto a window, it will let in only a certain fraction of the sunlight," he told the BBC World Service programme, Science In Action.

"When the window gets hot, it becomes more reflective. But it only becomes more reflective of the heat portion of the sunlight, not in the visible portion."

Heat control

Architects are increasingly using glass in modern designs - such as for the planned "Shard Of Glass" London Bridge Tower, potentially the highest skyscraper in Europe - as it creates light places for people to live and work.

Sometimes the chemistry doesn't behave on a three metre-wide piece of glass as it does on a piece 10 centimetres wide Dr Ivan Parkin, University College London

But many windows also let in great quantities of heat, especially in hot climates. This drives demand for air-conditioning.

In some cities, such as Tokyo, all the air-conditioning equipment creates a haze across the skyline in summer.

But the UCL scientists are hopeful that if their method can be scaled up, it could mean a dramatic cut in the amount of air-conditioning required.

Air-conditioning in summer frequently uses more energy than heating a building in winter.

Dirty windows

The UCL team have also found a way to choose at which temperature the glass begins to prevent heat coming in, by putting tungsten into the vanadium dioxide coating.

Glass is very much a feature of modern architecture

"By controlling the amount of tungsten dopent we introduce, we can change the temperature at which it will reject the heat," Dr Parkin explained.

"We can actually tune our system from anywhere between 70 degrees, which is the pure vanadium dioxide switching temperature, all the way down to zero degrees, if we wanted to."

However, currently the amount of glass they can cover at one time is around 10cm square.

The scientists are currently in negotiations with companies to look at scaling up the process.

Dr Parkin explained there were a number of problems that needed to be overcome to make it a full-scale commercial enterprise.

One is that the coating is a yellow colour that makes the window look dirty. The team are currently looking at a way of modifying this.

"Also there are issues when you scale up the chemistry. Sometimes the chemistry doesn't behave on a three metre-wide piece of glass as it does on a piece 10 centimetres wide," Dr Parkin said.

However he added that he was hopeful advances could be made very quickly.

"Unlike pharmaceuticals, in the glass industry it's generally a very rapid process to go from the laboratory bench scale all the way to the final product."