The idea of “switchable” windows has been around for 15 or 20 years. It’s an appealing concept.
Instead of closing a blind you simply touch a switch, your windows darken, the room cools. These windows can reduce heating and cooling costs but they have never really caught on in the marketplace.
Part of the reason, of course, is price. Even triple glazed windows have been cheaper.
Another part of the reason has been performance. So-called smart windows are installed in many passenger aircrafts. But they are made largely with tungsten oxide and change colour when charged with electricity.
Those windows tend to be expensive and often have a blue tint. They can also take more than 20 minutes to dim.
And as the material in the window ages, it becomes less opaque.
Different research teams are taking different approaches to the problem.
At Princeton University in New Jersey, researchers are working with specialized solar cells in an application that is both self-powered and flexible enough that it can be applied to existing windows via lamination. But when the window is darkened, it also turns to a dark blue, which some people find disconcerting.
Now we read that engineers at Stanford University in California have developed dynamic windows that can switch from transparent to opaque or back again in under a minute and do not degrade over time.
The prototypes are plates of conductive glass outlined with metal ions that spread out over the surface, blocking light, in response to electrical current.
Michael McGehee is a professor of materials science and engineering with a background in solar cells. He’s a member of the research team that came up with the new glass.
"We did not tweak what was out there; we came up with a completely different solution," he says.
"We’ve had lots of moments where we thought, ‘How is it even possible that we’ve made something that works so well so quickly?’ And now we’re running the technology by glass companies and other kinds of companies."
McGehee’s group’s prototype blocks light through the movement of copper and another metal in a solution over a sheet of transparent indium tin oxide modified by platinum nanoparticles.
When transparent, the windows are clear and allow about 80 per cent of surrounding natural light through. When the glass is dark, light transmission drops to under five per cent. The researchers switched the windows on and off at least 5,000 times and saw no change in the transmission of light. This indicates that the design is durable, McGehee says.
He is careful about what he says when speaking for publication. His group has just recently applied for a patent on their system and they’re still waiting for approval.
There is still work to do before scaling up, however. McGehee says there is currently a limit to how much area the prototypes can cover. During the study the team used small windows with a surface area of just 25 square centimetres. It’s not known, at least for publication, just how large the windows might be by the time they come to market. McGehee might not know that himself since he referred to window size as a problem to be addressed.
His group wants to do further work on the metal electrodes. The goal is to cut the cost of the prototype so that is it is at least half the cost of dynamic windows that are already on the market
"We’re excited because dynamic window technology has the potential to optimize the lighting in rooms or vehicles, save about 20 per cent in heating and cooling costs and even change the way people wear sunglasses," McGehee says.
"This is an important area that is barely being investigated at universities and there’s lots of opportunity to keep us motivated."
Korky Koroluk is an Ottawa-based freelance writer. Send comments to email@example.com.