Windows let light, warmth and a view into rooms, but we don't always want all three of those at once. That's why researchers in Switzerland are developing a relatively simple method to smarten up windows by embedding tiny reflective mini-mirrors in them and keep rooms comfortable all year round.

Smart window prototypes are coming in all kinds of forms in recent years. Electrochromic windows can change their tint on demand, going from clear to dark in minutes, while some can be adjusted to block either light or heat as needed. Some can harvest their own energy from the Sun as well, while other designs change their opacity for extra privacy without blocking light.

The problem is that most of these systems need power sources, and are relatively complex. Researchers at EPFL and Empa in Switzerland are creating simpler smart windows that remain transparent, but can selectively allow light into the room while keeping heat out.

To start with, the team used a precision laser to create a grooved microstructure on the surface of a film. Tiny mirrors, known as Compound Parabolic Concentrator (CPC) lenses, are then evaporated into the pores on the surface. The film can be applied to glass or inserted into double-glazed windows, where the array of mini-mirrors can then reflect sunlight into the room more efficiently, lighting up even the gloomiest corners.

In their tests, the researchers say the smart window material managed a light incidence of 60 degrees and was able to divert 80 percent of the incoming light, spreading it out into the room almost horizontally. For those concerned about a murky or tinted window, the team also says that the glass doesn't affect visibility, remaining transparent.

The team has designed the windows to be seasonal too, reflecting light and heat away in summer and into the room in winter. To do this, the mirrors are angled so that in summer, when the Sun is higher in the sky, the sunlight hits the window at an angle that sends it bouncing back outwards. In winter, the Sun's lower position makes it able to reflect more light and warmth into the building. In both cases, that should cut back on artificial heating and cooling costs.

The researchers are currently working on a pilot project for the reflective coating, which will involve installing prototype windows in an Empa building and monitoring how well it performs. The team is also currently developing mass manufacturing processes.

Source: Empa