[Originally posted at EcoGeek. This is not only an energy storage window, but the whole assembly has a U-value of 0.08 (which means it would be about equal to an R-12 wall) and that’s for a window! This could make it very easy to do a rather low energy building that needed almost no thermal input. This is a really cool high-tech system, though it is rather pricey right now.]


A remarkable new glazing system has been available in Europe for several years and is now being brought to the North American market. The GlassX window is an insulated glass assembly that incorporates a phase-change material (PCM) between two of the glass panes in the window. At lower temperatures, the PCM is a translucent solid. But, as it heats, the PCM melts and becomes transparent. This lets the window itself absorb heat from solar energy during the day, and then releases the energy again later on, as the material cools again.

The combination of good insulating windows along with heat storage makes these windows very useful for passively designed buildings. PCMs are excellent heat storage materials, and the GlassX windows are able to store as much heat as a 9″ thick concrete wall. Even in its solid, translucent state, the GlassX windows allow more than 25% of the exterior light through, so that daylighting is not entirely lost. The windows also incorporate a diffuser that reflects high angle light from the sun in summertime, while allowing low angle light in the winter to pass through more directly.

These are not ready replacements for the current windows in most homes. The GlassX windows are over 3 inches (8 cm) thick and weigh nearly 20 pounds per square foot (100 kg/m2). They are also rather expensive at $60-90 per square foot ($560-$970/m2), but the company expects payback on these to be under ten years. There are several installations of the material in Europe, but as yet there are none in North America.

via: @bglive