A remarkable Swiss window technology has just recently been introduced in North America. GlassX windows are super high-efficiency windows that use several technologies to make them possibly the most high-tech windows available. First of all, GlassX windows have a prismatic plane that deflects high-angle sunlight in the summertime, rather than transmitting it, to keep the building cool. Lower-angle winter light is transmitted through this layer, and even more interestingly, these windows incorporate a phase-change material (PCM) between two of the panes of glass. Polycarbonate spacers used between the panes to segregate the PCM give the windows a venetian blind appearance.
Windows are the weak point of almost any building for thermal control. However these GlassX windows serve to alleviate many of the problems found in conventional windows. The PCM material stores heat by melting, and then radiates it back into the space at night, as the material cools and re-solidifies. As a solid, the PCM still transmits about 25% of the visible light, and more than 40% is transmitted when the PCM is in its liquid state, so daylighting is not compromised.
Given that they are so high-tech, GlassX windows are very expensive — they weigh in at $60-90 per square foot. However the company states that the payback period on these windows is expected to be only 5 to 10 years. With all the layers involved, the thickness of the windows is another obstacle, but not one that cannot be overcome to produce some attractive and energy efficient designs.