“Low-E” Windows Maximize Buildings’ Energy Efficiency (photo: EWC)

Low-E stands for low emissivity, and these windows are constructed to minimize heat transfer through the glass. Since windows are essentially huge holes in the walls of a building, choosing a low-E window design that’s appropriate for local climate and architecture can greatly increase a structure’s thermal efficiency, while reducing energy use and utility costs.

How does it work? A low-E window uses a microscopically thin layer of metal or metal oxide as a coating on the glass, which prevents heat from being transferred through the window. This coating can either help to keep a building cool in hot weather, or it can prevent heat loss in the cold winter months.

Low-E glass can be customized for different amounts of solar gain, meaning heat from sunlight. For example, a structure in a cold climate would benefit from a window that prevents heat loss, but admits as much heat from sunlight as possible. Low-E glass is typically transparent to visible light, so it shouldn’t be confused with the tinted or mirrored glass commonly seen in commercial buildings.

Low solar gain design for hot climates (photo: EWC)

The performance of a low-E window is usually described as its “U-factor” or “U-value”. Simply put, this is the inverse of the “R-value” that is commonly referred to in describing insulation materials. So, whereas a high R-value (resistance to heat transfer) is a good thing for insulation, the U-value (heat flow) will be a very low number in an energy-efficient window.

Low-E glass is often combined with other design elements to maximize a window’s energy efficiency. Two or more panes of glass create added insulation, and the gaps between panes may be filled with an inert low-conduction gas like argon or krypton. And remember, the installation is just as important as the design of a window. To enjoy the efficiency benefits of low-E windows, the window frames must be properly mounted and sealed to eliminate any drafts or leaks.

(via Efficient Windows Collaborative)

MIT researchers have announced that they have created “organic solar concentrators” that could make windows become powerful solar panels in as little as three years. The concentrator is mixture of two or more dyes painted onto a pane of glass or plastic. The dyes absorb light across a range of wavelengths, re-emit it at a different wavelength and transport it across the pane to the solar cells at the edges. Focusing the light like this increases the electrical power generated by each solar cell by a factor of 40. The advantages are twofold: the dyes greatly increase the power of solar cells, and homeowner are much more likely to incorporate solar glass into their homes.
The work was funded by the National Science Foundation and the U.S. Department of Energy’s Office of Science.

Scientists had tried using similar solar concentrators in the 1970s, but abandoned the idea when not enough of the collected light reached the edges of the concentrator. The MIT engineers revamped the idea by using a mixture of dyes in specific ratios, which allows some level of control over how the light is transmitted.

More details can be found here.

Although windows can naturally heat buildings in the cold seasons, during the summer they can easily overheat a building. To help with this problem, Japanese scientists at AIST recently developed a thin film which can make transparent glass turn into mirrors. The use of such a window in buildings or automobiles could reduce the energy consumed by air conditioners by more than 30%. The scientists, Kazuki Yoshimura and Shanhu Bao developed the thin film material. Previous research works have focused on the use of thin films made of magnesium-nickel alloy that behave as switchable mirrors: these, however, all have a yellow tinge in their transparent state.

The switching mechanism in Yoshimura and Bao’s window, is created by altering the gas content between the two glass panes. By introducing a small amount of hydrogen into the atmosphere between the panes, the glass acts as a transparent window. Alternatively, adding a small amount of oxygen with no hydrogen forms a reflecting mirror.

"Pilkington
Activ"
glass needs little cleaning, reducing consumption
of detergent and water.

The active glass has a special nano-scale (extremely thin)
coating of microcrystalline titanium oxide which reacts to daylight. The
reaction breaks down dirt on the glass, and when the water hits the glass
the dirt slides off.

The chemical used — titanium
dioxide is a fairly benign compound found in toothpaste and paint.

The
glass is slightly more expensive that conventional glass, adding about
15-20% to the cost of installation. However,
it also provides an approximate
20 percent reduction in ultraviolet light transmission over
clear glass and equivalent energy efficiency.