Window Glass Athens GA
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Valuable Home Glass
Glass is an amazing substance - strong, durable, cheap and almost totally clear. Clarity comes at a cost, however - the cost of extra heating and air conditioning. Anyone who has ever sat on a vinyl car seat in the summer knows just how transparent glass is to the heat energy of the sun. Glass is also a great heat conductor, which means your windows leak heat in the winter but admit heat in the summer. Material scientists have spent decades (and millions, perhaps billions, of dollars) designing various ways of reducing the liabilities of glass while maintaining its assets. The result - smart windows - shows how careful engineering can produce materials with tailored properties. Any discussion of windows should start with light, the visible part of the electromagnetic spectrum.
The visual spectrum starts at red, goes through orange, yellow, green, blue and indigo, and ends at violet. Below the red part of the spectrum lies infrared (IR), invisible to the eye but felt as heat. Above the violet end of the spectrum lies the ultraviolet (UV), again invisible to the eye but experienced by lying in the sun and getting a terrific sunburn. Unlike our eyes, glass doesn't much discriminate between IR, visible and (some) UV light - all pass pretty easily through a pane of normal glass. That's too bad, since the IR just heats up the inside of a house while the UV fades carpets and wall coverings. Ideally, one would like to block UV from entering a window at all times, block the IR whenever it is warm enough on the inside, and adjust the amount of visible light to suit one's mood and taste. The first step in this direction was low-E coatings on windows. The E stands for emissivity (or emittance), which is a measure of how readily energy is emitted from an object. The easiest way to understand E-values is to remember that a mirror has an E-value around 0 percent; a perfectly clear piece of glass has an E-value of 100 percent. Regular pane glass has an E-value around 85 percent. With high-E windows, heat pours into your house through the glass in the summer and pours right back out in the winter. Either way, you're paying, whether to cool your home in the summer or to heat it in winter. Poor energy management, indeed. Low-E coatings are a pretty straightforward fix.
The coatings are alternate layers, just a few molecules thick, of metals and metal oxides sputtered onto one surface of a window. Low-E coatings have E-values around 15 percent for the heat-carrying IR radiation and about 90 percent for visible light. This is brilliant! Visible light passes right through the low-E layer, but the heat-carrying IR is largely reflected right back where it came from. Place a low-E coating on the No. 3 surface of a double-glazed window (No. 1 being the surface outside, No. 4 being the surface facing the room), and the heat in your house stays in the house. Place it on the No. 2 surface and heat is reflected back outside. (As you can imagine, the low-E layers are fragile, so manufacturers typically apply them to one of the interior surfaces of a double-glazed window.) Low-E coatings are a passive technology; they work perfectly well without any intervention from you or power from your house. Newer smart windows, on the other hand, are an active technology. You control the window's behavior to suit your needs by controlling the amount and type of light allowed to penetrate the window. Smart window technologies use electric voltage to control the transmissivity of the glass. (Transmissivity is the amount of light allowed to pass through a window, relative to the amount of light falling on the window.
We'll call it the T-value. Clean, flat glass has a T-value of 90 percent; a brick wall has a T-value of zero.) How the current modifies T-values varies between windows. You can divide the technologies into two camps: those that block light using opaque particles or crystals and those that block light using layers of atoms. The first, called stereogenic windows, are dark when the voltage is off. The second, chromogenic windows, are clear when the voltage is off. The technologies behind the windows are just as different. Stereogenic windows use particles or liquid crystals to block light coming through a window. Think of one of these windows as a sandwich with an outer layer of plastic as the bread. Every good sandwich has a thin layer of mustard on the bread; in stereogenic windows, the mustard is a transparently thin layer of metal oxides. Finally, the filling of the sandwich is a thick layer of (at the risk of straining the analogy) olive loaf.
The loaf material is an inert polymer, while the olives are the opaque particles. This is a mighty thin sandwich - about the same thickness as a third of a human hair. Despite the thinness of the polymer, stereogenic windows are a bit hazier than clear glass; even when in the clear mode, so expect some distortion of the view. Privacy windows are perhaps the best-known stereogenic windows. They use liquid crystals (like the displays in a calculator) as the blocking particles. The crystals are contained in beads suspended in the polymer. In the off state, the liquid crystals are arranged randomly. Incoming light is scattered (not blocked) by the liquid crystals, which gives the entire window a frosted, translucent look. The windows aren't dark; they admit almost as much light as clear glass. Turn the power on and the crystals form parallel rows, allowing the light to enter undisturbed. Unfortunately, these windows have just two states: on and off. One can't adjust the T-value, and they don't provide much control of IR or UV light.
While a great privacy tool, they don't provide lighting or spectrum control. A newly emerging stereogenic technology, called a suspended particle device, or SPD, may address some of these shortcomings. Developed by Research Frontiers of Woodbury, N.Y., SPDs use small opaque particles instead of liquid crystals as the light blockers. SPD windows use polymer suspension and voltage-sensitive control systems, similar to those in privacy windows, packaged as a thin laminate on the inside surface of a pane. Unlike privacy windows, SPD windows can be turned from clear to nearly opaque. A variety of films will soon be available, according to Mike LaPointe of Research Frontiers. Residential SPD films should have T-values of around 9 percent (dark) to 77 percent (light).
Unfortunately, Research Frontiers, which licenses their technology to manufacturers of SPD films, is not yet aware of how much IR light their laminates will block. Chromogenic windows are built on entirely different principles. T-values of chromogenic windows are sensitive to some signal from the environment. There are three types of chromogenic windows: photochromic (think of sunglasses that automatically darken in bright sunlight), thermochromic (think of mood rings, whose color changes with temperature), and electrochromic (EC). EC materials darken when an electric current is passed through them, and they appear to be the first chromogenic windows nearing commercial practicality. At the heart of EC windows is a multi-layer package of transparent metals and metal oxides. The outer layers are electrically conducting metal oxides. Between them is a layer of EC material, such as tungsten oxide. Ion-conducting and ion-storage layers complete the package. This package behaves much like a battery. When the current is off, everything is transparent and the window is clear. Turn on the current (less than five volts will do) and electrons flow from the conductive layer into the EC layer.
In response, ions stored in the storage layer also flow into the EC layer, where they combine and form new compounds that absorb (instead of scattering) incoming light. Increase the voltage, and more ions flow into the EC layer, increasing its opacity. The package is transparently thin but has plenty of light-absorbing capacity. One manufacturer of EC windows, Sage Electrochromics, is on the verge of marketing EC windows commercially, along with a number of business partners. Mike Myser, Sage's vice president of sales and marketing, says EC windows effectively reduce both UV and IR light entering a house, the latter to as little as 1 percent. A nice feature of Sage's design is the artful combination of EC and low-E technologies. The EC layer is sputtered onto the No. 2 surface, the inside of the outer glazing. All the light energy absorbed by the EC layer is converted into heat, which would ordinarily radiate into the house. To circumvent this, Sage adds a low-E layer on top of the EC layer, reflecting much of the unwanted heat back to the exterior side of the house.
They end up with a variably transmissive and relatively cool window. All of these technological marvels have "non-ideal" features. EC windows give a light blue-gray tint to the world, while liquid crystal and SPD windows are distinctly hazy in their clear state. Switching times might also disappoint some users. Myser estimates that a 2-by-3-foot EC window might take a few minutes to go from fully clear to fully dark, while liquid crystal and SPD windows switch pretty much instantaneously. EC windows may lose their full range of transmission after many switch cycles, even though they remain clear. All of these active windows require a low-voltage power supply to each window, which means additional installation and maintenance costs. It might also prove difficult to repair that hole knocked in a window by the neighbor's kid. Even the best do-it-yourselfers may not have the skills and materials needed to fix a complex piece of technology like EC glass.