Properties Primer

• Introduction
• Transmittance
• Reflectance
• Absorptance
• Emittance

Glass

• VT
• SHGC
• U-factor
• Low-E Coatings
• Reflective Coatings
• Tints
• Laminates
• Surface Treatments
• Applied Films

Assembly

• Multiple Panes
• Gas Fills
• Spacers
• Frames
• Air Leakage

Advanced

• Glass
• Dynamic Windows
• BIPV
• Automated Shading

Thermochromic Windows

Thermochromic materials for use in variable tinting windows can adapt to changing sunlight intensity to reduce heat load in buildings. Preferably the thermochromic layers in the windows change transmission continuously over a range of temperatures so they not only reduce heat loads (especially at times of peak demand), but they maximize daylighting. If properly designed, the thermochromic layers are minimally sensitive to changing outdoor or ambient temperatures but respond dramatically to changing amounts of direct sunlight on the windows. Sunlight responsive thermochromic windows use the sun's energy to tint the windows and block the sun's energy that otherwise would have ended up as heat load in the building. In addition to minimizing solar heat gain and maximizing daylighting, thermochromic layers help reduce glare, fading and noise, and increase safety.

Windows are known that change between clear and light scattering with changing temperature. This change is based on lower critical solution temperature (LCST), phase change materials that in the past were inadvertently called thermochromic. These materials are better described as thermotropic since there is a phase change or change in state of the materials. As there is no change in "chroma" in these systems they do not qualify as chromogenic. In addition to LCST systems, there are systems based on liquid crystals between crossed polarizers. At lower temperatures the liquid crystals rotate the light that passes through the first polarizer to allow some of that light to pass through the second polarizer. At higher temperatures the liquid crystals reorient and no longer rotate the light thus causing less light to pass through the crossed polarizers. While sometimes called thermochromic, these liquid crystal systems are also better described as thermotropic.

Thermochromic materials used in windows reversibly change from less light absorbing to more light absorbing as the temperature of the materials is increased. As they are reversible, they also decrease in light absorption as the temperature is decreased. By having some tint in the windows to start with, sunlight shining on the windows can warm the thermochromic layer and thereby increase the absorption of the layer. This is based on the conversion of light to heat and is not based on photochemical or photochromic changes. The sunlight responsive thermochromic layers darken based on the amount of direct sunlight on the window. The layers are lightly tinted when there is indirect sunlight or the day is cloudy. The sunlight responsiveness of thermochromic windows is refined to provide dimming that is nearly optimum for minimizing heat load while maximizing daylighting. Thus, unwanted the heat is blocked; especially when the thermochromic layer is combined with a low emissivity layer to help reject heat to the outside.

Thermochromic layers that adapt directly to changing sunlight intensity do not require wires, power supplies, control schemes or control equipment. Windows with these layers can be installed by glazing contractors, just like conventional windows. These features, along with lower cost, make thermochromic windows very attractive as compared to electrochromic and other electro-optic approaches to variable tinting windows.

The most promising approach to thermochromic layers for windows involves thermochromic materials incorporated into plastic films of polyvinyl butyral (PVB). PVB is one of the most widely used safety glass lamination film. This approach allows local glass laminators and window manufacturers to provide a value added product that represents the state of the art in adaptive glazing, simply by purchasing rolls of thermochromic PVB and processing it with conventional fabrication equipment. In addition, the window fabricator may use any tinted glass, thereby allowing architects to choose a desirable outside appearance for a building with thermochromic windows. Thermochromic PVB became commercially available late in 2010 from Pleotint, LLC and has been successfully laminated between sheets of glass on conventional lamination lines. Laminates up to 5 foot by 10 foot have been used to make windows that have been installed in a number of installations.

During a typical sunny day, a typical insulated glass unit incorporating a thermochromic laminate changes its visible light transmission from 50% to 10% and then back to 50%. The timing of these changes depends on the location of the window on the building. Each day of the year, any time of the day, and for any orientation on the building the windows simply tint to a desirable level based on sun exposure. The solar heat gain coefficient and shading coefficient vary down to 0.13 and 0.15 respectively when there is full direct sunlight.

Sunlight Responsive Thermochromic, SRT™, windows installed in corporate headquarters of Pleotint, LLC in Jenison, Michigan, USA. On the left the windows are tinted by direct sunlight and on the right the windows are clearer as they are only exposed to indirect sunlight. In both cases the heat load on the inside of the building is minimal and the daylighting is regulated in a desirable manner. Photo courtesy of Pleotint, LLC.