Tomorrow's Options for More Efficient Windows
Window technology has continued to advance over the years with the particular intent to decrease heat loss or gain. Window coatings/films are one enabling technology that involves low-emissive (low-e), spectrally-selective, heat-absorbing (tinted), and reflective surfaces. Another enabling technology is gas-filled windows which decrease heat conduction between panes of glass.
State-of-the-art superinsulating windows, or superwindows, combine all the typical advanced features, "low-e coatings, gas fill, good edge seals, insulated frames, and airtight construction", according to Edwin Datschefski at BioThinking. A clever alternative is to apply the low-e coating to one or two sheets of Southwall Corp.'s Heat MirrorTM, a transparent polyester film placed between the glass panes. The product gives windows whole-unit ratings of up to R-6 insulation value-about twice as efficient as the same thickness of fiberglass. Testing by Lawrence Berkeley National Laboratory's (LBNL) Mobile Window Thermal Test (MoWiTT) facility has shown that, for cold, overcast days, the 24-hour net average heat loss is smaller for the super-window than for an R-15 insulated wall. According to the Efficient Windows Collaborative, "Insulated superwindows with three or more layers will virtually eliminate condensation on the interior surface of the glass-even under extreme cold weather conditions."
Heat Mirror comes in roughly a dozen different designations that transmit varying amounts of solar radiation. The product designations generally represent the amount of light allowed to pass through the coated film. For example, Heat Mirror 77 will transmit more light than a Heat Mirror 22 product. Several manufacturers (such as Gilkey Window, Traco, EFCO, and Alpen) use Heat Mirror as the enabling technology for their superwindows. Superwindows are somewhat expensive, so they are most likely to attain a decent payback in only very cold climates. In hot climates, you are better off buying less expensive low-e windows and using shading techniques instead.
Also under development are chromogenic (optical switching) glazings that will adapt to the frequent changes in the lighting and heating or cooling requirements of buildings. According to the Lawrence Berkeley National Laboratory (LBNL), "Electrochromic coatings (EC) are switchable thin-film coatings applied to glass or plastic that can change appearance reversibly from a clear tint to a dark Prussian Blue tint when a small dc voltage is applied." The University of Minnesota Center for Sustainable Building Research describes a variation of the technology based on the orientation of suspended particles:
"Suspended Particle Device (SPD) windows utilize a thin, liquid-like layer in which numerous microscopic particles are suspended. In its unpowered state the particles are randomly oriented and partially block sunlight transmission and view. Transparent electrical conductors allow an electric field to be applied to the dispersed particle film, aligning the particles and raising the transmittance."
These smart windows are generally categorized as either passive or active glazing types. Passive glazings vary their light transmission as driven by changes in sunlight (photochromic) and their transmission of heat as driven by changes in ambient temperature (thermochromic). By contrast, active (electrochromic) window transmittance properties are driven by use of a small electric current from an outside power source.
LBNL further states that, "Low-voltage power is required to switch EC windows and for some types of windows, a small applied voltage is needed to keep the EC in a constant state, irrespective of the level of tint." For instance, SAGE Electrochromic's commercially available EC window requires constant power. LBNL field tested the window technology and reported the following power consumption levels (end use power at the wall outlet) for an array of (15) 35x18 inch windows:
- "If no power is applied, the EC window rests at the clear state. The level of tint at the clear state will vary slightly between windows (for example, visible transmittance (Tv) = 0.60-0.70) and may be discernable when comparing two side-by-side windows. The EC window can be left unpowered during the night.
- If the EC window is in the process of being switched, peak power consumption is 0.26-0.32 W/ft²-glazing (5-6 W for a 42.5x60 inch EC window).
- If the EC is being held constant at any level of tint, steady-state power consumption is 0.07-0.15 W/ft²-glazing (1.2-2.6 W for a 42.5x60 inch window), assuming a 1-to-1 relationship between the EC window unit and its window controller. This includes power to the window, electronic circuitry for control, and parasitic losses due to the efficiency of the power supply.
- Average daily power consumption of the EC system (window + controller + power supply) during a 12-hour day was monitored to be the same as steady-state power levels in the bullet above. These consumption levels can be reduced to 25% to 30% of current levels, if the control circuitry and power source are designed more efficiently."
In early 2009, Innovative Glass Corporation (Plainview, NY) completed the installation of 59 interior and 9 exterior windows using SPD-SmartGlass technology at the Indiana University Health Information and Translational Sciences Building. The SPD technology was developed by Research Frontiers and produced by Hitachi Chemical Co., Ltd. PPG Aerospace is supplying Alteos interactive electrochromic window systems for the Boeing 787 Dreamliner jet.
Like any emerging technology, EC windows are expensive (around $100/ft²-glazing). The cost is expected to decrease as production volumes increase and economies of scale begin to apply. With the cost of spectrally selective low-e windows averaging $10 to $15/ft²-glazing, the much higher cost of EC windows cannot be justified at this time by direct energy savings alone. You will also have to consider cost savings indirectly related to the use of EC windows such as the reduction in HVAC capacity and maintenance requirements, the reduced need for blinds or shading systems, and greater productivity of employees from increased comfort levels.
Sources
Windows Systems for High-Performance Buildings - Center for Sustainable Building Research. 2007. University of Minnesota and Lawrence Berkeley National Laboratory.
About the Electrochromic Window Technology - Lawrence Berkeley National Laboratory. July 2006. Environmental Energy Technologies Division, Buildings Technologies Department.
Advances in Glazing Materials for Windows - Office of Energy Efficiency and Renewable Energy. November 1994. U.S. Department of Energy.
Photo by Il conte di Luna
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