Figure 1. Variations in visible transmittance and shading coefficients for various glazing products. With the exception of electrochromics, the figure represents products available on the U.S. market in 1992. (From the Window Library in the DOE-2 building energy simulation program.) Note: The next revision to this database will report solar properties as Solar Heat Gain Coefficient, which is now gradually replacing Shading Coefficient in product literature as the key solar parameter.
Recent advances in technology have helped make the window an ally in efforts to save lighting energy. New window technologies can help minimize unwanted solar gains in summer as well as heat losses in winter, without squandering valuable daylight.
It is useful to think of a window as a luminaire. Windows are sources of light and have distinct optical characteristics and implications for visual comfort. Of course, they are also sources of heat gains and losses. Although the field of daylighting is as old as architecture itself, recent advances in window technology, aided by LBNL research, have opened up new opportunities for reducing electrical lighting requirements in buildings.
As is so often true, efforts to improve the energy efficiency of a technology--in this case windows--have led to a dramatic expansion of consumer choices. In addition to multiple glazings, efficient window options now include a host of shading systems, low-emissivity and selective coatings, gas fillings, and daylight integration (for example, via dimming ballasts). Smart, switchable glazings are just around the corner.
A host of non-energy-related benefits tend to accompany energy-efficient window systems. For instance, they generally have lower sound transmittance and reduce the amount of damaging ultraviolet rays entering a building. Efficient windows also offer better thermal comfort: their interior surfaces are cooler in summer and warmer in winter.
New Technologies
Windows are one of the most complex energy-using technologies in buildings. They
play a role in lighting, heating, cooling, and ventilation.
Aesthetics--appearance, view, and optical performance--are usually quite important
to the occupant. Indeed, the serious lighting designer cannot ignore the energy
implications of window choices.
New technologies help to resolve the historic problem of the trade-off between windows that reflect unwanted solar gains in the summer and those that admit a maximum amount of useful light. Todays window technologies can replace more primitive strategies for shielding a room from unwanted sunlight, such as tinted windows and curtains. Tinted windows have the disadvantage of absorbing solar radiation and can become very warm (up to 50 degrees C). Some of this heat is then radiated to the interior space, causing discomfort to anyone nearby. Tinted windows also impede the building occupants' view of the outside environment and require higher artificial lighting energy use to compensate for daylight loss. Similarly, multiple-paned windows can effectively retain heat in the winter but filter out useful daylight.
In hot climates, spectrally selective glazings admit visible light wavelengths while reflecting unwanted infrared wavelengths. The larger the ratio of a window's visible transmittance to its shading coefficient (a measure of solar transmission) the greater is its selectivity. This "coolness factor" ranges from a ratio of 0.25 to 1.6 for windows sold today.
In cold climates, low-e coatings are of interest. These nearly invisible, multilayer coatings are deposited on glass or plastic at the time of manufacture or as an off-line process. The coatings reduce radiative heat losses by reflecting heat back into the building. The bottom-line effect is an increase in the insulating value of the window. For even better performance, gaps between the layers of multiglazed windows can be filled with gases--such as argon, krypton, or xenon--that have better insulating properties than air. Windows with low-e coatings have already captured a 35% market share in the U.S, with sales of 25 million square meters (270 million square feet) per year.
Fig. 1 is based on a comparison of about 200 glazing products, including single-, double-, triple-, and quadruple-pane glazings with different tints, coatings, gas fills, glass thicknesses, and gap widths. Visible transmittance varies from 0.15 to 1.0. Shading coefficients vary from 0.05 to 0.9.
Fig. 2a plots the coolness factor as a function of the U-Value, showing that, for example, in the U-value range of 2 to 3, the coolness factor ranges from 0.2 (low visible transmittance in relation to solar gain) to 1.4 (very well-managed solar heat gain and good visible transmittance). Remarkably, visible transmittances vary from roughly 0.2 to 0.8 over the entire range of insulating values (Fig. 2b).
Figures 2a and 2b. Over a range of thermal efficiencies, (U-Value), windows vary considerably in their spectral selectivity (above) and visible transmittance (below).
"Smart" Windows
Since both thermal and luminous conditions are constantly changing, the ideal
window should have properties that can be dynamically controlled.
Once a fantasy, such "switchable" windows are now becoming a reality in the Center's research laboratories and are moving toward the marketplace. With one of the most promising types of switchable glazings--electrochromics--the optical density can be controlled as a functional parameter, such as a function of direct or total solar radiation, outside temperature, the previous-hour space-conditioning load, or the indoor daylight level. Electrochromatic window properties are changed by applying a very small electrical voltage across the electrochromic coating (Fig. 3). If used in conjunction with electronic dimmable ballasts, electrochromic windows can help attain considerable lighting energy savings relative to static window shading systems.
Figure 3. Cross section of prototype five-layer electrochromic coating in clear and colored states (layers not to scale).
Prototype electrochromics being developed by the Center's Building Technologies Program have shading coefficients that can be adjusted from 0.98 to 0.36 and visible transmittances from 0.85 to 0.13. Such windows will free designers from the historical rule-of-thumb that energy use eventually increases as a function of ratio of window-to-wall areas. Even in very hot climates, energy use can decline steadily with increasing window area if electrochromics are used with daylighting controls, whereas conventional windows inevitably increase energy use as their size increases.
Economics
Figure 4: Change in total energy use for cooling, fans and lighting for a west-facing perimeter zone in a prototype commercial office building located in Blythe, California. Results are shown for idealized electrochromic windows and a static glazing for varying window-to-wall area ratios. The electrochromic windows are controlled to maintain an interior daylight illuminance level of 538 lux. All systems use electronic ballasts with continuous dimming daylight controls and a lighting power density of 16.1 W/m2. (Calculations made with DOE-2 by E. Lee, Lawrence Berkeley National Laboratory.)
The market price of advanced window technologies vary widely. In markets where the technologies aren't well-known, prices can be extremely high (if the product is available at all). By contrast, in markets with considerable production and demand or where utility rebates or building codes call for such windows, prices can be quite reasonable.
System cost-effectiveness is determined by a combination of many factors. For new construction, the higher costs can often be partially--or even completely--offset by cost savings made possible by HVAC downsizing. In this case, the payback time is instantaneous, and any extra savings are pure profit.
Evan Mills
Center for Building Science
(510) 486- 6784; (510) 486- 5394 fax
An earlier version of this article appeared in the IAEEL Newsletter vol 3-4/95. This research is sponsored by the DOE Office of Building Technology, State and Community Programs.