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Solar control fašades

spectrally selective | angular selective | solar filters | exterior solar control

Spectrally selective solar control

Spectrally selective glazing is window glass that permits some portions of the solar spectrum to enter a building while blocking others. This high-performance glazing admits as much daylight as possible while preventing transmission of as much solar heat as possible. By controlling solar heat gains in summer, preventing loss of interior heat in winter, and allowing occupants to reduce electric lighting use by making maximum use of daylight, spectrally selective glazing significantly reduces building energy consumption and peak demand. Because new spectrally selective glazings can have a virtually clear appearance, they admit more daylight and permit much brighter, more open views to the outside while still providing much of the solar control of the dark, reflective energy-efficient glass of the past. They can also be combined with other absorbing and reflecting glazings to provide a whole range of sun control performance. 

Because of its solar heat transmission properties, spectrally selective glazing benefits both buildings in warm climates where solar heat gain can be a problem and buildings in colder climates where solar heat gains in summer and interior heat loss in winter are both of concern. In other words, different variants on these glazings are appropriate for residential and commercial buildings throughout the United States. The energy efficiency of spectrally selective glazing means that architects who use it can incorporate more glazing area than was possible in the past within the limitations of codes and standards specifying minimum energy performance. When spectrally selective glazing is appropriately used, the capacity of the building's cooling system might also be downsized because of reduced peak loads. 

Top Left: Solar spectral properties of an ideal spectrally selective glazing. The photopic response curve represents the eye's response to light.
Top Right: Solar transmission spectra of the best available spectrally selective glazings. (click on the picture for high resolution image)

Spectrally selective glazings screen out or reflect heat-generating ultraviolet and infrared radiation arriving at a building's exterior surface while permitting most visible light to enter. Spectral selectivity is achieved by a microscopically thin, low-emissivity (low-E) coating on the glass or on a film applied to the glass or suspended within the insulating glass unit. There are also carefully engineered types of blue- and green-tinted glass that can perform as well in a double-pane unit as some glass with a spectrally selective low-E coating. Conventional blue- and green-tinted glass can offer some of the same spectral properties as these special absorbers because impurities in tinted glass absorb portions of the solar spectrum. Absorption is less efficient than reflection, however, because some of the heat absorbed by tinted glass continues to be transferred to the building's interior. 

Spectrally selective glazings can be used in windows, skylights, glass doors, and atria of commercial and residential buildings. Note that it may not provide reduced glare control even if solar gain is reduced. This technology is most cost effective for residential and nonresidential facilities that have large cooling loads, high utility rates, poorly performing existing glazing (such as single-pane clear glass or dark tinted glass), or are located in the southern United States. In the northern U.S., spectrally selective low-emissivity windows can also be cost effective for buildings with both heating and cooling requirements. In general, the technology pays back in three to 10 years for U.S. commercial buildings where it replaces clear single-pane or tinted double-pane glass and for most commercial buildings in the southern U.S. where it replaces conventional high-transmission, low-emissivity, double-pane windows. Spectrally selective glazing is applicable in both new and retrofit construction.

References

Lee, E. S. 1998. "Spectrally Selective Glazings." Federal Technology Alert, New Technology Energy Management Program, Federal Energy Management Program, DOE/EE-0173, August 1998. http://www.eren.doe.gov/femp/prodtech/ fed_techalert.html

Schuman, J. 1992. "Technical Focus: Cool Daylight." Progressive Architecture 4.92:136-141.

Okasolar between-pane louver system (above) Serraglaze prismatic glazing (below)

Angular selective solar control

Angular selective fašades provide solar control based on the sun's angle of incidence on the fašade. The main technical objective is to block or reflect direct sun and solar heat gains during the summer, or during the majority of the cooling season for a given building type, but admit diffuse sky-light for daylighting. 

Several engineered, fixed louver systems have been designed specifically to address this technical objective for the European Union (EU) climates and latitudes. For example, the Okasolar between-pane louver system consists of 2-cm-wide mirrored aluminum louvers with a unique geometrical profile. Direct sun is blocked and reflected out while diffuse sky-light is admitted from the sky. The optimum vertical angle of blockage occurs along the north-south axis at solar noon. 

Research to develop angular selective coatings on glass has proven to be challenging and has not yet resulted in a commercial product. Thin film coating techniques can to create microstructures that in principle, selectively reflect visible or solar radiation based on bi-directional, hemispherical angles of incidence. Energy and daylighting performance of such structures has been evaluated by Sullivan et al. 1998 (see References below). 

Interesting variations on this theme include between-pane louvers or blinds with a mirrored upper surface, to be used in the clerestory portion of the window wall, or exterior glass lamellas (louvers) where the upper surface is treated with a reflective coating. These systems fully or partially block direct sun and redirect sunlight to the interior ceiling plane (see Daylighting Fašades description next), given seasonal adjustments. 

Conventional louvered or venetian blind systems enable users or an automated control system to tailor the adjusted angle of blockage according to solar position, daylight availability, glare, or other criteria. Another variant includes between-pane acrylic prismatic panels that are either fixed or used as a system of exterior louvers to block direct sun and admit diffuse daylight. For vertical windows, the panels must be adjusted at least seasonally to block sun and to prevent color dispersion. Fixed systems can be used in roof applications.

References

Bader , G. and V. Truong. 1994. Optical Characterization of An Angle Selective Transmittance Coating. IEA Solar Heating and Cooling Program Task 18 Report T18/B7/CAN/94. October 1994.

Maeda K., S. Ishizuka, T. Tsjino, H. Yamamoto, and A. Takigawa. Optical Performance of Angle Dependent Light Control Glass. Central Research Laboratory, Nippon Sheet Glass Co. Ltd.

Mbise, G.W, D. Le Bellac, G.A. Niklasson, and C.G. Granqvist. Angular Selective Window Coatings: Theory and Experiment. Upssala: Department of Technology, Uppsala University.

Sullivan, R., L. Beltran, E.S. Lee, M. Rubin, S.E. Selkowitz. 1998. "Energy and Daylight Performance of Angular Selective Glazings." Thermal Performance of the Exterior Envelopes of Buildings VII: Conference Proceedings, Clearwater Beach, Florida, December 7-11, 1998. LBNL Report 41694, Lawrence Berkeley National Laboratory, Berkeley, CA. http://eetd.lbl.gov/BTP/pub/BSpub.html 

Smith, G. 1997. Angle Selective Transmittance Coatings - Final Project Report. IEA Solar Heating and Cooling Program Task 18 Report, T18/B7/FPR/97, February 1997.

Smith, G., S. Dligatch, and M. Ng. Optimizing Daylighting and Thermal Performance of Windows with Angular Selectivity. Sydney: Department of Applied Physics, University of Technology.

Ceramic-enamel coatings on glass

Solar filters

"[Ceramic frit glass] had a minor effect on the building's energy performance for the Blue Cross/ Blue Shield Headquarters in Chicago (BD&C 10/98) but allowed extensive overhead glazing in the UA terminal at O'Hare in the late 1980s and to meet ASHRAE Standard 90ů Most projects use white-colored frit. Frits do reduce the shading coefficient of the glass, but low-E coatings provides more effective reductions." Building Design and Construction, July 2000.

Solar filters indiscriminately absorb or reflect a portion of both direct and diffuse solar radiation. Overhangs, fins, "lightshelves", or a secondary exterior skin made of filter material are applied to south, east, or west-facing fašades to cut down on incident solar radiation levels and diffuse daylight. Filters may be made with an opaque base material (woven or perforated, metal screens or fabric) or transparent base material (etched, translucent, or fritted glass or plastic). 

Generally, the effectiveness of solar control is normally in proportion to the percentage of opaque material and will vary with the thickness, opacity, reflectance/absorptance of the material, and position within the fašade. Interior fabric roller shades can provide modest solar heat gain control if its exterior-facing surface reflectance is high (white or semi-reflective). Translucent composite fiberglass panels (e.g., Kalwall) used as part of the window wall also provides modest solar control. 

Between-pane absorptive shade systems, such as those used in double-skin fašades, can also lead to thermal stress on the window system and to increased solar heat gain, if inadequately placed, due to the increased surface temperature of the absorbing shading layer. Localized solar absorptance can cause increased thermal stress and possible glass breakage with fritted glass. 

The architectural trend over the past one to two decades has been to use filtering material (fritted and etched glass). Ceramic-enamel coatings on glass (fritted glass) rely on a pattern (dots, lines, etc.) to control solar radiation. The pattern is created by opaque or transparent glass fused to the substrate glass material under high temperatures. The substrate must be heat strengthened or tempered to prevent breakage due to thermal stress. A low-e coating can be placed on top of the frit. To reduce long-wave radiative heat gains, it's best to use the absorbing fritted layer as the exterior layer (surface #2) of an insulating glass unit. 

Initially, filters were used in the non-view portions of the roof or window wall. There is an increased trend to use filters in the view portions of the window wall for aesthetic visual effect. Such use can impair view and increase glare significantly, particularly if backlit by direct sun, since the window luminance within one's direct field of view is significantly increased. Perforated blind systems provide solar control with daylight admission, and can improve visual comfort through the reduction of the luminance contrast at the window.

Exterior solar control

Exterior solar control can be provided by overhang, fin, or full window screen geometries - the shape and material of which defines the architectural character of the building. The general concept is to intercept direct sun before it enters the building. Once direct sun enters the building, the only way it can get back out is through reflection (only the visible and near-infrared wavelengths of solar radiation can be reflected back out) or indirectly by convection and long-wave radiation. Exterior solar control should be designed to intercept direct sun for the periods of the year when cooling load control is desired (which tends to be 6-8 months out of the year in California for most commercial buildings). Shading systems that cover the entire face of the window (screens, blinds, etc.) should be placed back from the exterior glass surface to allow free airflow. A prevalent type of solar control in Europe is retractable louvers and blinds and is discussed briefly here.

Louvers and blinds are composed of multiple horizontal or vertical slats. Exterior blinds are more durable and usually made of galvanized steel, anodized or painted aluminum or PVC for low maintenance. Appropriate slat size varies and tends to be wider for exterior use. Slats can be either flat or curved. With different shape and reflectivity, louvers and blinds are used not only for solar shading, but also for redirecting daylight.

While fixed systems are designed mainly for solar shading, operable systems can be used to control thermal gain, reduce glare, and redirect sunlight. Operable systems (whether manual or automatically controlled) provide more flexibility because the blinds can be retracted and tilted, responding to the outdoor conditions. Glossy reflective blinds can be used to block direct sunlight while redirecting light to the ceiling at the same time. This might generate glare, depending on the slat angle, if direct sun is reflected off the slat surface into the field of view.

Louvers and blinds perform well in all climates. For commercial buildings in hot climates, the system may be more energy-efficient if placed on the exterior of the building while blocking solar radiation. For buildings in cold climates, the system can be used to provide more daylight and absorb solar radiation.

 

Left: Sketches of various exterior shading systems (at left, from top to bottom) Horizontal overhang protects south fašades from high-angle sun during the day. Vertical fins protect window fašades from east and west low-angle sun. Overhang and fins combined can be applied to buildings in hot climates. Window setbacks, where the window plane is pushed inward from the face of the building, can provide good shading potential. Fixed or moveable horizontal louvers provide shading similar to an overhang with improved daylight potential. Interior blinds can be controlled to accommodate occupant preferences.

 

Above: Shading simulation of fins, overhangs, and overhangs and fins on south fašade over course of June 21. The combination of overhang and fins (right picture) protects the window the most throughout the day compared to no protection (left picture). This simulation is given for June 21st at 1-hour increments from 9:00 AM to 3:00 PM for a latitude of 34░N (San Francisco).

 


Question/Information: eslee@lbl.gov