Technology Spotlight: What's new in using solar for lighting

Lighting is the largest consumer of electric energy in commercial buildings. Since sunlight has a higher ratio of light to heat than any type of electric lighting, using sunlight for lighting has the potential to reduce energy use for both lighting and cooling in well-designed buildings. Daylighting is the traditional technique for capturing the sun as a light source. Aerogel glazing systems and hybrid solar fiber optic lighting are two more technologies now commercially available, and recent advances in both fields promise to further reduce lighting energy use.

Daylighting

"Daylighting" was a matter of course before the invention of electric lighting and has reemerged in recent decades as an energy-efficiency measure. The technique involves placing windows and reflective surfaces so natural light can effectively illuminate a building during the day. Energy savings are achieved if occupants leave electric lighting off or if photocells are used to dim electric lighting in response to light levels.

Reflective surfaces may be as simple as well-placed, light-colored walls, ceilings and window sills. Other strategies include the use of light shelves, clerestories, skylights and solar tubes. The Daylighting Collaborative provides examples and guidelines for incorporating these elements into a design. Light shelves, for example, can not only diffuse light deep into the space, but also reduce glare in perimeter zones. Solar tubes can "pipe" sunlight into interior spaces. In addition to saving energy, well-designed daylighting contributes to an attractive, comfortable space and can improve productivity.

The impact of daylighting strategies on heating and cooling requirements must be taken into account to ensure lighting energy savings win over HVAC penalties.

Aerogel skylights, facades and roofing products

Aerogels (aka nanogels or nanofoam) were first produced in 1931 by replacing the liquid component of a gel with a gas. Low thermal conductivity, low density and high light transmission are among aerogel's many remarkable properties. Also, silica aerogel is the most insulating material known. Aerogel used in glazing applications has R-values of 7 to 8 sq. ft-degrees Fahrenheit-h/Btu per inch. Yet, it has light transmission of 75 percent and so is semi-transparent.

When used in glazing products, cells between the panes are filled with silica aerogel in granular form. Since aerogel is not transparent, but translucent, aerogel glazing systems cannot replace windows in all applications. Research is still underway to increase their transparency. But their diffuse "museum-quality" light is attractive in building façade, skylight and roofing applications. Aerogel glazing appears blueish against a dark background and whitish against a bright background. Material costs are reportedly competitive with high-performance, conventional glazing systems. Pre-fabricated glazed roofing systems save on installation costs.

Cabot Corporation manufactures granular silica aerogel and has teamed up with several companies to produce aerogel building products, including Super Sky Products, Inc., Centerpoint Translucent Systems, Acralight International Skylights, Wasco Products, Inc. and Kalwall. Other companies that have worked on development of aerogels include BASF, Thermolux, Aerojet, Airglass and Aspen Aerogels.

Hybrid solar fiber optic lighting

There are essentially three components of a hybrid solar fiber optic lighting system: the solar collector, fiber optic cables and hybrid luminaires. The solar collector – either a parabolic mirror or lens concentrator – tracks the sun and focuses sunlight onto the ends of fiber optic cables that run to hybrid luminaires. Hybrid luminaires have both fiber optic illuminants and electric lamps (either incandescent or fluorescent). The electric lighting dims in response to light levels. A four-foot-diameter parabolic concentrator can illuminate approximately 1,000 square feet of floor area.

Fiber optic lighting can be run to interior spaces, provided the space is no more than about 50 feet from the collector. Light levels decrease with cable length and, according to Oak Ridge National Laboratory, 50 feet is currently the maximum practical length. Hybrid lighting systems are designed to transmit visible light through the cable, while filtering out other parts of the spectrum, such as UV light. The cable transmits very little heat, so the lights do not increase cooling loads.

At least three companies are manufacturing hybrid solar fiber optic systems:

1. Sunlight Direct in the U.S. is in the process of beta testing their HSL3010 solar lighting system.

2. Parans is a Swedish company that distributes their solar hybrid system through Huvco located in Maryland.

3. La Foret Engineering is located in Japan.

For more information

• Daylighting; U.S. Department of Energy, Energy Efficiency and Renewable Energy.

• Translucent and opaque nanostructured thermal insulating materials in building facades (PDF file); ZAE Bayern, Würzburg, Germany, 2006.

• Hybrid Solar Lighting Illuminates Energy Savings for Government Facilities (PDF file); U.S. Department of Energy, Federal Energy Management Program and Oak Ridge National Laboratory, April 2007.

Resources

Daylighting Collaborative

Sunlight Direct

Parans

La Foret Engineering

Daylighting

Translucent and opaque nanostructured thermal insulating materials in building facades

Hybrid Solar Lighting Illuminates Energy Savings for Government Facilities

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