Spiraling energy costs, energy conservation efforts and the beneficial effects of daylight exposure on worker productivity have spurred development of technology that enables natural light to be brought into areas of buildings where it was previously not possible. The technology has not been widely used, however, because of its cost.
A less expensive daylighting method is being developed under the auspices of the U.S. Department of Energy's (DOE) Small Business Innovation Research Program. Norwalk, Conn.-based lighting specialist Steven Winter Associates (SWA) won a competition to develop the technology, and is midway through the two-year project.
The research strives to improve the cost-effectiveness of a high-tech Japanese-developed system called Himawari. It uses a computer-controlled sun-tracking system and an array of Fresnel lenses that concentrate light onto the ends of 1 mm glass optical fibers.
"The Japanese system is our yardstick," says Ravi Gorthala, SWA's senior research engineer on the project. "We wanted to see if we could come up with a passive system." Early research concentrated on developing a system that did not require tracking the sun. "We concluded that it was not possible to develop a system that has no tracker," says Gorthala.
The research then turned to the development of a more economical system that accomplishes the same objective: Deliver natural light inside buildings. Instead of a computer-controlled sun-tracking system, Zomeworks Corp. of Albuquerque, N.M., devised a method using less costly photovoltaic panels mounted on a dual-access passive frame that requires no electricity for operation.
"For our technology to work, we need to access direct sunlight," says Gorthala. Dual-access tracking enables the system to follow the sun through its diurnal and seasonal variations. According to Gorthala, the sun's position in the sky varies plus or minus 23 degrees from summer to winter.
Liquid refrigerant is key
A liquid refrigerant moves the tracking system without the use of electricity. Using light-absorbing plates and sunshades, the refrigerant absorbs solar radiation and becomes vapor. The vapor pushes the liquid through a refrigerant line, shifting the position of the tracker as the sun moves across the sky. An "automatic wakeup" absorber plate receives radiation and swings the tracking system from west back to east to begin again the next morning.
The glass fibers used on the Himawari system were replaced with less-expensive plastic fibers. "Plastic optical fibers bring the overall cost down significantly, says Gorthala. "Glass fibers are needed in the Himawari system because sunlight in the system is highly concentrated. They are trying to catch very exact direct light, which means the system can only be off by plus or minus a half degree. In our case, if the system is off by more than 1 degree it can still receive direct sunlight.
To increase the angle of acceptance of the sun, Gorthala says SWA uses a series of two Fresnel lens light concentrators, whose optics were developed by FarLight LLC of Torrance, Calif. He adds, "We could not use only one Fresnel lens. A two-lens system is needed to extend the angle of access."
The prototype sun-collector system features a series of four Fresnel lenses that are visible to the eye. Below each lens is a second lens. These primary-stage lenses focus sunlight onto corresponding secondary-stage "totally internally reflecting" concentrators, which in turn transmit the daylight through 1/2-in.-diameter plastic fiber-optic cable.
Tailoring lumens
The amount of daylight that can be transmitted is limited only by the size of the system and the lighting fixture. "The amount of light can be tailored to the amount of light desired for a room," says Gorthala. "If 10,000 lumens to one light fixture are wanted, it can be done, based on the size of the collector and the size of the fiber-optic cables." Because the cables spot the light, diffused lighting would require the use of an additional mechanism.
The daylighting system is ineffective on cloudy days and at night. However, it can be integrated into electrical systems that are equipped with daylight dimmer control. Hybrid light fixtures are in development at Oak Ridge (Tenn.) National Laboratory. They feature both electronic and fiber-optic daylighting capabilities, says Gorthala.
The system has not been installed to date in the field, other than prototype testing at Gorthala's home in Asheville, N.C. According to Gorthala, SWA is identifying areas in which it can optimize the system by altering its size, weight, shape and function. "Then we'll apply the concepts and commercialize the technology," he says. SWA is searching for a manufacturer to take the patent-pending technology to market.
Possible applications for the product include commercial, industrial and institutional, as well as residential facilities. Secure buildings, such as detention facilities or others that receive little natural light, are particular examples of where the technology may apply. According to Gorthala, the Pentagon has contacted SWA about a possible installation.