Curt Maxey, Oak Ridge National Laboratory
Hybrid solar lighting systems focus highly concentrated sunlight into a fiber optic bundle to provide sunlight in rooms without windows or conventional skylights. The flexible sunlight bundles are easily routed through small openings and around obstacles to carry the light to where it is needed. The optical fibers terminate in hybrid luminaires where the sunlight is combined with electric light that is automatically adjusted to keep the overall light level constant within the lighted area.
The hybrid solar lighting concept was originally proposed at Oak Ridge National Laboratory in Tennessee in the mid-1990s, but funding hurdles prevented the idea from seeing daylight for more than five years. Hybrid solar lighting was touted as a means for using solar energy directly without any conversion losses and for increasing the visual quality of interior lighting. As such, it promised to be both energy-efficient and aesthetically appealing, but its technical complexity made potential sponsors wary. They had to be convinced that the lighting concept could be accepted into the marketplace and that the systems could be manufactured at an acceptable cost.
An earlier fiber-coupled daylighting system marketed in the early 1990s used expensive quartz optical fibers to distribute the light and served only a niche market that was willing to pay a premium for the novelty. By contrast, the hybrid solar lighting system proposed using inexpensive plastic optical fibers to distribute the light to hybrid (sunlight/electric) luminaires that would be visually and functionally identical to conventional luminaires. In this way, the lighting could be integrated seamlessly into existing design concepts and thus easily embraced by architects and lighting designers as a means for offering daylight as a lighting option.
Finally, after receiving its initial funding from the US Department of Energy, Oak Ridge National Laboratory unveiled in 2002 the first prototype hybrid solar lighting system, which provides lighting for one-half of a second-floor laboratory space. For the first time, visitors could see the contrast in the quality of the fiber-coupled sunlight on one side of the room versus the light from the conventional fluorescent system on the other.
The way it works is that a solar tracker up on the roof follows the sun across the sky (Figure 1). Then, daylight-harvesting sensors maintain a constant level of illumination, even with clouds blocking the sun. With the concept successfully demonstrated, subsequent research efforts were aimed at cost reduction and component reliability.
|Figure 1. Up on the roof, the tracker controller system moves with the sun, collecting light into a focused beam that enters the optical fibers. The IR and UV wavelengths are stripped out before the visible light is sent into the building.|