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By the year 2020, hybrid solar lighting - now in its prototype stages - could return economic benefits in the $5 billion per year range, and energy savings of 30 billion kilowatt hours.

The promise of this emerging technology was explored in the first Hybrid Solar Lighting Summit held at hybrid's "home", the Oak Ridge National Laboratory in Knoxville, Tennessee, in October. "We received a lot of excellent feedback from the lighting designers and architects who attended. As a result, we've refocused the first market for technology. Before, we had always focused on traditional downlighting, using a 2-by-4 troffer and fluorescent lamps," Muhs explains. "From our discussions with lighting designers at the summit, we realized that we should look at point-source lighting where color rendering is important, like the halogens and track-lighting used in high-end retail environments."

Paybacks Faster with Point Source Lighting http://www.ccbuptzsw.cn

Aside from leveraging the very high color rendering index of distributed sunlight, a major reason for the refocus is energy savings potential and ease of integration.

"Any time natural light can displace electric lighting that has an efficiency of 20 lumens/watt or less (such as halogen/incandescent bulbs) versus 90 lumens/watt or more (such as fluorescent lamps), it's pretty easy to see that the payback will dramatically improve," he explains.

"Because of the input from the summit, we went back and did some calculations on simple paybacks considering this new first market," he explains. "In some of the Sunbelt regions where there's a deregulated electrical market and quite a bit of sunlight, the payback could be less than a year or two. That includes a reduction in HVAC load, reduction in bulb replacement requirements, and so on."

"What's more, distributed sunlight emerges from optical fibers in much the same spatial pattern as it emerges from electric point source lamps. To match the spatial distribution of co-located electric lamps is easier and less expensive in point-source lighting applications, when compared to fluorescent downlighting applications," Muhs says.

"We already have collectors that are flexible enough to accommodate multiple end-use applications such as point-source lighting and traditional downlighing. At this point, it all boils down to reducing the cost of the collector and developing a suite of hybrid luminaries or light fixtures. As fast as organizations can develop light fixtures to go along with the roof-mounted collectors, we will begin to see winning combinations," he says.

Progress with hybrid solar lighting has fulfilled early expectations. "Our sponsors at the Department of Energy Office of Solar Energy rightfully have said that if we focus on getting distributed sunlight into the room, once you've done that successfully, it will be a case of: "If you build it, they will come"," says Muhs.

Technology Improvements Made in 2003

"In the past year, we've changed the collector to make it easier to assemble and align," Muhs says. "There are fewer parts and it can be used with multiple lighting applications."

For example, a previous iteration of the solar collector used an 8-sided secondary mirror that the light reflected off of before going into the fibers.

"Each facet of the mirror had to be aligned with an individual, large-core fiber. In our latest design, we replaced that component with a single secondary mirror, focusing light on a bundle of polymer optical fibers. Each one is three millimeters in diameter. So instead of having eight large fibers carrying 5,000 to 10,000 lumens, now we have over a hundred of much smaller fibers that can be bundled together or used separately in lots of different lighting scenarios. This allows lighting designers a more "plug-and-play" daylighting experience," he says.

Architects will like the fact that in the new design, the optical fibers have a much smaller bend radius and can be routed longer distances. In one demonstration, ORNL is routing optical fibers through walls to rooms located two stories below the roofline. Another nice feature of the new design is the fact that the low-cost polymer fibers have less attenuation losses per unit length and less color shift than large-core optical fibers used in the first prototype.

Milestones Set for 2004

"We need to continue evaluating this second generation collector," Muhs says. "We have some thermal management issues, because now we are focusing a square meter of sunlight at one location rather than splitting it into eight locations. We are completing a new design that eliminates thermal loadings that might otherwise degrade the fibers."

"We also want to make the control system more robust, both inside the room and on top of the roof where it tracks the sun," he says.