Walk into almost any store, office, or school during the day, and even if the sun is shining outdoors, chances are that the lights are blazing inside. Indoor lighting gobbles up about a quarter of the electricity used in the U.S., and conventional lighting is terribly inefficient, with less than 25 percent of the energy it consumes converted to visible light. On top of that, the heat given off by artificial lights increases by about 10 percent the energy needed to cool buildings. But a recent project at the federal Oak Ridge National Laboratory (ORNL) to develop hybrid solar lighting promises to make lighting more energy-efficient.
Hybrid solar lighting (HSL) uses a satellite dish–shaped solar collector mounted on a building’s roof to gather sunlight, which is delivered to light fixtures by fiber-optic cables. Just as hybrid cars switch between a gas engine and battery power, HSL switches between the natural light of the sun and artificial lighting as needed.
The idea to combine natural and artificial light was pioneered in Japan in the seventies, but remained simply an interesting concept for a long time. “Back then, the materials for this technology were very expensive,” says Duncan Earl, one of the co-inventors of HSL at ORNL. “They were using glass fibers, glass mirrors, and very extensive tracking equipment and electronics that aren’t as good as what we have today.”
Earl and his collaborator, Jeff Muhs, substituted plastic fiber-optic cables and lightweight plastic mirrors for glass ones to concentrate solar light. They also pioneered a light fixture that marries sunlight with artificial light to provide a steady amount of light regardless of the weather. High-tech microprocessors and GPS tracking equipment help maximize the amount of sunlight that strikes the collector.
One problem with using all that plastic was heat. “When we told people we were going to do HSL with plastic, which melts just a little bit above the boiling temperature of water, they thought we were crazy,” says Earl. The team came up with a smart solution: Use an additional mirror to strip off the infrared range of light, the invisible portion of sunlight that holds the heat. This turned out to be one of the biggest assets of the new technology. “We can capture the infrared light and send it to a photovoltaic module or a solar hot-water heater, so we can use the thermal portion of the sunlight as well as the visible light,” says Melissa Voss Lapsa, program manager for the HSL initiative at ORNL.
ORNL licensed the technology to Sunlight Direct, a company Earl formed to commercialize HSL. Together they are beta-testing the technology at 25 commercial facilities around the U.S., including a Staples store in New York, Aveda’s corporate headquarters in Minneapolis, and an office building at San Diego State University (SDSU). They plan to roll out HSL collectors and light fixtures for commercial buildings in early 2007; residential models will be available within a couple of years. “It’s not uncommon for some commercial users, such as retailers, to have 60 percent of their electricity expense attributable to lighting,” says Earl. “So we expect them to be our early adopters.”
New users might realize other benefits, too. According to a 2003 study sponsored by the California Energy Commission on Productivity and Interior Environments, shoppers spend more money in naturally lit spaces, and productivity increases among workers and students. Other studies show that daylit spaces reduce health problems such as sleep disorders, overeating, depression, and joint or stomach pain.
What do people think about HSL in the real world? “Everybody has a really positive view of it,” says Bill Lekas, the energy manager for SDSU, who’s overseeing the HSL demonstration project at the university. “The brightness of the artificial light adjusts automatically to the amount of sunlight available, so the light level is steady and of good quality.”
Utility companies are intrigued by HSL’s potential, and four of them in California recently hired the California Lighting Technology Center at the University of California Davis to perform an independent evaluation of the technology. “The utilities want a quantitative assessment,” says Konstantinos Papamichael, associate director of the center and an architect specializing in daylighting and energy efficiency. The tests will assess issues like how much light HSL generates, how reliable it is, and how quick the payback period is. “In the mid-eighties I did presentations that talked about this technology like it was science fiction,” Papamichael says with a laugh. “Now the future is here, and we see the technology being realized. It is really promising.”
Lighting facts from the U.S. Department of Energy
- Buildings use 72 percent of all electricity and account for 80 percent of all electric expenditures.
- On average, lighting consumes more energy than air conditioning in U.S. homes.
- Lighting is the second-highest contributor of carbon dioxide emissions from building energy use, representing about 7 percent of total emissions.
- There are over 156 billion square feet of lighted floor space in commercial buildings in the U.S.
- The heat given off by indoor lights increases the amount of air conditioning used in homes and commercial buildings by as much as 27 percent.