How many ways can you light a room, besides
screwing in an electric light bulb? You could pipe in sunlight through
optical fibers connecting the roof and the room. That would work wonderfully
well during the day, especially a cloudless one. Or you could use a
more efficient approach invented at ORNL: Maximize the use of the sunlight's
energy by piping in the visible portion of the spectrum while turning
the nonvisible, infrared part into solar electricity to power the light
bulb. That way you could provide hybrid lighting to the room.
Part of the light would come from the sun and part would come from the electric light bulb. To keep the room at a constant lighting level, smart sensors could be used to dim the electric light when the incoming solar light is bright. And the electric light could be turned up to its highest level at night.
In 1999 Jeff Muhs and his colleagues in ORNL's Engineering Technology Division devised this full-spectrum solar energy system, using a winning combination of existing technologies. They now call the concept "combined solar light and power."
"While we were doing our hybrid lighting studies, we realized that visible light contained only half of the energy of sunlight and that we were simply throwing the rest away," Muhs says. "We were not fully using what's available in sunlight. We decided we could collect the nonvisible light and use it to generate electricity or heat water while using the visible light to light the building's interior."
ORNL is negotiating with two industrial groups to license the system so that it can be further developed and marketed as a commercial product. ORNL and 10 companies have formed the Hybrid Lighting Partnership. Each of the companies manufactures one of the different components of the combined solar light and power system.
The heart of the system is a parabolic dish concentrator (primary mirror), which collects and separates the incoming sunlight. The concentrator, which resembles a satellite dish, is supported by a penetrating pipe that contains a bundle of very large optical fibers. The sunlight striking the concentrator is reflected to an attached secondary optical element positioned above the dish. This element consists of a spectral filter and a silicon-based photovoltaic cell.
Artist's conception of the combined solar lighting and power system showing the cutaway of the inside of a building illuminated by sunlight and lighting fixtures powered by solar energy. Drawing by John Jordan.
The spectral filter reflects the visible light
into the pipe containing the optical fibers, which channel the visible
light into the building's interior. The spectral filter also transmits
the infrared light into the photovoltaic cell, which absorbs this invisible
light and turns 20% of its energy into electricity.
"It is four times
more efficient to light a room using visible light from the sun and
electricity from the photovoltaic cell than it is to use traditional
solar electricity only for lighting," Muhs says. "It is more sensible
to use visible light from the sun directly for illumination of building
interiors than to convert sunlight into electricity and turn around
and reconvert only 30 to 40% of it to light."
The development was funded
by DOE's Office of Building Technologies, State and Community Programs.
This DOE office and DOE's Office of Power Technologies are expected
to jointly support further research. It is hoped that the system can
be commercialized in two or three years.
ORNL's full-spectrum solar
system may also be used with other technologies to better manage carbon
emissions from power plants. Because "carbon-eating" algae growth rates
require much less light per unit area than is available naturally on
a sunny day, it may be possible to drastically reduce the required acreage
needed to sequester carbon from power plants. A team including Ohio
University and ORNL have proposed a 10-acre chamber capable of replacing
a 250-acre lake. The system contains closely spaced vertical sheets
of algae illuminated by self-illuminating large-core optical fibers
that glow like fluorescent lamps but use ORNL's collector technology.
The infrared portion of the spectrum is captured by photovoltaic cells
to produce several megawatts of electricity to operate pumps and conveyor
belts needed to grow and harvest the algae.
ORNL's combined solar lighting
and power concept appears to have a bright future.