How would you like a $40-billion electricity bill? That's what artificial lighting costs the nation in a year, about one fourth the total electric bill. Interestingly, the demand for lighting peaks (up to 40% of total electricity use) during the day when natural light is most abundant. To reverse this paradox and conserve energy, the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) (Oak Ridge, TN) and several industrial firms plan to develop hybrid lighting which combines natural and artificial light. And any significant cut in lighting energy goes a long way in meeting goals for reducing green-house gas emissions.
The concept would supplement window light with external collectors to capture sunlight for waveguides to transmit to special fixtures within a building. These fixtures contain a combination of fluorescent and natural-light tubes to produce a desired output. Sensors dictate how much artificial light is required. As light levels change, sensor feedback to controllers adjusts the output accordingly. Previously, "there has been a lack of overall coordination and integration of these technologies on a systems level," says co-developer Jeff Muhs, of ORNL's Engineering Technology Division.
But several challenges remain before hybrid lighting is ready for the marketplace. Today, transparent, low-cost polymers for sunlight transmission from rooftop collectors have absorption light losses of about 1% per foot of length--unacceptable in a multi-story building. "Our goal is 1% loss per 10 ft," states fellow ORNL co-developer Art Clemons. "There are lots of material candidates, we're testing lots of compounds," says Hanafi Fraval, chairman of the TransLight division of Fiberoptic Technology, Inc. (FTI, Pomfret, CT). "The fundamental issue is light-guide development," Fraval adds, "which is science development, not engineering like the other parts."
These other technology hurdles include lighting uniformity and efficiency, optical materials, light dispersing elements, systems integration, and auto feedback and control systems. As an example, "Controllers allowing seamless switching from sunlight to artificial lighting will need quite a bit of engineering," notes Fraval. "When a cloud passes overhead, you don't want the occupants to notice it."
Let it shine. Today every hour of the day can be illuminated. However, there is a hidden cost of lighting beyond fixtures, replacement lighting elements, and electricity--namely heat due to inefficiency. "About 10% of our cooling and ventilation costs result from clearing away the heat generated from lighting," notes ORNL physicist Michael Cates. While major improvements have been made by the use of fluorescent fixtures; halogen, sodium, and mercury-vapor lamps; and automatic on-and-off controls, lighting costs will continue to rise from the expected doubling of world office floorspace by 2020.
Previously, DOE lighting efficiency improvement programs have centered on supplementing artificial light with window light. Hybrid lighting, says Cates, would reverse this emphasis, augmenting natural light with artificial. "For many hours, essentially all illumination could be provided by natural light. Fully hybridized lighting in working environments could cut artificial light requirements in half in some regions of the U.S." By combining natural and centralized high-efficiency light sources available today, energy costs for lighting might be reduced by about one third.
Cates notes manageable costs and significant savings are crucial to acceptance of hybrid-lighting schemes. Any higher initial lighting-fixture costs must be justified by long-term efficiency. "For maintenance it is important that hybrid systems cost little if any more than today's standard systems," he adds, "and individual investors will not cooperate for energy savings unless their bottom lines are in the black." Hybrid lighting must also be user friendly, with still just the flick of a switch needed. And last, but definitely not least, is aesthetic appeal. Lighting should be attractive, interesting, and unobtrusive.
Enlightenment. The modern information-based economy has brought more workers indoors, requiring them to have improved illumination. And after work, people spend more time indoors. To reduce energy consumption growth, lighting-equipment manufacturers have begun marketing higher-efficiency bulbs and fluorescent tubes, improved ballasts, and new designs for various replacement components. But little has been done to exploit natural light. In homes, this is limited mostly to skylights and picture windows. Large windows in commercial buildings are often mainly for the view.
"Hybrid lighting is beginning to make sense economically because it is beginning to make sense technically," says Cates. For example, during a clear midday, visible-wavelength sunlight power on the surface of the Earth can be more than 1,000 W/m2. "And compared with incandescent or fluorescent light bulbs, it is pure light," he notes. "A 100-W bulb produces only about 18W of light." One square meter of bright sunlight is thus roughly equal to 55 100-W bulbs. If two or three such bulbs can light a room, one square meter of sunlight can illuminate about 20 rooms. A building's roof area is roughly equivalent to the floorspace on a single floor, "so, theoretically, enough light strikes on a sunny day to light an entire building--even if it's more than a hundred stories high!"
Constantly tracking the sun's position and focusing the maximum amount of available light is the most effective way to collect light, but may be too complex and expensive for lighting. However, with more sunlight available than is typically needed to light a building, fixed, passive collectors, and concentrators are feasible. A typical collector might consist of only three fixed elements, one aimed straight up to catch mid-day rays, and two angled parts, for morning and afternoon illumination. Such a simple system is much cheaper in cost and maintenance than more efficient tracking or multi-element passive collectors. Scaling up the collecting area could make up for some of the inefficiencies.
Because it is not necessary to have image-quality optics, plastic lenses or mirrors--cast, molded, or extruded rather than finished with any great precision--can function as concentrators. Plastic filters may also have an advantage in filtering out ultraviolet light and sending infrared wavelengths into different optical circuits for non-lighting purposes. An added benefit of using mostly natural light of the solar spectrum is better visual acuity. "In principle," Cates adds, "hybrid lighting can generate any variety of spectra and intensity levels. In theory, if cost is no consideration, the user could dial a spectrum to fit the mood, very much like dialing the frequency equalizer of a stereo."
With some hybrid lighting concepts, a centralized light source eliminates separate light bulbs (and their wiring and replacement), as well as heating each room from the wasted energy of operation. Large high-efficiency centralized sources can be locally cooled and ventilated, but still be accessible, like a water heater or washing machine.
Optical fibers and hollow light guides would distribute the hybrid light. These need not be high-quality silica for data communication but could be plastic optical light guides, some even filled with a clear liquid or gel. With about the same size and weight as electrical wiring, handling and routing can be like their electron carrying cousins. A square meter of sunlight focused into a one square centimeter guide can be branched into smaller conduits.
Sensors, such as thin-film photodiodes and similar devices, in each room or light fixture, will sample several visible wavelength bands by filtering techniques or multiple sensors having peak sensitivity across fixed wavelengths. Microcontroller chips can be similarly configured. "In mass production these elements will not make up a large fraction of the lighting cost despite the fact that they will make the difference between a curiosity or decorative system and a full-time, hands-off lighting package," notes Cates.
Hybrid lighting may eventually be part of integrated building energy systems. As solar electric conversion improves in efficiency, sunlight piped into buildings can be used to produce electricity for controlling illumination as well as other uses. It's not inconceivable, according to Cates, that self-contained home energy systems could even furnish their surplus power to the utility grid.