Daylighting--Part 2: Bringing Daylight Deeper into Buildings
IMAGINE A DEVICE THAT SITS ON the roof of a building and focuses sunlight into cables the size of electrical wire. These cables are run through walls and ceiling plenums into light fixtures that beam natural, full-spectrum daylight deep into a building’s interior.
Sound like science fiction? It’s not. One such product, the Himawari®, has been commercially available for nearly 15 years, and more than 1,000 of these (expensive) fiber-optic daylighting systems have been installed in Japan and Western Europe.
In the U.S., research on cost-effective fiber-optic daylight distribution is still in its infancy, though many of the components and technologies, including optical fibers used for electric lighting, are well developed and already finding widespread use for specialized lighting applications.
The idea of using special devices to distribute daylight or light from a remote source has been around for a while. Ships in the 1800s were sometimes fitted with special prisms that channeled daylight down into the ship’s hold. In 1880, William Wheeler patented a mechanism for distributing light throughout a building using a network of pipes—though he did not succeed with implementing the concept.
This article takes a look at strategies for bringing daylight deeper into buildings. Last month we addressed daylighting with windows and conventional skylights—strategies that have been shown to boost worker performance in offices, increase test performance in schools, and increase sales in retail establishments. This month, we’ll look at strategies for increasing the penetration of daylight. In addition to high-tech fiber-optic systems, we’ll examine tubular skylights, which are finding widespread use in the residential market, as well as some high-performance commercial skylights that rely on highly reflective light wells or sun-tracking mirrors to boost performance.
Used primarily in residential buildings, tubular skylights are devices that channel daylight from an aperture on the roof down through an unheated attic to a ceiling-mounted diffuser, which looks much like a conventional ceiling-mounted light fixture. The light is transmitted through a cylindrical tube 8” to 24” (200 mm to 600 mm) in diameter with a highly reflective interior surface.
Tubular skylights were developed in 1989 by the Australian Steve Sutton, who founded Solatube, Inc. In the U.S., they were first developed and introduced by Greg Miller, who founded The Sun Pipe Co., Inc.
Today there are at least a dozen companies producing tubular skylights in the U.S. and Canada (see sidebar below).
The design of these systems is pretty straightforward, though there are some significant differences among the various products. Most have acrylic domes that mount on or above the roof; a few use polycarbonate for the domes to meet special hurricane design standards in effect in parts of Florida (where tubular skylights are especially popular). Polycarbonate has roughly 10% lower light transmission than acrylic, but it is much stronger. At least one product is available with a special reflective scoop to capture more sunlight. Several products include a prismatic or
collimation lens beneath the dome, which improves the capture and downward channeling of low-angle sunlight in the morning and afternoon. Special flashing kits are available from most manufacturers to adapt the units to various roof types.
As for the tube, most products use a highly reflective silver film on the interior to minimize light loss as the daylight is channeled downward. Both 3M and Alcoa produce silver film used in these products. A few manufacturers use highly polished anodized aluminum. Either approach appears to have advantages and disadvantages. Reflective silver films have the highest reflectivity, but some have been known to delaminate; the polished aluminum can’t delaminate, but the surface may oxidize over time, reducing its reflectivity. One manufacturer, Sun Tunnel Skylights, has a flexible tube that simplifies installation, but reflectivity is significantly lower and light output suffers.
Most ceiling diffusers are simply white acrylic, which spreads the light fairly evenly. A few products have prismatic diffusers, which increase the lateral distribution of light, and one manufacturer (Huvco) has recently introduced a unique holographic strip above the diffuser (at the bottom of the tube) to improve lateral lighting—though details on how this works were not available.
Most residential tubular skylights cost $300 to $500, not including installation. The least expensive we found were about $200 for 8” diameter (200 mm) models. Tubular skylights can be installed fairly quickly, and many products are targeted toward a do-it-yourself market. Light delivery varies according to the tubular skylight diameter, the transmissivity of the dome, the reflectivity of the tube inner wall, the transmissivity of the ceiling diffuser, and the outside sunlight conditions. The best-selling 10”, 14”, and 16” (250 mm, 360 mm, and 405 mm) Solatube units provide up to 3,750, 6,300, and 8,200 lumens, respectively, under ideal, full-sun conditions.
So are tubular skylights a good economic investment? Unfortunately, calculating the payback from residential daylighting systems, including tubular skylights, is very difficult. To accurately calculate electricity savings, we would need to know how many hours per day electric lighting is actually being saved by the skylight (though the skylight is producing light all day, it is only displacing electricity when the electric light would otherwise be on); the wattage of electric light(s) being displaced; the cost of electricity; and the installed cost of the tubular skylight.
For the sake of argument, let’s assume that a 10” (250 mm) tubular skylight that cost $500 to put in is able to displace a ceiling fixture with two 60-watt incandescent lamps (producing 870 lumens of light apiece, or 1,740 lumens total) for four hours per day. Under those conditions (with electricity costing 10¢/kWh), the simple payback on the tubular skylight would be 28 years. If the electric lights were displaced eight hours per day and the skylight cost only $300 to install, the payback would drop to 9 years. The bottom line is that, in most situations, tubular skylights cannot be justified economically based on the energy savings. On the other hand, there may be other benefits that cannot as easily be quantified—such as health and productivity benefits from full-spectrum, natural daylight.
Another issue with tubular skylights is their thermal performance—will the unit increase heating or cooling loads in a building? For this question,
EBN turned to several building scientists to our north. John Straube, at the Civil Engineering Department and School of Architecture at the University of Waterloo in Waterloo, Ontario, said that “tubular skylights offer more daylight per unit of heat energy gained than normal (flat) skylights,” which will be a benefit in warm climates and during summer months. They are also likely to have higher resistance to heat loss than a conventional skylight, he says, though that depends on the quality of the glazing in the conventional skylight.
Alex McGowan, an engineer with Enermodal Engineering in Kitchener, Ontario said that the thermal performance of a tubular skylight depends on whether the attic through which it runs is vented. “For unvented attics, tubular skylights are comparable [to conventional skylights] in terms of heat loss, but have slightly better solar and visible gains (because of the mirrored surface, which transmits light downward better than the diffuse surface of a conventional skylight well).” McGowan does not recommend tubular skylights for installation in vented attics because of excessive heat loss through the uninsulated tubes and because condensation can readily occur. McGowan’s company did some computer simulations a few years ago (using the FRAME program) and found that the heat loss from a tubular skylight is dependent on the length of the tube. “More tube exposed to the attic, whether vented or not, means more heat loss,” he said. Given these concerns, it would seem that performance of tubular skylights (at least in cold climates) could be significantly improved by constructing the tubes of an insulating material or by wrapping them with insulation.
Advanced Commercial Skylights
For the most part, tubular skylights are aimed at the residential market. From an economic standpoint, however, skylighting often makes much more sense in commercial buildings, which have greater daytime use. This hasn’t gone unnoticed by manufacturers of tubular skylights, several of which are moving aggressively into the commercial-building arena. Both Huvco (through their companion enterprise The Daylight Company) and SunPipe, Inc. are looking to commercial buildings for much of their future growth. Greg Miller of SunPipe told
EBN that commercial skylights account for just 10% of sales today but will eventually exceed the residential market.
Skylighting is nothing new for commercial buildings. Indeed there is a well-established industry of commercial skylight manufacturers. But companies like Huvco and SunPipe are trying to bring advances to that market. Traditional commercial skylights are typically square or rectangular; for example 4’ x 4’ (1.2 m x 1.2 m) or 4’ x 8’ (1.2 m x 2.4 m). SunPipe is marketing their largest round skylight for this market and plans to introduce such features as damper-activated output control, which Miller thinks will be a big plus for the school market, where teachers want to darken classrooms.
Huvco’s SkyLite™ is a wholly redesigned product for single-story commercial buildings, such as retail stores. This square 4’ x 4’ (1.2 m x 1.2 m) skylight includes a cast acrylic or polycarbonate dome, a collimation lens to improve light collection when the sun is low in the sky, highly reflective inner skylight walls, and a diffusing lens on the bottom.
Another way to boost the performance of commercial skylights is to use a mirror system that tracks the sun actively and reflects additional light down into the skylight—a process referred to as
active daylighting. The idea is to take better advantage of low-angle sun in the early morning and late afternoon that is not effectively transmitted through conventional skylight domes.
So-Luminaire® of Los Angeles and the Natural Lighting Company of Phoenix both manufacture such systems. Andersen Windows developed an active skylight prototype that was used on the City of Industry, California Wal-Mart Ecostore, though the company has not commercialized the product.
So-Luminaire uses an array of mirrors above the skylight that tracks the sun and reflects low-angle sun into the skylight. The mirrors are mounted on a center pole that is secured to the perimeter of the skylight. The mirror array rotates to follow the sun.
The Natural Lighting Company keeps the mirror assembly fully within the acrylic dome instead of having it extend above. This assembly pivots to face the sun using a photovoltaic-powered motor and infrared suntracking control unit. The company introduced its active UTD (Under the Dome) skylights five years ago and in January 1999 modified the unit to operate independently using PV power. The Natural Lighting Company also produces high-performance passive skylights targeted towards the school market—indeed passive commercial skylights (introduced about ten years ago) remain the company’s primary product. These skylights are available with a darkening shield so that teachers can darken a skylit room. All of the company’s skylights include a highly reflective lightwell. Company vice president Bruce Bilbrey told
EBN that the active mirror assembly increases the effective daylighting period by two to four hours by taking advantage of low-angle morning and afternoon sun.
Be aware, however, that the tracking mechanism adds complexity to an active skylight. Some architects have shied away from these units out of fear that they may fail. If specifying active skylights, pay attention to reliability and warranties offered by the manufacturers. As active skylighting grows in popularity—and assuming that the trackers function as designed—this concern should gradually diminish.
Because commercial skylights are larger than residential tubular skylights (providing a lot more light) and because they generally displace electric lighting throughout the day, they offer much better economic return (even if you only consider electricity savings—see
Vol. 8, No. 9 for other benefits). The Daylight Company of Huvco claims a two-year simple payback on its commercial SkyLite.
Tubular skylights and active skylights with tracking mirrors and reflective lightwells deliver daylighting effectively but only a limited vertical distance—usually a single floor of a building (through an unheated attic or ceiling plenum). The Holy Grail of daylighting is to distribute natural light anywhere in a building. To do that, companies and researchers are turning to high-tech fiber optics. Fiber optics is the transmission of light through special fibers—glass, plastic, gel, or liquid-core—such that the light waves bounce off the outer cladding of the fiber and remain inside, moving through the fiber to the other end. In the U.S., fiber-optic daylighting is at a very early stage of research, but in Japan the Asahi Glass company of Tokyo has been producing the Himawari® since 1985.
Himawari—“sunflower” in Japanese—is available in five different models. Each collector has 18 to 196 individual fresnel lenses, each 4.1” (105 mm) in diameter (3.1” or 78.5 mm for the smallest Himawari model). This collector follows the sun as it moves across the sky using PV-powered motors that provide two-axis tracking. Each lens focuses sunlight into a 1 mm-diameter optical fiber made out of quartz glass. In the process of feeding light into the fibers, the ultraviolet and infrared portions of the sunlight spectrum are filtered out, so the light being distributed is only the visible portion of the spectrum. The fibers are bundled into cables that can be run through a building more than 330 feet (100 m). At a distance of 130 feet (40 m), a single six-fiber cable under full-sun conditions provides 1,180 lumens—about the output of a 100-watt incandescent lamp. The company claims that there is only a 4% loss of light per 10 meters (33 feet) of fiber-optic length. The cables can be run through walls and ceiling plenums, and the company provides light fixtures to distribute the fiber-optic lighting at the point of use.
The downside to the Himawari system is its cost: about $8,000 for the smallest system, which provides the equivalent of about three 100-watt incandescent lights, and several hundred thousand dollars for the largest. Despite this high cost, the system is fairly popular in Japan, according to Kenneth Eben of the New York City-based Mitsubishi International Corporation, which is the U.S. distributor of the product. More than 700 systems have been purchased in Japan, including many of the smallest units by apartment dwellers who put a very high value on natural light. In the U.S., only a handful of systems have been installed—all for research or demonstration purposes.
There is growing interest in fiber-optic daylighting in the U.S., however. Researcher Jeffrey Muhs, of Oak Ridge National Laboratory (ORNL) is currently heading up a research initiative with several industry partners. He believes that there will be a significant market for daylighting the top two or three floors of commercial buildings with fiber optics if cost can come down. The ultimate target, according to Muhs, is a three- to five-year simple payback. He notes that the total conversion of solar energy into light is pretty high with distributed daylight—much higher than when we are using solar energy to generate electricity and then convert that electricity back into light. Because of cost, the research effort is looking to plastic optical fibers rather than glass.
One of the industry partners is Innovative Design, a firm that has been working with Duke Solar to commercialize solar-thermal power systems (see
Vol. 8, No. 7/8). They are responsible for the solar collector—the daylight harvesting end—and hope to have a prototype built this fall. “Our goal,” according to Innovative Design project manager Gary Bailey, AIA, “is within two years to have a product ready for commercialization.”
An important component of a fiber-optic daylighting system will be its integration with electric lighting. Some researchers envision that daylight will be fed into the optical fibers when the sun is shining, then a centralized, high-efficacy electric light source would be substituted for the daylight at night or when the daylight is inadequate. Muhs doesn’t think the integration between daylighting and electrical lighting will happen this way. Rather, he predicts that we will see hybrid luminaires (light fixtures) that can distribute light from both optical fibers and electrical sources, with appropriate controls so that the electric lights dim or turn off when the fibers are delivering adequate light.
Additional research on fiber-optic daylighting is being done by Steven Winter Associates (SWA) of Norwalk, Connecticut. SWA’s Ravi Gorthala told
EBN that a prototype is currently being built and should be ready for testing later this fall. “It’s looking very good,” she reports. As with the ORNL initiative, bringing down the cost is the overall goal. They are working with off-the-shelf components, including plastic optical fibers and advanced lighting controls.
The goal is simple enough: to bring daylight deep into buildings, providing occupants with healthy, natural light while dramatically reducing electricity consumption for lighting. Today, this is being accomplished to some extent with tubular skylights and advanced commercial skylights—some of which use tracking mirrors to direct additional light down into the skylights. Soon, perhaps (if current research efforts pan out), fiber optics will allow that daylight to be distributed hundreds of feet throughout our buildings.