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Need Daylight Deep in Buildings? Tube It In

GREENBUILD-- Tubular daylighting devices were on display by a few vendors at Greenbuild 2009. Solatube International gave Denis Du Bois a demo of their flagship product, the 750DS. It has features that gather sunlight very efficiently, and give commercial building occupants the ability to control how much of it is delivered.

Tubular daylighting devices aren't new this year, but their variety and sophistication is making them an attractive feature for certain types of facilities where windows and skylights aren't feasible. TDDs harvest sunlight at the roof, channel it down highly reflective tubes, and deliver it to diffusers inside the building.

Solatube photo on EnergyPriorities.com

Solatube's Todd Anderson demonstrates the daylight diffuser on a tubular daylighting device at Greenbuild.

Solatube launched its flagship product, the 750DS, a year ago at the Greenbuild Conference and Expo. The Energy Star rated product is an energy-efficient way to bring daylight deep into a building. With the time it takes for a new product to be specified and installed, the California-based company is just starting to see the popularity of the 750DS translate into sales.

Capturing and rejecting light

The Fresnel lens design at the top of the tube grabs as much daylight as possible and focuses it into a tight beam.

A Fresnel lens is like a magnifying glass that has been sliced into dozens of concentric rings and nearly flattened. The slices still act like a magnifier. Fresnel lenses have been used for almost two centuries in lighthouses.

Solatube's TDD doesn't just indiscriminately push daylight into a room. The other component at the top of the tube is a rejection lens that keeps the light from getting too bright.

"We're looking for a consistent light source," Solatube's Cynthia Sener said as we toured the company's exhibit at Greenbuild 2009. "Occupants want to rely on a daylighting system the same way they rely on electric lights, yet have the benefits of natural light."

The lens configuration brings in as much light as it can near dusk and dawn, when the sun is low. As the sun reaches its peak intensity in the middle of the day, the lenses keep it from overwhelming occupants with too much light.

Transferring light downstairs

A very shiny reflective tube carries the harvested light along its path to the room below at nearly its original intensity.

That path need not be straight. The 21-inch diameter tube, which can be as long as 50 feet, can bend to dodge obstacles such as HVAC or structural components.

"Warehouse retrofits are where we see a lot of our product installations," Sener says, "because it's easy to drop this product in from the rooftop of an existing building."

Installers can do all of the tube installation work, then quickly cut the holes and finish the installation without interrupting the occupants in the space below.

Delivering controlled daylight

Solatube photo on EnergyPriorities.com

The Solatube 750DS.

Inside the building, daylight arrives at a diffuser at the end of the tube or in the closed ceiling grid.

Occupants don't always want a room to be brightly lit. Boardroom PowerPoint presentations or video playback in classrooms require darkness.

Solatube previewed their dimming technology in Chicago at Greenbuild 2007. This feature nicely complements the light-collecting lenses in the 750DS.

The dimmer accepts an electric signal from a lighting control system or wall switch, just like a fluorescent dimming ballast. When occupants want to darken the room, they can dim the Solatube TDD from full to no daylight at all.

"You can turn the light on and off, or to multiple levels," Sener says. "Think of it just like the dimmer you would have for electric lighting, only it's actually blocking out the sunlight."

Points and productivity

TDDs can qualify a building for LEED credits in several categories. They're still new enough to submit for innovation points.

Daylighting saves energy because occupants can turn off electric lights during the day. With salaries representing a much larger expense line item than energy use in most businesses, productivity gains alone are paying for efficiency upgrades like TDDs, even without the energy savings.

"The thing about daylight is there are so many benefits for the people underneath that light," Sener explains. "Independent studies say that attention spans are greater, productivity is higher, health is better. With all of these great benefits, why not tap into the sun when you can, and reserve your electricity use for when the sun isn't available as a resource."

Model it first

Daylighting levels are difficult to calculate without the help of sophisticated modeling software. Solatube provides the necessary building information modeling (BIM) data to remove the guesswork.

BIM allows architects and designers to visualize and accurately calculate how a building will perform, before it is built. Daylight and shading can be precisely modeled throughout a year, accounting for the latitude and typical weather in the building's location. Modeling daylight avoids installing too much just-in-case electric lighting.

"This is a great resource for our building industry clients because it allows them to factor in space for their daylighting systems, without being concerned about conflicts with other structural components,” Sener said when the BIM resources were released at last year's Greenbuild conference.