Harnessing Daylight For Energy Savings
Designing buildings properly and controlling glare and solar heat gain can cut energy bills by more than half
The problem is the incredible amount of energy in sunlight. On a clear day, the sun provides 8,000 to 10,000 foot-candles of light. Even through the glass, it provides 5,000 foot-candles on a clear day and 1,000 foot-candles on a cloudy day. Most people need a mere 35 foot-candles to read. Compounding the problem is sunlight is as much heat as visible light. The sunlight that pours through a typical 4-by-8-foot window section in an afternoon can heat 15 to 30 gallons of water to a temperature hot enough to take a shower.
But why would a building owner even want to take on the challenge of using daylight to light a building? In a word, money. A good daylighting design can save up to 75 percent of the energy used for electric lighting in a building. The amount of daylight available, the occupancy pattern, and the control strategy can all affect energy savings. In addition, because significant daylight is often available during utility peak demand hours, a good daylighting design can reduce demand charges. Electric lights also generate significant heat in a building and by turning off or dimming the lights when not needed, 10 to 20 percent of the energy used to cool a building can be saved. On top of that, so-called soft savings attributable to increases in productivity and health of the building occupants can add to the hard savings, researchers say.
Using daylight correctly takes a multidisciplinary approach to design and an understanding of the strategies and technologies available to control the sun.
Good Idea, Bad Approach
A facility executive’s daylighting goals of holding down first cost and decreasing operating costs often go unmet when the entire building team isn’t brought into the design process from the beginning.
“The biggest problem with daylighting design is we still take a traditional building approach to a daylighting project,” says Eric Truelove, director of sustainable design and HVAC engineer for Renschler. In other words, the architect and owner design the building and then hand it off to the engineers, who hand it off to the contractors.
In a typical case, the architect’s design offers plenty of glass for light, but that light brings with it too much heat, forcing the engineer to increase the cooling tonnage when the building is complete. It also becomes difficult to decrease lighting energy costs because when the building is complete, occupants demand blinds to cut down on glare. As a result, the blinds are drawn much of the day, requiring the use of electric lights. An owner who thought daylighting was going to save money finds out that the design not only costs more upfront but costs more to operate as well.
Truelove says if engineers and contractors are brought in at the beginning, then issues such as the type of glass, cooling loads and lighting systems can be addressed during design, saving on change order costs, mechanical and electrical costs and operating costs.
Big savings can result from a single design change. If, for instance, a building that faces southwest and receives a good dose of afternoon sun were oriented just 20 degrees to the east, the cooling requirement could drop by 30 percent, saving the owner hundreds of thousands of dollars upfront plus thousands more annually on operating costs.
“When daylighting is done right, it’s a win-win,” says Barbara Erwine, senior consultant with Paladino and Company. “It’s win-win because it costs less to build and costs less to operate. When it’s done wrong, it’s a lose-lose.”
Both the location and interior characteristics of a building are important in daylighting design and should be the first things to consider. Because location varies, as does its effect on the interior design, there can be no cookie-cutter approach to daylighted buildings. A well-designed daylighted building in Phoenix would look and function differently than a daylighted building in Boston.
Location has everything to do with access to daylight. An urban building may be shaded during part of the day, which will affect, not rule out, a daylighting strategy.
Multiple stories also have an impact on how deep within the floorplate daylight can travel. There are solutions for multiple-story buildings, such as placing little-used, transitional or unoccupied spaces near the core of building — restrooms, copy rooms or elevators.
Good daylighting design will also affect a building’s interior spatial characteristics. A daylighted building’s interior should have the task areas close to daylight sources — whether that is sidelighting from the windows or top lighting from skylights. The placement of transitional or unoccupied spaces should be moved to the core or other areas with minimal windows.
“Any building can be daylighted,” Erwine says. “But daylighting could change the look of a building. You want to bring the task to the daylight as much as possible. Look at the hierarchy of tasks and what light is needed where, and use that to define the layout.”
Good daylighting will drive the building’s orientation as well. The optimal position is with the long sides of the building facing north and south. This presents only one long facade into the sun — the south side. And by facing the narrower sides of the building toward morning sunlight on the east and afternoon sunlight on the west, the building has already reduced the potential heat gain of the sun. The south side’s exposure is most direct in the winter when the sun is less intense. In the summer, the high arc it follows in the sky allows the south windows to be easily shaded with horizontal overhangs.
Good daylighting will have an impact on the size and number of windows. Generally speaking, an owner would want few, if any, windows on the east and west sides and not more than 35 percent of façade in windows on the south and north sides, says Abby Vogen, director of the Daylighting Collaborative at the Energy Center of Wisconsin. More windows could be used but very high-performance glazing would be necessary. Because windows are more expensive than walls, fewer windows also help reduce the overall cost of a project.
The taller and higher the windows, the farther light can reach into the interior. Optimally, the ceiling plane and top of the windows are at the same level. And ceilings higher than 10 feet are important to deeper penetration of light. A rule of thumb, Vogen says, is light penetration is equivalent to 1.2 to 1.5 times the height of the windows, assuming that the ceiling is also that high. In fact, properly designed windows and ceilings could distribute light as far as twice the height of the windows.
That doesn’t necessarily mean a huge expanse of glass. When window-to-wall percentage is less than 35 percent, the usual design includes some combination of solar control glass as view glass at the occupant level with clearer glass as clerestory windows. The clerestory windows become the primary source for distribution of daylight deeper into the space while the view glass offers buffered sidelighting.
To achieve the high perimeter ceiling height for clerestory windows without changing overall floor-to-floor height, ceilings can be sloped, or stepped, away from the windows to allow HVAC ducts to be accommodated in the center of the space, Erwine says.
There is also evidence to suggest that interior light shelves, used in some early daylighting projects, aren’t essential in good daylighting strategies. Vogen says they sometimes create shadows directly below them, near the windows, which is often a critical task space area. There may be instances when they are needed, but they aren’t required for good daylighting and may be more trouble and cost than they are worth.
Exterior light shelves and louvered overhangs, on the other hand, often prove useful to diffuse and bounce light into a space and to shade windows. The necessity, placement and size of these exterior features is controlled by geographic location.
Other physical characteristics of the space include light-colored ceilings, walls and furnishings. Reflectance of the ceiling and walls is particularly important, says James Benya, lighting designer. Ceiling materials now come with reflectance values of 90 percent and greater. A value of 80 percent or more is recommended. Walls should offer 50 to 70 percent reflectance and floors and furnishings from 20 to 45 percent.
The biggest challenges in daylighting design are controlling glare and heat. The building owner’s choice of glass for a daylighted building is really a function of geographic location. For instance, an acceptable visible transmittance — the amount of light the glass will allow through — can range from 20 to 50 percent in the Midwest but should range from 18 to 30 percent in the Southwest. The range could be as high as 70 to 80 percent in the Northwest, Vogen says.
“In the South you really need to control the sunlight more than in Portland, Ore., where there is much more leeway with the light because it’s less intense,” she says.
Typically, visible transmittance ranges from 30 to 50 percent. But even at 30 percent visible transmittance — with only 30 percent of available light entering — the glass looks clear from the inside.
Truelove says architects almost always want clear glass and specify glass with transmittance of 80 percent. “They often don’t realize until you show them that glass with 40 percent transmittance looks clear too,” he says.
By selecting for certain optical properties, the building owner not only maintains a view out the window and brings daylight in, but increases energy efficiency as well. By going from a transmittance of 80 percent to 45 percent, Truelove says, the building's cooling load can be cut in half.
Clerestory glazing, however, needs to bring light in and should be much clearer glass with visible transmittance of 70 to 90 percent. Plus, the glass should be as continuous as possible along the top of the wall. Wide structural breaks will cast shadows and create too much contrast. Clerestories also make up a much smaller percentage of the window-to-wall ratio.
With all the sunlight coming into the space also comes heat, unless the glass is equally specified to control for solar heat gain. With the use of various low-emissivity (low-e) coatings, glass can have a solar heat gain coefficient — a measurement of solar radiation, mostly as heat, allowed to pass through the glass — of as low as .25. That means only 25 percent of the potential heat from sunlight enters the building. A single pane of typical clear glass will allow 80 percent or more of available solar heat in.
So a good daylighting system is a balance of high visible light transmittance and low solar heat gain. An easy and increasingly accepted way of determining a good balance is to divide visible light transmittance by the solar heat gain coefficient. For instance, a visible light transmittance of .6 divided by a solar heat gain coefficient of .4 yields a glass with a combined rating of 1.5. Called the light-to-solar-gain ratio, 1.5 is a good place to start, Erwine says. “If the windows can’t be shaded from the exterior, then it’s very important to use a glazing product with as high a ratio as possible to minimize cooling loads,” she says. Many good products exceed 1.25; the best exceed 1.5.
Physical barriers such as exterior light shelves, trees or light-colored pavement, also can play a significant part in controlling the amount of both light and heat that enter the building. And even good daylighting designs may incorporate window features such as blinds or perforated roller screens that are either manually or automatically controlled to block direct sunlight.
“The movement of the sun varies throughout the year so it’s difficult to design any single feature that will take care of everything,” Erwine says. “Plus occupants like to have the ability to control the environment in their space and can often do so without affecting the quality of the daylighting design, if the design is a good one and they understand the design.”
A Balance of Light
Once the design effectively distributes cool daylight around the space, the electric lighting needs to be balanced with available daylight.
“The typical direct-indirect lighting system, with maybe some recess lighting where needed, is nearly the same for a daylighted space as it is for a non-daylighted space,” Benya says. There are two important differences: Daylighted lighting systems are zoned and lighting controls may be necessary.
Unlike other physical features in a daylighted space, whether it’s structural beams or ductwork that should run perpendicular to the windows, the lighting fixtures should run parallel to the windows. This is so the lights can be zoned — dimmed or turned off in sections according to the intensity and penetration of daylight into the space during various times of the day.
To save energy, the electric lighting needs only to augment the daylight. In some case, dimming or turning lights off manually can be an effective method and has worked in a number of schools where the staff has sufficient introduction into daylighting, Vogen says. But where this might not be possible, optical lighting sensors are used.
In poor examples of daylighting designs, lighting sensors are poorly located and rarely commissioned. The installation is handed off to contractors who have little understanding about the complexities of a daylighting system. Contractors often merely split the distance between the windows and the farthest wall with little regard as to how daylight is actually distributed.
If done right, however, lighting controls can be the heart of an energy-saving daylight design. According to Heschong Mahone Group, which has done extensive research on daylighting, energy savings from daylighting controls can range from about 50 cents per square foot to 75 cents per square foot depending on the building type, location, operation and local cost of energy.
“We have enough knowledge and information now to place the sensor so we can get good energy savings,” Erwine says. But like so much about daylighting, the placement of sensors is site and building design specific.
Good daylighting offers a number of benefits, including energy savings, improved occupant comfort and possibly greater productivity. Creating cool, controlled daylight is a complex design challenge that is only possible with a coordinated group effort by the whole building team.
“The energy benefits of a good daylighting system are critical to building owners,” Vogen says. “To achieve the benefits, building owners have to support a design strategy for their buildings that involves all the design professionals — architects, engineers, designers and contractors — working together from the beginning. That’s the only way to be successful and for the building to be cost-effective.”
David Kozlowski is a freelance writer who has covered facility management for more than 10 years. He was formerly senior editor for Building Operating Management.