Daylighting, The Other Renewable Energy FMI 2008 Energy Track Orlando Florida Eric T. Truelove, P.E., LAP, Associate AIA
Daylighting Using daylight as an alternative to artificial lights. The benefits of daylighting include: Reduced electric bills Improved health Improved productivity Higher sales Daylighting sounds easy, but it s not. Effective daylighting is a science that has been mastered by only a few designers.
Hours of Daylight Daylight is readily-available at all latitudes in the continental United States. Summer hours in Orlando: 6:30 AM to 8:30 PM Summer hours in Milwaukee: 5:12 AM to 8:35 PM Winter hours in Orlando: 7:15 AM to 5:30 PM Winter hours in Milwaukee: 7:20 AM to 4:20 PM Good daylight is available for 6- to 8-hours every day, depending on the time of year, but does not vary significantly based on location.
Intensity of Daylight The sun delivers more light than we need, much more in fact. Corridors and stairwells: 5 to 20 foot-candles Offices: 35 to 50 foot-candles Classrooms: 50 to 75 foot-candles Laboratories: 100 foot-candles Direct sunlight delivers upwards of 10,000 footcandles along with a fierce amount of unwanted heat. Cloudy skies can actually deliver more light than clear skies.
Is This A Good Daylit Room?
Reduced Electric Bills Depending on the hours of operation, 25 to 50% of a building s electric bill is energy for lights. A typical 30,000 square-foot retail building having 33 kilowatts of connected light load. If the facility operates 70 hours per week, the lights will consume 120,000 kwh of energy each year at a cost of around $14,000 (assume 12 cents/kwh). If you could shut off all of the lights using daylight 6- to 8-hours a day, you could save $10,000 a year or about $0.33 a square foot.
Improved Health People may be healthier when they have daylight in their lives. Hospital patients perceive less pain and take 22% less pain medication (Walsch, Rabin, Day, Williams, Choi, and Kang, - 2005) Students grow taller and have fewer cavities (Heschong Mahone Group - 1999 and Alberta Education 1992). Our eyes are built to accept daylight and our bodies seem to need it in ways we don t yet understand.
Improved Productivity People may be more productive when they have daylight in their lives. Students learn and comprehend 20 to 26% better and have a 1.6 to 1.9% higher attendance rate (Heschong Mahone Group - 1999 and Alberta Education 1992). Greater job satisfaction and lower stress reported (eb Environmental & Behavior - 1998). In addition to saving energy, more daylight can decrease medical costs and provide a productivity increase.
Higher Sales People seem to buy more in daylit environments. Retail stores with skylights sell 31 to 49% more than those without (Heschong Mahone Group 1999) When merchandise was moved to daylit areas, sales increased (Heschong Mahone Group 2005). Study results point to daylighting as a potential means to cut costs and increase sales. The challenge is to do the daylighting effectively and within your budget.
LEED Points Daylighting may help you achieve Leadership in Energy and Environmental Design points. Daylighting Credit IEQ 8.1 (One Point) Energy Credit EA 1 (One to Two Points) Skylights will not get the views Credit EA 8.2 The most energy-efficient approach is to daylight using only 5% of the roof surface. If you cover a greater area with skylights, the energy losses and gains through the skylight will negate the benefits of daylighting.
How Do Lights Switch On/Off You can t save energy if the lights stay on; therefore, combine fluorescent fixtures with one of the following approaches: Low budget, doesn t work Manual switching High end, too costly Continuous dimming Best approach Stepped dimming (1, 2, or 3) If you have metal halide fixtures, you will need to replace them with fluorescent fixtures. Metal halide does not work well with multiple on/off cycles.
Where To Put Light Sensors If you plan to value engineer your light sensor system, then just leave it out completely. Low budget, doesn t work One sensor on roof High end, too costly One sensor on each fixture Best approach One sensor every 1,500 square feet Sensors need to be located near floor level, and see light levels on the merchandise. Make sure they do not see skylights or artificial lights. Above all, consult with a Registered Professional Engineer first.
Skylights Daylighting will only work in a big-box store if you use skylights. No more than 5% of the roof area should be skylight area. Generally, skylights should be spaced 1.0 to 1.5 times the ceiling height (center to center). For example, if the ceiling height is 12-feet, the skylights should be spaced 12- to 18-feet apart. Don t cut cost here! Only use reputable, local contractors with a good skylight system, who offer an enforceable, blanket roof warranty.
Skylights Additional Don t let your designers talk you into clear skylights. You will cook in the summer! ASHRAE Standard 90.1 recommends a maximum solar heat gain coefficient (SHGC) of 0.2 to 0.3 depending on the building location. This standard can be ordered at the following web site: www.ashrae.org. You want diffused light in the spaces, not direct sunlight. The sun is directly overhead at the warmest time of the year, so don t go with clear skylights.
Interior Colors If your interior designer insists on grey or dark colors on the ceiling and floor, stop now. Daylighting won t work People will not tolerate dark or shadowed ceilings. Bright white would be nice, but off-white is a good compromise. Surface reflectivity of 0.8 or higher. A white ceiling mimics the natural sky. If you have to look up to see if the lights are off, your daylighting is working.
Additional Enhancements Many products are available that claim better results. Review these with a Registered Professional Engineer who has experience with daylighting and skylights. Skylights with sun-tracking ability. The trade off is better performance versus more complexity and maintenance. Drop ceilings. The trade off is another $1.00/s.f. Refer your designers to Skylighting Guidelines, Your Guide to Energy Efficient Design Practices (1998).
Cost Summary These values are general rule-of-thumb numbers. Get a specific quote from your general contractor before making a decision. All values are per square foot (sf) of floor area. Skylights cost $25 to $45/sf of skylight assembly. At 5% roof area, the overall cost is $1.25 to $2.25/sf of building floor area. Lighting controls add $0.50 to $1.00/sf. Total installed cost is about $2.00 to $2.50/sf. So, what s the payback?
Payback Would you believe an immediate payback is possible? Total installed cost is about $2.00 to $2.50/sf. Let s go with the higher number of $2.50/sf. What if you finance the cost of the skylights (which you were planning to do anyway) on a 15-year commercial loan with a 9% APR? Annual repayment = $2.50 X 0.1241 = $0.31/sf. Remember our previous example where we calculated an annual savings of $0.33/sf just for energy?
Conclusion Skylights are an effective means for daylighting. They can reduce energy cost, improve employee health and productivity, and increase sales. Skylights can deliver immediate payback. Above all, hire professionals who know what they are doing. Check their references carefully. See the Energy Center of Wisconsin - Daylighting Collaborative Web Site for more information: www.daylighting.org
Questions? Eric T. Truelove, P.E., LAP, Associate AIA Director of Sustainable Design Services The Renschler Company Madison, Wisconsin 608-833-2321 erict@renschler.com