The Light Stuff--Which Bulb to Use: Nitty-gritty

August 31, 2011

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Q: I'm a fan of the new LED bulbs because they seem to be more energy efficient than compact fluorescents (CFLs). However, I'm perplexed by terminology, confusing packaging and lack of labeling—how can I compare the efficiency of one light bulb to another? How do I know what size LED bulb should replace a 60W incandescent or 15W CFL? Also, are there any lifecycle analyses on the overall environmental footprint of each kind of bulb?

Asked by Michael McCrystal, '91, San Francisco, Calif.

The back of the label

The back of the new light bulb labels will include a few more important pieces of information for consumers.

  • Light Appearance: Color temperature, measured in degrees Kelvin, is a measure of the color of the light emitted. Higher color temperatures, in the range of 3500 to 5500, indicate a "cooler" color of light—think blue-green hues, similar to the light emitted from an overcast sky. Lower color temperatures, in the range of 2700 to 3500, represent "warmer" colors of light with red-yellow hues more similar to candlelight. If you're lighting a desk-space, cooler light produces higher contrasts and is likely the best choice. For ambient light in living spaces, a warmer color temperature is generally preferable.
  • Life: How many Stanford alumni does it take to screw in a light bulb? Hopefully just one, but in any case most of us usually prefer to do something other than change burnt-out bulbs. In the longevity category, LEDs burn out the competition and can last up to an extraordinary 15 years. Furthermore, unlike CFLs, their longevity is not affected by frequent on-off switching.
  • Mercury content:All bulb packages will be required to divulge mercury content—a lingering environmental drawback of compact fluorescent bulbs, as discussed below.

Lifecycle analysis

Your question brings up the important point that efficiency isn't a tell-all metric of environmental impact. Life Cycle Assessment (LCA), which reviews the impact through all stages of a product's life, from harvest of virgin materials to manufacturing, use and final disposal, is a great approach to further ponder your best bulb choice.

Two recent LCAs conducted by the Carnegie Mellon Green Design Institute and by OSRAM, a prominent German lighting manufacturer, compared the three main residential bulb types we've discussed: LED, CFL and incandescent. The OSRAM analysis broke bulb life into five phases: virgin material, manufacturing, transport, use and end-life. For each bulb, they examined effects on global warming, environmental acidification caused by GHG emissions, eutrophication of water resources, photochemical ozone creation, human toxicity and depletion of crude energy resources.

Both studies found that for all types of bulbs, the "use" phase of the life cycle was vastly dominant for both energy use and environmental impact. The OSRAM study found that LEDs and CFLs are currently similar with respect to most of the environmental categories defined, and both far outshone incandescent bulbs. The OSRAM study concluded, however, that as LEDs become increasingly efficient, they will increasingly be the best choice.

Global Warming Potential for manufacturing and use of all three lamps
Global Warming Potential for manufacturing and use of all three lamps: A representation of Global Warming Potential from the OSRAM study. 25X GLS, 2.5 CFL and 1 X LED lamp represent the numbers of each bulb analyzed, in order to match the lifetime of one LED bulb.

CFLs and LEDS are perhaps most different with regards to human toxicity. Mercury is present in all CFL bulbs and is a human neurotoxin that bio-accumulates in the environment. This issue is particularly relevant to the end-use phase of the LCAs. Breakage of the bulbs during disposal releases mercury vapor and can cause serious environmental contamination. Proper recycling of CFL bulbs is mandatory; however, many bulbs still end up in the trash despite free CFL recycling programs offered by major retailers such as IKEA, Home Depot and Walmart. LEDs do not contain mercury and broken bulbs do not represent a hazard.

LEDs: our light in shining armor?

Light Emitting Diodes (LEDs) have come a long way since their introduction as the ubiquitous red pimples of light used as indicators on electronic devices. LEDs use a chip of semi-conducting material, like a computer chip, "doped" with impurities to create a positive-negative or "p-n" junction. When electrons flow through this p-n junction, they fall to a lower energy level and emit a narrow spectrum of visible light. LEDs are extremely efficient because the wavelengths emitted are entirely visible light. In contrast, incandescent and fluorescent bulbs release wavelengths outside of the visible spectrum that are "lost" as heat, reducing the efficiency of the bulb while also causing it to heat up. LEDs already equal or exceed the efficiency of CFL bulbs. And while CFL bulb development has leveled off, the efficiency and economics of LEDs continue to improve. And while the maximum efficiency of CFL bulbs has leveled off at about 60 lumens per watt, the efficiency and economics of LEDs continue to improve. The DOE projects up to 160 lumens per watt efficacy could be achieved by 2025, approximately doubling the efficiency of LEDs currently on the market.

With the promise of such astounding efficiency, LEDs have quickly become the rising stars of residential lighting. The DOE has poured support into LED development and estimates that if solid-state lighting replaced all existing lights, customers would save $115 billion by 2025 and result in a 10 percent reduction in greenhouse gas emissions.

Despite the buzz, LEDs are not yet as commonplace in home and businesses as their efficiency-stardom should warrant. To explore this apparent contradiction, I spoke with Brad Powley, MS '08, co-author of the report "Projecting LED Competitiveness," and a PhD student at Stanford's Precourt Energy Efficiency Center. Powley noted that penetration of LED technology into commercial and industrial sectors has been more fluid than in the residential sector. "Industries are more likely to use sophisticated decision-making processes, which consider not only the upfront costs [of the bulb], but also the long-term cost-benefit of using LEDs over other technologies," Mr. Powley explained. For example, the City of San Francisco is making the shift to LED bulbs for street lamps. The expense and inconvenience of maintenance and bulb replacement for thousands of 40-foot fixtures around the city made LEDs a clear choice when analyzing the long-term costs.

According to James Broderick, the DOE's lighting program manager, the next 10 years will be a "game changing decade" for LEDs in the home market. The Energy Independence and Security Act of 2007 calls for phasing out low-efficiency bulbs starting in 2012, and a minimum efficacy of 45 lumens per watt by 2020. Increasing availability and affordability of LED bulbs, along with education and outreach programs, will be key factors in boosting LED sales.

Broderick cautions, however, that not all LED bulbs are up to par. "It's important to note that for any LED lighting product, light output and efficacy aren't enough. You still need to consider such factors as color quality, light distribution and lifetime to ensure that product performance is acceptable to consumers. And right now, the data show that many LED replacement lamps do not meet these other product performance metrics." Broderick and the DOE are confident that LEDs will rise to this challenge in the coming decades and recommend Lighting Facts, a voluntary certification and labeling system for LEDs, to ensure you purchase a top quality bulb.

Whole-home lighting design and efficiency

Efficient bulbs alone do not necessarily equate to an energy-efficient home. Thoughtful lighting strategies and habits are also essential to improve lighting efficiency. A few basic tips are outlined below.

  • Lighting scenario: The American Lighting Association defines three main lighting scenarios: ambient, task and accent. Carefully selecting the bulb type and brightness appropriate for each space in your home can reduce the energy demand. For example, task and accent situations play to LED strengths, as they require more directional light, often in tight spaces. CFLs generally provide more diffuse light with warmer color temperatures and greater lumen output. They can be a good choice for ambient lighting, as long as the bulbs are carefully recycled.
  • Sunlight is free: Take advantage of natural light delivered daily by our very own sun. Daylighting not only reduces energy demand in your home—it has also been shown to enhance mood and increase productivity. Thoughtful, integrative design is required for successful daylighting, though. One must consider many factors including climate, orientation of windows and skylights, wall-to-window ratios and reduction of unwanted side effects, such as glare. The Daylighting Collaborative is a good place to begin exploring daylighting strategies.
  • Lighting Controls: It's easy to forget to hit the light switch on the way out the door—thankfully, lighting controls can remember for us. Occupancy and motion sensors detect when you've left the room or when nobody's home. Photosensors register the amount of light in a room and can be used together with daylighting to keep electric lights off when daylight is doing the job.

A recent study on LED efficiency by Sandia National Laboratories notes that historically, technological advances in lighting efficiency are reliably countered by an equal increase in overall light consumption. With sound lighting knowledge and ever-more efficient LED bulbs on the market, we have the opportunity to defy this trend and reduce our environmental footprint from lighting energy demand. Will this be the "game-changing decade" where we make that leap?

MAIKA NICHOLSON is a graduate student in civil and environmental engineering.

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