How Well Can LEDs Meet the Energy Challenge?
As president of LED Lighting Technologies, Dr.Nisa Khan consults in the solid-state lighting industry and educates consumers about LED lighting. She has a bachelor’s degree in physics and mathematics, and master’s and Ph.D. degrees in electrical engineering. Email her at email@example.com
The energy challenge can be significantly surmounted by reducing energy consumption and adopting energy-efficient solutions in such areas as transportation, heating and air conditioning, refrigeration and lighting, and such media appliances as TVs, computers and more.
It’s not news that Washington and the news media customarily focus on energy use and related environmental challenges – but today’s momentous reality is BP’s massive oil spill in the Gulf of Mexico. Because the out-of-control spill and its related recovery costs will be counted in billions of dollars, routine environmental news has leaped to the front page. Albeit momentous, the spill fits alongside other critical, but less headlined, energy challenges, because energy conservation and its related costs have become the world’s most pressing economic and political concern.
Because I’m a scientist, columnist and investment adviser, I often research energy and technical related issues and their interrelated uses and costs. I can’t control an oil spill, but I can help LED lighting become more efficient and concurrently educate LED users so they make intelligent buying choices. Combined, these two aspects will help move the SSL industry forward and, in part, aid energy conservation.
The U.S. Dept. of Energy (DOE) says lighting accounts for more than 20% of all energy usage, both domestically and globally. A significant part of this 20% is industrial-lighting applications. For example, the most common commercial lighting unit, the T8 linear fluorescent lamp (LFL) tube is commonly arrayed in office and workplace ceilings. Typically, offices are lit with multi-lamped, 4-ft. long, T8-based luminaires while large stores and warehouses – and cabinet signs – are often illuminated with multi-lamped, 8-ft. long, T12-based luminaires.
Many older, installed luminaires have lower efficacies and consume a great deal more energy than the newer, high-efficiency lamps. According to Dr. James Broderick, the DOE’s emerging technology program manager, tens of millions of these fixtures are switched on daily. Upgrading them to more efficient luminaires would lead to tremendous energy savings, and, because LED lamps are touted as the next-generation, energy-efficient lighting solution, the question rises: How do LED-based T8-equivalent luminaires fare against the current T8s?
The answer is extensive, but, because LED technologies have demonstrated significantly higher efficacies, light output and color quality in the last few years, many SSL manufacturers are convinced that LED-based, T8-equivalent luminaires are ready to outperform their LFL counterparts. Their contention? White, HB LEDs can produce the same (or higher) efficacies as LFLs. Thus, a T8-equivalent constructed with numerous, surface-mounted LEDs will easily match or outperform T8-LFLs.
My analysis reveals the concept as presently invalid. Additionally, a report by the DOE Commercially Available LED Product Evaluation and Reporting (CALiPER) program, which supports testing various SSL products available for general illumination, confirms my comparative, T8 analysis. I contend the energy-efficiency argument with LEDs for all major light sources is premature at this time.
However, LEDs are well suited to replace such-small area illumination uses as retail, display, refrigeration and task lighting, but warehouses, large stores and office buildings are better served with fluorescent lights, from both cost and energy-saving perspectives.
Compared to other light sources, inorganic LED lamps are small area devices, particularly in relation to LFLs. Another important difference is that LEDs are flat or planar devices whereas LFLs are cylindrical tubes that distribute light outward, radially, from the lamp’s glass tube and along its length. Large areas – offices, shops -- can be more uniformly and brightly illuminated with ceiling-mounted LFLs arrays, because, for LED lighting to achieve a similar effect, individual LED light sources must be densely arranged around a T8 sized tube – which isn’t currently possible with surface-mount technology (SMT) LED lamps that have a crowned, lamp device as the light source.
Even with a denser (than currently possible) arrangement of LED chips installed within a T8-tube structure, the LED-sourced illumination wouldn’t be as broad or uniform as a standard T8, because LED lamps output light directionally. An accordingly equipped LED-based T8 would produce considerably less light than the LFLs, if the lumen per square meter were similar.
For illumination, increasing the unit-area brightness beyond a certain level isn’t vital. It’s more vital to acquire a brightness level that uniformly spans a broader area. Substantially increasing unit-area brightness (for example making each LED lamp brighter with either higher feed current or higher efficacy) beyond this level isn’t useful as long as LEDs themselves remain small, planar and directional.
To construct a 4-ft LED, T8-equivalent lamp, the experimenting engineers must pack multiple, SMT LEDs into a 4-ft.-long, 1-in. diameter tube. Using current LED technology, this experiment, at best, would produce a 50% duty cycle for the spatial light sources and leave the other 50% dark. The measurable light that emits from such an LED-based T8 would be limited to half the optical power of a conventional T8 with similar unit-area brightness, because the LFL produces fairly continuous light (almost 100% duty cycle) all along the tube length and 360º from the tube surface. An LED-based T8 wouldn’t illuminate as uniformly. Further, the illumination would be confined to a contracted region because light emanating from each discrete LED is directional (narrow), which makes such as system unsuitable for illuminating such large spaces as stores, shops and warehouses.
Some argue that when the unit area brightness is increased, meaning, when individual LEDs produce more lumens per square area without driving them harder (improving the efficacy by a factor of 2, that is, to 150 lm/W), the LED T8’s light output will be the same, or more than, that of the LFL T8.
This may be true when the total light output is compared from an integrated sphere or with goniophotometer measurements, but the uniformity of the light distribution won’t improve, and each exposed LED will appear too bright for the naked eye. Such high brightness may not be good for the human eye, and I wouldn’t be surprised if it would be rated hazardous for viewing. Using a translucent cover on top will reduce the luminaire efficiency and lessen the energy efficiency argument for LED T8s.
The LED lamp-quantity requirements for a large-scale LFL replacement are a further challenge. Typical LED-chip sizes, quantities, overall module sizes and practical gap limits, when arranged on a 4-ft.-long T8 tube, yield approximately 450 SMT units per T8. For example, to replace 20 million T8s would require more than 9 billion HB-LEDs -- a nearly impossible challenge for many years due to the high demands in LCD-screen backlighting, manufacturing and LED-chip, material-shortage disputes. Fewer LED lamps per T8 would produce lower uniformity and higher brightness requirements – not necessarily a better solution.
The DOE recently compared 12 LED T8s from several SSL manufacturers to typical LFL T8s. The results? Even the best-performing LED T8s produced roughly 1,500 lumens, whereas the LFL counterparts routinely produced more than 3,000 lumens. The efficacy comparison yielded 75 lm/W for LEDs and only slightly less for LFLs. This comparison proved the impracticality of doubling the SMT LEDs or LED-chip count in an LED T8 to produce twice the output, because of the 50% duty cycle practical limit mentioned earlier.
The study also showed the luminaire efficiency in LED T8s mounted in similar fixtures is higher (83%) than in LFL T8s (66%), which makes sense because LED light escapes directionally in T8 structures; thus, it encounters less obstruction from fixture components.
Nor is there much use in putting the LED SMT modules on the T8-type tube’s topside, the segment that faces the ceiling, because LED lamps’ highly directional light won’t efficiently escape from the fixture.
Additional challenges arise when attempting to replace LFL T8s with LED T8s in commercial buildings, because LFL fixtures enclose ballasts that aren’t compatible with LED-driver requirements. Removing the ballasts would impose both regulatory and cost barriers; leaving them installed would entail adopting special LED drivers for handling high-voltage outputs from the ballasts.
The DOE-sampled LED T8s’ costs ranged from $50 to $150, whereas a LFL T8, with twice the optical power, costs $3. And, the long-life argument for LED T8s is weak, despite the LED lifespan claim of 50,000 hours. The LFLs are rated for 24,000 to 36,000 hours.
Nine dollars worth of LFL T8s would outperform the expensive,T8-style LEDs.
LFLs remain superior for broad-area illumination because of their large sizes and cylindrical shapes. Theoretically, LEDs with much broader areas and flexible properties, such as OLEDs, may be better suited for general illumination -- but as I have written in earlier columns, currently, OLEDs are less mature and have shorter lifespans. Thus, LEDs remain a special type of light source -- one that is certainly compatible with digital technology and sign-illumination applications where the lamps are placed close to illuminated sign faces, such as in channel letters and cabinet signs. In these applications, also, overly bright LEDs effectively remain behind translucent surfaces. n