Solid-State Lighting Can Reduce C02 Emissions
October / November 2008
Volume 6 Number 5
Recent issues surrounding CO2 emissions and energy-consumption efficiency are closely tied to the arena of home and business lighting, which account for about 20 percent of global electricity use, or about 7 percent of global energy use.
With standard incandescent lighting, we have, for over a century, been generating much more heat than light—€”for every watt of electricity consumed, incandescents use only 5 percent of that energy to produce light. While, in contrast, fluorescent lighting reaches 25 percent efficiency, scientists at Sandia National Laboratories fully subscribe to the possibility of at least doubling that efficiency, to 50 percent, with solid state lighting, known as SSL.
A common experience of such LEDs (light-emitting diodes) occurs in the red lights on cell phones, and mp3 players, where they sip energy from the devices' battery. There are also several types of diode lasers, as well—€”these can be thought of as super-LEDs, where the semiconductor's light energy is optically or electrically amplified.
In the public sector, SSL is best exemplified by red-LED traffic Lights and the savings in electricity and dollars to municipalities are significant.
Unfortunately, while red and infrared LEDs have been relatively easy to optimize, some other spectral colors have proven more difficult. A phenomenon known as nonradiative recombination severely constrains green-wavelength emission, diminishing the number of lumens per watt (and hence, the efficiency) obtained. And since combining green, red and blue LEDs is one efficient route to white LED lighting (and thus, everyday home and business illumination), this issue is crucial.
Hence, based on 1990s Sandia research into optoelectronics, the science of producing and extracting light from electronically excited materials, the Laboratory Directed Research and Development program at Sandia funded a Solid State Lighting Grand Challenge from FY 2001 through FY 2003 to the approximately $7 million level. From its inception, the scope was quite broad, ranging from fundamental materials physics problems to assembly of these materials into the layered structures comprising LEDs, to methodologies for enhancing light extraction, and even into related issues such as practical applications for LEDs in areas other than lighting (for example in biothreat detection).
And from the beginning, there were interested corporate partners, such as the Silicon Valley lighting company, Lumileds. Significantly, it was also during the grand challenge period when DOE recognized the great need for a roadmap.
Combined private/public arousal marked a recognition of the enormous market and global impact potential for LED technology. Given that, in addition to markedly increasing efficiency, LEDs can bring lighting to regions where candles and oil lamps still form the primary lighting mechanisms, these developments presaged a transition of inherently inexorable momentum. From the standpoints of peak oil and global climate change, there may possibly be no more important single transition than LED lighting (symbolized by LED lighting of the entire swimming venue at the 2008 Olympic games in Beijing). In response, DOE initiated a series of technology roadmap workshops and reports, with Sandia chosen as a major contributor and reports editor. With technical support from Sandia, DOE established an SSL R&D program and concomitant product development program, collectively known as the Next Generation Lighting Initiative. In 2007, Sandia was chosen as the lead lab of the National Center for Solid State Lighting R&D. "We worked hard . . . the center didn't just fall into our laps," emphasizes senior manager Jerry Simmons.
With the center came FY 2007-2008 funding of $5 million, $3.2 million for several Sandia projects; but despite a strong backing from Senator Jeff Bingaman (D, N.M.) it has been unclear whether additional center funding will be forthcoming; Bingaman had earlier played a key role in SSL funding authorization that appeared in the 2005 Energy Bill.
The Lumileds partnership has since failed to develop in a completely meaningful fashion, although the connection with Sandia is sustained on an intellectual level, according to project staff. Lumileds became a wholly owned Philips Inc.
subsidiary in 2006, and in a $40 billion annual market, part of the block to collaboration is legal, tracing back to a change in federal law defining the means whereby small companies, DOE labs and universities may retain control of intellectual property developed through federally funded research.
Although the basic physics of incandescent, fluorescent or LED is straightforward, the nuances can be excruciatingly challenging. Add electrical energy to matter and that extra energy can serve to raise electrons to higher energy levels. As certain of these electrons drop back to lower energy states, some of the energy difference is liberated as emitted light (some as heat). Depending on the exact nature of the energetic transitions (governed by quantum mechanics), that light can be infrared, visible as various energies (colors), ultraviolet (UV) or beyond.
Key among the current LDRD-supported projects are nanofabrication and characterization of various types of photonic crystals. These crystals are essentially periodic arrangements designed to improve the emission of photons and their escape to the surroundings as useful illumination. This latter research area is every bit as critical as that of light generation. Understanding how to improve light extraction and better appreciating the sources and mechanisms of energy loss are both critical to improving efficiency, the name of the game for making SSL competitive.
A related piece to the materials puzzle is cost-competitive fabrication using the most-favorable materials and chip manufacturing technologies. Along the way, ingenious LDRD-funded developments—€”such as the tungsten photonic lattice—€”have shown promise. Yet, their ultimate contribution remains to be realized although commercial ventures based on Sandia nanoengineering have arisen and are now reputedly poised to contribute product solutions.
It is also important to note that although refinements involving infrared-emitting solid-state devices do not always contribute to solutions for visible lighting, IR remains the light-encoded medium of choice for a huge number of applications, used in electronics of all types—€”remote controls for TVs and other electronics exemplifying a small slice of that market.
A key development from the Sandia standpoint is the 2006 formation of the Sandia-led National Institute for Nano-Engineering. NINE members include many corporations, such as Intel, IBM and Lockheed Martin, as well as numerous prominent universities. Perhaps SSL may ultimately receive some attention as the "flip-side" of photovoltaic technology: in the former, electrons drop to lower-energy states accompanied by light emission; in the latter, sunlight absorption raises electrons to higher-energy states, thereby generating electrical currents (the fundamental energy transformation in green-plant photosynthesis).
A current LDRD in partnership with Rensselaer Polytechnic Institute exemplifies the basic materials science effort to understand the path toward high-efficiency green LEDs in the context of the more-fundamental physics and nanoengineering issues. The solution should eventually turn the planet toward an economically feasible display of solid-state lighting; a globally extended version of the 18-million LEDs currently populating the NASDAQ display perennially blazing above New York's Times Square.
Vin LoPresti writes for Sandia National Laboratories.
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