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energy R&D, on the move: light-emitting diodes


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energy R&D, on the move: light-emitting diodes


David Parekh on Energy Use Performance

Since 70 percent of electricity is consumed in our residential and commercial buildings, an energy-efficient built environment is crucial for maximizing the value of our supply-side energy infrastructure. When we talk about energy, ultimately what we really want are energy servicesthe proverbial cold beers and hot showersnot the kilowatt-hours themselves. So, if we can double the number of cold beers we get from one coal fired power plant though better end use energy productivity instead of doubling the number of power plants we build, then without even touching the supply-side infrastructure, we get a more affordable and more environmentally responsible energy system.

A recent example worth championing: the light-emitting diode (LED) that requires only 20–25 percent of the electricity as compared to an incandescent bulb. Though first patents for LEDs were filed in the 1960s amidst work from numerous US corporate and university research groups and the technology gradually found commercial applications as indicator lighting, advances through the 1990s significantly improved LED color production and brightness enough to broaden their use in consumer goods. Shuji Nakamura of Japan’s Nichia Corporation, in particular, is credited with a breakthrough demonstration of the first high-powered blue LED in 1994. Through continued university and private-sector R&D into the chemical composi- tion of the LED, energy efficiency and brightness have approx- imately doubled at a pace of every three years. Costs have also come down. Even though the upfront purchase prices for “drop- in” LEDs remain highif falling rapidlylife-cycle costs over LEDs’ 25x incandescent bulb lifetime are already attractive, especially in commercial-sector general lighting applications in which such savings can be realized sooner.  

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LEDs - Available Today


Technologies that give you greater efficiencies—efficiencies in lighting, air conditioning, heating, and transportation—all have the potential to decrease the cost of doing business for US companies.

— William Perry, Stanford University professor emeritus

LEDs - Available Today


Technologies that give you greater efficiencies—efficiencies in lighting, air conditioning, heating, and transportation—all have the potential to decrease the cost of doing business for US companies.

— William Perry, Stanford University professor emeritus

photos: DOE/flickr

LEDs represent a sustained R&D program that has truly delivered: a no-regrets, enabling technology that has progressed from niche applications to the mainstream. Academic and industry efforts have together resulted in this generation’s contribution to a long, unbroken arc of gains in what is perhaps humanity’s defining technology: light. Today’s household LEDs are approximately 30 times as effective at producing light as the first electric filament lamps, 300 times as efficient as a kerosene lantern, and 1800 times as efficient as a Babylonian sesame oil lamp. Put another wayas estimated by the economist William Nordhausa typical laborer at the outset of the 19th century needed to work 40 hours to afford enough whale oil candles for the equivalent light output of an evening’s use of a single incandescent bulb; an American citizen today, with an LED-equipped lamp fixture, earns that much illumination every second.  

Related: Professor Jim Sweeney of Stanford's Precourt Energy Efficiency Center describes research on efficiency markets and consumer behavior →

High-quality fluorescent lighting replacements

University of Michigan

A University of Michigan clean energy startup looks to inform investors and future customers about a new lighting technology that offers a cleaner, longer-lasting, and higher-quality alternative to today’s fluorescent tubes.

Headquartered in the U-M Tech Transfer’s Venture Accelerator, Arborlight LLC has developed an LED-based, drop-in replacement for the linear, fluorescent tubes commonly used in overhead lighting and other commercial applications. These replacements are mercury free, last more than 50,000 hours, and provide a cost-effective source of uniform, bright light.

“Our current designs suggest that our lamps will be considerably more efficient, more durable, and robust than today’s glass fluorescent tubes,” said Max Shtein, a U-M materials science and engineering professor, who with U-M electrical engineering and computer science professor P. C. Ku conceived of the lighting architecture at the heart of Arborlight. “In addition to this being a commercial opportunity, we could improve energy efficiency in lighting and eliminate over five metric tons of mercury from the waste processing stream each year in the US alone.”

Credit: Amy Mast for the University of Michigan Energy Institute, 2013 

Associate Professors Max Shtein and Pei-cheng Ku, University of Michigan  (photo Laura Rudich)

Associate Professors Max Shtein and Pei-cheng Ku, University of Michigan  (photo Laura Rudich)

Affordable, off-grid LED indoor lighting

Stanford

Outside of the United States, where perhaps one billion people globally rely on kerosene or other direct biomass combustion for lighting, cheap long-lasting battery-powered LEDs are already expanding access to indoor illumination, which the United Nations Foundation estimates to increase family income by about 15 percent. Moreover, this illumination comes without contributing to indoor air pollution from the combustion of solid fuels, which, according to the World Health Organization, is otherwise responsible for perhaps two million deaths worldwide each year—more than 3 percent of all global mortality.

Encouraging wider deployment of lighting is both an engineering and a design question. On the one hand, LEDs that are long lasting, high intensity, and very low power draw have come from decades of basic and applied materials science laboratory research. These are game-changing product attributes. But translating those engineering successes into compelling, problem-solving applications is not automatic.

This challenge is at the core research ongoing at the Stanford mechanical engineering department’s “D.School.” Here, faculty and graduate students are developing cross-disciplinary frameworks that can be used to integrate engineering, business, and behavioral science through a user-centric process they call “design thinking.”

One recent spinoff: an affordable, solar-powered LED lantern for the developing world called “d.light.” The four cofounders were students together at Stanford. Their experiences pointed both to the need for safe and clean indoor lighting in poor countries and to the potential for newly available LED technology to help. Years of prototyping and user testing followed: designing a usable LED and battery package, improving durability and reliability, and testing the distribution, manufacturing, and financing models needed to get their devices into the right hands. Three million lamps have since been sold to customers in Africa and India. 

Related: the Stanford d.School's user-centric "design thinking" approach to innovation education →

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LEDs - Near at Hand


The potential of organic LEDs, similar to organic solar cells, is now only beginning to be fully realized because up until now we have been struggling to improve their lifetime. . . . When we made the first ones in 1980s, they would last an hour. Then we learned how to package them a little better and they would last a month. Today they last a million hours, which is 100 years of continuous use.

— Vladimir Bulovic, MIT School of Engineering associate dean for innovation 

LEDs - Near at Hand


The potential of organic LEDs, similar to organic solar cells, is now only beginning to be fully realized because up until now we have been struggling to improve their lifetime. . . . When we made the first ones in 1980s, they would last an hour. Then we learned how to package them a little better and they would last a month. Today they last a million hours, which is 100 years of continuous use.

— Vladimir Bulovic, MIT School of Engineering associate dean for innovation 

Nanophosphor "Q"LEDs

MIT

Challengers to the technological descendants of Thomas Edison’s incandescent filament light bulb, including halogens, fluorescent tubes, and compact fluorescents, have time and again stepped into the ring. Each promised one major improvement—less energy use per unit of emitted light—but none has successfully delivered the knockout punch. A major weakness? Color production. But today, “quantum dots” that have been commercialized after multiyear research from the lab of MIT professor Vladimir Bulovic are set to make LEDs—Edison’s latest challenger—into a true contender.

LEDs are a type of solid-state lighting—essentially, microchips—that emit a clean light with little energy lost to heat. But even after decades of improvement in their lifetime and performance, LEDs have remained impractical for general indoor illumination in part because of the cold, bluish white light they emit. This is because a common LED is actually blue—it only turns “white” to our eye when the blue photons it emits excite a phosphor coating applied around the bulb itself. The result of this coating trick? Some blue light, and some yellow light.

With Bulovic’s “Q”LED quantum dots, tiny nanophosphor crystals of cadmium selenide again coat the convention blue LED bulb. But tuning the dots’ diameters optically down-converts its light: 1.7 nanometers gives blue, 3 nanometers green, and 5 nanometers red. With the addition of red, not available in conventional phosphors, a cheap gallium nitride blue LED’s color rendering index reaches 91 (near incandescent), looks warm to eyes, and overall efficiency remains high at over 60 lumens per watt. Apples look a rich red again, and LEDs start looking like a smart wager.

Bulovic and partners have formed a spinoff startup company named QD Vision, which brought quantum dots to the market in 2009. The technology is now being applied to LED displays, concentrating solar PV cells, and other military needs.

Related: a 2013 research overview by MIT colleagues, "QLEDs for displays and solid state lighting" →

Professor Vladimir Bulovic demonstrates the similarities between an OLED display and a pickle to the MIT Alumni Association

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LEDs - On the Horizon


Population and electricity use do not correlate. There are about 1.4 billion people who do not have access to electricity, and another 1.6 who have very limited access. So about three billion people have very limited access to electricity, or no electricity. And we’re adding three more billion people within the century in most of those regions. If they start using electricity, what happens then. . . . This is a major issue, because that’s where the growth is going to be.

— Arun Majumdar, Google vice president for energy* 

LEDs - On the Horizon


Population and electricity use do not correlate. There are about 1.4 billion people who do not have access to electricity, and another 1.6 who have very limited access. So about three billion people have very limited access to electricity, or no electricity. And we’re adding three more billion people within the century in most of those regions. If they start using electricity, what happens then. . . . This is a major issue, because that’s where the growth is going to be.

— Arun Majumdar, Google vice president for energy* 

High-efficiency piezoelectric microwire LEDs  

Georgia Tech

Researchers have used zinc oxide microwires to significantly improve the efficiency at which gallium nitride LEDs convert electricity to ultraviolet light. The devices are believed to be the first LEDs that will have performance that has been enhanced by the creation of an electrical charge in a piezoelectric material using the piezo-phototronic effect.

Because of the polarization of ions in the crystals of piezoelectric materials such as zinc oxide, mechanically compressing or otherwise straining structures made from the materials creates a piezoelectric potential—an electrical charge. In the gallium nitride LEDs, Georgia Institute of Technology researchers used the local piezoelectric potential to tune the charge transport at the p-n junction—the diode formed where two semiconductor materials interface. The effect was to increase the rate at which electrons and holes recombined to generate photons, enhancing the external efficiency of the device through improved light emission and higher injection current.

The devices produced increased their emission intensity by a factor of seventeen and boosted injection current by a factor of four when compressive strain of 0.093 percent was applied to the zinc oxide wire. “By utilizing this effect, we can enhance the external efficiency of these devices by a factor of more than four times, up to eight percent,” said Georgia Tech material science and engineering professor Zhong Lin Wang. “From a practical standpoint, this new effect could have many impacts for electro-optical processes—including improvements in the energy efficiency of lighting devices.”

Credit: John Toon for Georgia Tech Research News, 2011 

A low-power polariton laser 

Stanford

Lasers are an unseen backbone of modern society. The physics powering lasers, however, has remained relatively unchanged through fifty years of use. Now, an international research team including Stanford professor Yoshihisa Yamamoto and research associate Na Young Kim has demonstrated a revolutionary electrically driven polariton laser that could significantly improve the efficiency of lasers.

All lasers are based on Einstein’s principle of stimulated emission. Charged particles, such as electrons, exist in discontinuous energy levels like rungs on a ladder. An electron provided with enough energy can become excited and “jump” up to a higher energy level. Excited electrons can spontaneously fall down to an available lower energy level, shooting off the difference in energy as a bit of light called a photon. The process, however, is inefficient: conventional lasers waste energy, unnecessarily exciting electrons to higher energy levels even when the lower levels are too full to accept the excited electrons when they fall.

A polariton laser, however, pairs electrons with so-called “holes” to form another type of particle, an exciton. These excitons are bosons, and an unlimited number of them can inhabit any given energy level. Using bosons in lasers has been a scientific goal for decades, but Yamamoto’s team is the first to successfully build an electrically driven laser using bosons. This change drastically reduces the amount of power required to run the laser. The current iteration of the polariton laser requires two to five times less energy than a comparable conventional laser, but could require 100 times less energy in the future.

“We’re hoping we can replace conventional semiconductor lasers with these polariton lasers in the future,” Kim said. The device is already being utilized by Stanford researchers developing quantum computers and quantum simulators.

Learn More: Thomas Sumner for the Stanford News Service, 2013 


Synthetic magnetism photon control

Stanford

An interdisciplinary team of physicists and engineers has created a device that tames the flow of photons with synthetic magnetism. In fashioning the device, the team has broken what is known in physics as the time-reversal symmetry of light. Breaking time-reversal symmetry in essence introduces a charge on the photons that reacts to the effective magnetic field the way an electron would to a real magnetic field.

For engineers, it means that a photon traveling forward will have different properties than when it is traveling backward, the researchers said, and this yields promising technical possibilities. “The breaking of time-reversal symmetry is crucial as it opens up novel ways to control light. We can, for instance, completely prevent light from traveling backward to eliminate reflection,” said Stanford professor Shanhui Fan.

The Stanford solution capitalizes on recent research into photonic crystals—materials that can confine and release photons. To fashion their device, the team members created a grid of tiny cavities etched in silicon, forming the photonic crystal. By precisely applying electric current to the grid, they can control—or “harmonically tune”—the photonic crystal to synthesize magnetism and exert virtual force upon photons.

The researchers reported that they were able to alter the radius of a photon’s trajectory by varying the electrical current applied to the photonic crystal and by manipulating the speed of the photons as they enter the system. Providing a great degree of precision control over the photons’ path, this dual mechanism allows the researchers to steer the light wherever they like. In essence, once a photon enters the new device it cannot go back. This quality, the researchers believe, will be key to future applications of the technology, as it eliminates disorders such as signal loss common to fiber optics and other light-control mechanisms.

Credit: Andrew Myers for the Stanford School of Engineering, 2012

Professor Zhong Lin Wang, Georgia Institute of Technology

Professor Zhong Lin Wang, Georgia Institute of Technology

Professor Yoshihisa Yamamoto, Stanford + National Institute of Informatics, Japan

Professor Yoshihisa Yamamoto, Stanford + National Institute of Informatics, Japan

Professor Shanhui Fan, Stanford

Professor Shanhui Fan, Stanford

 

 
We’re on the right track with the very significant energy research effort we have today in our research universities, but the key is to continue to innovate—to continue to think in terms of game changing technologies—because without this forward thinking, we will stagnate and fall short of the challenging goals ahead.
— Robert Armstrong, MIT Energy Initiative director and Chevron professor of chemical engineering