Pushing the boundaries of energy—theoretical, practical, and personal

Professor Chen

Mechanical Engineering Professor Gang Chen

Gang Chen—one of the pioneers on the front lines of energy science—is an energy phenomenon himself. The Carl Richard Soderberg Professor of Power Engineering is one of the world’s leading researchers in the nanoengineering of energy transfer. He heads the new Solid State Solar Thermal Energy Conversion Center (S3TEC) with a staff of 50+, serves on the editorial boards of five scientific journals, teaches undergraduate and graduate classes, advises 21 graduate students, 12 postdocs, and two research scientists, and delivers lectures and seminars from one end of the planet to the other. And in whatever time is left at the end of the day, Gang Chen is helping to redefine the laws of physics.

Chen made headlines last summer when he and his colleagues, MIT graduate student Sheng Shen and Columbia University Professor Arvind Narayaswamy (who was also Chen’s student), solved a centuryold problem in the physics of heat transfer. Since 1900, when German physicist Max Planck formulated his blackbody radiation law, scientists have relied on the fact that the law describes the maximum thermal emission possible from any radiating object. Up to a point, that is.

Beyond Planck’s Law

As Planck himself suspected, the theory breaks down when the distance between objects becomes minute. But over the ensuing hundred years, no one has found a way to measure this anomaly precisely. A big part of the challenge has been mechanical—keeping two objects in very close proximity without allowing them to touch. “We tried for many years using parallel plates,” says Chen. The best that he and his colleagues could achieve with this approach, however, was a separation of about one micron (one millionth of a meter).

The ground shifted when the team  reconsidered the shape of one of the objects. They replaced one of the flat plates with a small silica glass bead. “Because there is just a single point of near-contact between the bead and the plate, it proved much easier to close the gap to the nanometer scale,” explains Chen. Using the bead, they’ve reduced the distance between the objects to 30 nanometers (30 billionths of a meter)—one-thirtieth of the gap they had accomplished with two flat surfaces.

Once they had transformed the distance parameter, the team then introduced a bimetallic cantilever from an atomic force microscope. This significantly increased the precision of the temperature and heat flow measurements and produced startling results. It turned out that at the nanoscale, heat transfer between two objects can be 1,000 times greater than Planck’s Law predicts.

Although the force of Chen’s accomplishments is palpable to students and colleagues, the man himself does not betray the extraordinary dynamism that propels his accomplishments. The modest, affable Chen, who is known for his collaborative zeal, will never be the first person in the room to point out that his findings on Planck’s Law, published in the August 2009 issue of the journal Nano Letters, could well have seismic impact.

Chen’s discovery can have many potential applications. The magnetic data recording systems used in computer hard disks, for example, typically have spacing in the five to six nanometer range. Because the recording heads tend to heat up in these devices, researchers have been looking for ways to manage or even exploit the heat to control the spacing. The fundamental insights revealed by Chen and his colleagueswill allow designers to improve the performance of such devices.

Chen also is becoming increasingly excited at possibilities for the development of a new generation of thermophotovoltaics (TPVs)—energy conversion devices that harness the photons emitted by a heat source. “The high photon flux can potentially enable higher efficiency in existing technologies as well as new energy density conversion devices,” Chen says. “We don’t yet know what the limit is on how much heat can be dissipated in closely spaced systems. But current theory won’t be valid once we push down to a one nanometer gap.”

Professor Chen in his office at MIT

Professor Chen in his office at MIT

The birth of S3TEC—and the rebirth of solar

Of course, the plain truth about science in modern times is that discoveries are just a gleam in the eye of the researcher without the funding to move them beyond the theoretical. Gang’s leadership in micro- and nanoscale thermal and mechanical phenomena recently helped MIT secure a $17.5 million grant from the U.S. Department of Energy to establish an Energy Frontier Research Center (EFRC). Chen heads the new initiative, called the Solid State Solar-Thermal Energy Conversion Center or S3TEC. He also oversees a staff of 50+, including 12 co-investigators from MIT, Boston College, and Oak Ridge National Laboratory, 32 graduate students, and six postdocs.

S3TEC’s mission is to lay the scientific groundwork for transformational solid-state solarthermal to electric energy conversion technologies. Working at nanoscale, researchers will be developing materials that can harness and convert solar energy and other man-made heat into electricity using thermoelectric and TPV technologies. “At the fundamental level, we’ll be advancing our understanding of how electrons and phonons move through materials and interfaces,” Chen explains. “This will enable us to engineer materials with significantly improved heat to electricity conversion efficiency.” They also are designing surfaces that maximize radiation absorption and minimize thermal loss so that solar radiation can be efficiently converted into electricity via solid-state heat engines.

S3TEC researchers may well prove to be game-changers for solar electricity generators. Whereas current siliconbased photovoltaic solar cells produce electricity at $3 to $4 per Watt, the next generation of thermoelectrics could reduce that cost to as low at fifty cents per Watt.

Scaling up to gigawatts

S3TEC scientists also will be pushing the structural design of solar TPVs toward its theoretical limits. The highest recorded efficiency of multiple junction solar cells is slightly more than 40%, but theory indicates that it is possible to nearly double that performance with a nanoengineered single junction TPV cell. “If we can understand the fundamental science at nanoscale,” he says, “we’ll stand a better chance of successfully scaling up to gigawatts.” Both thermoelectric and TPV technologies can be applied to terrestrial heat sources such as geothermal and waste heat from industrial production, transportation, and buildings. Thermoelectrics, too, can be combined with existing solar technologies and used for refrigeration and air-conditioning without producing any greenhouse gases.

With a characteristic mix of modesty and candor, Chen pays tribute to the Mens et Manus credo. “Before I came to MIT in 2001, I was primarily focused on fundamental science,” he admits. “I’ve definitely seen an expansion in my thinking to include the real-world impact of research. It’s one of the great strengths of the Institute. Everyone is continually asking the question ‘What’s the potential impact of these findings? How can it be put to work to make life better?’”

Gang Chen has clearly made it his life’s work to find out.meche logo

Explore the work of S3TEC at http://s3tec.mit.edu.