Solar Cells Like LEDs
Researchers from the University of California at Berkeley have suggested and demonstrated a solar cell concept more like that of LEDs to allow solar cells to be able to emit light as well as absorb it—a counterintuitive concept because currently to produce the maximum amount of energy, solar cells are designed to absorb as much light from the Sun as possible.
Principal researcher and UC Berkeley professor of electrical engineering Eli Yablonovitch said, “What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce.”
Yablonovitch and his colleagues have been trying to understand why there has been such a large gap between the theoretical limit of 33.5% and the limit that researchers have been able to achieve of 26%. As they worked, a “coherent picture emerged,” explained Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch’s group. They came across a relatively simple, if perhaps counterintuitive, solution based on a mathematical connection between absorption and emission of light. “Fundamentally, it’s because there’s a thermodynamic link between absorption and emission.”
Designing solar cells to emit light—so that photons do not become “lost” within a cell—has the natural effect of increasing the voltage produced by the solar cell. If there is a solar cell that is a good emitter of light, it also makes it produce a higher voltage, which in turn increases the amount of electrical energy that can be harvested from the cell for each unit of sunlight.
The Berkeley research team will present its findings at the Conference on Lasers and Electro Optics (CLEO: 2012), to be held May 6-11 in San Jose, Calif.
Harvesting Energy with Nanotechnology
With 58% of the energy generated in the United States wasted as heat, researchers at Purdue University are working on a technique that uses nanotechnology to harvest energy from hot pipes or engine components to potentially recover energy wasted in factories, power plants and cars. “If we could get just 10% back, that would allow us to reduce energy consumption and power plant emissions considerably,” said Yue Wu, a Purdue University assistant professor of chemical engineering.
The technique utilizes glass fibers coated with a thermoelectric material containing nanocrystals of lead telluride they developed, then exposed to heat in an annealing process to fuse the crystals together. When the materials are heated on one side, electrons flow to the cooler side, generating an electrical current. These coated fibers could be used to create a solid-state cooling technology that does not require compressors and chemical refrigerants or even be woven into a fabric to make cooling garments. The fibers could be wrapped around industrial pipes in factories and power plants, as well as on car engines and automotive exhaust systems, to recapture much of the wasted energy. The ‘energy harvesting’ technology might dramatically reduce how much heat is lost, Wu said.
These findings were detailed in a research paper appearing last month in the journal Nano Letters. The paper was written by Daxin Liang, a former Purdue exchange student from Jilin University in China; Purdue graduate students Scott Finefrock and Haoran Yang; and Wu.
Today’s high-performance thermoelectric materials are brittle. The devices are formed from large discs or blocks, which requires using a lot of material. But the new flexible devices would conform to the irregular shapes of engines and exhaust pipes while using a small fraction of the material required for conventional thermoelectric devices, researchers said.
“This approach yields the same level of performance as conventional thermoelectric materials but it requires the use of much less material, which leads to lower cost and is practical for mass production,” Wu said. Further, this approach promises a method that can be scaled up to industrial processes, making mass production feasible.
—Ann Steffora Mutschler