July 28, 2011

More Efficient Sun-free photovoltaics

Using new nanofabrication techniques, MIT researchers made these samples of tungsten with billions of regularly spaced, uniform nanoscale holes on their surfaces. In their TVP system, this type of photonic crystal serves as a thermal emitter, absorbing heat and then—because of its surface structure—radiating to the PV diode only those wavelengths that the diode can convert into electricity. The inset shows a digital photo of the full 1 cm-diameter sample, illuminated by white light. The color suggests the diffraction of white light into green as a result of the surface pattern.

A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. While the principle involved is not new, a novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths of light — selected to match the wavelengths that photovoltaic cells can best convert to electricity — makes the new system much more efficient than previous versions.

They used a slab of tungsten, engineering billions of tiny pits on its surface. When the slab heats up, it generates bright light with an altered emission spectrum because each pit acts as a resonator, capable of giving off radiation at only certain wavelengths.

In this novel MIT design, input heat from an energy source raises the temperature of the tungsten photonic crystal, which transmits radiative heat at selected wavelengths to the PV diode. A second photonic crystal—mounted on the face of the PV diode—lets through heat at wave- lengths that the diode can convert into electricity and reflects the rest back to the tungsten photonic crystal, where it is reabsorbed and reemitted. Electricity from the PV diode passes to an electronic circuit that adjusts its voltage to match the external device being powered.

Prototypes of their micro-TPV power generator are "pretty exciting," says Celanovic. The devices achieve a fuel-to-electricity conversion efficiency of about 3%—a ratio that may not sound impressive, but at that efficiency their energy output is three times greater than that of a lithium ion battery of the same size and weight. The TPV power generator can thus run three times longer without recharging, and then recharging is instantaneous: just snap in a new cartridge of butane. With further work on packaging and system design, Celanovic is confident that they can triple their current energy density. "At that point, our TPV generator could power your smart phone for a whole week without being recharged," he says.

This diagram demonstrates how manipulating the nanostructure of the tungsten photonic crystal can affect the spectrum of the light it emits. (Emittance is an indicator of radiation efficiency.) In this example, the three colored spectra come from heated tungsten samples that contain nanoscale holes of differing diameters, depths, and spacing. Those differing geometries dramatically change the dominant wavelengths in the emitted light. The spectrum drawn in black is from a sample of tungsten with a smooth surface

The key to this fine-tuned light emission, described in the journal Physical Review A, lies in a material with billions of nanoscale pits etched on its surface. When the material absorbs heat — whether from the sun, a hydrocarbon fuel, a decaying radioisotope or any other source — the pitted surface radiates energy primarily at these carefully chosen wavelengths.

Based on that technology, MIT researchers have made a button-sized power generator fueled by butane that can run three times longer than a lithium-ion battery of the same weight; the device can then be recharged instantly, just by snapping in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope that steadily produces heat from radioactive decay, could generate electricity for 30 years without refueling or servicing — an ideal source of electricity for spacecraft headed on long missions away from the sun.

Half a century ago, researchers developed thermophotovoltaics (TPV), which couple a PV cell with any source of heat: A burning hydrocarbon, for example, heats up a material called the thermal emitter, which radiates heat and light onto the PV diode, generating electricity. The thermal emitter's radiation includes far more infrared wavelengths than occur in the solar spectrum, and "low band-gap" PV materials invented less than a decade ago can absorb more of that infrared radiation than standard silicon PVs can. But much of the heat is still wasted, so efficiencies remain relatively low.

The solution, Celanovic says, is to design a thermal emitter that radiates only the wavelengths that the PV diode can absorb and convert into electricity, while suppressing other wavelengths. "But how do we find a material that has this magical property of emitting only at the wavelengths that we want?" asks Marin Soljačić, professor of physics and ISN researcher. The answer: Make a photonic crystal by taking a sample of material and create some nanoscale features on its surface — say, a regularly repeating pattern of holes or ridges — so light propagates through the sample in a dramatically different way.

"By choosing how we design the nanostructure, we can create materials that have novel optical properties," Soljačić says. "This gives us the ability to control and manipulate the behavior of light."

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