Photovoltaics Without Sunlight

It has long been known that photovoltaic (PV) cells needn't always run on sunlight. 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 that occurring 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 thermal emitter radiates only the wavelengths that the PV diode can absorb to convert them into electricity, while suppressing other wavelengths. A photonic crystal is made by taking a sample of material and creating 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. A research team at MIT 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.

A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. A novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths oflight - selected to match the wave-lengths that photovoltaic cells can best convert to electricity - makes the new system much more efficient than previous versions.The key to this fine-tuned light emission, lies in a material with billions of nanoscale pits etched on its surface. When the material absorbs heat, the pitted surface radiates energy primarily at these carefully chosen wavelengths.

MIT researchers made use of this technology to make a buttonsized 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.

A variety of silicon chip micro-reactors developed by the MIT team contains photonic crystals on both flat faces, with external tubes for injecting fuel and air and ejecting waste products. Inside the chip, the fuel and air react to heat up the photonic crystals. In use, these reactors would have a photovoltaic cell mounted against each face, with a tiny
gap between, to convert the emitted wavelengths of light to electricity.

The button-like device that uses hydrocarbon fuels such as butane or propane as its heat source has at its heart a micro-reactor. The device achieves a fuel-to-electricity conversion efficiency three times greater than that of a lithium-ion battery of the same size and weight, This powerful approach has been widely used to improve lasers, lightemitting diodes and even optical fibres to use it to create several novel electricity-generating devices. The resear-chers are excited about how fundamental research in materials can result in new performance
that enables a whole spectrum of applications for efficient energy conversion.