Battery-powered mobile gadgets allow us do almost anything anywhere: call, shoot video, check stocks, read PNAS First Look until, of course, the power runs out. A new microchip-scale form of power generation could replace common batteries and dramatically prolonging the time we can spend on our favorite devices.
This week in PNAS Early Edition, Ivan Celanovic, John Joannopoulos and colleagues report the prototyping of the first small-scale thermophotovoltaic energy converter. The system, roughly the size of a pinky nail at 1 cm2, runs on propane to produce heat, converting heat into electricity via a photovoltaic cell. Once optimized, researchers predict fuel-burning generators will perform five to 10 times better, pound-per-pound, than current systems.
Due to their chemistry, batteries are inherently limited. One 1 kilogram of lithium-ion battery is able to release roughly 1 kilowatt for about 12 minutes. By contrast, 1 kilogram of hydrocarbons, running at 20% efficiency, holds enough energy to release 1 kilowatt for two hours.
Celanovic and colleagues make a start at harnessing this power through, he says, “a complete power plant system, inside a chip size package.” The generator holds four main components. The microcombustor burns propane and oxygen, though in the future other fuels could be used. A photonic crystal, capable of altering the properties of light that passes through it, filters the thermal energy into a confined band of infrared photons. That thermal radiation is converted to electricity as it passes through photovoltaic cells. The final main component converts the electricity into useful current and voltage levels. Though the temperatures released in the device are very high (700–1,100°C), they are contained within a vacuum, keeping the device surface at room temperature.
“Although conceptually this (system) is a simple idea,” says Celanovic, “building and demonstrating a real system, that works across energy conversion domains ranging from chemical, thermal, optical, and electronic, was a daunting task.”
Celanovic says it’s difficult to predict when chip-scale thermophotovoltaic will be widespread. They will be replaceable devices: once a generator runs out of power, a new fuel cartridge will be swapped in. Before they appear in a laptop, robot, or airplane near you, however, the prototype must be optimized. It now runs with 2.5% conversion efficiency, but this, the team estimates, could be pushed to 32% with the right tweaking.
“We are working hard to make this practical and get it to real-life devices and systems,” Celanovic says.