Photovoltaic cells are best known for turning sunlight into electrical power — and they're big business. But did you know that there's a type of PV cell that eats heat instead of light to make power? It could replace the Li-ion battery in your cell phone, and it may also be used to scavenge waste heat from almost anything that normally dumps it into the environment, from your TV's electronics to your car's engine (even an electrical one).
Thermal PV tech has been around ages, and works the same way as the solar variation: Incoming radiation excites the atoms in its semiconductor structure, which then push electrons out — generating current. And much as is the case for solar PV cells, the advances in the tech have all been about improving their efficiency.
Scientists at MIT have recently honed this tech, pushing the efficiency up so far that thermal PV cells are now a viable alternative to all sorts of other tech. MIT's breakthrough was to add a layer of tungsten to the front of a PV cell, with a surface that's been etched on a nanoscopic scale so that when heated it emits infrared light (heat) at wavelengths precisely tuned to the best efficiency of the PV cell behind it.
Right now, MIT is building the tech into tiny silicon micro-reactors. These are tiny furnaces that burn butane to generate heat, and then extract the heat to produce electricity. If that sounds convoluted, then this will impress you: The microreactors are small enough to replace the button cell Li-ion batteries you find in devices such as watches, and convert the chemical ingredients that make them tick with three times better efficiency than Li-ion can match. Better yet, when they run out of fuel you simply snap in an extra cartridge of butane to recharge them.
But because MIT's system is so very efficient, and is based on a material that's not too rare or expensive, it could be used to build fuel-less heat-scavenging units that are stuck inside all sorts of devices to recover the wasted heat energy that nearly every machine we've ever made kicks out (thanks to the lovely laws of physics). How about the hot back of your TV? Or the hot chassis of the electric motor in your Nissan Leaf? Let's get fanciful and imagine it would be possible to claw back a few milliwatts of energy from the hot shower water you simply let run down the drain.
The MIT team is confident it can further improve the energy density of its micro-reactors though, and they're confident it could get to the point your smartphone could go for a week without needing a recharge, just from its own heat.
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