Power, electrical power is the life blood of our modern world, a constant heartbeat that drives every gadget on the planet. MIT scientist have demonstrated a huge breakthrough in power, a Zippo lighter sized power station that’s three times as powerful as lithium-ion batteries and runs off a tiny cap of methanol. The technology behind the tiny device is able to convert any source of heat into electricity. Called a Thermo-Photovoltaic system – TPV -, the Zippo generator is able to convert heat into specific frequencies of light which is used by a tuned PV cell – solar cell – to generate electricity. While it may sound complicated, there are no moving parts, nothing to wear out and it is made of cheap materials. The team of MIT researchers have managed to produce a cheap and efficient take on the technology previously only available to NASA. Will the Zippo make a come back as every iPhones best friend, maybe.
Themo-Photovoltaic cells – TPV – like the Zippo convert heat into photons which are converted to electricity by solar cell like layer. First demonstrated in 1956 by Henry Kolm, TPV’s didn’t become practical to produce until Pierre Aigrain’s lectures on the subject in 1960. While the basic TPV principle here is the same the technology leap here is based on very tiny holes etched into tungsten. The source of the heat used by this system is virtually unlimited, a camp-fire battery charger could be placed near a fire to charge your phone, exhaust heat from combustion engines becomes a power source. The most contentious heat source of all can even be used, sources of the radioactive type. Used for a long time in space craft there are actually radioactive variants that are very safe, last 30-50 years and burn the radioactive material completely.
The tungsten plays the part of a photonic crystal – fancy way of saying light emitting chip – in the power system. Like a butterflies wings the MIT team covered the surface of a tungsten with tiny specially sized holes. The specifically sized and spaced pits work on the principle that the tiny holes can control the way light behaves when being emmited. Much computer time was spent analysing the effects of different patterns on the light emitted by the tungsten. Varying the size and spacing proved a viable way to tune the system to emit different spectrums of light, using the pattern to tune the Tungsten in effect. Why is it so you ask ? The nano sized pits act as resonators amplifying certain frequencies – waves – of light being emitted by the tungsten, giving preference to certain frequencies and not others allowing the tungsten to emit the best frequencies for the PV – Solar cell – part of the system.
If you have ever seen metal glowing white-hot then you have seen an example of a photonic crystal in action, given all metals are crystal structures and the glowing white light is well light, photons. The MIT team chose to use Tungsten as the photonic emitter in their system for a number of reasons. As you may have noticed with the old incandescent light bulb, Tungsten glows very bright white when hot, it can sustain those temperatures for long periods of time, is relatively cheap and commonly used already.
There are currently a number of systems designed around materials that can convert heat into electricity directly, all of them are very in-efficient and expensive. Called Thermo-electric -TE – materials these substances have the ability to convert heat differentials into voltage. Used most commonly as the power supply for satellites and space craft the ability to create electricity without oxygen – like all combustion technologies – along with zero moving parts, long life spans of typically 30-50 years and are very hardy -able to take a pounding in space hardy- makes these types of power systems the only option for satellites and space probes. The Voyager space craft uses a Plutonium core to heat a thermoelectric material that generates the power for the little space explorer. The TPV system developed by MIT will be in direct competition with TE systems like that used in space craft like Voyager. The cost advantages may even win some customer for the MIT team. Till now the cost to produce TPV’s and TE’s has made wide-spread use cost prohibitive, made using exotic materials and complex processes till now only NASA could afford this technology. Using Tungsten and standard semi-conductor production equipment should change the cost equation, allowing this technology to be used in more than space-craft.
Further efficiencies are gained recovering waste heat, as with all systems there is waste, in this case waste heat. Not all heat passing through the Tungsten is converted, what isn’t converted is reflected back to the tungsten for reabsorption.
Applying TPV heat harvesting systems to modern machines has only just begun. As more efficient techniques are developed more application will be found for the Zippo sized power station. Most mechanical systems that generate waste heat, combustion engines, power transformers, microprocessors could all gain improved efficiency by converting waste heat to electricity. Zippo power has already won our hearts but can it win our wallets. Having to compete with the likes of a world of batteries and fuel cells Zippo has a lot of ground to make up. Although with the worlds growing portable power needs growing so fast can one Energizer Bunny be enough, we think not.
The MIT team consisted of YiXiang Yeng (EECS), Ivan Celanovic ScD ’06 (ISN); associate professor Marin Soljačić and Walker Chan, (EECS),