Portable, sustainable power source converts chemical to electrical energy
The development is based on a discovery announced in 2010 by Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering at MIT, and his co-workers which found that a wire made from carbon nanotubes was capable of producing an electrical current when it is progressively heated from one end to the other. By coating the nanotube with a combustible material and then lighting one end to let it burn like a fuse.
The discovery represented a previously unknown phenomenon, but experiments at the time produced only a minuscule amount of current in a simple laboratory setup. Now, Strano and his team have increased the efficiency of the process more than a thousandfold and have produced devices that can put out power that is, pound for pound, in the same ballpark as what can be produced by today’s best batteries. The researchers caution, however, that it could take several years to develop the concept into a commercializable product.
The research results have been published in the journal Energy & Environmental Science, in a paper by Strano, doctoral students Sayalee Mahajan PhD ’15 and Albert Liu, and five others: “Sustainable power sources based on high efficiency thermopower wave devices.”
“It is actually remarkable that this [phenomenon] hasn’t been studied before,” said Strano. Much of his team’s work on the project has focused on not just improving the efficiency of the process but also “developing the theory of how these things work.” Strano reckons the latest experiments show good agreement between theory and experimental results, providing strong confirmation of the underlying mechanism.
The effect arises as a pulse of heat pushes electrons through the bundle of carbon nanotubes, carrying the electrons with it like a bunch of surfers riding a wave.
One key finding that helped to verify the theory is that sometimes the wave of heat produces a single voltage, but sometimes it produces two different voltage regions at the same time. “Our mathematical model can describe why that occurs,” said Strano. Alternative theories cannot account for this. According to the team’s theory, the thermopower wave “divides into two different components” which sometimes reinforce one another and sometimes counter each other.
According to Strano the improvements in efficiency “brings [the technology] from a laboratory curiosity to being within striking distance of other portable energy technologies,” such as lithium-ion batteries or fuel cells. In the latest version, the device is more than one percent efficient in converting heat energy to electrical energy, the team reports — which is “orders of magnitude more efficient than what’s been reported before.” In fact, the energy efficiency is about 10,000 times greater than that reported in the original discovery paper.
“It took lithium-ion technology 25 years to get where they are” in terms of efficiency, Strano pointed out, whereas this technology has had only about a fifth of that development time. And lithium is flammable if the material ever gets exposed to the open air – unlike the fuel used in the new device, which is much safer and also a renewable resource.
While the initial experiments had used potentially explosive materials to generate the pulse of heat that drives the reaction, the new work uses a much more benign fuel: sucrose, otherwise known as ordinary table sugar. But the team believes that other combustion materials have the potential to generate even higher efficiencies. Unlike other technologies that are specific to a particular chemical formulation, the carbon nanotube-based power system works just on heat, so as better heat sources are developed they could simply be swapped into a system to improve its performance.
The device is already powerful enough to show that it can power simple electronic devices such as an LED light. And unlike batteries that can gradually lose power if they are stored for long periods, the new system should have a virtually indefinite shelf life, suggested Liu. That could make it suitable for uses such as a deep-space probe that remains dormant for many years as it travels to a distant planet and then needs a quick burst of power to send back data when it reaches its destination.
In addition, the new system is scalable for use in the increasingly tiny wearable devices that are emerging. Batteries and fuel cells have limitations that make it difficult to shrink them to tiny sizes, said Mahajan, whereas this system “can scale down to very small limits. The scale of this is unique.”
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