How ultracapacitors work (and why they fall short)  

Earth2tech has a look at the state of the ultracapacitor market - How ultracapacitors work (and why they fall short).

Already, Schindall believes some electric vehicle manufacturers are using ultracapacitors for acceleration. The devices also appear in hundreds of other applications, from cell phone base stations to alarm clocks (as backup power) to audio systems.

For most music, Schindall explained, a high-end audio system with big speakers might do just fine with a 1-watt amplifier. “But then the kettle drum comes in,” demanding a sudden power surge of 1-kilowatt. One solution, Schindall said, is to build a 1-watt supply, plus an ultracapacitor to handle the peak.

Ultracapacitors hold promise for a similar job on the electric grid. Today, transmission lines operate below full capacity (often somewhere above 90 percent), said Schindall, in order to leave a buffer for power surges. Banks of ultracapacitors could be set up to absorb power surges, enabling transmission lines to run closer to 100 percent capacity.

It might not seem like much, especially considering that it would take warehouse-sized banks for ultracaps to do the job. But installing ultracapacitors to handle the peaks would actually be much cheaper, Schindall said, than adding even 5 percent more capacity with new transmission lines.

In cars, ultracapacitors could play a role in the growing market for “microhybrids,” which cut the engine during idling. In these “start-stop” systems, Schindall explained in an email, “The ultracapacitor would provide power during the stop (lights, radio, air conditioner, etc.).” It would also provide power for the restart, and then be “recharged during the next interval of travel.”

How to build better ultracapacitors

There are two basic ways to improve the performance of ultracapacitors: increase the surface area of the plate coating, and increase the maximum amount of voltage that the device can handle.

Recall old Faraday again. Capacitance, measured in Farads, is how much electric energy our device will hold given a certain voltage. Increase the voltage, and you can increase the amount of energy our device holds (energy is equal to half the capacitance, multiplied by voltage squared).

Schindall is tackling the surface area challenge using carbon nanotubes (more like a shag carpet or paintbrush than the sponge-like activated carbon). Other researchers, he noted, are working with graphene or better activated carbon. In addition to boosting the surface area, carbon nanotubes and graphene can also “withstand a somewhat higher voltage” than activated carbon, said Schindall.

The voltage challenge, meanwhile “seems to be a tougher road,” he said. Researchers are experimenting with ionic liquid electrolytes (all ion, no solvent, behaves like a liquid), which under the right conditions can operate at up to three times the voltage of conventional electrolytes.

But ionic liquids are “fussy,” Schindall said. “They don’t like being liquids,” and tend to freeze below room temperature. They’re also expensive, and they have higher resistance than conventional electrolytes, which means you can’t get energy out as fast. The maximum power—one of ultracaps’ key advantages—is decreased. As Schindall put it, “There’s always a tradeoff.”