One of the most intriguing energy storage stories of last week was news of a solid-state supercapacitor so tough it could potentially be built into laptop casings, electric vehicle panels and even walls to become the basis of a structural energy storage device.The discovery, uncovered by a team from Vanderbilt University’s Nanomaterials and Energy Devices Laboratory, adds to near-daily reports of potential game-changing improvements to the technology.
So it seemed like a good time to take stock of supercapacitors (and ultracapacitors, but more of that later): what they are, what they can and can’t do in energy storage, how they can be improved, and what the future might hold for the sector.
The first issue to address is the difference between an ultracapacitor and a supercapacitor. And the short answer is: there isn’t any. The terms are interchangeable and refer to electric double layer capacitors, or EDLCs.
Whichever name you chose to use, they store charge from ions in an electrolyte with high surface area electrodes that are made from activated carbon and, more recently, more exotic materials, as we shall see.
Delivering a huge kick of power
The ‘super’ and ‘ultra’ prefixes are due to the fact that they have capacitance values that are typically orders of magnitude higher than traditional capacitors.
Because they deliver capacitance in the Farad rather than microFarad range, supercapacitors (let’s stick with that term from now on) can deliver one huge kick of power in a short space of time, and much quicker than batteries.
Michael Sund, vice president of Maxwell Technologies, a major manufacturer of supercapacitors, goes so far as to describe them as “lighting in a bottle”.
Another advantage as a storage medium is that as charge is stored electrostatically rather than electrochemically, removing the wear and tear of constant chemical reactions, supercaps have long lifetimes, with over half-a-million cycles possible.
They are also resilient in cold conditions, cannot be over-charged and have less tendency to overheat (and burst into flames) than batteries. However, to compare them directly with batteries at all seems a little disingenuous.
Supercapacitors versus batteries?
Despite Elon Musk of Tesla saying that supercaps will “supercede” batteries for grid and electric vehicle applications, we must also consider that he is planning a giant battery, not supercapacitor, factory in the US.
That’s because with current technology, supercaps don’t have the capacity (as opposed to capacitance) to rival similar-sized batteries. They only reach as little as 5%, in fact.
But they are ideal for delivering short bursts of energy, such as acceleration in an electric vehicle. Equally, they can absorb a sudden jolt of electricity, such as power spike on the grid.
Right now, supercaps have their own unique roles to pay in the energy storage pantheon, which is why they are predicted to show a 27% compound annual growth rate from this year to 2020.
Current uses for supercapacitors
Current uses for supercaps include energy recovery in the automotive and rail sectors. Electric buses use them to absorb energy generated when a bus brakes for one of its many stops and then releases the power to help the bus get started from its dead stop.
In Europe, Citroen and Peugeot cars use supercapacitors for the same reason during start-stop urban driving.
Recently, a Korean metro network installed supercapacitor-based braking energy recuperation systems as a money-saving measure and to stabilise voltage throughout the system.
There are other similar projects around the world, including on the Long Island Rail Road. Supercaps have a role to play on the grid, too.
According to Chad Hall, the founder of Ioxus, another major player in the sector: “About 90% of all voltage dips and sags last less then two seconds. Ultracaps can handle these. The last 10% can be handled by batteries.”
A clear role within energy storage
Hall’s company is clearly feeling confident about the future of supercaps, having recently opened a new factory in New York State. Another current application is in wind turbines, for adjusting blades during gales.
Here, supercaps are replacing lead-acid batteries, thanks to their much longer lifespans of 15 to 20 years, as opposed to 18 to 24 months for batteries. The ability of supercaps to operate in a temperature range of -40ºC to 65oC is also an advantage.
So supercaps clearly have their own niche roles and a larger potential working in concert with batteries. But what will it take to improve their capacity, so they can combine their own inherent advantages with those of lithium-ion batteries, for example?
As with batteries, increasing the surface area of the electrodes using various combinations of carbon nanotubes and graphene is one avenue that is currently being explored.
And as the cost of manufacturing of at least some types of graphene seems set to plummet, it looks like supercaps will be able to muscle in on battery territory in the longer term, at least.
Until that time, supercapacitors can step up to the plate and do some of the important energy storage tasks that are beyond the abilities of batteries, and every bit of news about further research is welcome.
Written by Mike Stone