Supercapacitors Amp Up as an Alternative to Batteries
Recent breakthroughs have made supercapacitors a more viable and potentially safer charging option, but batteries still have advantages in cost and size.
Imagine charging your cell phone in just a few seconds. Or consider how transportation would be transformed if it took only a few minutes to fuel up an electric car.
The technology for rapid-fire power-ups has been around for decades—in supercapacitors. Supercapacitors not only charge faster than batteries, they last longer because they don't suffer the physical toll in charging and discharging that wears down batteries. They also have a number of safety advantages. However, supercapacitors' super size—they have to be much larger to hold the same energy as batteries—and their super-high cost have held them back.
But a number of scientists believe that recent breakthroughs put the fast, reliable, and potentially safer power storage in supercapacitors, sometimes known as ultracapacitors, well within reach of competing better with batteries.
"Ultracapacitors are kind of like lightning in a bottle, if you will," said Michael Sund, vice president of Maxwell Technologies, a leading manufacturer of the new technology, which is selling thousands of the units to charge buses in China.
Safety Woes for Batteries
Anyone who has run low on charge during an important phone call, or has tried to calm a child whose toy truck has suddenly stopped dead knows the limits of batteries. Batteries take a long time to charge, are relatively heavy—a big problem for the electric car market—and their safety often arises as an issue.
Just this summer, a major retailer had to recall thousands of replacement batteries for laptops made by Apple, only one of many makers of laptops and cell phones that have seen their own batteries recalled because of safety concerns. (See related quiz: "What You Don't Know About Batteries.")
Fires involving batteries earlier this year also helped temporarily ground Boeing's new Dreamliner. In one of the worst tragedies traced to battery failure, two crew members died in the 2010 crash of a UPS airplane in Dubai that investigators tied to flames rising from a cargo of batteries. (See related story: "Reshaping Flight For Fuel Efficiency: Five Technologies on the Runway.")
The dangerous pitfalls of battery use are part of what's helping boost renewed interest in supercapacitors.
Safety is much more of an issue than it has been in the past, said Peter Harrop, chairman of IDTechEx, a market research firm based in Cambridge, U.K. He and other fans of the emerging technology contend supercapacitors will thrive as companies seek new and more robust power sources that are also safer than today's batteries.
Instead of the chemicals that make batteries difficult to manage, supercapacitors use a sort of static electricity for storing power. That means their performance is more predictable, their materials are more reliable and less vulnerable to temperature changes, and they can be fully discharged for safer shipping, Harrop said. (See related photos: "Seven Ingredients for Better Electric Car Batteries.")
An Opening for Supercapacitors?
Scientists long have known that energy can be stored as electrical charge, instead of in chemical reactants as in batteries. Benjamin Franklin's famous experiment with rows of Leyden jars, which he called a "battery" after the military term for weapons functioning together, actually was an early version of a capacitor.
But recent breakthroughs in supercapacitor materials could make them competitors to batteries in more applications. "Supercapacitors are improving much faster than are batteries," Harrop said.
Then again, supercapacitors have been on the verge of commercial success for years. A 1995 headline, for example, suggested that ultracapacitors were "surging ahead." But they have remained a small business compared to rechargeable batteries—primarily because they store relatively little energy compared to conventional cells.
In batteries, storing an electrical charge is called "energy density," a distinction from "power density," or how quickly energy is delivered.
The energy density of supercapacitors pales against lithium ion batteries, the technology typically used today in phones and laptops. Lithium ion batteries store perhaps 20 times the energy of supercapacitors for a given weight and size. That means the iPhone 5 might have to be two or three inches thicker to hold a supercapacitor, making for a device that is hardly svelte.
Supercapacitors, on the other hand, excel when it comes to power density. They pack tremendous power—they can be charged quickly and release that power in fast bursts of current. Think of those sharp electrical jolts that can occur after one rubs the wrong way on shag carpeting. Or, perhaps better, think of the streaks of electricity that light up a summer storm.
Supercapacitor manufacturer Maxwell Technologies' largest sales have gone to bus manufacturers, Sund said. Operators use the supercapacitors to capture energy generated when a bus brakes for one of its many stops, and then discharge the power to help the bus get started from its dead stop. For that purpose, supercapacitors can replace batteries entirely on hybrid buses, while all-electric buses require fewer batteries.
That's likely the best way to continue selling supercapacitors, as supplements to batteries or engines that run on fuels, Sund said. "Supercapacitors will often be complementary to batteries," he said. "So we try to stay away from what we call 'battery bashing.'"
Still, there are other places where supercapacitors are replacing batteries entirely. One example is in wind turbines, particularly those that are offshore and difficult to reach. Supercapacitors can provide, for example, the bursts of power needed to adjust turbine blades in changing wind conditions.
Batteries traditionally have met that need. But batteries wear out because their chemicals lose potency over time. Because they don't rely on chemicals for storing electricity, capacitors last much longer, an important factor for turbines, whose height and remote positioning make them expensive to maintain.
Some European cars also use supercapacitors in a fashion similar to buses. The European "micro-hybrid" cars turn themselves off when they would normally be idling. That "start-stop" technology normally runs off batteries alone, but French carmaker PSA uses Maxwell supercapacitors in some of its Citroen and Peugeot cars.
Batteries, though, continue to grab most of the micro-hybrid market because the supercapacitors and accompanying electronics can add a couple hundred dollars to a car's cost. The technology's backers argue that the supercaps cost less in the long run because they last longer than batteries and save more fuel because they work more reliably.
Still, when it comes to micro-hybrid cars, initial purchase price so far trumps efficiency and long-term cost of ownership, Sund said.
Hurdles to Overcome
New materials might help supercapacitors compete better on energy density. Many scientists are focusing on graphene, carbon that is only one atom thick and that has generated much excitement since it was refined about ten years ago. Graphene has proven expensive to manufacture. But a lab recently showed that a cheap, common household device could make graphene in inexpensive, high-quality sheets. A graduate student used a DVD burner to make graphene in a chemistry lab run by Ric Kaner, a professor at the University of California, Los Angeles.
The DVD drive had a feature called LightScribe that etches images onto the surface of the DVDs. It turns out that the laser also converts a common material, graphite oxide, into sheets of graphene. The discovery was described last year in the journal Science.
The laser produces graphene with a characteristic that makes it especially promising for supercapacitors: It emerges with holes, or pores. This highly porous graphene can be stacked many layers deep while both sides of each layer remain accessible. In experiments, that has doubled or tripled the energy density of supercapacitors made with graphene.
A single layer of carbon atoms doesn't store much energy, Kaner said. "It's when you can stack hundreds or even thousands of layers—and that's what we're doing."
He won't predict when the new material might appear in commercial supercapacitors, other than saying he hopes it won't be ten or even five years. Even then, the supercapacitors will likely work in tandem with batteries. "They are not a replacement for batteries, as of yet," Kaner said.
But if they eventually can store enough energy to compete with batteries, supercapacitors have key advantages, including that they can provide high power and be used for millions of cycles, Kaner said. "Also, unlike batteries, they don't overcharge and they don't overheat."
The safety advantages of supercapacitors will grow as the demand for portable energy grows, said Harrop of IDTechEX. Capacitors themselves present their own concerns with safety, because any technology that stores energy is potentially dangerous. But makers are phasing out toxic and flammable chemicals that have been used in supercapacitors, and even those supercapacitors have better safety records than lithium ion batteries, he said.
Meanwhile, battery safety will become more of an issue as the cells get larger, such as those now being used in electric vehicles. The larger a battery, the more likely it is that something can go wrong, Harrop added. "It's easier to make a battery safe for something like a phone than it is for a car."
This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.