When it comes to electric-vehicle design, supercapacitors have always taken a back seat to batteries. A battery not only produces and stores energy but also delivers energy over a relatively long period of time, which translates into an extended EV driving range.
A supercapacitor, in contrast—which is essentially a latter-day Leyden jar—dispenses its energy over a much briefer period of time, making the technology suited for short power boosts.
Supercapacitors can be charged rapidly and thus can repeatedly release power in quick bursts of current so they are useful for regenerative braking and stop-start systems, according to Cosmin Laslau, an analyst at Lux Research, a high-tech business research and advisory firm. A few European and Japanese microhybrids employ supercapacitors so they can turn themselves off when their engines would normally be idling or so braking energy can be captured.
The largest application of the technology is in a fleet of some 10,000 hybrid diesel buses in China.
The current annual market for superconductors, which also includes wind turbine blade controls and specialized consumer electronics, is a modest $366 million, but it is expected to grow at 18% a year, he said. Lux Research estimates that by 2018 the global annual market for heavy truck applications of supercapacitors will reach $323 million while the trade for passenger vehicles should hit $152 million, constituting the largest industry segment of a total predicted market of $836 million.
Lux recently published these numbers in a market forecast report titled “Power Play: Supercapacitor Innovation for Growth in Transportation and Electronics.”
Rapid power at hand
Unlike batteries, which create current using kinetically slow chemical reactions, supercapacitors store energy through the separation of highly mobile electrical charges on the surfaces of electrodes, which means they produce current rapidly. Each "symmetric" supercap typically contains a pair of identical metal plates that are coated with activated carbon. Activated carbon is made more porous, thus greater surface area and charge-holding through either physical or chemical means.
The electrode pair is immersed in a liquid organic electrolyte that speeds the transport of charge. When fully charged, each carbon electrode has two layers of charge carriers coating its surface. This is why supercapacitors are sometimes referred to as double-layer capacitors (or alternatively, ultracapacitors). The cells can look like Red Bull cans or flexible pouches.
Supercaps in cars
One of the first auto industry applications of supercapacitors was by Honda. Its in-house-developed ultracapacitor was used in the 2002 version of the company’s FCX fuel-cell research car to provide power boosts for passing and hill climbing.
Production use in the auto industry began a few years ago when PSA began installing $40 pairs of supercap cells from Maxwell Technologies for the e-HDI systems in some Citroën and Peugeot microhybrids. Likewise, some Mazda microhybrids contain an i-ELOOP system with 10 Nippon Chemi-con cells (about $130) that adds brake-energy-recapture functions to the stop-start operations.
Proponents say that supercapacitors can cost less in the long run because they can operate reliably much longer than batteries can. But batteries dominate the microhybrid energy-storage market because supercapacitors and the accompanying electronics—a dc-to-ac converter, a generator—can add a couple hundred dollars to a car’s price tag, not to mention take up scarce space and add weight, Laslau said.
Car makers are considering even more powerful supercapacitor units that could function as a mild hybrid or peak-load enhancer, he noted. “Several OEM engineers mentioned interest pursuing more aggressive applications that use from 30 to 50 supercapacitor cells to run mild hybrids. It could provide efficiency savings of 5 to 7%.”
Diesel hybrid buses
The biggest installation of supercapacitors that have been deployed thus far operate more than 10,000 hybrid diesel buses in China, he reported. Each contains a $15,000 pack of about 300 supercapacitors. The key adoption driver in this case was a government subsidy that fully financed the green power storage systems, which would otherwise double the $70,000 cost of a standard diesel bus.
Bus operators charge the supercapacitors to capture energy during braking and then discharge the cells to get going. In this case, supercapacitors can replace batteries entirely, whereas all-electric buses can use fewer batteries. The hybrid systems boost fuel efficiency by 25% to 30%, the analyst said, who added that “since the government subsidy expired, sales have plummeted.”
Global supercapacitor makers include Maxwell, Nippon Chemi-con, LS Ultracapacitor, Ioxus, Elton, Nesscap, Vina Tech, and Cap-XX.
Available supercap electrodes provide a specific capacitance (the main figure of merit) of 100 F/g at a cost of $28/kg, but materials developers are coming up with potentially better performing options. One high-end chemically activated carbon is made from a coke feedstock by Power Carbon Technology, a joint venture of GS Caltex and Nippon Oil, and offers 135 F/g at $110/kg.
And new nanostructured carbons are starting to enter pilot production, said Laslau. One firm, NanoCarbons, is producing a chemically activated material that is based on rayon fiber (135 F/g,), while another, EnerG2, is using a sol-gel process to make high-purity, stable carbon with “tunable porosity” that could reach 150 F/g, perhaps at $60/kg.
Another electrode possibility is "supermaterial" graphene—one atom-thick carbon. But Laslau urged caution: “Though graphene could provide better capacitance, it’s still in the early stages. There are stability issues to address, such as avoiding aggregation from vibration, and costs are sky high while supply is limited.” Two firms working in graphene supercaps are XG Sciences and Angstron Materials.
Today’s devices use either propylene (ethylene) carbonates or acetonitrile as electrolytes. Although the latter performs better, potential fire hazards have led to Japanese proscription, the analyst said, adding: “Ionic liquids keep coming up as potential alternatives, but they pose more questions than answers.”
Perhaps a better strategy, Laslau suggested, would be to boost the voltage of the entire system from 2.7 V to 3.5 V. Higher voltage means better energy density, so fewer cells are needed. But it also can lead to breakdown of the electrolyte, and it requires high-purity carbon, which could be expensive. But a voltage upgrade could yield a 40% improvement in cell price/energy unit, he said. Both eSionic and Silatronix work in this area.
Laslau emphasized lastly that broader adoption of the supercaps also depends on cutting the costs of the balance of systems such as power electronics, since the cells do not solely drive the overall costs.