Let's say it's 2010, and you're boiling off midlife ennui or burnishing your golden years in time-honored fashion: by zooming around in a high-performance road machine. The car accelerates powerfully, and yet it moves quietly and nimbly, slaloming through curves like a go-cart. Best of all, it sips gas like a connoisseur enjoying 40-year-old Armagnac. Would you believe you owe these rejuvenating, guilt-free thrills to a bunch of capacitors?

Not just any capacitors, of course. To understand what's going on under the hood of this car, you'll need to leave behind the Lilliputian world of the picofarad and the microfarad and enter the realm of the kilofarad. It is a place where NessCap Co., in Yongin, South Korea, holds sway.

NessCap is one of about 10 makers of ultracapacitors, devices that can store so much charge that they are beginning to blur the functional distinction between the capacitor and the battery. And according to some experts, nobody does it better than NessCap, which offers a unit rated at an impressive 5000 farads at 2.7 volts in a package a little bigger than a half-liter soda bottle. NessCap's capacitors "perform as well as or better than any others we've ever tested, in terms of energy and power density," says Marshall Miller, a research engineer at the University of California at Davis, where he specializes in testing advanced capacitors and other devices.

Ultracapacitors made by NessCap and others are just now starting to show up in products ranging from toys to experimental buses, basically as alternatives to batteries. The worldwide market isn't large; it was just US $38 million in 2002, the most recent year for which figures are available, according to the research firm Frost & Sullivan, in San Antonio. But NessCap and the handful of other makers of the largest ultracapacitors all have their sights set on the automotive market, which could do for their business what the iPod did for sales of MP3 songs. Frost & Sullivan, at least, is a believer; the company optimistically predicts total 2007 revenues for ultracapacitors of $355 million.

On paper, anyway, the idea is not far-fetched. In comparison with batteries, ultracapacitors can put out much more power for a given weight, can be charged in seconds rather than hours, and can function at more extreme temperatures. They're also more efficient, and they last much longer—in tests at the Idaho National Engineering and Environmental Laboratory, in Idaho Falls, upwards of 500 000 charge-discharge cycles have been recorded. Automotive traction batteries, for comparison, have much shorter lifetimes, particularly if they are discharged deeply.

Pondering the relative strengths of capacitors and batteries, Joel Schindall, associate director of the Laboratory for Electromagnetic and Electronic Systems at the Massachusetts Institute of Technology, in Cambridge, says: "In all ways other than energy density, an electric field is superior to chemistry for storing energy regeneratively, because it is completely reversible" and therefore intrinsically efficient and durable. Part of Schindall's research focuses on advanced materials that could be used as electrodes in future ultracapacitors.

Ultracapacitors are now establishing themselves in niches demanding a power source that can recharge quickly, be sealed into a system that has to last for years, or put out prodigious amounts of power in short bursts. Tokyo-based Ricoh Co. is using them in copier machines to store the energy needed to warm up the machines quickly, minimizing time spent in the energy-wasting standby mode. Makers of high-end car stereo amplifiers are using ultracapacitors to deliver the surges of power demanded by musical crescendos, without straining the vehicle's battery.

Another use is in solar tiles; a new twist in landscape architecture, they're used to guide pedestrians at night, by storing solar-generated electricity during the day and using it to power a small light-emitting diode panel after dark [see photo, "Bright Idea"]. Sealed into a walkway, wall, or staircase, these clear, rugged tiles have to last for a decade or more, working without fail night after night, withstanding subfreezing and sweltering temperatures alike—criteria only ultracapacitors can fulfill.

And then there are cars. The hybrid-electric vehicle, in its various forms, is poised for an increasing share of the automotive market in several parts of the world, including the United States. And ultracapacitors have already found their way into hybrids, albeit in a minor role: hardly noticed among the Toyota Prius's many celebrated technical breakthroughs is the fact that it uses ultracapacitors, from Panasonic, to power an electric-hydraulic pump in the mechanical braking system.

It's just the start of what some experts say ultra-capacitors will do for hybrids. For example, with their lightning-fast charge and discharge capability, ultracapacitors could handle the power surges needed for accelerating, allowing engineers to use a smaller battery pack in the vehicle (and eventually, perhaps, no battery pack at all). Shielded from high-current pulses, the batteries would last longer, too.