In the early 1980s, developers of personal computers achieved substantial performance enhancements with an intermediate memory cache that improved processing speeds by making better use of available processor power. Today, in a similar manner, designers of portable and battery-based electronics systems are taking advantage of a new concept, known as power cache, that provides a balance between peak power requirements and available stored energy.
All batteries have peak-power limitations-including those used in laptops, phones, pagers, PDAs, toys with motors, burst-mode transmitters and newly developed hybrid electric automobiles. Until recently, electronics designers have been forced to compromise between power and energy because battery-based systems required widely varying power levels at different times during normal use.
Designers have been forced to select large batteries sized to meet the peak-power requirements of a device, even if the peak was reached less than 10 percent of the time. As a result, the battery's volume, cost and chemistry were often inappropriate for the overall application. Nonetheless, consumers paid the price in terms of dollars, size and extra weight.
The power solution that electronics designers have been needing is not an oversized battery but a temporary storage place for holding the power required for peak demands. They need a device with the ability to balance power requirements with the available energy storage-a power reservoir, or a cache of power, similar in concept to the memory cache now common in personal computers.
An applicable new technology has been created as a result of research and development on a double-layer capacitor device known as an ultracapacitor.
Ultracapacitors are being commercialized and moving toward large-scale manufacturing.
Internationally, such companies as NEC and Panasonic have developed small ultracapacitors capable of sustaining the clock functions on VCRs, radios and televisions. U.S. organizations producing higher-performance ultracapacitors include Maxwell Technologies, Pinnacle Research and PolyStor, all in the San Francisco Bay Area.
Conventional capacitors are capable of delivering high power, but they hold very little energy. In many emerging applications, power must be available for more than a few microseconds, and that is where normal capacitors fall short.
Like a traditional capacitor, an ultracapacitor stores electric energy by accumulating an electrical charge between two conducting plates separated by a dielectric material.
Ultracapacitor storage capacity can be measured by its capacitance or total stored energy. While the capacitance of standard capacitors is rated in micro-Farads, ultracapacitors can easily achieve capacitance values in excess of 2,000 Farads. Described another way, ultracapacitors can provide power densities of 2,000 W to 4,000 W/kilogram, or about 20 times more power stored than in batteries of similar size.
Much of the commercial activity around ultracapacitor design and production is being funded by automotive interests, because of the potential market size and because of government mandates for pollution-free, energy-efficient vehicles.
But the fact is that ultracapacitors stand to find homes in a wide variety of applications. They are ideally suited for the load-leveling requirements of energy-efficient electric vehicles. There are dozens of battery-powered handheld devices that will benefit from the power-cache concept. And in the utility industry, companies are already beginning to take advantage of ultracapacitors.
— Jim Nickerson is vice president of sales and marketing for Maxwell Technologies (San Diego).
