[Technology Report]
Ultracapacitors Branch Out Into Wider Markets
Known for their muscle, these devices are scaling back in size but not in power to provide juice for a broader range of devices—including portables.
Once the staple of brute-force power supplies and large industrial and consumer power applications, ultracapacitors are now finding their way into products of all sizes, particularly portables. Also called supercapacitors, these components are notable for capacitance values ranging into the thousands of Farads and fast charge/discharge rates.
With the ability to store massive amounts of energy for long periods of time, ultracapacitors behave more like a battery than a standard capacitor. In fact, as things progress, they will replace rechargeable batteries in a plethora of products, from computers and digital cameras to cell phones and other handheld devices.
WHAT'S AN ULTRACAPACITOR? Basically put, the ultracapacitor or supercapacitor is a very large, polarized electrolytic (electro-chemical) capacitor. In describing these components, though, “large” refers to capacity, not necessarily their physical size.
True, when it comes to generic electrolytic capacitors, the greater the capacitance and/or voltage values, the bigger the overall package. Electrolytic caps typically offer capacitance values in the micro-Farad range from about 0.1 µF and topping off at approximately 1 F with voltage ratings up to 1 kV dc. Usually, the higher the voltage rating, the lower the capacitance value, and the bigger the capacitor and the higher the capacitance value, the bigger the package even as operating voltage may decrease.
The same rules of thumb essentially apply to ultracapacitors. These components come with capacitance values from 1 F and up and operating voltages ranging from 1.5 to 160 V dc or greater. As both values increase, so does capacitor size.
Early-generation ultracapacitors with values in the tens of Farads were hefty clunkers, relegated to large power-supply applications. Smaller ultracapacitors with miniscule voltagehandling capabilities found employment as short-term power backups in consumer electronics.
As capacitor technology evolved, new developments haven’t only leveled the playing field, they’re also beginning to tip it favorably for the ultracap market. Despite the vast similarities between supercapacitors and their electrolytic counterparts, there is a significant variation in their electrical and physical proportions.
For example, a general-purpose 10-µF electrolytic with a 25-V dc rating may measure only slightly smaller or even the same as a 1- to 10-F, 2.7-V dc ultracap. And, with recent advancements, boosting the operating voltage to 25 V dc may translate into a size increase of less than double for the supercap, which may or may not be significant depending on the application.
ANATOMY OF AN ULTRACAPACITOR Basically, one can view an ultracapacitor as a rechargeable battery. It stores a charge proportional to its capacitance and releases a charge when called upon to do so. What sets the ultracap apart from its electrolytic brethren is an electrical double-layer architecture, which enables higher capacities.
Standard capacitors sandwich a dielectric substrate between two electrodes attached to plates (Fig. 1). Depending on the type of capacitor, the dielectric can be aluminum oxide, tantalum tetroxide, titanium oxide barium, or polyester polypropylene, each of which determines capacitance and voltage capabilities (Fig. 2). The amount of dielectric and the distance between the plates also affects capacitance levels. However, the maximum allowable distance between the plates limits the amount of dielectric.
In this single-layer topology, increasing the amount of dielectric to boost capacitance is usually achievable in one of three ways: widen the package and increase plate size, lengthen the package and increase plate distance, or a combination of both. Any of the three solutions translates into a physically larger capacitor as a tradeoff for more micro-Farads.
Electric double-layer capacitors (EDLCs), as the name implies, solves this problem by adding a second dielectric layer within the same package that works in parallel across a separator with the first layer (Fig. 3). EDLCs also employ nonporous dielectrics such as activated carbon, carbon nanotubes, carbon aero gels, and select conductive polymers, which exhibit higher storage capabilities than standard electrolytic materials. This combination of the extra layer and more efficient dielectric material enables a capacitance boost in the neighborhood of four orders of magnitude.
There is a tradeoff in terms of voltage capability, traceable to the dielectric. In EDLCs, the dielectric is extremely thin, measured in nanometers, creating a large surface area that’s responsible for higher capacitance. However, these thin layers lose some of the desirable insulating properties of conventional dielectrics and therefore require lower operating voltages.
SUPERCAP APPS In relation to standard capacitors and batteries, EDLCs have several advantages that make them desirable alternatives. These benefits include a greater number of charge/discharge cycles than rechargeable batteries, efficiencies in the realm of 98%, a lower internal resistance, high output power, better thermal capabilities, and better safety margins than both batteries and standard capacitors.
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