Li-ion, NiMH and other battery technologies have made great strides in providing a reliable energy storage solution. They have found their way in many designs and worked out many of the early cost issues. But in the end designers are faced with the same dilemma they had when using lead-acid batteries. All these technologies in the end are based on a chemical reaction and suffer from limited life along with temperature restriction. High current demands will also directly affect their life. Therefore, for long lasting, reliable applications they posses some challenges.

On the horizon there is a very attractive technology called fuel cells. These devices have been publicized lately and are finding their way into many applications. The final frontier for these devices is automotive applications, but in the interim the backup power markets have embraced these devices. The key issue with using fuel cells for back-up power as well as main power is the startup time and the dynamic power reaction of these devices. Although they have excellent energy density they do suffer from low dynamic power. Therefore, they need an augmenting technology for power assist and startup.

Along comes ultracapacitors, or properly known as Electro-chemical double layer capacitors. Based on an existing 100 year old technology, these devices possess a very high power density along with a substantial energy density compared with electrolytic capacitors. In the past several years these devices have found their way in many applications ranging from consumer electronics to industrial and automotive applications.

Figure 1 - Power Density

Today, the best ultracapacitors are extremely high-power devices, with power densities up to 20 kW/kg, while their energy is still a fraction of that of batteries. Compact in size (small-cell ultracapacitors are often the size of a postage stamp, or smaller), ultracapacitors can store much more energy than conventional capacitors and can release that energy quickly or slowly. They have an extremely long life and are designed to last the lifetime of the end product. When combined with the newest technologies, high-energy batteries and/or fuel cells, the best of both worlds is possible: high power features and a long operating lifetime.

Even though there are several ultracapacitor manufacturers around the world offering a variety of products, most double layer capacitors are basically constructed in a similar manner. As it can be seen in Figure 2, ultracapacitors are constructed very similarly to electrolytic capacitors or batteries, the main difference being the electrode material used. In ultracapacitors, the electrode is based on a carbon technology, which allows for a very large surface area. The combination of this surface area along with a very small charge separation gives the ultracapacitors the high energy density they possess. Most ultracapacitors are rated in farads and typically can be found in the 1F to 5,000F range.

Figure 2 - Ultracapacitor

Depending on the application needed, ultracapacitors may be suited as battery replacements or enable smaller, economical battery selection. Ultracapacitors have low equivalent series resistance (ESR), allowing them to deliver and absorb very high currents. The "mechanical" rather than chemical charge carrier mechanisms enable long, predictable life with a more graceful performance change over time. Applications benefiting from these characteristics include regenerative braking and other quick-charge scenarios such as toys and tools.

Some applications are suited for battery/ultracapacitor systems. Designs can be optimized to prevent battery over-sizing for power demands. Examples include consumer electronics such as digital cameras, in which an inexpensive alkaline battery is combined with an ultracapacitor (rather than using expensive lithium ion batteries), and automotive applications such as hybrid power trains.

Another fuel cell technology is the Proton Exchange Membrane (PEM), which is a high efficiency energy conversion device that can operate continuously for as long as hydrogen fuel is available. It is environmentally benign and can provide a reliable source of backup power for many applications. Several characteristics of ultracapacitor and PEM fuel cell systems make them ideal as complimentary components for use together. Both are low voltage high current components. With low ESR and high charge storage capacity, the ultracapacitor can ramp up large currents with minimal change in voltage creating a short term buffering response to peak power demands. This permits the fuel cell to maintain its quiescent operating point without inefficiency.

In all back-up fuel cell applications the need for power is immediate after the loss of main power. Since fuel cells typically have a 10-60 second start-up time to reach full power there is a need for an energy buffer. This can be achieved with either batteries or ultracapacitors. Since the amount of energy needed is very limited and reliability is an absolute must, ultracapacitors are a better choice for this application. Today, we are seeing more and more fuel cell companies looking at the ultracapacitor as an integral component of their total package for back up power requirements.

To satisfy this demand, ultracapacitor manufacturers all around the world are offering a variety of cells and modules for the power backup market. These cells and modules can be placed in a series/parallel format to create and satisfy different capacitance and voltage requirements. With more exposure and availability in the market, design engineers are using ultracapacitors similar to any other passive component.

Ultracapacitors have made great strides in becoming a standard component in the back-up power world. Being viewed as a lab experimental product about 10 years ago these products were only sold in handfuls a year with prices in the $1.00-$2.00 dollars per farad range. These components are viewed today as a standard product which is in mass production, driving the prices in $0.01 to $0.02 cents per farad. With such availability and reduction in price many designers are using ultracapacitors as standard a energy storage option for their high power needs and high reliability backup power requirement.