Energy storage: Ultracapacitors answer power needs

In today’s electronics world there is a constant cry for more power, including for motion control applications.

By Control Engineering Staff April 26, 2007

In today’s electronics world there is a constant cry for more power, including for motion control applications. There are various electronics functions associated with every device which makes the need for energy storage a must. Because of these complexities, most of the energy needs are not easily fulfilled by your common alkaline batteries anymore.

Ultracapacitors are a fairly new device type in the energy field that can be used alone as a primary energy source, or in combination with batteries as a secondary energy source. Their main attributes are high power capability and long life. Desirable characteristics for use in high power applications include: extremely long cycle life, wide operating temperature ranges, low weight, flexible packaging, zero maintenance and environmental friendliness. Based on these attributes, ultracapacitors are becoming the component of choice for engineers and designers with applications requiring short-term or peak (burst) power.

The units range in capacitance from several farads to several thousands of farads, giving designers the ability to customize energy storage to their exact needs and reducing overall system size and cost. A key advantage, compared to batteries, lies in their safety and ease of use.

Batteries, especially Li-ion and Ni-MH , require extensive monitoring and charging circuitry, whereas ultracapacitors are easily maintained, and require no or minimal care. State of health is easily monitored during any cycle, and the life expectancy is not a surprise to the end user.

A fundamental difference between batteries and ultracapacitors is that batteries have a high energy density while ultracapacitors have a high power density. Unlike with batteries, the processes of storing and discharging energy within an ultracapacitor are purely electrostatic. No chemical bonds are made or broken, so the energy storage mechanism is highly reversible, leading to over one million full charge/discharge cycles with minimal degradation.

What limits battery life is not the energy requirement, but internal impedance that prevents delivering the energy as needed. Ultracapacitors provide a more efficient energy extraction solution. Placing a low-impedance ultracapacitor in parallel with a battery lowers the overall energy storage system impedance.

Some of the benefits gained from this combination are:

• Cost improvement, since the ultracapacitors can be sized for power and the battery for energy.

• Improved temperature response, since the ultracapacitors have a very good cold temperature performance.

• Life improvement, since the ultracapacitors will be handling most of the short term energy needs associated with high cycling.

By understanding the technologies available, engineers can recognize the subtle yet critical difference between power and energy, and choose the right combination for their application.

Bobby Maher, director of business development, Maxwell Technologies

Edited by C.G. Masi, senior editor, Control Engineering Machine Control eNewsletter

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