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Hybrid Storage

Ultracapacitors and batteries work together to solve power quality problems.

Fortnightly Magazine - June 2010

, they can deliver a great deal of current from a small package. What differentiates ultracapacitors from their traditional counterparts, electrolytic capacitors, is their high energy density, allowing them to store a vast amount of energy in a small package.

The capacitors with which most design engineers are familiar have short time constants, which means their voltage cycles quickly, whereas ultracapacitor arrays have time constants between tens of seconds and minutes in length. The large capacitance and extremely low frequency time constants allow ultracapacitors to be used in applications that haven’t been practical or economical for other types of capacitors. Using such capacitor supplies in concert with power electronic techniques brings the design and cost of power conditioning equipment within reach of most volume users of electrical energy. Further, as the sophistication of power conditioners increases, their costs will come down, and such systems probably will become available for a much wider spectrum of power consumers.

Because ultracapacitors have a much lower internal resistance and much faster charge rate than batteries do, they can make a battery-powered system run much more efficiently. An array of ultracapacitor cells in series, coupled to a load in parallel with a storage battery, creates a hybrid power source with higher power and energy density than either device in a stand-alone configuration. By gradually taking on a load, batteries are insulated from high current drains that cause thermal, chemical, and mechanical stresses. And by reducing current spikes, the internal temperature of batteries is decreased substantially, extending the life of the batteries by as much as 400 percent, depending on the application. Additionally, there are times when a battery simply can’t deliver the current needed for an application. In this situation, an ultracapacitor can be used to augment the battery.

The primary limitation associated with ultracapacitors is their low voltage rating, which may be overcome to some extent in lower voltage applications by constructing a parallel-series array of devices; the series connections increase voltage standoff, while the parallel connections increase capacitance and reduce equivalent series resistance (ESR). The arrays can be interconnected easily and allow for capacitor banks that will function well up to intermediate voltage levels (400 to 600 V).

In most instances, it’s necessary to incorporate a controller and appropriate power electronic circuitry to meet specific needs. For example, an array of ultracapacitors can be used as a power source to compensate for power sags of short duration; however such systems require, in addition to the capacitor bank, a controller and power electronic circuitry to make them really useful. With these additions, a module can be constructed to compensate for power sag and do real-time power factor correction for loads of various sizes. The cost-to-benefit ratio of such a system is at present questionable, but as prices of ultracapacitors decline, such applications will become widespread. These systems also have the ability to be scaled up to higher voltage levels. Experimental applications at grid level still are being evaluated.

Ultracapacitors allow design engineers to separate energy and power needs. In most applications there’s a continuous power demand