Back-up doesn’t have to mean batteries

Even if they are rarely used, batteries deteriorate over time, and most chemistries only have a 3-5 year operating life.

This means that the maintenance team will have to be tasked with checking and replacing them regularly – not an insignificant cost if the equipment is widely distributed and/or inaccessible.  If they see regular power peaks, their life can be reduced, and performance is also impaired by extremes of temperature. As every car owner knows, they are particularly ineffective in cold temperatures, as they operate through a temperature dependent chemical reaction.

These and other considerations have led to the growing popularity of ultracapacitors as an alternative energy storage and power delivery technology with an operating life that can be extended to over ten years. This article reviews the benefits of this technology briefly, looks at specifying them correctly and concludes with a review of a real-life application: ultracapacitors in a bridge power application.

Ultracapacitor: the ‘non-chemical’ battery
By contrast with batteries ultracapacitors are very stable over a wide operating temperature due to the chemistry and physical make up of the products. Their organic based electrolyte has a low freezing point, allowing them to be used over a wide range of temperatures with relatively consistent performance.

Ultracapacitor life is predominantly affected by a combination of operating voltage and operating temperature. The typical degradation behaviour of the ultracapacitor resembles that of an exponential decay. The majority of the performance change occurs during the initial use of the ultracapacitor and this performance change then levels off over time. The most dramatic effect of the life degradation is on the internal resistance of the device. The ultracapacitor does not experience a true end of life, rather the performance continually degrades over the life of the use of the product.

Since the energy storage mechanism of the ultracapacitor is not a chemical reaction, charging and discharging of ultracapacitors can occur at the same rate. Therefore, the rated current for the ultracapacitor applies for both charge and discharge: efficiency values of charge and discharge are essentially the same. Hence a variety of methods can be used for charging, either through constant current or constant power charging via a DC source, or through AC charging methods. This flexible "opportunity charging" allows a system designer to make best use of their energy sources.

Ultracapacitor selection
The basic questions in specifying an ultracapacitor are not too different from any other back-up power source: what is the maximum system voltage, the minimum allowable application voltage, typical current or power needed, peak current or power needed and load duration.

The second step is to analyse the system environment, including typical and extreme ambient conditions, together with self-created conditions around the ultracapacitor. Not all ultracapacitors are equal. Maxwell Technologies’ BOOSTCAP ultracapacitor is a double-layer capacitor incorporating a unique metal/carbon electrode and an advanced non-aqueous electrolytic solution. When a voltage potential is applied across the terminals, ions migrate to the high surface area electrodes. The combination of available surface area and proximity to the current collector provide an ultra-high capacitance for this electrostatic process. As a result, performance of these ultracapacitors is very stable over a wide operating temperature.

By contrast with batteries, for ultracapacitors, operating temperature should be kept as low as practicable. Consideration should be made for the duty cycle and resulting capacitor temperature as well as the anticipated ambient temperature the device will be operating under. The combination of the two should not exceed the operating temperature for the ultracapacitor. Vendors such as Maxwell Technologies provide tools that enable designers to apply known thermal profiles to their projects, for example to assess whether heat sinks are required. Cooling at the capacitor ends or terminals is the most efficient where devices have electrically insulating shrink sleeving around the capacitor body.

Operating voltage has an impact too
In many applications, the ultracapacitors will be maintained at working voltage until needed. The datasheet will show the degradation in rated capacitance for ultracapacitors held at typical working voltages for long periods of time, and at different temperatures.

A further consideration is the number of charge/discharge cycles the device has to endure. Again, manufacturers should be able to provide the data designers need, backed-up by real-life test results. For example Maxwell’s cycle testing demonstrates that, under typical conditions, the BOOSTCAP ultracapacitor family is expected to provide in excess of 1 million duty cycles with an approximate 20% reduction in rated capacitance. Details of the testing and plots of capacitance versus cycles are available to designers on request.

Bridge power
The benefits of ultracapacitors in practice are best seen by considering a real world application, such as the provision of bridge power for high availability systems such as telecoms network equipment. Historically, a single power generating solution, typically a diesel generator, was used with a simple battery-fed inverter uninterruptible power supply (UPS) as a bridge. For installations where a minor power glitch was only a nuisance, this is adequate, considering that the typical UPS only can supply the load for 8-20 minutes. One of the major issues with diesel generators and battery UPS systems is reliability and maintenance. Telecom companies require much more reliability that a typical genset/battery combination provides. More complex architectures are now being fielded to address the growth of telecommunications and data systems and factory processes that cannot tolerate any power interruption. “Waterfall” systems use a cascading set of different continuous power technologies (e.g. engines, fuel cells, micro-turbines), bridging between each transition with short-term bridge power technologies (e.g. batteries, ultracapacitors, flywheels). With the many options in bridge technology, one must consider the overall reliability requirement. With the maturation of the ultracapacitor industry, ultracapacitors are highly competitive with, and in many cases superior to, older legacy bridge technologies.

Ultracapacitors offer the functionality, life cycle costs, and reliability necessary to make mission-critical power backup systems successful. Since the ultracapacitor is used strictly as a bridge, its high power density is ideally suited to supply high power for short periods of 30-100 seconds. A battery is more typically sized to deliver power over longer periods, making them larger than necessary. If a battery is sized for the actual duration required, it may have difficulty supplying the necessary power.

Additionally, since ultracapacitors operate on a different principle than batteries, the ultracapacitor is capable of sitting on a charge voltage for extended periods without any loss of capacity. Batteries are notorious for loosing capacity when held on charge for extended periods.

Ultracapacitors have another distinct difference when compared with batteries, which makes them ideally suited to support fuel cells. A fuel cell’s output varies with load (which is then regulated by power electronics). A battery’s output is fairly fixed, and therefore will affect the fuel cell’s performance by loading the fuel cell’s output (unless it is employed on the output of the power electronics in a DC system, in which case the battery output is then unregulated).

One key challenge with batteries is the difficulty in measuring their state of charge. Numerous algorithms and circuits are employed to give the operator an indication of how much capacity remains in a battery. An ultracapacitor, on the other hand, is measured solely by its voltage; know the voltage, know the state of charge.

Conclusion
An interesting use model that is increasingly emerging with wireless systems is the use of ultracapacitors to provide ‘top-up’ back-up power. The back-up battery is sized to provide a maintenance supply that covers the main system functions, with the ultracapacitor cutting in to provide additional power to support peak loads, for example during the transmission of wireless data. This can allow a much lower capacity back-up supply to be used than would otherwise be the case.  In so many ways, ultracapacitors provide an excellent replacement for, or complement to, other back up power options including UPS, fuel cells, generators and batteries.

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