Using Ultracapacitors for Bridge Power in Mission-Critical Installations

Defined as short-term power that is necessary to “bridge” one long-term power source to another, bridge power is necessary when typical standby power generation equipment is not immediately available, taking additional time to be brought online.

A good example of this is a diesel generator set used to power a hospital. During a utility outage, the genset may experience power failure, resulting in the loss of lighting and other non-critical loads. In these situations, batteries or capacitor banks are employed locally to temporarily supply power to mission-critical equipment — as either a stand-alone uninterruptible power supply (UPS) or integrated into equipment such as monitors and infusion pumps — until the genset can be restarted and brought back online.

The combination of both bridge and long-term power generation is necessary because the cost associated with extending bridge power beyond a few minutes is high. In addition, maintenance and reliability issues where batteries are used may make a single “long-term bridge” costly. More complex architectures are now being fielded to address the growth of telecommunications and data systems as well as factory processes that cannot tolerate any power interruption.

Critical installations are using what the industry calls “waterfall” architectures, in which multiple power sources are available for backup power. These systems use a number of different continuous power technologies, such as engine generators, fuel cells, and micro-turbines, to provide a long-term bridge between each transition. Short-term bridge power technologies may include batteries, flywheels, and ultracapacitors.

With this many options in bridge technology available today, it's important to keep up with the advancements on all fronts. This article will focus on the ultracapacitor.

How an ultracapacitor works. The National Renewable Energy Laboratory (NREL) provides an easy-to-understand explanation of how an ultracapacitor works via its Web site (http://www.nrel.gov/vehiclesandfuels/energystorage/ultracapacitors.html).

NREL explains that the device, also known as a double-layer capacitor, polarizes an electrolytic solution to store energy electrostatically. So, even though it's an electrochemical device, there are no chemical reactions involved in its energy storage mechanism. This mechanism is highly reversible, and allows the ultracapacitor to be charged and discharged hundreds of thousands of times.

The Figure (click here to see figure) shows an ultracapacitor, its modules, and an ultracapacitor cell. Basically, an ultracapacitor can be viewed as two nonreactive porous plates, or collectors, suspended within an electrolyte, with a voltage potential applied across the collectors.

In an individual ultracapacitor cell, the applied potential on the positive electrode attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts the positive ions. A dielectric separator between the two electrodes prevents the charge from moving between the two electrodes.

Once the ultracapacitor is charged and energy is stored, a load can use this energy. The amount of energy stored is very large compared to a standard capacitor because of the enormous surface area created by the porous carbon electrodes and the small charge separation (10 angstroms, or 10-7 centimeters) created by the dielectric separator. According to www.worldandi.com, today's ultracapacitors achieve capacitances ranging up to 2700 farads.

Ultracapacitors are true capacitors in that energy is stored via charge separation at the electrode-electrolyte interface.

Comparing ultracapacitor and battery functionality. An ultracapacitor stores a much smaller amount of energy than does a battery. But since the rates of charge and discharge are determined solely by its physical properties, the ultracapacitor can release energy much faster (with more power) than a battery, which relies on slow chemical reactions.

Like batteries, ultracapacitors are energy storage devices. They use electrolytes and configure various-sized cells into modules to meet the power, energy, and voltage requirements for a wide range of applications. But batteries store charges chemically, whereas ultracapacitors store them electrostatically.

Currently, ultracapacitors are more expensive (per energy unit) than batteries, but that differential is decreasing as a result of improved manufacturing methods.

Since an ultracapacitor is used strictly as a bridge, its high power density is ideally suited to supply high power for short periods of 30 seconds to 100 seconds. A battery, on the other hand, is typically sized to deliver power over longer periods.

Additionally, an ultracapacitor is capable of holding a charge voltage for extended periods without any loss of capacity. Batteries, in contrast, can lose capacity when held on charge for extended periods.

Ultracapacitors have a distinct trait that 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's used on the output of the power electronics in a DC system, in which case the battery output is then unregulated. An ultracapacitor, on the other hand, has no fixed operating voltage, and therefore can operate directly across the output of the fuel cell, directly into the power electronics.

One key challenge with batteries is that it's somewhat difficult to measure their state of charge. Numerous algorithms and circuits are used to give an indication of how much capacity remains in a battery. With an ultracapacitor, you measure its state of charge solely by its voltage.

Making the case for ultracapacitors. According to Frost & Sullivan, a market research company with offices all over the world, battery chemistries may have higher energy densities in comparison to ultracapacitors, but their power delivery capability — when the application demands it — remains poor. In comparison, ultracapacitors are a powerful energy source and a good option where power surge requirements have to be met.

Another industry source, The Energy Blog cites an even more detailed listing of reasons on its Web site why ultracapacitors are becoming a popular alternative to batteries.

First of all, they can be charged and discharged almost an unlimited number of times. Ultracapacitors can discharge in a matter of milliseconds or as long as tens of seconds or several minutes, and they can be charged in seconds to minutes. Featuring high power density, the devices do not release any thermal heat during discharge, and there is no danger of overcharging because the ultracapacitor simply quits accepting a charge when fully charged. They are not affected by deep discharges as are chemical batteries, and they have a long lifetime, which reduces maintenance costs.

Anecdotal evidence suggests that they lose about 80% of their storage capacity after 10 years, with a lifetime estimated to be 20 years. The DC-DC round-trip efficiency is 80% to 95% in most applications. Operating temperatures range as great as between -50°C and 85°C. Capacity increases as temperature decreases below the rating temperature, and they do not release any hazardous substances that can damage the environment.

Everett is vice president, chief technical officer, Maxwell Technologies, San Diego.