[Technology Report]
Get The Lowdown On Ultracapacitors
Now that you're armed with more capacitance than earlier generations of engineers ever dreamed of, what do you do with it?
Applications The most basic applications for ultracaps lie in stabilizing dc bus voltages. Ultracaps have become widely used in automobiles to protect the various engine control units and other microcontrollers from voltage dips associated with the application of sudden transient loads. (Voltage spikes are handled differently.)
Those sudden loads often are associated with motors. But if the speaker output of the car’s entertainment system is sufficiently robust, the load could come from audio peaks. In lieu of simply putting an ultracap on the 12-V input to the entertainment system, an application note by Australian ultracap-maker Cap-XX shows a way of increasing the voltage for a class-D output amplifier’s H-bridge (Fig. 3). It uses a small boost converter and stores the power needed for those occasional peaks in a pair of ultracaps.
Elsewhere in transportation, the ultracap’s ability to absorb and discharge energy rapidly makes it far better than batteries for regenerative braking schemes. Most of these applications have been in public transportation (Fig. 4). The Bombardier rail cars in the light-rail system in Mannheim, Germany, use packs of 600 2600-F ultracapacitors for braking energy recapture. The stored energy is used to boost acceleration and to bridge non-powered sections and intersections. Operation there represents between 100,000 and 300,000 load cycles/year. This is an all-electric rail system, so recaptured braking energy reduces demand on the grid. In that regard, the prototype has demonstrated a potential for energy savings of up to 30%.
Mannheim installs the ultracaps on the rail cars themselves. An alternative is to install the ultracaps alongside the tracks. Demonstrating this approach, Siemens Transportation Systems uses ultracapacitors for regenerative braking in its Sitras SES system, which is used in Cologne’s and Madrid’s metro rail lines. In a typical trackside implementation, the ultracapacitors absorb the braking energy from all trains within a 3-km radius.
In hybrid transportation applications in the U.S., ISE Corporation’s buses now run in Elk Grove and Long Beach. The buses accelerate more quickly than standard buses. At gross vehicle weight, the bus can accelerate from zero to 31 mph in 17 seconds and can reach a maximum speed of 62 mph. Preliminary data indicates better average fuel efficiency compared to competitive battery-based hybridelectric drive systems. These ultracapacitorplus- battery hybrid buses recuperate 38% of the propulsion energy, which translates into more than 3.9 miles/gallon of fuel-economy gain on average.
ISE developed its own thermally controlled modules, each of which uses 144 18-F ultracaps. The modules provide 360 V at 400 A. A pair of the modules is used in series to take the voltage to 720-V nominal (800-V peak). This dual-pack configuration allows charge/discharge cycles at power levels up to 300 kW and can store approximately 0.6 kWh.
Regenerative braking means capturing kinetic energy. These applications also recapture potential energy. One recent example is a forklift, but the much wider potential lies in building-elevator systems.
For forklifts, General Hydrogen offers retrofit and new “Hydricity Packs,” fuel-cell systems sized for direct lead-acid battery replacement in conventional factory equipment. Its ultracapacitor bank stores power every time the loading fork descends with a pallet and releases it when power bursts are required for heavy lifting. Figure 5 plots typical power usage in a forklift, demonstrating the synergy between fuel-cell and ultracapacitor power.
The short discharge time doesn’t adversely affect some ultracapacitor applications. In European wind farms, the latest turbines have 160-ft blade diameters, with hubs 250 ft above the ground. In high winds, the blades must be feathered, lest the turbines over-rev. That requires high-torque pitch motors for each blade, along with a power source for those motors.
Although that looks simple enough for lead-acid storage batteries, the wind-turbine designers chose ultracapacitors. Batteries would need regular servicing, while ultracaps do not. Of course, the utility needs to employ some skilled service people to climb the towers. But it can get by with fewer of them if they can concentrate on serious maintenance work and not be continually clambering up and down thousands of towers just to babysit batteries.
Circuit design Combining ultracaps, batteries, fuel cells, and solar panels constitutes an interesting design exercise. Much of what follows comes from papers presented at the Power Electronics Technology conference in Dallas earlier this month and represents the state of the art.
In a paper titled “Storing Power with Super Capacitors,” Thomas DeLurio of Advanced Analogic Technologies describes portable applications such as wireless data cards for GSM, GPRS, or WiMAX that require a peak current during data transmission of signals that exceeds what’s available under PC Card, CF Card, or USB standards.
DeLurio also notes a similar problem with flash LED illumination in camera phones. “The challenge for designers is determining how to most efficiently interconnect the battery, dcdc converter and super capacitor in a way that will limit the super capacitor charge current and continually recharge the capacitor between load events,” he says.
The problem with ultracaps, DeLurio says, is their low equivalent series resistance (ESR). When the capacitor is initially discharged, it looks like a lowvalue resistor to the charging circuit. The resulting large in-rush current would essentially short-circuit the device’s battery. Additionally, he notes, “Any circuit of this type also requires short-circuit, overvoltage, and current flow protection.”
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