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Put a CAP in it!

Aug. 9, 2012
High current in a small size gives ultracapacitors the edge when there’s a need for a fast energy jolt.

Authored by:
Chad Hall Vice President Ioxus Inc. Oneonta, N. Y. Edited by Robert Repas
Key points:
• Ultracapacitors store electrical energy in electrostatic fields, not electrochemical reactions.
• Ultracapacitors are noted for their high charge acceptance, offering rapid charge and discharge cycles.
• The number of charge and discharge cycles for ultracapacitors can exceed 1 million cycles.
Ioxus Inc.

The spotlight may currently be on renewable energy, but energy storage has a high priority as well. The erratic nature of power available from most clean energy sources, notably solar and wind, has brought a need to store energy for those times when the sun doesn’t shine or the wind doesn’t blow. While batteries have historically been the primary energy storage devices, a number of new technologies hold promise for situations that put a premium on quicker charge times, high energy densities, and light weight.

One such device is the ultracapacitor. It stores electrical power in an electrostatic field similar to a capacitor. Ultracapacitors sport high energy densities, short charging cycles, and can operate over a wide temperature range making them well suited for energy storage in numerous applications. However, batteries still reign supreme in overall energy capacity. Battery systems also do not exhibit a fundamental operating quirk of any capacitorbased storage: a continuously falling output voltage as energy is taken from the device.

Automotive applications
Buses, trucks, and automobiles increasingly rely on ultracapacitors where applications include frequent start/stops, energy capture and regeneration, and cold temperatures. The technology has now moved into the mainstream with vehicles known as microhybrids that save fuel by shutting off their engine when the vehicle stops. An electric motor is then used to start the vehicle moving again before the gasoline engine is restarted. During vehicle acceleration, the rapid discharge ability of ultracapacitors can provide up to 10× more power than batteries alone, thus giving the electric drive motor a quick burst of energy for rapid acceleration.

Ultracapacitors also enjoy a 95% efficiency, meaning only 5% of their power is lost in the charge/discharge cycle. That’s a significant boost over the 70% average for automotive batteries. It also means a greater portion of recovered energy, such as that obtained from regenerative braking, is stored for later use.

Another area of anticipated growth for ultracapacitors is in hybridized energy storage where an ultracapacitor is paired with a smaller battery. The ultracapacitor handles peak power demands which lets the manufacturer size the battery based on average energy needs. The net result is that batteries can be smaller and lighter. In a nutshell, hybridized energy storage lets the strengths and weaknesses of each technology balance out the other.

Urban transit buses typically rely on diesel engine propulsion. Yet most of their operation consists of stop-and-go driving with the diesel engine swinging between idle and the high power needed for acceleration. Neither condition is conducive to high efficiency. So increasingly an electric motor gets installed in tandem with the diesel power train to assist the diesel during acceleration.

Energy to operate the electric motor may come from a small generator attached to the diesel engine or from regenerative braking. The rapid charge and discharge rate of ultracapacitors makes them candidates for capturing the electrical energy from regenerative braking.

The electric assist offloads the diesel so it can be smaller. Drawing power from the battery and ultracapacitor system helps meet the temporary demands the electric motor may need to deliver.

Use of an ultracapacitor can let the battery be smaller as well. The battery supplies more usable energy per charge because the ultracapacitor handles the peak current demands. And the relatively high impedance of a battery won’t permit the peak power level of a battery/ultracapacitor combination.

Hybrid-bus applications need a lot of cooling. A hot battery doesn’t last long. With its low-equivalent-series resistance (ESR), ultracapacitors can handle the high current flows found during acceleration and regenerative braking. This evens out the charge and discharge rate for the battery and, in turn, reduces its internal heating. As internal heating is a function of the square of the current (I2R), halving the peak demand reduces heating to a quarter of its prior level. The lower temperature lengthens the cell’s life.

Light rail
The same problems that plague hybrid buses exist in hybrid and electric light-rail service, including the need for a low ESR storage device to handle peak current demands and reduce internal heating. Trains are heavier than buses, so regenerative braking develops even greater amounts of energy for storage. Ultracapacitors are the only energy storage device with enough volumetric power capacity, charge/discharge times, and longevity to handle these high-cycle life requirements.

Esthetically speaking, they also offer a way of eliminating high-power overhead pantograph and catenary systems: The ultracapacitors can supply the peak current demands of the train, evening the draw from external power sources. Power lines can be smaller because they needn’t be sized to meet the peak demand of all trains starting at once.

Renewable energy
Another industry which has consistently benefited from ultracapacitor technologies is that of renewable power, notably the solar and wind energy sector. Worldwide estimates for new wind-turbine installations through 2015 suggest a growth rebound with new cumulative installations of 236 GW. This roughly translates to 118,000 new turbine installations by that year.

Ultracapacitors may be a primary means of storing the energy that renewables generate until utilities need it. Even in cold temperatures, ultracapacitors possess high charge acceptance, high efficiency, and cycle stability.

But ultracapacitors are not limited to storing output power. Within the wind turbine they are replacing banks of batteries once used to power safety actions, such as blade-pitch control. Wind turbines change the pitch of their blades to get the most energy from the wind and to limit rotation speed in high winds. However, should pitch control be lost, the blades can rotate at catastrophic speeds. The turbine may literally fly apart.

To prevent such an occurrence, self-contained safety systems within the rotating blade monitor blade pitch and rpm, verifying control is maintained. If the pitch-control signal is lost or the blades spin too fast, the safety systems immediately force a fail-safe mode placing the blades in a neutral pitch to stop rotation. Currently, these systems are powered by batteries that must be serviced regularly: neither an easy nor an inexpensive task when the hub sits 300Šft or more above ground level. However, ultracapacitors are starting to replace these batteries because they operate over a wide temperature range, last a long time, and need little to no maintenance.

Industrial use
Ultracapacitors are also finding myriad uses within the industrial sector. Applications such as power tools, motor start and braking, peak shaving, power quality, and actuation systems can take advantage of the ultracapacitors’ quick charging and high-cycle life.

One such use is in automated-guided vehicles (AGVs), which transport materials without the need for onboard operators or drivers. However, AGV adoption and use have been hampered by the inefficient energy-storage options, typically batteries. The continuous energy demand for travel between stations is punctuated by bursts of high power for lifting or dumping. Such requirements drain battery life, forcing a battery switch or recharge several times daily. With a typical work shift of 8 hr, AGV owners lost production time while replacing or recharging batteries.

The adoption of ultracapacitors into AGV power storage has helped eliminate much of that lost production. Instead of swapping out battery packs or waiting for AGVs to charge up , userscan quickly recharge ultracapacitors through the use of in-floor inductive charging stations. These stations can be positioned within the normal work pattern of the device, offering a quick energy refill as the AGV passes. The constant recharge means the AGVs can operate continuously for a full 8-hr work shift, and possibly even more.

In addition, the estimated overall lifespan of the ultracapacitor-storage system within the AGV extends to 10 years with little to no maintenance — a far cry from battery-based storage systems that need regular service. This frees manufacturers from constant battery swaps and replacements. And again, the low internal resistance of the ultracapacitors helps meet peak current demands for lifting, dumping, or other applications that need large or frequent bursts of power.

The combination of ultracapacitors and batteries in a hybrid supply boosts power density while reducing the size of the energy-storage system. The result is an AGV that weighs less and uses less energy.

Ultracapacitor technology is also poised to replace lithium-ion batteries as power backup in memory storage functions, for both IC memory chips and hard-disk drives like those in redundant-array-of-independentdisks (RAID) server applications. The typical two-year lifespan of batteries is short compared to the 10-year lifespan for ultracapacitors. The longer lifespan reduces maintenance and, more importantly, boosts reliability for applications where short outages can be extremely costly, possibly costing millions of dollars per unplanned shutdown. Ultracapacitors are especially beneficial as a backup helping with short-term ride-through in the case of power outages or sags in available power.

Handheld consumer electronics, such as toothbrushes, shavers, clippers, and flashlights, are turning to ultracapacitor technology to provide a fast charge. In addition, ultracapacitors typically outlast such devices. This means less waste in landfills and an overall greener consumer product.

Also catching on is the use of ultracapacitors in LED lighting, notably LED emergency lighting and LED flashlights. For example, an ultracapacitor in an LED flashlight produces a handheld light that can recharge in as little as 60 sec, yet produces light for a full day.

Looking ahead
In addition to traditional cylindrical cells, ultracapacitors increasingly are packaged as laminated pouch cells. Compared to cylindrical packages, thin-film prismatic, high-energy cells offer extremely high power and energy densities. Because these pouches create a much smaller module than one using cylindrical cells, they can sit in hitherto unused cramped quarters. These flexible ultracapacitor energy packages are particularly useful in parts of the world where the utility grid does not provide the smooth voltage input or peak current necessary to power machines. Here, tapping a bank of ultracapacitor modules provides the necessary power to carry-on.

What is an ultracapacitor?

Ultracapacitors are described as a cross between a battery and a capacitor, although that description can be misleading. Like a battery, the ultracapacitor has both positive and negative electrodes with an electrolyte between them. But like a capacitor, there is a dielectric or insulating layer between the electrodes that blocks the flow of current inside the device. Energy storage takes the form of an electrostatic field, like a capacitor, rather than in a chemical reaction like a battery. This lets the ultracapacitor charge and discharge more rapidly than a battery, making it a candidate for transportation, renewable energy, server memory, and consumer electronics applications.

Ultracapacitors weigh one-fifth as much as batteries of a comparable size and are capable of more than a million charge/discharge cycles. They can operate in a temperature range of –40 to 70°C. The high number of charge and discharge cycles in some applications means the life span of an ultracapacitor often exceeds that of the machine or device into which it is built.

One of the most-appealing benefits of ultracapacitors is the potential for cost savings. Over the past decade, the price of ultracapacitors has dropped 90%, compared to only 30 to 40% for battery prices. This price drop comes from several factors: lower costs for raw materials because of bigger ultracapacitor demand and volume discounts, as well as greater competition between manufacturers.

There is one critical operating parameter to remember when working with ultracapacitors. Unlike batteries whose chemical reaction produces a fairly constant output voltage at various current levels, the output voltage of a charged ultracapacitor begins to fall the instant current flows from the device and the electrostatic charge starts to deplete.

The rate at which the voltage drops is measured in time constants (T). The value of T (in seconds) is a function of the capacitance of the ultracapacitor in Farads (Fd) and its connected resistance in Ohms (Ω): T = R × C. Thus, a 300-Fd ultracapacitor connected to a 100-ΩŒ resistor would have a T equal to 30,000 sec.

One time constant (1T) is the amount of time it takes for the output voltage to drop 63.2%. At the end of the second time constant (2T), the value has dropped another 63.2% of the value at 1T. The ultracapacitor is said to be totally discharged after 5Ts, although its output voltage has dropped below usable levels well before this time is reached.

© 2012 Penton Media, Inc.

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