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VOLUME -22 NUMBER 8
Publication Date: 08/1/2007
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Ultracapacitors: How they Power the Future
Ragone chart shows relative power densities.
By Tim Koeske, Director of Engineering, Tecate Products, San Diego, CA
Technical advancements have generated an array of devices offering solutions for the needs and conveniences of society. What once was a tethered world is now portable, and the tethered world is becoming simplified by integrating power and communication over a single channel. Demand from the power grid sometimes exceeds supply and a momentary event can have catastrophic and costly effects on sensitive electronic systems. The transportation and automotive industries are aggressively seeking technologies that are greener and more efficient. The electrochemical solutions we have relied on for decades are proving insufficient for powering existing and emerging technologies.
Ultracapacitor solutions based on nanotechnology are now powering portable products and extending the performance of existing electrochemical solutions that are incapable of keeping pace with innovation. Engineers now recognize that ultracapacitors offer an elegant, simple, and low cost solution to a variety of technical problems. Ultracapacitors will be further developed to target niche applications optimized for energy density and power delivery needs. Electrolytic capacitor manufacturers seek to increase energy density by etching the surface areas of parallel aluminum foil films and reducing the thickness of the oxidized aluminum surface serving as the dielectric interface. The oxidized aluminum film serves as the anode and the electrolyte and second aluminum film serves as the cathode.
Electrochemical double layer capacitor (EDLC) manufacturers employ porous carbon-based electrode material with a very high surface area, a separator between the electrodes, and an ionic liquid layer less then 10 Angstroms thick to increase energy storage capacity. When an electrostatic field is applied to the ultracapacitor electrodes, charge separation occurs in the electrolyte, resulting in a double layer dielectric interface. Ultracapacitors can be cycled numerous times (1000 times greater compared to batteries) without degradation because no chemical reactions are involved during ionic separation.
Within the classification of ultracapacitors the two predominant types of electrolytes are aqueous and organic. Aqueous electrolyte generally has lower resistance and lower decomposition voltage resulting in faster cycling, but lower energy storage density compared to organic electrolyte formulas. Regardless of the electrolyte used, ultracapacitors generally need to be series connected to serve useful applications.
Sizing an ultracapacitor module for an intended application requires knowing the minimum and maximum operating voltages, output current, and run time. Energy is proportional to the square of the voltage, and effective utilization of the capacitor's energy requires wide margins in operating voltage. A discharge to half the voltage from a fully charged capacitor yields approximately 75 percent of the total energy stored in the capacitor. Sizing a module begins with factoring an approximate system capacitance, selecting an appropriate cell size, and determining the number of series/parallel interconnected cells.
Understanding how the parameters change with time, temperature, and frequency also need to be considered when sizing modules. Generally, additional capacitance needs to be factored in to compensate for aging effects over time, ambient and cell temperature levels, and self-heating caused by high duty cycles. The instantaneous voltage drop attributed to the internal resistance of the cell must be factored into the operating voltage margins in high-current applications. Excursions above the rated voltage, if excessive, will reduce the life of the cell. Excessive over-voltage operation causes the electrolyte to evolve gas, and a vent is designed into the cell housing to release this gas once a certain pressure level is reached resulting in catastrophic cell failure. Cell balancing may be required to evenly distribute an equal portion of the overall system voltage across each cell. Balancing schemes include active balancing, passive balancing, and over-voltage limiting. For applications such as backup memory, self discharge due to leakage current needs to be considered, and the type of balancing scheme should be chosen to minimize overall current consumption.
A Ragone Chart is a useful benchmark when comparing ultracapacitor solutions to other electrochemical devices. We see from the chart that ultracapacitors have higher energy density compared to conventional electrolytic capacitors and higher power density compared to batteries and fuel cells. By understanding the performance characteristics of each electrochemical technology one can determine whether an ultracapacitor is best used as a replacement for, or a supplement to, an existing electrochemical device. Ultracapacitor modules can effectively be used in main power, backup power, and pulse/bridge power applications.
Battery technologies, such as Nickel-Metal Hydride, Lithium Ion, and Lithium Polymer require complicated charging methods and require one to several hours for charging. Performance varies considerably for temperature, load current, and cycling behavior. Demanding applications and usage profiles generally reveal the weaknesses of contemporary battery technologies resulting in premature battery failure.
PowerBurst 5.4V capacitors.
Ultracapacitors have superior low temperature performance compared to batteries and have simple charging requirements, making them a preferred choice for the main source of power in applications such as automated meter reading, remote sensing, and energy harvesting; where the ambient temperature of the environment is extreme and replacing a failed device is costly. Ultracapacitors can also replace batteries in portable devices requiring a quick charge from a charging station, where a slow charge is available and an infrequent burst of energy is required, and when long term memory backup is needed.
Solar energy, Power-over-the-Ethernet, and Universal Serial Bus all provide a simple and effective means to charge ultracapacitors. Batteries that are charged for prolonged periods of time, or are briefly cycled often lose cell capacity, but ultracapacitors can remain on charge indefinitely with no loss of storage capacity. In many circumstances, temporary energy storage is needed to allow a circuit to power down reliably, provide emergency power for mechanical actuators, or alert a technician that the main power is off-line and a system requires servicing. An ultracapacitor is ideally suited as a cost-effective robust solution.
Ultracapacitors are effective when used in parallel with batteries, fuel cells, and generators. Batteries generate excessive heat when delivering continuous pulse discharges and prefer a constant current drain. Batteries have poor low temperature performance, and in contrast, ultracapacitors are stable down to -40°C which allows a reliable means for cold cranking power in automobiles and trucks. Batteries can be sized smaller for continuous operating currents and ultracapacitors for peak requirements for load leveling and extending battery life. Ultracapacitors are ideal for capturing energy for reuse (as in regenerative braking) or for protecting sensitive equipment from spikes and surges. Low power wireless systems often require a continuous trickle of power with periodic bursts of energy for transmission, and again ultracapacitors offer a practical solution.
No Interruptions Permitted
Large telecom centers, hospitals, and factories require reliable power with absolutely no interruption in power. Diesel generators and batteries have traditionally been used to provide a means of reliable power, and fuel cells are now being used in place of uninterruptable power supplies. Fuel cells and generators are not brought online instantly, and ultracapacitors are ideal for providing bridge power. With the state of current ultracapacitor technology, bridge power for 30-100 seconds of duration can be expected. Waterfall architectures consisting of all existing technologies are employed for providing the optimum level of performance by exploiting the strengths of each power technology, and ultracapacitors are being used more frequently due to ease of implementation, long life, and wide temperature range.
PowerBurst capacitor modules.
Ultracapacitors are now in widespread use. Engineers who recognize the benefits of ultracapacitors have a technological edge over their counterparts. Engineers that understand the transient effects of power consumption can design products that are more robust, and have longer operating life by employing ultracapacitors. Success in the market place is not only about the target application, but also about consistent and predictable operation and longevity. It's easy to identify products that do not live up to expectations due to the constant need of recharging or requiring frequent replacements of rechargeable batteries.
Radio and television commercials broadcasting to conserve energy on hot days to avoid brownouts serve to remind us that energy production is not as reliable as we once considered it to be. Ultracapacitors provide an effective and environmentally friendly means to keep our world powered. They are being used in greater abundance and in ever widening applications. Ultracapacitors are no longer an interest of research, but an effective and viable solution for today's portable electrical needs and infrastructure.
For more information, contact: Tecate Products, 7520 Mission Valley Road, San Diego, CA 92108-4400
619-398-9750 fax: 619-398-9797 E-mail: email@example.com Web:
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