Energy storage devices are considered enabling technologies for future automotive designs, particularly hybrid electric vehicles (HEVs) and fuel cell cars. Consequently, there are ongoing efforts to develop better performing batteries for these applications. Most of this work involves either NiMH or Li-ion/Li-polymer chemistries, which hold great promise for automotive designs because of their high energy density and other attributes.
While these battery developments should have a significant impact on automotive designs in the long run, battery technology tends to develop slowly. The same assessment cannot be made for another type of energy storage device — ultracapacitors. As ultracapacitor manufacturers develop these components for a wide range of applications, ultracaps are making fast and steady progress in performance and cost. These gains could soon affect automotive designs.
In terms of performance, Maxwell Technologies has achieved a 50% increase in energy density (14 J/cc to 21 J/cc) in four years. The increase in power density during the same period was even greater, rising from 3 kW/liter to 9 kW/liter. These advances resulted from increases in the carbon density of the ultracapacitors and improvements in packaging. While performance has been going up, the cost per ultracapacitor cell has been going down. As a result, the vendor has been able to cut ultracapacitor prices in half every two to three years.
Both performance and cost trends will be critical to the deployment of ultracapacitors in automotive electrical systems, which may demand dozens of ultracapacitor cells per vehicle. The combination of many cells into an ultracapacitor bank produces a high-power energy storage device that may be used in tandem with a battery or fuel cell to enhance the performance or reliability of those devices.
Although they have less energy density than rechargeable batteries, ultracapacitors offer greater power density. This enables ultracapacitors to function effectively as short-term, high-power storage devices as already have been demonstrated in trains, trucks and buses. And now, ultracapacitors are getting closer to widespread commercialization in cars.
HEVs may be among the first vehicles to benefit. Consider that Maxwell Technologies is developing standard automotive ultracapacitor power sources aimed at dual 12 V and 42 V electrical systems where currently only a 12 V battery is used. These standard ultracapacitor storage units will deliver power for acceleration during start-stop operation and recovery of energy during regenerative braking. These units will also support electric power steering, computer power net stabilization and other needs. Prototypes of these energy storage units are being performance tested against automotive requirements.
For an ultracapacitor-based energy storage unit such as this one to be commercialized, cost targets become critical. For example, a 42 V storage device might employ 16 ultracapacitor cells, each rated at 2700 F and a peak voltage of 2.75 V. Such cells now cost about a penny per farad, according to Maxwell. However, pricing for the same cells is expected to reach 0.5 cents by 2006, leading to a cost of less than $250 per energy storage device. (Electronics are also required in each box.)
The cost goal for widespread use would be less than $150 per box, which the company expects to reach around 2008. Achieving that goal will demand further increases in carbon density as well as raising the cell's operating voltage, perhaps as high as 3 V in the next two years. Concurrent with cell development will be efforts to optimize and shrink the electronics required for cell balancing, charging and dc-dc conversion.
All of these efforts bear watching, as they will play a critical role in the ongoing electrification of the automobile. Designers interested in the ultracapacitor's potential in automotive designs may do well to follow the development of ultracapacitor energy storage in other areas such as telecom back-up power, where the required power or energy levels may be roughly comparable to those required in cars.