Batteries have high-energy capability while ultracapacitors have high power capability. In an optimal hybrid storage system, both technologies could be combined in a way that maximizes the benefits of both.

ULTRACAPACITORS -Used in Hybrid-Electric Drive Systems

Contributed by | Maxwell Technologies

Batteries have high-energy capab
Batteries have high-energy capability while ultracapacitors have high power capability. In an optimal hybrid storage system, both technologies could be combined in a way that maximizes the benefits of both.
Ultracapacitors used in Hybrid-Electric Drive Systems
Maxwell Technologies

Background

The depletion of natural resources, air pollution, traffic congestion and the rising price of fossil fuels are all issues pushing communities and individuals to search for alternative means of transportation. Fortunately, something is being done. "Transportation for a Clean Planet" is the motto of the ISE Corporation, who has been building hybrid-electric diesel, gasoline, hydrogen and fuel-cell-based drive systems for buses and trucks since 1996.

These alternative drive systems are gaining favor throughout the automotive industry because they improve fuel economy and reduce harmful emissions.  ISE specializes in production of "series" hybrid-electric drive systems, where the engine is not coupled to the driveline and is used only to generate electrical power.  This type of hybrid architecture is especially attractive for large vehicles in stop-and-go driving, such as urban transit buses and delivery trucks.  Conventional buses and trucks of this type use huge amounts of fuel and produce high levels of toxic emissions because they have large (typically diesel) engines that are constantly ramping up and down - the least efficient way to operate a power source. 

In the ISE series hybrid system, engine efficiency is increased with the use of a smaller engine mated to a generator and operated at constant, efficient rpms and power output levels.  When vehicle power requirements temporarily increase - such as during acceleration or hill climbing - additional power is drawn from an onboard energy storage system comprised of batteries and/or ultracapacitors. During deceleration, regenerative braking recaptures energy while slowing down the vehicle and recharging the energy storage system.  At other times when vehicle power requirements are low, the generator can recharge the energy storage system. 

The Problem

In hybrid applications, batteries create many design challenges for automotive engineers. Firstly, batteries need a temperature management system to function well in extreme hot and cold weather. Secondly, batteries require charge equalization management to prevent premature cell failure. Thirdly, batteries have limited cycle life under deep discharge conditions, which can result in high-cost replacement throughout the life of the vehicle. A new battery has to be purchased and installed; the old battery has to be removed and disposed. Battery disposal can be problematic unless the manufacturer has a recycling program. All of this adds to the cost of a battery-based system, not to mention downtime of the vehicle itself.

Perhaps most importantly, batteries are limited or inefficient in their ability to quickly capture and regenerate energy, or, in other words, provide bursts of high power during short duration events, such as acceleration and braking. This high power limitation reduces the efficiency of the hybrid electric drive system design.  Because city buses spend much of the time either braking or accelerating, the ability to capture and regenerate braking energy was central to ISE's design. The limiting factors of the battery proved a particular challenge in ISE's goal of designing highly efficient bus and truck systems that would overcome traditional inefficiencies while delivering rugged, all-weather, stop-and-go traffic capability.

The Solution

To bring a truly alternative solution to market, ISE would need to design an electric power and storage system that overcomes the limitations of hybrid electric batteries as well as those of trucks and buses themselves. ISE turned to its partner, Maxwell Technologies, to assist in the development of ultracapacitor solutions. This resulted in what Maxwell calls the BOOSTCAP® ultracapacitors, which are integrated into ISE's Thunderpack II™ ultracapacitor packs.

Compact, high-performance, with exceptional long-life, ultracapacitors fulfill many of the functions of batteries, but with dramatically higher reliability, advanced storage features and overall performance.  Ultracapacitors can be used to compliment battery performance, or can replace batteries altogether.

Compared with battery power solutions, the key benefits offered by Maxwell's BOOSTCAP® ultracapacitors are as follows:

  • Ultracapacitors perform well in cold weather, down to -40° Celsius, whereas, without heating, batteries do not operate reliably below 0° degrees Celsius,
  • Ultracapacitors are a safe solution, as a pack is easily discharged overnight or for immediate maintenance.
  • Ultracapacitors have a long life cycle, essentially lasting the lifetime of the machine into which they are incorporated.  This low maintenance results in cost savings.
  • Ultracapacitors are 85-95% efficient, compared to an average of 70% or lower for batteries in similar applications (as measured by ISE).
  • Ultracapacitors are environmentally friendly, as they are 70 percent recyclable and do not include heavy metals.
  • Ultracapacitors provide a significant high power energy storage solution, capable of capturing energy from braking and storing it for the next acceleration.

Following is a comparison of a specialized, heated ZEBRA battery solution also used by ISE Corporation as compared to the proprietary Thunderpack II™ ultracapacitor solution.

 

ZEBRA Battery Pack

Thunderpack II™
Ultracapacitor Pack

Typical Usable Energy

23.5 kWh

0.3 kWh

Maximum Discharge Current

224 Amps

400 Amps

Specific Energy

113 Wh/kg

4 Wh/kg

Specific Power

174 W/kg

1500 W/kg

Expected Life

2.5-5 year

10-12 years

System Cost

400 $/kW

100 $/kW

Life Cycle Cost

$1200/kW

100$/Kw

As is clearly seen, batteries have high-energy capability while the ultracapacitors have high power capability. In an optimal hybrid storage system, both technologies could be combined in a way that maximizes the benefits of both.

As Tom Bartley, Manager of New Business at ISE Corporation states, "Due to their excellent power characteristics, Maxwell ultracapacitors are essential to the design of ISE drive systems.  Ultracapacitors increase the performance, reliability and durability of on-board energy storage and provide a key link in accelerating the societal impact and viability of environmentally-friendly transportation."

Since successful testing in 2003, ISE has incorporated ultracapacitors into its gasoline, diesel and fuel-cell hybrid electric vehicles with extraordinary results.

Conclusion/Benefits

Since developing the ThunderPack II™ ultracapacitor solution, ISE has introduced its drive systems in the designs of gasoline hybrid electric, diesel hybrid electric, hydrogen and fuel cell powered buses.  The clean running, quiet, low maintenance vehicles are operating in a number of US urban areas including Long Beach, Palm Springs, Oakland, Gardena, Elk Grove, San Bernardino, Montebello, and New Jersey.  ISE has formed partnerships with Siemens, New Flyer Industries, Optima Bus, Van Hool, and various key suppliers for the manufacture of the vehicles.  There are over 100 buses in revenue service with an expected demand to double that number during the next year.

ISE estimates that over 30,000 ultracapicitors are at work today in its hybrid and fuel cell bus drives, delivering over 75 million farads of electric drive and regenerative breaking power.  As of early 2006, Tom Bartley estimates that the ultracapacitor powered bus fleets have put in over 1.5 million miles of clean, reliable service, delivering on ISE's promise of creating transportation solutions for a clean planet.

ISE Corporation is a leading supplier of hybrid drive systems and components for heavy-duty vehicles such as buses, trucks, trams, airport equipment, and military vehicles, and is a world leader in electric, hybrid-electric, and fuel cell technologies. For more information, visit www.isecorp.com.

Maxwell Technologies is a leading developer and manufacturer of innovative, cost-effective energy storage and power delivery solutions. Our BOOSTCAP® ultracapacitor cells and multi-cell modules and POWERCACHE® backup power systems provide safe and reliable power solutions for applications in consumer and industrial electronics, transportation and telecommunications. Our CONDIS® high-voltage grading and coupling capacitors help to ensure the safety and reliability of electric utility infrastructure and other applications involving transport, distribution and measurement of high-voltage electrical energy. Our radiation-mitigated microelectronic products include power modules, memory modules and single board computers that incorporate powerful commercial silicon for superior performance and high reliability in aerospace applications. For more information, visit www.maxwell.com.

 

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