ISE hybrid-electic bus built for city of Long Beach, California
This ISE hybrid-electric transit bus utilizes its jointly developed Thunderpack II ultracapacitor bank.

Better Than Batteries

Case study of application of ultracapacitors in heavy hybrid bus applications

By Tom Bartley

The depletion of natural resources, air pollution, traffic congestion and the rising price of fossil fuels are all issues driving 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, which has been building hybrid-electric hydrogen and fuel-cell-based drive systems for busses 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 completely decoupled from the driveline and is used only to generate electrical power.  This type of hybrid architecture is especially attractive for large vehicles that perform large amounts of stop-and-go driving, such as urban transit buses and delivery trucks.  Conventional buses and trucks of this type are highly inefficient 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, a smaller engine is mated to a generator and operated at a constant, efficient speed and power output level.  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. When vehicle power requirements are low, the energy storage system is recharged.  Not only is engine efficiency increased, but also the vehicle is able to recapture energy whenever it slows down through a process called regenerative braking.

A privately held company, ISE Corporation was founded in 1994 by David Mazaika and Michael Simon in San Diego, California.

The Problem
ISE's Hybrid Electric Vehicle (HEV) technology combines the best characteristics of fuel-driven engines, electric motor drives and energy storage components. Their solution is designed with a combustion engine that functions as the primary power source, and an electric motor with a power storage system that functions as the secondary power source. Designers are able to size the combustion engine for cruising power requirements, thanks to the presence of the secondary power source that handles peak power demands for acceleration. Additionally, regenerative braking energy is captured by the secondary power system. That energy is applied for further acceleration or for the basic energy needs of supplementary electrical systems.

Standard hybrid electric design, which uses only batteries to provide electrical power storage, has drawbacks. These deficiencies are multiple, and they create many design challenges for automotive engineers.

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 though, batteries are limited in their ability to capture and regenerate energy, or 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 most city busses are in a constant brake-acceleartion state, 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. 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 busses themselves.

The Solution
To do this, ISE turned to its partner, Maxwell Technologies, to assist in the development of ultracapacitor solutions. 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:

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 18kWh 0.3 kWh
Charge and Discharge Current 32kW Over 150 kW
Energy Density 87 Wh/kg 4 Wh/kg
Power Density 0.16kW/kg 1.5kW/kg
Expected Life 2.5-5 year 10-12 years
System Cost 375 $/kW 100 $/kW
Life Cycle Cost $1125/kW 100$/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 are combined in a way that maximizes the benefits of both.

At ISE we have found that 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.

Since developing the Thundercap II ultracapacitor solution, ISE has introduced its drive systems in the design gasoline hybrid electric, diesel hybrid electric, hydrogen and fuel-cell powered busses. The clean running, quiet, low maintenance vehicles are operating in a number of US urban areas including Long Beach and Sacramento. ISE has formed partnerships with Siemens and Flyer Inc. for the manufacture of the vehicles.

ISE estimates that over 30,000 ultracapacitors are at work today in its hybrid and fuel cell bus drives, delivering over 75 million farads of electric drive and regenerative breaking power. We currently estimate in early 2006 that the ultracapacitor powered buss fleets have put in over 2 million miles of clean, reliable service, delivering on ISE’s promise of creating transportation solutions for a clean planet.

Times Article Viewed: 12130
Published: 05-Apr-2006


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