As electrification of transportation becomes more important, designers and manufacturers are looking for energy storage solutions to increase fleet efficiency. Already, many are using ultracapacitors to replace or enhance batteries in transportation markets such as hybrid buses and trains. Ultracapacitors store a large amount of power in a small package and have a large capacitance with a low RC time constant, making them ideal for hybrid bus, rail and electric vehicle applications. Ultracapacitors work best in conjunction with a battery, as they make a battery-powered system run more efficiently. There are several reasons why hybridizing public transportation is the best way to decrease wasted energy. First, ultracapacitors operate at a 95 to 98 percent efficiency range, which is far beyond that of batteries. Second, ultracapacitors function well in wide-ranging temperature conditions, from +70 degrees Celsius to -40 degrees Celsius. With the added benefits of a long lifespan and little required maintenance, ultracapacitors are enticing to manufacturers. Finally, ultracapacitors are more cost effective for high power, high cycle applications. Ultracapacitor prices have declined by 99 percent in the past decade compared to a less than 40 percent cost decrease in batteries. All of these factors put ultracapacitors way ahead of batteries, yet many public transportation sectors are still wary of adopting the technology. New technology advances in the development of ultracapacitors are set to change this and make ultracapacitors an industry standard in mass transit markets. ESR, or equivalent series resistance, is the measure of resistance within the capacitor. A lower ESR results in higher efficiency, which ultimately is the desired end result. By increasing that efficiency, even if by only 10 percent, the power capabilities of the ultracapacitor will increase by a parallel amount. The boost of power translates into higher performance, which is attractive to mass transit manufacturers and operators. Energy storage technology has advanced significantly in the past decade and will continue to improve rapidly in the future. As such, ultracapacitor volumes are forecasted to see more than 50 percent compounded annual growth during the next 10 years. As a result of the innovation and research occurring in the energy storage industry, it is likely that we will see the availability of ultracapacitors with decreased ESR of at least 10 percent on the market in the near future. A lower ESR will increase energy performance in a number of mass transit applications. These include the following: Light rail: Light rail applications require a significant number of ultracapacitors to provide the power and energy required to propel a train up to speed and across distances as long as 500 meters. The number of cycles can be up to 350,000 per year. These systems require cooling due to the heat generated from the system and cell resistance. By lowering the cell resistance by 10 percent, mass transit manufacturers can also reduce their cooling requirements by 10 percent, making the systems smaller, lighter and more efficient. Hybrid buses: Similar to the train application described above, ESR plays a large role in the efficiency and cooling requirements used in hybrid bus applications. The lower the ESR, the more efficiently the bus can recapture energy during braking or use the energy captured during acceleration with fewer cooling requirements. Mass transit markets are eager for cells that reduce the excessive cooling requirements and suppress the cycle life. Technology capable of meeting that demand is on the horizon. In the near-term, ultracapacitors with lower ESR will fill the needs of manufacturers in the rail and hybrid bus markets, leading to lower fleet maintenance costs and increased efficiencies. Jeff Colton is vice president of sales with Ioxus, Inc. Colton has 20 years of success leading technical sales and marketing teams at several companies, including General Electric Corp., Sanyo Electric Corp., Saft Battery Corp. and Pentadyne Power Corp.