One problem most common with transit bus fleets is starting failure, especially on cold mornings or after a long weekend when the bus has been idle for several days.

The Regional Transportation District (RTD) in Denver, Colo., has taken a novel approach to combat this common no-start problem with the aid of supercapacitors.

The supercapacitor is an electrical energy storage device that operates by means of static charge capture that is similar to an electrical charge built up on a carpet that gives you a jolt when you walk on it in dry weather conditions. The supercapacitor uses special negative and positive electrode plates separated by an electrolyte. There are several benefits of the supercapacitor. It has very high rates of charge and discharge and can generate a burst of high current power necessary for cranking bus engines. It has an essentially unlimited charge/discharge cycle life, and its power is unaffected by cold temperatures.

RTD uses a supercapacitor design employed to start military tanks in Siberia. In the bus, a supercapacitor is wired-in parallel with the batteries that charge it. During bus starting, a solenoid is activated to complete the circuit between the supercapacitor and batteries. Supplementing the battery power, the supercapacitor provides a short burst of current two to three times higher than that of regular batteries to overcome the engine parasitic load even at very cold temperatures to crank the engine to high rpm. After the engine is started, the supercapacitor stays in the circuit for five minutes to fully recharge, then disconnects and waits until the next starting cycle. The process control for charging and discharging the supercapacitor is handled by the bus multiplexing system.

RTD started to look into the use of supercapacitors in 2003 when it retrofitted 10 buses with them. This demonstration showed a potential benefit in reducing no-start problems and battery usage. When RTD awarded a five-year contract in 2005 to replace its old transit bus fleet, it included in its bus technical specification a requirement for a supercapacitor in each bus. The new buses started to arrive in the summer of 2005. Currently, RTD has 270 transit buses in service with supercapacitors. Even though the supercapacitor experience has been relatively short, we have seen several benefits. We are seeing a decrease in bus battery usage. The buses equipped with supercapacitors are showing a battery usage rate equal to only one-third the usage rate of the buses without supercapacitors. We are also seeing a decrease in starting/charging roadcalls. This winter, one of the harshest winters we have seen in many years with weeks of temperatures below freezing, we have seen the starting/charging roadcalls on buses equipped with supercapacitors equal about only 20 percent of the roadcalls on the buses without supercapacitors. In addition, supercapacitors seem to be reliable. So far we have not replaced a single one due to failure.

We are continuing to equip our new buses with supercapacitors in an attempt to reduce starting problems to a level that excludes it from the top 10 maintenance problems of RTD's transit bus fleets.

Lou Ha is the manager of technical services for the Denver Regional Transportation District.