Many mass-transit systems are electrically powered - for example urban light-rail and underground-train systems. In these applications, energy-storage devices can be used to capture energy when the vehicle brakes, and then release this energy to assist in acceleration. This can reduce energy demand significantly, saving money and allowing more or longer vehicles to use an existing track without upgrading infrastructure.For buses, both fully electric and hybrid designs can be made more efficient through the use of energy storage to capture braking energy. In hybrid vehicles, start-stop operation is also made possible: the internal combustion engine is turned off when the vehicle is stationary and initial acceleration is powered only by the electric motor. Fuel consumption, noise and pollution are all reduced - and the low- speed performance can be improved - with the electric motor assisting the main engine. As well as for the recuperation of braking energy, energy storage can also be used to help meet peak power demands. This can improve reliability in rail applications by avoiding voltage drops when demand is highest (e.g. when more than one train is accelerating away from a station at the same time).
Energy storage for transportation
Heavy transportation vehicles place particular demands on energy-storage devices. They must be very robust and reliable, with a long lifetime and low maintenance requirements. They must be able to operate efficiently under harsh conditions, and be able to deliver high peak currents. They must also be capable of working on a high duty cycle and cope with frequent deep discharging. Finally, they must be straightforward to integrate into vehicle designs. The obvious energy-storage device might be a rechargeable battery, but in fact these have some serious limitations for this kind of application: batteries are heavy, large in size, have a limited charging rate and potentially high maintenance; they suffer from degraded performance at low temperatures.Recently, newer designs have looked at another energy-storage component: the ultracapacitor. Ultracapacitors - or double-layer capacitors - provide high charge acceptance, high efficiency, cycle stability and strong low-temperature performance, and they are virtually maintenance-free. The combination of ultracapacitors and batteries is also an option if high power and pure electric driving are required. Batteries have a higher energy density than ultracapacitors, but a lower power density. They are suited to delivering high power for relatively short periods, typically less than 20s, while batteries can provide lower power for longer periods. This means the two devices can often complement each other. Compared to batteries, ultracapacitors have a number of key advantages for transportation applications: They offer up to 10 times the power of batteries, helping acceleration of the vehicle. Their low-temperature performance is excellent down to -40°C, whereas without heating, batteries do not operate well below 0°C. Moreover, ultracapacitors are extremely safe because a pack featuring equalisation is discharged over night and recharged at the start of its drive cycle the next morning, thereby ensuring cell balance. T The life cycle of an ultracapacitor is very long - usually the life time of the vehicle they are designed into - thus reducing maintenance costs (they can typically be used for 1 million charge cycles, which typically equates to 7,500 operational hours or 15 years of useful life). Finally, ultracapacitors are more efficient: up to 95% compared to below 70% for batteries. Maxwell Technologies has introduced a heavy-transportation module: the 125V HTM Boostcap ultracapacitor module. Based on the 3000F cells operating at 2.7V, the module can store more energy per unit volume, deliver more power per unit volume and weight, and last longer than other commercially available solutions. The ultracapacitor cells have very low internal resistance to achieve high efficiency. This efficiency and careful thermal management in the module design allow it to operate at very high currents - up to 150A continuous and 750A peak - to deliver the high power demanded by transportation applications.
Ultracapacitors in practice
In transportation applications worldwide, ultracapacitors have proven their value over a number of years. For example, in Germany, a prototype light-rail vehicle developed by Bombardier has been in passenger operation since 2003, and has demonstrated that it can potentially save up to 30% of energy as compared to a modern regenerative light-rail vehicle. Ultracapacitors are used to store energy from braking, which lies somewhere between 100,000 and 300,000 load cycles per year in a typical light-rail vehicle - which also means that batteries would not be suitable for this application. Ultracapacitors are also lighter than a comparable battery.Siemens has also used ultracapacitors for regenerative braking in its Sitras SES system, which has been proven in the metro systems of Cologne and Madrid, for example. In this implementation, the ultracapacitors are located in trackside units, which can absorb the braking energy from all trains within a 3km radius. Hybrid electric bus applications have also shown good results: one example is ISE's buses, which run in the US cities of Elk Grove and Long Beach. The systems have worked reliably at temperatures from -25 to +45°C. The response of the drive system is significantly better than that of a standard bus, and fuel economy is improved through the efficient capture of more braking energy. Preliminary data indicates the average fuel efficiency of a bus with an ISE ultracapacitor-based hybrid-electric drive system is significantly better than that of a bus with a competitive battery-based hybrid-electric drive systems and a bus with a standard drive system. Field experience shows that the ultracapacitor-plus-battery hybrid bus recuperates 38% of the propulsion energy, which translates into an average fuel economy gain of more than 3.9 miles/gallon.
