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Energy Storage for the Rail Industry, Part II: Ultracapacitors for Bridging Power, Last Mile and Diesel Starting

Energy Storage for the Rail Industry, Part II: Ultracapacitors for Bridging Power, Last Mile and Diesel Starting

| Alpbora Oruc, Sr. Sales Applications Engineer
To effectively switchover between lines, trains need bridge power so that electricity to the train continues uninterrupted. Ultracapacitor (supercapacitor) energy storage systems serve this application well.

In Part I of this blog series, I discussed the difference between wayside and onboard energy storage systems for the rail industry. Today, let’s look specifically at ultracapacitor energy storage and its ability to contribute to higher efficiency and emissions reductions for various rail applications.

Bridging power for catenary-free trains

Catenary-free trains refers to trains that do not have overhead lines. Many trains are partially catenary-free, meaning there are some points when the train is on and off an overhead line. There are various energy challenges related to catenary-free trains.

One difficult scenario is when there is a change of tracks between two lines. At this intersection point, there is no overhead source of power, resulting in loss of electricity to the train and power for rider services, including air conditioning and Wi-Fi. To effectively switchover between lines, trains need a bridging system, or "bridge" power, so that electricity to the train continues uninterrupted.

An ultracapacitor (supercapacitor) energy storage system can serve as the backup power source when overhead power isn’t available. Approximately 20 seconds of bridging power (which translates to about 100 meters of bridging power) is required when trains change tracks at lower speeds. Ultracapacitors are very well-suited for these applications because they are specialized for these types of short-term, high power demands. When applied, an ultracapacitor energy storage system bridges the energy gap with highly efficient bursts of power, effectively eliminating short-term power interruptions. Once the train is back on an overhead line, the ultracapacitor system recharges from the line power.

Last mile

The last mile or last meter application involves stretches of the train’s journey where it doesn’t have access to power. This could be when a train enters a tunnel, when there is a short-term grid power loss, or when the train is approaching a maintenance facility where there are no lines or traction power and the train needs to travel the last mile (or kilometer) without catenary power.

This is an application that doesn’t require a lot of energy but does need several seconds of power at low traveling speeds. Again, ultracapacitors are well-suited for this application. A few dozen ultracapacitor modules, which are light-weight compared to most battery energy storage systems, can be installed as an onboard solution to serve this short-term power need and carry the train through its last stretch of travel.

Diesel starting

Traditional diesel trains continue to operate where rail systems are not yet electrified. Diesel trains have massive engines that need reliable starter systems, especially in extremely cold European regions where icy conditions cause starting failures. Ultracapacitors have proven their resilience in cold weather engine starting, performing in temperatures as low as –40°C. They can be flexibly dimensioned and installed in a series and parallel connection to provide the necessary voltage for starting.

An advantage of using ultracapacitors for engine starting is that the train’s batteries can be downsized significantly. A hybrid ultracapacitor-battery system reduces weight while providing the same power capability of the original full battery system.


Another method for achieving the above applications is synchronized inverters that transfer energy from one line to another in a dynamic way. For the inverter method, the rail must have highly impacted rail stations so that the process can be executed in perfect time: one passenger train should be braking while the other train is accelerating so that the inverters can instantaneously transfer the energy directly between the lines.

Ultracapacitor energy storage doesn’t rely on start-stop events and therefore provides more flexibility. In the case of an overload or if utilization is not that high, ultracapacitor energy storage makes it possible to save excess energy for a long period of time and dispatch it when it’s needed and with very high efficiency. Ultracapacitors eliminate the synchronization issue of the inverter method.

Battery energy storage is another option. Batteries are ideal storage applications for long-term energy delivery; however, placing high power demands on batteries often results in low efficiency and the battery systems need oversizing to achieve high power discharge. Systems operators should consider the advantage of batteries for long-term energy and ultracapacitors for short-term high power, with many applications being best approached with a hybridized system.

I hope this blog series has provided you with valuable insight into energy storage solutions for the rail industry.

RELATED CONTENT: Energy Storage Options for Rail Take the Spotlight at TRAKO and Expo Ferroviaria (PHOTOS)

Mike EverettAlpbora Oruc
Sr. Sales Applications Engineer


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