Energy storage for infrastructure
Wayside energy storage systems at traction substations capture electrical energy generated by braking trains and release it to supply accelerating trains or reduce peak demand on the grid connection.
On AC-fed railway networks, regenerative braking energy can be fed back into the utility grid via inverter substations.
The recovery problem on DC networks
On DC traction networks — operating at 750 V, 1.5 kV, or 3 kV — the catenary is a passive conductor that cannot accept energy unless a receptive load, typically another accelerating train, is present on the same electrical section.
When no receptive load exists, the braking train must dissipate the energy in resistor banks. Wayside energy storage resolves this constraint by temporarily absorbing braking energy at the substation and reinjecting it when a train accelerates.
Storage technologies
Three technologies serve wayside railway energy storage applications. Lithium-ion batteries provide high energy density and are suited to applications where energy is stored across minutes rather than seconds — for example, capturing energy from one train and releasing it to the next in a lower-frequency service pattern.
Supercapacitors (electric double-layer capacitors) offer lower energy density but much higher power density and cycle life measured in millions of cycles, making them effective for high-frequency urban services where trains brake and accelerate every 90–120 seconds.
Flywheel systems, which store energy as kinetic energy in a spinning rotor, provide similar power density characteristics to supercapacitors and have been deployed in several European metro applications.
Siemens SITRAS supercapacitor-based wayside storage was deployed on the Cologne tram network, where measured energy savings of up to 30% were achieved.
Brussels metro and tram lines using onboard energy storage have reported energy savings of 18–36%, according to peer-reviewed reviews of onboard energy storage deployments in European urban transit.
Grid benefits and sizing
Beyond recovering braking energy, wayside storage can reduce peak demand at the grid connection point, smoothing the highly variable load that traction systems present to the utility network.
This has value both in reducing network charges and in deferring the need for substation capacity upgrades. Storage systems are sized by simulating the traffic timetable, the recoverable braking energy per train, and the time between braking and acceleration events on the same electrical section.
The return period on investment depends primarily on the frequency of train movements and the local energy pricing structure.
EN 50163 defines the voltage quality requirements that the storage system must maintain at the catenary interface. Connection to the substation DC busbar requires protection coordination with the existing substation protection scheme.