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.
Energy recovery is one of the structural asymmetries in the railway’s power supply and electrification architecture: AC networks can return braking energy to the grid, DC networks cannot. Wayside storage exists to close that gap.
The recovery problem on DC networks
On AC-fed railway networks, the substation transformer is inherently bidirectional, so regenerative braking energy is fed back into the utility grid directly, without extra equipment. 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. Lithium-ion batteries provide high energy density and suit applications where energy is stored across minutes rather than seconds — 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, with cycle life measured in millions of cycles. That makes 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, according to Siemens’ own figures for the installation, energy savings of up to 30% were achieved. Onboard, rather than wayside, energy storage is a related but distinct approach, covered under battery systems for rail.
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 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.