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Siemens installing Sitras supercap-based stationary energy storage unit for TriMet light right substation

Sitras SES. Click to enlarge.

Siemens is currently installing the first Sitras SES (stationary energy storage) Energy Storage Unit with supercapacitor technology in the US on the new TriMet Portland-Milwaukie Light Rail Transit Line. The southeast Portland Tacoma substation location will house the first US storage unit that allows for energy created during braking to be stored and then re-used in one of two applications: energy savings or voltage stabilization during peak demand times. TriMet will utilize the system in voltage stabilization mode.

In voltage stabilization mode, the Sitras SES allows the system to avoid problems that have led to disruptions in mass transit operations. If a number of vehicles accelerate simultaneously, system voltage can drop below a critical level and result in instances of undervoltage tripping in vehicles and, consequently, lead to disruptions in passenger service. The Sitras SES ensures the system voltage always remains within the required range and voltage-related disruptions no longer occur.

The regenerative energy storage unit is an important piece of the many sustainable elements being incorporated in this light rail project.

—Dan Blocher, executive director of TriMet Capital Projects

American Maglev Technology (AMT) is installing supercapacitor-based braking energy recuperation systems on the TriMet light rail vehicles. (Earlier post.)

In TriMet’s system, energy generated by decelerating vehicles is fed into the traction electrification system for use by other vehicles on the system. TriMet is using the TIGGER funding to retrofit 27 vehicles with the energy storage system, which captures and stores regenerative braking energy as it is generated, uses the stored energy for acceleration, and releases surplus energy to other vehicles on the system as needed. The 27 retrofitted vehicles are being paired with non-capacitor equipped vehicles so that stored energy can be used by both vehicles.

Sitras SES
Input voltage 600 / 750 Vdc
Supercap modules 2 x 12
Total capacitance 94 F
Usable energy content 2.5 kWh
Max energy saving per hour 80 kWh/h
Peak power 0.7 MW

The Sitras SES unit comprises four double cubicles for installation in existing or in new substations. The system can be connected either directly to the traction power supply system or to the busbar in the substation by means of the connection unit, comprising the disconnector, the DC high-speed circuit-breaker and the precharging unit. The connection between the connection unit and the capacitor banks is made by the standard vehicle converter, which functions as a step-up / step-down converter.

The Sitras SES operates in two modes:

  • As a voltage stabilizer, the energy content is constantly kept at a high level and energy is discharged when the system voltage falls below a specified limit.

  • In energy saving mode, it absorbs the energy generated by braking vehicles and stores it until the energy storage system can feed it back into the power supply system at a later point when vehicles are accelerating.

Installation of the Sitras Energy Storage Unit also allows TriMet to avoid placing a utility-connected substation in the same location.

The system can be switched at any time between these two operating modes so that it can adapt perfectly to the prevailing operational requirements.

The Sitras Energy Storage unit is part of Siemens Smart Grid Rail Electrification portfolio, providing solutions to connect railway systems to the power grid.

The Sitras SES has been successfully installed in Spain and Germany, reducing the energy demand at Cologne Transit Authority substation by 15,000 kWh in one month. The use of just a single energy storage unit could save a maximum of 500,000 kWh per year, Siemens says. In addition to the cost saving, the storage unit can reduce CO2 emissions by 300 tons per year with this reduction in energy generation.

Siemens is providing additional solutions for the overall 7.3-mile line extension project that will connect the southeast corridor to downtown Portland, and other regions that TriMet MAX serves. Siemens Rail Systems is manufacturing 18 rail vehicles at its Sacramento, CA facility for the Portland-Milwaukie extension and Siemens Mobility and Logistics will be implementing its Rail Automation signaling and communication (SCADA) technologies to center control on the line extension.

The first substations will be installed beginning in October 2013. The Portland-Milwaukie Light Rail Transit Project is set to open in 2015 and will expand the MAX system to 60 miles and 97 stations.



At $100/MWh, the electricity cost savings come to only about $50,000/year.  This could be increased if demand charges are also reduced.

There are probably other opportunities for savings in the system, such as reducing the rate of acceleration when multiple trains are leaving stations at the same time, or staggering departures.  If this could further reduce demand fees it might pay for itself.


I suppose you have to build some of these and see how well they work and cost.
Sometimes you have to spend the money to learn the lesson.

Learning how to use supercaps in general is a good thing and could benefit many fields (HEVs, trains, etc.)

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