Photo: Siemens

TRANSFER OF POWER: As this train in Oslo slows to a stop, its motors feed power back into the grid for other trains to use.

Oslo has one of the world’s smallest carbon footprints for a city of its size, but it wants to get even greener. To that end, it’s replacing 63 of the T-bane Metro’s trains with new three-car trains from Siemens that are 30 percent more energy efficient than the best cars currently in service there. The key is in the trains’ ability to generate electricity while braking and transfer that power to other trains.

When a train’s operator applies the brakes, the four 140-kilowatt, 750-volt dc electric motors are engaged as generators that use the kinetic energy of the turning wheels to send current back into the metro’s power grid. This technique, called regenerative braking, allows the trains to recover up to 44 percent of the energy used to bring the trains up to speed.

Slowing vehicles down by transforming their inertia into electric current is by no means a new idea. Hybrid-electric cars use regenerative braking to charge onboard battery packs and help boost their fuel economy. The challenge with train systems is that the energy generation occurring in one train must be timed to coincide with a demand for power from a nearby train that is accelerating. The more these stops and starts can be paired, the less electricity the operating authority has to draw from the grid.

Regenerative braking is not to be confused with dynamic braking, employed in many diesel-electric trains to limit wear on the mechanical brakes. In dynamic braking, the current generated by a train’s motors during deceleration goes to a set of large onboard resistors. They release the energy as waste heat or use it to warm the passenger compartments.

The environmental benefits from the new trains do not stop with the regenerative braking system. The 94-metric-ton, 54-meter-long MX3000 trains are made mainly of aluminum, so they are lightweight, and therefore require less energy than the average steel-bodied train to accelerate from a dead stop. What’s more, 85 percent of the materials used to build each train are recyclable. Much of the rest can be burned at thermal energy plants.

Because of the MX3000’s higher efficiency, plus the fact that most of Oslo’s electricity is generated by hydroelectric plants, as little as 2.6 grams of carbon dioxide will be added to the atmosphere per kilometer traveled and per metric ton of vehicle weight, Siemens estimates. In other cities, the average electric train or tram contributes upward of 25 grams per kilometer traveled.

Two prototypes delivered to Oslo in 2005 for testing lived up to energy-­efficiency expectations. The city has so far received a quarter of its 63-train order. By 2009 all of Oslo’s metro system will rely on regenerative braking.