Metro Braking Energy for Station Electric Loads: The Business Case of a Smart Hybrid Storage System

The utilization of excess energy produced through vehicle movements stands in the center of efficiency measures in the transport sector. In case of electric trains, the excess energy of vehicle regenerative braking is mostly wasted as heat. Instead of an instantaneous waste, a later re-use of this energy requests the adoption of an electric storage system. The paper describes real data obtained through on-site and train on-board measurement schemes and a methodology to achieve metro system energy savings redirecting unused energy produced from braking metro trains to the metro station grid consumption. An emphasis is on cost/returns analysis and environmental benefits of the storage system. The Hybrid Energy Storage System (HESS) design developed for the Athens Metro combines efficiently the higher power density and (dis)charging cycles of supercapacitors (coping the high frequency of train stops producing energy) with the superior energy density of batteries (matching a slower release and a longer energy consumption time of stations’ current drain). A smart energy management and control strategy allows upon demand for an internal energy transfer between both storage technologies. So far, single-technology, onboard or wayside storage systems servicing mainly the traction of accelerating trains were available. The novelty here is the dual-technology HESS, located at stations servicing the energy demand of the latter. Preliminary results confirm the feasibility of the energy saving concept indicating a large potential for the MetroHESS reuse of 5000–6000 kWh/day per rectifier substation of otherwise unused braking energy of a metro line and a subsequent s sizing of the stationary HESS is performed. About 30% of the braking energy accrued can be reused through the MetroHESS to cover about 90% of the station energy demand while the residual braking energy will be dissipated in the train braking resistors. An implementation of the stationary storage system to Line 2&3 rectifier substations would cost 17 mi.€, saving on an annual base about 4 mi.€ electricity expenses for the operator as well as 8.600 tons CO₂ for the sake of the community.