Hybrid Rail Technology Review: an Intermediate Pathway For Electrifying the Freight and Commuter Rail Sector - a Technical and Operational Assessment

Abstract

Abstract The transport industry, as any other sector, has been permanently challenged by both the continuously stringent environmental standards and the energy efficiency requirements, which has driven a set of initiatives focused on both the fuel burn reduction and the environmental performance improvement. The rail sector currently relies on the efficient and local zero emission electrical traction for the medium to heavy density corridors. However, for the light to medium density corridors (both passenger and freight), given the high upfront costs associated with the electrical infrastructure, they are currently required to rely on fossil fuel based traction (often, the diesel-electric) architecture, with an inherent efficiency and environmental burden. The advent of hybridization, i.e. the use of more than one power source in a powertrain (mainly — but not restricted to — an internal combustion engine (ICE) and electric motors (EM), associated with an electrical energy storage device - ESD) — currently a feasible approach for the automotive sector — has opened the way for the rail industry, as an opportunity to improve the energetic efficiency and reduce the environmental footprint for the aforementioned low to medium density rail corridors, without the cost burden of an electrical infrastructure. The hybrid powertrain efficiency drivers are basically: i) kinetic energy recovery, through the use of the regenerative braking (i.e. using electric motors as generators, to recover part of the train’s kinetic energy); ii) improved engine performance, avoiding the low efficiency (low load) engine range and iii) engine downsizing (engine power requirement reduction, as it is assisted by the electric traction on power bursts). From an environmental perspective, the reduced fuel consumption also means lower emissions. Moreover, hybrid configurations might also reduce noise and gaseous engine emissions within/nearby stations or urban rail yards, by switching off internal combustion engines, running the train and powering auxiliary systems with the previously stored electrical energy on the ESD. Finally, for electrified rail lines, the hybrid rail configuration might also provide the so called last mile capability, used to circumvent non electrified rail stretches, like bridges or tunnels, as well as small extension non electrified rail segments. This work presents a review of hybrid rail technology, covering hybrid configuration and energy storage devices, from both a technical, operational and environmental perspective, supported on current available technical literature, as well as on simulation and field test reports, followed by a near to mid term outlook of hybrid rail technology for both freight and passenger segments.