More on de-carbonising public transport

In our 2121 Zoom AGM, our Vice-chairman and Strategy Officer (and retired engineering professor), David Murray-Smith, gave a presentation on the work he has done on decarbonising public transport since our 2019 AGM in Perth (see previous presentation).

David has also written (in collaboration with Peter Fisher of Arcola Energy) a further report outlining developments: Hybrid trains for the highlands? Computer simulations of fuel cell/battery-electric trains on secondary routes in Scotland.

In a cut-down form it was published in the first issue of a  magazine entitled, Electric and Hybrid Rail Technology, which appeared in print at the end of July this year.

This is a progress report on an industry and university collaboration. Early in 2020, Transport Scotland and Scottish Enterprise announced financial support for development of a hydrogen fuel-cell/battery-electric multiple unit for trials in Scotland (on important secondary routes, such as the West Highland Line, where the business case for electrification is not strong). A contract for converting a former ScotRail Class 314 three-coach electric multiple unit to a hybrid configuration has been awarded to a group of companies led by Arcola Energy Ltd.

The report notes that hydrogen fuel-cell stacks are characterised by a sluggish response to demanded power-level changes and their efficiency depends on the operating condition. Powertrain control strategies may therefore involve fuel-cell stack operation with slow rates of change that capture power demand, with fast dynamic changes and peak loads being supplied by the battery pack. The battery pack recharges through regenerative braking or from available power from the fuel-cell stack. Optimal powertrain component sizes depend on route characteristics, with relatively flat routes and operation at constant speed favouring large fuel-cells, while routes with prolonged and steep gradients or larger accelerations require larger batteries. Specifications for lengthy routes involving steep and prolonged gradients such as those encountered in the Scottish Highlands present significant difficulties.

Mathematical models for longitudinal train motion provide a basis for conventional forward simulation of a train with power or tractive force variables as an input, and acceleration, speed and distance travelled as outputs. In contrast, the analysis of road-vehicle powertrains often involves a reverse procedure which starts from a duty cycle based on a record of speed versus time with static or quasi-static models being used to estimate steady-state power and energy demands.

The simulation methods applied in this paper are based on dynamic models which allow transient power requirements to be understood more fully. These methods have been applied to the assessment of hydrogen train designs for some specific routes and also for test routes having profiles that are chosen to be typical  of the routes of interest, but with simplified profiles. These simplified test routes provide useful physical insight into the effect of train characteristics on the specifications for fuel cell stacks, battery packs and other powertrain components.

These methods enable straightforward investigation of performance sensitivities, not only in terms of the longitudinal train dynamics but also the powertrain parameters and route characteristics. The impact of trade-offs made to reduce the weight and volume of powertrain components and the cost of the train can be modelled.

Fuel-cell efficiency can also be considered, as larger cells allow operation over a wider range of conditions. Findings from the test routes considered allow estimation of powertrain ratings and storage requirements for operation of a three-coach hybrid hydrogen fuel-cell/battery electric train on the Glasgow to Fort William line.

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