Weighing up the alternatives to electrification

4 March 2019
To quote the Rail Delivery Group’s long-term passenger rolling stock strategy, “over time, electric trains have generally proved to be more reliable, efficient, environmentally friendly and cheaper than those powered by diesel engines”.
DAVID SHIRRES compares the various ways we can electrify our network.

Chris Grayling’s parliamentary statement cancelling various electrification schemes included the comment that “bi-mode trains can improve journeys without the need for electrification”. Yet, when he decided to cancel these schemes, he was advised that bi-mode rolling stock with the required speed and acceleration did not exist.

At full power, the overhead line supplies a nine-car electric Pendolino train with about 6MW – equivalent to the amount of power needed to supply six thousand homes. 5MW are needed to power the train, with a further 1MW or so suppling auxiliary machines and the train’s hotel load.

A nine-car bi-mode train has five underfloor diesel engines with a total power output of 3.5MW, of which about 2.8MW is available to power the train. When these trains operate in electric mode they draw 4.5MW of traction power from the wires, sixty per cent more than in diesel mode.

Dirty diesels

Electric trains are much cheaper to operate and maintain, and modern EMUs are more than twice as reliable as modern DMUs. The Great Western IEP fleet costs £4m per coach more than the electric East Coast one, mainly because it operates more miles in diesel bi-mode. Thus, the additional diesel maintenance cost of the GWML bi-mode fleet is around £1bn over the period of the maintenance contract.

Diesel fuel is also significantly more expensive than electric traction, in part because electric trains can recover the huge amount of energy generated during braking back into the wires. Diesel fuel accounts for 40% of Virgin West Coast’s traction cost, despite only 15% of its fleet being diesel-powered.

Electrification offers not just higher speeds but higher acceleration. A modern EMU can accelerate to 60mph in about 50s, compared with 110s for a modern DMU. For this reason, electrification offers significant reductions in journey time on routes with frequent stops.

Former Transport Minister Jo Johnston called for rail industry proposals to remove diesel-only trains from the rail network by 2040. Johnston considered that, as battery technologies improve, diesel engines in bi-modes would be replaced altogether and that perhaps both batteries and diesel engines will be replaced by hydrogen units.

Batteries versus hydrogen

A trial in 2015 used a four-car EMU that had a 7.2 tonne, 424kWh battery pack fitted under one coach. Running under electric power for seventy percent of the time was sufficient to provide a battery charge that provided performance comparable to an EMU over 77km.

The trial demonstrated the feasibility of using batteries to power a train for short distances off the electrified network. However, the use of batteries beyond electrified wires for much longer distances, such as Kettering to Sheffield and back (305km), would require the development of batteries with a much higher energy density.

Hydrogen has been shown to be a viable traction technology for medium-range applications. However, it has a low efficiency. When produced by electricity, hydrogen trains require around 3.4MW to deliver 1MW of traction power. If electricity is fed directly into a train, only 1.2MW is required.

...hydrogen cannot be used for long-distance high-powered trains unless passenger space is sacrificed.

Like batteries, hydrogen also has a low energy density – a tenth that of diesel. Hydrogen traction equipment consists of fuel cells, a traction battery and traction converter. Incorporating this and the hydrogen fuel tanks into a train requires it to be built around this equipment. Hence hydrogen cannot be used for long-distance high-powered trains unless passenger space is sacrificed.

What's the answer?

For the foreseeable future, the only viable self-powered rail traction technology for high-powered or high-speed trains is the diesel engine. This is particularly true for freight trains that require around 2.5 MW.

Thus, the only way to decarbonise the rail network is further electrification with the use of hydrogen and batteries on branch lines and rural routes only. Electrification eliminates pollution at the point of use and offers potential carbon reductions by enabling rail traction to be powered by renewable energy sources.

The benefits of electrification are recognised by many countries that have a high percentage of their rail network electrified. The industry needs to convince the UK Government that it has learnt lessons from over-expensive projects and can now deliver electrification at an affordable cost.

Author's face
David Shirres

David Shirres joined British Rail in 1968 as a scholarship student and graduated in Mechanical Engineering from Sussex University. His roles in British Rail included Maintenance Assistant at Slade Green, Depot Engineer at Haymarket, Scottish DM&EE Training Engineer and ScotRail Safety Systems Manager.

He retired from Network Rail in 2009 after a 37-year railway career. At that time, he was working on the Airdrie to Bathgate project. David was appointed Editor of Rail Engineer in January 2017 and, since 2010, has written many articles for the magazine on a wide variety of topics.

He is also an active member of the IMechE’s Railway Division, having been Chair and Secretary of its Scottish Centre.

Permanent Rail Engineering is an engineering consultancy focussed on planning, design and management for sustainable transport, including railways. The views of our guest bloggers aren't necessarily exactly the same as ours, but you can be sure that there's a good overlap on transport matters.