Railway must be decarbonised to help meet ambitious environment targets

David Shirres

Hydrogen fuels cells on Alstom's iLint hydrogen train (Credit: Alstom)
Hydrogen fuels cells on Alstom's iLint hydrogen train (Credit: Alstom)

The 2008 Climate Change Act requires the UK government to ensure that, by 2050, greenhouse gas emissions are 80% lower than those of 1990*.

By 2017, emissions were 42% of the 1990 baseline, largely because renewables now produce 30% of the nation’s electricity.  

But further reductions will be more difficult. And a new net-zero target by 2050 has been recommended by the Committee on Climate Change (CCC). The committee considers that this new target is achievable with known technologies but requires urgent policy changes.

This requires deep decarbonisation of the national grid with a major expansion of renewable and low-carbon generation plus carbon capture and storage. A CCC report calls for extensive electrification, with domestic gas boilers replaced by electric heating and widespread use of battery-powered road vehicles. As a result, electricity production needs to double by 2050. 

This report barely mentions the rail sector. Rail greenhouse gas (GHG) emissions per passenger are typically a quarter of those from road transport. Furthermore, in 2017 rail transport produced only 2m tonnes GHG, compared with 69.6m, 29.4m, 3.4m and 20.8m tonnes respectively for cars, vans, buses and HGVs.

Nevertheless, rail has a part to play, particularly in respect of modal transfer. A shift of 3% of passengers and 3% of freight from road to rail would give annual GHG emissions savings of 2m tonnes – the rail sector’s total emissions. Such a modal shift is only possible if rail has the capacity to accept it. This is one reason why High-Speed 2 is required.

Diesel dilemma

The rail industry must also address environmental concerns about diesel particulate emissions. As more cities create ultra-low emission zones, diesel train engine fumes in stations will become increasingly unacceptable. Although this is not a climate-change issue, the introduction of zero-GHG emission trains will also solve this problem.  

Last year, the government called for diesel-only trains to be off the tracks by 2040 and challenged the industry to develop alternative traction. Transport secretary Chris Grayling has said that the new East-West rail line will not be electrified and that instead it will have “a completely new generation of low-emission trains”. Having experienced electrification cost overruns, the government’s view seems to be that new types of self-powered traction are needed to deliver a zero-carbon railway.

Indeed, Grayling justified the cancellation of electrification schemes by declaring that bi-mode trains are the “best available technology”. Yet the reality is that they will spend most of their lives lugging around idle diesel engines, which constitute 8% of their weight, under electric wires and, in diesel mode, have the same performance as the trains they replace. Bi-modes are not ‘diesel-only’ and so are exempt from the 2040 deadline to remove other diesel trains.

The only viable alternative self-powered rail traction with range and performance comparable to diesel is hydrogen trains. Yet these cannot deliver the power of electric trains and, due to conversion losses, require three times their energy. The low energy density of compressed hydrogen requires a pressure vessel eight times the size of a diesel tank.

Go electric

Electric trains are the only form of transport that offers high speed and high acceleration with potentially zero-GHG emissions, as they take power directly from the grid. So their carbon footprint will reduce as electricity supply is decarbonised. Currently, the respective annual average GHG emissions for electric and diesel rail passenger vehicles are 93 and 352 tonnes. Projected grid decarbonisation will reduce electric vehicle emissions to 39 tonnes by 2040. 

For freight locomotives, the annual electric and diesel emissions are 122 and 866 tonnes, with emissions from electric locomotives forecast to be cut to 52 tonnes by 2040. Electrification is the only way to decarbonise rail freight as there is no low-carbon option for self-powered locomotives.  

For all these reasons, the CCC report requires a more ambitious programme of rail electrification. The government needs to heed this call if it is to meet its Climate Change Act obligations.

This article appears in Issue 5, 2019, of Professional Engineering. Since the time of writing, the government has signed stricter targets into law

Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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