Comment & Analysis

Decarbonising Rail: Trains, energy and air quality

Dr Jenifer Baxter, Head of Engineering Policy

Decarbonising Rail: Trains, energy and air quality
Decarbonising Rail: Trains, energy and air quality

Transport Minister Jo Johnson’s recent announcement to phase out diesel-only trains by 2040 aligns policy for rail with that recently announced for new petrol and diesel only passenger road vehicles. However, although there are similarities, there are also differences and specific challenges to phasing out diesel for different modes of transport. We need appropriate and distinct solutions for each part of our transport network.

Our recent report A Breath of Fresh Air: New Solutions to Reduce Transport Emissions outlines significant opportunities to reduce emissions in the transport sector, and proposes some of the signals and policies that need to be introduced to achieve these reductions.

One recommendation from the report is that:

The Department for Transport (DfT) conducts a series of trials on existing diesel railway rolling stock, the new bi-mode trains and major stations, to understand the level and effect exposure of pollutants has on commuters and railway workers. We already know that emissions from diesel powered transport include nitrogen dioxide as well as particulates of various sizes are affecting the health of thousands of people every year. In addition to this burning diesel emits Greenhouse Gases (GHG) that contribute to climate change.

The current situation for rolling stock (these are the trains in everyday use), operating on UK railways is that they are run using either electricity or diesel. Electric powered trains are AC or DC from the fixed electrified infrastructure feeding traction motors connected to the wheels of the train. Diesel trains use an on board engine that powers the axles of the train or generates electricity to run electric motors in diesel-electric trains. Fully electric powered trains running on the fixed electrified network are the most efficient, cost effective and environmentally friendly. For example, on its West Coast services, the traction cost of diesel for Virgin trains is four times that of electricity. 1 One reason for this is that, unlike self-powered vehicles, electric traction can absorb the huge amount of energy generated during braking and feed it back into the grid.

For trains, the weight and space limitations of self-powered diesel trains is such that they cannot achieve the same performance as electric traction especially when the requirement for the train’s hotel load (power required for non-propulsion purposes such as climate control, communications, entertainment, lighting, refrigeration etc…) is considered. This can be up to a quarter of the traction power and on diesel trains, has to be powered by the engine. Thus, electric commuter trains have typically, twice the acceleration of their diesel equivalent.

With its high initial capital cost, electrification is best suited for busy routes. Many countries have a high percentage of their rail network electrified. These include the Netherlands (76%), Italy (71%) and Spain (61%). In the UK just 42% of the network is electrified. The railway sector in the UK is now in the position that it is currently managing ageing diesel-powered trains, some of which were made prior to 1993 and are set to remain in service for another ten years or more. This means that engineers across the railway sector are looking for solutions that will meet the requirements of the diesel-only train phase-out particularly on lines where electrification is cost prohibitive.

Bi-Mode trains are designed to operate on both electrified lines and non-electrified lines. Those currently being introduced on Great Western and East Coast routes are able to switch between the electric powered mode and an on board diesel powered engine. However, while flexible, the electric-diesel bi-mode train suffers with increased emissions when operating in diesel mode, has higher fuel, capital and maintenance costs than pure electric trains and are less powerful when working in diesel mode (8.6 kw/tonne) compared with electric mode (11.2 kW/tonne). 2 Bi-mode trains offer a solution to non-electrified lines and reduce the requirement to invest in electrification, but do not provide the required performance or offer the most efficient or environmentally friendly solution.

Battery powered trains offer zero emissions at the point of use. However the energy density of batteries (2.63 MJ/litre for a lithium ion battery) is less than a tenth of diesel (35.8 MJ/litre). Furthermore, unlike diesel trains, batteries have a long charging time, limited range and a poor environmental footprint. For this reason battery powered trains are mainly suitable for journeys from an electric line onto a short non-electrified branch. Such as the Independently Power Electric Multiple Unit (IPEMU) application that was recently trailed in Essex on the Harwich branch where it ran for 50km under electric power and 30km under battery power. 3

Hydrogen trains are powered in a similar way to electric trains, they use a fuel cell that provides the electricity from hydrogen. The fuel cell works by creating positively charged hydrogen ions, which are passed through an electrolyte, to a cathode and provide electrical power. The emissions from this process are water and heat. If the fuel cell is supplied with hydrogen and oxygen it will continue to create electricity. One key difference between the fuel cell and conventional electric train is that fuel cell powered trains are less efficient when it comes to rail traction, they are similar to diesel powered trains at around 30%4. The traction energy efficiency depends on a number of factors from air resistance and inertia to comfort functions and efficiency losses. Hydrogen also offers fast refuelling.

The production of hydrogen as an energy vector is also a hot topic. The use of excess renewables or nuclear to generate hydrogen through the electrolysis of water to help store energy, balance the grid and provide transport fuels is being investigated. Research and development in improving electrolysis efficiencies is still required to make this a realistic and green option. In addition to this the use of carbon capture utilisation and storage is being trialled in industry to capture CO2 emissions during hydrogen production through steam methane reforming and this too will reduce the environmental impact of hydrogen production.

Like battery-powered trains, hydrogen has a low energy density (2.7 MJ/litre when compressed at 350 bar) compared with diesel. Alstom’s iLint train is a hydrogen / battery hybrid which overcomes this limitation by using both batteries and hydrogen for acceleration and uses its fuel cell to keep the traction batteries fully powered. In this way the iLint has the same performance and range as a passenger diesel multiple unit. However, with its low energy density, hydrogen is unlikely to give the same power output as electric trains.

With the need to decarbonise trains, hydrogen trains are becoming an increasingly popular option for lines where diesel multiple units currently operate, such as the Great Western. It seems likely that we could see hydrogen trains operating in the UK in pilot schemes as early as 2020. Exciting times for those of us who live out West!

LNG is a fuel that is often suggested as a transition fuel to replace diesel, this is the case in HGVs and shipping as well as for trains. The LNG train is powered using a combustion engine like diesel, but fuelled by natural gas that has lower CO2 and particulate emissions than diesel. LNG engines also tend to be quieter and create less vibration reducing the impact of a railway line on its neighbours. Only a few trials have been conducted using this technology in Spain and Russia5.

This is another area where the transport network can link up with the wider energy network. LNG can be replaced by BIO-LNG generated from waste materials. This can be stored and transported in the same way, but reduces the overall environmental footprint of gas powered trains.

These are just some of the possible technologies that we could see, there are also hybrid versions, plug in hybrids and maybe one day we will see renewable technologies directly powering trains.

However even with all of the options mentioned above, electrification of the rail network is the likely to be only way to reduce emissions from trains to ultra-low or zero at the point of use on busy routes requiring high-powered rail traction. As electric routes carry more traffic, reducing emissions on electrified routes offers far greater emission reductions than the use of alternative fuels which are more suited to less busy routes.

Furthermore with the decarbonisation of the grid, electrification offers a zero-carbon, zero-emissions alternative. For example Dutch operator, NS, is now operating all its electric trains by wind power.6 Electrification also offers major opportunities to reduce the unit cost of train operation and maintenance and to provide improved capacity, journey times and reliability as well as environmental benefits.

Unfortunately recent electrification schemes have been delivered at an unacceptably high cost. For example the Great Western electrification scheme rose from £874 million in 2013 to £2.8 billion in 2015. At today’s prices, this is seven times the cost of British Rail’s electrification of the East Coast route. 7 For this reason Government has cut back further electrification and, wrongly, claims that its benefits can be delivered by bi-modes and other technologies.

With this in mind, the Institution recommends that DfT instruct Network Rail to develop an appropriate specification for railway electrification, which will achieve an affordable business case for rolling programme to complete the electrification of main lines between Britain’s principal cities and ports, and of urban rail networks through our major city centres. As the pollution within our cities continues to cause health problems and climate change becomes increasingly evident across the globe, this is now an urgent requirement.

List of references:


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