How can we develop the next generation of rail vehicles to deliver on decarbonisation?

Researchers are working to integrate hydrogen and battery power to the country’s rolling stock to meet the government’s ambitious decarbonisation targets.

Governments are setting bold emissions targets as the political energy to tackle the climate crisis intensifies, requiring radical overhauls of our current technological base. Researchers at the University of Birmingham, a world leader in rail engineering research, are working to integrate hydrogen and battery power to the country’s rolling stock to meet the government’s ambitious decarbonisation targets.

The transport sector is critical to achieving current emissions pledges. In the UK, for instance, it accounts for roughly a quarter of greenhouse gas emissions. The government’s commitment to reduce carbon emissions by at least 80% against 1990 levels by 2050 cannot be achieved without transitioning all segments of the transport system to cleaner fuels. The country also plans to remove all diesel-only vehicles by 2040.

The rail sector, while not a leading source of emissions, has to comply with a phase-out whose deadline is near. Rolling stock companies effectively cannot purchase any diesel vehicles now because the time they take to be delivered, and the roughly three decades of use necessary to earn the returns, will overshoot 2040. The sector also needs to phase out diesel to respond to growing public anxiety about poor air quality. 

Rail has moved through four energy sources to date; from horsepower to coal-fired steam to electricity and diesel today. Researchers are now exploring two new energy sources - hydrogen and batteries - and ways to increase electrification, especially as the grid draws more power from renewables. “I don’t think there will be another form of traction power in our lifetime, so we’re at a critical transition,” says Dr Stuart Hillmansen, Head of the Traction Research Group at the University of Birmingham’s School of Electronic, Electrical and Systems Engineering.

Professor Hillmansen is leading the university’s research in exploring how to integrate fuel cells and hydrogen into UK rail. This partly hinges on using batteries to help trains cover longer distances in parts of the country where electricity supplies are patchy, by drawing power from overhead lines or rail-side infrastructures and storing it for later. This is especially important for the UK, compared to a country like Germany where a larger number of mid-sized towns has led to broader rail electrification coverage. 

The second focus is unleashing hydrogen as a power source. “Hydrogen is appealing because, per kilogram, it has high energy density; around three times as much energy as diesel,” says Professor Hillmansen. HydroFLEX, the partnership between Birmingham Centre for Railway Research and Education and Porterbrook, a railway rolling stock company, is developing the UK’s first hydrogen-powered train, by equipping an existing model with a hydrogen fuel cell so it can power itself from the existing electrification system and combine it with on-board hydrogen stores. 

HydroFlex train on mainlineHydroFLEX train on the mainline, September 2020.

Transitioning to new energy sources is a huge project - a twenty-year undertaking, says Hillmansen. This is partly due to the rigorous safety requirements needed for this mass transport medium. “Rail is the safest form of travel, thanks to years of carefully developed engineering which means any introduction of new technology gets highly scrutinised. Standards need to be adapted for hydrogen and battery-powered vehicles, so there’s a lot of work to be done to understand how new energy systems perform in terms of safety and risk”. The workforce itself also needs education. “Everyone in the railway sector understands electricity and diesel but now they need to learn a new system of providing power, either battery or hydrogen, so you have to train all the depots and fleet managers and so on”.

The hydrogen used in the HydroFLEX project is sourced from steam methane reforming, which splits methane into carbon dioxide and hydrogen mixed with water. The long-term vision, says Professor Hillmansen, is to produce hydrogen via electrolysis, a zero-carbon, zero fossil-fuel method. “We want to take power from the grid and use it to make hydrogen, which is compressed in a depot, put in a train and then converted back”. 

Fuel cell on the trainFuel cells onboard the HydroFLEX train.

Some might question the logic of using electricity to make hydrogen, squashing it and storing on a train and turning it back into electricity, since a lot of power is lost in each transition. But this is offset, says Hillmansen, by the flexibility that hydrogen power brings. “As our grid becomes more decarbonised through renewables and nuclear, there will be times where wholesale cost of electricity becomes cheap and even negative. It already does this quite often. In recent weeks, due to the COVID-19 shutdown, there have been times where the wholesale price of electricity has gone negative and people are getting paid to use it. 

In the future there could be a clever way of making hydrogen where you make hydrogen almost free, because the electricity companies are paying you to burn their electricity”.  And crucially, hydrogen can be bought at an economically favourable cost using off-peak electricity, compared to a rush hour train that needs power at 9 a.m. “With hydrogen you could be using three times as much electricity, but you are buying it at less than a third of the price, so the cost of providing power ends up being the same”. 

Power conversion for railways

Integrating new power sources requires a twin research effort to convert the electricity coming in different formats and ensuring control of the overall power balance. “The rail sector wants to use fewer diesel trains and more alternative propulsion like hydrogen and batteries, and there is an appetite for further electrification,” says Dr Pietro Tricoli, senior lecturer in electrical power and control at the University of Birmingham’s department of electronic, electrical and systems engineering. “This means coping with an evolving public grid which is seeing more renewable sources and less fossil fuel. Being able to control power correctly is key to enable these technologies to be deployed on trains”. 

There are two main electricity form: direct current (DC) - a unidirectional flow - and alternating current (AC), which periodically reverses direction. Turning electricity from one into another requires converters but each transformation costs energy. Decarbonising the rail system requires the integration of more diverse electricity forms which, without a corresponding improvement in converters, could lead to inefficiencies and energy waste. “Every time you want to integrate a new power source - to add batteries, or interface with a new renewable source, you need a power converter. My research is exploring how to design converters to minimise the number of conversion steps”. Tricoli is also working to minimise the size of converters - to increase their ‘power density’ - so they can easily fit into rail carriages where space is at a premium. He predicts that more efficient converters will improve overall power control and optimisation. “When you have different power sources, you want to be able to share this power in the most intelligent way, to avoid overloading and to make sure you are maximising the efficiencies of different power sources”. 

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Tricoli works by developing proof-of-concept prototypes in a laboratory, a necessary initial phase for exploratory research, given the safety constraints of deploying in the rail industry. “You scale things down and develop it in a lab, demonstrating that it can work technically. We build bespoke systems starting from individual components, building a converter and then a control system”. He then works with industry partners to explore performance requirements and gradually bring these innovations into alignment with the operational realities they need to function in. This process gives the “freedom to discover new types of control and power systems, with an understanding of the performance targets needed in real-world settings”. Tricoli is working to deploy newly developed power converter technology in the Madrid metro, as part of an EU-funded effort to maximise the uptake of renewable energy by improving the interchange of power between electricity systems and transport networks. 

The push for decarbonisation has never been stronger globally, as governments, companies and civil society grapple with the climate crisis. Academic work conducted at the University of Birmingham, in partnership with the transport industry, can help navigate the complex transition involved in phasing out harmful fuels and smoothly integrating next-generation power sources. 

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