Since the 1970's NASA has used liquid hydrogen to propel the space shuttle and other rockets into orbit. Hydrogen fuel cells power the shuttle's electrical systems, producing a clean byproduct - pure water, which the crew drinks. Hydrogen can be generated from renewable energy sources and an engine that burns pure hydrogen produces almost no pollution. For this reason, hydrogen is currently one of the main contenders in the race to find a green fuel. At HSL we are at the forefront of research into the use of hydrogen as an alternative fuel.

We recently carried out work for the Department for Transport (DfT) to look at a possible future in which all cars, lorries and other vehicles will be powered by hydrogen fuel. DfT wanted to understand the risks from a future hydrogen transport infrastructure, i.e. from the transport and storage of hydrogen from its point of manufacture to hydrogen refueling stations (the equivalent of current petrol stations). They also wanted to know about any problems that might prevent the risks from these activities from being assessed accurately, such that more research will be needed.

The Hazards

Hydrogen has a number of properties that affect its hazards. It is lighter than air and so tends to disperse readily upwards into the atmosphere.  It can also explode and is more likely to detonate than other fuels. It is easy to ignite and hydrogen flames may be virtually invisible to the naked eye. Many of these properties had been investigated experimentally by colleagues at HSL.The two images below show a barely visible hydrogen flame and the extent of the same flame using thermal imaging.

One of the challenges of this particular project was to imagine what a hydrogen transport infrastructure might look like. We decided that it would need to involve transport of very cold liquified hydrogen by rail to storage depots, from where it would be transported to refueling stations in special road tankers. Storage in the refueling station itself might be either as very cold liquid or as very high pressure gas.

Understanding the Risks

We commenced a very approximate risk assessment for two purposes. One was to start to understand the risks, and the other was to discover the gaps in current knowledge that prevented us from estimating those risks very accurately.

Our assessment showed that, in common with the current systems for delivering fuel to petrol stations, incidents could not be ruled out, but would be very unlikely. The most catastrophic events would be caused by total failure of a liquid hydrogen railcar or of a large storage tank at a depot. A number of less severe, but slightly more likely events, were also identified. This analysis helped to target areas where the development of engineering standards and controls to minimise the risks were most important.

Knowledge Gaps

We also showed that there are a number of key knowledge gaps at present, which prevent an accurate risk assessment from being done. This includes the lack of clarity about what a hydrogen transport infrastructure would look like, in terms of tank sizes and operating pressures and temperatures. It also included details of how to calculate the consequences of hydrogen fires and explosions and information about how often equipment containing hydrogen would be expected to fail.

DfT asked for the outcomes of the project to be presented by HSL at a seminar in London to an audience of relevant policy makers from DfT and other government departments.

Read the full paper: MOONIS, M., WILDAY, A.J., WARDMAN, M.J. Semi-quantitative risk assessment of commercial scale supply chain of hydrogen fuel and implications for industry and society. Process Safety and Environmental Protection. March 2010, 88(2), 97-108.