Over the hydrogen rainbow


Not all hydrogen is created equal. Image credit: Suzanne Rowcliffe, Pixabay.

Globally, countries are attempting to prevent climate change by moving towards energy sources that do not release greenhouse gases. One exciting and promising area of development is hydrogen, which can be reacted with oxygen in a fuel cell to produce electricity. With water being the only by-product, this is as clean a reaction as anyone could hope for. Over 30 countries have implemented hydrogen strategies, all around the world, as they strive to achieve fully renewable energy systems.

The challenge of using hydrogen fuel emerges at the point of production. Hydrogen exists naturally as a gas but cannot simply be captured from the atmosphere and, instead, must be chemically produced. The different methods of production have been classified historically by a rainbow system:

  • Black hydrogen—produced by burning coal, which releases carbon into the atmosphere.
  • Grey hydrogen—produced by superheated steam breaking down methane or other natural gases, which releases greenhouse gases. In 2022, grey hydrogen accounted for 92% of the hydrogen produced.
  • Blue hydrogen—produced like grey hydrogen but carbon capture, utilisation, and storage (CCUS) is used to prevent the release of carbon into the atmosphere.
  • Purple hydrogen—produced using nuclear energy.
  • Green hydrogen—produced using renewable energy sources. This process hypothetically produces no harmful emissions.
Different types of hydrogen - hydrogen rainbow
Schematic representation of the rainbow system. Image credit: Skillen et al., 2022, CC BY 4.0.

This rainbow system highlights the important fact that not all types of hydrogen are equally environmentally friendly. If a company is using grey hydrogen, for example, then this may not be the improvement on fossil fuels that consumers are hoping for. By communicating using the rainbow system, the public is given an easy tool with which to hold companies and governments accountable. This can be seen in government policies, such as the House of Commons Committee report on the role of hydrogen in achieving Net Zero. In this report, a large emphasis is placed on the need to expand the amount of blue and green hydrogen produced relative to grey hydrogen. It can also be witnessed in how energy companies communicate about hydrogen fuel—Shell, for example, highlights the fact that ‘all hydrogen power supplied at Shell’s retail stations in the UK is green hydrogen.’

Whilst the rainbow system enables simple communication about the environmental impact of hydrogen, it is in danger of oversimplifying. An example of this is that assigning the label “green” to hydrogen does not account for the environmental cost of manufacturing, installing, and transporting the components for the renewable energy facilities. Similarly, it is often overlooked that blue hydrogen is only a fully clean source of hydrogen if CCUS is 95% efficient– a level currently reached rarely.

Similarly, it is often overlooked that blue hydrogen is only a fully clean source of hydrogen if CCUS is 95% efficient—a level currently reached rarely.

Such considerations could be communicated more effectively if, rather than using a qualitative system, a quantitative classification system was used. There is already traction for this in the UK, where weighted average carbon intensity (WACI) is increasingly adopted. WACI is a more accurate and precise measure of environmental impact, which allows for better founded policies to be implemented.

When carbon intensity is investigated in this way, green hydrogen is shown to be 20-40% less efficient than using renewable energy directly. This means that green hydrogen is only a sustainable and economic option when it is produced using the surplus energy from pre-existing renewable energy sources. The H100 Fife project is doing just that—excess energy from wind warms off the East coast of Scotland is being used to produce hydrogen to heat homes. This illustrates how a more critical appraisal of green hydrogen can lead to productive results.

Similarly, both the government and energy companies have recognised that hydrogen has great potential as a method of energy storage. During summer months, less energy is required for heating, so this excess renewable energy could be used to produce hydrogen, which can then be stored safely until demand increases in the winter months.

Green hydrogen is, however, an inefficient energy source. This means that no matter how green the hydrogen is, it is never the greenest option available.

However, there are still areas which could benefit from better quantitative evaluations of the environmental impact of hydrogen. For example, the UK government 2022 hydrogen strategy emphasises the need to increase blue hydrogen production by expanding CCUS infrastructure, with the plan for increasing deployment outlined in the Energy White Paper (2020). This paper, however, only focuses on introducing CCUS to new industrial sectors. It ignores the need for the technology behind CCUS to be substantially improved if the 95% efficiency required for blue hydrogen to be carbon neutral is to be achieved.

The UK government clearly believes that hydrogen will be a key player in the fight against climate change and its confidence in this fact only seems to be growing—in 2022, the UK doubled its hydrogen production capacity target relative to its 2020 target. However, it is debatable whether this is the wisest policy. Given the negative environmental impact of black, grey, and arguably blue hydrogen, the first step should be the deployment of green hydrogen to displace all other hydrogen sources. Only once this has been achieved is it sensible to start considering expanding the role of hydrogen.

It is likely that the future of hydrogen will see it being deployed in sectors that cannot be easily electrified, such as to fuel industrial processes and heavy transport. Green hydrogen is, however, an inefficient energy source relative to using renewable energy directly. This means that no matter how green the hydrogen is, it is never the greenest option available. The future of hydrogen fuel is therefore limited and, unfortunately, unlikely to be the silver bullet that the UK government is hoping it to be.