The role of hydrogen in reaching net zero

The good news from the latest release of the UK government’s energy statistics is that the fraction of electrical power generated from renewable sources in 2019 reached a record high of 37.1%, driven largely by an increase in offshore wind of 20%, to a new high of 32 TWh a year. The bad news is how little difference this makes to the UK’s overall energy consumption – of the 2300 TWh used, 78.3% was obtained from burning fossil fuels. This is a decrease from last year’s fraction – 79.4% – but progress remains much too slow.

It’s tempting to focus on the progress we are making in decarbonising the electricity supply, and this isn’t insignificant. But while the UK used 346 TWh of electricity in 2019, the country directly burnt gas to provide 512 TWh heat for domestic and industrial purposes (not counting here the gas converted to electricity in power stations), and 152 TWh of petrol and 301 TWh of diesel to power vehicles. We’ve no chance of reaching net zero greenhouse gas emissions by 2050 without displacing this directly burnt fossil fuel contribution. And given the longevity of energy infrastructures, we haven’t got long to start building out the technologies to do this at scale.

Can hydrogen help? This technology – or more accurately, group of potential technologies – is having a moment of attention, not for the first time. I think it could well make a significant contribution, but there are some awkward choices to make. Implementing any use of hydrogen in our energy system at scale will involve massive, long-term investments, and making the right choices involve difficult economic judgements, not just about the technologies as they currently exist, but as they may evolve under the pressures of energy markets across the world. Of course that evolution can be steered by incentives, regulation, and targeted support for research and development.

To begin with the basics, because there aren’t any reserves of molecular hydrogen lying around, it isn’t a source of energy, but a way of storing, transmitting and using energy. When burnt, or combined with oxygen in a fuel cell, it produces nothing but water. So the issue is how you make it without producing carbon dioxide in its manufacture. There are three broad options:

How then might the hydrogen be used to attack the carbon dioxide currently produced by the nearly 1000 TWh of energy we derive from burning gas, petrol and diesel for heating and transport?

All of these ways of making hydrogen and using it are technically possible. They’re also all potentially enormously expensive, with the potential for locking the country into solutions which turn out to be inappropriate or made redundant by rival technologies. Some experimentation is necessary, and some blind alleys are probably inevitable, but what needs to be taken into account as we make our choices?

To start with the basic physics and chemistry, hydrogen is a light gas which burns completely and cleanly to yield only water vapour. Perceptions of hydrogen are inevitably shaped by the Hindenburg disaster – but all flammable gases are potentially dangerous, and these are risks of the kind that industrial societies have got used to managing. Hydrogen is more easily set aflame than methane and it burns hotter, but on the other hand at atmospheric pressure burning a given volume of hydrogen produces less energy than the equivalent volume of methane, and much less than petrol vapour. In fact it’s this low volumetric energy density of hydrogen that poses the biggest problem. Even compressed to 70 MPa (as it would be in typical compressed gas tanks) its energy density is only 1.3 MWh per cubic meter, compared to petrol or aviation spirit eat about 10 MWh per cubic meter. Even liquified its energy density is still only 2 MWh per cubic meter, and this needs a temperature of -250 °C, considerably colder than liquid nitrogen.

Moving on to economics, how can we find the most cost-effective solutions? The problem is that technologies don’t stand still – indeed, it’s essential that costs come down, and substantial research efforts are needed to make sure that happens. Where can we hope to see the biggest cost reductions? Existing technologies – like steam reforming of natural gas with carbon capture – are probably the cheapest options with current technology, but being mature further improvements are likely to be more difficult to find than with newer technologies like proton exchange membrane or high temperature electrolysis.

It’s important to remember that the UK accounted for just 1.4% of the world’s energy consumption in 2018, and this fraction will inevitably (and desirably) fall over the next few decades. The choices we make must take into account what the rest of the world is likely to do; while the UK might hope to influence that path, perhaps by helping develop new technologies cheap enough for wide adoption, the UK isn’t a big enough market to be able to make unilateral decisions about technology directions. If battery electric vehicles win the race for zero-carbon personal transport, it would be pointless for the UK to develop a hydrogen network for fuel cell cars. Likewise, if the UK is the only country to back hydrogen boilers for domestic heating while the rest of the world chooses electric heat pumps, it won’t be a big enough market to justify the development of hydrogen domestic boilers by itself, so its plans would be left high and dry.

We have well developed existing energy distribution systems, so the question for any new energy vector is whether these systems can be incrementally adapted, or do new ones need to be built out entirely from scratch? We currently have a well developed electricity distribution system. Distributed PEM electrolysis plants could take zero-carbon from the grid, and produce hydrogen locally. We also have systems for distributing natural gas: it’s likely that the core high pressure network would have to be entirely rebuilt for hydrogen, but the low pressure local distribution system could be adapted. We don’t have a cryogenic liquid distribution system at scale, and this is likely to limit global trade in hydrogen.

Finally, we have to consider our plans for low carbon electricity. Whatever we do, we need to replace the 512 TWh of gas we use for heating, and the 453 TWh of petrol and diesel we use for transport, with zero carbon alternatives. If this involves electrification – either directly or through the production of hydrogen from zero-carbon electricity – this will need a huge expansion of power generation capacity from the current 346 TWh/yr. I find it difficult to see how this can happen without both a massive increase in offshore wind – possibly including floating offshore wind – and new nuclear build, possibly next generation nuclear able to produce high temperature process heat for production of additional hydrogen.

These are difficult choices, but we haven’t got much time. Let’s get on with it!

Some references:

Current UK energy statistics from DUKES 2020.
Hydrogen supply chain evidence base.
On hydrogen storage (US Dept of Energy PDF) –
Royal Society Policy Brief Options for producing low-carbon hydrogen at scale.