There is much discussion in the industry on how hydrogen transportation and storage can be best facilitated. Diverse methods are being proposed. This article will focus on hydrogen storage and transportation at massive (i.e. infrastructure) scale.
Hydrogen Transportation
At massive scale, there are two principal ways to transport hydrogen: pipeline and ship. Transportation can happen as pure hydrogen, or as another carrier such as ammonia (NH3) or methanol (CH3OH).
Hydrogen Carriers
Ammonia and methanol each carry three hydrogen atoms per molecule. This makes them energy-dense – i.e. lots of hydrogen per unit volume – and both are liquid at reasonable temperatures.
Both should be kept pressurised because both are volatile. In hot weather, the chemicals can reach explosive temperatures – but that’s no different from hydrocarbon liquids and gases. Technologies for transporting both in high volumes, by both land and sea, in ships, lorries and pipelines are well known.
Ammonia and methanol can be synthesised renewably from electricity, hydrogen and elements found in the atmosphere or waste. However this adds cost over and above the hydrogen cost; further energy and cost is required to split the hydrogen from it when there is energy demand. That further cost is avoided if it is used directly as a feedstock for the next process, for example:
- Certain mixes of ammonia and hydrogen have similar combustion characteristics to methane, and so can replace natural gas directly with affordable levels of modification;
- Both can be burned in suitable internal combustion engines, e.g. to power shipping;
- Both are widely used as precursor chemicals to make other substances.
Pipelines
Pipelines can be converted to carry hydrogen, provided the pipes and seals are of suitable standard. Special pumps and valves are also needed because the hydrogen molecule (molecular weight = 2) is much smaller than a methane (16) or air (29). Therefore materials whose structure can contain methane or air may allow hydrogen through like a sieve. Hydrogen can also get embedded into some materials, making it brittle (“hydrogen embrittlement”).
Conversion is expensive and time-consuming, though many countries have for years been replacing pipelines with ones suitable for hydrogen. This will reduce the time and cost of conversion. These technologies are well known if not widespread.
Putting Hydrogen into Pipelines
There are two ways to convert the gas grid to carry hydrogen: gradually mixing hydrogen into the grid, or a one-off conversion to 100% hydrogen. There are two primary considerations to make: increasing demand for hydrogen, and the uses of hydrogen.
It is important to increase hydrogen demand, otherwise electrolysis and other hydrogen formation technologies will never have enough volume to reduce in cost. Gas pipelines can take about 5-15% hydrogen without any significant conversion of either the current pipeline or equipment. To that extent, dilution is beneficial to the development of the hydrogen economy, and to the energy transition in general.
Uses of Hydrogen
Only by considering the uses of piped hydrogen can we determine the best strategy for the remainder of the transition.
There are many uses for hydrogen, which is why it and electricity are considered the two major fuels of the future. Indeed, hydrogen and electricity can be combined (via synthesis) to form most of the rest, too. Hydrogen is proposed for:
- Combustion at low temperatures, e.g. domestic and commercial heating;
- Combustion at high temperatures, e.g. special industrial and chemical processes;
- Fuel cells;
- Feedstock for fuel synthesis;
- Feedstock for different chemical and industrial processes, e.g. making iron and steel.
Of these, only the first two can take a mix. Of the first two, the question must always be asked: are there cheaper or more efficient ways to reduce emissions? Heat pumps, arc furnaces, large-scale long-duration electricity storage (if considering combustion in power stations) are viable alternatives. In general, making hydrogen with electricity is more expensive than using the electricity directly and more efficiently.
Of the other uses, nearly all require pure hydrogen. Therefore, putting a mix of hydrogen and natural gas in pipelines will not help them grow, and will therefore stymie the hydrogen economy.
Another consideration: the gas mixture at different concentrations of hydrogen will have different performance, energy density and flame characteristics. Therefore, a stepped increase in the mix may entail multiple conversions of plant and equipment, which would be more expensive and labour-intensive than a single larger conversion.
After the first 5-15% mixing, the remainder of the transition should focus on industrial hydrogen hubs. Their pipelines will carry pure hydrogen. These pure-hydrogen pipelines can then be extended into neighbouring grid sections gradually, as conversion work is done and as hydrogen availability increases. These hubs will therefore grow outwards until they eventually merge into large regional or national networks.
Hydrogen Storage
Hydrogen is difficult to store in large volumes. A typical lorry only carries 400kg (though specialist vessels can carry up to 900kg), because of its exceedingly low density. For such large volumes of hydrogen, high pressures are necessary. The vessels therefore need to be made of very high grade material to avoid both leakage and embrittlement, to withstand the pressure, and to offer appropriate levels of safety.
Therefore geological storage is required, the main proposed methods being:
- Depleted hydrocarbon wells;
- Aquifers;
- Salt caverns.
Hydrocarbon Wells and Aquifers
Depleted hydrocarbon wells are not empty, they just contain too little material to be economically viable; typically ~35% of their original hydrocarbons. This means that, unless the hydrogen is injected and withdrawn very slowly indeed, there will be considerable mixing. Moreover, most hydrocarbons will get stuck in pores in the rock and gradually come out over time. Both of these processes contaminate the hydrogen so it is no longer usable for non-combustion purposes; and combustion would lead to low but significant levels of greenhouse gas emissions.
Moreover, just as with other materials, the rocks may be porous to hydrogen though they can hold methane; each rock structure will require an individual assessment. This applies to both hydrocarbon wells and aquifers.
Salt Caverns
The best-proven way to store large volumes of hydrogen is in salt caverns. There is huge geological potential for this. The caverns are well known (currently used for both natural gas and compressed air energy storage, among other uses), though the casings and equipment (wellhead, pipes, seals, valves, pumps, safety equipment) need to be of suitable grades.
Storing hydrogen in salt caverns need not compete with other things being stored in them; the geological potential is so extensive. And the pressures at which it will typically be stored are similar to the pressures of hydrogen from electrolysis and of pipelines. This would save energy by conserving pressure.
Moreover, many applications (e.g. Storelectric’s Hydrogen CAES™ and H2 Hybrid CAES™, fuel/chemical synthesis, even steel making) require both hydrogen and electricity storage, so having the two in adjacent caverns makes a lot of sense.
They make even more sense when powered by intermittent renewables. Intermittency reduces the efficiency of electrolysers and heavy industry process equipment. But storing the electricity near the hydrogen can deliver baseload or near-baseload energy to the process.