Energy Transition Series – Hydrogen Pros & Cons V

Some Simple Chemistry …

Hydrogen is the most common element both in the Universe (90%) and in our Solar System (70%).  It is believed to be the first element to have formed in the universe, with all other elements having subsequently been produced from hydrogen by nuclear fusion. While on earth hydrogen is significantly less common than the percentages quoted above, there’s no shortage of it in the crust, in the hydrosphere, in the atmosphere and in our own bodies.  Hydrogen is the simplest and lightest element; at the Earth’s surface it exists as a colourless, odourless gas, with the lowest density of all gases; and it forms the smallest molecule.

Like hydrocarbons such as oil and gas, hydrogen burns readily in air, thereby being oxidised and – in the process – releasing heat.  In today’s world, however, there is a critical difference between those two burning processes.  Burning hydrogen produces steam – water – only.  Conversely, hydrocarbons contain carbon; burning them produces carbon dioxide and water; and there are widespread concerns that this process is ultimately contributing to global warming or climate change.

Hydrogen in the Context of Climate Change

As the previous Opinion Piece in this series pointed out, in Paris in 2015 the great majority of the world’s nations pledged significantly to reduce their emissions of greenhouse gases or GHGs, principally CO₂, on the presumption that this will prevent a rise in global temperatures.  In order to meet these nations’ GHG reduction objectives, coal-, oil- and gas-fired energy sources must be dramatically reduced and replaced by energy sources which emit significantly lower quantities of CO₂ … in short: renewables.

Even 5-7 years ago it was predicted that the heavy lifting in the renewable context would be carried out by solar power, wind power and biomass.  The key realization over that period, based partly on high-profile decreases in energy reliability in some developed nations, was that solar power and wind power are not reliable at scale and that biomass is limited by geographical and other factors.  Yet, over that same time interval, national governments imposed increasing pressures on their countries to accelerate “decarbonization”, meaning the reduction of their nations’ emissions of carbon dioxide.

In that context, the “hydrogen economy” is a view of the future in which hydrogen is used as fuel for heating, hydrogen-fuelled vehicles, energy storage and long-distance transport of energy.  Since hydrogen can be created from water using intermittent renewable sources such as wind and solar, and its combustion only releases water vapour to the atmosphere, hydrogen is potentially a key technology to phase out fossil fuels.

So hydrogen, a dispatchable energy source, has been hailed as the solution to the problem of the intermittency of renewables.  But like anything it has its advantages and disadvantages.  We’ll look at those a little further down the page … but first an explanation of what to some may be a confusing aspect of recent discussions of hydrogen fuel: the various “colours” of hydrogen.

Any Colour You Like …

A tendency has commenced to label hydrogen generated by different processes with different colours as a kind of shorthand.   Here is a summary of the principal categories … but be warned, I have seen as many as ten (!) different colours of hydrogen:

• Black hydrogen: formed by gasification of coal (brown = lignite)
• Grey hydrogen: formed by steam reformation of natural gas
• Blue hydrogen: black or grey hydrogen with the equivalent carbon dioxide emissions stored in subterranean reservoirs (CCS)
• Green hydrogen: formed using renewable power sources (e.g. surplus wind or solar power).

Broadly, blue and green hydrogen are in principle both carbon neutral, but the latter is intrinsically so whereas the former relies on a balance of carbon dioxide generation and storage.

Hydrogen’s Pros and Cons

As stated above, hydrogen – like hydrocarbons such as oil and gas – can be burned to produce energy.  The combustion reaction produces no carbon dioxide or other non-condensing greenhouse gases, so that reaction is “green”.  In addition, hydrogen can be piped and stored, which means that it is a dispatchable source of power – in other words it can be turned on and off as required – which wind and solar are not.  In fact, hydrogen may be a far more environmentally friendly dispatchable source than batteries.

On the other hand, hydrogen has some significant disadvantages as a dispatchable energy source.  Firstly, hydrogen is significantly less energy dense than hydrocarbons such as natural gas, which is principally comprised of methane.

Secondly, though natural subterranean accumulations of molecular hydrogen (H₂) are known to exist, they are tiny by comparison to natural gas accumulations.  We can’t produce significant quantities of hydrogen … we need to manufacture them.

Thirdly, hydrogen is an extremely reactive element and, consequently, in nature – at least in the earth’s crust, hydrosphere, atmosphere and biosphere – it typically exists in an oxidised state, chemically bonded to something else.  It has been joked that hydrogen is the “Elizabeth Taylor of molecules” (a joke which perhaps appeals more to my older readers) – because in chemical terms it’s always “married” to something else.  There’s a serious point to be made here … because the subsequent “divorce” requires more energy input than the hydrogen produces for human use when burned, enormous surplus quantities of solar and wind power are required in order to produce “green” hydrogen as defined above … though at the time of generation that solar and wind power may be surplus to human requirements, so using them to generate hydrogen may be considered a sensible thing to do.

Fourthly, some operational and HSE aspects.  Replacing methane with hydrogen in a developed economy such as the USA or the UK involves enormous infrastructure expense, because metal gas pipelines and appliances may become embrittled by prolonged contact with hydrogen; there are other safety issues (one of which is that the human eye finds it almost impossible to see a hydrogen flame, which sounds trivial but could be anything but); and storing hydrogen is difficult because the extremely small size of the molecule makes it prone to leakage.

Like everything, hydrogen as a source of energy for our advanced world has its pros and its cons.  If green (generated from emissions-free energy sources) or if blue (emissions offset by CCS) then it can certainly play a part in emissions reduction programmes.  However, there are a number of potentially expensive obstacles to surmount, and like all activities intended to reduce AGW it must be coordinated globally … there’s only one atmosphere, and spend on emissions-free hydrogen in one country can be counteracted by emissions in another.