Last week we talked about batteries for grid scale electricity storage. There is another way we can use chemical bonds to store spare renewable electricity – Power-to-X.

The idea behind Power-to-X is to convert renewable electricity into a transportable and stable chemical, typically a fuel. When you want the energy back, you take your high-energy chemical and break the bonds you have just made, releasing the stored energy to do useful work.

‘X’ can be many chemicals:

  • Hydrogen (H2)
  • Ammonia (NH3)
  • Methane (CH4)
  • Methanol (CH3OH)
  • e-Fuels (synthetic diesel and kerosene)

You can burn all of these as fuels for power or electricity generation. Hydrogen, ammonia and methanol can also be used in fuel cells to generate electricity directly.


It all starts with hydrogen

Although Power-to-X allows you to store spare energy in a whole range of chemicals, it all starts with hydrogen. Once you have a ready source of hydrogen, add carbon dioxide, nitrogen and water and you can recreate almost the entire chemical industry using no fossil fuel at all. It may not be cost effective today, but all the reactions and processes are available.

How do we get hydrogen from renewable electricity? Green hydrogen is made by electrolysing water (H2O). You probably remember the school experiment with two electrodes in water connected to a battery. Bubbles collect on each electrode; hydrogen on the negative electrode and oxygen on the positive. In a more sophisticated version, you put a water-filled test tube over each electrode to capture the gas. Industrial electrolysers work the same way – just on a huge scale.

Structural diagram of a polymer electrolyte membrane electrolyser

Polymer Electrolyte Membrane Electrolyser Cell

There are two main types used commercially. In alkaline electrolysis, the two electrodes are immersed in an alkaline electrolyte solution (potassium or sodium hydroxide). The electrodes are separated by a permeable diaphragm that creates two compartments that keep the generated gases apart. Polymer electrolyte membrane (PEM) electrolysis uses an acid electrolyte in the form of a perfluorosulphonic acid polymer film, coupled with electrodes coated with expensive noble metals like platinum and iridium.

Alkaline electrolysis units are simple and cheap to manufacture but are not as productive. PEM units can operate at much higher current densities and are more productive but have a higher capital cost. However, BNEF expects alkaline electrolysers to be the biggest chunk of the market in the next few years. The technology is cheaper and suited to large projects. Researchers are squeezing the best performance out of these technologies. In alkaline electrolysis, the electrolyte is corrosive, so materials have to be carefully chosen and electrode degradation is a problem. The goal with PEM electrolysers is to find cheaper materials for the electrode coatings and catalysts.

Two more technologies rapidly developing are solid oxide electrolyte cells (SOEC) and anion exchange membrane electrolysers (AEM). SOEC electrolysers operate at high temperatures (700°C – 850°C), use cheaper electrode materials than PEM, but they don’t like being turned on and off. So not such a good fit for intermittent renewable electricity sources. AEM electrolysers offer the simplicity and performance of PEMs, but can use cheaper materials. Unfortunately, they don’t yet have a long enough life.

Containerised unit for producing hydrogen from water and renewable electricity

Containerised PEM Electrolyser from ITM-Power

Each electrolyser type has its strengths and weaknesses, applications where it fits best, and an ongoing development pathway. None is a knockout cost/performance winner, and each finds use.

Modern electrolysers have an efficiency of around 80%. Not bad for producing a chemical intermediate. However, to get the electricity back you use the hydrogen in a fuel cell, which is only 60% efficient. Overall, you get back just under half the electricity you started with.

There will be an immense demand for green hydrogen production capacity both to supply hydrogen to heavy industry and to soak up excess renewable electricity. 458 MW of electrolysers were added in 2021, and the estimate for 2022 was an additional 1.8 – 2.5 GW. Beyond that, there is a pipeline of projects totalling 100 GW by 2030. A very fast adoption pathway.


Where will hydrogen be used?

Clean Hydrogen Ladder showing relative priority for uses of hydrogen.

Clean Hydrogen Ladder

Where will the hydrogen be used? There are a whole range of options, from heating homes, through transport and energy storage to heavy industry like chemicals and steel. Michael Liebreich and various colleagues have developed the ‘Clean Hydrogen Ladder’, a graphic that shows which use cases for clean hydrogen offer the best benefits, and which will be uncompetitive with alternatives. Right at the top of the list are applications in the chemical industry. This vital industry cannot be decarbonised without green hydrogen. Next level down is the steel industry. Using electric arc furnaces with green hydrogen as the reducing agent, it is possible to produce green steel entirely without fossil fuels. For an energy intensive industry, a major step forward.

On the same rung of the ladder is long term energy storage. If you have plenty of renewable electricity, and no other cheap and convenient way to store it, such as pumped hydro, you can use hydrogen as long term storage. With 80% efficiency in electrolysers and 60% efficiency for fuel cells, you have a round trip efficiency just below 50%. That is pretty poor compared to pumped hydro at 75% – 85% and batteries at 80% – 90%. But if you don’t have the geography and capital for pumped hydro, or the high initial costs of batteries, it could make sense.

Other applications of hydrogen fall much further down the ladder. There will be a huge demand from the chemical industry, steel, and long-distance shipping and aviation (through e-fuels). Applications where there is no practical alternative to hydrogen. Using it for electricity storage will struggle compared to other options, and the chances of using hydrogen in volume for passenger cars and domestic heating are vanishingly small.

Next up – Last thoughts

Previous articles:

Electricity Storage 7 – Power-to-X, Chemicals as an Energy Reservoir
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