Hydrogen storage – final piece in the renewable energy puzzle ?

Hydrogen cavern storage
Photo © MCI

Today we have an energy crisis. We all know this. And its effects are global. Furthermore global electricity demand growth is set to rise in 2024-2025 at a rate well above the pre-pandemic level of 2015-2019.

According to the IEA Electricity Market Report of Feb 2023, by 2025 world electricity demand will increase to 2,500 TWh above 2022 levels, meaning that over the next three years the annual increase in electricity consumption will be approximately equivalent to that of Germany and the United Kingdom combined. A sobering thought!

Renewables and nuclear energy will dominate the growth of global electricity supply over this period, together meeting on average more than 90% of the additional demand, with their share of the power generation mix rising from the 2022 level of 29% to 35% in 2025. China is set to meet more than 45% of renewables generation, and the EU 15%.

But with all this increased renewable energy production, if a lot of it isn’t to be wasted, there will need to be an equivalent development in energy storage facilities.

And with hydrogen forecast to be the fuel of the future, in addition to its use in fertiliser production needed to help feed the world’s growing populations via its ammonia derivative, energy companies around the world are racing to develop hydrogen storage, believing that it will help to reduce dependence on natural gas and boost energy security.

Why is hydrogen energy storage so crucial?

Firstly let’s take a look at hydrogen itself. Hydrogen is able to address two major challenges in the global drive to achieve net zero emissions by 2050. First, it can help tackle the one major issue with renewable energy sources such as wind and solar. And that is their intermittency. By converting excess power generated on windy or sunny days into hydrogen, the gas can store renewable energy that can then be deployed at times of peak demand as a clean fuel source for power generation.

Second, hydrogen can replace fossil fuels to decarbonise sectors where electrification alone isn’t adequate, such as domestic heating, industry, shipping and aviation. As an energy source it is very scalable.

The difficulty is that, while being an excellent medium for renewable energy storage, hydrogen itself is tricky to store.

This is because it has a low volumetric energy density compared with other gases — such as natural gas — meaning it takes up much more space. Also, hydrogen has a boiling point close to absolute zero and so in its liquid form requires cryogenic storage. Furthermore, under certain conditions, it can cause cracks in metals, particularly in iron and high strength steel. This is known as ‘hydrogen embrittlement’. However it’s a potential issue that can be resolved.

Here are four hydrogen storage solutions that could help address these challenges.

1. Liquefied hydrogen

As well as being stored in its gaseous state, hydrogen can also be stored as a liquid. In fact the space industry has been using liquefied hydrogen to fuel rockets for many years.

But liquid hydrogen storage is technically complex and thus quite costly. It has to be cooled to -253°C and stored in insulated tanks to maintain the low temperature and minimise evaporation. This requires complex and expensive plant which has limited the use of liquefied hydrogen.

The semiconductor chip industry is also a major user of liquefied hydrogen. However, with the proliferation of renewable hydrogen supply and demand, greater economies of scale will make liquefaction a more viable storage and transport option in the future.

2. Compressed hydrogen storage

Like any gas, hydrogen can also be compressed and stored in tanks, and then used as needed. However its volume is much larger than that of other hydrocarbons relative to their weight — nearly four times greater than natural gas. So for practical handling purposes, hydrogen needs to be compressed. For example, fuel-cell powered cars run on compressed hydrogen contained in highly pressurised containers.

If an application requires hydrogen volume to be reduced further than attainable by compression, it can be liquefied. The two techniques — compression and liquefaction — can also be combined.

Hydrogen’s low energy density, high volume, and need for cryogenic storage are some of the biggest barriers to its adoption. This is especially true for use in transportation, where a balance must be struck between passenger space and range.

3. Geological hydrogen storage

The storage of hydrogen underground, both onshore and sub-sea in caverns, salt domes, depleted oil and gas fields, and aquifers (porous rock or sediment saturated with groundwater) is fast-becoming a serious contender for hydrogen storage at scale. In fact gas storage in salt caverns is a long-established method, making the technology easy to adapt.

Depending on their depth, salt caverns may be operated at pressures up to 200 bar, thus allowing for large-volume storage, sometimes exceeding 6,000 tonnes per cavern. In fact hydrogen storage in caverns is scalable to such an extent that it can function as grid energy storage.

Cavern storage is available throughout much of the world including the USA, Europe, Middle East, and China.

Taking the UK as an example, the maximum theoretical capacity for onshore salt cavern storage has been estimated by one study at 2150 Twh, with a peak load deliverability of 1876 GW being technically possible, thus meeting peak domestic heat demand.

Hydrogen Economy
Photo © American Chemical Society

4. Materials-based storage

Another option for storing hydrogen is in other materials. In materials-based hydrogen storage, solids and liquids that are chemically able to absorb or react with hydrogen are used to bind it. This includes creating metal hydrides from elements such as palladium (capable of absorbing 900 times its own volume in hydrogen), magnesium, and aluminium.

Using ammonia as a carrier for hydrogen is, arguably, the option with the most potential. Its energy density by volume is nearly double that of liquefied hydrogen, making it far easier to store and transport. This means that hydrogen is converted to ammonia, transported to its destination and then “cracked” to release the hydrogen at its point of use.

Of the more than 100 low-carbon hydrogen projects listed in Wood Mackenzie’s Hydrogen Project Tracker, over 85% integrate ammonia and hydrogen to serve domestic and export markets.

Transitioning to hydrogen

But what happens when a country has not yet fully transitioned away from fossil fuels to a future ‘hydrogen economy’ ?

There is one business with a time-tested solution. And that is the UK’s Manchester-based Storelectric whose patented CAES™ technology has been deployed since 1978 in Huntorf, Germany, and since 1992 in McIntosh, Alabama, USA. Both of these installations work by using compressed air stored in underground salt caverns, and regenerate electricity by feeding it into a gas-fired power station. Thus the acronym CAES™, standing for Compressed Air Energy Storage.

Storelectric CAES energy map
Photo © Storelectric Limited

Storelectric’s more recent Combined Cycle Power Plant (CCGT) technology has the advantage of also being able to burn hydrogen. But until sufficient low-cost hydrogen is available, it can burn either methane or a combination of hydrogen and methane.

The company states that the hydrogen can be obtained either from the gas grid — which many countries are converting to carry hydrogen — and, more immediately, mixtures of hydrogen and methane, or from dedicated local production such as produced by electrolysers.

The schematic above shows the use of Storelectric’s latest Green CAES™ technology integrating renewables and the green hydrogen economy. According to Mark Howitt, CTO of Storelectric, plants built using their technology are both cheaper and easily configurable, not least because they employ standard and widely-available equipment, making them an ideal choice in a wide range of energy generation scenarios.

Ending the renewable energy storage conundrum

And so it can be seen how the objections to a swifter adoption of renewable energies of many types can be overcome relatively simply, and at a reasonable CAPEX. The storage solution is already with us, right under our feet and oceans. And the technology to work with it is already proven, just waiting to be used.



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