Underground storage of hydrogen - Catalyst of sustainable energy transition

Renewable sources of energy, such as the sun and wind, are crucial for transitioning to a sustainable energy system. There is, however, one major drawback to these renewable sources: we do not have control over when they deliver their energy. In summer, there’s likely to be an over-production of solar energy. You need to be able to convert this solar energy into an efficient carrier of energy, which must, moreover, be easy to store in order to compensate for the shortfall during the winter months. Hydrogen might just be the most promising candidate for doing this, says Maartje Boon, who has been leading the ADMIRE Hydrogen Lab at the department of Geoscience and Engineering at TU Delft.

Solar and wind energy can be converted to hydrogen by a process of electrolysis. Hydrogen’s main advantage is its high energy content per mass and the fact that this ‘green gas’ produces combustion products that are free of CO2. There is one major issue that needs to be addressed first, though, in order for hydrogen to be used on a large scale: large amounts of space are required to store the gas, which has a very low energy content per volume.

“This space is not available above ground, and even large-scale storage of liquid hydrogen – which takes up less space – would be too costly, besides possibly being unsafe,” Boon explains. For this reason, the researchers of the ADMIRE Hydrogen Lab are examining the options the earth offers for subsurface storage, i.e. storage of hydrogen in underground reservoirs.

Storage chamber or ‘pantry’

Maartje Boon has been investigating the earth as a storage facility for a decade now. While working towards her PhD at Imperial College London and as a post-doctoral researcher at Stanford University she focused on subsurface storage of CO2. We cannot avoid carbon capture and sequestration (CCS) if we are to achieve the global climate objectives. What this boils down to is capturing CO2 before it enters the atmosphere and then transporting it to somewhere inside the earth where it will remain stored for centuries. Science and industry have been grappling with this for over twenty years. “And yet the upscaling of subsurface CO2 storage is unfortunately proceeding much too slowly,” says Boon. “We have both the knowledge and the technology, but policy and the statutory framework are proving to be an obstacle.”

Her valuable research experience into subsurface CO2 storage has now brought Maartje Boon to the recently established ADMIRE Hydrogen Lab. There is an important difference between CO2 and hydrogen, though: whilst CO2 needs to be captured in the earth forever, hydrogen storage is meant to be a temporary solution – meant to last until you want to use the hydrogen. The earth does not serve as a storage chamber here, but as a ‘pantry’. And this is proving to be an entirely different matter.

Simulator

“The objective is to be able to retrieve the hydrogen that we inject into the earth again at some stage in as pure a form as possible,” says Boon, “preferably without losing too much in terms of quantity.” Injecting hydrogen into underground cavities such as salt caverns – which is possible in the north of the Netherlands – would minimise this challenge. However, one alternative is to use the porous rock in certain geological strata as the hydrogen pantry. This raises questions: how does gas travel through this rock, how far does it spread, and more particularly, how badly will the hydrogen be contaminated by mixing with the reservoir fluids and gases already present in the porous stratum?

In order to get answers to these and other questions, associate professor Hadi Hajibeygi launched an ambitious research programme, ADMIRE, two years ago. His specialism is designing simulators that recreate rock, and that can therefore be used to study how hydrogen behaves in an underground reservoir. “The reliability of the research results of simulators like this depend on good input parameters,” says Boon. This is exactly what her work involves: she conceives and conducts experiments to yield increasingly more accurate input parameters in order to optimise these models – and to get a steadily more faithful image of what might or might not be possible in the future outside of the laboratory.

From one pore to many pores

Step by step, the experiments are meant to give the researchers a better idea of the complex interaction between the hydrogen to be injected, the rocks, and the reservoir fluids and gases that are already present. One of the experiments conducted at the ADMIRE Hydrogen Lab involves the use of a glass model to simulate the pores in porous rock. This glass is filled with brine, which is found in the rock as well, and then hydrogen is injected. The researchers then follow the movement of the hydrogen and the brine through the pores on micrometre scale. This involves measuring the contact angle between the brine/hydrogen interface and the glass, as this contact angle determines the distribution of the brine and the gas. This distribution in turn greatly influences how easily the hydrogen flows through the porous rocks. The research has shown that hydrogen will be located in the large pores especially. “This is a positive result,” Boon explains, “as it makes it easier to inject hydrogen into rock, and less hydrogen will remain in the rock when it is extracted again for use.”

But as the substrata do not consist of uniform rock, of course, one experiment of this kind is not enough. Put in technical terms, Boon explains, geological strata have multiple ‘scales of heterogeneity’. For this reason, a new experiment will be starting soon in which the matter will be observed on centimetre scale rather than micrometre scale. The key question is: how will the hydrogen behave now? Boon has designed this second experiment to involve the use of a medical CT scanner to visualise the distribution of the brine and the hydrogen in actual reservoir rock. Measurements will be made of how the injectivity and the trapping of the hydrogen is influenced by the use of rocks with various heterogeneous structures. Following this, there will be many further questions to be answered in new experiments, not least of which will be whether it is possible for hydrogen, the smallest molecule, to pass through the top seal of hard rocks and thus escape from the underground reservoir.

Pioneers

This research into underground storage of hydrogen puts associate professor Hajibeygi’s research group in the international vanguard. In July this year, people from across the world will be coming together in Delft for the very first International Summer School on Underground Hydrogen Storage, organised by the TU Delft research group. Academics from Boon’s network – one that is extremely relevant to this subject – will be taking part, as well as industry experts who are involved in pilot projects for underground hydrogen storage, in Argentina and Austria, for example. And, with the lessons learnt from the extremely slow upscaling of CO2 storage in mind, Boon and her colleagues have consciously chosen to involve European policy­makers in the event.

The interdependence of science and policy in tackling the climate crisis effectively is a fact. The fact that women scientists like Maartje Boon sometimes take the lead in this respect is a great asset. Used to being judged on her scientific merits just the way her male colleagues are, Boon does not find this exceptional, even though geoscience remains a discipline dominated to a large extent by men. “It wasn’t any different at Stanford,” says Boon, “but I did have an inspiring role model there.” Her post-doc supervisor, Sally Benson, was recently appointed to be Deputy Director for Energy and Chief Strategist for the Energy Transition in the White House Office of Science and Technology Policy. As far as Benson and Boon are concerned, the earth under our feet should become a challenging lead actor in the transition to a more sustainable energy system.

-- by Dr Ellen Lammers, 3 March 2022