BY JASON DEIGN:
A French research project has provisionally concluded using methane in a closed-loop power-to-gas-to-power (P2G2P) process could be a “credible solution” for seasonal storage from 2030.
The FluidSTORY project is looking into the viability of an electrolysis-methanation-oxyfuel (EMO) cycle for P2G2P, using underground storage for methane, carbon dioxide and oxygen.
EMO aims to overcome an important challenge with conventional power-to-methane: the fact that carbon dioxide supplies, say from carbon capture and storage, may be insufficient for methanation.
To deal with this, the carbon dioxide used in EMO is stored underground, in salt caverns, and cycled in a closed loop, so it can be reused endlessly.
There are other differences between EMO and the standard power-to-methane process, where hydrogen is transformed into methane, using biological or catalyst-based methanation, with the addition of carbon monoxide or dioxide.
Methane with underground storage
In the standard process, for example, when water is split to create hydrogen, the oxygen from electrolysis is wasted. In EMO, it goes into underground storage, so it can be used later for methane combustion.
And because the methane is burned using pure oxygen, the only by products are water and carbon dioxide. This carbon dioxide gets fed back into the methanation process.
The French Geological Survey (Bureau de Recherches Géologiques et Minières or BRGM in French) has looked into suitable sites across France and identified at least three potential deep-storage locations.
These are in addition to ample shallow storage sites in the Paris Basin, which could be used to store the gases at low pressure.
An analysis of the full-cycle (grid-to-storage-to-grid) efficiency of the process has shown it might be comparable to that of hydrogen, and significantly more efficient that traditional methanation.
To deliver a single megawatt-hour energy
To deliver a single megawatt-hour of energy to the grid would require 4.3MWh of electricity with conventional methanation, 3.1MWh with hydrogen electrolysis and fuel-cell conversion, and 3.4MWh with EMO, studies show.
But methane beats hydrogen on energy density and on ease of transport across the existing gas grid. In Europe, approximately 70m consumers can be served using a 2.2m km gas grid, BRGM notes.
As befits an early-stage research project, though, there are still many unknowns around the FluidSTORY storage concept.
For example, it is unclear how methane might be stored with oxygen and carbon dioxide underground.
“Oxygen has never been stored in salt cavities,” admitted André Burnol, head of the BRGM’s safety and performance of underground uses unit, presenting the project’s findings to date at Electrify Europe last month.
Set to end in 2019
The project, which has now been running for three years and is set to end in 2019,envisages two scenarios, he said.
In the first, carbon dioxide would be stored with oxygen, separated by their different densities, and with injection of the latter being used to extract the former.
Another option is to store oxygen and carbon dioxide in different cavities, but that would increase the cost, said Burnol.
A second challenge for the concept is that the oxyfuel part of the EMO process is still relatively new, and the concept overall has never been tried out on anything like the scale envisaged by FluidSTORY.
So far the project team, which includes eight partners including BRGM and Areva, has modelled the process using Aspen Plus chemical industry simulation software, but has not carried out a test or pilot.
Looking for commercial partners
FluidSTORY is currently looking for commercial partners that could help the project take this next step, Burnol said. But the quest to find takers could be hampered by a final challenge facing EMO: cost.
A levelized cost of energy analysis shows EMO could pan out at around €700 per MWh delivered to the grid, which is above €670/MWh for conventional methanation and €540 for a hydrogen electrolysis and fuel-cell combo.
Despite this, the FluidSTORY project team believes EMO could become viable as the fraction of renewable energy grows on the grid and the value of seasonal storage increases.
“In-depth analysis of 24 recent European energy scenarios showed that power-to-gas appears as credible solution from 2030 onwards, when the share of renewables become significant or even dominant,” said the team in a paper.
“In this frame, the closed-loop power-to-gas EMO concept appears as a credible alternative that overcomes shortcomings in the conventional methanation and pure hydrogen fuel cells options.”
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