The idea behind power-to-gas has been around for a while. It uses electric power to produce gaseous fuel, usually hydrogen. The advantages are that it can be used to take up excess electric energy from renewable generation in a chemical form. Later, the fuel can be used as needed.
The gas produced does not need to be for generating electricity. It can be used to power internal combustion engines, to heat buildings, or even as an industrial feedstock. And there has been a lot of research done to create the hydrogen from electric power in the most efficient way possible, to make the economics of the process work well.
A recent report from the Institution of Mechanical Engineers, “UK ‘should store excess renewable energy in hydrogen,’” suggests that power-to-gas facilities be used in the UK as a backup for renewable energy, as a less-expensive alternative to a battery. The idea is to mix hydrogen with natural gas in existing gas facilities and use the mixture as fuel.
Regardless of how it is done, storing hydrogen is somewhat problematic. Part of the problem is that hydrogen molecules are so small they can fit between the atoms in alloys, including the steel used in gas cylinders. Though it does so slowly, hydrogen can leak right through the sides of a cylinder, making long-term storage difficult. Another problem is that because it is chemically very active, it can react with metals and weaken them. Yet another problem is that hydrogen does not contain as much energy as the same volume of methane. Also, it cannot be compressed to make it liquefy, even at the low temperatures used to maintain liquid natural gas.
The CleanTechnica article, “How US Shale Gas Could Torpedo UK’s Hydrogen ‘Gas Battery,’” provides another problem for the economics of hydrogen. It will have trouble competing with liquefied natural gas from US fracking fields, which is now being shipped to Europe in increasing amounts. And, of course, if increasing amounts of gas are available from fracking in the UK or the European continent, that will only add to the competition hydrogen faces.
An alternative for a power-to-gas scheme is to use the hydrogen to synthesize methane, which is functionally the same as natural gas. This was reported in a Reuters article, “Germany’s Uniper makes head start in converting wind power to gas.” The path from hydrogen to methane only requires that one carbon atom be attached to the atoms in two hydrogen molecules to produce methane. And the carbon can be taken from a carbon dioxide molecule.
In Uniper’s case, the carbon will come from carbon dioxide captured at a bio-ethanol plant, according to information in “Power-to-Hydrogen: Legal Barriers and Regulation,” a document Uniper supplied to the European Commission. It is an important point because it means that the carbon in the methane would otherwise have been released it directly into the atmosphere. In other words, the synthetic methane could be viewed as containing “free” carbon, and burning it does not increase the carbon dioxide in the atmosphere any more than otherwise would have been the case.
The bio-ethanol process is basically the same as the fermenting proces used to make beer, wine, and other beverages. The carbon dioxide from these processes is normally released into the atmosphere, where it is a greenhouse gas, just as if it came from a car’s tailpipe. There is a lot of this carbon dioxide available, and it is distributed through all parts of the world where brewing is done. Once captured, it can be compressed to liquid and shipped much more easily than natural gas. It could be processed with hydrogen created as a by-product of the production of renewable energy, and this could be done wherever it is needed.
The methane produced in Uniper’s process is unlike the hydrogen product of most power-to-gas schemes in several important ways. It can be stored easily without directly degrading metals it contacts, it can be compressed to liquid and shipped in that form, it has more energy per volume, and it is a drop-in replacement for natural gas.
We should also remember, however, that methane need not be burned to be useful. Methane can be used as a basic feedstock for a variety of chemicals, and a lot of research is being done to increase its value for such use. Synthetic methane has an advantage over fossil natural gas because can be created in a fairly pure form.
The products that can be created from synthetic methane range from paint and pharmaceuticals to plastics. And though some, such as plastics, may be horrible in several important ways, one nice thing about some possible products of synthetic methane is that the carbon atoms in them would be sequestered. If we can manufacture and use organic derivatives of mathane responsibly and dispose of them properly at the end of their lifetimes, they can be used to draw down the carbon dioxide in the atmosphere.