Researchers have succeeded in getting bacteria to convert CO2 to methane in the laboratory. This discovery may constitute a step on the way towards storage of excess energy from wind turbines.

When bacteria convert biomass to methane, they produce a relatively large amount of carbon dioxide in the same process. This means that the finished biogas often consists of approximately 40–50 percent CO2, which is a residual product lowering the energy density of biogas. However, a group of researchers have now described how excess power from wind turbines can convert carbon dioxide to biogas in anaerobic microbial processes with a possibly 100 percent methane yield. The next step is to study options for creating economically viable and safe full-scale operating solutions.

What do we do with the excess power?
Our current energy system is based on demand-controlled energy production that always has the capacity to meet the most extreme user demands. This entails increased implementation of renewable sources that depend on immediate wind and sun conditions, which means that we often have an excess of energy that we have to waste or export to other countries at a price that might be much lower than average production costs.

“This excess power constitutes one of the main challenges in the conversion to a sustainable energy society, and there’s an urgent need for new ways to store power from fluctuating energy sources,” says Associate Professor Lars D. M. Ottosen.

Together with his research colleagues, he has identified a new solution that may make it possible to store wind power at biogas plants.

Using hydrogen to feed bacteria
The principle is quite simple. The researchers take the surplus power from wind turbines and use it to split water into oxygen and hydrogen by means of electrolysis. The hydrogen is used to feed the bacteria that can then convert CO2 to methane.

“By adding hydrogen to the reactor, we can shift the microbial balance and significantly increase the methane yield from a given amount of biomass. Today, biogas from standard plants consists of CO2 and methane in equal proportions. In the laboratory, we can increase the methane proportion to almost 100 percent by adding hydrogen,” says Associate Professor Ottosen.

Using hydrogen to produce methane is nothing new. The method has previously been used in connection with fuel cells, but this technology is so expensive that it is unlikely to be used outside the laboratory.

“We use the excess power and let the bacteria do the work for us. All we really do is use a process that’s been around for billions of years. The challenge we’re facing is to fully understand this process in detail and create a system that allows us to control it,” says Associate Professor Ottosen.

The researchers expect that the new microbiological method for methane production can be implemented within the next five years and become an important technology for the storage of electricity within the existing energy infrastructure.