Against Dark Calms: How Electricity Can Be Stored

With little sun and hardly any wind, Germany experiences a "dark lull." This winter has already seen several periods during which solar plants and wind turbines provided barely any electricity due to seasonal and weather-related conditions. Even in recent days, the share of renewable energy looked meager. 

Then electricity has to come from elsewhere, currently primarily from controllable coal and gas power plants. These are planned to be shut down in the future. And then? 

An electricity system that primarily relies on solar and wind, aiming for up to 80 percent by 2030, needs balancing mechanisms for a reliable supply. On the one hand, consumption could be more closely aligned with the availability of electricity in the future. Cross-border electricity trading and grid expansion are also important. Additionally, other producers need to step in - a range of options is possible in this regard.

Lithium-ion batteries: currently market-dominating

Battery storages can bridge shorter periods, such as a few seconds to keep the grid stable, or a few hours during the night. Large storages made of lithium-ion batteries, similar to those in electric cars, can swiftly absorb excess electricity from the grid and store it again later. According to the Jülich Research Centre, they exhibit a high efficiency of 80 to 90 percent.

This storage and reconversion into electricity are increasingly taking place, with lithium-ion battery storages being set up in many locations. Other battery storages still play hardly any role. In recent years, lithium-ion batteries have not only become lighter and smaller but also much cheaper.

"They are also very durable, almost like combustion power plants - it's not like with cell phones," says Martin Winter, head of the MEET Battery Research Center at the University of Münster and the Helmholtz Institute Münster, an external branch of the Jülich Research Centre.

Germany needs large capacities. The Fraunhofer Institute for Solar Energy Systems ISE expects that by 2045 a total of 180 gigawatt hours (GWh) of central and decentralized electrical storage will be needed. The country is far from that: currently, the storages have a capacity of 17.8 GWh, according to the Battery Charts of RWTH Aachen.

Sodium-ion batteries: lower energy density

Since the raw material lithium is not endlessly available everywhere, other types of batteries are being researched and tested. According to Maximilian Fichtner from the Helmholtz Institute Ulm, lithium will increasingly be replaced by sodium - known, for example, as a component of sodium chloride, or table salt.

This material is environmentally friendly, cost-effective, and available worldwide, unlike lithium, which comes from only a few countries, and thus one becomes dependent, said Fichtner in a conversation with his university. Sodium-ion batteries are now market-ready and are used, for instance, in small electric car prototypes.

However, sodium-ion batteries have a lower energy density, meaning they are heavier and larger than lithium-ion batteries.

"You always need more, in extreme cases up to three times as much," Winter points out. This means: "Up to three times as many materials, cell housings, fine chemicals, also three times as many ovens, trucks for transport, and space."

Hydrogen: for long-term storage

Extended lulls in wind energy lasting several days cannot be covered by battery storage; even with significant expansion, the capacities will not suffice.

“We need to rely on chemistry, meaning to produce molecules with green energy that can be burned,” says Winter.

With the help of electricity - ideally from renewable sources in the future - water can be split into hydrogen and oxygen. The hydrogen is then used in fuel cells or power plants - partly this should happen in old gas power plants. Alternatively, the hydrogen can be further used to obtain synthetic fuels.

“Unfortunately, all of this is very inefficient,” says Winter.

However, the hydrogen can also be produced elsewhere - such as in sun-rich countries - and brought to Germany in ships or pipelines. It is conceivable that hydrogen could also be used to build seasonal storage in the future.

Nuclear Fusion: still a long way off

Deliberately fusing atomic nuclei to gain energy is already possible - but real fusion power plants are likely to come online only in several decades. It is also difficult to obtain sufficient quantities of the raw material tritium, as explained by the Office of Technology Assessment at the German Bundestag in a recent report.

In fact, these power plants would have to run at high capacity continuously, otherwise, the investment wouldn't be worthwhile, suggests fusion researcher Christian Linsmeier from the Jülich Research Center. They cannot be switched on just for a few days during a dark doldrum. Rather, it might be conceivable to produce hydrogen instead.