The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt or DLR) is investigating whether Germany’s coal plants could be reused as energy storage assets.  

The research body, which has a track record in concentrated solar power (CSP) development, is planning a pilot that will involve ripping out the boiler from an old coal plant and replacing it with a molten salt thermal storage tank that will be heated using excess renewable energy.

If the concept works, then advocates say it could help safeguard coal generation jobs while giving Germany tens of gigawatts of storage capacity for renewable energy load-shifting on the German grid. 

Furthermore, a single pilot could be enough to prove the commercial viability of the concept, since the technology, described as a Carnot battery, is based on commercially available industrial components and standard engineering practices. 

Dr. Michael Geyer, senior adviser at DLR’s Institute of Engineering Thermodynamics in Almeria, Spain, said the center is preparing a commercial-scale pilot in association with an unnamed German utility. A feasibility study for the pilot had already been awarded, he confirmed.

Geyer explained that engineering proposals would take 12 to 18 months and construction could take another year and a half, meaning the pilot plant could be up and running within three years. The pilot is being financed as a public-private initiative, he said.   

"A commonsense application"

According to its website, DLR has been researching Carnot batteries since 2014. Experience with molten salt storage in CSP plants, meanwhile, stretches back almost a decade.

This is significantly longer than the operational track record of grid-scale lithium-ion battery plants, Geyer noted. Thermal storage tanks are relatively low-tech, low-risk engineering concepts, he said, requiring only a steel tank, concrete base and the salt itself.

The principal engineering task is to fit the tank into a coal plant, he said. These plants were not designed to house molten salt storage containers, he admitted, and a Carnot battery built from scratch would likely not resemble a coal plant. 

But in Germany utilities are interested in the concept as a way of extending the lifespan of coal generation assets that have now been given a final cutoff date of 2038. Geyer said 7 gigawatts of coal generation were due to close down by 2023, rising to 23 gigawatts by 2030.

Most if not all of this capacity could potentially be turned into Carnot batteries, he said. “This is a commonsense application to make use of existing infrastructure,” he said.

Combining with lithium-ion and renewables

The cost of converting the coal plants is not expected to be overly significant because a large amount of the existing infrastructure would be reused and permits plus grid connections are already in place.

For operation, meanwhile, the batteries would rely on renewable energy that is currently being curtailed. Carnot batteries would likely complement lithium-ion battery plants on the German grid, Geyer said, with the latter delivering primary and secondary frequency control services.

The combination of lithium-ion and Carnot batteries could maintain grid stability and deliver load-shifting to keep grids running on high levels of renewable energy for hours or possibly days, he said. 

However, to cope with extended periods of low wind and poor sunlight, for instance during winter calms, some of the converted coal plants could be equipped with gas boilers, said Geyer.

Nevertheless, the Carnot battery concept only makes sense as a way of taking advantage of coal plant equipment and excess renewable energy that would otherwise go to waste, Geyer conceded, because the round-trip efficiency of the storage process is only about 40 percent. 

DLR hopes to raise this level to 60 percent using heat-pump technology, in a manner analogous to that being pursued by the Alphabet spinoff Malta.

Jonathan Walters, an economic consultant working for the World Bank on routes to transition away from coal, said lignite mining areas could cover distressed land with solar panels to power Carnot batteries.

“You’re going to lose the lignite jobs,” he said, but “you’re going to save the power plant jobs and you’re going to save some of the physical assets that would otherwise be written off.”

Geyer and Walters are due to release further details of the Carnot battery concept in a webinar next month.