The flagship of an innovative ‘hot rocks’ energy storage system concept being developed by Stiesdal Storage Technologies (SST) is to be set up with power and fibre-optic group Andel on Lolland, a renewables-rich island off Denmark in the Baltic Sea.

The demonstrator, to be charged with surplus power from wind and solar farms on the island, will be the first full-scale test of the SST GridScale technology, which heats up pea-sized crushed stones in insulated steel tanks using a pump-based system and releases the stored energy via a turbine to produce electricity.

Located at Rødby, the long-duration thermal energy storage project – backed most recently along with several other SST technologies by a major investment from PensionDenmark – is expected to run for 10-15 years connected to the island’s power grid.

“We have found the perfect geographical location and can speed up the construction of our hot rock energy storage so that we get one step closer to storing power from renewable energy sources”, said Jesper Hjulmand, CEO of Andel, which in April invested DKr75m ($12m) in the technology.

SST CEO Peder Riis Nickelsen stated: “For us, Rødby is a great location, both in relation to its neighbouring district heating plant, Rødby Varmeværk, as well as the regional supply situation where Lolland has a large surplus of renewable energy.

“It is precisely in such a context that our storage technology can make a difference and contribute to a far more extensive integration of power from sun and wind than what has been feasible up till now.

“The market for storing electricity from renewables is huge, and we expect that GridScale’s combination of a long discharge cycle and low cost will attract international interest,” he said.

Hjulman added that the location at Rødby allowed for “space to expand the storage facility, and perhaps also test and develop new storage methods”.

“As a society, we are facing an absolutely crucial and comprehensive task in reducing climate change. Seen in the light of the most recent IPCC [Intergovernmental Panel on Climate Change] climate report, the task has not diminished,” he stated.

“At Andel, we want to be part of the solution and lead the way by investing in the green transition, for instance through the extension of the charging infrastructure as well as using our knowledge to develop new green technologies.”

Early mover

An early mover in the renewable energy space, Lolland by 2006 was already producing 50% more power from wind than the resident population could consume, with the island even becoming a test bed for a pioneering energy storage pilot using hydrogen fuel cells.

Today, the island’s offshore wind arrays alone generate over twice the local demand, but without a high-cost investment in power infrascture to export it to the mainland, much of he produced power goes unused, “a clear-cut example of one of the challenges included in the green transition”, said Hjulmand.

“There is an abundance of renewable energy, but it cannot be transported off Lolland without very large investments in the electrical infrastructure,” he noted. “Therefore, you must use the electricity for something else, or you must be able to store it.”

Holger Schou Rasmussen, the mayor of Lolland municipality said: “We are often asked why more green power should be produced on Lolland when we are already self-sufficient, and the hot rock energy storage is part of the answer.

“The utilisation of excess power through storage or production of gas, hydrogen and ethanol will be an important industry in the future”.

Construction of the GridScale demonstrator is expected to start in the autumn, with the facility slated to be to store energy “approximately one year from now”.