Andel and Stiesdal partner for large-scale, stone-based energy storage

20. April 2021

Energy and fibre-optic network group Andel is investing 75 million DKK in Stiesdal Storage Technologies. Their ambition is to take stone-based energy storage to a new level.

The green transformation is in full swing, and greater amounts of energy are being produced sustainably, in such forms as wind power and solar power. Many expect that within ten years, we will have transitioned to using 100% renewable energy sources for electricity. But when the sun isn’t shining, the wind isn’t blowing, and traditional power plants are no longer operating, what will we do? This is one of the major challenges facing the green transformation. Stone-based energy storage is a strong answer to this challenge.

Electricity from hot rocks

The potential for stone-based energy storage has been documented by two Danish innovation projects conducted at DTU Risø, one by Andel and one by Stiesdal Storage Technologies. In both projects, electricity is stored in stone in the form of heat — and that heat can be used to produce electricity on demand.

The knowledge produced by the projects at Risø have taken large-scale energy storage from a simple idea to a realistic, technically feasible technology. Andel has been searching for a strong partner in the industry for continued development. Jesper Hjulmand, executive director at Andel, is thrilled by the new partnership with Stiesdal Storage Technologies:

“I’m really looking forward to the start of our close collaboration with Henrik Stiesdal and his colleagues. Henrik is a pioneer who has been involved in the green transformation since 1976, and the technological capabilities of his business are truly unique. Together, we will be producing a prototype for testing and demonstrations. For Andel, strengthening our focus on energy storage is a great fit for our strategy. If we’re to solve this problem, which has been an obstacle to increasing our use of renewable energy sources and bringing electricity to whole societies, this is the way forward.”

Commercial solution in the works

Henrik Stiesdal has been working with energy storage since 2010 and is just as excited about the new partnership:

“The only really significant challenge standing in the way of 100% green electricity is that we can’t take the power generated when the wind is blowing and the sun is shining and store it for later use. Production and consumption are just not balanced here. So far, there are no commercial solutions to this problem, but we hope to change that with our GridScale energy storage system. We can’t express how happy we are to have Andel as a strategic partner for this project. Technological developments and advanced equipment aren’t enough here. We also need to collaborate with an experienced operator that has extensive knowledge of the electrical grid and knows how to co-ordinate production, storage, and consumption. In that regard, Andel is a strong, ambitious team-mate. They know how to test the technology and deploy it on a large scale.”

The energy storage system the two companies are working on involves pea-sized pieces of broken rocks, held in insulated steel tanks. When the grid has a surplus of power, the storage system is charged using a specially designed heat pump, which transfers heat from one group of tanks to another. The stone filling in the source tanks becomes cooler, while the filling in the destination tanks becomes much hotter — up to around 600°C, in fact. Heat can be stored in the stone-filled tanks for many days. When the grid experiences a greater demand for electricity, heat from the hot tanks is returned to the cold tanks using a kind of gas turbine, which generates electricity. The energy loss with this solution is low, making it highly efficient. The solution can be scaled up by simply adding more stone-filled tanks.

Affordable and efficient

Ole Alm, Andel’s head of development, is in charge of the company’s energy storage projects. He has high expectations for the new storage system:

“Stone is a cheap, sustainable material. It can store large quantities of energy without taking up much room, and it can withstand countless charge-discharge cycles. We learned this from the tests we ran at Risø. Now, we need to create units that are flexible and relatively easy to work with. They can be installed at solar and wind power facilities, at transformer stations and industrial facilities, and perhaps also on the ‘wind islands’ that are currently in development. For projects like these, we need a partner in the industry, like Stiesdal Storage Technologies. Together, we can create functional, large-scale solutions.”

Peder Riis Nickelsen, co-CEO of Stiesdal Storage Technologies, is looking forward to the project’s next steps:

“Commercially sustainable storage of large quantities of energy requires a few things: First, you need to have a very cheap storage medium; and second, you have to be able to industrialise the supporting equipment. Our GridScale technology meets both of these criteria. Compared to essentially every other energy storage medium, the price of crushed stone is on a totally different level. Our charge-discharge system uses well-known technologies that have been around for a century in other industries, and they’re suited to mass production. For those reasons, we see a lot of opportunities in this concept — not only here in Denmark, but especially in export markets.”

Exactly where the first prototype of the new storage system will be installed and tested has yet to be decided. However, it has been confirmed that the site will be in either South Zealand, West Zealand, or on the islands of Lolland and Falster. Production of new, large-scale solar power facilities is increasing in these areas, but without a corresponding increase in power consumption. In order to avoid bottlenecks in the grid, which would require renewable energy generation to be suspended, it needs to be possible to store generated energy or transport it elsewhere.

Millions in support from the Danish Energy Agency

To further accelerate development, eight Danish partners have been awarded grants from the Energy Technology Development and Demonstration Programme, which is overseen by the Danish Energy Agency.

This innovation project, entitled “GridScale: A cost-effective, large-scale, electricity-to-electricity storage system”, will run for three years on a budget of 35 million Danish kroner. In addition to Stiesdal and Andel, partners include Aarhus University, the Technical University of Denmark (DTU), Welcon, BWSC, Energi Danmark, and Energy Cluster Denmark. The partners will develop an energy system analysis and optimised design for a stone-based storage system.

One planned analysis involves combining the European power distribution system model developed at Aarhus University with DTU’s gas turbine optimisation model. This will not only offer insight into the storage system’s potential role throughout Europe, but also make it possible to optimise a conceptual design.

“The goal is to establish how stone-based storage can best help Denmark and Europe in their green transformation. We hope to have an option ready for installation on ‘wind islands’ and in many other locations where there is a need to store renewable energy,” says Glenda Napier, CEO of Energy Cluster Denmark.


From 2016 through 2019, Andel conducted a stone-based storage project together with DTU, AU, Energinet, Rockwool, Dansk Energi, and EUDP. The project’s purpose was to investigate the possibility of using existing technology to build a high-temperature energy storage system filled with stone, making it possible to cheaply store surplus electricity for later use when there is a deficit. The project was also intended to shed light on the role of a storage system and its business potential within the context of the power distribution system as a whole.

The project confirmed that stone can withstand repeated heating, that the energy can be retrieved from storage at a constant temperature, and that a large-scale storage system can help to address challenges in the grid.


From 2018 through 2020, Stiesdal Storage Technologies collaborated with DTU, AAU, Welcon, Frecon, Blue Power Partners, and Energy Cluster Denmark on a project supported by the EU’s European Regional Development Fund. The purpose of the project was to validate models for hot stone storage using experiments in steel tanks at a scale of 1:10. The project showed that heat could be transferred from the air to the stones as calculated, and with less of a pressure drop than projected.