Innovative energy storage: 600-degree hot stones are used to store green electric power

Innovative energy storage

A view of the top of the energy storage model at DTU Risoe. The majority of the ball-shaped steel capsule, which is filled with hot stones, is cast in the foundation. Photo: SEAS NVE

A test model of a new type of energy storage has been inaugurated at DTU Risoe. The innovative technology has a large potential for storing wind and solar energy. NIRAS consulted on the model’s design.

March 22, 2019
  • SDG: #7, #9, #13
  • SECTORS: Energy
  • COUNTRIES: Denmark
  • DONOR: Danish Energy Agency’s EUDP

What started as a master’s project at DTU in 2016 was on Monday, 18. March 2019 inaugurated as a ground-breaking test model of a new type of energy storage. At the inauguration of the energy storage system, located at DTU Risoe near Roskilde, the Minister for Higher Education and Science, Tommy Ahlers unveiled the innovative model to the press and all the project parties.

The 3.5 m3 large energy storage capsule is a test model, with the purpose of testing a new technology with exceptionally great potential. The energy storage consists of a ball-shaped steel capsule filled with stones. The stones’ temperature is increased to 600 °C with large electric heaters powered by sustainable energy.

Subsequently, the heat may be changed back to electricity using different types of energy systems.

In order to retain the heat as well as protecting the boiler shell, consisting of armoured concrete, the storage is isolated with skamol between the hot inner steel basin and the exterior concrete shell. Furthermore, the construction reduces the power impact in the boiler shell with its geometric design.

Preview of a large-scale solution

The energy company, SEAS-NVE, being head of the project, hopes for development of the new method for storing energy for large-scale solutions. With its 3.5 m3, size, the test storage at DTU Risoe is able to store an energy quantity corresponding to 875 kWh. It is the rough consumption of electricity in a household with two adults and two children over a two-month period.

Together with its partners, SEAS-NVE will analyse the results and measurements from the test storage prior to deciding on whether to proceed with larger test plants. An energy storage the size of a modern IKEA warehouse at approximately 200,000 m3 has been considered during the project.

A storage this size would be divided into cells, with each capable of storing approximately 2.5 GWh. With 20 of these cells it will correspond to approximately 9,500 households’ annual consumption.

Great potential in storage of renewable energy

Large-scale energy storage may solve some of the challenges covered by conventional electric power plants. The large energy storage units have the potential to accommodate consumers’ fluctuating electricity demand when renewable energy is not available. Sun, wind and waves have a way of coming when they want and not when their energy is needed. With an increasing share of renewable energy in the energy sector, energy storage will be necessary in the future in order to cover energy demand.

This method of energy storage has great export potential. This is especially valuable as more countries convert to renewable energy but are limited by their infrastructure. The implementation of energy storage has the potential to increase the share of renewable energy and thereby reduce emissions of CO2.

As a consultant for SES-NVE, NIRAS coordinated the numerous ideas for design, and prepared a final storage prototype, which can be scaled and complies with the technical requirements made for the final product.

Project partners:

SEAS-NVE is project manager

Other partners: Technical University of Denmark, Dansk Energi, Rockwool, Energinet, Aarhus University

NIRAS is consultant for SEAS-NVE on the energy storage design

The project is funded by the Danish Energy Agency’s EUDP – Energy Technology Development and Demonstration Program

One of our greatest challenges as a community is to minimize the use of fossil fuels. There is a great demand for even more energy from renewable energy sources and the possibility to store this energy. The project’s investigations indicate that storage in heated stones may contribute to meet this demand. Stone storages are an environmental friendly and low-cost solution that in a larger scale may contribute to fulfil the objective regarding a 100 per cent fossil free energy supply without loss of supply safety.

Jesper Hjulmand, managing director, SEAS-NVE

The energy storage solution in short

  • Electricity production from wind turbines or solar cells is converted to 600 °C hot air

  • The hot air is blown into the energy storage capsule and heats the stones in the storage

  • The storage is designed to store the energy on a daily basis

  • As required, the process is reversed and the heat in the storage is converted to electricity for consumers

  • Learn more about the project:

600 °C

This is how hot the stones get in the core of the energy storage.


Households’ annual electricity consumption in potential giant cells.

Get in touch

Niels Bahnsen

Niels Bahnsen

Project Director

Allerød, Denmark

+45 4017 2884

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