GridScale: Storing Renewable Energy in Stones Instead of Lithium Batteries

When there is a surplus of electricity from wind or solar, the energy storage is charged. This is done by a system of compressors and turbines pumping heat energy from one or more storage tanks filled with cool stones to a corresponding number of storage tanks filled with hot stones. This makes the stones in the cold tanks very cold, while it gets very hot in the hot tanks, up to 600 degrees. Credit: Claus Rye, Stiesdal Storage Technologies

The concept of storing renewable energy in stones has come one step closer to realization with the construction of the GridScale demonstration plant. The plant will be the largest electricity storage facility in Denmark, with a capacity of 10 MWh. The project is being funded by the Energy Technology Development and Demonstration Program (EUDP) under the Danish Energy Agency.

Pea sized stones heated to 600°C in large, insulated steel tanks are at the heart of a new innovation project aiming to make a breakthrough in the storage of intermittent wind and solar electricity.

The technology, which stores electrical energy as heat in stones, is called GridScale, and could become a cheap and efficient alternative to storing power from solar and wind in lithium-based batteries. While lithium batteries are only cost-effective for the supply of energy for short periods of up to four hours, a GridScale electricity storage system will cost effectively support electricity supply for longer periods – up to about a week.

“The only real challenge with establishing 100 percent renewable electricity supply is that we can’t save the electricity generated during windy and sunny weather for use at a later time. Demand and production do not follow the same pattern. There are not yet commercial solutions to this problem, but we hope to be able to deliver this with our GridScale energy storage system,” says Henrik Stiesdal, founder of the climate technology company Stiesdal Storage Technologies, which is behind the technology.

In brief, the GridScale technology is about heating and cooling basalt crushed to tiny, pea-sized stones in one or more sets of insulated steel tanks. The storage facility is charged through a system of compressors and turbines, which pumps heat energy from one or more storage tanks filled with cool stones to a similar number of storage tanks filled with hot stones, when there is surplus power from wind or the sun.

This means the stones in the cold tanks become very cold, while they become very hot in the hot tanks; in fact up to 600oC. The heat can be stored in the stones for many days, and the number of sets of stone-filled tanks can be varied, depending on the length of storage time required.

When there is demand for electricity again, the process reverses, so the stones in the hot tanks become colder while they become warmer in the cold tanks. The system is based on an inexpensive storage material and mature, well-known technology for charging and discharging.

“Basalt is a cheap and sustainable material that can store large amounts of energy in small spaces, and that can withstand countless charges and discharges of the storage facility. We are now developing a prototype for the storage technology to demonstrate the way forward in solving the problem of storing renewable energy – one of the biggest challenges to the development of sustainable energy worldwide,” says Ole Alm, head of development at the energy group Andel, which is also part of the project.

The GridScale prototype will be the largest storage facility in the Danish electricity system, and a major challenge will be to make the storage flexibility available on the electricity markets in a way that provides the best possible value. Consequently, this will also be part of the project.

The precise location of the prototype storage facility has yet to be decided. However, it will definitely be in the eastern part of Denmark in south or west Zealand or on Lolland-Falster, where production from new large PV units in particular is growing faster than consumption can keep up.

The full name of the innovation project is ‘GridScale – cost-effective large-scale electricity storage’, and it will run for three years with a total budget of DKK 35 million (EUR 4.7 million). The project is being funded with DKK 21 million (EUR 2.8 million) from the Energy Technology Development and Demonstration Program (EUDP).

In addition to the companies Stiesdal and Andel, the partner group comprises Aarhus University (AU), the Technical University of Denmark (DTU), Welcon, BWSC (Burmeister Wain Scandinavian Contractor), Energi Danmark and Energy Cluster Denmark.

The partners will provide an energy system analysis and design optimization for a stone storage facility as well as optimize the technical concepts and mature the GridScale technology to a ready-to-market scalable solution.

For example, the European energy system model developed by AU will be combined with the model for optimizing turbines developed by DTU to gain insight into the potential role of the stone storage facility in a European context and to optimize the design:

“The transition to renewable energy changes the way the energy system works – simply because wind and solar energy are not necessarily produced when we need it. Therefore, we need to find out how the technical design can best be adapted to the energy system and in which countries and when in the green transition the technology has the greatest value. We will look to identify the combination of energy technologies that will provide the greatest value for the storage solution. I think that stone storage technology has a huge potential in many places around the world and could be of great advantage in the green transition,” says Associate Professor Gorm Bruun Andresen from the Department of Mechanical and Production Engineering at Aarhus University.

Aarhus UniversityBattery TechnologyEnergyGreen EnergyPopular
Comments ( 19 )
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    … nice, but wouldn’t it be easier to use two lakes on different levels and pumps, when you have a Sun you pump the water back, when you need power, well you just let water run down the generator…

  • Xa745763226700

    A concept known as “pumped hydro”.

    Now, this is Denmark. There are no lakes at “different levels”.

    • xABBAAA

      … the one can build a building and there you go, and any form of chemical energy would do it…However, there should be better alternative to the Sun energy in Denmark, where is something rotten, hope the Denmark CIA get it, … it is a William Shakespeare, Hamlet, just to make sure, that, well you know the drill…

  • John Jakson

    Isentropic was a UK company that explored this idea using pumped argon and gravel in hot and cold tanks, but they ran out of steam in 2016. Their expertise was in gas turbines so it seemed like a good idea at the time.

    They claimed that at scale the round trip efficiency could be near 70% which compares well to pumped hydro. While pumped hydro has severe limits on where it can be used, gravel energy banks could be placed anywhere at almost any size.

    Now several smaller scale models have been created that are closer to 60% eff to be expected but should do better if scaled up. At 70% they should be vastly cheaper than electric batteries and have no toxic or rare components.

    On the other hand stored heat in molten salt tanks makes far more sense, generate high temperature heat from fission, fusion or solar and store in salt tanks. Nuclear heat would be at constant production, but the electrical output can be staged far below to far above average to match the grid even with lots of variable RE inpuits. These would completely replace gas peakers. Terrapower, Moltex are planning on this.

    If the heat is from solar, you are back to the land problem of low energy density and variable daytime production, with the salt tanks supplying electricity at a lower rate through the day/night as per tank capacity like Gemasolar.

  • Tom Smith

    I can’t see how this technology isn’t killed by thermodynamic irreversibilities. The second law losses must be enormous. What are the round trip energy efficiencies (electrical energy out/electrical energy in) like after a week of storage? My money’s on pumped hydro and vanadium flow-cells any day.

    • John Jakson

      Search for Isentropic uk, the lead story is a system that was built in 2019 and appears to work and the first of its kind. Isentropic was the original inventer, after failing in 2016, their project was transferred to Newcastle University Swan Centre PHES, and the structure was built as per the original design. See the The Engineer article and the comments including from the original design engineer at Isentropic Ltd, Jonathon Howes. The 60-65% eff exceeded his expectations too, but to get to 70% it has to go to 1MW power levels.

      The Swan centre reports 150KW power with 600kWh of storage, not very much, thats only $60 worth of energy. It also claims 75-80% eff round trip for a bigger version. I saw a $100/kWh price somewhere, but the picture shown looks a lot more expensive than $60k.

      I found the pdf project summary on the Swan PHES site, it costs EU 18M, or $22M, Wow. They want to scale to 10MW. There is a statement that says Low Cost, 18-22 EU/kWh and capotal cost 550-800 EU/KW. Energy density similar to lead acid. If these lower costs can be achieved, then that is far better than Li ion, pumped hydro or Vanadium batteries.

  • 123xyz

    The question with systems that claim to store electrical power at a larger scale and/or lower prices than batteries is always how much efficiency do you sacrifice to get the greater storage capacity. This article gives no numbers for that.

  • Dan

    Which should be recognized here is that this technology has a potential to be backed up very very long-term with fossil fuels. I am not advocating the storage be small thereby leaning on fossil fuels more, but I am advocating that in those tail risk events there is a possibility to have very long-term storage.

  • gene hunter

    What percentage efficiency does this process yield

  • Jh

    There are currently no tanks with stones in them either

  • DanC

    “when electricity is in demand, the process reverses”
    If they simply tap into the hot or cold tanks to heat or cool buildings, that would seem more sensible than producing electricity from storage.

    • Alan Peery

      For heating or cooling, a direct approach is indeed more efficient in a local area. But if you want flexibility or the ability to transmit over distance electricity is much more flexible.

  • Frankie Ku

    Best small gas turbines efficiency too low maybe 55% and compressor efficiency can be 80-90%; then adding in losses for system ditch this quick.
    World including China & India needs fast tracks to safe NPP / SMR’s replacing closed and soon to close old coal and other plants including nuclear. Include SCO2 cycles to reduce power plant system size and cost – see

  • Max Harrison

    Interesting, although water has a better specific heat capacity, well, it boils!

    I bet the efficiency losses can be marginalised with insulation

    Quite a novel idea where gravity pumps are not an option, or a SMES 🙂

  • Derek ryan

    They should use torsion springs like that in a wound watch to store energy. Like gigantic cassette tapes that we store all this engery into by just winding this wheel or cog. And once it’s to it’s max capacity we just either store these gigantic disks down old missile silos or in near earth orbit. And then use them as we need. But maybe there is to much to that. That’s why it hasn’t been thought of yet

  • Kane

    Pumped hydro is limeted by suitable sites and the space it takes up. Ireland is investing heavily in pumped hydro, but even being a hilly country there aren’t enough suitable sites to provide all of the countries needs and many areas of natural beauty would have to be destroyed to create the resovoirs. I think the key to future success is going to be combining many methods which have different pros and cons; pumped hydro for long term storage, something like heated rocks for short term storage. I’m just happy to see how much is being invested in different energy storage solutions, eventually a few very good solutions will come from all of these different products.

  • Gene

    How about to storidge in pressure air in big tank

  • Questioner

    Can someone PLEASE concisely explain the process for withdrawing energy from the temperature difference? Storing it is easy, conventional refrigeration tech. But Withdrawal?

  • Lathechuck

    Questioner: to withdraw the energy, you simply force a small volume of cold gas into the hot-rock tank, let it expand as it warms, then use the higher volume of hot gas to run a turbine. Though it takes energy to force the cold gas in, the hot-gas turbine produces more. It’s not much different from an internal combustion engine (cold air in, hot air out), but the heat comes from the rocks, not from a chemical reaction. Read about various thermodynamic cycles: Rankine (water – steam), Brayton (constant pressure), Stirling (inert gas) which are all “heat engines”. The Carnot efficiency limit applies, as well as frictional losses in the moving parts, heat leakage through the insulation, and the cost of capital to build and maintain the equipment.