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Hot rocks could be the next big energy storage technology

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Hot rocks could be the next big energy storage technology

Soapstone, widely used in cooking appliances and countertops, could be an ideal low-cost material to store the sun’s heat for electricity
June 8, 2023

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Granite and soapstone likely conjure up an image of kitchen countertops for most people. These rocks, among the most plentiful in the world, have become common building materials. But the same properties that make them excellent kitchen workhorses—strength, durability and resistance to heat and chemicals—could also make them ideal for storing energy.

Researchers have found that granite and soapstone could be well-suited to store the sun’s heat, which could then be used to produce electricity or to dry foods. Soapstone from Tanzania is particularly good, packing a lot of heat energy by weight and staying stable at high temperature, the team reports in the journal ACS Omega. “Using rocks as a storage medium offers the potential of affordability due to the abundance and low cost,” they write.

Storing energy for long periods of time will be critical as intermittent wind and solar power generation increases globally. Batteries are today’s go-to storage technology, but they are expensive. Other experimental storage methods being tested or put into use today can be complicated to operate.

Thermal energy storage, in which energy is stored as heat in materials such as water, oils, or molten salts, offers a promising alternative. The heat can be collected directly from the sun by concentrating sunlight, or by converting extra wind or solar power using heat pumps. When needed, the heat is released to drive a generator that produces electricity or used directly for industrial uses.

 

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Using natural rocks to store heat could be cheaper than using molten salts and oils. Some demonstration projects such as GridScale in Denmark, and a larger gigascale system in Israel, are already underway. They store energy in tanks full of crushed stone.

But the properties of rocks can vary based on where in the world they were formed. Some rocks can be much better at storing heat than others. The team led by Thomas Kivevele from Nelson Mandela African Institution of Science and Technology set out to investigate the properties of soapstone and granite found in Tanzania, where the Craton and Usagaran geological belts meet.

Granites are the most abundant rocks in the continental crust. Soapstone, meanwhile, has been used since ancient times to make cooking pots and the internal linings of stoves, but no one has studied its potential for thermal energy storage.

The researchers collected several rock samples from the Craton and Usagaran belts for analysis. They found a large amount of silicon oxides in the granite, which added strength. Craton granite, however, contained compounds that made the rock unstable at high temperatures. Soapstone had the mineral magnesite, which imparted high energy density and heat-holding capacity.

Tests showed that both soapstone samples and the Usagaran granite had no visible cracks at temperatures over 980°C, but the Craton granite crumbled. The soapstone was better than the granite at releasing its stored heat.

The team found that the Craton soapstone performed best as a thermal energy storage rock. It absorbed, stored and transmitted heat effectively while staying stable and strong. This makes it ideal for electricity storage applications. The other rocks could be used for a lower-energy application, such a solar food dryer.

This work points to a low-cost, reliable, efficient, and environmentally friendly way to store the sun’s energy, the researchers say.

Source: Lilian Deusdedit Kakoko, Yusufu Abeid Chande Jande, and Thomas Kivevele. Experimental Investigation of Soapstone and Granite Rocks as Energy-Storage Materials for Concentrated Solar Power Generation and Solar Drying Technology. ACS Omega, 2023.

Top image: ©Anthropocene Magazine

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