Soapstone, granite rocks used as energy storage materials

Using a new approach known as concentrated solar power, researchers have stored then used heat from the sun to dry foods and create electricity. A team reporting in ACS Omega has found that certain soapstone and granite samples from Tanzania are well suited for storing this solar heat, featuring high energy densities and stability even at high temperatures.

Energy is often stored in large batteries when not needed, but these can be expensive and require lots of resources to manufacture. A lower-tech alternative is thermal energy storage (TES), which collects energy as heat in a liquid or solid, such as water, oil or rock. When released, the heat can power a generator to produce electricity. Rocks such as granite or soapstone are specifically formed under high heat and found across the globe, which could make them favourable TES materials. However, their properties can vary depending on where in the world they are formed, possibly making some samples better than others.

In Tanzania, the Craton and Usagaran geological belts meet, and both contain granite and soapstone. A team of researchers from the Nelson Mandela African Institute of Science and Technology and Ardhi University investigated the properties of soapstone and granite found in each of these belts. The researchers collected several rock samples from the belts and analysed them. The granite samples contained a large amount of silicon oxides, which added strength. However, the Craton granite contained other compounds, including muscovite, which are susceptible to dehydration and could make the rock unstable at high temperatures.

Magnesium was found in the soapstone, which conferred a high density and thermal capacity. When heated to temperatures over 1800°F, both soapstone samples and the Usagaran granite had no visible cracks, but the Craton granite fell apart. The soapstone was more likely to release its stored heat than the granite. In all, the Craton soapstone had the best performance as a TES, able to absorb, store and transmit heat effectively while maintaining good chemical stability and mechanical strength. The other rocks may be better suited for a lower-energy TES solution, such as a solar dryer. The researchers said that though further experiments are needed, these samples show promise in being a sustainable energy storage material.

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