Soapstone and granite from Craton in Tanzania’s Dodoma region and Usagaran in the Iringa geo-tectonic settings have been found to be ideal for thermal energy storage (TES), which involves storing solar heat for later use.

Unlike fossil fuels and other energy sources, solar energy storage has proven to be a difficult thing making solar batteries expensive.

The total cost varies based on the manufacturer, battery type, power capacity, installation fees and other factors, with the cost of popular solar batteries ranging from $9,500 to $23,000.

Energy is often stored in large batteries when not needed, but these can be expensive and require lots of resources to manufacture.

Thermal energy storage, which collects energy as heat in a solid such as a rock or liquid such as water or oil, presents as a lower-tech alternative.

Using rocks as a storage medium offers the potential of affordability due to the abundance and low cost of rocks. Some of the rocks that show high-energy storage potential include basalt, micro-gabbro/dolerite and granite.

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Low-tech raw materials

Researchers from Tanzania found that using a new approach known as concentrated solar power, heat from the sun can be stored in rocks to create electricity. The rocks are specifically, soapstone and granite, due to their special thermo-properties.

“We 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,” they said. “When released, the heat can power a generator to produce electricity.”

They say because of this, they think the next generation of sustainable energy technology might be built from some low-tech materials — rocks and the sun.

The scientists are investigating further the properties of soapstone and granite found in each of the Craton and Usagaran geological belts.

The team say the granite samples contained a large amount of silicon oxides, which added strength.

This study explores the potential of the rocks and influence of the sites’ geo-tectonic setting to soapstone and granite rocks as thermal energy storage materials.

From preliminary studies, the rock with the most desired properties for thermal energy storage was soapstone from the Craton setting. Magnesite was found in the soapstone, which conferred a high density and thermal capacity.

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The Craton granite was found to contain other compounds, including Muscovite, which are susceptible to dehydration and could make the rock unstable at very high temperatures.

“When heated to temperatures over 1800 degrees Fahrenheit, both soapstone samples and the Usagaran granite had no visible cracks, but the Craton granite fell apart,” say the scientists in their paper, “Experimental investigation of soapstone and granite rocks as energy-storage materials for concentrated solar power generation and solar drying technology,” published in the scientific journal American Chemical Society-ACS Omega, last month.

“Additionally, the soapstone was more likely to release its stored heat than the granite. In all, the soapstone from craton had the best performance as a thermal energy storage material for both CSP and solar drying,” say the researchers, adding that though further experiments are needed, these samples show good promise in being a sustainable energy storage material.

For applications in drying of agricultural food products TES is used at a moderate and steady temperature of around 40–75 °C.