Researchers Point to Greener, Cheaper Solar Power Through a Low-Tech Storage Material: Soapstone

Researchers from the Nelson Mandela African Institution of Science and Technology (NM-AIST) and Ardhi University have come up with an approach for using a very low-tech material for harvesting energy from the sun: blocks of soapstone and granite.

"The intermittence of solar energy resource in concentrated solar power (CSP) generation and solar drying applications can be mitigated by employing thermal energy storage materials," the team explains of the reason for its works. "Natural rocks are well recommended thermal energy storage materials as they are efficient for CSP generation."

Traditionally, solar power is either used directly from the sun — to dry food for preservation, for example — or captured using high-tech solar panels and converted to electricity. Neither work well when the sun isn't shining, which is where thermal storage systems come in: turning the sun's energy into heat, capturing it, and releasing it over a period of time — a sort of heat-capacitor, in effect.

Key to the efficacy of such systems is finding the right material, which is where the research team's efforts come in: investigating granite and soapstone samples from where the Craton and Usagaran geological belts meet in Tanzania to see how well they held up under high temperatures and how efficiently they could serve as thermal energy storage materials, taken straight from the ground.

"The overall results show thermal properties for both soapstone and granite vary with geo-tectonic settings and are site specific," the researchers found. "The soapstone from Craton (CS) [has] good performance as a thermal energy storage material for both CSP and solar drying surpassing the other three rocks in terms of thermal capacity and conductivities which contribute to good absorption, hence good storage and transmission of heat per degree change in temperature. It also has good chemical stability at higher temperatures and has the highest mechanical strength."

"The soapstone from Usagara had the second-best thermal capacity and thermal conductivities," the researchers continue, "but are susceptible to deterioration at elevated temperatures and have the lowest mechanical strength and thus are easiest to disintegrate due to rock-bed loading. UG [Usagaran Granite] rock has low thermal capacity and conductivity thus needing a high temperature change to store an equal amount of energy to the soapstone rocks."

The team's work, which point to a direction for more ecologically friendly ways to harness the sun's energy, has been published in the journal ACS Omega under open-access terms.