Using rocks as heat batteries for renewable energy storage

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New Mexico-based CSolPower LLC is partnering with Sandia National Laboratories to research and develop the use of landscape gravel as a thermal energy storage medium for intermittent sources of generation like solar and wind.

The system is comprised of rocks held in a bed that are heated or cooled with air to store thermal energy. Sandia’s researchers said the gravel from landscaping companies can be successfully used for the system without requiring extensive washing or preparation.

Sandia designed a small 100 kWh test project at its National Solar Thermal Test Facility. PV panels are installed at the site, which is being tested for its ability to store intermittent generation.

“One of the advantages of thermal energy storage in rocks is that it can be built anywhere,” said Walter Gerstle, co-founder of CSolPower. “It can be commodified and doesn’t require extensive permitting. We believe it can be implemented more quickly and economically than other approaches.”

The rock bed is a long-duration energy storage system, a category of energy storage that has introduced creative solutions like gravity-based storage, rusted iron pellets, thermal bricks, and more.

Sandia tested the bed by charging it with heated air greater than 900 degrees F and maintained that temperature for up to 20 hours. The system was successfully discharged, and Sandia said the performance of the system has been in-line with modeling and predictions. The lab said prototype testing will continue until June 2024.

CSolPower said it intends to make the product ready for utility-scale use, but first it will develop small-scale versions.

“A natural approach is to store excess electricity generated during the day as heat, and then use it to heat water and homes at night. This is an example of the small-scale use of this storage option,” said Gerstle.

If the current phase of testing is successful, several greenhouses in northern New Mexico are prepared to use the rock bed storage in another pilot project.

“Instead of curtailing solar energy production, we would store it and use it during cold nights to keep the greenhouses warm enough to grow plants year-round,” said Gerstle.

The project is financed by the Technology Readiness Gross Receipts Initiative. Sandia’s economic development department administers the tax-funded program, which enables New Mexico businesses to work alongside scientists and engineers from Sandia and Los Alamos national laboratories.

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