The use of landscape gravel as a thermal energy storage medium for intermittent sources of generation like solar and wind is being explored by U.S. Sandia National Laboratories (SNL) and New Mexico-based CSolPower.

The long-duration storage system is composed of rocks held in a bed that are heated or cooled with air to store thermal energy. Gravel from landscaping companies can be successfully used for the system without requiring extensive washing or preparation.

Experiments were conducted on a small 100 kWh test project at the photovoltaic (PV) panel-equipped SNL National Solar Thermal Test Facility. When charged with air heated to greater than 900° F, the bed maintained that temperature for up to 20 hours. The system was successfully discharged, demonstrating performance in-line with modeling and predictions.

“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.”

Thermal energy storage technology is gaining traction for expanding the accessibility of energy derived from renewable sources. Researchers in Europe are engineering a hybrid system that uses PV and solar thermal energy separately to provide steam in the food and beverage industry. The system combines a solar thermal plant based on parabolic trough collectors connected to water storage and a PV facility coupled to a sand-based high-temperature heat storage system. The two facilities are planned to provide heat separately to the same industrial facility, with the PV unit being used when the solar thermal plant is unable to meet the heating demand.