Researchers at the National Renewable Energy Laboratory (NREL) are in the advanced stages of prototype testing a new thermal energy storage technology— Economic Long-Duration Electricity Storage by Using Low-Cost Thermal Energy Storage and High-Efficiency Power Cycle (ENDURING)— which uses inexpensive silica sand as a storage medium.

ENDURING uses electricity from surplus solar or wind to heat thermal storage material, in this case, silica sand. The silica sand is fed through an array of electric resistive heating elements to heat them to 1,200°C. It is then gravity-fed into insulated concrete silos for thermal energy storage. The baseline system is designed for economical storage of up to a staggering 26 GWh of thermal energy. Storage capacity can be scaled up or down with relative ease with slight changes in modular design.

When energy is needed, the heated silica sand is gravity-fed through a heat exchanger, heating and pressurizing a working gas inside to drive the turbomachinery and spin generators that create electricity for the grid. The system discharges during periods of high electricity demand and when limited intermittent power sources like solar or wind are unavailable.

Once discharged, the spent, cold sand is again fed into insulated silos for storage until conditions are appropriate for charging.

The NREL report states silica sand is an abundant, stable, and inexpensive storage medium, costing $30‒$50 per ton. It has a limited ecological impact both in extraction and end of life. It also asserts that particle thermal energy storage is a less energy-dense form of storage when compared to lithium-ion batteries but is very inexpensive at $2‒$4 per kWh of thermal energy at a 900°C charge-to-discharge temperature difference.

ENDURING systems can also be constructed using existing infrastructure from retired coal- and gas-fired power plants.

In addition to providing grid storage and building heat, ENDURING offers a steady source of heat for industrial and chemical processes that are otherwise incompatible with the intermittency associated with solar and wind power.

“Sand and concrete silos with refractory insulation are very inexpensive materials that can lead to low-cost energy storage,” said Patrick Davenport, an NREL researcher. “Traditional four-hour storage technologies don’t scale well to the grid or city scale. Now that we need large-scale energy storage, this technology makes much sense.”

General Electric, Allied Mineral Products, Worley, Purdue University, Babcock & Wilcox, and Colorado School of Mines are some of the industry and academic research partners that contributed to the ENDURING project.

The researchers at NREL recently won the American Society of Mechanical Engineers Advanced Energy Systems Division and Solar Energy Division 2021 First-Place Best Paper Award and several U.S. Department of Energy technology funding awards. Patents on concentrating solar power integration have been awarded, and several more are being filed.

Zhiwen Ma, the principal investigator of the ENDURING project, is confident that ENDURING technology will offer great potential to support renewable integration for future carbon-free energy supply if the prototype tasks are successful by this year’s Fall.

NREL and clean-energy technology firm Babcock & Wilcox have an exclusive intellectual property option agreement to license the ENDURING particle thermal energy storage technology. The ENDURING prototype heaters and heat exchangers are currently undergoing testing in high-temperature conditions.

NREL analysts found significant market potential for utility-scale diurnal storage (up to 12 hours) in the United States power system by 2050, predicting over 125 GW growth for grid-scale storage capacity in the United States. NREL researchers also found that energy storage can provide a range of grid services and has the potential to play an essential role in the development of a cost-effective power sector for India. They predict that energy storage becomes cost-competitive with other technologies due to projected cost declines through 2030.