NREL Researchers Use Silica Sand to Store Renewable Power

Renewable energy sources like solar and wind are changing how we power our buildings, industries, and grid; however, they are intermittent, giving rise to the need for energy storage to ensure continuous power. Lithium-ion batteries are great for short-duration energy storage, but our buildings, businesses, industries, and grid need more storage, at lower cost, for longer durations, and at larger capacities than batteries can provide to displace fossil fuels for a sustainable future.

To meet this energy storage challenge, researchers at the National Renewable Energy Laboratory (NREL) are in the late stages of prototype testing a new thermal energy storage technology that uses inexpensive silica sand as a storage medium. Economic Long-Duration Electricity Storage by Using Low-Cost Thermal Energy Storage and High-Efficiency Power Cycle (ENDURING) is a reliable, cost-effective, and scalable solution that can be sited anywhere.

The ENDURING Mechanism: Storable, Electrically Heated Sand Delivers On-Demand Electricity

ENDURING uses electricity from surplus solar or wind to heat a thermal storage material—silica sand. Particles are fed through an array of electric resistive heating elements to heat them to 1,200°C (imagine pouring sand through a giant toaster). The heated particles are then gravity-fed into insulated concrete silos for thermal energy storage. The baseline system is designed for economical storage of up to a staggering 26,000 MWh of thermal energy. With modular design, storage capacity can be scaled up or down with relative ease.

When energy is needed, the hot particles are 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 solar photovoltaic or wind power are available, such as early in the morning and evening, during dinner preparation, and when TVs are on. Once discharged, the spent, cold particles are once again fed into insulated silos for storage until conditions (and economics) are appropriate again for charging.

How Hot Sand in a Silo Is Revolutionizing Energy Decarbonization

ENDURING offers several advantages relative to other electricity storage technologies.

As a storage medium, abundant silica sand is stable and inexpensive at $30‒$50/ton, and has a limited ecological impact both in extraction and end of life. For comparison, lithium-ion batteries have an exceptional energy storage density―important for certain sectors such as transportation, where weight matters―but it comes at a high cost. Particle thermal energy storage is a less energy dense form of storage, but is very inexpensive ($2‒$4 per kWh of thermal energy at a 900°C charge-to-discharge temperature difference). The energy storage system is safe because inert silica sand is used as storage media, making it an ideal candidate for massive, long-duration energy storage.

ENDURING systems have no particular siting constraints and can be located anywhere in the country. These systems may also be constructed using existing infrastructure from retired coal- and gas-fired power plants.

Multiple Potential Economical Use Cases Support Decarbonization by 2050

The Biden Administration seeks to achieve a carbon-free power sector by 2035 and a net zero emissions economy by 2050. Zhiwen Ma, principal investigator of the ENDURING project, sees an important role for particle thermal energy storage in achieving these goals. “While decarbonization of electricity has a clear path, decarbonization of the whole economy―which includes things like building heat and industrial processes―is more challenging because natural gas is very cheap, making it hard to displace,” he said. “Decarbonizing industrial processes and building heat is very tough.”

Early Achievements and ENDURING Promise

The ENDURING project is seeing promising progress and early interest. The team 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.

The ENDURING prototype heaters and heat exchangers are currently undergoing testing in high-temperature conditions. If the prototype tasks are successful this fall, Ma is confident that ENDURING technology will offer great potential to support renewable integration for future carbon-free energy supply.