Using Hot Sand To Store Energy
As communities, cities, and states develop ambitious energy efficiency and decarbonization goals, energy storage is an increasingly critical component of our energy economy. Renewable energy sources like solar and wind are changing how we power our buildings, industries, and grid; however, they are intermittent ― we need continuous power even after the sun sets or the wind dies down. As such, energy storage is critical to ensuring continuous power and allows energy producers to take full advantage during times of overgeneration on sunny (or windy) days.
When it comes to short-duration energy storage, lithium-ion batteries are considered the front-runner, but batteries are not the whole story. 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 game-changing 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.
Particle thermal energy storage systems can be constructed with existing infrastructure from retired coal and gas power plants. Image by Al Hicks and Besiki Kazaishvili, NREL
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.
ENDURING technology can support the expansion of renewable energy generation across our country. Building these cost-effective particle thermal energy storage systems around the United States could help utilities to continue using solar and wind without running the risk of destabilizing the grid or needing to curtail renewable energy generation. Particle thermal energy storage will also provide energy reserves so our communities can better navigate through extended weather events, whether a week-long cold front or a summer heat wave.
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.”
Converting renewable electricity into heat is one way to decarbonize these sectors. Ma sees an opportunity for particle thermal energy storage to play a role in cost-effectively supplanting natural gas. By using a heat pump, one unit of electricity is transformed into two to three units of heat, which can be stored in the particle thermal energy storage system and then later delivered to the end user (depending on the coefficient of performance of the heat pump or the use of an emerging pumped thermal energy storage technology). These technologies can be used for building and industry process heating to replace coal or natural gas.
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.
According to NREL researcher Patrick Davenport, the economic environment, decarbonization goals, and technology have aligned for particle thermal energy storage. “Sand and concrete silos with refractory insulation are very inexpensive materials that can lead to low-cost energy storage,” he said. “Traditional four-hour storage technologies don’t scale well to the grid or city scale. Now that we are in need of large-scale energy storage, this technology makes a lot of sense.”
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.
Ma is not the only one who sees promise: NREL and clean-energy technology firm Babcock & Wilcox have an exclusive intellectual property option agreement to license the ENDURING particle thermal energy storage technology. Babcock & Wilcox are among several industry and academic research partners that contributed to the ENDURING project, including General Electric, Allied Mineral Products, Worley, Purdue University, and Colorado School of Mines.
Learn more about NREL thermal systems and concentrating solar power research.
Article courtesy of NREL.
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Paris’ bike-share system, Vélib has long been considered one of the shining success stories of urban micromobility. With a massive fleet of over 20,000 pedal and electric-assist bicycles around Paris, the service has helped millions of residents and tourists get around the City of Light without needing a car or scooter. But lately, a growing problem is threatening to knock the wheels off this urban mobility marvel: theft and joyriding.
According to city officials and the service operator, more than 600 Vélib bikes are now going missing every single week. That’s over 30 bikes a day simply vanishing from the system – some stolen outright, others taken on “joy rides” and never returned.
“At the moment we’re missing 3,000 bikes,” explained Sylvain Raifaud, head of the Agemob company that currently operates the Velib system. That’s nearly 15% of over 20,000 Vélib bikes across Paris.
The sticky-fingered culprits aren’t necessarily professional thieves or organized crime rings. Instead, they’re often regular users who treat the shared bikes like disposable toys.
The city estimates that many people have figured out how to pry the bikes out of the system’s parking docks, unlocking one for a casual cruise and then ditching it somewhere far from a docking station.
Once pried free, the bikes are technically usable for the next 24 hours until their automatic locking feature kicks in. At that point, the bikes are often simply abandoned. Some end up in alleyways. Others get tossed in rivers. A few just disappear completely.
And since the bikes are intended to be parked at their many docking stations around the city, they don’t have GPS chips, further complicating recovery of “liberated” bikes.
The issue started small but has grown into more than an inconvenience – it’s beginning to undermine the entire purpose of the service. With bikes going missing at such a high rate, many Vélib docking stations are left empty, especially during rush hours.
Riders looking for a quick commute or a convenient hop across town are increasingly finding themselves without available bikes, or having to walk long distances to find a functioning one.
That kind of unreliability chips away at user confidence and threatens to drive potential riders back into cars, cabs, or other less sustainable forms of transport at a time when Paris has already made great strides to dramatically reduce car usage in the city.
The losses are financially painful, too. Replacing stolen or vandalized bikes isn’t cheap, and the resources spent on tracking down missing equipment or reinforcing anti-theft measures are stretching thin. Vélib has faced theft and vandalism issues before, especially during its early years, but this latest surge has officials sounding the alarm with renewed urgency.
Officials acknowledge that there’s no easy fix. Paris, like many cities with bike-share systems, walks a fine line between accessibility and accountability. Part of what makes Vélib so successful is its ease of use and widespread availability. But those same features make it vulnerable to misuse – especially when enforcement is limited and the consequences for abuse are minimal.
The timing of the problem is especially unfortunate. In recent years, Paris has seen impressive results in reducing car traffic, expanding bike lanes, and promoting cycling as a key part of its sustainable transport strategy. Vélib is a cornerstone of that plan. But if the system becomes too unreliable, it risks losing the very people it was designed to serve.
Meanwhile, as Parisians increasingly find themselves staring at empty docks, the challenge for the city and Vélib will be to restore confidence in the system without making it harder to use. That means striking the right balance between freedom and responsibility, between open access and protection against abuse.
In a city where cycling is supposed to be the future of mobility, losing thousands of bikes to joyriders and sticky fingers isn’t just frustrating; it’s unsustainable.
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