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Anybody who has ever hot-footed it barefoot throughout the seaside on a sunny day walks away with a better understanding of simply how a lot warmth sand can retain. That potential is anticipated to play a significant function sooner or later, as know-how involving heated sand turns into a part of the reply to power storage wants.

Batteries are seemingly what most individuals take into consideration when it comes to storing power for later use, however different applied sciences exist. Pumped storage hydropower is one widespread technique, albeit one which requires reservoirs at totally different elevations and is proscribed by geography. One other strategy depends on what is called thermal power storage, or TES, which makes use of molten salt and even superheated rocks.

TES exhibits promise as a low-cost various to current storage applied sciences, and storing power in stable particles equivalent to sand offers a prepared reply, with out geological restrictions.

In spite of everything, sand, like air and water, is all over the place.

“Sand is simple to entry. It’s environmentally pleasant. It’s steady, fairly steady, in a large temperature vary. Additionally it is low value,” mentioned Zhiwen Ma, a mechanical engineer within the laboratory’s Thermal Vitality Methods Group.

The Want for Lengthy-Time period Storage

Patented know-how developed and prototyped at NREL reveals how heaters powered by renewable power sources like wind and photo voltaic can elevate the temperature of sand particles to the specified temperature. The sand is then deposited right into a silo for storage and use later, both to generate electrical energy or for course of warmth in industrial functions. A laboratory-scale prototype validated the know-how and allowed researchers to create a pc mannequin that exhibits a commercial-scale system would retain greater than 95% of its warmth for at the very least 5 days.

“Lithium-ion batteries have actually cornered the market at two to 4 hours of storage, but when we need to obtain our carbon discount objectives, we are going to want long-duration power storage gadgets—issues that may retailer power for days,” mentioned Jeffrey Gifford, a postdoctoral researcher at NREL.

Gifford, who already shares two patents with Ma on warmth exchangers that convert saved thermal power to electrical energy, mentioned using sand or different particles to retailer thermal power has one other benefit over batteries. “Particle thermal power storage doesn’t depend on rare-earth supplies or supplies which have complicated and unsustainable provide chains. For instance, in lithium-ion batteries, there are loads of tales in regards to the problem of mining cobalt extra ethically.”

Along with TES, Gifford’s experience is in computational fluid dynamics. That information is vital as a result of the sand must stream by way of the storage system. Different TES media contains concrete and rocks, which may simply retain warmth however stay solidly in place. “Your warmth switch is way larger and far faster and rather more efficient when you’re transferring your media,” Gifford mentioned.

TES additionally has one other key benefit: the associated fee. Ma has calculated sand is the most affordable choice for power storage when in comparison with 4 rival applied sciences, together with compressed air power storage (CAES), pumped hydropower, and two varieties of batteries. CAES and pumped hydropower can solely retailer power for tens of hours. The associated fee per kilowatt-hour for CAES ranges from $150 to $300, whereas for pumped hydropower it’s about $60. A lithium-ion battery would value $300 a kilowatt-hour and solely have a capability to retailer power from one to 4 hours. With a length lasting tons of of hours, sand as a storage medium would value from $4 to $10 a kilowatt-hour. To make sure low value, the warmth could be generated utilizing off-peak, low-price electrical energy.

Ma, who holds a handful of patents on the know-how, beforehand served because the principal investigator on an ARPA-E funded challenge often called ENDURING, for Financial Lengthy-Length Electrical energy Storage by Utilizing Low-Value Thermal Vitality Storage and Excessive-Effectivity Energy Cycle. The prototype got here from this challenge. Subsequent up is the groundbreaking in 2025 on an electrical thermal power storage (ETES) system at NREL’s Flatirons Campus exterior Boulder, Colorado, that will likely be designed to retailer power for between 10 and 100 hours. The stand-alone system is free from any siting restrictions that restrict the place CAES or pumped storage hydropower might be established.

The DOE-funded demonstration challenge, Ma mentioned, is meant to point out the industrial potential of sand for TES.

Molten salts are already in use to quickly retailer power, however they freeze at about 220 levels Celsius (428 levels Fahrenheit) and begin to decompose at 600 C. The sand Ma intends to make use of comes out of the bottom within the Midwest of america, doesn’t must be saved from “freezing,” and might retain significantly extra warmth, within the vary of 1,100 C (2,012 F) that may retailer warmth for energy technology or to switch burning fossil fuels for industrial warmth.

“This represents a brand new technology of storage past molten salt,” Ma mentioned.

Deciding What Will Retailer the Warmth

However will simply any previous sand do? Not based on NREL researchers, who examined varied stable particles for his or her potential to stream and to retain warmth. In a paper revealed final fall, Ma and others experimented on eight stable particle candidates. Among the many particles thought-about have been man-made ceramic supplies utilized in fracking, calcined flint clay, brown fused alumina, and silica sand. The clay and fused alumina have been rejected due to thermal instability on the goal temperature of 1,200 levels Celsius (2,192 levels Fahrenheit).

The ceramic supplies outperformed the sand in all classes, however the marginal efficiency positive factors have been thought-about inadequate to justify the upper value. Whereas the sand prices from $30 to $80 a ton, the costs of the ceramic supplies have been about two magnitudes larger. The sand is within the ultra-pure type of alpha quartz and available within the Midwest.

Increasing the quantity of power that may be saved in sand is so simple as including extra sand, mentioned Craig Turchi, supervisor of the Thermal Vitality Science and Applied sciences Analysis Group at NREL.

“That’s a marginal value so as to add further storage capability,” he mentioned. “We’d like storage starting from minutes to months. Batteries labored very well within the minutes to hours house when it comes to how they scale. And whenever you get into months of storage, you’re often making a gas like hydrogen to offer that long-term storage. However within the interval between a number of hours and two weeks, there’s not a very good match proper now. Hydrogen is just too costly for that. Batteries are too costly for that.”

The elements wanted to transform the superheated sand again to electrical energy does require an upfront value. “However when you’ve paid for that,” Turchi mentioned, “when you simply need to have extra length on your energy it’s a lot, less expensive so as to add extra sand than the choice, which is to maintain including batteries.”

By Wayne Hicks. Courtesy of Division of Vitality, NREL.