Solution to energy storage may be beneath your feet

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

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

TES reveals promise as a low-cost different to current storage applied sciences, and storing vitality in strong particles resembling 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 easy to access. It is environmentally friendly. It is stable, quite stable, in a wide temperature range. It is also low cost,” mentioned Zhiwen Ma, a mechanical engineer within the laboratory’s Thermal Vitality Techniques Group.

The necessity for long-term storage

Patented expertise developed and prototyped at NREL reveals how heaters powered by renewable energy sources like wind and photo voltaic can increase 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 expertise and allowed researchers to create a pc mannequin that reveals a commercial-scale machine would retain greater than 95% of its warmth for at the very least 5 days.

“Lithium-ion batteries have really cornered the market at two to four hours of storage, but if we want to achieve our carbon reduction goals, we will need long-duration energy storage devices—things that can store energy for days,” mentioned Jeffrey Gifford, a postdoctoral researcher at NREL.

Gifford, who already shares two patents with Ma on heat exchangers that convert saved thermal vitality to electrical energy, mentioned the usage of sand or different particles to retailer thermal vitality has one other benefit over batteries. “Particle thermal energy storage doesn’t rely on rare-earth materials or materials that have complex and unsustainable supply chains. For example, in lithium-ion batteriesthere are a lot of stories about the challenge of mining cobalt more ethically.”

Along with TES, Gifford’s experience is in computational fluid dynamics. That information is necessary as a result of the sand must circulate by means of the storage machine. Different TES media contains concrete and rocks, which may simply retain warmth however stay solidly in place. “Your heat transfer is far greater and far faster and rather more efficient for those who’re shifting your media,” Gifford mentioned.

TES additionally has one other key benefit: the price. Ma has calculated sand is the most cost effective possibility for vitality storage when in comparison with 4 rival applied sciences, together with compressed air vitality storage (CAES), pumped hydropower, and two forms of batteries. CAES and pumped hydropower can solely retailer vitality 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 price $300 a kilowatt-hour and solely have a capability to retailer vitality from one to 4 hours. With a period lasting tons of of hours, sand as a storage medium would price from $4 to $10 a kilowatt-hour. To make sure low price, the warmth could be generated utilizing off-peak, low-price electrical energy.

Ma, who holds a handful of patents on the expertise, beforehand served because the principal investigator on an ARPA-E funded undertaking referred to as ENDURING, for Financial Lengthy-Period Electrical energy Storage by Utilizing Low-Value Thermal Vitality Storage and Excessive-Effectivity Energy Cycle. The prototype got here from this undertaking.

Subsequent up is the groundbreaking in 2025 on an electrical thermal vitality storage (ETES) system at NREL’s Flatirons Campus outdoors Boulder, Colorado, that shall be designed to retailer vitality 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 will be established.

The DOE-funded demonstration undertaking, Ma mentioned, is meant to indicate the industrial potential of sand for TES.

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

“This represents a brand new era of storage past molten salt“What he mentioned.

Deciding what’s going to retailer the warmth

However will simply any previous sand do? Not based on NREL researchers, who examined varied solid particles for his or her capacity to circulate and to retain warmth. In a paper revealed final fall in Photo voltaic VitalityMa and others experimented on eight strong particle candidates. Among the many particles thought-about had been man-made ceramic materials utilized in fracking, calcined flint clay, brown fused alumina, and silica sand. The clay and fused alumina had been rejected due to thermal instability on the goal temperature of 1,200°C (2,192°F).

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

Increasing the quantity of vitality 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 cost to add additional storage capacity,” he mentioned. “We need storage ranging from minutes to months. Batteries worked really well in the minutes to hours space in terms of how they scale. And when you get into months of storage, you’re usually looking at making a fuel like hydrogen to provide that long-term storage. But in the period between multiple hours and two weeks, there’s not a good fit right now. Hydrogen is too expensive for that. Batteries are too expensive for that.”

The elements wanted to transform the superheated sand again to electrical energy does require an upfront price. “But once you’ve paid for that,” Turchi mentioned, “for those who simply need to have extra period to your energy it is a lot, less expensive so as to add extra sand than the choice, which is to maintain including batteries.”