Silica sand mining – “Expanding the amount of energy that can be stored in sand is as simple as adding more sand” IMAGE © Homerun Resources which is working with NREL on using its silica sand for thermal energy storage
“Expanding the amount of energy that can be stored in sand is as simple as adding more sand” Craig Turchi, manager of the Thermal Energy Science and Technologies Research Group at NREL has said.
In 2018, a team of researchers at the US National Renewable Energy Laboratory (NREL) won $2.8 million in funding in the ARPA-E DAYS ENDURING Project to develop long-duration thermal energy storage in sand.
Now, it has attracted interest for potential industry applications.
Initially, the project team envisioned that long-duration thermal energy storage would utilize the assets of decommissioned US coal plants.
Thermal power plants like coal plants have assets that can be repurposed to store and deliver thermal energy; their existing access to transmission in and out to the grid, existing turbines and generators to convert the stored heat back to power, and tall coal silos that could instead store the hot sand.
Several industry partners, including New York Power Authority, and Homerun Resources, a silica materials company, see the value of hot sand storage and are working on different applications for NREL’s innovation.
Additionally, the leading green architect and smart city planner, Jiri Skopek advises Toronto city managers on sustainable development. Skopek is advocating for incorporating NREL’s Sand Thermal Energy Storage (TES) technology in district heating.
Skopek also believes that energy storage in sand using nuclear plants’ thermal waste would make baseload power sources more flexible to be better able to respond to peak demands in the new ‘lumpy’ grid as more intermittent renewables are added. Ontario has nuclear potential, and while it also has plenty of hydropower for now, Skopek foresees that Ontario’s future non-fossil energy development will need new nuclear power in addition to wind and solar.
Why long-duration storage is necessary
While batteries excel at quick, short-term energy storage over several hours, they become expensive if doubled-up in series to cover longer periods. So we must also find ways to store energy economically for longer durations, even over several days. That is the thinking behind the ARPA-E DAYS ENDURING award.
Thermal energy storage makes the grid more stable. Just as parking lots were a necessary component when initially building the interstate highway system, now that the world is adding more intermittent power, it needs places to “park” surplus electricity.
“This kind of distributed energy system can avoid grid congestion that is currently happening almost everywhere in the developed world,” Zhiwen Ma confirmed, in a call from NREL in Colorado, where he leads this ARPA-E-awarded thermal energy storage research.
“A lot of renewable energy just cannot go up to the grid. It’s not because the demand is not there. It’s just because, with congestion, the energy cannot be uploaded to the grid. If you’re not locked to the grid by 2027, they’re actually going to increase the fee to connect. So that’s a global issue right now.”
Why store energy in sand?
Sands are cheap, and stable at high temperatures to over 1400°C without chemical degradation. They are an abundant, non-toxic, easily accessible energy storage medium, and it is relatively simple to scale up the volume for longer durations.
Because of these qualities, researchers are investigating various sands, such as silica, bauxite and desert sands, for thermal energy storage.
The sand would be heated with electricity when the grid doesn’t need extra power and when it does, the heat would be sent to run a power block to generate electricity. Converting the heat back to electricity for the grid does add a conversion inefficiency, but this is countered by the extremely low cost of most types of sands.
How silica calcining is synergistic with thermal storage
For the last year and a half, British Columbia-based Homerun Resources has supported the NREL team in developing a dual-use case: thermal energy storage for the grid while simultaneously using the stored heat to purify the firm’s unique silica sand. Once it is refined, it is used in many ways, including in ceramics, glassmaking, and silicon chips.
In particular, cyclical heating of silica sand can refine its quality and convert it into higher-value silica which is the critical raw material for many industrial products and energy solutions like solar and batteries.
Zhiwen Ma and his NREL team innovates ARPA-E-awarded thermal energy storage in sand IMAGE © Joe DelNero / NREL
“When we discovered the NREL system, we contacted Zhiwen with the idea that the thermal energy storage system represented a unique opportunity to integrate our HPQ silica, a high-purity silica sand, into the system for the purposes of potentially purifying the silica while the system was cycling for energy storage,” said Homerun Resources CEO Brian Leeners.
“To purify this silica sand, it first goes through a calcining process that requires heat at 1000 °C for 10 hours. And then the silica can be cooled and transported to market, and the next batch of unrefined silica gets heated.”
Homerun and NREL’s research suggests that with a dual use like this, the economics are very attractive. At $100 a ton, using this particular silica sand could produce a thermal energy storage cost of under $10/kWh.
Homerun Resources would utilize the heat stored in the silica sand in the NREL thermal energy storage system as a calcination chamber. The silica industry already uses heat to purify silica sand, a process that requires 1000°C heat. The idea would be to purify successive batches using heat while each batch also acts as a heat storage medium.
The sand is at an ambient temperature at the start, and after the multiple heating cycles, a heat exchanger cools down the hot purified silica so it can be loaded onto trucks and delivered to the market.
The extracted heat is used by the thermal storage market, delivered as power or heat. Meanwhile, the next batch of raw silica is heated, while the hot silica stores thermal energy.
“Silica is an interesting market, because as you purify it, the price goes up exponentially,” said Homerun Resources CEO Leeners, who has a Venture Capital background.
“Unpurified silica is extremely cheap; less than $100 dollars a ton, but super-purified silica is thousands of dollars a ton. While supplying heat with your sand, you are actually getting a better price for it once it comes out of the system.”
Homerun Resources sources this silica sand for a global market from its unique silica resource which it controls in Brazil.
Silica is refined into useful solar products while supplying thermal energy storage IMAGE © Homerun Resources
The competitive advantage of using silica sand for thermal energy storage
Technically, heating the silica is free because you’re actually heating the sand for another purpose, the storage market. However, sharing heat is not always easy. Many industrial processes driven by heat must have consistent and predictable temperatures around the clock.
“The primary market is storage, so the charge and discharge cycles will be variable, as they are 100% dictated by the storage end-use requirements; power or heat outputs. ” Leeners explained.
“In some cases these cycles may be quick, in others, up to 10 hours. The key is that the silica gets held at the 1000°C level until it is calcined and ready for leaching. The calcination process is unaffected by the Enduring System Cycles, whether short or long, as the silica can reside in the system for as long or as short a period as we want.”
This means that this silica processing can be unusually lenient in sharing the stored heat with any unexpected demands made during that process, due to it being used simultaneously in the energy storage market. It may turn out that heating the silica over potentially superfluous extra cycles in the calcining process, due to storage market demands, could reduce the subsequent leaching cycle.
NREL has tested various heating conditions with silica purification potentials funded by Homerun Resources over the last 18 months, designing and demonstrating that the process works. Dorfner in Germany, the foremost world experts in silica treatment, have helped the Homerun team in working on specific process flows for their particular silica which can be applied to this particular system.
“Even if the 1000 °C heat is extracted at any point for the storage market during the 10 hours, the silica structure has already been permanently altered each time it is heated, because its calcination process won’t reverse,” concluded Leeners.
“This means that if the process is interrupted at any time by the storage market’s needs, the calcining can simply be picked up where it left off.”
NREL Awarded $2.8 Million to Develop a Long-Duration Thermal Energy Storage Technology
NREL Results Support Cheap Long Duration Energy Storage in Hot Sand
Long-duration thermal energy storage in sand begins NREL demo
