Dear colleagues and friends,
Wayne Hicks wrote this article, which is shared by an esteemed colleague who works on the subject of energy. The article was published in the news section of the National Renewable Energy Laboratory (NREL) website and translated by us for this space. Let's see what it's all about...
Anyone who has ever walked barefoot on the beach on a sunny day will have a better idea of how much heat sand can hold. That capacity is expected to play a vital role in the future, as technology involving hot sand becomes part of the answer to energy storage needs.
Most people probably think of batteries in terms of storing energy for later use, but there are other technologies. Pumped storage hydroelectric energy is a common method, although it requires reservoirs at different elevations and is limited by geography. Another approach is based on what is known as thermal energy storage, or TES, which uses molten salts or even superheated rocks.
TES seems promising as a low-cost alternative to existing storage technologies, and storing energy in solid particles such as sand provides an immediate response, without geological constraints.
After all, sand, like air and water, is everywhere.
“The sand is easily accessible. It is environmentally friendly. It is stable, quite stable, over a wide temperature range. It's also low-cost,” said Zhiwen Ma, a mechanical engineer in the lab's Thermal Power Systems Group.
The need for long-term storage.
The patented technology developed and prototyped at National Renwable Energy Laboratories (NREL) reveals how heaters powered by renewable energy sources such as wind and solar can raise the temperature of sand particles to the desired temperature. The sand is then deposited in a silo for storage and later use, either to generate electricity or to process heat in industrial applications. A laboratory-scale prototype validated the technology and allowed researchers to create a computer model that shows that a commercial-scale device would retain more than 95% of its heat for at least five days.
“Lithium-ion batteries have really taken the market with two to four hours of storage, but if we want to achieve our carbon reduction goals, we'll need long-lasting energy storage devices, things that can store energy for days,” said Jeffrey Gifford, a postdoctoral researcher at the NREL.
Gifford, who already shares two patents with Ma on heat exchangers that convert stored thermal energy into electricity, said that using sand or other particles to store thermal energy has another advantage 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 the case of lithium-ion batteries, there are many stories about the challenge of mining cobalt more ethically.”
In addition to TES, Gifford's experience focuses on computational fluid dynamics. That knowledge is important because sand must flow through the storage device. Other TES media include concrete and rocks, which can easily hold heat but remain solidly in place. “The heat transfer is much greater, faster and more effective if you move the material,” Gifford said.
TES also has another key advantage: cost. Ma has calculated that sand is the cheapest option for energy storage compared to four rival technologies, including compressed air energy storage (CAES), pumped hydropower and two types of batteries. CAES and pumped hydroelectric power can only store energy for tens of hours.
The cost per kilowatt-hour for CAES ranges from 150 to 300 dollars, while for pumping hydroelectric energy it is about 60 dollars. A lithium-ion battery would cost $300 per kilowatt-hour and would only be able to store energy for one to four hours. With a lifespan of hundreds of hours, sand as a storage medium would cost between 4 and 10 dollars per kilowatt-hour. To ensure a low cost, heat would be generated using low-cost electricity and outside of peak hours.
Ma, who holds several patents on the technology, previously served as principal investigator on an ARPA-E-funded project known as ENDURING, for long-term economic electricity storage using low-cost thermal energy storage and high-efficiency energy cycling. The new prototype emerged from this project.
The next step is the innovation in 2025 of an electrical thermal energy storage system (ETES) at NREL's Flatirons campus outside Boulder, Colorado, which will be designed to store energy for 10 to 100 hours. The stand-alone system is free of location restrictions that limit where CAES or pumped storage hydroelectric power can be established.
The DOE-funded demonstration project, Ma said, aims to show the commercial potential of sand for TES.
Molten salts are already used to temporarily store energy, but they freeze at about 220°C (428°F) and begin to decompose at 600°C. The sand that Ma intends to use comes from the soil of the Midwest of the United States, there is no need to prevent it from “freezing” and can hold considerably more heat, in the range of 1,100°C (2,012°F) that can store heat for energy generation or to replace the burning of fossil fuels with industrial heat.
“This represents a new generation of storage beyond molten salts,” said Ma.
Decide what will store the heat
But will any old sand do? No, according to NREL researchers, who examined several solid particles to determine their ability to flow and retain heat. In a paper published last fall in Solar Energy, Ma and others experimented with eight solid particle candidates. Among the particles considered were artificial ceramic materials used in fracking, calcined flint clay, brown fused alumina and silica sand. Molten clay and alumina were rejected due to thermal instability at the target temperature of 1200 °C (2192 °F).
Ceramic materials outperformed sand in all categories, but the marginal gains in performance were considered insufficient to justify the higher cost. While sand costs between 30 and 80 dollars a ton, the prices of ceramic materials were approximately two quantities higher. Sand is found in the ultra-pure form of alpha quartz and is readily available in the Midwest.
Expanding the amount of energy that can be stored in sand is as simple as adding more sand, said Craig Turchi, manager of the Thermal Energy Science and Technology Research Group at NREL.
“It's a marginal cost to add additional storage capacity,” he said. “We need storage that ranges from minutes to months. The batteries performed very well in the space of minutes to hours in terms of how they scale. And when you get to months of storage, you're usually looking to make a fuel like hydrogen to provide that long-term storage. But in the period between several hours and two weeks, there is no good option right now. Hydrogen is too expensive for that. Batteries are too expensive for that.”
The components needed to convert superheated sand into electricity require an upfront cost. “But once you've paid for that,” Turchi said, “if you just want to have more battery life, it's much, much cheaper to add more sand than the alternative, which is to keep adding batteries
