Concrete has been used widely since Roman times, with a track record of providing cheap, durable material for structures ranging from the Colosseum to the Hoover Dam. Now it is being developed for a new purpose: cost-effective, large-scale energy storage.
EPRI and storage developer Storworks Power are examining a technology that uses concrete to store energy generated by thermal power plants (fossil, nuclear, and concentrating solar). Recent laboratory tests validated a Storworks Power design, setting the stage for a pilot-scale demonstration at an operating coal-fired power plant.
The Potential of Concrete
As variable renewable energy gains share in the electricity system, the result is excess power and low power prices during certain periods. In response, thermal power plants may cycle up and down in output or shut down temporarily. Most conventional generating units were not designed for such cycles, which can diminish plant performance and damage components. Large-scale energy storage is emerging as a more viable option for handling load fluctuations. BloombergNEF forecasts that global energy storage deployment will grow from 9 gigawatts (GW) to 1,095 GW between 2018 and 2040—a 122-fold increase.
Today, more than 160 GW of pumped hydro storage account for about 94% of grid storage worldwide, though deployment has slowed in some regions due in part to restrictions on land and water use. Batteries represent the largest share of recent bulk storage deployment.
It’s likely that a mix of technologies is necessary to provide the enormous storage capacity in the future. In particular, there is growing need for sustained storage over longer periods when renewable energy generation is not available. Relative to lithium ion batteries, concrete can provide thermal energy storage for longer durations and at lower cost.
“In the same way that different generation technologies help balance the grid today, a range of storage technologies can serve various applications,” said EPRI Principal Technical Leader Scott Hume. “Grid operators can dispatch the lowest cost storage technology on a case-by-case basis, and grid planners can select the optimal mix of technologies for reliability.”
How It Works
With concrete thermal energy storage, large concrete blocks are stacked in a location adjacent to a thermal power plant. When the plant’s power output is not needed by the grid, its steam is redirected from the plant’s turbines to tubes embedded in the blocks, storing the steam’s heat in the concrete. When plant power production needs to be increased again, heated feedwater from the plant is pumped into the tubes and converted to superheated steam for power generation at a separate steam turbine. At the same time, steam generated by the power plant is diverted back to the plant’s main turbine to generate additional output (see diagram). This approach can extend the time for the plant to run at full load, boosting efficiency and reducing damage that can result from cycling up and down and other dynamic modes.
Using readily available, cheap concrete can potentially enable energy storage at capital costs of less than $100 per kilowatt-hour—well below the capital costs of lithium ion batteries. Because concrete is a strong material, systems can be assembled in stacks, resulting in significantly smaller footprints per unit of energy relative to battery systems.
“At about $65 per ton, concrete is less than 10 percent of the cost of the molten salts currently used for thermal storage,” said Hume. “With heat losses of about 1 percent per day, concrete systems can potentially provide several days of storage, which is what’s needed in wind- and solar-dominated energy markets. That’s well above the four hours of storage possible with today’s grid-scale battery storage systems. In the future, several days of storage will be needed to shift solar and wind energy from periods of excess production to periods of limited production.”
When thermal plants are retired, thermal storage systems can then be retrofitted to store renewable energy and use the plant’s power cycle to generate emissions-free power.
Putting Concrete to the Test
To simulate plant operating conditions in the laboratory, researchers cycled samples of 3 different concrete mixes from 400°C to 600°C more than 1,500 times and continuously exposed other samples to 600°C for 5,000 hours, periodically assessing material properties. One mix outperformed the other two, meeting or exceeding targets established by modeling a full-scale system. It had no damage at the tube-concrete interface.
“1,500 thermal cycles are equivalent to more than three years of operation, so these tests give us a reasonably good indication of how the system will perform long-term,” said Hume.
One concern with concrete thermal storage is that corrosion or defects in the tubes could result in steam leaks that create cavities in the concrete. If steam pressure were to build in these cavities, the concrete blocks could potentially rupture. To examine this possibility, the team drilled pinholes in the tubes and examined the impacts of the resulting steam leaks. They found that the concrete formed small cracks that enabled the steam to escape without significant damage to the blocks.
Demonstration in Alabama
In collaboration with Southern Company, Storworks Power, and engineering company United E&C, EPRI plans to demonstrate the optimized design at Alabama Power’s Plant Gaston. The project is supported by a $4 million award from the U.S. Department of Energy. The system will consist of 60 blocks, each weighing 18 tons with approximately 200 kilowatt-hours of storage capability. In total, the system will measure 50 feet long, 25 feet wide, and 30 feet tall, and provide 10 megawatt-hours of thermal storage. The blocks are designed to be transportable to the site. While the system is pilot-scale, larger, commercial-scale systems could be deployed by simply adding blocks.
Researchers will use the demonstration to examine the system’s ability to accept steam from the plant and to generate steam of appropriate temperature, pressure, and flow to enable rapid ramping. Over 11 months of testing, it will be cycled more than a thousand times to verify the concrete blocks’ ability to withstand thermal cycling. To reflect the grid’s flexibility needs, it will be charged and discharged at various rates.
With engineering underway, participants expect construction to be complete in September 2021, with the demonstration completed by the end of 2022.
“A technology portfolio that includes new forms of energy storage will be essential as our generating fleet adapts to the operational demands of intermittent renewable resources,” said Josh Barron, Southern Company senior research and development engineer. “Southern Company expects to gain important knowledge on concrete thermal energy storage from this new collaborative research with EPRI, the Department of Energy, and our industry partners.”
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Artwork and infographic by David Foster Graphics