Commercializing Standalone Thermal Energy Storage

Molten salt storage tanks at the Solana Generating Station in Arizona. Credit: Abengoa.

Two innovators in highly efficient thermal energy storage materials believe that thermal storage could work as a standalone storage play, not just as part of a more familiar Concentrated Solar Power (CSP) project designed for electricity generation.

Anoop Mathur, founder and CEO of Terrafore Technologies, is a DOE awardee for his work on super-efficient advanced storage materials. He first suggested this possible new market for thermal storage as a pure storage play. His thinks that this kind of solar thermal energy plant would be designed to store all of its energy and be called upon on-demand like a battery.

“I believe, in the immediate future, we should be looking at this market,” said Mathur. “Analysis has shown that using a large turbine with a small solar field and up to three hours of storage is significantly more profitable, if allowed to participate in ancillary services such as spinning reserve and regulation rather than just the day-ahead energy market. The benefit-to-cost ratio was calculated to be more than 25 percent for the California market.”

The way to make this form of solar act purely as storage would be to oversize the turbine in a full CSP project, and then run the plant just to fill the storage tanks, Mathur suggested.

“For example, a solar field designed for 100 MWe with a 200 MWe turbine and up to three hours of storage, supplying 300 MWhs a day, can have higher revenues to justify the capital costs than a plant designed for day-ahead energy markets.”

Generating entirely from the storage on demand, this kind of grid storage would look like a solar thermal power plant but act like a battery.

“The utility industry pays high dollars for guaranteed power when they need it. For these markets the thermal storage route is definitely profitable today,” Mathur said.

Solving Power Block Cost

Halotechnics founder and CEO Justin Raade has also explored deploying stand-alone thermal storage in a similar pure storage play, because of its economy: storage with molten salts in a tower CSP plant costs about a tenth of the cost of battery storage, at around $30 per kWh, compared to $250 per kWh for batteries.

Initially, he envisioned a standalone storage tank of molten salt with PV or wind supplying electricity to heat the molten salts, and a steam turbine at the other end to turn the heat back to electricity for the grid. But after estimating the cost of standalone storage fed by electricity from PV (or wind) Raade found that most of what stood in the way of cost-effectiveness was the inclusion of the turbine used to transform the heat energy back into electricity.

Power block costs canceled out the savings of standalone thermal energy storage. When thermal energy storage is included as part of a CSP project, the cost is minimized because the power block is already needed for electricity generation, so it is part of the plant cost, not of just storage, which is just tanks, piping, heat exchangers and cheap molten salts. But if not amortized by a full power plant, the power block is costly.

Armed with this realization, Raade took another approach. Instead of building a standalone thermal storage unit, fed by electricity from PV or wind, he now envisions offering this storage as a unit that can be piggybacked on an already existing turbine, cutting power block cost. The thermal storage unit would be attached to the steam turbine power block in a combined-cycle natural gas plant, and be commercialized as an efficiency improvement. By turning stored energy back into electricity using an existing steam turbine, costs are minimized. And as Glasspoint has done, with 1 GW of solar thermal for industrial steam production, Raade believes that the deeper pockets of the fossil fuel industry could help him get from innovation to commercialization.

Chile’s Atacama Desert will soon be home to the first hybrid CSP with Molten Salt Storage plus PV projects to help power mining operations, like this copper mine. Credit: Shutterstock.

Raade’s long-term plan is to use earnings from this thermal energy storage using bankable molten salts to fund deployment of his signature molten glass storage. Halotechnics is an ARPA-E award winner for high-temperature molten glass phase-changing material, a storage medium for the advanced CSP storage of the future.

The costs of concentrating solar power (CSP) with storage is expected to fall to less than 6 cents per kWh by 2020. Credit: U.S. Department of Energy Sunshot Initiative.

“Our next generation product would utilize our molten glass energy storage at 1,500°C and would integrate into a combined cycle power plant in much the same way,” said Raade.

Artist rendering of the SolarReserve Copiapó plant. After construction began, a second tower was added to the design. Credit: SolarReserve.

These temperatures are significantly ahead of their time for the concentrating solar power industry, which now operates at a high of 1,050° F and “freezes” at 550° F. The DOE SunShot Initiative is funding research into developing the receivers and tank materials needed to contain higher temperature storage materials for commercial deployment by 2020.

Thermal Renewables To Replace Peaker Plants?

The grid needs an alternative to the polluting single-cycle peaker plants, both for instant response and for several hours of dispatchable generation on demand. Could solar thermal storage units displace these dirty peaker plants? Well, maybe not.

Single cycle plants can respond within a minute, which is why they are deployed as peakers. But there is a penalty for that fast action. They emit 1,365 pounds of CO2 per MWh. At 825 pounds of CO2 per MWh, combined-cycle natural gas plants are somewhat cleaner, but too slow to be peakers. Their response is limited by steam turbine ramp rates, which is typically about 10 percent a minute. CSP plants use the same kind of back-end power block, so they also cannot be instant-on like the higher-emitting peaker. However, if a CSP plant is running off its stored heat, it could start to respond in just ten minutes, Mathur said.

As a Solar Battery for PV

In general, the thermal storage included with CSP decouples solar generation from energy that must be taken whenever it is generated — into a form of solar that could be available whenever it is needed. Molten salt thermal energy storage used in conjunction with CSP supplies a dispatchable form of solar energy. Some of the solar can be stored to be available when the grid requires it.

But CSP could also be built in a hybrid with a PV plant, where virtually all of the CSP is stored for use on demand, creating what amounts to a solar-powered “battery” for PV.

For the first time, such a combination is under construction now in two PV/CSP plants in Chile’s Atacama Desert, with the CSP being stored for use as a battery. This region primarily powers mining operations, which need power 24/7, and solar is able to supply this round-the-clock need by combining two forms of solar generation.

At Atacama -2 Abengoa is building a hybrid combining 100 MW of PV with 110 MW of CSP with a record 17.5 hours of thermal storage. To make up the initial ten-minute gap, Abengoa will also include a small (12 MW, 4 MWh) actual battery to supply the first seconds and minutes of instant-on power.

SolarReserve has completed permitting for a second such solar hybrid, combining 260 MW of CSP with 150 MW of PV at Copiapó — to supply 24-hour base load generation, by oversizing the solar thermal aspect and generating from its storage, as Mathur suggested.

Whether by marrying CSP’s storage with PV as Abengoa and SolarReserve are doing, or piggybacking it onto traditional thermal generation as Halotechnics is proposing — the economy of thermal energy storage is now beginning to be utilized. 

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