Innovation: Thermal storage for solar thermal plants?

The U.S. Department of Energy and Southern Research, a nonprofit R&D organization, have teamed up for a three-year test of new thermal storage technology for solar thermal power plants.

Ultra-Mega Project
Ultra-Mega Project

If successful, the technology could help make solar thermal power substantially more economically competitive – which might help solar thermal power catch up more with photovoltaics.

The project is being done at the pilot scale, just 1 MWh of storage. Design work is now beginning. The storage unit will be installed in 2017 at a new CSP plant at a solar research center in Birmingham, Alabama, and will operate through 2018.

If the pilot is successful, it’s expected that DOE and Southern Research will team up again for a full-scale commercial deployment test at an operating CSP plant.

Solar thermal energy (also called concentrating solar power, or CSP) has been in use for decades longer than PV; the earliest solar thermal plant started operating in 1910, for steam generation. However, CSP deployment lags far behind PV in the U.S., mostly because it’s relatively costly, especially given the dramatic drop of PV panel prices in recent years.

“Bolting a technology like this onto a CSP plant would make CSP more competitive,” said Bill Grieco, vice president for energy and environment at Southern Research. “So far, CSP can’t quite compete at scale, in terms of cost.”

So, why thermal storage, instead of battery storage? For starters, thermal storage offers some capabilities that battery storage lacks. Battery storage generally is not easily dispatchable. If you draw large amounts of power quickly from battery storage, that tends to degrade the battery over time. In contrast, stored thermal energy can either be dispatched quickly to meet sudden energy needs, or to allow constant operation of turbines.

On the other hand, CSP plants can operate without thermal storage, but only when the sun shines – which hinders their economics. According to a 2012 National Renewable Energy Laboratory report, by adding a six-hour thermal storage capacity, CSP plants can increase the operational value of a utility’s generating capacity by $5.50/MWh, compared to PV solar power.

“This is why thermal storage is highly recommended for large-scale CSP operations,” said Grieco. “It amortizes the capital cost of installation over more MWh of energy produced. However, realizing this economic benefit requires optimizing plant size, storage size, cost, and energy output.”

U.S. CSP plants that do include thermal storage (which includes all CSP plants currently being designed) mostly rely on molten salt thermal storage technology. Molten salt is less expensive than battery storage, but it’s still fairly costly. Southern Research has been developing thermal storage technology which uses a calcium-based fluid which reacts with carbon dioxide to store thermal energy in a chemical reaction.

Santosh Gangwal, one of Southern Research‘s principal investigators for the project, noted that while molten salt thermal storage currently costs about $35-$40/kWh to install, calcium carbonate technology is expected to have an installed cost of about $15/kWh.

Tim Hansen, the another principal investigator for this project, noted that the lower cost is due to the higher energy density of this system’s components, which reduces the size of the thermal storage system.

“Our target, and DOE’s, is to eventually get the cost of solar thermal power down to a very competitive 6 cents perkWh,” said Gangwal.

In addition, the calcium carbonate technology can operate at temperatures up to 200 degrees Celsius higher than molten salt technology, which increases its conversion efficiency to 50% (vs. 40% for molten salt). Also, the calcium carbonate technology is more durable.

CSP isn’t the only possible application for calcium carbonate thermal storage. Hansen noted that it could be applied to any thermal cycles: stationary or shipboard power plants fueled by coal, natural gas or nuclear. Also, Southern Research is investigating a modification of this thermal storage technology that would support carbon capture.

In general, the Solar Energy Industry Association sees thermal storage as an opportunity. In a statement, the organization noted that thermal storage is “a proven, cost-effective technology that has been deployed at scale. The experience the industry has gained with thermal storage in previous CSP projects has allowed solar companies the chance to improve future projects.”

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