An innovative thermal energy storage system that uses sand, water and carbon dioxide as its core components promises to be among the lowest-cost long-duration options available, and a potential game-changer for renewable energy back-up.

Ohio-based Echogen says that its PTES (Pumped Thermal Energy Storage) system will deliver a levelised cost of storage (LCOS) of $50-60/MWh for a 100MW facility once it reaches commercial maturity around 2030 — prices that would equally apply for eight or 100 hours of storage.

The company says this would make it the lowest-cost build-anywhere energy storage on the market. By comparison, a four-hour 100MW/400MWh lithium-ion battery system currently has an LCOS of $132-245/MWh, according to investment bank Lazard.

Long-duration energy storage market leader Highview Power offers a comparably low LCOS for its liquid-air system, which is currently about $100/MWh for a 100MW system and could fall to $50/MWh by 2030, its chief executive Javier Cavada told Recharge in 2019. And Siemens Gamesa told Recharge in February that its thermal energy storage system would deliver power for $48-60/MWh when converting existing coal-fired power plants, but this does not apply to newbuild projects.

Future LCOS calculations depend on a wide range of assumptions — on everything from the cost of capital to the number of hours per year that a system is in operation — so they are hard to compare. Echogen says its own calculations show that the LCOS of its PTES system will be 20-30% lower than Highview’s at 100MW/1GWh on a like-for-like basis, largely due to lower-cost equipment and the lower complexity of its system.

How does the Echogen system work?

Like other thermal energy storage systems, Echogen’s converts electricity to heat, which is stored for hours or days until required and converted back to electricity when required.

Echogen’s system uses electrical power to move heat from a cold reservoir filled with water/ice to a hot reservoir containing grains of sand (or potentially concrete) using a heat pump. The heat is stored in the sand until needed, and then is run through a heat engine to generate electricity as and when required.

Echogen’s technology is unique because it uses supercritical carbon dioxide (sCO2) as its heat transfer medium, or working fluid — which is 15-30% more efficient than the water/steam traditionally used for heat-to-power conversion.

In Echogen’s closed-loop system, inexpensive beverage-grade carbon dioxide is heated at high pressure using compressor-pump heat exchangers until it becomes a supercritical fluid — a strange physical state (see video below) which is neither liquid nor gas but somewhere in between. Supercritical CO2 expands to fill its tank like a gas, but with a density like that of a liquid.

In the charging phase, the sCO2 is heated to up to 300-350°C, with that heat being transferred to the sand via a fluidised bed heat exchanger.

In the generating phase, heat from the sand is used to reheat the sCO2, which is then quickly cooled by “cold energy” from the cold reservoir, causing it to rapidly expand and drive a gas turbine that is roughly 20 times smaller than a steam turbine with the same capacity.

“Because of the high density of CO2, the turbomachinery is really small. I can put the spinning part of a 10MW turbine, the impeller, in your hand,” Echogen chief technology officer Tim Held tells Recharge.

Echogen uses sCO2 as its working fluid because of the way it stores heat. Unlike boiling water, sCO2 uniformly rises in temperature as heat is added, and its temperature falls uniformly as heat is removed, making it far more energy efficient than steam.

The company has been using its tried-and-tested closed-loop sCO2 system for more than a decade in waste heat recovery, which has been its core business to date.

If a PTES project was co-located with a source of waste heat, such as a steel plant, its LCOS would be even lower than $50/MWh.

The storage duration can easily be increased by adding more tanks of sand and water/ice, while the power can be raised by drawing more electricity from the grid and using bigger CO2 turbines. Due to the low cost of this equipment, the LCOS would barely rise as more storage capacity is added.

“There is no upper limit,” chief executive Phil Brennan tells Recharge, explaining that the storage duration could technically be increased to days or weeks.

Stage of development

Echogen has already built and tested a small-scale 200kW PTES system at its research facility in Akron, Ohio, in partnership with the Technical University of Vienna in Austria — which was funded by the US government’s energy innovation programme, Advanced Research Projects Agency–Energy (ARPA-E).

The US company has now prepared the groundwork for a 25MW/250MWh commercial project, and is in “very late stage discussions” with developers interested in building it — at sites ranging from Texas to China and Singapore, says Brennan.

“We’re looking to have those commercial commitments with notice to proceed in the third quarter of this year, and then we'd be able to deploy it at the very end of 2022,” he says.

Echogen CEO Phil Brennan. Photo: Echogen

According to Echogen’s calculations, if this first 25MW/250MWh ETES system was installed in west Texas — a region where large amounts of energy storage are needed, judging by the recent collapse of the grid during an extreme cold spell — it would be extremely profitable.

Based on a business model by which wholesale power is bought when electricity prices are low and sold when prices are high, and ancillary services are provided to the grid during idle hours, this project would generate a 30-year internal rate of return (IRR) of up to 16.2%, Echogen says. If the project was 100MW/1GWh, the IRR would reach 23.6%.