Molten salt storage: Is upside down the way forward?

Turning the solar tower concept on its head looks as though it could solve a lot of the current technological challenges associated with molten salt storage – in theory, at least.

By Jason Deign in Barcelona

Researchers at the Massachusetts Institute of Technology (MIT) reckon they may have solved some of the technological issues associated with molten salt storage. They have flipped the solar tower concept on its head.

In a paper published in Solar Energy, MIT Pappalardo Professor of Mechanical Engineering Alexander Slocum and co-workers have proposed building a heliostat field on a hillside and focusing sunlight onto a combined heating and storage tank below it.

The tank, filled with molten sodium-potassium nitrate salt, would admit concentrated sunlight through a narrow opening on its top and would contain a movable plate to separate the hot liquid at the surface from colder layers below.

The top layer of hot molten salt, which the researchers say could reach more than 500 degrees Celsius, would be used to drive a turbine and then returned to the bottom of the tank. In daytime operation the plate would move down the tank as the volume of hot salt on top increased.

The team believes that a Concentrated Solar Power on Demand (CSPoD) tank measuring 25 metres across by five deep could provide 20 MW of electric power around the clock, and store enough heat over 10 days of sunshine to continue generating for a full day without sunlight.

The beauty of the team’s proposal is that it appears to deliver 24-hour power using on a tried-and-tested storage medium. Other lines of research have relied on untried storage media such as graphite or compressed air, or exotic plant figurations such as CSP with nuclear backup.

At the same time, though, CSPoD removes much of the complexity, and cost, associated with tower configurations; since the heat receptor and storage medium are the same, there is less need for plumbing, which simplifies construction and maintenance.

Hill formation

Using a standard US Department of Energy analysis tool, Slocum’s team has calculated the concept could deliver power at a cost of anywhere between USD$0.33 and $0.07, which at the lower end would make it competitive with conventional energy sources.

One potential setback is that the heliostat field would ideally require a fairly specific type of hill formation, shaped like a portion of a shallow funnel, to work best; and any hillside might be subject to greater shading than flat land.

However, MIT’s Danny Codd, who based his doctoral thesis on CSPoD, says his colleague Alexander Mitsos has carried out a geographical analysis of the US, combined with solar resource mapping, and discovered plenty of good potential sites.

“He looked across all of the US and found there are dozens of gigawatts of solar potential,” Codd says. “And that was even after taking into account shading and blocking.”

Another potential stumbling block for CSPoD, which so far has only been tested conceptually in the MIT basement, is that it may lose out to the increasing number of other storage options now available.

Nevertheless, says Mark Mehos, principal programme manager for CSP at the US National Renewable Energy Laboratory: “It represents some innovative thinking around reducing the cost of thermal energy storage.”

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Image: Courtesy of Alexander Slocum et al.