Graduate student Adam Caldwell pours molten salt during an experiment.

No sun? No problem. Engineers work to cheaply harness stored solar heat

Solar power is growing in popularity, but it still only makes up an estimated 2% of the electricity used in the U.S. That’s due, in part, to some challenges: Solar remains more expensive than some other energy generation modes, and it’s less available at night and on cloudy days. Purdue University engineers are working on solutions to those problems.

The engineers are improving how concentrated solar power plants produce electricity. The facilities store heat captured from sunlight to provide power at off-peak times. They can produce energy at a large scale and for a lower cost than some other solar methods, like rooftop solar panels. 

Currently there are only 11 concentrated solar power plants in the country. Their energy production costs have dropped 50% since 2010, and the Purdue engineers are working to push that even lower. Improving the technology could help make this solar generation more competitive with fossil fuel generation options. 

“Since storing solar energy as heat is already cheaper than storing energy via batteries, the next step is reducing the cost of generating electricity from the sun’s heat,” Kenneth Sandhage, a materials engineering professor at Purdue, said in a news release

Dr. Kenneth Sandhage
Dr. Kenneth Sandhage. Credit: Rebecca McElhoe / Purdue University

From sun to sea

Concentrated solar power plant turbines operate at a peak temperature of 1,022 degrees Fahrenheit. But electricity production costs drop if the turbines operate at much higher temperatures. Running at 1,382 degrees Fahrenheit or higher increases the efficiency of converting heat energy to electricity.

The system also would benefit from storing energy at higher temperatures. The facilities can store energy from sunlight by heating molten salts, but the commonly used salts degrade at the higher temperatures. The researchers, therefore, have been using seawater to access a different type of salt that stays more stable at high temperatures and does not oxidize when exposed to air.

Heated exchange

Other parts of the system also need to be able to withstand higher temperatures to improve efficiency. 

For instance, heat exchangers transfer the heat from the molten salt to a high-pressure fluid, and the fluid spins the turbine to create electricity. But cost-effective heat exchangers are usually made of stainless steel alloys that would become too soft at 1,382 degrees. The Purdue team created a ceramic-metal composite that could withstand the higher temperatures, and it has been successfully tested in rocket nozzles at more than 4,000 degrees.

“If you want to run the whole system hotter, you can’t have a weak link anywhere in the chain,” Sandhage said.

What’s next

The research team has filed patent applications for some of the concentrated solar power plant technologies they developed.

“Ultimately, continued developments will allow for large-scale penetration of renewable solar energy into the electricity grid,” Sandhage said. “This would mean dramatic reductions in man-made carbon dioxide emissions from electricity production.”

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