Lowering the costs of using solar-powered electricity when it’s not sunny

Using the sun’s heat, researchers work to bring cheaper renewable electricity

WEST LAFAYETTE, Ind. – Solar power accounts for about 2% of US electricity, but it could become more widespread if it were cheaper to generate this electricity and make it readily available on cloudy days and at night.

To reduce these costs, Purdue University engineers are developing ways to improve the way structures called concentrated solar power plants produce electricity. These systems provide energy during off-peak hours by storing the heat captured by sunlight that is focused by thousands of mirrors on a small area.

The developments in this research are important steps to put solar heat generation into electricity in direct cost competition with fossil fuels, which generate more than 60% of US electricity.

Solar panel systems installed on farms and rooftops are commonly used to generate electricity from the sun and already store energy for later use using batteries, but concentrated solar power plants can offer large-scale energy storage at a lower cost.

“Since storing solar energy as heat is already cheaper than storing energy in batteries, the next step is to reduce the cost of generating electricity from the sun’s heat,” said Kenneth Sandhage, Reilly Professor of Materials Engineering at Purdue.

There are only 11 concentrated solar power plants in the United States, but the cost of generating electricity using these plants has fallen by more than 50% since 2010. Purdue researchers are working to further reduce the costs of concentrated solar energy to compete with. fossil fuels.

To make it cheaper for concentrated solar power plants to produce electricity, the turbines in these plants must operate at much higher temperatures. The turbines currently operate at a maximum temperature of around 1,022 degrees Fahrenheit.

By improving the molten salt used for low-cost heat storage at higher temperatures, researchers can help concentrated solar power plants generate electricity more efficiently and economically. (Photo by Purdue University / Rebecca McElhoe)

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By running turbines at 1,382 degrees Fahrenheit or more, a concentrated solar power plant could convert heat energy into electricity more efficiently. This also requires low-cost storage of solar heat at higher temperatures so that plants can produce electricity 24 hours a day and respond quickly to peaks in electricity demand.

Concentrated solar power plants can store energy from the sun by heating molten salts, but the molten nitrate salts currently used would degrade to 1,382 degrees Fahrenheit. Sandhage and other researchers in the field have turned to seawater for chloride-based salts that can remain fairly stable at higher temperatures.

But molten chlorides derived from seawater, such as the magnesium chloride and potash-containing salts that researchers have examined so far, degrade by oxidation in ambient air at 1,382 degrees Fahrenheit.

In a study published in Materials Today, Sandhage’s research team predicted and demonstrated that a different molten salt derived from seawater, a composition of calcium chloride and sodium chloride, is highly resistant to oxidation in the air. environment at 1,382 degrees Fahrenheit.

Sandhage’s research team has also developed a way to make nickel corrosion resistant so that it can better contain a molten salt such as calcium chloride and sodium chloride composition.

“We have identified and demonstrated a salt that is stable and abundant, and we have a containment strategy to hold the salt for extended periods to store energy for use when the sun is not shining,” Sandhage said.

Raising the flame, one step at a time

To continue to reduce the cost of generating electricity from concentrated solar power plants, other stages of the electricity generation process must also handle higher temperatures.

One of these steps is the transfer of heat from a molten salt to a high pressure working fluid through devices called heat exchangers. Upon heating, the working fluid expands and spins the turbine to generate electricity.

Compact and inexpensive heat exchangers are typically made from stainless steel alloys that would become too soft for use at 1,382 degrees Fahrenheit with a high pressure working fluid.

In 2018, Sandhage and his collaborators published an article in Nature revealing how heat exchangers made of a ceramic-metal composite material would be able to withstand the higher temperatures and pressures needed to generate electricity more efficiently. Purdue researchers invented this composite-based heat exchanger using materials that had been successfully tested at over 4,000 degrees Fahrenheit in solid fuel rocket nozzles.

Molten salt and heat exchangers that can handle heat are just two pieces of the puzzle. Other researchers in the field, for example, are making improvements to receivers – metal tubes that carry molten salt to be heated in sunlight – and to mirrors that concentrate sunlight.

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

And in some respects, the temperatures need to be lower. Despite the advantages of the calcium chloride-sodium chloride composition, this salt melts at higher temperatures than currently used molten nitrate salts. Sandhage’s research team is working to modify the composition of the molten chloride salt to melt at lower temperatures while remaining stable at higher temperatures.

“Ultimately, continuing developments will enable large-scale penetration of renewable solar energy into the power grid,” Sandhage said. “This would mean drastic reductions in man-made carbon dioxide emissions from electricity generation.”

Sandhage’s research team has filed patent applications through the Purdue Research Foundation’s Office of Technology Commercialization for various technologies developed to improve the way concentrated solar power plants generate electricity and store solar thermal energy. The research is funded by the United States Department of Energy.

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Writer, Media Contact: Kayla Wiles, 765-494-2432, wiles5@purdue.edu

Source: Kenneth Sandhage, sandhage@purdue.edu