Renewable storage breakthrough boosts efficiency, cuts costs

Source: Christa Marshall, E&E News reporter • Posted: Monday, October 16, 2017

Scientists have developed a ceramic pump that they say could transform backup power for renewables by storing heat in extremely hot liquids.

In a study this week in Nature, researchers reported the system could operate at temperatures of 1,400 degrees Celsius and transport liquefied metals like molten tin. That is hundreds of degrees hotter than temperatures allowed in existing storage systems, including those using molten salt to store heat with concentrated solar power.

If scaled up, the pump could allow grid-scale storage for renewables that is more efficient and inexpensive than current options, researchers say.

“It appears likely that storing energy in the form of heat could be cheaper than any other form of energy storage that exists,” said Asegun Henry, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering, in a statement. “This would allow us to create a new type of battery. You would put electricity in when you have an excess, and get electricity back out when you need it.”

Henry was part of a Georgia Tech team that led the research in conjunction with scientists from Purdue University and Stanford University. The Advanced Research Projects Agency-Energy at the Department of Energy provided funding.

Storage via extremely hot liquids could lower costs in theory because hotter temperatures allow greater conversion of stored thermal energy to mechanical or electrical energy per unit of material.

To date, storage of liquids at 1,400 C was a challenge because pumps and pipes couldn’t handle that level of heat. Ceramic was considered too brittle to be an option.

The team got around the hurdle partly by using graphite, which can withstand much higher temperatures than the flexible polymers currently used, to form seals in the pump and surrounding pipes. The team also took advantage of machinery advances that can shape materials precisely.

“What is new in the past few decades is our ability to fabricate different ceramic materials into large chunks of material that can be machined,” Henry said. “The material is still brittle and you have to be careful with the engineering, but we’ve now shown that it can work.”

Scientists operated the pump continuously using heated molten tin for 72 hours continuously at a few hundred revolutions per minute.

The technology has a ways to go before it could be a commercial application. Scaling up new technologies can add new technical and cost challenges. The pump showed wear after the 72-hour window, and at 36 millimeters in diameter, it currently is too small for industrial-scale power.

The team is working on a new pump made of silicon carbide that could be hardier, according to Henry. It is also examining different ceramics and using the pump as part of a larger system to produce hydrogen from methane without producing greenhouse gases.

Theoretically, that’s a possibility because liquid tin can split methane molecules apart to produce hydrogen without also generating carbon dioxide.

Henry said he was optimistic because an increase in pump size of about four times would allow heat transfer consistent with utility-scale power plants. Some liquids such as molten silicon might be able to improve efficiency more by operating at temperatures closer to 2,000 C, he said.

“The hotter we can operate, the more efficiently we can store and utilize thermal energy,” he said.