Pump that can move molten metal offers new energy conversion and storage technologies

By Process Online Staff
Wednesday, 15 November, 2017


Liquid metal flow

Research conducted at the Georgia Institute of Technology has demonstrated the possibility of pumping molten metal at over 1400°C using ceramic pump components, opening up new possibilities in high-temperature heat transfer and storage — a critical factor in efficient energy conversion.

A ceramic-based mechanical pump able to operate at record temperatures of more than 1400°C can transfer high-temperature liquids such as molten tin or silicon.

The research was supported by the Advanced Research Projects Agency-Energy (ARPA-E) and reported in the journal Nature. The pump was developed by researchers from Georgia Tech with collaborators from Purdue University and Stanford University.

“Until now, we’ve had a ceiling for the highest temperatures at which we could move heat and store it, so this demonstration really enables energy advances, especially in renewables,” said Asegun Henry, an assistant professor in Georgia Tech’s Woodruff School of Mechanical Engineering. “The hotter we can operate, the more efficiently we can store and utilise thermal energy. This work will provide a step change in the infrastructure because now we can use some of the highest temperature materials to transfer heat. These materials are also the hardest materials on Earth.”

Thermal energy, fundamental to power generation and many industrial processes, is most valuable at high temperatures. Liquid metals such as molten tin and molten silicon could be useful in thermal storage and transfer, but until now engineers didn’t have pumps and pipes that could withstand such extreme temperatures.

Ceramic materials can withstand the heat, but they are brittle — and many researchers felt they couldn’t be used in mechanical applications like pumps. But Henry and graduate student Caleb Amy decided to challenge that assumption by trying to make a ceramic pump.

The researchers used an external gear pump, which uses rotating gear teeth to suck in the liquid tin and push it out of an outlet. The gears were custom-manufactured by a commercial supplier and modified in Henry’s lab in the Carbon Neutral Energy Solutions (CNES) Laboratory at Georgia Tech.

“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 explained. “The material is still brittle and you have to be careful with the engineering, but we’ve now shown that it can work.”

Addressing another challenge, the researchers used graphite to form the seals in the pump, piping and joints. Seals are normally made from flexible polymers, but they cannot withstand high temperatures. Henry and Amy used the special properties of graphite — flexibility and strength — to make the seals. The pump operates in a nitrogen environment to prevent oxidation at the extreme temperatures.

The pump operated for 72 hours continuously at a few hundred revolutions per minute at an average temperature of 1473 K — with brief operation up to 1773 K in other experimental runs. Because the researchers used a relatively soft ceramic known as Shapal for ease of machining, the pump sustained wear. But Henry says other ceramics with greater hardness will overcome that issue, and the team is already working on a new pump made with silicon carbide.

Among the most interesting applications for the high-temperature pump would be low-cost grid storage for surplus energy produced by renewables — one of the greatest challenges to the penetration of renewables on the grid. Electricity produced by solar or wind sources could be used to heat molten silicon, creating thermal storage that could be used when needed to produce electricity.

“It appears likely that storing energy in the form of heat could be cheaper than any other form of energy storage that exists,” Henry said. “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.”

The Georgia Tech researchers are also looking at their molten metal pump as part of a system to produce hydrogen from methane without generating carbon dioxide. Because liquid tin doesn’t react with hydrocarbons, bubbling methane into liquid tin would crack the molecule to produce hydrogen and solid carbon — without generating carbon dioxide, a greenhouse gas.

The ceramic pump uses gears just 36 mm in diameter, but Henry says scaling it up for industrial processing wouldn’t require dramatically larger components. For example, by increasing the pump dimensions by only four or five times and operating the pump near its maximum rated speed, the total heat that could be transferred would increase by a factor of a thousand, from 10 kW to 100 MW, which would be consistent with utility-scale power plants.

For storage, molten silicon — with still higher temperatures— may be more useful because of its lower cost. The pump could operate at much higher temperatures than those demonstrated so far, even past 2000°C.

Image: Liquid metal flowing at 1400°C in a laboratory at Georgia Tech. Even though all the surrounding materials are glowing, the tin remains reflective and the ripples from the pool of tin below are visible via reflections from the stream. Image credit: Caleb Amy.

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