The pump was developed by researchers at the Georgia Institute of Technology in collaboration with researchers from Purdue University and Stanford University. The research was supported by the Advanced Research Projects Agency-Energy (ARPA-E) and reported in the October 12 issue of the journal Nature.
The newly developed pump could make it easier to achieve high efficiency, low-cost thermal storage, providing a new way to store renewable energy generated by wind and solar power, as well as improving the process for generating hydrogen directly from fuels such as methane.
“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 utilize 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 most valuable at high temperatures
The temperature of liquid metals we can pump usually caps out at about 1,300 Kelvin (1,027 degrees Celsius) simply because there are very few pump-building materials that can stay solid or maintain chemical stability at higher temperatures than 1,300 Kelvin.
However, thermal energy is fundamental to power generation and many industrial processes and is most valuable at high temperatures because entropy – which makes thermal energy unavailable for conversion – declines at higher temperatures. With the newly developed pump, molten metals such as molten tin or molten silicon could replace molten salt used in thermal storage and transfer.
“The hotter you can operate, the more you can convert thermal energy to mechanical energy or electrical energy,” Henry explained. “But when containment materials like metals get hot, they become soft and that limits the whole infrastructure.”
A carefully engineered ceramic pump
It is well known that ceramic materials can withstand high heat, but they become brittle. And many researchers have said ceramics can’t be used in mechanical applications like pumps because of this. However, Henry and graduate student Caleb Amy, the first author of the paper, challenged that idea and tried to make a ceramic pump, anyway.
“We weren’t certain that it wouldn’t work, and for the first four times, it didn’t,” Henry said. They developed an external pump with rotating gear teeth to suck in the liquid tin and push it out of an outlet. This technology is different from centrifugal and other pump technologies.
For one thing, materials for the pump had to be chosen to allow for expansion and contraction of the pump with heat, while an appropriates material for sealing the pump had to be used. The researchers used graphite for the piping, joints, and seals, as well as a ceramic material called Shapal, “a machinable aluminum-nitride-rich composite,” because it had similar heat expansion properties as graphite.
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.”
The pump operated for 72 hours continuously at a few hundred revolutions per minute at an average temperature of 1,473 Kelvin – with brief operation up to 1,773 Kelvin in other experimental runs. And while the team used Shapal because of the ease of machining the material, the pump did sustain some wear. However, 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.
“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.”
