Pumping of liquid metals has previously been limited to 1,300 °K (or 1,027 °C), because of the risk of the pumps themselves disintegrating under the stress of the heat, becoming chemically unstable, or melting themselves. Now engineers from the Georgia Institute of Technology, Stanford, and Purdue University have raised that cap to 1,673 °K (1,400 °C).
The research team built a ceramic mechanical pump that is able to operate at those higher temperatures, allowing the transfer of high temperature liquids such as molten tin, which could enable a new generation of energy conversion and storage systems. Potential applications include high efficiency low-cost thermal storage, where the energy generated by wind and solar power is stored as thermal energy in super-heated salts and molten fluids. It could also facilitate an improved process where hydrogen is generated directly from fuels such as methane, without producing carbon dioxide.
Normally ceramic components are considered too brittle for mechanical systems, but the engineers used precise machining and seals made from graphite to enable them to build the ceramic mechanical pump.
The research was supported by the Advanced Research Projects Agency – Energy (ARPA-E).
“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 is most valuable at high temperatures because entropy declines at higher temperatures, and that increases the amount of thermal energy that can be converted to mechanical or electrical energy.
While liquid metals such as molten tin and molten silicon could theoretically be used for thermal storage and transfer, the lack of pumps and pipes capable of withstanding such extreme temperatures limited their use.
To challenge the assumption that ceramic materials are too brittle for mechanical applications like pumps, the researchers used an external gear pump. This pump uses rotating gear teeth to suck in the liquid tin and push it out of an outlet. The design is relatively simple and able to operate at lower speeds. Additionally, technological improvements in the past few decades allowed the engineers to fabricate different ceramic materials into large pieces, that could then be machined.
The pump operates in a nitrogen environment to prevent oxidation at the extreme temperatures. The team ran it for 72 hours continuously at a few hundred revolutions per minute, at an average temperature of 1,473 °K, briefly going up to 1,773 °K. Because the pump used Shapal, a relatively soft ceramic, the pump sustained wear from the run. However, the team is now working on a new pump that utilises ceramics that are harder, such as silicon carbide.
[Caption: Georgia Tech Graduate Student Caleb Amy shows how two ceramic gears mesh in a pump developed to transfer molten tin at more than 1,400 degrees Celsius. (Credit: Christopher Moore, Georgia Tech)]