Fundamental to power generation and many industrial processes, thermal energy is most valuable at high temperatures because entropy – which makes thermal energy unavailable for conversion – declines at higher temperatures. Liquid metals such as molten tin and molten silicon could be useful in thermal storage and transfer, but these materials cannot withstand such extreme temperatures.
U.S. academic researchers engineered a solution in the form of a ceramic-based mechanical pump. The device was fabricated by precision machining and includes seals made from graphite, another high-temperature material. The pump operates at record temperatures of more than 1,400 C (1,673 K) and can transfer high temperature liquids such as molten tin, enabling a new generation of energy conversion and storage systems.
In addition to its contribution to high-efficiency, low-cost thermal storage, the pump offers new opportunities for wind and solar energy storage. It could also help advance the production of hydrogen from methane without coincident carbon dioxide emissions.
Researchers from Georgia Institute of Technology, Purdue University and Stanford University used an external gear pump, which uses rotating gear teeth to suck in the liquid tin and push it out of an outlet. This pump technology, featuring gears custom-manufactured by a commercial supplier and modified by the researchers, was selected for its simplicity and ability to operate at relatively low speeds. The pump operates in a nitrogen environment to prevent oxidation at the extreme temperatures.
The system was operated for 72 hours continuously at a few hundred revolutions per minute at an average temperature of 1,473 K – with brief operation up to 1,773 K in other experimental runs. The pump sustained some wear, which is attributed to the use of Shapal, a relatively soft ceramic that is easily machined. Other ceramics with greater hardness are expected to overcome that issue, and the team is already working on a new pump made with silicon carbide.
The researchers envision deployment of the new pump in low-cost grid storage systems for surplus energy produced by renewables, which remains a major challenge to renewables diffusion. Power produced by solar or wind sources could be used to heat molten silicon, creating thermal storage that could be tapped as 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,” said Asegun Henry, an assistant professor in Georgia Tech’s Woodruff School of Mechanical Engineering. “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 pump could also be used to allow higher temperature operation in concentrated solar power applications, where molten salts are now used. The combination of liquid tin and ceramics would have an advantage in being able to operate at higher temperatures without corrosion, enabling higher efficiency and lower cost.
The ceramic pump now uses gears just 36 millimeters in diameter, but it can be scaled up for industrial use without the need for markedly larger components. 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.
Molten silicon – with still higher temperatures – may be more useful for storage due to its lower cost. The pump could operate at much higher temperatures than those demonstrated so far, even past 2,000 C.