Thermal energy is of greatest value—that is, has the highest available work or ‘exergy’—when it can be transported, stored and converted at the highest possible temperatures. However, the manipulation of heat at extreme temperatures (well above 1,300 K) has proven to be extremely difficult for many applications. Even in cases where high-temperature fluids are used, such as gas turbines and rocket engines, the pumps and compressors are not operated at high temperatures. Instead, the pumps and compressors are kept in a relatively cold portion of the system, with mechanical force transmitted through the fluid itself to the hot region. Very few applications involve the flow of high-temperature liquids. The high-temperature pumping of liquids is severely restricted by limitations on pump materials because there are only a few classes of materials that remain solid, are chemically stable with such liquids, possess sufficient strength and exhibit long life at temperatures well above 1,300 K.

Molten metals can be optimal high-temperature heat-transfer fluids because they: (1) tend to have low viscosities near their liquidus temperatures; (2) have high electrical conductivity and therefore high thermal conductivity (because convective heat transfer is directly proportional to thermal conductivity, the use of molten metals instead of fluids such as oils and salts that are electrically insulating can result in an increase in the heat transfer coefficient of two to three orders of magnitude); (3) can have a very large liquid range (for example, 505–2,876 K for tin), thereby enabling single-phase operation and high volumetric energy density via sensible heating; and (4) can be relatively abundant (mass-produced) and inexpensive. These desirable properties lead to compact and high-volumetric-power-density heat transfer and thermal storage.

… Although highly attractive, liquid-metal heat transfer has historically been limited by the availability of suitable containment materials. … the key question is whether clever approaches that use ceramics—which accommodate their brittle nature—can be developed to pump and circulate liquid metals at extreme temperatures. This is an important question to address because a technology that enables liquid-metal pumping and heat transfer above 1,300 K would unlock new strategies and concepts in, for example, electric power generation and chemical and metals processing.

—Amy et al.

Thermal energy, fundamental to power generation and many industrial processes, 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 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. Asegun Henry, an assistant professor in Georgia Tech’s Woodruff School of Mechanical Engineering, and graduate student Caleb Amy—the paper’s first author—decided to challenge that assumption by trying to make a ceramic pump.

Model of the pump. A graphite reservoir primes the pump (left). The gears then pressurize the fluid, causing it to flow through the graphite piping where it then falls back into the reservoir. An image showing tin flowing at 1,673 K is provided bottom left. A model of the pump gears is shown on the right, illustrating the direction of rotation with a plot showing how the volume near the outlet changes with rotation. The two discontinuities are transitions at which the volume highlighted in orange is gained (VG) and then lost (VL). The slope of the volume versus angle or time is the theoretical volumetric flow rate for an incompressible fluid. Amy et al. Click to enlarge.

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. That technology differs from centrifugal and other pump technologies, but Henry chose it for its simplicity and ability to operate at relatively low speeds. 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. The material is still brittle and you have to be careful with the engineering, but we’ve now shown that it can work.

—Asegun Henry

Addressing another challenge, the researchers used another high-temperature material—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 1,473 Kelvin (1,199.85 ˚C)—with brief operation up to 1,773 Kelvin (1,499.85 ˚C) 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. 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. 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.

—Asegun Henry

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.

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 uses gears just 36 millimeters in diameter, but Henry says scaling it up for industrial processing wouldn’t require significantly 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 temperature—may be more useful because of its lower cost. The pump could operate at much higher temperatures than those demonstrated so far, even past 2,000 degrees Celsius, Henry said.

On the basis of these results and this initial proof-of-concept demonstration, the use of ceramic and refractory metal components in an inert atmosphere should now be considered a viable approach to pumping liquid metal at extreme temperatures. This technology has broad applications and it is important to begin to rethink how various industries operate, to identify opportunities where substantial energy savings or increases in efficiency could be achieved.

—Amy et al.

Resources

• Caleb Amy, et al. (2017) “Pumping Liquid Metal at High Temperatures Up To 1,673 K,” Nature doi: 10.1038/nature24054