Researchers at the Georgia Institute of Technology, Stanford and Purdue University have built a ceramic mechanical pump that can move liquid metal at up to 1673 K (that’s about 1400 degrees Celsius). Typically, the temperature of liquid metals that you can pump tends to reach 1,300 K (1,027 degrees Celsius), because there are few pump building materials that remain solid and chemically stable beyond that. The existing materials tend to crack or break quickly under the pressure of such heat.
But this new pump, made from carefully engineered ceramics, could be good news for concentrated solar energy, as well as the associated thermal energy storage.
An example of this type of system is SolarReserve’s 110 MW Crescent Dunes plant outside Tonopah, Nevada. The facility consists of a series of heliostats — or panels that reflect sunlight off a thermal tower — and a molten salt tank. The molten salt is pumped to the thermal tower and captures the heat from the sunlight projected onto the tower. It is then stored at 566 degrees Celsius (1,050 degrees Fahrenheit) and used to transfer heat through a heat exchanger to a working fluid that spins a conventional turbine and generates electricity.
This latest research could improve that system by allowing operators to trade that molten salt for liquid tin, which has a higher thermal conductivity than molten salt and, at 1.673K, could deliver more energy to a system with less effort. In fact, the researchers say, “using molten metals instead of liquids 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.”
How did they do?
The trick to creating the ceramic pump lay in the careful engineering of the pump itself. Materials had to be chosen to accommodate the expansion and contraction of the pump with heat, as well as a suitable material to seal the pump. The researchers used graphite for piping, joints and seals, as well as a material called Shapal (“a machinable aluminum-nitride-rich composite”) because it had a similar coefficient of expansion to graphite under heat. Tungsten was used for the outside of the sealed area.
The external gear pump was then built with the gear teeth of the pump and the motor deliberately misaligned at room temperature; when the pump experienced very high temperatures, it would expand into proper alignment. The pump expanded more than 1 mm vertically while in operation.
To accommodate liquid metal at a very high temperature, the atmosphere in the pump was replaced with inert N2which, according to the researchers, “could also be used on an industrial scale.”
The pump system was tested at an average temperature of 1,473 K (1,200 degrees Celsius) – with peak temperatures above 1,673 K (1,400 degrees Celsius) – for 72 hours. The researchers noted that the temperature of the liquid metal “was the result of limited heating power and is not a fundamental limitation on the pump or other system components.” After 72 hours, the test was stopped and there were no component failures during the test or during cooling. Although the gears showed “visible wear,” the researchers offered some design suggestions that would improve component life in future tests. “Shaft wear was reduced by more than an order of magnitude over previous tests simply by polishing the shafts,” the paper said.
Liquid tin’s high volumetric energy density would allow a relatively small pump to be used in a large-scale power plant. The researchers estimate that using very hot liquid metal in their pump would result in “a relative increase in efficiency of about 50 percent” in concentrated solar thermal systems, “which would reduce costs by 20 to 30 percent.”
One thing worth noting is that this research was funded by the Advance Research Projects Agency-Energy (ARPA-E). The agency distributes grants to institutions such as the Georgia Institute of Technology to promote energy-related technology, especially projects that may be unprofitable for private companies to fund. But projects like this may not be possible in the future as the Trump administration has asked to cut all ARPA-E funding in the 2018 budget.
Nature2017. DOI: 10.1038/nature24054 (About DOIs).
Frame image by Christopher Moore, Georgia Tech