Why a pump to move molten metal is awesome

The conversion of one form of energy into another is more efficient at higher temperatures.1 For example, one of the most widely used components of any system that involves the transfer of heat from one part of the system to another is a device called a heat exchanger. When it’s transferring heat from one fluid to another, for example, the heat exchanger must facilitate the efficient movement of heat between the two media without allowing them to mix.

There are many designs of heat exchangers for a variety of applications but the basic principle is the same. However, they’re all limited by the explicit condition that entropy – “the measure of disorder” – is higher at lower temperatures. In other words, the lower the temperature difference within the exchanger, the less efficiently the transfer will happen. This is why it’s desirable to have a medium that can carry a lot of heat per unit volume.

But this is not always possible for two reasons. First: there must exist a pump that can move such a hot medium from one point to another in the system. This pump must be made of materials that can withstand high temperatures during operation as well as not react with the medium at those temperatures. Second: one of the more efficient media that can carry a lot of heat is liquid metals. But they’re difficult to pump because of their corrosive nature and high density. Both reasons together, this is why medium temperatures have been limited to around 1,000º C.

Now, an invention by engineers from the US has proposed a solution. They’ve constructed a pump using ceramics. This is really interesting because ceramics have a good reputation for being able to withstand extreme heat (they were part of the US Space Shuttle’s heat shield exposed during atmospheric reentry) but an equally bad reputation for being very brittle.2 So this means that a ceramic composition of the pump material accords it a natural ability to withstand heat.

In other words, the bigger problem the engineers would’ve solved for would be to keep it from breaking during operation.

DOI: 10.1038/nature24054
DOI: 10.1038/nature24054

Their system consists of a motor (not visible in the image above but positioned to the right of the shaft, made of an insulating material), the gearbox, a piping network and a reservoir of liquid tin. When the motor is turned on, the pump receives liquid tin from the bottom of the reservoir. Two interlocking gears inside the pump (shown left bottom) rotate. As the tin flows between the blades, it is compressed into the space between them, creating a pressure difference that sucks in more tin from the reservoir. After the tin moves through the blades, it is let out into another pipe that takes it back to the reservoir.

The gearbox. Source: https://www.youtube.com/watch?v=Y_6K-Xo4nH8
The gearbox. Source: https://www.youtube.com/watch?v=Y_6K-Xo4nH8

The blades are made of Shapal, an aluminium nitride ceramic made by the Tokuyama Corporation in Japan with the unique property of being machinable. The pump seals and piping network are made of graphite. High-temperature pumps usually have pipes made of polymers. Graphite and such polymers are similar in that they’re both very difficult to corrode. But graphite has an upper hand in this context because it can also withstand higher temperatures before it loses its consistency.

Using this setup, the engineers were able to operate the pump continuously for 72 hours at an average temperature of 1,200º C. For the first 60 hours of operation, the flow rate varied between 28 and 108 grams per second (at an rpm in the lower hundreds). According to the engineers’ paper, this corresponds to an energy transfer of 5-20 kW for a vat of liquid tin heated from 300º C to 1,200º C. They extrapolate these numbers to suggest that if the gear diameter and thickness were enlarged from 3.8 cm to 17.1 cm and 1.3 cm to 5.85 cm (resp.) and operated at 1,800 rpm, the resulting heat transfer rate would be 100 MW – a jump of 5,000x from 20 kW and close to the requirements of a utility-scale power plant.

And all of this would be on a tabletop setup. This is the kind of difference having a medium with a high energy density makes.

The engineers say that their choice of temperature at which to run the pump – about 1,200ºC – was limited by whatever heaters they had available in their lab. So future versions of this pump could run for cheaper and at higher temperatures by using, say, molten silicon and higher grade ceramics than Shapal. Such improvements could have an outsize effect in our world because of the energy capacity and transfer improvements they stand to bring to renewable energy storage.

1. I can attest from personal experience that learning the principles of thermodynamics is easier through application than theory – an idea that my college professors utterly failed to grasp.

2. The ceramics used to pave the floor of your house and the ceramics used to pad the underbelly of the Space Shuttle are very different. For one, the latter had a foamy internal structure and wasn’t brittle. They were designed and manufactured this way because the ceramics of the Space Shuttle wouldn’t just have to withstand high heat – they would also have to be able to withstand the sudden temperature change as the shuttle dived from the -270º C of space into the 1,500º C of hypersonic shock.

Featured image credit: Erdenebayar/pixabay.

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