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.

Curious Bends – nuclear Himalayas, tiny Indians, renewables victory and more

1. 50 years ago a bomb’s worth of plutonium was lost on India’s second highest mountain. The mystery remains unanswered

“In October 1965, the US’ Central Intelligence Agency (CIA) and India’s Intelligence Bureau (IB) joined hands in a clandestine mission to install a nuclear-powered sensing device on the summit of India’s second highest peak, also one of its most revered: the 25,643ft (around 7815m) Nanda Devi in Uttarakhand’s Garhwal Himalayas.” Then they lost the plutonium-filled device. Fifty years later we still didn’t know where it is. (20 min read, livemint.com)

2. India’s preference for sons has created a nation of tiny people

“Indian children are among the shortest in the world, and the country’s preference for sons might be to blame. Globally, one in four children under the age of five is stunted—that is, they grew at a slower rate than a healthy child would. This stunting is manifest in shorter than average height. About half the stunted children live in Asia and another one-third live in Africa. India has the fifth-highest stunting rate in the world—nearly 40% of the children were stunted in 2005. This is a worrying proportion, even if you didn’t know that by 2020 India is projected to have the world’s youngest population.” (3 min read, qz.com)

3. The Ghanian puzzle: “Water, water, every where; not a clean drop to drink”

“Despite an abundance of water sources, most people in Ghana can’t simply turn a knob in the wall to get it. The water infrastructure in the country does not even come close to meeting demand; to call it patchwork would be an insult to quilts. Ghanaians have to balance their time, money and safety to determine where they will get a drink. Millions of them choose to get their water in 500-ml plastic sachets. And some of them get their sachets from Johnnie Water.” (25 min read, mosaicscience.com)

+ The author of this story, Shaun Raviv, is a freelance journalist. He’s “currently American, formerly Ghanaian and Swazi.”

4. The Indian government heavily subsidises private healthcare at the cost of public amenities

“Since medical insurance payments are tax-deductible, up to a quarter or more of the insurance premia that support the private corporate hospitals is probably claimed as a tax waiver. In other words, the government is paying Rs 6,000 crore for the sustenance of these corporate hospitals; those insured pay the rest. On top of this, state and local governments have provided land at subsidised rates to these hospitals, in return for free or subsidised treatment to poor patients, who were to account typically for a quarter of the total patients. There is no corporate hospital that has met its obligations on this score; in one infamous case, the hospital said it had promised free treatment but not a free bed or bed linen.” (3 min read, businessstandard.com)

5. Suicide will soon become India’s #1 killer

“On being asked whether he thought the government of India was doing enough for mental health problems in India, Patel told TOI “Nowhere near the need, witness the complete absence of public health approach to suicide for example”. An earlier research by professor Patel on suicides in India had thrown up shocking findings. Four of India’s southern states — Tamil Nadu, Andhra Pradesh, Karnakata and Kerala — that together constitute 22% of the country’s population were found to have recorded 42% of suicide deaths in men and 40% of self-inflicted fatalities in women in 2010.” (4 min read, timesofindia.com)

Chart of the Week

“The race for renewable energy has passed a turning point. The world is now adding more capacity for renewable power each year than coal, natural gas, and oil combined. And there’s no going back.” (bloomberg.com)

Capacity addition by energy sources, divided as fossil fuels and 'clean' energy.
Capacity addition by energy sources, divided as fossil fuels and ‘clean’ energy.