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17th July 2014
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Helium SOS

Jasmin Fox-Skelly, 18 July 2014

Aerogel cell

Forged in the hot core of stars by the fusion of two hydrogen atoms, helium is one of the most abundant elements in the universe. Despite this, the Earth’s reserve of helium is running out, as it is too light for gravity to prevent it from floating into space. Scientists estimate that we will have used up our supply of helium within 25–30 years, proving problematic for the many industries that rely on it.

The most common type of helium is helium-4, used mainly as a cooling agent in MRI machines, and in the manufacturing of semiconductors, LCD panels and fibre optic wires. Helium has the lowest boiling point of all gases in a liquid state (–269°C, or 4.2 degrees above absolute zero), making it the coldest liquid on Earth. This coolness has given it a key role in the field of cryogenics.

For MRI machines to work, special superconducting wires surrounding the magnet are bathed in liquid helium to cool them and reduce their resistance to zero, allowing them to conduct more electrical current, which is necessary to maintain a strong magnetic field. A typical MRI scanner contains 1700 litres of liquid helium.

Helium is also used to uncover the electronic and magnetic properties of new materials, such as graphene,1 by cooling the materials to very low temperatures and subjecting them to high magnetic fields.

Whilst helium-4 has a vital role in cryogenics, helium-3 – a much rarer He isotope – is used to protect people from terrorism.2 Helium-3 has a special ability to absorb neutrons, which are emitted from radioactive materials as they decay, meaning that it can detect radioactive material. The US federal government uses helium-3 neutron detectors at the country’s border to prevent smuggling of nuclear material. Although the world also faces a shortage of helium-3, alternatives such as boron trifluoride, boron-10 and lithium-6 could be used for neutron detection.2

Helium-4 is produced by the very slow radioactive decay of geological rocks, and is a byproduct of extracting natural gas from reservoirs. US natural gas producers extract approximately 80bn  litres/year of helium from gas reserves.3

The majority of the world’s helium-3, meanwhile, has been produced as a result of the US nuclear weapons programme as tritium, a radioactive isotope of hydrogen used in nuclear warheads, decays into helium-3. In 1960, the US government began to maintain a stockpile of raw helium at a facility near Amirillo in Texas.4 The facility provides 42% of US helium, and 30% of the world’s supply.5 In 2009, it was estimated that more than 500bn litres of helium remained in the stockpile.6

However, the reason that helium is running out is because few natural gas wells contain enough to extract it economically, so we are reliant on existing supplies, notably the Amirillo stockpile. The situation was not helped when, in 1996, the US Congress ordered the sale of all but a small portion of the stockpile by 2015. In order to achieve the sale of the reserve, helium was sold below its market value, which made it economically not worth recycling. Fortunately, the US House of Representatives voted in September 2013 to delay closing the reserve, averting a potential supply crisis. However, the days of cheap helium are over and many researchers are struggling to cope with the rising costs.

There is as yet no economically viable way to manufacture helium and unfortunately it costs around 10,000 times more to extract helium from air than it does from rocks and natural gas reserves.7 Nor is it always economical to extract helium from gas wells, meaning that gas companies frequently do not collect it when extracting natural gas. Much more helium is lost through not being extracted in this way than is actually being used in the world.  However, as the price of helium increases this may change.

Solutions to the problem of helium shortage could involve preserving supplies we already have; extracting it more economically from gas wells or other sources; or finding alternative supercooling technologies.

As helium has become more expensive, most cryogenic research laboratories have put in place a system to recycle it. In the past, helium would be lost as it boiled off from the cryostats, which house the magnets. However, by using what is called a helium liquefier machine, scientists can recover the helium gas as it boils off, and turn it back into liquid helium. The liquid helium is then reinjected back into the system and used again.

The liquefier machine works by expanding a pressurised gas to achieve cooling. This approach has been used at the Large Hadron Collider at CERN in Switzerland, where the helium gas is completely trapped. However, this is costly, and most research laboratories struggle to capture, purify and reliquify 100% of the gas.

Meanwhile, alternative technologies to achieve cryogenic temperatures are already on the market. Antony Carrington, professor of physics at the University of Bristol, UK, explains: ‘Pulsed tube cryocoolers are the main ‘helium-free’ technology, which can produce very low temperatures. They are basically mini-liquification plants where helium is cooled by expansion, then cycled back around and recompressed. It is a very similar cycle to that used in a helium-liquefier, but the helium does not leave the closed system.’

Jeremy Good, director of London, UK, based company Cryogenic, is convinced that the future lies in these ‘cryogen-free’ or ‘dry’ mechanical systems. In August 2013, he announced that he had developed a new way to cool magnets for MRI machines to near absolute zero. ‘Traditional liquid helium systems require a continuous supply of liquid helium to create the cooling,’ says Good. ‘Dry systems, however, create cooling through the compression and expansion of a small fixed amount of helium gas, which is recirculated throughout the system.’

The technology uses mechanical refrigerators that cool to cryogenic temperatures using only electrical power.  The company is offering its magnets for use in a variety of imaging techniques, including MRI, nuclear magnetic resonance spectroscopy and electron spin resonance spectroscopy.

While very valuable for many applications, however, Cryogenic’s pulse tube coolers are not always an adequate substitute for helium, according to Shaun Fisher, vice chair of the Low Temperature Group at the Institute of Physics and professor of physics at Lancaster University, UK. ‘Pulse tube coolers are a very nice development, which could reduce the demand for liquid helium. They have the added advantage of being simple to use, as you just switch them on, and they can thermally cycle quite quickly, which is often important for research purposes,’ says Fisher. ‘There are issues, however, with vibrations. The pulse tube coolers necessarily generate large amounts of vibration. For many applications, this can be damped/ isolated from sensitive equipment. In our lab, however, since we work at the lowest accessible temperatures, vibrations are a big issue and pulse tube coolers are simply not an option. So for us, liquid helium is essential.’

So could other technologies replace helium in the field of cryogenics? According to Carrington, ‘the big issue is economics. At present, a conventional magnet system, which you fill with liquid helium, costs about five times less than a “helium-free” system based on a cryocooler. Of course, the running costs of the latter are much lower because you don’t need to pay for helium but it would take 5-10 years to break even.

‘At the moment there are no viable alternatives to helium for getting things really cold. On the other hand, there are materials known as high temperature superconductors, which in the future could get rid of the need to cool the magnets to such low temperature. More research needs to be done to improve these materials to the point where they can compete commercially.’

References

1. J. Good, Materials Today 2014, 17( 1), 2.
2The Helium-3 Shortage: Supply, Demand, and  Options for Congress Congressional Research Service Report for Congress 22 December 2010.
3Department of the Interior, U.S. Geological Survey, Mineral Commodity Summaries 2010, http://minerals.usgs.gov/minerals/pubs/mcs/2010/mcs2010.pdf, p. 72.
4Selling the Nation’s Helium Reserve. Washington, DC: The National Academies Press, 2010
5United States extends life of helium reserve. Nature. http://www.nature.com/news/united-states-extends-life-of-helium-reserve-1.13819
6Mineral Commodity Summaries 2010.
7The world is running out of helium: Nobel prize winner. Physics Org. http://phys.org/news201853523.html

Jasmin Fox-Skelly is a freelance science writer based in Cardiff, UK

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