Time to engineer: fusion energy.
Scientific investigations are the foundation of the technological world, but there comes a time when science needs to give way to engineering. Sometimes, things just need to be made.
"Take the internal combustion engine," says Dr David Kingham, chief executive of Tokamak Energy (pictured right). "When the first ones worked, people knew far less about the physics of combustion than they do now, but they knew enough to make it work. We're in that sort of position with fusion. We know enough to take it on as an engineering challenge."
Fusion is the holy grail of nuclear energy technology. With fusion, the legacy problems of radioactive waste are diminished, and the fuel is cheap and widely available. It's clean, green and sustainable; the trouble is, it doesn't i yet work at the commercial scale.
For over half a century, the Culham Laboratory, or Culham Science Centre as it is now known, has been at the core of fusion research in the UK. It was nearly 20 years ago that scientists there managed to produce 16MW from a fusion process ... the downside being that it required 24MW of heat to make it work. Superficially, not much has happened since then, although the path of scientific discovery continues to be walked.
Tokamak Energy was launched six years ago with a view to change that and shake up the industry. As Kingham points out: "Progress has been really slow, but that is an engineering challenge. Scientific progress has been fine. We have this feeling that the science is already understood well enough. What we need to do is build high-performance devices and get the best out of them, rather than do more and more science."
Tokamak Energy believes building prototypes quickly, demonstrating performance and then moving on to the next device is the fastest route to realising commercially viable power plants. Of course, 'quickly' is a relative term and, since Tokamak Energy has been in existence, it has produced two prototypes. Its third will be completed in the next few months.
Fusion is what happens in the sun. Superheated hydrogen atoms collide to produce helium and a lot of energy. This combination of extreme temperature and pressure, provided by gravity in the sun, strips the outer electrons away from the positively charged hydrogen nuclei, creating a neutral, but fully ionised plasma.
Attempts to develop a controlled fusion reaction revolve around forcing two hydrogen isotopes (deuterium and tritium) to collide in a similar way. However, to do so requires phenomenal temperatures to create the superheated plasma, which contains hydrogen isotopes. The plasma, at more than 100,000,000[degrees]C, is contained by magnetic fields inside a tokamak--the name given by Russian scientists to the vacuum vessel used for fusion experiments. The phenomenal temperatures highlight why there was a net energy loss in the early Culham experiments.
Fusion reactions have proven extremely difficult to attain and, as yet, impossible to maintain. When a reaction does happen, helium and a single neutron are produced, along with a lot of energy. Four-fifths of this energy is carried out of the plasma by the neutron, which is captured outside the inner vacuum vessel of the tokamak. The energy of this particle is extracted as heat, which can then be used to produce steam for electricity generation.
These early designs were doughnut shaped, but there is fresh promise seen in spherical tokamaks. This breakthrough is what led to a couple of scientists at Culham, along with Dr Kingham, to form Tokamak Energy. The idea is to combine spherical tokamaks with high-temperature superconductors to overcome the problems created by pumping millions of amps through copper magnetic coils.
Kingham comments: "The short-term roadmap is to produce plasma temperatures hotter than the centre of the sun in 2017. Then we need to go hotter, to 100 million [degrees]C, in 2018. We then need to get as close as we can, in 2019, to fusion energy conditions.
"The conventional view is you have to go to bigger and bigger devices, but we're saying you can keep tokamaks relatively small. You have to go to very high magnetic fields, so the engineering is very challenging, but you can get fusion in a device that's not tens of metres across, but just a few."
There is a minimum size of device that's feasible. If a device is too small, then particles are more likely to hit the sides as plasma. As a consequence, temperature is constantly being lost. Equally, the cost of larger devices escalates exponentially and, arguably, can become more inefficient.
"There's an advantage in keeping things as small as possible, certainly, during the R&D process," says Kingham. "We want to quickly tackle the engineering challenges with relatively small devices and then scale up later, if necessary; although, we think devices just a few metres across are quite viable as 100MW power plants." The actual vessel is made of stainless steel, though more exotic materials would be required for production reactors in order to protect against erosion. Keeping the plasma off the vessel walls requires careful control of the magnetic field pressure, and this is provided by the positioning of magnetic coils and the current passed through them. It aims to produce a gap of a few centimetres between the tokamak's wall and the superheated plasma.
In the device that is currently being built, there will be up to 250 million amps in each of the 24 coils and 6 million amps down the centre column.
"The engineering challenge of doing that is substantial," concedes Kingham. "There's twisting forces and compressive forces on the joints, and you need to get them just right. If you're trying to pass these currents through a joint between two copper limbs, you don't want too much extra resistance at those joints or you can have a real problem. You're getting towards the limits of what you can sensibly do in mechanical engineering."
More particularly, it is getting towards the limits of what can be done with copper, as too much energy is wasted on resistive heating, which is why Tokamak Energy is leading research into high-temperature superconductor magnets.
"Conventional superconductors, as used in MRI, are brilliant materials, but they're limited in terms of magnetic field strength. High-temperature superconductors, particularly if you cool them down to around 20K, will deliver very high current densities in a high magnetic field."
The particular material under trial is the yttrium barium copper oxide second-generation high-temperature superconductor. "We've actually built one of these small tokamaks with high-temperature superconducting magnets. We've managed to run the magnets for 29 hours with nice and stable magnetic fields."
The company has grown to 30 employees, including five design engineers, while the increasing complexity of the third device, the ST40, has meant that 3D visualisation is important, meaning a move to 3D CAD from the drawing board ... literally.
Siemens Solid Edge was chosen as the design platform 18 months ago, as Paul Tigwell, mechanical design consultant and another recruit from Culham, explains. "Two of us started the same day," he recalls. "The other engineer had worked with Solid Edge, but knew nothing about tokamaks. I didn't know anything about Solid Edge, but I knew tokamaks, so we complemented each other quite well."
Much of Tigwell's early work was taking the original paper drawings and building them into 3D models. This process in itself highlighted the issues and clashes within the design; issues that the first prototype had to address by 'an engineer with a hammer' while assembling the device.
One of the features of Solid Edge that appeals to Tigwell is the ability to have 'alternate assemblies'. This allows the detail to be turned down, so the computer and graphics processors can update the display a lot faster.
"We've got the assembly and an overview," he states. "If I bring something new in, it affects both lines, so it's not two separate models you have to keep updated."
For modelling the magnetic fields, the company is using multi-physics simulation software from ANSYS. There are a number of design engineers working on both the current and next-generation prototype, meaning the company is also investing in Team Centre, which comes out of the same Siemens stable as Solid Edge.
"With Team Centre, you can look at the history and see when parts were changed, who changed it and what the change was. And we can attach documents to it, so, if one of the physicists gives us a design note, we can attach that. It means we have the history and evolution of the design all contained in one area."
Tokamak Energy's approach of getting its hands dirty and, to a certain extent, a degree of trial and error, is a bold one in such a long-term and high-cost branch of engineering. However, the rewards could be enormous.
"If we could demonstrate high performance in a relatively compact, relatively inexpensive device, that's a game changer for fusion," concludes Kingham "Even before we demonstrate really high performance, if we can get close to what tokamaks like JET have done in the past, in something that's a lot cheaper and quicker, then that really will reset things."
Article courtesy of Eureka magazine
TOKAMAK ENERGY'S FAMILY TREE
ST25--The spherical tokamak (ST) with a 25cm outside radius of the plasma
ST25 HTS--Proved that High Temperature Superconducting (HTS) magnets worked
ST40--40cm plasma radius version In construction, due for high temperature testing, starting spring 2017
ST60 HTS--60cm version with HTS magnets, targeted for testing in 2019
Caption: Above: Cut away design of the ST40. Below left: Dr David Kingham
Caption: Above: One of the limb assemblies of the ST40 Coil System
Left: Super-heated plasma inside a tokamak
Below: The ST25 undergoing tests at Tokamak Energy.
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|Title Annotation:||NUCLEAR FUSION|
|Date:||May 1, 2017|
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