Very cool: superconductors made simple

To use HTS “you need to get below a temperature of around 90K (-193°C)”, says Manuel La Rosa of Neutron Star Systems, but at that transition point, “a whole lot of things get far easier, just because wires lose their resistance”. And achieving this is no longer far-fetched, even for commercial vessels.

Superconductors have certainly come a long way since HK Onnes discovered the effect by cooling mercury: the next big step forward was arguably the cuprate-based YBa2Cu3O7-d (YBCO) innovation of the 90s as this meant the required temperature could be reached by, for example, liquid nitrogen cooling, giving rise to the slightly misleadingly named ‘high-temperature superconductors’ (HTS).

Now, the technology has “reached the second generation” explains Dr Markus Bauer of THEVA, a company specialising in thin, flexible tapes. While earlier wires centred on drawing and annealing a silver cylinder filled with HTS material, these later developments coat either a nickel-based or stainless steel foil with a nonreactive buffer. On top comes a micron-thick superconductor layer. Cabling solutions shape it into a standard spiral form and enclose it in cooling jacket, both electrical and thermal insulation, finishing it with a metal sheath.

The whole thing is surprisingly compact, coming – on average – to between 40mm and 60mm in diameter for a typical 5MW DC line. By contrast, standard copper wiring for the same power can be five times the girth says La Rosa. Even an off-the-shelf cryocooler such as the one from Stirling isn’t that cumbersome, taking up just a few cubic metres at one end of the link.

But the reason for entertaining the idea is simple: the losses are very, very low.

In fact, according to research by Nexans, a typical 5MW (5kA,1kV) shipboard system loses just 0.56% at 20 Kelvin – and that reduces even further for lower temperatures. Put that against a medium voltage 250V DC copper wire distribution: operating with DC transformers and the system accumulates a total loss of around 5%. By comparison, the HTS is more efficient – and it works out “far less bulky overall” adds La Rosa.

A portion of these gains result from discarding the typical high-voltage grid used to mitigate the resistance across long, 30m to 300m runs. Replacing power cables with HTS between generator and bus-bar, or bus-bar and motor allows low or medium voltage operation, ditching the standard converters and transformers necessary to ramp it up and down.

According to the Nexans report, that means a 100m-long, 5MW superconducting power distribution system in operation for 300 days per year can save between 150 and 250MWh per year over a conventional DC installation.

There are few considerations – the length of the run has to be long enough to be worthwhile, probably over 30m, as the coolant retains its low temperature more effectively given greater mass. Another is that the ambient links need care if the system is to retain efficiency. Further, it requires time and energy for cool down – “hours or possibly a day” says Bauer, though because of its light operational load, that might just mean the system is kept running during shorter port stays.

What makes this of particular interest to shipping is that there’s a neat tie-in with alternative power. There are already a handful of fuel cell vessels under build, including cargo carriers running LNG – and some designs are looking at incorporating liquefied hydrogen.

urther, cruise ships have a large, fairly continuous onboard power draw – often over 40MW. “A number of cruise operators are investigating the possibility of fitting fuel cells,” says La Rosa, but tight space and distribution over hundreds of metres can present a challenge, so it’s possible that HTS could provide a solution.

The most significant point is these fuels are themselves cryogenically stored. For LNG-propelled ships, the tank temperature is already more than halfway to the HTS operating temperature, so the liquefied gas can be used for precooling the nitrogen, allowing a smaller unit for the last step down. Further, with liquefied hydrogen “cooling comes for free as the H2 has to be warmed up anyway before use”, explains Bauer.

Most importantly, HTS tape or wire can also replace copper windings and permanent magnets, doubling the magnetic field’s power and allowing the development of more compact motors. However, while the current rises with decreasing temperatures, it also falls in relation to increases in the magnetic field, says La Rosa. In cables that field is low, therefore temperatures where nitrogen is liquid (65 to 77 Kelvin) are sufficient. Motors, with their magnetic fields, demand a lower temperature to achieve a reasonable current, so neon or helium gas will probably be necessary to bring the temperature down to 20 or 30 Kelvin.

But as Bauer underlines once reached, maintaining this temperature is much easier than might usually be expected. As he explains, replacing a motor’s traditional copper with HTS means “there’s virtually no resistive heat during operation… so only very low cooling power is required”.

Moreover, overall performance is also considerably enhanced since a fall in resistance is accompanied by a drop in reactance, allowing for improved stability across transient conditions.

It has to be said, this is not exactly new technology. Nearly two decades ago, Kawasaki came up with a 1MW, HTS pod motor and followed this with a 3MW variable speed unit which returned tested efficiencies of 98%. Siemens has also demonstrated a 4MW motor, but AMSC and Northrop Grumman’s 36.5MW machine for the US Navy in 2016 has been a massive jump in scale. La Rosa points out that the total system came in less than half the size of the conventional version… and a third of the weight.

Despite warship utilisation, merchant marine take up “has been comparatively slow” says La Rosa, although there’s headway in renewable energy applications: THEVA recently helped apply a 3MW generator to a wind turbine.

But as La Rosa and Bauer both explain, the markets are changing. There’s the rise of cryogenic fuels, and HTS systems are themselves developing.

So far, most HTS motors are divided into a warm and a cold side. This usually positions the superconducting field windings on the rotor, while retaining conventional copper on the stator.

La Rosa explains that the reduced mass of the HTS element results in less time and energy for cool-down. A disadvantage is that it requires connecting components at very different temperatures. Bauer explains it needs a cryostat to keep the boxed in, turning rotor at the right temperature – slightly tricky, but do-able. It also requires vacuum and superinsulation between warm and cool sides, though La Rosa adds this configuration is still useful for larger applications.

While bringing both rotor and stator windings to cryo temperatures might create an even lighter machine, significant AC and cooling losses means “there is a long way to go till it makes economic sense”, says Bauer.

However, all these options still have a common element that limits the achievable flux explains Bauer. If the iron teeth supporting the winding reach magnetic saturation, it leads to a rapid temperature rise and corresponding losses. As a result, while this more conventional stator still lends some efficiency improvements, the reduction in scale is limited to about half that of a conventional machine.

A further, completely new development sees the stator windings wrapped around a non-magnetic core such as aluminium or GRP.

This promises further advantages. While it demands more HTS material – and makes both heat and torque transfer more complex as it means getting rid of a solid lump of metal – this design avoids the vibration and harmonic field distortions generated by the iron teeth. Most importantly, it could also increase flux density four or five times, reducing the motor’s scale for the same output.

There are challenges ahead: there’s still a lack of familiarity and there’s no ready-made modelling solution to show how they’d perform as part of an onboard power plant… as yet. But while the superconductor chemistry itself is still fairly expensive, “it’s coming down” says Bauer. Production volume is rising with demand from other more exotic sectors, including ‘small’ nuclear fusion reactors and the space industry “so for the same capacity the price will eventually be comparable to copper”, he predicts.