The Motorship January 2021

Page 16

SHAFT POWER

VERY COOL: SUPERCONDUCTORS MADE SIMPLE Superconductors haven’t often been mentioned in relation to shipping. However, not only could the technology cut the space requirement for both onboard grid and motors, some alternative-fuel vessels might even give it a leg up, writes Stevie Knight 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 micronthick 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

16 | JANUARY 2021

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. Further, 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,”

8 High-temperature superconductor (HTS) tapes

8 Transition temperatures have changed over time, taking off in the 90s

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