Cold power heats up for commercialisation

Tom Shelley reports on the latest advance along the path to commercialisation of superconductivity in motors and generators The world's first, large, commercial design, high temperature superconducting generator is already being run on its test bed. Intended for use in ships and on offshore platforms, its design builds on an already successful 400kW prototype, now preserved as a landmark development for posterity. Advantages include small size, extreme efficiency, and very high overload capacity. One day, we believe, all large motors and generators will be like this. The new HTS unit on a test stand in the Siemens plant in Nuremberg is rated at 4MVA and is designed to function as a ship's generator. It is the latest development to come from the Siemens joint team with Corporate Technology, Large Drives and Marine Solutions. It represents the culmination of a development process that started with experiments on large superconducting motors cooled by liquid helium for possible naval applications in the late 1960s in the UK, and the discovery of High Temperature Superconductors in 1986. According to Siemens director product management, Thomas Bartes, the basic concept of the new machines is to have a synchronous machine, whose rotor is equipped with high temperature superconductor windings carrying DC, and a conventional copper wound stator carrying AC. The DC supply for the HTS windings is taken from slip rings. Superconductors tend to lose their superconducting abilities when used to carry high current AC. Cryogenic cooling is undertaken by a neon filled heat pipe to move heat from the inside of the stator to the external cooling system. Because a heat pipe is a closed system, losses of neon are not detectable. Thomas Bartes said that the performance of the earlier 400kW machine, developed from 1999 to 2001 "Exceeded all expectations" during its test programme which started in 2001 and finished in August 2004. Output was 400kW, but it was found during tests that the machine had a short term overload capacity of around 700 per cent, much greater than the maximum 150 per cent overload capacity of conventional synchronous machines. The 400kW HTS machine ran at 1500 rpm and the rotor operated at 25 to 30 deg K at which point rotor losses were just 20W. While high temperature superconductors will function at the boiling temperature of liquid nitrogen, - 196 deg C or 77 deg K, their current carrying capacity is limited in the presence of strong magnetic fields unless the temperature is lowered further. Thomas Bartes commented, "It was not easy to build these coils". HTS conductor made out of thin tape is basicly ceramic, held within a silver alloy matrix. A special winding technology - not adaptable from available copper winding processes - has had to be developed, and the rotor has to be bandaged to hold it together and provide heat insulation. Overall efficiency was found to be 96.8 per cent including energy consumed by the cooling compressor for the cryogenic system. The machine may now be viewed in the entrance hall of the test facility at Nuremburg, where inspection of the plate declares it to have been built for the Hanover Messe 2005, in partnership with Large Drives and Corporate Technology. The new machine delivers 4MVA, and runs at 3,600 rpm. It is the world's first HTS machine ever designed as a generator, synchronised to the grid for the first time in June 2005. The coils are made from HTS tape supplied by European Advanced Superconductors in Hanau. Its power output would be sufficient to propel a 50m luxury motor yacht and provide all other electrical services on board. The test and improvement programme is scheduled to continue until Spring 2006, although what the company is now looking for, is a suitable opportunity to field test a prototype design in a real world ship or offshore application. Advantages include a 20 to 30 per cent weight and size saving and around 2 per cent higher efficiency. As regards cost, the main problem is the expense of the superconducting material, but this is coming down as sales increase, mainly because of its increasing use in power applications such as HTS cables which employ a manifold of HTS tape. Target markets are passenger ships and naval vessels taking advantage of lowered noise and vibration, as well as generators and large motors for offshore platforms, where weight is often at a premium. About 30 per cent of all new ships built today have electrically driven propellers including almost all new cruise ships. This is because electric drives are quieter, and cruise ships do not make long voyages across the oceans at steady speed, but call in different ports, often ones that demand complicated manoeuvring to get into and out from. The favoured configuration for such vessels is now to have motors and propellers in azimuth rotatable pods. In addition, one third of the power used on cruise ships is for cooking, lighting and luxury amenities. Future superconducting machine designs are expected to include fast running generators and motors directly driving compressor turbines without the need for gearboxes and situations requiring a high overload capacity. The company already produces very large conventional motors and generators with shafts running at up to 15000 rpm in magnetic bearing supports. The company believes routine commercial production is probably "five or six" years away. Prospective customers are already said to be enquiring about possible versions capable of delivering 20MW. To the best of our knowledge, the technology is presently commercially ahead of developments in both Japan and the US, despite multiple claims from the latter country as to having made all technical breakthroughs in the subject and having the most near to market development machines. Siemens justifies its own development efforts on the grounds of needing to keep, "One to two steps ahead of competition." Financial support was provided by the German Research Ministry. Siemens Eureka says: Big high temperature superconducting motors and generators are coming. The advantages of reduced weight and size and enhanced electrical efficiency make them an obvious way to go, now that materials advances have done away with the need to cool to liquid helium temperatures Pointers * Siemens has a running 4MVA generator on long term test * Weight and size is 20 to 30 per cent less than that of a conventional machines and efficiency is around 2 per cent higher * Overload capacity is enhanced several times over conventional designs