Superconducting coil to slash costs and improve efficiency of direct-drive wind turbines
Conventional offshore wind turbines are expensive and complicated pieces of machinery – in a large part because of their complex and maintenance-intensive gearboxes. Dr Shahriar Hossain from the University of Wollongong (UOW) in Australia is looking to slash production costs and drastically improve efficiency replacing these gearboxes with a superconducting coil.
Wind turbine gearboxes connect the low-speed shaft, which is turned by the rotation of the blades, to the high-speed shaft that drives the generator, increasing the rotational speed of the low-speed shaft from around 30-60 rpm to the rotational speed required by the generator to produce electricity – which is usually around 1,000-1,800 rpm.
To avoid the cost, maintenance and efficiency-loss problems associated with the use of gear boxes, Dr Hossain, a materials scientist from the UOW's Institute of Superconducting and Electronic Materials with funding by the Australian Research Council in 2013 under the Discovery Early Career Researcher Award (DECRA) scheme, is developing a magnesium diboride superconducting coil made from magnesium and boron that he says is very cheap and easy to manufacture and would allow wind turbines to operate with no gearbox at all.
Unlike a conduction loop made of conventional copper wire that loses about seven to 10 percent of energy due to resistance when an electric current is sent into it, a superconducting loop would have no loss of energy as it has no electrical resistance. This would allow the current to circulate indefinitely, even after the power is cut off.
When we reached out to Dr Hossain to ask about the problems surrounding the low temperatures required for the superconductors to work, he admitted this is the most challenging part of developing the system around the magnesium diboride superconducting coil.
To address the problem he plans to use off-the-shelf cryocoolers to cool the rotating components of the system in a two-stage process. The first crycooler will drop the temperature to -218° C (-360° F), while the second will then lower it further to -253° C (-424° F). Dr Hossain says that in comparison to well-established niobium-based superconductors, his magnesium diboride-based superconductors have achieved very high critical current density.
Despite this two cryocooler arrangement, Dr Hossain says it will still be cheaper than using high temperature superconductors (HTS), which can exhibit superconductivity at temperatures as high as -135° C (-211° F), but cost around AUD$25 (US$21) a meter. Dr Hossain's US industry partner, Hyper Tech Research, predicts that magnesium diboride coil will cost just $1 (US$0.85) a meter to manufacture by 2015.
Additionally, unlike niobium-based low temperature superconductors (LTS) that require increasingly pricey liquid helium to operate, Dr Hossain says the cryocooler system will run with ambient temperature helium gas supplied by compressors and entering into the rotor and returning through a rotary coupling in a closed loop.
Dr Hossain says that 10 MW-class wind turbines will require up to 200 km (124 mi) of superconducting coil to generate electricity, with each HTS-based coil costing between AUD$3 to $5 million (US$2.5 to $2.4 million) to manufacture. However, he claims that the same length of magnesium diboride superconducting coil would cost just AUD$180,000 (US$153,000), with that figure expected to drop significantly.
"Australia desperately needs sustainable energy sources," says Dr Hossain. "Wind is cheap, clean and we can get it day and night and on rainy and sunny days. And considering Australia has more than 35,000 km of coastline, there is ample room for offshore wind farms. With industry support, we could install superconducting offshore wind turbines off the coast of Australia in five years, no problem."
Dr Hossain details his work in the video below.
Source: University of Wollongong