Friday, July 4, 2008

Cold war 'caterpillar drive' could harvest sea power

  • 02 July 2008
  • From New Scientist Print Edition. Subscribe and get 4 free issues
  • David McNeill
  • Miguel A. Quintana

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WHEN it was launched in June 1992, Yamato 1 seemed to herald the future of marine transport. With its sleek lines and glass-encased cockpit, this 166-tonne catamaran resembled a cross between a bullet train and Thunderbird 2, but its most advanced feature was hidden from view: a revolutionary electromagnetic engine that used superconducting magnets rather than propellers to drive it through the water. The same technology featured in Tom Clancy's novel The Hunt for Red October, published eight years earlier, as the "caterpillar drive" that powered the fictional Soviet stealth submarine. According to Yamato 1's Japanese builders, this vibration-free and virtually silent engine would make their vessel the forerunner of a new generation of high-speed cargo ships. And that would also bring stealthy subs like Red October a step closer.

Sadly, the tests that followed told a different story. The vessel's electromagnetic drive was extremely inefficient and needed huge amounts of electricity, generated by two large diesel engines, to provide the necessary power. This made Yamato 1 far less economical to run than a conventional vessel of similar size, and with a top speed of just 7 knots (13 kilometres per hour), what should have been an ocean greyhound turned out to be a sea slug. Eventually the tests were abandoned. Today the vessel lies almost forgotten outside the Kobe Maritime Museum in Japan.

Yet one Japanese scientist believes that the effort invested in Yamato 1 was not wasted. Minoru Takeda, a physicist at the Faculty of Maritime Sciences at Kobe University, is convinced that this apparent technological dead end is actually a crucial step on the road towards one of technology's most elusive goals: the production of cheap, non-polluting renewable energy. The trick, Takeda says, is to run the electromagnetic drive in reverse. Instead of using electrical power to move seawater, the device could use tidal currents to generate electricity. With no moving parts, such a generator should be far more reliable than a conventional underwater turbine. "The potential of this is very high," he says. "It could one day replace nuclear power in Japan."

That's quite a boast, given that nuclear plants supply about a third of the country's power needs, but Takeda is the first to admit that the technology has a long way to go. So far, his lab-based prototype can generate only a fraction of a watt of electrical power, but he is convinced he can improve its efficiency and scale it up into a practical device. He forsees a future where we will extract energy from strong tidal currents in estuaries or narrow channels using arrays of electromagnetic generators anchored to the seabed.

Dean Peterson, head of the Superconductivity Technology Center at Los Alamos National Laboratory in New Mexico, thinks Takeda may be on to something. Peterson says his preliminary assessment of this kind of device appears promising, and with superconducting wires now commercially available, "electric power applications such as this should be a reality within the next few years".

The theory underpinning Takeda's electromagnetic generator is hardly new. In the 19th century, Dutch physicist Hendrick Lorentz showed that if you place a wire horizontally in a vertical magnetic field and pass a current through it, the wire experiences a force perpendicular to both the magnetic field and the current flow. If instead of the wire you use a conducting fluid, this "Lorentz force" can be used to create a pump (see Diagram). Scientists investigated this in the 1950s as a way of moving conducting liquids such as blood or liquid metals - which are either too fragile or too corrosive to be moved using conventional pumps.

The silent service

With the cold war in full swing, naval engineers began to wonder whether the phenomenon could be harnessed to build a "magnetohydrodynamic" (MHD) drive for warships and submarines. The idea was that seawater would enter a cylindrical duct running through the hull. Then the interaction between a vertical magnetic field and an electric current passed horizontally through the water in the duct would push the water out of the stern, generating thrust. Since propellers become inefficient at high speed, a vessel with an electromagnetic engine would be able to offer revolutionary performance.

"An electromagnetic engine would be able to offer revolutionary performance"

Detailed studies during the 1960s and 1970s seemed to confirm this theory. A team in Japan calculated that an electromagnetic drive could propel a 9000-tonne submarine at up to 60 knots (111 kilometres per hour), with an overall efficiency of 60 per cent. In addition, Stewart Way at Westinghouse Electric Corporation in Pennsylvania calculated that such a vessel could cruise at 29 knots with a propulsion efficiency of over 80 per cent.

Soon, however, some snags became apparent. Engineers realised that to reach these performance levels they would require magnetic fields of 10 tesla or more. At the time, the magnets capable of generating such intense fields were far too heavy to make this practical, and research was abandoned when it became clear that such a vessel would sink under the weight of its engine.

In the 1980s, optimism returned with the discovery of "high-temperature" superconductors. Though these materials still have to be cooled to low temperatures to function, it suddenly became feasible to make superconducting wires that could be used to build compact electromagnets capable of generating large fields. Researchers in the US, the then Soviet Union and Japan began to develop prototype drives, culminating in the launch of Yamato 1 in 1992.

Costing ¥5.5 billion ($52 million) and taking six years to build, Yamato 1 used two thrusters, each with six ducts. Each duct contained a pair of electrodes and was encased in superconducting magnets that generated a field of 4 tesla - though these used conventional superconductors that had to be cooled to 4.2 kelvin (-269 °C) by liquid helium. The electrodes and other components of the thrusters needed a total current of over 24,000 amps to operate, which was supplied by onboard diesel-powered generators. Disappointingly, this relatively modest magnetic field meant the engines were just 1 or 2 per cent efficient. What's more, the magnets used a dipole design that could not be scaled up. "It wasn't practical for commercial propulsion," Takeda says.

During the 1990s, Takeda joined forces with engineers from Japan and China to try to up the drive's efficiency. The best way to do this, they decided, was with a design previously investigated by scientists in the Soviet Union. With a superconducting solenoid magnet creating field lines along the axis of the duct, and with the duct's outer wall acting as a cathode and a cylindrical anode running along its centre, this design generates a Lorentz force that swirls the water around the central anode. A helical wall running the length of the duct like a screw thread creates thrust by converting this rotation into an axial flow. In 2000, they published findings showing that their design could generate a magnetic field of up to 14 tesla and reach an efficiency of over 20 per cent (Cryogenics, vol 40, p 353). Given the same electrical power, a single helical thruster could generate more propulsive power than all 12 ducts used in Yamato 1.

Even so, the electromagnetic drive remained too complex and expensive for marine propulsion, so Takeda decided to change tack and try running the device in reverse. "I realised that using the helical propulsion system could lead to the production of electricity," he says. If a conducting fluid such as seawater were forced through the duct, the magnetic field should generate a small voltage across the electrodes. Given a strong tidal current, the drive would become an electricity generator.

But would it work? To find out, Takeda, along with his colleagues Xiaojun Liu and Tsukasa Kiyoshi from the Tsukuba Magnet Lab in Japan, first constructed a computer model of an MHD generator. This showed that an 11-litre-per-second flow of seawater through the helical duct in a 6-tesla magnetic field would generate up to 1.2 volts at the electrodes with a current of around 0.1 amps. To test their calculations, they constructed a prototype, pumped saltwater through it and measured the trickle of electrical current flowing out. The results, published in 2006, showed the device generating a current of less than 100 milliamps (Cryogenics, vol 46, p 362), but Takeda and his colleagues claim that it can be scaled up. A 10-fold increase in size should result in a 100-fold increase in output, Takeda says. He believes the technology could make a significant contribution to Japan's energy supply if arrays of generators are positioned in areas with strong tidal currents (see Diagram).

"A 10-fold increase in size should result in a 100-fold increase in power output"

One problem with any electricity generator sited at sea is how to get the power ashore, but Takeda has original ideas here, too: use the electricity where it is generated, to make hydrogen gas. He demonstrates this in his lab, slowly turning up the water flow through the generator until it creates enough power to electrolyse seawater in a nearby tank. We watch as bubbles of hydrogen begin to rise to the surface. The sea provides an effectively unlimited supply of hydrogen, he reminds us, and this generator could offer a sustainable source of hydrogen for fuel cells.

Super cool engine

Whether Takeda's optimism is warranted remains to be seen. Building high-power superconducting magnets is still a significant challenge, and the liquid helium needed to keep them cold is expensive and difficult to handle. However, superconducting technology is coming on in leaps and bounds. The Japanese engineering company IHI is currently developing a ship's engine that uses a bismuth-based superconducting wire cooled by liquid nitrogen. Meanwhile, Idaho-based American Superconductor has begun to build offshore wind-powered generators using magnets made from bismuth-based superconductors which are cooled by commercially available liquid-helium refrigerators, and the company's superconducting wires are being used as power transmission lines. "Superconducting wires are commercially available," Peterson says, and "advances are rapidly being made in several areas such as high-temperature superconducting tapes and transmission-cable cooling systems." If Takeda's generator makes use of these developments, it should be cheaper to construct and cost less to run.

A preliminary assessment of electricity production using MHD technology has persuaded Peterson that the technique shows promise. However, Nobuhiro Harada, an electrical engineer at Nagaoka University of Technology, Japan, is not yet convinced. Without detailed figures obtained from field tests, it is difficult to compare Takeda's device with conventional generators, he says.

Engineer Thomas Lin, a veteran of electromagnetic drive research at Pennsylvania State University in University Park, says Takeda's design looks better than its predecessors. And though Lin says it won't compete with nuclear power, he believes it could still make a valuable contribution. "It could be useful if it takes advantage of natural ocean currents, tides or elevation drops, much like a hydroelectric power plant."

Takeda's electromagnetic generator also has several advantages over other renewable-energy generation systems, one of them being robustness. The sea is a harsh environment for machinery, where strong currents and prolonged exposure to the corrosive saltwater can wreak havoc. Last year, for instance, a test of tidal turbines in New York's East river ran into trouble when strong currents broke the blades and damaged their anchoring bolts. Like ships' propellers, underwater turbines can also suffer from another problem called cavitation, which causes pitting of the blades when they turn at high speed. This also reduces their efficiency. With few moving parts, Takeda's generator should be more hard-wearing, and will also pose less risk to sea creatures.

Takeda and two assistants are continuing to work on the MHD generator in a cramped building next to Kobe's docks. Takeda says he would like to build a full-size drive and deploy it in shallow water off the coast. Yet even with fuel prices at record levels, he has been unable to secure the funding he needs to pull this off. Poor links between Japan's private and public sectors are to blame, he says. Most of his limited funding comes from the Ministry of Education, which usually backs basic rather than applied research. He says his work does not fit the framework of research targeted for funding by the ministry, and that he had been too busy to look for private money himself. "I'm a poor salesman," he admits.

The civil servants who hold the purse strings say they are merely treating Takeda's device on its merits. Several ministry officials involved in science and technology-related projects, who requested anonymity, told New Scientist that Takeda's proposal sounds promising, but that they have yet to be convinced it deserves more money. "It lacks persuasiveness as far as sustainability is concerned," said one.

Takeda rejects the idea of seeking foreign investment. "I'd prefer to hold out for funding in Japan," he says. An international partnership could cause problems later on when the Japanese authorities eventually see the light, he says. "Since we're determined to do all we can, we carry on with our research with limited funds, step by step."

So what's next for Takeda's generator? The good news is that it has now been selected for additional funding from the Ministry of Education and, in June, the Japanese government announced plans to cut the nation's greenhouse gas emissions by up to 80 per cent by 2050. This could be just the spur needed for renewable-energy projects like Takeda's. For now, he has even started to apply for private investment to get his design out of the lab. If Takeda succeeds, he could soon be conducting trials in the powerful currents off Kobe, not far from its maritime museum and the sleek craft that languishes outside.

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David McNeill is a freelance writer. Miguel A. Quintana is a researcher and translator. They are both based in Tokyo


From issue 2663 of New Scientist magazine, 02 July 2008, page 40-43

Caterpillar drive

Yamato 1 may not have been a success, but that doesn't mean engineers have given up on the electromagnetic drive.

Since 1996, researchers at the Institute of Electrical Engineering at the Chinese Academy of Sciences in Beijing have been developing their own superconducting ship-propulsion technology. By 2000, the researchers had built and tested a helical drive in a 3-metre-long boat called HEMS-1. During one month of trials in a test tank, the researchers fine-tuned the drive's shape and magnetic field to optimise its thrust. HEMS-1 eventually reached a speed of about 2 knots(3.6 kilometres per hour).

Yan Peng, the engineer in charge of the project, says several key challenges remain, including the need for more powerful "high-temperature" superconducting magnets and corrosion-proof electrodes. Peng says her team plans to tackle these problems soon. "We are now designing a new lab, to build a bigger magnet that should work at higher efficiency," she says. The facility should be finished next year. So is the idea of building a working magnetohydrodynamic (MHD) ship realistic? "Yes, I think so," she says.

Dean Peterson, who runs the Superconductivity Technology Center at Los Alamos National Laboratory in New Mexico, agrees. He has a similar list of challenges to solve: as well as large magnets cooled by liquid nitrogen rather than liquid helium, he says he needs support structures capable of withstanding the very high Lorentz forces that these magnets will generate. His research suggests that new thruster designs offer the possibility of overall efficiencies greater than 50 per cent at speeds of 40 knots (75 kilometres an hour) or higher.

"Even higher efficiencies could be achieved for low-speed movement of cargo at lower magnetic fields," he says. "MHD propulsion is challenging and not presently practical. However, advances are rapidly being made."

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