Pictures of the Future    Spring 2006

Cruising on Cold Power

The world's first generator with a high-temperature superconductor (HTS) is designed for marine propulsion. Smaller, lighter and more powerful than conventional systems, HTS systems are suitable for use in a range of vessel types—and even on drilling platforms.

When Georg Bednorz and Alex Müller discovered the high-temperature superconductor (HTS) properties of rare-earth ceramics back in 1986, the response was ecstatic. Right away there were visions of a host of new applications, especially in the fields of power transmission and advanced electronic components. Superconductivity is the property of having zero electrical resistance; that is, being able to conduct electricity with practically no loss. It's a property found in certain substances at temperatures close to absolute zero (-273 °C). Although this phenomenon had been known for a long time, the need for a complex helium-based cooling system made its use impractical for all but very expensive equipment such as particle accelerators. By contrast, the new class of HTS substances discovered by Bednorz and Müller were superconducting at temperatures above -196 °C, making it practical to use an inexpensive liquid nitrogen-based cooling system.

Twenty years later, a research team at Siemens is developing one of the first commercial HTS systems, although it has little in common with the visions scientists had in the '80s. "Our goal is to use HTS for marine generators," explains Dr. Georg Nerowski of Automation and Drives (A&D) in Nuremberg. "If we're successful, such generators will soon be providing electricity for ship propulsion and onboard electrical systems." The principle works, and over the last six months, the world's first high-speed HTS generator has undergone extensive testing at A&D in Nuremberg (Motor Testing). The system is the size of a small car and runs at 3,600 rpm.

"We want to determine the operating characteristics of the HTS generator and gauge its reliability," says Nerowski, whose team designed the generator. This involves intentionally short-circuiting the system, for example, or letting it race with no load. So far, it has come through its endurance tests without a hitch. Although it has an output of 4 MVA—enough for thousands of households—it's much smaller than standard generators. Like its conventional counterpart, the HTS generator has a rotor within a cylindrical housing called a stator. When the rotor is turned by a drive shaft, its magnetic field generates a voltage in the coils of the stator. This electrical energy can then be harnessed.

"In the HTS generator, the rotor winding is made of HTS ceramic," Nerowski explains. When cooled to a sufficiently low temperature, the HTS winding can carry much more current than the copper wire traditionally used. As a result, the 4 MVA HTS generator's weight and volume are only about 70 % of those of its conventional equivalent. Similarly, energy losses are halved, which improves efficiency. "At these dimensions, the advantages are pretty amazing for this power range," says Bernd Wacker, project coordinator at Corporate Technology (CT) in Erlangen. "With a conventional generator, the only way to boost efficiency is to use more material and greatly increase mass and volume."

Slimline motor yachts. HTS generators are being designed primarily for use in "all-electric" ships (AES). In such vessels, the screws are not driven directly by diesel engines. Instead, the fuel is burned to power a gas turbine, which drives the generator. The power thus generated is fed via cable to smaller electric motors for the propellers, which saves space. Instead of using a massive diesel engine, the vessel is powered by a number of smaller generating units, which can be better accommodated in the ship's hull. This will mean sleeker designs for motor yachts, significantly reducing water resistance and therefore energy consumption.

Only a few of today's ships are all-electric, although this alternative type of propulsion is increasingly found in cruise liners, an area where almost every new craft is an AES. This is also because electric drives are much quieter than chugging diesels. And the electricity produced can be used to power onboard systems for passengers, with about one third of the power used for cooking, lighting and amenities. "We expect this trend to continue," says Wolfgang Rzadki, who works for Marine Solutions—part of Siemens' Industrial Solutions and Services (I&S) Group—in Hamburg. Rzadki is the man responsible for launching this technology. And there is another good reason for the system's popularity. Cruise ships and luxury yachts mostly move at a leisurely pace, stopping at many ports, with an occasional burst of speed. Since larger AES craft are equipped with three or so small electric motors per screw, an optimum number of turbines and generators can be switched on, depending on need. This is more efficient than running one large diesel engine at a reduced load. Another likely application for the new technology will be in military craft. But the diesel engine remains (at least for now) the system of choice for large container ships, because they travel at a constant speed for long periods.

Three Siemens teams—from A&D, I&S and CT—worked on the development of the HTS generator. In addition, researchers at CT also came up with a new cooling system that can chill the high-speed rotor to -248 °C. Their ingenious solution uses the generator shaft as a tunnel for the cooling agent—the noble gas neon. The gas is first liquefied in two bucket-sized containers and then flows via the hollow shaft to the rotor, thus cooling it to the requisite operating temperature. As the neon becomes warmer, it reverts to a gas and flows back via the same route to the cooling chamber. "The neon flows back and forth fully automatically," Wacker explains. "No pumps are needed, and the system is hermetically sealed and maintenance-free." This is ideal for a marine application, where simplicity of design increases reliability. And even a short downtime of the cooling unit wouldn't affect overall operation, since the system has a large thermal reserve and the rotor is well insulated.

In addition to marine propulsion, researchers are also looking at other potential uses. These include a compact unit comprising a fuel tank, turbine and generator that could be packed into a container and used to generate power on a remote island, for example, or on container ships or drilling platforms. It would also be possible to combine the unit with the powerful, lightweight generators used in wind-power plants.

Naturally, an HTS generator requires a high-quality superconductor. Here, Siemens is working with Hanau, Germany-based European Advanced Superconductors, which can supply the HTS wire in the requisite quality and length. According to Managing Director Burkhard Prause, large HTS generators will grow in importance in coming years. "They provide savings on two fronts," Prause explains. "First, they use much less primary energy as they're more efficient and flexible than conventional generators; and second, they can be up to 75 % smaller, which means savings on raw materials such as copper and iron."

First of its Kind. But there's still work to do before an HTS generator can go into commercial operation. To begin with, the system must undergo additional testing, so the research engineers can get to know its idiosyncrasies, especially under simulated operating conditions. "In some areas, we're still involved in basic research," Nerowski explains. "After all, this is the first generator of its kind." To ensure that the system is suitable for marine engineering purposes, engineers are also working with Germanischer Lloyd (GL) in Hamburg, which determines and verifies standards for shipping. The generator will have to withstand a range of adverse conditions, including the effects of seawater, having to operate at a tilt, and taking on jolts and vibrations as a ship's hull plunges into foaming seas.

According to Rzadki, it will be five to ten years before a commercial product is available. "Sure, a 4 MVA generator can propel a handsome motor yacht," he says, "but larger ships require a system with 30 to 40 times as much torque." Nevertheless, Rzadki is confident that this can be achieved. "It takes time to develop, optimize and dimension a completely new system, but there's no doubt that we'll get there."
                                                                                                               Tim Schröder