The Lure of the Electric Drive

By Stuart C. Karon and Dr. Swarn Kalsi, American Superconductor

In the September 2002 issue of Maritime Reporter and Engineering News, a detailed discussion was featured on the advantages of future High Temperature Superconductor (HTS) machinery for propulsion of electric drive ships. Since then, development of HTS ship propulsion motors has moved ahead — at a rapid pace — specifically in three areas:

• Motors A 5-MW 230-rpm machine for the U.S. Navy's Office of Naval Research (ONR) is undergoing factory tests. ONR is now funding the next step - design, fabrication and factory testing of a 36.5-MW 120-rpm machine to prove the technology at full-scale for warships.

• Generators Development of HTS generators has also leapt ahead with the initial production of 10-MVAR synchronous condensers for utility voltage adjusting service. These condensers are very similar in design and operation to 10-MW, 1,800-rpm generators.

• Very large systems HTS technology is particularly well-suited to high torque marine propulsion systems, and the small size, light weight, high efficiency and inherent quietness offer unique solutions to propulsion of very fast and very large ships.

While diagnostic devices using superconducting magnets are common in hospitals and some industrial applications, virtually all rely on low temperature superconductor wire that must be maintained within a few degrees of absolute zero to function. Commercially available high temperature superconducting wire, developed over the past decade at American Superconductor Corporation (AMSC), achieves superconductivity at temperatures up to 100 degrees above absolute zero, in the vicinity of the temperature of liquid nitrogen. While very cold by human standards, this temperature is easy and inexpensive to achieve and maintain. Commercially available refrigerator systems and straightforward insulation systems combine to make HTS superconducting machinery practical and affordable in any industrial setting, including ship propulsion.

Motors

AMSC and ALSTOM Power Conversion have teamed to design and build the Navy's 5-MW 230-rpm marine propulsion motor, now undergoing factory tests at ALSTOM's facility in Rugby, U.K. (see figure 1 below)

The HTS field coils on the rotor are cooled by a small quantity of helium gas, using commercial off-the-shelf Gifford-McMahon refrigerators, that is fed to the rotor through a rotating seal at the non-driven end on the far side of the motor. The refrigerator system used in this prototype motor has several times more capacity than is required for a 5-MW motor, and was purposely designed in this manner to demonstrate the refrigeration system required for a large marine propulsion motor. (see figure 2 below).

A brushless exciter power is used to energize the HTS coils in a manner analogous to conventional synchronous motors. This same circuit is also used to quickly de-energize the coils if needed in response to a system casualty or for machine performance monitoring. The stator is liquid-cooled for maximum power density. The motor is a six pole, 4.2kV machine, and has characteristics as outlined.

Because of the very high magnetic flux densities provided by the HTS rotor coils, the stator requires no iron teeth to distribute the flux. A primary source of noise is thereby eliminated, and additional stator copper can be packed into the space formerly occupied by the stator iron teeth to further enhance the machine's power density. Other than their extraordinary advantages, AMSC's HTS motors are essentially common AC synchronous electric motors, and can therefore be driven by standard electronic motor drives. The AMSC 5-MW motor will be delivered to the Navy with an ALSTOM VDM 5000 commercial electronic drive system.

Preliminary 5-MW motor testing results confirm the motor's design basis. Efficiency is extremely high, as predicted. Other machine operating parameters are also as expected, including the performance of the HTS coils, the coil exciter, and the refrigeration system. Factory testing will be completed shortly, and HTS motors in the 5-MW size range will soon be available for commercial delivery.

Conventional motors of this size are already used in thousands of moderate-sized ships, of which more than 100 are powered by diesel electric propulsion systems. Among these ships in this size range are cruise ships, product and chemical tankers, RoRo passenger ships, cable layers and icebreakers. Each of these ship types, as well as many others, will benefit from the small size and weight of HTS motors, and any that spend a significant time operating at part load will also see a substantial gain in fuel efficiency.

The commercialization of HTS marine propulsion motors comes at a time when both naval and commercial ships are rapidly transitioning from mechanical to electric propulsion. The Navy recognizes the extraordinary capabilities of HTS technology to deliver (in small, light, quiet and affordable packages) the very large power warships require. It therefore recently conducted a competition for the design, fabrication and factory testing of a large HTS ship propulsion motor. This Navy solicitation set rigid requirements for a 36.5-MW motor's performance and physical characteristics, particularly weight.

An AMSC-led team won this competition, has proven its HTS design algorithms and technology in the Navy's 5-MW program, and is now applying its HTS design algorithms and technology to the design and fabrication of a 36.5-MW (49,000-shp) 120-rpm motor. This motor is sized for the Navy's future DD(X) class ships, and its delivery is scheduled for spring 2006. The design effort is proceeding in accordance with the proposed schedule, and the program is on track to deliver the motor on time.

Generators And Synchronous Condensers

At the same time, AMSC has begun fabrication of a prototype for a 10-MVAR SuperVAR dynamic synchronous condenser (DSC) ordered by the Tennessee Valley Authority (TVA). DSC's ensure proper VAR levels are maintained in electric power transmission and distribution systems, thereby allowing the unimpeded flow of power through the lines and lowering costs. The SuperVAR machines are similar to 10-MW 1,800-rpm generators, and thus the production of these TVA machines is setting the stage for the future commercialization of HTS generators, both in marine and land-based applications. Like their motor cousins, HTS generators are smaller, lighter, and more efficient than conventional generators.

For example, a 36-MW marine generator and its support equipment will weigh about 40 tons and will measure less than 6.5 ft. (2 m) in diameter and less than 13 ft. (4 m) in length overall. Coupled with a GE LM 6000 or Rolls Royce MT-30 gas turbine (GT) at about 22 tons, a combined GT generator set will weigh only slightly more than 60 tons. These lighter and compact GT generator sets can now be located in a ship's superstructure, thereby minimizing the space requirements for the engine room and GT ducting.

Brian Ackerman, a marine propulsion consultant, recently evaluated this concept. He studied nine different diesel-direct drive container ships for the impact of a deckhouse GT generator set electric drive using conventional (and not HTS) generators. He estimated that the GT deckhouse system would make room for 4-16.4 percent more containers in the nearly vacant engine room, depending on the size of the ships. Ackerman further estimated that the added revenue from the container increase would pay back the greater first cost of the GT electric-drive and it's more expensive fuel in an average of 2.7 years, again depending on the specific ship. Ackerman noted that conventional generators control the weight of the GT generator sets and sometimes lead to arrangement problems at the higher ratings. With HTS generators, such difficulties will be ameliorated.

Very Large Systems

Ackerman's study also assumed electric pod drive in his analyses, which also maximized the revenue space made available in the former engine room. What he did not discuss was the difficulty that conventional electric drive or even direct diesel drive faces at very high power — the larger ships in his study need up to 77-MW of propulsion power, preferably in a single shaft for simplicity. This much power is not available with today's electric drive technology and is also problematic with direct drive diesels, currently the most common propulsion system in modern container ships. A recent issue of Marine Engineers Review suggests that direct-drive diesels may be reaching their practical size limit because of the vibration, heat stress and ship hogging they cause. Here again, HTS motors offer a solution. Because of the high flux densities and gap shear stresses generated by the superconducting field coils, HTS motors follow fundamentally different scaling laws than do conventional motors. HTS motors scale according to a 1/5 (0.2) power rule instead of the conventional 1/3 (0.3) power rule. This means that very high horsepower HTS motors are not much bigger than more moderate-sized HTS motors. For example, a 25-MW 120-rpm will be only 1.4 times dimensionally larger than AMSC's 5-MW HTS motor, despite having five times the power and ten times the torque of the smaller machine. Replacing the diesel with a gas turbine will remove the design limitations created by the diesel engine's great size and weight.

HTS technology also greatly improves today's electric ship propulsion systems. Consider the conversion to electric propulsion of the Queen Elizabeth 2 in 1987. The electric propulsion modification designers were constrained by the existing engine room layout and volume in which they could install, leading to the selection of two 44-MW 150-rpm synchronous pancake motors. Each weights about 400 cu. tons and measures about 29.5 ft. (9 m) in width and height and about 20 ft. (6 m) in length. Each motor is so large, and the ship's installation restrictions so severe, that each was built, delivered and installed in quarters — two half stators and two half rotors. The motors and the other ship's electricity needs are met by nine 10.5-MW medium speed conventional diesel generator sets with a total weight of more than 2,000 tons.

Conversion of the QE2 to electric drive was a remarkable achievement. The installation has worked well and the ship continues to operate today as the premier transoceanic cruise ship of our time. But what would have been possible if HTS technology were available in 1987? An HTS 36.5-MW motor, developing the identical torque of the QE2 motors, 2.1 million ft.-lbs., would be very much smaller and lighter.

The HTS motor would have been far easier to ship to the construction site and install. Further, because the HTS motor is an AC synchronous machine, it can be driven by any conventional synchronous drive, including the technology used on the QE2. Nothing new is needed here.

As to the nine diesel generators, three 36-MW GT HTS generator sets would provide more power, would weigh far less at about 210 tons, and could have been mounted in the superstructure to open up vast areas in the engine room for productive revenue-generating purposes.

Water jet drive motors for large, ultra-high speed ships are another attractive target for HTS technology. HTS GT generator sets located in the ship's superstructure and HTS electric water jet drive motors below may be an enabling combination for such ships.

HTS motor and generator commercialization is occurring at a rapid rate, prompted by the Navy's marine propulsion motor development contracts and TVA's order of five SuperVAR dynamic synchronous condensers, both involving AMSC. Earlier predictions of significant size and weight benefits, as well as greater efficiency, are being validated through these efforts. Further, no technology shortfalls have been revealed and none are foreseen.

The space, weight, efficiency and quietness advantages of HTS technology can translate directly into unique propulsion system arrangements and more revenue producing space in future passenger and cargo ships, as well as spawn the development of new ship designs of novel hull forms. The benefits to the maritime industry will be profound.

About the Authors

Stuart C. Karon is Director, Government Programs and Director, Business Development for the SuperMachines Business Unit at American Superconductor. Formerly a U.S. Navy Officer and subsequently marketing & sales director in the commercial sector, Stuart is responsible for structuring effective government-funded R&D and system design/development programs, and for bringing about the business relationships necessary for the American Superconductor Corporation SuperMachines Business Unit to achieve its objectives. For more information, you can reach him at [email protected].

Dr. Swarn Kalsi is Director of Advanced Design for the SuperMachines Business Unit at American Superconductor. He has more than 35 years of directly related experience in all aspects of superconducting magnet technology and electrical engineering.