13 minute read
PM shaft generators
ELECTRIFICATION RAISES PM SHAFT GENERATOR ROLE
Shaft generators are increasingly becoming part of ship electrifi cation designs, and the advantages of permanent magnet systems are being recognised
Demand for shaft generators is increasing across a wide range of vessel types, and, concerned about delivery time, Daewoo Shipbuilding & Marine Engineering (DSME) signed an MOU with Hyosung Heavy Industries in May to jointly develop domestic manufacturing capability for largecapacity permanent magnet (PM) shaft generator motors. The companies already have an arrangement for induction motors and have been supplying them to LNG, LPG and crude oil carriers. However, with the container ship market opening up, they will no longer have to rely on imported equipment for their larger power solutions. The switch to PM technology can increase fuel effi ciency by about 3%, says DSME, and is more compact.
PM generators are highly efficient over a wide range of speeds, says Emil Cerdier, Product Director at Berg Propulsion. Also, running smaller and/or fewer engines onboard reduces maintenance effort. “Variable speed gensets will have an increasing impact on fuel economy as the portion of power produced on a vessel shifts from main engines to gensets,” he says. Berg Propulsion is involved with an increasing number of vessels designed with more sophisticated power generation systems including hybrids combining diesel main engines with electric motors and batteries as well as fully electric-motor-powered propellers.
“Direct coupled diesel engines are great for constant high load, but for situations where less power is required an electric system is often more efficient,” he says. Designing vessels with this in mind can achieve fuel savings of 20% or more. “We are continuously increasing our focus on electrical integration to offer our customers complete and optimized propulsion systems. We have active systems that adapt propulsion parameters to always seek optimum efficiency. We also have in-house comparison software to evaluate alternative propulsion and power generation concepts to help our customers find the best alternative for their specific vessel and operation profile.”
Lighter, simpler and more effi cient
He cites a recent example of a bulk carrier currently being commissioned where the controllable pitch propellers are powered by PM motors mounted in two fins below the hull. The unique feature of the patent-pending design developed in partnership with The Switch is that the motor has no bearings of its own but rather relies on the ones in the shaft line. “We are including the power management system in our scope of supply, and with mode selection integrated in our control system it is easy for the captain to always operate the vessel in the most optimum way.”
Dr Jussi Puranen Head of Product Line, Electric Machines at Yaskawa Environmental Energy/The Switch, has seen a huge shift towards PM machines in marine applications over the last five years. The main reason for this has been the power conversion efficiency of the technology. Also, PM machines are mechanically simpler and thus less likely to fail. The technology allows a reduction in generator size and weight by up to 50% compared to a conventional electric machine.
PM machines can be applied as gensets, shaft generators or as propulsion motors to improve the energy efficiency of a vessel. Coupling a PM shaft generator with a frequency converter provides maximum freedom in optimizing engine and propeller efficiencies. These options can be particularly relevant for reefers or when a scrubber is being installed. Instead of adding gensets to cover the increased power demand, a greener and more feasible solution can be to demand, a greener and more feasible solution can be to upgrade a vessel that has a 2-stroke main upgrade a vessel that has a 2-stroke main engine with an in-line PM shaft generator. engine with an in-line PM shaft generator.
Puranen says that in a typical 2MW Puranen says that in a typical 2MW shaft generator application, a PM shaft generator application, a PM generator means around 50 tons generator means around 50 tons less fuel burnt annually compared less fuel burnt annually compared to conventional electrically-excited to conventional electrically-excited generator. “The neodymium magnets generator. “The neodymium magnets which are currently being used in which are currently being used in high power density megawatt-class high power density megawatt-class PM machines were invented in the PM machines were invented in the early 80s. Since then, there have early 80s. Since then, there have
8 A permanent
magnet shaft generator from The Switch
8 Dr Jussi
Puranen Head of Product Line, Electric Machines at Yaskawa Environmental Energy/The Switch
been huge improvements, for example, regarding energy density (which is already close to theoretical maximum value), temperature resistance and manufacturing methods such as grain boundary diffusion.”
Asbjørn Halsebakke, Manager Technical Solutions Marine at Yaskawa Environmental Energy / The Switch, notes that using a DC system onboard makes it much easier to integrate multiple energy sources. “I've spoken with several large shipowners, and the only thing that they can be sure of is that if they build a vessel today, they will surely have to change the way it is producing power at some point.” He agrees that future fuel choices and even power generation systems cannot be specified for certain at present and says the company's solutions are specifically designed to be future-flexible.
Redundancy and gear-less solutions
Svein Kleven, Senior Vice President - Energy & Systems Integrated Solutions at Kongsberg Maritime, is seeing interest from owners of a wide range of vessels for hybrid systems which include shaft generators, such as that installed by Tärntank for its latest chemical tanker newbuildings. The vessels' main engine is a Wärtsilä 10V31 DF. Two auxiliary Wärtsilä 8L20 engines are connected to a GESAB SCR-Catamiser for NOx reduction and waste heat recovery. A 500kWh battery system supplied by Corvus Energy continuously provides power to the system to boost propulsion, provides an alternative to running auxiliaries during narrow passages and harbour manoeuvring and allows for peak shaving. The battery system alone could power the vessel for a short time in case of blackout, but combined with the 1.5kV WE Tech shaft generator, the vessels could potentially sail at around eight knots if required during an emergency.
Going forward, Kleven sees more systems being developed without the main reduction gear often associated with a shaft generator. The gear typically has losses of 2-4%, or up to 8% at very low rpm. "That is significant, because the main reduction gear is working on the main power source of the ship, hence the main engine 2-4% saving on that amount of power is significant over a year, over a lifetime. Added to that is the reduction in installation and maintenance costs, making the proposition very interesting. "The drawback with some of the novel gear-less solutions developed up to now has been that they do not have the proper level of redundancy - if the generator breaks down, you may lose power entirely. So, we have been looking into a you may lose power entirely. So, we have been looking into a solution with a direct shaftline PM motor driven from the solution with a direct shaftline PM motor driven from the main engine without reduction gears, but without loss of main engine without reduction gears, but without loss of propulsion power in case of the PM-motor fails. With this propulsion power in case of the PM-motor fails. With this configuration you can continue the operations even in case configuration you can continue the operations even in case of the unlikely failure of an inline PM generator. This can be of the unlikely failure of an inline PM generator. This can be done essentially by ensuring the rotation of the shaftline done essentially by ensuring the rotation of the shaftline from the main engine to the propeller is continuing unfrom the main engine to the propeller is continuing uninterrupted even if the PM motor fails. This setup would interrupted even if the PM motor fails. This setup would significantly change the game, because it would give a more significantly change the game, because it would give a more reliable and more efficient solution. With Kongsberg battery reliable and more efficient solution. With Kongsberg battery solutions and energy management control systems, it will solutions and energy management control systems, it will also provide a take-me-home solution if either the main also provide a take-me-home solution if either the main engine or the generators were lost."
WE Tech is seeing increased demand from the container segment and also ropax vessels, ship types that have significant energy consumption on-voyage. Jan Backman, Sales Director of WE Tech, says shaft generators make sense in this case, as the specific fuel consumption on a main 2-stroke engine, regardless of whether if it's dual-fuel or burning new fuels such as methanol, is lower per kilowatt hour than on a four stroke genset running on part load. “However, if you need a lot of generator power input, you still you can't avoid diesel generators. So, the question is, does it make sense to invest in both the general diesel generators and the shaft generator? Many times, the answer 'yes' because the payback time of the shaft generator, specifically a PM system, is relatively short.”
To DC or not to DC, that is the question
Backman notes the benefits of a DC power distribution system: “DC simplifies the electrical grid onboard, because you don't need to add the large consumers into the main switchboard. Instead, a dedicated DC converter provides the power to the large consumers. In the case of a bow thruster,
8 The 26,000dwt
SUL (Self Un Loader) Bulker is under construction at CSSC Chengxi Shipyard
8 Far left:
Jan Backman, Sales Director of WE Tech Solutions and Svein Kleven, Senior Vice President - Energy & Systems Integrated Solutions at Kongsberg Maritime
for example, this can reduce the power spike when it is turned on.” Additionally, the energy storage system can act as a peak shaver and power reserve directly in the DC circuit, which allows the complete electrical system to be run more efficiently than conventionally.
WE Tech's shaft generators feature variable frequency drive technology, variable speed generator technology, DClink switchboards with dedicated inverter units and a power management system. With the DC-link distributing electrical power, energy efficiency can be increased by up to 35%, the main switchboard can have a smaller footprint with less copper used. Total harmonic distortion is low as is the reactive current flow in the electrical system. This enables the use of smaller inverters and less cabling.
Backman is keen to team the company's PM motors with variable speed gensets if engine manufacturers make them available. “It would suit our technology very well to modulate that variable frequency, and it would increase the efficiency a lot in existing ships if they could upgrade their existing gensets without large modifications.” mtu recently published a white paper on the potential advantages of its Series 4000 variable speed gensets, saying that they can reduce fuel consumption by up to 15% and increase time between overhauls by 20%. Ramp up times of 11 seconds have been achieved in both factor acceptance tests and real operation. mtu says power density is increased in less installation space. With its variable speed mtu gensets, Rolls-Royce says it is able to create customised driving curves according to specific customer requirements through simulation testing. This includes extreme operating conditions as well as any specific safety needs. parallel operation of different power producers and consumers. Synchronization is not required in this case since DC systems do not require any phase or frequency synchronization. Our power management system for isolated DC grids is standard software similar to electric or hybrid propulsion.”
So far, shaft generators are mostly used in power take-off (PTO) mode, where electrical machines run in a generator mode and feed power to a vessel's auxiliary AC network. In most cases, this requires accurate control of the fixed speed main engine. However, Fedorov says the next generation of shaft generators will be shaft generator systems with broader functionality. These systems will utilize different power control modes, including new power sources such as batteries, fuel cells and solar panels. In addition to PTO functionality, power take-in (PTI) mode will also be widely used. In this mode, the shaft generator system will feed power from the auxiliary grid to the propulsion, while the shaft machine will act as a motor and rotate the propeller. Vessels will obtain emission-free operational capability when batteries or other clean power sources are used to run the shaft machines.
New generations of PM motors
Fedorov says PTO and power take-in PTI modes require new technologies. “Traditional asynchronous and synchronous generators are not perfectly suited for dual operation or other shaft generator control modes. PM machines are the most suitable technology for new generation shaft machines. Danfoss Editron produces newer generations of PM motors, called synchronous reluctance assisted PM motors. The innovative stator design and location of the magnets inside machine frames offer even higher efficiencies across the whole operational range. Additionally, the machines are extremely compact and lightweight, crucial advantages in the design of hybrid systems.”
Looking to the future, he says: “We believe that the DC grid concept will be the leading topology for marine hybrids. To maintain high Energy Efficiency Design Index in new vessels and retrofits, we believe that the diesel engines should be removed from the propeller shafts, with electric machines running the propellers instead. In this scenario, various power sources and control modes can be implemented into current vessel designs and future upgrade projects.”
Bearing down on noise emissions
Noise emissions are be lower, with a comparison of surface noise (sound pressure level) at a power of 500kW (about 20 percent load) showing a 6dB reduction. “At first glance, the difference of 6dB does not seem to be very significant. However, +6dB means twice the measured sound pressure, and when considering that humans perceive an increase of 6-10 dB as being nearly twice as loud, this value shows in a very impressive way the advantages of a variable speed unit,” says mtu. This is particularly important as crew members are often exposed to noise and vibration emissions over very long periods.
Philipp Fedorov, Head of Marine Sales at Danfoss Editron, says slow speed vessel operation will be much more fuelefficient with variable speed diesels, though the integration of variable speed generators will be quite demanding for auxiliary power network design. Traditional AC systems require fixed frequency and fixed machine revolutions per minute, while additional frequency converters with heavy transformers and filters also have to be implemented.
“Danfoss Editron goes the other way. We implement isolated DC grids for the variable speed generators, batteries, fuel cells and other clean energy sources. These isolated DC grids feature their own power management systems, allowing easy
8 Philipp Fedorov,
Head of Marine Sales at Danfoss Editron
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