18 minute read
PM EMG variant
ADVANCES IMPROVE SHAFT GENERATOR CASE
Shaft generators can save operating costs, reduce emissions and raise a ship’s environmental profi le, but they require more thought than ever writes Stevie Knight
As Jussi Puranen of The Switch explains, “although auxiliary engines are still needed as a backup, using a shaft generator means that these can stay idle most of the time, signifi cantly reducing OPEX costs on fuel and genset maintenance”.
It mostly comes down to two-stroke’s specific fuel-oil consumption (SFOC) being lower than four-stroke auxiliaries but Puranen adds: “Conventional gensets run at full-speed - regardless of the power demand - to maintain a constant grid frequency, while an SG system can operate in variable speed mode.”
While the biggest fuel savings are typically achieved by vessels moving at less than a flat out pace, the focus also needs to cover those (sometimes lengthy) periods when the ship is in busy waters or on the berth approach. This can present a few challenges - though it isn’t about the SG itself, says Puranen: that “can typically operate even near zero speed, but main engines typically have much narrower range”.
The parameters take some consideration, says Michael Kranz of Wärtsilä, that is, “which engine speeds will deliver what power”. While for most operational profiles a range down to 70% of the maximum rpm is broad enough, occasionally the requirement goes as low as 50%. That is possible for the shaft generator - for a price. “The physics
Photo: HHI
makes the difference: if you want to have the same power from a lower speed, you need higher torque; this requires a bigger machine”, says Kranz, “although that can kill your business case.” And, he adds, it might still be unfeasible for the main engine itself.
Further, there is a small, nubby issue “that’s often overlooked” says Jan Backman of WE Tech. “While a hotel load of a few hundred kilowatts - as you’d find on, say, an oil tanker - can be met from normal engine margins, that’s not the case for something like a big container ship providing reefer slots.” Puranen adds “in practice” it can often mean pairing with a 15% larger main engine.
For the new, emissions sensitive ships, shaft generation is also interesting because it can produce electricity from a fuel for which there isn’t, yet, a DF auxiliary explains Backman. For example, WE Tech is supplying SGs to a couple of new Geogas Maritime LPG carriers to be delivered this spring: these SGs allow LPG to cover all the electrical load in transit, limiting the diesel gensets to harbour and manoeuvring operations.
ADDED EXTRAS
While the standard solution generates power for the grid, it’s not such a big jump to consider reversing the direction and supporting the main engine with the electric drive. “Using a
8 HHI and KSOE
have got together to develop a ‘direct on’ Engine Mounted Generator, the fi rst 1.3MW model being combined with a large 24.5MW main engine onboard SK Shipping’s new 318,000dwt VLCC
shaft generator as a booster motor for peak shaving can mode,” he says, “though you have to get into the relative
allow you to select a smaller main engine,” says Puranen - a declutching of the engine and easy maintenance.
step down in engine size, rather than up.
It’s a fairly straightforward argument. He continues: “If 99% of the time the necessary propulsion power is under 90% of the maximum, it makes sense to produce that remaining 10% of the power by putting a shaft generator into reverse, using it as a boost motor for moving against a heavy head-wind or through ice.”
Of course, this power reversal means firing up the gensets or relying on a battery installation to feed the shaft generator.
Further, there’s ‘take-me-home’ mode, where if there’s a problem with the main engine, it’s possible to leave the shaft generator to drive the propeller. It’s a much slower, lowpower transit, but it can get the ship back to safety.
The advantages also include better energy handling.
“Onboard electrical demand has been decreasing, the lights are modern, and the fans and other systems now have speed control,” says Kranz. So, it’s now primarily thruster draw that’s holding back efficiency.
For some vessels, shaft generators could make all the difference. A broad range of ships, such as bulk and gas carriers, have a relatively large 1.5MW or 2MW thrusters, “but their hotel load is about the same size or even lower” he explains. Given this, it’s possible to switch the power from one to the other, with full speed control via the variable frequency drive.
These VFD’s have been the saving of shaft generators, adds Thomas Hartmann of DNV GL, pointing out that older versions used to employ a constant gear ratio and were often rather simply connected to the grid, but “fluctuations in propeller shaft speed created fluctuations in frequency”. This strained parallel-positioned gensets on the ship’s grid and threw up more issues when using controllable pitch propellers. As a result, VFDs are now pretty much central to most modern installations.
They have also helped free up the dimensions of other systems. “Sometimes you might want to invest in a slightly bigger SG than you need for everyday use, just to cover the demand,” says Kranz. Either “it means you don’t have to install oversized and expensive gensets for the sake of running your thrusters just for, say, thirty minutes once a week”, or alternatively, it could allow for broader utilisation of controllable pitch propellers, “as some ships can take most of the berth approach on SG’s and CPP’s in combinatory merits during the design phase”.
FOUR-STROKE
The most obvious home for SGs may be the big two-stroke ships - but they’re a long way from being the only application: LNG carriers have usefully paired SG’s with four-stroke DF engines burning boil-off gas for some time, says Kranz.
While admittedly not as efficient as their larger, low-speed cousins, four-stroke SG installations still hold a trick or two. For example, these can position cost-effective induction generators on the gearbox spur. The ‘induced’ magnetic field requires a very close fit between squirrel cage and rotor - this isn’t easily accomplished on a typical in-line two-stroke; it requires a larger airgap to accommodate shaft movement.
Further, the gearbox tie-in also allows straightforward
THRUSTERS
HYBRIDS
Even more potential opens up by pairing the SG’s with a battery... “especially as now batteries are getting much cheaper and more effective,” says Puranen.
However, there is more than one hybrid configuration. “There’s the familiar method - you buy a battery and inverter and tie it in via the main switchboard,” says Kranz. “but we saw that for a little extra money, you could extend the PTO/PTI and connect the battery to the main SG frequency converter.”
It does yield advantages. Routing the power via a DC bus to the dedicated inverters of big consumers such as bow thrusters or large compressors, “can increase efficiency by up to 35%”, says Backman, as it avoids distribution and conversion losses as well as allowing more control on start-up.
This approach can be applied to both two-stroke and fourstroke engines; Wärtsilä’s solution is currently being explored by Finnlines, (part of the Grimaldi Group) onboard the two Superstar ferries employed on Baltic crossings. Kranz explains these newbuilds will have induction SGs paired with Wärtsilä 46F main engines: this incorporates thruster management, an SG that works as a generator or motor to give an engine boost mode, plus energy storage. “They also
8 There is a broad
range of potential shaft generators confi gurations, including hybrid and four-stroke main engines
Image: Wärtsilä
asked for efficient recharging of the battery,” explains Kranz. “You can either connect to shore power, when available, or you can use the battery instead - so you don’t need to burn fuel at berth.”
AT THE OTHER END
Certainly, supplying a typical long-haul VLCC with a shaft generator is an excellent bet, as it can usually reduce fuel consumption “by around 5%” overall says Mun Hwa Jung of DNV GL Korea. Both OPEX costs and environmental savings are, potentially, even greater when it comes to dual-fuel ships.
Despite all this, physical dimensions also play a large part in determining whether SG installation is even possible, explains Byoung Hun Kwon of KSOE’s Advanced Research Center.
In fact, two-stroke in-line installations require extending the intermediate shaft by 2m or more.
As a result, Hyundai Electric & Energy Systems, KSOE and HHI have got together to develop a ‘direct on’ Engine Mounted Generator (EMG), the first 1.3MW model being combined with a large 24.5MW main engine onboard SK Shipping’s new 318,000dwt VLCC.
Both Kwon and his HHI colleague Jung Ho Son underline, the EMG’s basic electrical generation principle is “proven technology”, but its main difference lies in the compact disclike structure layout and its direct coupling with the main engine.
This has given rise to developmental challenges, says Son.
The EMG’s capacity is governed by size, but at the same time, a larger mechanism either in diameter or depth raises the static load on the crankshaft. This provides a physical limit, he explains, and so the design requires careful tailoring to the specific engine loads.
Further, it’s in the location usually occupied by a torsional vibration damper or tuning wheel. In fact, the extra mass of the EMG itself can mitigate the issue, but it needs careful integration with the engine or it stands to increase - rather than reduce - torsional vibration on the propulsion shaftline.
Given all this, the EMG’s output stability during main engine running conditions has been thoroughly tested and verified by DNV GL, HHI Group and SK Shipping.
It is a worthwhile development, despite the effort: “These features reduce the length of the installation space by 40% compared to a conventional SG,” adds Son, “with no intervening gearbox”.
The result is a net gain in tank capacity. Kwon explains that a study for a reasonably standard LPG carrier showed that exchanging the EMG for a shaft generator in a conventional engine room allowed for enlarging the cargo space, “which went up from 90,000m3 to 91,000m3, adding 1,000m3 of gas”.
Further, it promises to minimise shipyard costs. Being able to fit and test both the shaft generator and engine before delivery cuts down installation time and effort says Kwon: delivering a fully integrated unit makes the usually tricky alignment procedure far simpler and shorter.
PM MACHINES
More recently shaft generators have been evolving to embrace Permanent Magnet (PM) machines of the kind developed by The Switch and WE Tech.
While smaller-sized PM units have been around more than a century, those of the necessary megawatt-scale suitable for shipping have only been plausible given the recent developments on permanent magnet material composition, resulting in a step up in power density.
These have no need for an external energy source as the magnetic field is inherent rather than powered, explains Puranen, adding this yields a “significantly more compact and mechanically simpler, more reliable generator”. Further, the higher power density means “they’re easier to install inline where space is limited”, says Backman, adding that these generally require little-to-no maintenance.
However, “there’s no such thing as a free lunch”, says Hartmann, and Puranen admits there’s been something of a challenge to overcome. These PM machines’ magnetic field is intrinsic, so - unlike other varieties - they don’t stop working when the power is pulled.
As a result, the most efficient, in-line PM SG’s have come under extra scrutiny by the class societies. If there’s no clutch to decouple the generator “all the time the shaft is moving .... and even when there is a malfunction in the machine, it will carry on pouring energy into the fault”, says Hartmann.
Further, though ideally the SG should be disconnected inside a few minutes, “that’s just not plausible with the big in-line PM machines”, he adds.
Kranz notes that for the same reasons “when installing them in a single shaft vessel discussion is critical, as given some failure in the SG you have to stop the main engine and still fulfil the specific redundancy rules”.
“All PM machines may require some added electrical protection,” says Hartmann. However, DNV GL’s answer is to allow for a sliding scale of response. He explains: “In what’s called Class 3 redundancy, we balance the risk of failure by asking for more protection of the machine plus higher quality winding and other construction and testing requirements so that we get to a point where an internal fault is reasonably unlikely. Then we will accept that it takes longer time before the rotating magnetic flux can be stopped.”
So, PM shaft generators need care, but they’re an important part of offering. It’s worthwhile noting that when pressed, the KSOE and HHI teams told MS that a “permanent magnet machine version of the EMG” is under development. Given this, factory pre-integration with the engine stands to make a significant difference. Handling these large magnets is “not easy”, explains Hartmann; that may be an understatement: Kranz is only partly joking when he adds the unwary “might never see their toolbox again”.
8 Permanent
magnets yield a signifi cantly more compact and more reliable generator but there are particular challenges to overcome as the magnetic fi eld doesn’t disappear when the power is pulled
VERY COOL: SUPERCONDUCTORS MADE SIMPLE
Superconductors haven’t often been mentioned in relation to shipping. However, not only could the technology cut the space requirement for both onboard grid and motors, some alternative-fuel vessels might even give it a leg up, writes Stevie Knight
To use HTS “you need to get below a temperature of around 90K (-193°C)”, says Manuel La Rosa of Neutron Star Systems, but at that transition point, “a whole lot of things get far easier, just because wires lose their resistance”. And achieving this is no longer far-fetched, even for commercial vessels.
Superconductors have certainly come a long way since HK Onnes discovered the effect by cooling mercury: the next big step forward was arguably the cuprate-based YBa2Cu3O7-d (YBCO) innovation of the 90s as this meant the required temperature could be reached by, for example, liquid nitrogen cooling, giving rise to the slightly misleadingly named ‘high-temperature superconductors’ (HTS).
Now, the technology has “reached the second generation” explains Dr Markus Bauer of THEVA, a company specialising in thin, flexible tapes. While earlier wires centred on drawing and annealing a silver cylinder filled with HTS material, these later developments coat either a nickel-based or stainless steel foil with a nonreactive buffer. On top comes a micronthick superconductor layer. Cabling solutions shape it into a standard spiral form and enclose it in cooling jacket, both electrical and thermal insulation, finishing it with a metal sheath.
The whole thing is surprisingly compact, coming - on average - to between 40mm and 60mm in diameter for a typical 5MW DC line. By contrast, standard copper wiring for the same power can be five times the girth says La Rosa. Even an off-the-shelf cryocooler such as the one from Stirling isn’t that cumbersome, taking up just a few cubic metres at one end of the link.
But the reason for entertaining the idea is simple: the losses are very, very low.
In fact, according to research by Nexans, a typical 5MW (5kA,1kV) shipboard system loses just 0.56% at 20 Kelvin - and that reduces even further for lower temperatures. Put that against a medium voltage 250V DC copper wire distribution: operating with DC transformers and the system accumulates a total loss of around 5%. By comparison, the HTS is more efficient - and it works out “far less bulky overall” adds La Rosa.
A portion of these gains result from discarding the typical high-voltage grid used to mitigate the resistance across long, 30m to 300m runs. Replacing power cables with HTS between generator and bus-bar, or bus-bar and motor allows low or medium voltage operation, ditching the standard converters and transformers necessary to ramp it up and down.
According to the Nexans report, that means a 100m-long, 5MW superconducting power distribution system in operation for 300 days per year can save between 150 and 250MWh per year over a conventional DC installation.
There are few considerations - the length of the run has to be long enough to be worthwhile, probably over 30m, as the coolant retains its low temperature more effectively given greater mass. Another is that the ambient links need care if the system is to retain efficiency. Further, it requires time and energy for cool down - “hours or possibly a day” says Bauer, though because of its light operational load, that might just mean the system is kept running during shorter port stays.
What makes this of particular interest to shipping is that there’s a neat tie-in with alternative power. There are already a handful of fuel cell vessels under build, including cargo carriers running LNG - and some designs are looking at incorporating liquefied hydrogen.
Further, cruise ships have a large, fairly continuous onboard power draw - often over 40MW. “A number of cruise operators are investigating the possibility of fitting fuel cells,”
8 Transition temperatures have changed over time, taking off
in the 90s
8 High-temperature
superconductor (HTS) tapes
says La Rosa, but tight space and distribution over hundreds of metres can present a challenge, so it’s possible that HTS could provide a solution.
The most significant point is these fuels are themselves cryogenically stored. For LNG-propelled ships, the tank temperature is already more than halfway to the HTS operating temperature, so the liquefied gas can be used for precooling the nitrogen, allowing a smaller unit for the last step down. Further, with liquefied hydrogen “cooling comes for free as the H2 has to be warmed up anyway before use”, explains Bauer.
Most importantly, HTS tape or wire can also replace copper windings and permanent magnets, doubling the magnetic field’s power and allowing the development of more compact motors. However, while the current rises with decreasing temperatures, it also falls in relation to increases in the magnetic field, says La Rosa. In cables that field is low, therefore temperatures where nitrogen is liquid (65 to 77 Kelvin) are sufficient. Motors, with their magnetic fields, demand a lower temperature to achieve a reasonable current, so neon or helium gas will probably be necessary to bring the temperature down to 20 or 30 Kelvin.
But as Bauer underlines once reached, maintaining this temperature is much easier than might usually be expected. As he explains, replacing a motor’s traditional copper with HTS means “there’s virtually no resistive heat during operation... so only very low cooling power is required”.
Moreover, overall performance is also considerably enhanced since a fall in resistance is accompanied by a drop in reactance, allowing for improved stability across transient conditions.
It has to be said, this is not exactly new technology. Nearly two decades ago, Kawasaki came up with a 1MW, HTS pod motor and followed this with a 3MW variable speed unit which returned tested efficiencies of 98%. Siemens has also demonstrated a 4MW motor, but AMSC and Northrop Grumman’s 36.5MW machine for the US Navy in 2016 has been a massive jump in scale. La Rosa points out that the total system came in less than half the size of the conventional version... and a third of the weight.
Despite warship utilisation, merchant marine take up “has been comparatively slow” says La Rosa, although there’s headway in renewable energy applications: THEVA recently helped apply a 3MW generator to a wind turbine.
But as La Rosa and Bauer both explain, the markets are changing. There’s the rise of cryogenic fuels, and HTS systems are themselves developing.
So far, most HTS motors are divided into a warm and a cold side. This usually positions the superconducting field windings on the rotor, while retaining conventional copper on the stator.
La Rosa explains that the reduced mass of the HTS element results in less time and energy for cool-down. A disadvantage is that it requires connecting components at very different temperatures. Bauer explains it needs a cryostat to keep the boxed in, turning rotor at the right temperature - slightly tricky, but do-able. It also requires vacuum and superinsulation between warm and cool sides, though La Rosa adds this configuration is still useful for larger applications.
While bringing both rotor and stator windings to cryo temperatures might create an even lighter machine, significant AC and cooling losses means “there is a long way to go till it makes economic sense”, says Bauer.
However, all these options still have a common element that limits the achievable flux explains Bauer. If the iron teeth supporting the winding reach magnetic saturation, it leads to a rapid temperature rise and corresponding losses. As a result, while this more conventional stator still lends some efficiency improvements, the reduction in scale is limited to about half that of a conventional machine.
A further, completely new development sees the stator windings wrapped around a non-magnetic core such as aluminium or GRP.
This promises further advantages. While it demands more HTS material - and makes both heat and torque transfer more complex as it means getting rid of a solid lump of metal - this design avoids the vibration and harmonic field distortions generated by the iron teeth. Most importantly, it could also increase flux density four or five times, reducing the motor’s scale for the same output.
There are challenges ahead: there’s still a lack of familiarity and there’s no ready-made modelling solution to show how they’d perform as part of an onboard power plant... as yet. But while the superconductor chemistry itself is still fairly expensive, “it’s coming down” says Bauer. Production volume is rising with demand from other more exotic sectors, including ‘small’ nuclear fusion reactors and the space industry “so for the same capacity the price will eventually be comparable to copper”, he predicts.
8 Cable section for
high-temperature superconductor (HTS) cables
8 Superconductor
