SHAFT POWER
ADVANCES IMPROVE SHAFT GENERATOR CASE
Photo: HHI
Shaft generators can save operating costs, reduce emissions and raise a ship’s environmental profile, 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, significantly 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
12 | JANUARY 2021
8 HHI and KSOE have got together to develop a ‘direct on’ Engine Mounted Generator, the first 1.3MW model being combined with a large 24.5MW main engine onboard SK Shipping’s new 318,000dwt VLCC
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
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