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Variable Speed AC generators and DC transmission in marine

Variable Speed AC generators and DC transmission in marine

Variable Speed AC generators and DC transmission in marine

There seems to be a small trend lately in potential future diesel electric propulsion platforms.  I first noticed it in a few whitepapers out there.  Then Caldonian Maritime moved towards it with their hybrid propulsion architecture (see pg 14, 18, & 19 of ppt):


But then these two major companies made the following announcements at almost the same time with a full DC grid and variable speed AC generators:



While Siemens has Østensjø Rederi signed up for an OSV, I like the ABB system better even though no word yet of a vessel to be fitted with it.  Siemens DC bus is only part of a switchboard lineup while ABB envisions an actual DC transmission network.

I even noticed that Washington State Ferries in the US may be interested in this type of hybrid system for their Hyak ferry with this Request For Information (RFI):


I believe wind power has been using something similar for a few years.  But, what is everyone's opinion on this development.  The potential fuel savings appears to be significant.  Any foreseen problems?  Big names like Siemens and ABB must carry a lot of weight, right?

RE: Variable Speed AC generators and DC transmission in marine

Electric transmission for propulsion has been around for a long time. Early steam turbines need to turn at high rpm while the propeller has to turn at a much lower speed. Before double reduction gearing the US Navy used steam electric propulsion on battleships built during the late WW 1 era and the most famous examples are the carriers "Lexington" and "Saratoga". Navy ships run at various speeds, low speed for extended range and high speed for operational reasons as wind-over-deck on aircraft carriers. Commercial vessels built with turbines in the 30's had double reduction gears and ran full speed at sea all the time with the system speed optimized for for best speed of turbine and propeller. During WW 2, the Navy continued with turbine-electric and diesel-electric propulsion on its destroyer escorts due to lack of gear cutting capacity. The of course there are the submarines which can use either diesel or battery power generally didn't do it at the same time. Once again for operational reasons.

Getting back to recent times, ferry boats have used DC diesel electric  propulsion since the Staten Id. ferries of the "American Legion" class of 1964. It was a cumbersome arraignment but allowed for full power either end more or less as in the direct drive reciprocating steam engines. The trouble with DC transmission is that there is an 15% energy loss compared with about 5% with all mechanical. With variable speed available for all AC transmission the energy loss is cut in half from all DC. Washington State Ferries has an variable AC transmission on the largest ferries the Jumbo mark 2's which have been in service for about 12 years now. I've noticed that all the private ferry companies run single ended ferries that are diesel drive through reduction gears and a large bow thruster to flip them end-for-end at the end of each run. They get the benefits of a "hybrid" system this way. Even the new small WSDOT just put in service are direct drive and a running change is to use a controllable pitch propeller instead do a reversing gear.

Hybrid marine propulsion is currently being tried out by Foss Marine with at least 2 tugs in service in California (where else) where they probably charging the batteries from shore power to boost the figures on fuel efficiency. All this at the expense of the grateful California taxpayer. Harbor tugs usually run at the low end of their power with only short burst of full power required  during ship docking so they can get by with undersized engines about 90% of the time and use battery boost for the rest, just like a hybrid car. In the case of the "Hyak" they need to change the controls as in the sister ships so instead of running 4 engines at 60% power they can run 3 at 80% power. The yet to be built 144 car ferries (same size as the "Hyak") are designed to use 2 3300-hp engines at each end to get the same speed. The engines were ordered ages ago and have been in storage since 2005. It turns out that they are the same model engine used on the design that is the basis for the new 58 car ferry mentioned above and have been appropriated for their construction instead. So variable speed transmission seems to be the way to go for large double ended ferries at least.

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