Saturday, October 26, 2013

Hybrid Propulsion

The amphibious ship Makin Island, LHD-8, differs from her sisters in having a hybrid engine system consisting of both a pair of GE LM2500 gas turbines and a pair of electromotors which draw their power from six diesel generators.  Propulsion is split between the gas turbines and the diesel generators.  At slow speeds, less than 12 kts or so, the diesel electric propulsion is used and allows significant fuel savings.  At higher speeds, the gas turbines can operate near their peak efficiency.  The Navy estimates fuel savings of $250M over the life of the ship.

By comparison, the Makin Island’s sister ships and many other conventional Navy ships use standard steam turbines.  Smaller ships such as the Burkes use only gas turbines.

Eliminating steam boilers allows for faster startup and reduced manning requirements, among other benefits.  Additionally, the gas turbines use the same fuel, JP-5, that aircraft use, thus simplifying the fuel storage situation.

Note that there is nothing inherently new about hybrid engine technology, gas turbines, or electric drives.  I assume that the motivation for the Navy’s switch to hybrid propulsion is due to a desire for fuel cost savings.  I hope that we aren’t giving up combat performance to obtain cost savings.  I don’t know enough about the propulsion technology, new or old, to be able to assess any impact on combat capabilities.

The America class, LHA-6, will also be fitted with a hybrid propulsion system and it would seem that all future amphibious ships will have some variation of this technology.

Assuming that there are no combat drawbacks to the propulsion arrangement, this would seem to be a reasonable development.  It’s good to see the Navy do something right!


  1. It is also worth noting that an increasing number of commercial ships are also going to hybrid electric drive systems. This is being driven by a number of factors. One is that the Electrical propulsion motors have gotten very good over the past 2 decades. In addition, the generator sets have gotten increasingly more efficient and reliable driven by the wide varieties of industries they are now being used in from data centers, commercial power generation, etc.

    For ships, the systems are fairly nice because it get rid of all the big mechanical linkages and replaces them with much more reliable and as efficient if not better electrical linkages. It also allows you to isolate the power generation from the propulsion drive. This means that you flexibility is greatly increased vs mechanical systems. Any combination of generators can be used to power the propulsion drive. It also allows the engines to run in their most efficient and reliable profiles.

    Another point that is driving the transition to hybrid electric is the increasing electrical loads on ships as well. By having the complete propulsion system being completely electrical, it allows power to be dynamically allocated where it needs to be at any given time. Critically for military ships, it also allows for APU/non-propulsion generators to be used in damage situations to provide power to propulsion.

    Another major advantage, is that by removing the mechanical linkages, it allows the generators to be placed anywhere on the ship with minimal impact. This is actually one of the major bullet points behind the US Tango Bravo Navy/DARPA submarine project: remove the mechanical linkages between the nuclear reactor and the propulsion which will allow for significant size reductions and layout options that will reduce costs. The same space and layout efficiencies also apply to surface ships using hybrid propulsion.

    Going forward it has significant advantages for the destroyer/cruiser class vessels as their overall power requirements are going significantly up. Where as with the current designs, you would basically need 2x LM2500 just to have redundancy for the radar/electrical systems AND 2x2 LM2500 for propulsion redundancy, with a hybrid electric system, you can go to a simple N+1 system. This provides for the real possibility to both completely remove 1 or more engines in the vessel and fit them pretty much anywhere where they will fit.

    Going forward, I would be very surprised to see new large scale combatants designed without hybrid electric propulsion systems. One needs only look at the Type 26 and the Zumwalt Class.

    1. It appears to be a significant success story (one of the Navy's few!) and the interesting aspect of it is that it's based on existing, indeed, old technology rather than a failed attempt at Star Wars technology. I bet there's a lesson in there, somewhere!

    2. The basic idea isn't new, but the underlying manufacturing and electronics are fairly new technology. Just like the idea of an electric powered submarine isn't new while the Tango Bravo electric powered submarine is very much new technology.

      The propulsion system for the Zumwalt-class couldn't of been produced 50 years ago for instance but that doesn't mean they didn't have IEP designs back then, they just weren't really viable with the past technology.

  2. A detailed write up of the fuel savings enabled by the hybrid electric drive of Makin Island, it appears to have been a total success.

    Naval analyst Ron O’Rourke said adding hybrid electric drive to the DDG-51′s mechanical-drive propulsion could reduce fuel use by about 16%. This would reduce the ship’s annual operating (steaming) cost from $15.7 million to about $13.2 million,

    The Philadelphia laboratory of the Naval Surface Warfare Center’s Carderock Division tested a prototype hybrid electric-drive system for the DDG51 developed by General Atomics and DRS Technologies. Under the 2009 contract GA and DRS aimed for significant fuel savings, up to 12,000 barrels of fuel per year per ship (492,000 gallons). DRS Technologies completed assembly and component testing of a hybrid electric-drive permanent magnet motor in 2010 planned for backfit to the Burke destroyers already in service. The hybrid electric system was to be integrated with the conventional mechanical drive components, with a DRS permanent magnet motor, motor drive, power converter, and system control electronics provided by General Atomics.

    Northrop Grumman also developed a HED for the DDG51 under contract with Curtiss-Wright and L3
    The Curtiss-Wright EMD electrical machine to the main reduction gear (MRG),it can act as a generator or as a motor. The LM2500 gas turbines are also connected to the electrical machine. A L-3 PPI Bi-Directional Power Converter links the electrical machine to the RR Allison gasturbine generators, and to ship electrical systems. Power can flow in, to drive the screw shaft. It can also flow out, to other parts of the ship e.g. the radar.

    The system operates in 2 modes. The first, Electric Propulsion System (EPS), propels the ship using power from 2 Ship’s Service Gas Turbine Generators, instead of the LM2500s. At higher speeds, the ship operates in Propulsion-Derived Ship’s Service (PDSS) Power Generation mode. In that mode, it can shut 1 Ship’s Service Gas Turbine Generator down, while excess power from the LM2500s is directed to combat systems and other ship needs.

    It appears nothing has come of these HED's, it was also rejected for the Burke Flight 111. To supply the extra electric power required by the AMDR it is more of the same, with the 3 RR Allison gas turbine generators to be uprated. The HED would have given the Burke extra range, saved on operating costs and the flexibility for the installation of the electromagnetic railgun if it comes to fruition.

  3. Saving Fuel saves money
    But saving fuel also saves fuel.

    If I told you a conventional ship can spend 45 days before it needs a dock or a tanker, but a hybrid can spent 90 days at sea, thats a fantastic increase in combat power.
    Numbers are made up, but fleets have fought and lost battles because fuel concerns meant it was fight then or go home. In some cases, there hasnt been fuel at port even if they made it back.

    1. TrT, a very good point. Reduced fuel requirements translates to tactical flexibility and reduced logistical footprint, both highly desirable.

  4. I'm very curious about these. I always thought the old hybrid Standard BB's were very neat designs. I loved the ability to re-arrange the ship according to protection needs rather than having to design massive parts of the ship around the propulsion plant.

    Some questions though:

    A) Would it be possible to do this for CVN's? Would it give them any advantages because of power production and propulsion configuration changes?

    B) Are there any concerns about battle damage? The old BB's did it because they could increase protection schemes. Nowadays, without any big armor protection scheme, what happens to an electric ship when you blow up part of it? That's alot of power that could arc and cause problems in a ship flooding with salt water.

    C) Is the Navy eligible for the Federal electric vehicle tax credit? ;-)


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