Hybrid Ferry with DC grid

I recently was notified of a rather interesting hybrid ferry project in the states. Washington State Department of Transportation has put out a requests for proposals for the conversion of their Hyak ferry to a hybrid propulsion system. Here are the details:


On page 46, they seem quite intent on a rather large quantity of lithium ion batteries. Of course, it may not be so large compared to CMAL’s hybrid ferries when one considers the size of the ship.

Also, an interesting simplified diagram of the propulsion system can be found on page 12. It appears that they are proposing a DC transmission network yet utilizing AC generators and AC motors. I have seen such concepts from ABB and Siemens so I don’t know if they are possible suitors to this project.

They are also proposing new diesels and I’d welcome any comments on what diesel engines might be best to maximize fuel efficiency?

All together, very interesting and I appreciate hearing from others as to such a novel concept.

Man, they never asked me to do research like this while I was in the academy.

But I do like to talk about technology.

Just remember, AC generators are highly efficient and simple/inexpensive to construct. They are also reliable do to the lack of brushes and commutators. AC propulsion motors fall under the same benefits. You need a VFD or Variable Frequency Drive to run a AC propulsion motor at varying speeds. This is nothing new, the railroads have been doing it for decades. Now, let’s examine the very first component of a VFD, it is a rectifier that converts the AC input to DC to both provide a constant voltage source for the SCR to chop up to simulate an AC output at varying frequencies. AC motors and VFDs also have an advantage that they can use a high frequency 300hz input signal to saturate the windings of the motor and limit current draw at reduced speed preventing overheating of the motor. This means AC propulsion motors can run at 100% rated torque and more continuously (usually 400%) while DC motors are limited on their torque output based on the duration of the draw (same with railroad stuff from decades ago). The only thing new about this technology is the concept of getting rid of the individual rectifiers and capacitors in each VFD for each motor on the vessel and using a single rectifier with a battery bank to replace the capacitors. DC died years ago due to the extraordinary cost of the switch gear due to the difficulty in extinguishing the arc in a DC contactor. Now everything is solid state so that is no longer an issue.

A Lithium Ion battery that larger would be pretty scary. Laptop battery can be impressive if they run away, I know of a underwater scooter batteries around 1150Wh that have burned up a motel when they were being recharged, and Boeing has had issues with 787 batteries that were sized to start the APU. I can’t imagine the prospect of a battery sized to power main propulsion on the Hyak going up. I don’t know the CFR’s but I can’t imagine that they address this risk yet.

You might want to check out the paper in the January issue of SNAME [I]Marine Technology[/I] about DC grid concept. Authors are from ABB makers of (among many other things) big AC drives for propulsion/thrusters as used on many of later generation drillships.

tons of large drives are DC now, its just a change in tech and the mainstream seems to swap ac to dc about every 10 years ( just went to an ABB presentation on propulsion)
For small stuff you get to run variable speed gen sets, propagate that around the world and you have major fuel savings.
I can see the EPA mandating all gen sets under a certain size to be DC due to the emission savings one day soon