As harbor tug operators get a better understanding of variable duty cycles, hybrid propulsion is looking more and more appealing.
The first hybrid-powered ship-assist tug, Foss Maritime's Carolyn Dorothy (see story on opposite page), has been operating in Southern California for almost 18 months and is setting the bar for what can be accomplished.
Other hybrid-power designs are on the drawing boards and at least one is close to installation.
It's called the PowerRing hybrid system. It's from EcoMarine Propulsion Systems , a subsidiary of Ocean Tug & Barge Engineering in Milford, Mass. Industrial Power Systems in Jacksonville, Fla., and Comprehensive Power Inc., in Marlborough, Mass., are partners in the project.
The basic elements are generators, electric motors and battery packs. If the batteries were topped off by shore power the night before, a harbor tug with PowerRing propulsion might go all day without starting the engines, if there are no ships to assist. At the same time, the tug can operate at full power off just its batteries.
As a model-railroad enthusiast, Ocean Tug & Barge's Bob Hill was aware of the work that Canadian rail company RailPower Technologies had done to develop hybrid-powered yard locomotives. And because he's a naval architect/marine engineer, he wanted to apply the concept to marine applications.
"They were the first people to have very large hybrid locomotives and wanted to apply it to battery-operated vessels," Hill said. For a while, Hill's company and RailPower were teamed up to adapt the locomotive hybrid drives to boats, but RailPower and Hill went their separate ways. Then Hill joined forces with Industrial Power Systems, which builds switchboards, switchgears and electrical controls, and Comprehensive Power.
Comprehensive Power builds PowerRing's power conversion equipment and the TeraTorq permanent magnetic motors. The motors are designed to produce as much as five times the power of induction or synchronous motors of the same size and weight. The magnetic motors are comparable to traction motors used in locomotives.
"With this group, we can do the entire shipboard electrical system, propulsion through DC, down to the navigational light system, to the ship's service, everything," said Hill.
Not only does EcoMarine Propulsion have the capability to build a propulsion system, "but we can completely test it before going to the shipyard. That will help separate us from the crowd," said Industrial Power's Glenn Beaupre.
PowerRing is designed to match up diesel-electric power with hybrid-power options such as batteries or fuel cells.
DC BUS
"One thing that makes the system different from anyone else is that we have a DC bus. We don't use AC for the main bus," Hill said.
The electricity that's generated from a conventional generator as AC current is immediately converted to DC current. "The advantage is that you don't have to synchronize and parallel generator sets as AC units," said Beaupre. "By going to DC you eliminate a lot of sophisticated controls for the generators. On the DC bus you only really need to control voltage to control load sharing duties."
Thus a boat's basic system would include a port and starboard power system, each with a DC bus, a bank of batteries and a couple diesel generators. The two systems would be tied together but could be separated.
"If something goes wrong with the port or starboard system, you can shut it down, open a switch and run on the other bus on reduced power," Beaupre said.
PowerRing propulsion is still an unproven concept, but two harbor tugs are due to have the system installed. One will be installed in a new tug and the second will be a repower. Hill wouldn't reveal the names of the tug companies.
"In our proposal for the tugs, the baseline system has sufficient battery capacity to give tugs full power - 5,000 horsepower - for about 20 minutes. Or if it is sitting on station and loitering, it might run 10 to 12 hours on the batteries," said Beaupre.
He added that some other hybrid systems couldn't get full power on batteries. But if a customer can accommodate the space and the weight of the batteries, a tug can pull full power from a PowerRing's batteries.
And because batteries can supply so much power, some fuel storage can be eliminated. "At eight pounds per gallon, that compensates for some of the battery weight. That's significant," said Beaupre.
A unique aspect of the system is how the batteries are monitored. Even with the least-expensive batteries - lead acid - the PowerRing's cell-monitoring system "constantly checks the condition of every cell of a battery," Beaupre said. "If a cell begins to degrade, we know about it and the cell can be replaced before it drags down anymore."
PowerRing can operate with lead-acid batteries, but EcoMarine is also looking at lithium batteries from Corvus Energy in Richmond, British Columbia.
Corvus batteries have high power density and capacity levels, which makes them lighter and smaller than lead-acid batteries. That means weight savings. They also have a much longer life, according to Beaupre.
PLUG-AND-PLAY
PowerRing is designed as a plug-and-play system. That means that 10 years after you install it, any new technology would be fairly easy to integrate into the existing propulsion system.
This makes it relatively easy to add a new battery system. "It's not like having to tear the boat apart to replace it, which is what you do with shafted conventional propulsion," Beaupre said.
Modular plug-and-play also means that if one of the components in the propulsion drive goes down, a replacement can be borrowed from somewhere else, such as the fire-pump drive.
Hill estimates that fuel savings with the hybrid system is anywhere from 10 to 45 percent. If you are going with straight diesel-electric propulsion, fuel savings could be as much as 30 percent.