Hybrid Propulsion—what is it, and when does it make sense for ships?

Excerpt from U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine, by Mr. Chris B. McKesson, P.E., consultant; and Mr. Thomas P. Risley, Alion Science & Technology Inc.


What Do We Mean by “Hybrid?”
A “hybrid” drive means there is more than one power source that can turn the shaft, via an electrical interconnection. There might be a combination of batteries and diesel generators, all of which can feed their power into an electric propulsion motor.

What Are the Advantages?
Fuel efficiency. In an ideal hybrid drive system, the system automatically determines the most efficient source of power for a given load demand. In the case of a large passenger vessel, the demands of the “hotel” load and “propulsion” load can be coupled together electrically and powered by a combination of power sources including generators, batteries, and alternative power sources.

In hybrid operation, the engine in the system runs at a constant load. When this load suits the vessel’s propulsion needs, this power is sent to the propellers and consumed in propulsion. But during those times when this power is not needed—during low-speed maneuvers, for example—the engine still produces the power, but it is “banked” in an accumulator array. Then, when required, the accumulator array is drawn upon and its power is added to the still-continuous output of the engine. This steady load is optimal for best fuel consumption.

In the above photo, the propulsion generator is shown in the engine room on a hybrid drive vessel. The cabinet on the right holds propulsion control electronics.

Easy to upgrade. A hybrid drive is inherently modular. Consider a typical hybrid, having one or more diesel generators, a battery bank, and a propulsion motor. In a case like this, you can change out a major component of the system without disturbing the rest. A related advantage is that the individual components are generally smaller and lighter than their traditional counterparts. Thus, the manipulation of any one component is likely to be physically easier.

Arrangeability. This modular or “component architecture” nature of a hybrid drive also brings some potentially important design flexibility: It is possible to put the components nearly anywhere on the boat.

Operational flexibility. In a hybrid drive there is no need for all the generators to be the same size. This system architecture gives the operator the opportunity to decide which power source to have online depending upon the immediate needs of the ship.

What Are the Disadvantages of a Hybrid?
There Ain’t No Such Thing as a Free Lunch. It’s true that a hybrid drive can lower fuel consumption and be more flexible and arrangeable than a conventional mechanical drive. Unsurprisingly, those advantages also come at a cost. In some cases, this cost is financial, but some of the “costs” are measured in other units, such as complexity or weight.

Be Advised: Batteries Are Consumables. Battery choices include lead-acid, advanced glass mat, gel, nickel metal hydride, and lithium ion. For most operators, the lead-acid or advanced glass mat battery is a competitive battery candidate for a hybrid ship, but a lead-acid battery can only survive approximately 1,000 charge/discharge cycles. Eventually it reaches a point where it no longer holds a useful amount of energy. The photo at left shows stainless steel cases on the right-hand side that house large battery banks on a hybrid vessel.

When Does a Hybrid Make Sense?
The best place for a hybrid drive is in an application with a varied duty cycle. Consider the following examples:

A commuter ferry like those in Puget Sound or San Francisco Bay. In these cases the ferry runs are between 30 and 60 minutes long, and turnaround times are fairly short. This type of operation is probably not suited to hybrid drive. We studied hybrid drive for the San Francisco Water Transit Authority and found that, for their nominal 45-minute runs, a hybrid drive would actually result in increased emissions compared to a clean diesel installation. This is because the weight increases associated with hybrid drive resulted in reduced passenger capacity on the ferry, so that the fuel burned was moving fewer people, and the increases in fuel efficiency were not sufficient to compensate for this.

A water taxi. Here, we envision a small-capacity boat making short hops between many closely spaced locales, with passengers hopping on and off frequently. The water taxi may spend as much time idling at the dock as it does underway, like the National Park Service’s Arizona Memorial taxis in Pearl Harbor. These services are ideal for hybrid drive. A quite small generator can be used, which might have a power output as little as one-fourth of the propulsion motor power.

Escort tugs. The tug spends much of its time merely being “available” to a ship, and then providing a significant push for a short time as part of harbor maneuvers. In such a case, it makes little sense to have a 5,000 hp engine running at idle just to lean on it for five minutes per hour. Instead, the hybrid tugs use a substantial battery bank and two small generators that feed the battery bank.

Another situation where a hybrid makes sense is where there is a substantial amount of shore power available. This could apply to a vessel that makes only one or two harbor cruises a day, or a vessel whose turnaround time is so long that it makes sense to plug in and charge up at each dock call.

Given the number of variables and areas for optimization presented by the hybrid concept, it is very important to approach it as an integrated, comprehensively engineered solution.

For more information:
Full article and “Environmental Protection” edition of USCG Proceedings is available at http://www.uscg.mil/proceedings/Winter2008-09/.

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