[QUOTE=Steamer;181861]Someone is completely missing the point.[/QUOTE]
The whole purpose of this thread is efficiency so maybe you’re missing the point?
[QUOTE=Steamer;181861]
There is more to an efficient tugboat operation than the thermal or mechanical efficiency of the powerplant.[/QUOTE]
Would you mind expanding on this?
Controllable pitch propellers are what come to my mind.
[QUOTE=OK Corral;181865]Would you mind expanding on this?
Controllable pitch propellers are what come to my mind.[/QUOTE]
Yes, CP wheels are another alternative to DE, in either the engine outputs the power required to perform the task. The type of operation or task might be more “efficiently” or effectively performed with that system or a DE than an engine driving a shaft through a reduction gear … which can actually be more fuel efficient than either DE or a CP wheel.
Fuel efficiency at part load is always less than at some ideal load but there has never been a boat made that only operates at optimum loading all the time. As someone else stated, that is the nature of the diesel (or any other heat engine) and frequent part load running is the nature of the type of operation that engine serves.
The original post seemed to focus on the fact that part load fuel efficiency is not as high as optimum load … so what?, that is one of the reasons hybrid drive systems are becoming popular in some applications. One could make a good argument that paying for a huge lithium battery bank is not an efficient allocation of corporate finances even though it increases fuel economy when the diesel is not running but it might lead to getting a contract that a conventional power system would not. You pick which “efficiency” is most important to the owner or provides the greatest return on investment.
Picking a single point of efficiency is an inefficient way to determine if a system is efficient in its entirety or economical or satisfies sufficient of the many aspects of ownership that lead to its purchase.
I was talking primarily about fuel efficiency. While a diesel-electric power plant may be better at partial load if you can run fewer engines closer to their optimal operating point, in many operational situations you need to have all engines running at low load in order to be instantly available (within the limits of the loading ramp, of course) if there is a sudden need of propulsion power. I’ve been on a diesel-electric vessel that ran for extended periods of time with three to four gensets at 20-30% load because of this. On top of that, there are those few percent of additional drivetrain losses.
Of course, diesel-electric propulsion has so many other advantages in the vessel types I’m working with that the fuel efficiency is rarely a sales argument.
[QUOTE=OK Corral;181777]HSecondly, a turbocharger at low loads is not as effective as at high loads because the exhaust gas spinning the compressor is cool and at a lower volume. As the load on the engine increases, the turbocharger is introduced to more hot air and spins faster creating a more efficient burn. Thus the gallons used per unit of power created is greater under low load conditions.
Is this a sound explanation? If not can someone point me in the right direction? I am trying to prove why electric power is a viable option to increase efficiency and reduce emissions.
[/QUOTE]
No, this is not right, really. A simple explanation;
At low loads, less exhaust passes the turbocharger because less fuel is being burned. This means the turbo is not spinning as fast and therefore not compressing as much air on the compressor end. As the load increases, more exhaust gases are forced through the turbine of the turbocharger, causing it to spin faster and compress a greater quantity of air. More air mixed with the fuel creates more complete combustion and therefore better efficiency. As far as the temperature of the air, it is always better for the air to be cool because cool air is more dense, therefore more oxygen molecules can mix with your fuel, this is why you have charge air coolers.
Anyway, the most efficient theoretical boat would be designed pretty much like a Chevy Volt (with a smaller engine running at peak efficiency to charge the batteries) But the battery tech and storage capacity is not there yet. It would be interesting to see how much space you would need just in batteries with today’s technology.
[QUOTE=OK Corral;181777]Is this a sound explanation? If not can someone point me in the right direction? I am trying to prove why electric power is a viable option to increase efficiency and reduce emissions.[/QUOTE]
What exactly is your objective? Are you trying to clean up the air around a port? Make some groundbreaking improvement in diesel engine design? Or simply justify the obvious?
I still think you are focusing a bit too sharply on one single aspect of a much more complex system. If your objective is to reduce emissions in a port then simply gaining a percentage point or two of diesel engine efficiency isn’t going to do much for you. Hanging your argument on a tug that operates at the offending condition for only a few minutes a day is a waste of time and your own energy in my opinion.
I think you need to spend a great deal of time looking at how ships come and go from a port. Look at how ship assist tugs operate, time their periods of operation in different modes, look at the companies that operate them. Are those dedicated assist boats or are they multipurpose boats that are tasked with ship assist when they happen to be available?
Electric ship assist boats are probably a big part of the solution for a port’s air quality problem but where does that electricity come from? If it is a coal burning plant in someone else’s backyard then moving the problem is not exactly a solution is it?
Your focus on turbocharger efficiency is a fly speck issue in a huge and complex problem. In any event, variable geometry turbochargers have and will just about eliminate the part load compressor efficiency issues and have/will greatly increase the power band at which they operate. Next generation turbos will probably have electric drive assist when needed and act as turbogenerators when the engine load allows. Look at larger issues …
Stop looking at trees and watch the forest. If you are looking for real life solutions that can be implemented today, look at LNG fueled tugs. They only burn half the diesel a conventional boat does. Combine that with a hybrid and forget about trying to squeeze a microscopic efficiency improvement out of a system that might benefit from that for only a few moments each working day.
If electricity can be supplied by solar, wind, tide, hydro, a Mr. Fusion machine then going electric is a no brainer and hardly needs another paper to promote the issue.
[QUOTE=OK Corral;181777]Hello,
I am trying to make a case for hybrid/diesel-electric propulsion over traditional diesel-mechanical. I’ve found that the vast majority of the lifespan of a tug’s engines are spent at under 30% load.[/QUOTE]
OK Corral, when making comparisons, the best approach would be to evaluate in detail what the tug operating profile and business model is.
For example (harbor tugs), if your tug is “on-hire” to assist larger vessels for only 6 hours out of an 8 hour shift, that’s 25% of idling. Of the 75% portion while “on-hire” the tug has to rendezvous with the ship, escort the ship into the harbor, and maneuver the ship into the port. If we guess on the engine throttling, lets say 50% to 75% MCR (maximum continuous rating) to rendezvous with the ship, then less throttle to follow the ship on standby, then high throttling for maneuvering the ship to the dock, we can graph the engine use over time and use that to make informed decisions on propulsion options.
Some other useful issues to evaluate might be: how long is the tug intended to be in service before it is retired? That question alone plays a huge role if you intend on keeping the tug for 20 years, because the cost of building the tug will be tiny compared to the cost of fuel over 20 years. Another useful question would be what capital is available upfront? because that limits options such as hybrid electric systems as they are likely to have a higher up front cost but hopefully have a lower “fuel burn” cost (even though maintenance costs might be higher on a hybrid).