Hybrid Propulsion - I Just Don't Get it

[goboatingnow]

Originally Posted by David M

RMS Queen Mary 2

I was on her, she generates more electricity then the City of Southampton, from where she regularly sails


Diesel Electric just like Diesel hydraulic is a power transmission system, not a hybrid system. A hybrid system uses both diesel and electric as a power system, with or without stored electric power.


Dave

[Captain Bill]

I saw a show a couple of months back on the discovery channel about the building of the world's largest cruise ship. It used diesel/electric propulsion. The electric motors were mounted in pods that could be swiveled 360 degrees to allow unprecidented maneuverability in a large ship. They noted that it also did away with the propeller shafts and associated shaft tunnels. They noted that in large vessels the shafts were often several feet in diameter and ran about 1/3 the length of the ship. The diesel/electric drives allowed the space to be used for other things.

[Sand crab]

This has been covered in other threads....
http://www.cruisersforum.com/forums/...cat-62699.html
I just want to point out again that the worlds largest sailboat manufacturer tried this with the Lagoon 420 Hybrid and almost all have now been converted to diesel. Why? Limited power, limited range (I remember 27 miles on batteries, are you frickin kidding me?), slow speeds, repair issues, etc. Frankly it just sucks. I posted a link about a boat that had gone through 3 complete hybrid systems in about as many years. (sucks times 3) I just want a boat that doesn't suck or at least sucks less. Maybe I'm a dreamer. BOB

[Jd1]

I just have to add my 2 cents .....
There are lots of reasons for a diesel electric drive system and I would not be surprised if, despite additional complexity, the overall economics prove diesel electric to be a winner. Let me explain ....
Let me make one important assumtion: a large battery system is part of the setup!

1) Diesel engines are most efficient and have the highest life expectancy if run in the 85 to 95% area of output power. It is a terrible thing to run a diesel for 30 minutes motoring in and out of the anchorage. I do not have a definitive answer on the engine life expectancy in regard to short runs but I would not be surprised to see a 50% or more engine life reduction in that kind of a service.
2) Being able to run a smaller engine at 90% power will provide better fuel economy (because it is smaller but also because it is run at 90% power) and better engine life.
3) Being disconnected from the drive motor means the engine can go pretty much anywhere (good access)
4) Engine load can be distributed producing a good safety factor - 2 small engines instead of one large engine.
4a) If you want to run economical you can do it on one engine, if you want full power you run two engines. Battery charging can be used as a load to run the engine (s) at best efficiency.
5) There is no necessity to run a separate generator
6) Although charging efficiency and battery efficiency is a serious issue, batteries (as well as solar power) are undergoing a huge push in R&D and great discoveries are sure to come along improving the overall efficiency.
7) It is up to the captain to select efficiencies - for example, it is easy to configure a mode of operation that runs all propulsion requirements with running engines at which point charging and battery efficiencies are irrelevant (since power goes straight from the generation stage to the use stage)
8) You may still say that you have to generate electricity which is less than 100% efficient and then turn an electric motor that is less than 100% efficient but have you considered the loss of efficiency in your transmission ? It's likely that there is less of an efficiency hit from the transmission but electricity isn't as bad as it first looks.

Overall, I have no doubt that coupling directly from engine to prop is the most efficient but I dare say that it might not be the most economical when all things are added up. I personally would be prepared to pay extra for the flexibility and redundancy of such a system if it could be implemented in a reliable way .... and that might take a few years before the pitfalls are ironed out.
Oh, if you know that you are going to be motoring 90% of the time then maybe the diesel electric system doesn't offer that much advantage but if you are sailing it has great potential IMHO.

[Jd1]

Originally Posted by twistedtree

Also, the efficiency of a diesel is pretty constant across the operating range that you would use for propulsion - say 25% to 80% throttle.

Definitively incorrect even if you don't include the reduced life expectancy of a diesel engine run at low output power and for short runs.

[twistedtree]

Originally Posted by Jd1

Definitively incorrect even if you don't include the reduced life expectancy of a diesel engine run at low output power and for short runs.

Here's a real example, using the propulsion engine in my boat, using data taken from the manufacturer's spec sheet.

20% power - 17HP/Gal/Hr
40% power - 19HP/Gal/Hr
60% power - 21HP/Gal/Hr
80% power - 20HP/Gal/Hr
100% power - 20HP/Gal/Hr

It peaks at 60%, but otherwise only varies by 5%-10%. Worst is at 20% load with a drop of 15% from peak. The difference between the worst and best operating efficiencies (15%) is less than the loss from just one of the three stages in a hybrid, let along all three.

Re transmissions, numbers I've seen attribute less than 5% loss to a transmission.

And motoring out at 4-6 kts shouldn't be lightly loading your engine.

And your assertion that a diesel should be run at 85% to 95% of full power is not correct either. To the contrary, engines rated for intermittent use (which pretty much all pleasure boat engines are) are specifically prohibited from running at more than 80% power for more than 1 out of every 8 hours of operation (exact numbers vary slightly between manufacturers). Other than operating below minimum temp, Cummins, for example, places no lower limits on operating power.

[Adelie]

Originally Posted by twistedtree

It seems to me that a hybrid diesel-electric (or gas-electric) propulsion system makes absolutely no sense on a boat. But some manufacturers are building them, and some people seem to think they do make sense. Am I missing something?

It seems that in all cases running a boat off electric from batteries is less efficient that running off it's propulsion engine. This is because the propulsion engine (or some other engine) has to run to turn a generator which is say 80% efficient, then store that power in a battery (another 80% efficiency), then draw the power out to run a motor which is again 80% efficient. So the shaft HP from the engine, rather than directly turning a prop, instead generates, stores, and runs an electric motor connected to that same prop, but at an end-to-end efficiency of 51% (.8 x .8 x .8).

Also, on a boat, you can't do any of the things you do in a hybrid car that are the real sources of efficiency, namely:

- Regenerative braking
- Engine loading so it operates in a more efficient range.

Overall it strikes me as a way to add more equipment, weight, and complexity in exchange for lower efficiency.

What am I missing? I just don't get it from a basic physics/thermodynamics viewpoint.

You aren't missing anything, it's a fad like winged keels on general purpose production boats. In 5-15yr it will fade away.

[Jd1]

Originally Posted by twistedtree

Here's a real example, using the propulsion engine in my boat, using data taken from the manufacturer's spec sheet.

20% power - 17HP/Gal/Hr
40% power - 19HP/Gal/Hr
60% power - 21HP/Gal/Hr
80% power - 20HP/Gal/Hr
100% power - 20HP/Gal/Hr

It peaks at 60%, but otherwise only varies by 5%-10%. Worst is at 20% load with a drop of 15% from peak. The difference between the worst and best operating efficiencies (15%) is less than the loss from just one of the three stages in a hybrid, let along all three.

Re transmissions, numbers I've seen attribute less than 5% loss to a transmission.

And motoring out at 4-6 kts shouldn't be lightly loading your engine.

And your assertion that a diesel should be run at 85% to 95% of full power is not correct either. To the contrary, engines rated for intermittent use (which pretty much all pleasure boat engines are) are specifically prohibited from running at more than 80% power for more than 1 out of every 8 hours of operation (exact numbers vary slightly between manufacturers). Other than operating below minimum temp, Cummins, for example, places no lower limits on operating power.

I am sorry, I am not familiar with a rating of HP/Gal/hr. All my diesel experience is based on land based stationary engines.
I would agree that a 15% to 20% change in efficiency sounds reasonable
I would also agree that 5% loss in the transmission sounds reasonable
My (limited) experience for out/in harbour movements is idle or slightly above idle and this is what I call lightly loaded. If you are operating with a squirrel on a treadmill than I am sure the squirrel is sweating profusely.
I have not seen lower limits of power output yet ask any diesel mechanic and the story you will hear is to not baby your engine. I have no explanation in regard to 80% power figures, maybe other people can give us additional input.
Keep in mind that I have suggested that for major propulsion requirements one doesn't go through the battery systems (in/out of the harbour is fine) but rather directly from the engine to the generator and to the electric motor.
One other comment - most reliable small diesel engines get between 30000 and up to 50000 hrs of life yet most boat engines do a major service at maybe 3000 hrs and I doubt that you will see many engines go to 5000 hrs before a re-power. This is the reason why I believe running the engine at low power and short duration is not healthy (besides what all the diesel mechanics will tell you).

[44'cruisingcat]

Originally Posted by twistedtree

It seems to me that a hybrid diesel-electric (or gas-electric) propulsion system makes absolutely no sense on a boat. But some manufacturers are building them, and some people seem to think they do make sense. Am I missing something?

It seems that in all cases running a boat off electric from batteries is less efficient that running off it's propulsion engine. This is because the propulsion engine (or some other engine) has to run to turn a generator which is say 80% efficient, then store that power in a battery (another 80% efficiency), then draw the power out to run a motor which is again 80% efficient. So the shaft HP from the engine, rather than directly turning a prop, instead generates, stores, and runs an electric motor connected to that same prop, but at an end-to-end efficiency of 51% (.8 x .8 x .8).

Also, on a boat, you can't do any of the things you do in a hybrid car that are the real sources of efficiency, namely:

- Regenerative braking
- Engine loading so it operates in a more efficient range.

Overall it strikes me as a way to add more equipment, weight, and complexity in exchange for lower efficiency.

What am I missing? I just don't get it from a basic physics/thermodynamics viewpoint.

IMO you're not missing a thing. Hybrids work on cars because they have to accelerate/decelerate frequently and climb hills. (Although there are now very efficient turbo-diesels which can match or better hybrid fuel economy, without the environmental disaster of their battery banks)

They also spend a lot of time with their engines running but not moving. This means most conventional cars have much bigger engines than they'd need to operate at a good cruising speed on level ground. These engines spend most of their time operating fairly inefficiently.

Boats don't climb hills and don't suffer stop/start traffic. Most displacement boat engines operate in their most efficient range almost all the time. So there's unlikely to be any real gain in fuel economy from a hybrid.

[Jd1]

Originally Posted by 44'cruisingcat

Boats don't climb hills and don't suffer stop/start traffic. Most displacement boat engines operate in their most efficient range almost all the time. So there's unlikely to be any real gain in fuel economy from a hybrid.

I would agree 100% based on the point of view you are taking. If you expand your premise though and include many short runs in and out of harbours for a sail boat, maybe running the main engine just for battery charging and advantages such as flexible engine placement or engine redundancy and all of a sudden things look better for the hybrid setup.
Please note that I do not believe the hybrid systems are mature enough YET but IMHO they will have a lot to offer once they are reliable enough.

As a side note, there are a lot of wonderful things one could do with electronics to improve efficiencies as well (if they can be reliable enough). As an example, think of a generator, towed or hull attached, that provides controlled charging. It provides charging only when there is extra power available such as when you are close to hull speed. As soon as speed drops, the water based generator load is reduced letting the boat run at maximum speed. If the wind picks up and extra energy is available, the towed generator load is increased and the extra energy is converted to electricity rather than being wasted on the futile effort to exceed hull speed.
I would not be surprised if we see a lot of alternative propulsion and power systems over the next ten or 20 years. The ocean is a wonderful place for alternative power once we learn how to harvest it properly.

[downunder]

Originally Posted by Jd1

I would not be surprised if we see a lot of alternative propulsion and power systems over the next ten or 20 years.

They are certainly not here yet.

[ozskipper]

Its just my personal opinion, but i feel that people buy these sort of power plants so they can have the warm fuzzy 'i am doing good for the environment' feelings. Perfect for the guy with too much money on his hands. But in reality, just give me a boring old yanmah with a gearbox and a propshaft straight out the back.

Having said that, there was probably some guy who made similar comments about the slow and noisy horseless carriage. So I promise I will eat my words in a century or two if I am wrong

[nwdiver]

I’ll wait for the mini nuclear plant to run my electric motor, no charging, no batteries just clean complex nuc power, maybe some of the nuc/electric designs will trickle down from Navy use

[Tropic Cat]

One lightning hit to a diesel electric GRP boat and you're suddenly without power when you need it the most. There's a reason Lagoon stopped making their boats with the 'green' option.

If you want to fantasize, look up Rossi e-cat in Google and consider how to couple this to a marine rated steam engine. Diesel electric is a non starter for a pleasure boat.

[Tellie]

Leave this to the slide ruler guys. Real world applications for a small cruising sail boats makes no sense. This year alone I've had three boats I've worked on rip the electric drives out and replace them with diesels. It all sounds great on paper and electric drive forums but many have been very disappointed with the real world results. I'm all for building a better mouse trap but try as they may, electric drives just aren't there yet. If you're trying to get through MIT great. If you want to go cruising... go diesel, have a brew, and be happy.