Generating power from main engine vs generator

[poiu]

Originally Posted by tanglewood

Per Valhalla360's suggestion, I inserted data for two other Cummins/Onan gensets, one 7.5KW, and the other 11.5KW. Revised graphs are attached.

The 7.5KW set isn't very efficient, with the engine alternators often doing as well or better. But the genset still does better at full load.

The 11.5KW set looks more like the 20KW from an efficiency perspective, and as compared to the engine alternators.

Nice charts. I think the curves would represent the performance in their output power ranges better with a log scale on the x axis.

[John Holbrook]

Originally Posted by valhalla360

I suppose your comment is technically correct the same way I would technically be correct saying your recommended alternator "can" produce 10A continously but It's meaning in the context of your sentence implies limited to. Just own up and admit that your original post was at best misleading and when context is considered completely wrong.

I'm not recommending any particular alternator and especially not one that produces 10A. I think you are loosing the plot

Originally Posted by valhalla360

I think what you were trying to say is that the alternator option is cheaper to buy, which I don't necessarily disagree with.

Great deduction Sherlock. Maybe the give away clue was when I wrote "reduced cost" responding to you in my post 24

[a64pilot]

I'm one that thinks if you could power the boat when underway without running the generator and only run it when at anchor, you will end up with way less costs, fuel being irrelevant, I think. Range is a different issue, then I woudl assume you turn off everything you can and slow down?
When we had our power boat, we cranked the generator before disconnecting shorepower and turned it off after reconnecting it, so we had three engines running when underway, if it had only been two, that would have reduced costs and hassle

[tanglewood]

Originally Posted by Dockhead

In any case, it is interesting as a well-done intellectual exercise which overturns some things which might seem obvious. I love it when that happens

Maybe this is why people are pushing back on it in every conceivable way.

[tanglewood]

Originally Posted by Dockhead

Once you get to that practical question, then you need to start figuring the value of generator hours saved, by not running genny and mains at the same time.

Your situation is different, with much greater AC power loads (I don't have aircon for example), but FWIW, my boat is set up with enough alternator capacity (2.5kW) and inverter capacity (3kW) to run practically any AC loads I might have without running the generator, so long as the main is running, at least individually (in order to be able to run different devices at the same time, I might like to have twice this capacity). If I'm motoring in a calm, for example, I will often run laundry (but can't run the kettle, microwave or espresso machine at the same time). Unlike you, I have not studied the incremental fuel burn from doing this vs. using the genny, but saving the genny hours is a no-brainer for my case.

For running really big loads from main engine alternators, there is one other issue to consider, however -- are you sure you're not overloading the mains? Might be worth looking at a propeller curve to see how much headroom you have at the RPMs you're planning to do this. You have fixed props, I presume, on a big power boat, and I guess at lower RPMs you must be fine. I have a variable pitch prop which pitches up to suck up the last bit of power available, and I would worry about this if I had any bigger alternator than I do. I tend to run the main a bit faster than usual if I'm planning to have an extended heavy electrical load on the alternator, to be sure.

We are set up the same way with matched (more or less) alternator and inverter capacity. While underway we run AC appliances at will, and they are all indirectly powered from the main alternators. This includes the clothes washer and dryer. I think it works really well, and eliminates needing to run the generator to do laundry which ends up being long runs at light loads on the gen. But we are not set up to run HVAC off the inverters, though it could operate one or two units if I wired it accordingly. But this little exercise has confirmed that with the larger load of HVAC, I might as well just run the generator.

As for main engine overload, I'm not too worried about it. As you guessed I have a fixed pitch prop. Even at WOT the engine doesn't reach full power (I'm slightly under propped), so if the alternators were at max output I can't see it being a problem. And besides, I never run at WOT except as a test. Our main is usually running in the 40% to 60% load range.

One thing I mentioned in the article is that engines have specifications for the max PTO load off the pulley's with belts, other PTO points, and of course the main flywheel. It's not a spec that is front and center for most engines, but I've been told that 10% is a good rule of thumb. So for me, I could pull about 30HP off the front of the engine. In practice with my current system, I'm pulling about 12-15hp, so around 5%.

It does make me wonder, though, on sail boats with much smaller engines, whether some alternator systems are overloading the engine's front end pulleys.

[tanglewood]

A number of people have brought up the amortized cost of running a generator or any other piece of equipment. I did not factor that in, yet the costs are very real.

But I have a slightly different take on it. The cost of the machinery is a sunk cost. Whether I run it or not, that money is spent and gone.

Also, I don't think I'll outlive either the main or generator. I know plenty of people with 10,000 to 15,000 hrs on these machines with nothing but regular maintenance.

Put these two factors together, and I have no qualms about running either machine when and if needed. That's what they are there for. I think we have a tendency to tip toe around machines, afraid to use them, when they actually exist solely for that purpose - to be used and to serve us.

So when you look at the marginal cost of running the generator, it's fuel cost and the maintenance cost. Whether you put one hour or 200 hrs on the generator each year, you need to change the oil, filters, and impeller annually, so there is an argument that those costs are part of the sunk costs of ownership. So it's really just to cost of an oil change when you run for more than 250 hrs per year (change interval for my gen).

Now I'm not going to run it for the sake of running it. Heck, I spent a lot of time modifying my alternator system and inverter system so I wouldn't have to run the generator to do laundry while underway.

And even though I'm a power boater, I'm actually pretty energy conscious. I have solar on my boat. And I have reduce my at-anchor power load to about 1/3 of other same-model boats. And my underway electric loads are about 1/2 other same-model boats. Heck, that's why I did this little study in the first place - because I care about efficiency. But it's within the context of the conveniences I want in life. Others may choose different conveniences.

[Bjorn_O]

Originally Posted by tanglewood

Per Valhalla360's suggestion, I inserted data for two other Cummins/Onan gensets, one 7.5KW, and the other 11.5KW. Revised graphs are attached.

The 7.5KW set isn't very efficient, with the engine alternators often doing as well or better. But the genset still does better at full load.

The 11.5KW set looks more like the 20KW from an efficiency perspective, and as compared to the engine alternators.

The graphs are confusing.
I believe that KW is kW (kiloWatt)
but I don't understand WKg/gal

[Typhoon]

I never charge from my main engine, I do have an alternator on it but I use it just to tension the belt. I will be rigging it up with a decent alternator just as back up to my generator.

You can see my generator here . Cruisers & Sailing Forums - ColdEh Marine's Album: Generator build

It also makes 40 gallons of water as i charge the 600 amp hours of Lithium batteries at 225 amps hot. at 1800 rpm alternator speed , no belts other then the ones on the water maker and they are overkill with a double pulley.

Regards John.

[StuM]

Originally Posted by Bjorn_O

The graphs are confusing.
I believe that KW is kW (kiloWatt)

Correct.

but I don't understand WKg/gal

It actually says "WKh/gal" which I took to be what would normally be shown as kWh/gal (kiloWatt hours per gallon)

Wkh (Watts x (1000 x hours)) and kWh ((1000 x Watts) x hours ) are mathematically equivalent but it's very unusual usage.

[tanglewood]

Originally Posted by Bjorn_O

The graphs are confusing.
I believe that KW is kW (kiloWatt)
but I don't understand WKg/gal

Sorry, sloppiness and bad typing on my part. The efficiency number is killowatt-hours per gal of fuel.

[tanglewood]

Originally Posted by StuM

Correct.



It actually says "WKh/gal" which I took to be what would normally be shown as kWh/gal (kiloWatt hours per gallon)

Wkh (Watts x (1000 x hours)) and kWh ((1000 x Watts) x hours ) are mathematically equivalent but it's very unusual usage.

The real explanation is that I can't type worth a darn. But I think you guys figured out what I meant.

[valhalla360]

Originally Posted by tanglewood

Right, I only have the gear that I have, so that's what the data covers. If others want to collect similar data it would certainly be interesting to see.

The propulsion loading, BTW, based on the prop curve for the engine it appears to be about 130hp out of 325hp max, so 40% load.

I think it's typical for a propulsion engine to be significantly more powerful than a generator engine, and for a generator's output to get a good bit greater than an engine alternator output. As such, I expect it's typical that the output overlap will be limited to the lower end of the generator range.

One of the questions I was trying to answer for myself is whether I should continue to load up the main engine with more alternator capacity to, for example, run HVAC while underway. I know a number of power boaters who have done that, or are thinking of doing it. So that would be pushing up the output capability of the main alternators, reaching towards the 50% (10kw) load point on the generator. In that range, the generator is clearly the more fuel efficient of the two, especially for an AC load like HVAC.

As for the generator data points, I agree it would be great to have more than 4, but I am again limited by the available manufacturers data.

But what is your counter theory? That engine alternator power generation is always more efficient? In what way does the data run counter to what you believe?

Just to be clear, I wasn't saying your conclusion was "wrong" but that we don't have enough info to determine if it's right or wrong.

Also, I think you have a different setup than typical (if there even is such a thing) for forum members.

You have a 325hp propulsion motor and your generator motor is probably around 25hp. Resulting in a HP difference of 13 to 1.

I would say the typical forum member with sailboat is on the order of 50hp propulsion motor and 10hp motor in the generator resulting in a 5 to 1 ratio between the HP. So for similar power production, the alternator option will eat up a much larger percentage of the propulsion motors capability.

Edit: I see you posted some fuel consumption graphs on a later post.

[tanglewood]

Originally Posted by valhalla360

You have a 325hp propulsion motor and your generator motor is probably around 25hp. Resulting in a HP difference of 13 to 1.

Not too far off. The genset engine is about 36hp, so 9:1

Originally Posted by valhalla360

I would say the typical forum member with sailboat is on the order of 50hp propulsion motor and 10hp motor in the generator resulting in a 5 to 1 ratio between the HP. So for similar power production, the alternator option will eat up a much larger percentage of the propulsion motors capability.

I guess what I'm not following is why it matters. It seems to me that as long as the main engine is pushing the boat, it's running in a reasonable efficiency range. Not necessarily it's peak efficiency, but just not in it's crap efficiency low power range. As such, adding alternator loads comes at good efficiency for the drive engine, and any handicap comes from the losses inherent to an alternator, primarily belt drive losses and rectifier losses.

For generator power, the key to efficiency is to run it at enough load to get the engine out of the crap efficiency low power range. If you run it in it's reasonable efficiency range of say 30%-100% load, then it's pretty darn good, at least partly because it doesn't have the drive losses and rectifier losses.

Take these principals and apply them to whatever equipment you have, scaled to that equipment, and I think you can come up with the right trade-offs and efficiency cross over points for your boat. They may well be different from mine, but I think the principals still apply.

[valhalla360]

Originally Posted by John Holbrook

I'm not recommending any particular alternator and especially not one that produces 10A. I think you are loosing the plot

That went way over someone's head.

Of course saying a 300a alternator "can" put out 10a is silly. It's technically true but a silly statement.

Just like saying a generator capable of generating over 500a (@ 12V) "can" put out 100a is silly.

[valhalla360]

Originally Posted by tanglewood

Not too far off. The genset engine is about 36hp, so 9:1



I guess what I'm not following is why it matters. It seems to me that as long as the main engine is pushing the boat, it's running in a reasonable efficiency range. Not necessarily it's peak efficiency, but just not in it's crap efficiency low power range. As such, adding alternator loads comes at good efficiency for the drive engine, and any handicap comes from the losses inherent to an alternator, primarily belt drive losses and rectifier losses.

For generator power, the key to efficiency is to run it at enough load to get the engine out of the crap efficiency low power range. If you run it in it's reasonable efficiency range of say 30%-100% load, then it's pretty darn good, at least partly because it doesn't have the drive losses and rectifier losses.

Take these principals and apply them to whatever equipment you have, scaled to that equipment, and I think you can come up with the right trade-offs and efficiency cross over points for your boat. They may well be different from mine, but I think the principals still apply.

Basically, your big 325hp engine is running at around 1/3 of max HP for propulsion. Enough to get the engine up to temperature but really not taxing the engine at all. Adding 5hp for some big alternators barely makes a difference in terms of stressing the engine. Even if you pulled 30hp, the engine is barely breaking 50% of peak HP. It has tons of excess capacity under normal conditions.

On the other hand a 50hp propulsion engine on a 45' sailboat running at hull speed is likely putting out around 40hp for propulsion. Adding 5hp for big alternators and you are pushing the engine to it's limit for continuous operation.

Efficiency can drop if you push an engine to it's limits. Since you are nowhere close to the limits, you won't see this effect but for those with smaller engines, it's easy to overload the engine with big alternators and engine driven compressors.