Generating power from main engine vs generator

[tanglewood]

Here is a comparison of the fuel efficiency of generating power while underway using the alternator on your main engine vs running a generator. Some of you might find it interesting.

Adventures of Tanglewood: Engine Alternator or Generator: Which is More Efficient?

[Paul L]

Thanks for the test data, interesting. Not sure it would change any behavior, ie genset when anchored or sailing, engine alt when motoring.

[noelex 77]

Thanks for the information

A real world, measured output, from the alternator of 258 A at 24v + is very impressive.

What model alternator do you have?

[tanglewood]

Originally Posted by Paul L

Thanks for the test data, interesting. Not sure it would change any behavior, ie genset when anchored or sailing, engine alt when motoring.

Right, the goal wasn't necessarily to change anyone's operations, just to shed some light on the tradeoff's. Lot's of people happily burn fuel in exchange for convenience, for example. Heck, I have a power boat, so that would be me :-)

But for example, I've also encountered a number of people looking to add additional alternator and inverter capability to run HVAC off the main rather than run their generator. One interesting take-away at least for me is that such an approach is probably counter productive.

[tanglewood]

Originally Posted by noelex 77

Thanks for the information

A real world, measured output, from the alternator of 258 A at 24v + is very impressive.

What model alternator do you have?

It's actually two alternators running in tandem. Both are Balmars, one 190A and the other 85A, so 275A theoretical max.

My boat is a big power boat, so very different from most here, but I don't think it really matters. I think the real key as that the generator is more efficient provided you have it reasonably well loaded - say above 30% - especially when powering AC loads. That can be applied at any scale, large to small. For lighter loads the main engine is better. Fortunately, as Paul L demonstrates, that's a good match for typical usage.

[john61ct]

Maximizing solar first, but, if not enough, I like the idea of a small motor (5-8HP?) driving a big-amp alternator to charge the banks, skipping AC-DC charger conversion losses.

Per-hour costs would be very low, and very efficient with a high-acceptance chemistry, especially LFP.

A DC water-maker could give great flexibility too.

[valhalla360]

A big issue with the analysis is determining where the propulsion motor is in it's operating range.
- At anchor, it's probably less efficient.
- Running at near hull speed, the alternator may be more efficient because friction losses make up a smaller percentage of the energy being consumed by the engine.

So if you are at anchor 90% of days, a dedicated generator makes more sense.
If you are underway or at dock most days, a big alternator can be a good option to save the weight and cost associated with a dedicated generator.

[John Holbrook]

Maybe. Most dedicated genset and battery charger costing 10-15k can charge the batteries at say 100amps.

Two 200amp Balmars can consistently charge at say 300amps, cost >5k and use about 5hp each. At 1200 -1500rpm that is efficient loading on a 40-50hp sailboat engine.

Times are changing. Many boaters only want a backup source for their solar panels and have big battery bank capacities. I know there are many other considerations.

Originally Posted by valhalla360

A big issue with the analysis is determining where the propulsion motor is in it's operating range.
- At anchor, it's probably less efficient.
- Running at near hull speed, the alternator may be more efficient because friction losses make up a smaller percentage of the energy being consumed by the engine.

So if you are at anchor 90% of days, a dedicated generator makes more sense.
If you are underway or at dock most days, a big alternator can be a good option to save the weight and cost associated with a dedicated generator.

[valhalla360]

Originally Posted by John Holbrook

Maybe. Most dedicated genset and battery charger costing 10-15k can charge the batteries at say 100amps.

Two 200amp Balmars can consistently charge at say 300amps, cost >5k and use about 5hp each. At 1200 -1500rpm that is efficient loading on a 40-50hp sailboat engine.

Times are changing. Many boaters only want a backup source for their solar panels and have big battery bank capacities. I know there are many other considerations.

Define what you think a $10-15k generator is? 100amps is a bit on the light side unless you are talking about a small 3-4kw generator where most of the money is eaten up by installation costs. I would expect a $!0-15k generator to be able to put out 200-30amps if the boat has room for a simple straight forward installation.

I think you are comparing apples and oranges but as I said, you need to define what you are trying to do. Large generator or large alternator really doesn't make a lot of sense unless you are trying to run air/con or similar high load/long duration devices while the engine is running.

If you are just looking for backup for the solar, why on earth would you spend $5k for massive alternators? Most days you wouldn't need to run the alternator/generator at all. When you do, a couple hours with a 100amp alternator would typically be sufficient to keep things going until you get a sunny day and the solar can take over.

[Dockhead]

Interesting and fun analysis.


One other thing to consider, though, is the idle overhead (and per hour maintenance and amortization) compared to varying loads.

I don't know what your maintenance and amortization costs are per engine hour compared to fuel, but I usually reckon they are about equal.

That greatly changes the results when you compare to running the main while also propelling the boat, versus running the generator separately.

Also, the idle overhead (on my 6.5kW Kohler generator equal to about 25% of max fuel burn) will eat you up during periods of low loads, if you are running the generator while using loads which vary a lot.

All of this tilts the whole thing towards using the main engine with large alternators, wherever possible, which is what I do. I can produce about 2.5kW from my Leece Neville schoolbus alternator, compared to about 5kW from my generator (I derate the 6.5kW generator for longevity), and I always use that power first, when it is available because I'm running the main for propulsion.

On multiday passages under sail, I will also usually start the main for battery charging, rather than the generator, even if I don't desperately need the propulsion.

[John Holbrook]

10-15k Genset - any installed name brand 7 - 10kva. If you want more battery charging power then add roughly $1500 per 100amps installed.

Large alternators make sense if you want to charge up the batteries fast at anchor or out sailing when you don't really need engine propulsion. Nothing to do with aircon. For aircon you probably do need a genset.

AGMs can be charged at 20-25% of their capacity so you would need at huge bank to untilize 300amps of charging. But Lithium and Firefly can be charged at rates near equal to their capacities. For me Lithium is expensive or complex. Fireflys look reasonable.

Why spend 5k on alternators - because the standard alternator is good for less than 100amps and I would have to put a smart regulator on it anyway. 2 x Balmar 200amp alternators $1,100 each, one Balmar regulator that handles both $700, 2nd alternator brackets from Yanmar $500? and cables, easy self and instal say $4k.

Originally Posted by valhalla360

Define what you think a $10-15k generator is? 100amps is a bit on the light side unless you are talking about a small 3-4kw generator where most of the money is eaten up by installation costs. I would expect a $!0-15k generator to be able to put out 200-30amps if the boat has room for a simple straight forward installation.

I think you are comparing apples and oranges but as I said, you need to define what you are trying to do. Large generator or large alternator really doesn't make a lot of sense unless you are trying to run air/con or similar high load/long duration devices while the engine is running.

If you are just looking for backup for the solar, why on earth would you spend $5k for massive alternators? Most days you wouldn't need to run the alternator/generator at all. When you do, a couple hours with a 100amp alternator would typically be sufficient to keep things going until you get a sunny day and the solar can take over.

[a64pilot]

I don't think its enough difference to matter. I hope to never crank the main engine just to make electricity, but if its running anyway I want to harvest some of the excess power and not run my generator.
To me its more about each machine has X number of hours in it and trying to make them last as many years as is possible.

However from my admittedly little experience I doubt very seriously if you can get anywhere near rated power from a small frame alternator, at least not close to continuous. They are capable of huge power generation but at least in my case they have to cut way back soon or they will overheat.
However my generator will make rated power until it runs out of fuel.
Be nice if the alternator manufacturers or re-sellers included a duty cycle, that I think would open some eyes, cause not all alternators were created equally

[John Holbrook]

I hear you on small frame alternators but the Balmar AT200 is rated at 198amp at 90 degrees Celsius so 150amp per alternator is severely cut back.

[john61ct]

Wow on those costs! I plan to spend well under $1000 for a DC genset charger, small Robin-Subaru motor driving a reco large-frame LN alt. And that's including the propane conversion and the MC-614 to allow 100+A net direct into Firefly or LFP.

Too bad this sort of setup isn't available off the shelf, true I'm not costing in my tinkering hours.

And only suitable for solar backup, maybe ten hours a week.

[ayates]

This fellow has information on engine vs generator efficiency: ZRD - Generators DC and AC Thought no doubt with a possible bias.

No connection...


Allan.