Generating power from main engine vs generator

[DeepFrz]

Originally posted by a64pilot: 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.

A fairly inexpensive fan and some ducting can increase the continuous output of your alternator quite substantially as was shown in another fairly recent thread.

[John Holbrook]

Don't be confused just Google the word "most" it's meaning is not the same as limited. Thanks for the flowers on this special day.

Originally Posted by valhalla360

snip from your prior post:
"Maybe. Most dedicated genset and battery charger costing 10-15k can charge the batteries at say 100amps.
[/B][/COLOR]"

Sure sounds like you were saying the generator would be limited to 100amps.

Randomly throwing out comments, some of which are flat out wrong only provides confusion. Why comment if it's just random statements with no coherent meaning or purpose?

Maybe your wife needs to step it up a bit to get you thinking more clearly.

[Pelagic]

I look at this whole power generation thing initially from a seaman's perspective .

Reliability, Reserves and Redundancy!

To me, that means the largest 24v battery bank I can safely fit for storing reserve energy.

Reliable Solar and the redundancy of two alternators on my main engine and a Generator to power 2 Victron chargers.

A key decision I made was to order my large capacity water maker with a 24vDC high pressure pump, so that I am not dependent on my AC generator to make Fresh Water.

As an efficient motorsailor, the main engine is often on at 1000rpm in winds below 10 knots, so making water under way between anchorages, gives me that DC option

My 10kw Northern Lights is used Only at Anchor, loaded at 80% for charging batteries, Air Cons, hot water heater, so I see both engines as complementing each other, rather than competing

[Pizzazz]

One can always find an excuse to overbuild, especially with boats. The reality is that sailboat engines average 100 hours annually, so if properly maintained, they would outlast the boat. Further sailboats have built in redundancy, sail and power. A small solar panel for emergency power and a spare alternator for $100 is plenty of redundancy for most people.

An interesting alternative would be a generator, an electric pod and slightly larger batteries (no main engine). Will see more of this in the near future.

[Pelagic]

Good points Pizzazz, but a lot depends on your location and whether you are constantly cruising new grounds.

Most sailboat installations don't allow for proper maintenance and a generator squeezed into a corner with sound shield are often repair nightmares.

I agree, Diesel/Electric propulsion will become more popular as the Hybird technology becomes marinized and flexible with propeller regeneration under sail.

I chose a motor sailor because I wanted a decent engine room, but completely gutted and redesigned because of those dam sound shields!

[capngeo]

My background is commercial boats... the Gennys (yes plural) always ran, at least one was going 24/7 It wasn't uncommon to go 5-6 years between overhauls.. 20,000+ hours. Now of course these were low speed BIG diesels.

On my sailboat, the main produces the juice when it is running, a 2Kw Yamaha when its not... and a whopping 100Ah battery bank for when neither is burning dead dinosaurs. I usually take a slip every third day to re-store, re-ice, and be a tourist....

[valhalla360]

Originally Posted by John Holbrook

Don't be confused just Google the word "most" it's meaning is not the same as limited. Thanks for the flowers on this special day.

As soon as you Google "can".

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 think what you were trying to say is that the alternator option is cheaper to buy, which I don't necessarily disagree with. I do disagree that 300A is needed if, as you indicated, it is just backup for solar.

[tanglewood]

There are a few things I should have been clearer on in the article.

- Don't get hung up on the size of the generator or the capacity of the alternators, or how much power I consume. That's not the point. The point is that a reasonably loaded generator is more fuel efficient that adding alternator load to your main while underway.

- Fuel efficiency is definitely not the only consideration in how to generate power. I just think it's useful to know as part of my own decision process. We all need to work within the confines of the equipment we have, the kind of cruising we do, maintenance costs, and our own values.

- The experiment looked at the fuel efficiency of adding alternator load to the main engine WHILE UNDERWAY. This is an important distinction from running your main at anchor to ONLY generator power. Underway, your engine has a reasonable load on it and is likely operating in a efficient part of it's operating range. It's just like the generator when it's got a reasonable load on it. In contrast, if you are running your main to generate power while at anchor, and the only load is the alternator, you are probably operating in a very inefficient power range for that engine. The exact answer will depend on your boat, the engine, the alternators, etc. etc., but it's very likely to be just like operating a generator while lightly loaded.

- The article isn't really meant to tell you how to power your boat. It's just looking at one very narrow question about the relative fuel efficiency of two different power generation sources. Hopefully the results will help inform your decisions about how to best power your boat. And if it's not applicable, no problem, carry on as you were.

[valhalla360]

Originally Posted by tanglewood

There are a few things I should have been clearer on in the article.

- Don't get hung up on the size of the generator or the capacity of the alternators, or how much power I consume. That's not the point. The point is that a reasonably loaded generator is more fuel efficient that adding alternator load to your main while underway.

- Fuel efficiency is definitely not the only consideration in how to generate power. I just think it's useful to know as part of my own decision process. We all need to work within the confines of the equipment we have, the kind of cruising we do, maintenance costs, and our own values.

- The experiment looked at the fuel efficiency of adding alternator load to the main engine WHILE UNDERWAY. This is an important distinction from running your main at anchor to ONLY generator power. Underway, your engine has a reasonable load on it and is likely operating in a efficient part of it's operating range. It's just like the generator when it's got a reasonable load on it. In contrast, if you are running your main to generate power while at anchor, and the only load is the alternator, you are probably operating in a very inefficient power range for that engine. The exact answer will depend on your boat, the engine, the alternators, etc. etc., but it's very likely to be just like operating a generator while lightly loaded.

- The article isn't really meant to tell you how to power your boat. It's just looking at one very narrow question about the relative fuel efficiency of two different power generation sources. Hopefully the results will help inform your decisions about how to best power your boat. And if it's not applicable, no problem, carry on as you were.

Sorry if I got side tracked correcting some misinformation by another poster.

But I don't agree with your assessment though. It appears to apply to the narrow set of test cases with your specific motors, generators and alternator setup but we don't have a clear picture of the propulsion loading and if you look at the data, there is only 1 data point below 10kw for the generator and the alternator peaks at around 7kw. Given wide variability of the 4 alternator data points with no clear pattern, I don't think we can say much with any certainty.

At around 5kw output is the only place where you have comparable output but if you look at the graph, this is far below peak efficiency for the generator, so not really comparable.

Interesting data but far from enough to make conclusions. What would have been interesting would be to see the results for a 7-10kw generator, so you could develop a series of comparable data points (though I assume you have a 20kw generator for other reasons).

[tanglewood]

Originally Posted by valhalla360

Sorry if I got side tracked correcting some misinformation by another poster.

But I don't agree with your assessment though. It appears to apply to the narrow set of test cases with your specific motors, generators and alternator setup but we don't have a clear picture of the propulsion loading and if you look at the data, there is only 1 data point below 10kw for the generator and the alternator peaks at around 7kw. Given wide variability of the 4 alternator data points with no clear pattern, I don't think we can say much with any certainty.

At around 5kw output is the only place where you have comparable output but if you look at the graph, this is far below peak efficiency for the generator, so not really comparable.

Interesting data but far from enough to make conclusions. What would have been interesting would be to see the results for a 7-10kw generator, so you could develop a series of comparable data points (though I assume you have a 20kw generator for other reasons).

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?

[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.

[conachair]

What sort of state of charge are the batteries at to accept these big currents?

To get anywhere near fully charged won't you be charging at much lower currents?

[Dockhead]

Originally Posted by tanglewood

There are a few things I should have been clearer on in the article.

- Don't get hung up on the size of the generator or the capacity of the alternators, or how much power I consume. That's not the point. The point is that a reasonably loaded generator is more fuel efficient that adding alternator load to your main while underway.

- Fuel efficiency is definitely not the only consideration in how to generate power. I just think it's useful to know as part of my own decision process. We all need to work within the confines of the equipment we have, the kind of cruising we do, maintenance costs, and our own values.

- The experiment looked at the fuel efficiency of adding alternator load to the main engine WHILE UNDERWAY. This is an important distinction from running your main at anchor to ONLY generator power. Underway, your engine has a reasonable load on it and is likely operating in a efficient part of it's operating range. It's just like the generator when it's got a reasonable load on it. In contrast, if you are running your main to generate power while at anchor, and the only load is the alternator, you are probably operating in a very inefficient power range for that engine. The exact answer will depend on your boat, the engine, the alternators, etc. etc., but it's very likely to be just like operating a generator while lightly loaded.

- The article isn't really meant to tell you how to power your boat. It's just looking at one very narrow question about the relative fuel efficiency of two different power generation sources. Hopefully the results will help inform your decisions about how to best power your boat. And if it's not applicable, no problem, carry on as you were.

And on top of everything else, it is surprising, hence especially interesting.

Kudos on an interesting study, and you've really framed the question well here

You can count me as one of those here who would never be doubling hours on my machinery for an incremental improvement in fuel consumption under certain circumstances, but then again -- I sail thousands of miles a year and put hundreds of hours each on main and genny, so I am conscious about conserving machinery hours. Someone else might not care or might even want to put some hours on, and might find this directly applicable.

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

[Dockhead]

Originally Posted by tanglewood

. . .

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. . . .

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.

[poiu]

It is interesting to see the figures laid out and to translate to my situation is pretty easy. My Onan genset is the one in your study and burns fuel at a rate of .5 lt per KWh as it is ridiculously oversized and therefore not efficient, so say 40c in fuel. Add 30c amortisation of 10,000 hrs life on a $30,000 installation and then a bit for oil and parts and my labour. 80c per KWh generated say. Pretty damn expensive power. Nearly ten times the grid power costs. A good thing I don't use much.

If the main engine is running anyway it is just the marginal cost of fuel to account for and in my case the alternator fuel efficiency is not too different from the genset, so he same cost in fuel, but no other costs. The engine (when motoring) is a far cheaper source of power therefore. I will run down the battery if I know I will have an into wind motoring trip.

For you, running your motor a lot, you may want to look at what is available in the market for very high efficiency alternators, much better than the usual Leese Neville based units. There are some super high efficiency units out there that might give you 40% extra power for free depending on what you have. Then tie it, or a balanced pair to your engine or with perhaps a clutch controlled off-take from your prop shaft so as to avoid belt losses and to avoid any side loads. You would be building a combined engine/DC genset. Mecc Alte can do the alternator.

Actually, I think it's all a bit upside down. As a priority I'd look at how to avoid using power rather than how to make it. Life is much nicer, simpler, quieter and cheaper without it. The impact is measured in multiples, rather than percentages. Breeze and shade instead of aircon. Use one fridge freezer instead of 5. (Yes, I have 5 if you include the ice maker). LEDs, efficient equipment, etc. Once that is done, solar for the roof, then big LFP batteries to power the now really small efficient aircon at night on occasions where you must. You may find like that that you can or do come close to going without the genset, or just to have a mini one for redundancy. This is my ambition, mostly achieved.