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Sunday, February 12, 2017

Engine Alternator or Generator: Which is More Efficient?

This question keeps coming up, and is often debated at length.  Like most endless debates, they go on and on because nobody really knows the answer.  Here's an attempt at getting to a real answer, but I'll spoil it for you - the answer still isn't super clear.....

The question, in a nut shell, is which is more efficient, generating electricity with a big alternator on your main engine, or running a stand alone generator?

One of the first misconceptions is that power from your main engine alternator comes for free because the engine is already running.  Unfortunately that's not true.  Generating that power puts an increased load on the engine which in turn burns more fuel.  It's that old conservation of energy law in physics.  It just won't go away.

Once past that first realization, the debate quickly turns to other power losses, and which power generation source has more losses.  Here's a little score card tabulating the most obvious losses for each generation source.  Once again, there is no clear winner.




Generator
Alternator
Drive losses
There is virtually no loss in the direct drive between the engine and alternator
Belt drives definitely have friction losses.  How much?  I really have no idea.
Initial form of electricity
AC
AC.  Many people don't realize that your alternator actually generates 3-phase AC.  Inside the alternator it is rectified to DC, with associated losses
Engine efficiency
This will vary with the load.  At light loads, the efficiency is noteably lower
Your main engine will be propelling the boat, so likely operating in a favorable efficiency range.
Powering AC loads
The output is already AC, so no extra losses to power AC loads
The alternators DC output has to go through an Inverter to get AC at about 90% efficiency
Powering DC loads
The generator's AC output needs to go through a battery charger to get DC at about 85% efficiency
The output is already DC, no no extra losses to power DC loads


Of course we all know that running our generator burns fuel too, so the real question is which burns more, the increased load on the main engine, or the generator?  Sitting at the helm while cruising can sometime lead one's mind to wander, and mine always seems to wonder to technical things like this, so one day I decided to measure the fuel burn to generate power from my main engine alternator.  Generator manufacturers publish fuel burn rates, so that data is know.  With corresponding data for generating power from a main engine alternator, perhaps some light could be shed on the question.

To measure the fuel burn required to generate power, I logged my fuel burn for the first few hours after weighing anchor.  The whole time I ran the main engine at a constant setting, then logged the fuel burn as the alternator output went from full output initially, down to just maintaining the underway house loads.  Using the fuel burn rate and power load at the end of the experiment, I was able to tabulate the incremental fuel burn and corresponding power generation from each of the higher output stages.

Here's what the data looks like:


Main Eng burn (gph)
Alt output (A)
Power (KW)
Inc Power (KW)
Inc fuel (gph)
kWh/gal
7.25
258
7.33
4.91
0.55
8.9
7.15
210
5.96
3.55
0.45
7.9
6.90
150
4.26
1.85
0.20
9.2
6.80
115
3.27
0.85
0.10
8.5
6.70
85
2.41




So if you consider the last line as the baseline, each line above it represents some incremental amount of power generated (Inc Power column), and corresponding incremental fuel burn (Inc fuel column).  The fuel efficiency to generate that incremental power can then be calculated as show in the last column.  The resulting numbers jump around a bit, I think because the precision of my fuel burn numbers are limited, but they are clustered together enough to be generally believable, even if not exact.

With this data in hand, it can now be compared to the fuel efficiency of a generator.  My generator is a 20KW Northern lights, and unfortunately they only publish 2 fuel burn data points at 50% and 100% load.  I wanted more than that, so picked the published data from an Onan 21KW unit.


Power (KW)
Fuel Burn (gph)
KWh/gal
21.5
2.2
9.8
16.1
1.5
10.7
10.8
1.1
9.8
5.4
0.8
6.8

At higher power levels, you can see that the generator is more efficient than the alternator, but it's not a huge difference.  And very noteworthy is the significant drop in efficiency at lower power levels where the diesel engine is less efficient.

Plotting all this on a chart helps put it all in perspective.  This chart shows the fuel efficiency of generating power in each device's native form; AC for the generator, and DC for the alternator.


From this you might conclude that for any loads greater than around 35% of the generator's capacity, the generator is the more efficient way to generate power.  That's probably not a bad rule of thumb, but there is still a bit more to the story.

Depending on your generation source, and the type of load you want to power, there might still be another conversion step involved with associated losses.  Let's look at two cases to illustrate this.

First we can look at powering DC loads, including getting your batteries charged up after a night on the hook.  In this case, the power coming out of the alternator requires no further conversion to charge your batteries.  But to charge the batteries from the generator, you need to run the generator's AC power through a battery charger, and they are typically around 85% efficient.  This favors the alternator and handicaps the generator.  The graph below shows the adjusted fuel efficiency of charging batteries from each power source.


You can see that the generator needs to be loaded to around 50% (10KW in this example) of it's rated power to match the efficiency of the main engine alternator when powering DC loads.

The second example is looking at powering AC loads like air conditioning.  In this case, the generator's output is ready to use with no further conversion, but the alternator's power output needs to be run through an inverter at about 90% efficiency.  This favors the generator and handicaps the alternator.  The graph below shows the adjusted fuel efficiencies of powering AC loads.


Now you can see that the generator load only needs to be a bit above 25% to match the alternator efficiency, and quickly becomes a good bit more efficient.

So what's the take-away from all this?  I think two rules of thumb:

1) When powering DC loads, it only makes sense to do so from your generator if you can keep the total generator load up over about 35-40%.  Note that this is the total load on the generator, not just the DC loads.  So if you have your generator on to make water or run your air conditioning and you load is up over 35-40%, then by all means use it for your DC loads as well.   But if you are just plodding along with loads less the 35%, let your alternator do the work.

2) When powering AC loads, the generator pretty quickly becomes the preferred power source, starting at around 25% load, and quickly becoming far more efficient.  For those thinking that giant alternators and inverters might be a good idea to run air conditioning on your boat, think again.  That generator is most likely the more fuel efficient way to power it.  Only modest loads make sense to power from your alternator.

Caveat emptor:

This is just an analysis of one boat and one generator, and not even my own generator.  So please just take this as what it is; just one example and not an exhaustive study.  Maybe I picked a particularly efficient generator, or maybe it's particularly inefficient.  I think it's a representative example, but honestly don't know.  And I have no idea how the efficiency of power generation from my main engine alternators compares to other, but I again expect it's a good representative example.