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Saturday, February 25, 2017

Electric Power Strategy

Since talking about the original design of our power system back when we were specifying the boat, I really haven't said much more about how it's working, what changes we are making, and why.
I've posted this article on our solar system, this article on reworking my main engine alternator system, and this article on the relative efficiency of generating power via the main engine vs a generator.  I have several more articles is process for other modifications, but realize I've never stepped back and provided an overview of what I'm doing, where it's heading, and why.  So here's the big picture.

Solar Panels

As you read through this, think in terms of cruising for extending periods of time, underway some days, but not all, and anchored at night or for several days.  Notably absent are marinas and shore power, so the boat needs to be completely self-sufficient from a power perspective.  Loads need to be powered, and batteries need to be recharged.

If you return to a dock each day and plug into shore power, this is a much easier problem to solve.  All you need is enough power to last until you return to the dock, and shore power can recharge everything.

Similarly, if you run a generator all the time, you will have plenty of power.  As boats get larger, this becomes the norm, and if you are in a climate where you want air conditioning all the time, a constantly running generator is the practical solution.

For other boats, the power we consume comes from an alternator on our engine while underway, from a generator run periodically, or from power stored in a battery bank which of course needs to be recharged periodically.  Increasingly common are solar arrays that generate power as well.  Operating comfortably like this is our goal.   We ran for over a month last summer, never tying up to a dock, and never plugging into shore power.

All of these elements interact in infinite ways depending on how the boat is used, and how it is equipped.  It's important to understand that there is no right answer here, however I always find it interesting to see how other people have solved the problem, and hopefully our approach will prove equally useful to others.

The diagram below shows our power system.  We have ways to generate power; a generator, main engine alternator, and a solar array.   We have a way to store power in the batteries.  And we have things that consume power, some more than others.  It's worth spending a little time on the things that consume power, since they have a large influence on the over-all power system design.



Here's what we have discovered and done so far.


First, we have and continue to pay close attention to our electric loads, and select appliances that are as power efficient as possible without giving up the features we want.  I think we have been pretty successful so far, and have an at-anchor battery load that 1/2 to 1/3 what other boats of the same model report.

Relatively small loads can run off the batteries.  This includes things like lighting, entertainment systems, and various boat systems.  Heavier loads can also run off the batteries just fine as long as they are not prolonged loads.  Our house water pump, microwave, and coffee maker are good examples.  They draw a lot of power, but only for a limited time, so the total power consumption isn't huge.  The greater the cumulative load of everything, the larger batteries you will need and/or the more frequently they will need to be recharged.  Everyone needs to find their happy compromise between convenience of having gadgets, and the inconvenience of powering them.

For heavier, longer lasting loads, running off batteries usually doesn't make sense.  The batteries get drained quickly and need recharging, so it tends to make sense to just run the generator from the onset, powering the load directly, and taking the opportunity to charge the batteries at the same time.

This raises a couple of interesting questions for future articles.  For example, what is the dollar cost of generating power from a generator?  This article compares the fuel efficiency of generating power from a generator vs and engine alternator, but it is comparative only and doesn't quantify the costs in dollars.  And a related question is the cost to store and retrieve power from a battery.  It's not free.  But that can be the subject of another future article.

Batteries eventually need to recharged, and larger loads need to be powered.  We have a solar system, and it contributes nicely, but is limited by the space we have available for solar panels.  On a sunny day it more or less covers our standby loads while at anchor, but there is nothing left over to contribute towards recharging our batteries from the night before.  That still requires some other form of charging, even though the solar makes it less frequent.

That leaves us with two forms of power generation;  alternators on our main engine, and a generator.  Of the two, the generator is the more powerful with a 20kw output.  The main engine alternator is about 5kw.  If we are getting underway, we will typically just let the alternator recharge the batteries.  It can almost always do it before we reach our next destination.  But if we are staying at anchor for the day, we will run the generator and take advantage of the available power to do laundry, heat water, and of course charge the batteries.

But the decision becomes more complex when we are both underway, and when we want to do laundry or run some other larger load like our electric oven or air conditioning.  One option is to run off the alternator and inverters to power these devices.  The other option is to run the generator to power the loads, removing all loads from the alternator.

This brings us to the first enhancement we wanted to make, and the ensuing ripple effect of changes.

While underway, it is a great time to do laundry.  But it required running the generator, yet it's a very light load for the generator.  And our inverter system was 120V only, so we didn't have the option of running off the main engine alternator.  This spawned a project that I will write about in the future where we expanded our inverter system to run a few select 240V appliances - the washer, dryer, and oven to be specific.

Our inverters are capable of powering 7kw which will run two out of three appliances at the same time, and perhaps all three at once.  But our main alternator is only about 5kw, so we needed more alternator output or heavy loads will drain the bateries.  This triggered the first ripple effect and a project to increase our alternator output to about 7kw to match the power of the inverters.

Many people, including me, have increased their alternator and inverter capacity to operate some of these larger loads while underway.  Our initial focus was on being able to do laundry while underway without running the generator.  Laundry by itself isn't enough of a load to keep the generator busy, and not too large a load to power with our existing alternators, so it seemed like a good starting point.   The end result has been quite successful, I think.
One thing I have been wondering through the whole process is how far to take the alternator+inverter approach vs just running the generator.  The next questions I was facing was whether to allow for powering some or all of my air conditioning units via the alternator+inverter.  This question brought about a set of experiments that I ran last summer and reported in this article comparing the fuel efficiency of generating power via alternators vs a generator.  As a result of the experiments, it's unlikely that I will expand my alternator capacity since larger loads are more efficiently powered by the generator.

One related side effect benefit of moving a few 240V appliances to our inverters is that we now have a way to run them while on 50hz shore power.  Our washer, dryer, and oven will only run on 60hz power, or at least that's what the manufacturer says.  I haven't tried it, so am going on what they say.  That created a problem when we get to locations with 50hz shore power.

Our 120V service is 100% serviced through inverters so that always runs at 60hz.  The 240V service, however, was direct connected to either the shore power or generator.  The generator is 60hz, so that would work fine, but not sure power.

By moving the laundry and over to our inverter service, it too now always run at 60hz, so it at least partially solves the 50hz problem.   However, on 50hz shore power our inverters never switch to charge mode and just keep inverting, drawing their power from the batteries.  So there is a need to keep the batteries charged and the inverters fed from shore power.

Back when we first designed our electrical system we considered this and added a universal shore charger.  It's a 100A charger that can run on either 50 or 60hz, and pretty much any input voltage that you throw at it.  So when on 50hz shore power, the charger provides DC that keeps the batteries charged and powers the inverter, and the inverter in turn powers the 120V loads and the 60hz sensitive 240V loads.  It works well, but with the increased inverter load from the laundry, there are times when the charger won't keep up.  So, in the same way we needed to increase alternator output to support the washer and dryer loads, a project for this year is to add a second charger to double its capacity and provide some redundancy when running on shore power.  That will happen sometime before we leave North America.

Hopefully this provides a little more context for some of my more recent projects, and a few articles that I have not yet written.

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.