Originally Posted by Pizzazz
Here is a link to a paper that describes alternator efficiency in detail. The heart of the matter is that low voltage (12V) and high current losses (in the stator windings) lead to very inefficient power generation. While it is possible to operate a school
bus alternator at 70% efficiency (24V helps), this only happens in a narrow rpm band. At higher speeds, the generated current increases and the efficiency drops down to 40%. So, the average could be 55%, see the paper if you want more details.
The inverter generators are very efficient because they use high voltage ac generators (to minimize the losses) which is then rectified, converted and then inverted to 110V AC.
The bottom line for our application is that the Honda
2000i is as efficient as it gets. If you want a 12V DC generator, then you need to look for an inverter generator or an optimized alternator that runs at a specific speed, reaching the 70% quoted above.
If we take the energy content of fuel
, lose 60% in the diesel combustion, another 30-40% in the power generation, add mechanical losses and solar
becomes really attractive. It also means that using electricity for heating water
or the boat
is highly inefficient but we all knew this already. Off I go with my diesel heater
This is great information -- I learned a lot from it. Thanks for sharing it.
However, shouldn't we look at the bigger picture?
First of all, ordinary car-type alternators are not good for producing bulk power in any case. They're fine for capturing a bit of energy incidentally to propulsion
, but not for running the house systems on an energy intensive boat. And efficiency is not the main issue.
So if we're serious about producing bulk power, we're going to have a school bus alternator anyway. And if it's on the main engine, what difference is +/- 10% or even 30% in the conversion efficiency, anyway? If we're charging
with the main engine not doing propulsion
, then that inefficiency will be lost
in the big inefficiency of running a big diesel engine at a small fraction of its appropriate load. That's on the one hand. On the other hand, if we are producing power while the main IS doing propulsion, then the power is almost free, so who cares if you lose an extra 10% or 30% of it?
So if an alternator on the main engine is all you've got, then a Honda generator does make sense to use instead of running the main at anchor
. And the efficiency of the Honda's electrical
side is not the first or even second reason why you would do that -- the health
of the main engine is by far the most important reason why you would prefer to have another way to do it.
A real DC generator run with an appropriately sized engine will be far more efficient than a Honda -- and far more long lasting -- just because it's a diesel engine. It will have a really good alternator on it -- why would you choose anything else? -- so the electrical
efficiency won't be worse than the Honda, but again, that is not at all the main factor.
In my opinion, priorities for mechanical power generation should be something like this:
1. Reliable, long lasting, easy and cheap
3. Inexpensive to acquire and install.
4. Convenient to operate.
I think the cost of fuel doesn't even make the list, it's so minor compared to the other factors. Whether the power produced is AC or DC is EVEN LESS important.
Honda scores high on 3, is OK on 2, but poor for 4 (you have to store it and set it up every time you use it). On 1 -- good for reliability
, but poor for long-lasting (compared to diesel). Very good for being easy to just take down to a repair shop or even replace if needed.
Heavy duty AC generator (like mine) scores very high on 4 and 2, very poor on 3, and again mixed on 1 -- very long lasting, very reliable, but expensive and difficult to repair if it breaks.
Simple DC generator like Noelex's scores high on everything except 3, where it is medium. Scores exceptionally high on 1 for being extremely easy and cheap
to repair if it breaks. In fact, you can carry an entire spare alternator and regulator
at modest cost and weight.
So according to these priorities, the case for a simple DC generator is pretty compelling, in my opinion.
As far as operating costs are concerned --
As I said, I don't think operating costs even belong on the list. But for what it's worth:
1. Fuel cost on the Honda will be not less than for other methods, and will be much more if large quantities of power are generated. Issue is the small gasoline/petrol issue versus diesel. Honda is cheap to acquire but has a much shorter useful life, so amortization per hour should be roughly the same.
2. Cost of maintenance
is going to be close to the cost of fuel, and is a function of hours. A bit more or less of incremental load on the engine is not going to change this -- yet another reason why you don't care so much about alternator efficiency.
P.S. -- I had a Honda generator on board my boat, which I used mostly for putting a long finishing charge on my batteries
in order to save running the diesel generator for hours without a load. But I also played around with it running the boat on it. So I'm speaking from experience. If you generated a lot of power with it, it used a lot of fuel, and it was a PITA to keep it fueled, not to mention paying for the fuel. For generating small amounts of power, it was great. And you didn't have to worry about its health
running very small loads (gas engines don't care), so it was great for the finishing charge.
I finally got rid of it -- not for any of these reasons, but because it was really hard to store. You can't keep it inside the main hull
volume because of the dangers of gasoline/petrol vapors.