Hi. This is my first post here. Nice site!
I can actually contribute somthing here as we recently went through the excercise of trying to DIY a wind/prop tow generator and the original idea was to use off the shelf alternator parts,
My wife decided to run the numbers for it as part of her university maths project in alternative energy sources.
The very very brief summary is as follows.
The very best marine alternators are about 30% efficient. The ones that come standard on an automobile are about 15% efficient. The reason for this is that they do not need to be better, and by the manufacturer's POV, they need to be as cheap per unit as possible.
So why such a low efficiency?
Well, for starters, Magnets. Commercial alternators use electro magnets that eat about 30-50% of the charge in order to "work".They also use a very narrow winding which means that a lot of the magents area of effect is missed.
The reason they can be like this is that alternators are hooked up to huge monsterous things called engines....And they have power to spare, so wasting it is acceptable.
If you are ever in sailing conditions that reproduce the sort of torque and rpm on your wind/tow alternator/generator that a gas or diesel engine gives off through its pulley...check the liferaft and radio the people on land that you love them. It is possible that ABM AMRO1 in the Volvo race could pull along a tow prop driving an alternator at somthing like the revs and torque of a Suzuki Micra...maybe. Almost.
So the answer becomes using a "real" generator. This means genuine rare earth magnets, a broader coil (this means that instead of being the size of a standard alternator, it is about the diameter of a standard bucket) that collects more of the magnetic field, a regulator that does not automatically shut out low revs (cars idle at around 900revs, most units cut out anything less then 600) and improved bearings for reduced friction.
Now you are at about 85-90% efficiency, including at low revs, and do not need a lot of torque to turn the unit. All good...but you have spent a fair bit of money.
So what happens to all that lovely wasted effort that does not turn into electricity to gladen the cockles of your AGM?
Yep...heat. Now alternators form cars have a lovely notion of how to deal with this...they have great big holes cut in them and catch the passing breeze of both the car's rapid passage along the kinds of uncluttered roads only found in car commercials and the radiator cooling fan. Mostly fine, and it is not like it needs to be all that efficient.
Commercial tow and wind units are built more or less sealed, with cute little heat dispersal fins that radiate the heat out...but those fins actually are pretty carefully designed to take heat form critical areas inside the casing and make the whole thing work within its tempreture parameters. Of course, if it is over 100degrees F and in direct sunlight....it is going to be working outside of its temp range and be producing less power...but that's life.
Now you CAN just shove a commercial alternator into a waterproof can you make for it...and even try for some cooling fins...but it is going to further reduce its usefull output...and salty moisty goodness getting in there is not going to improve things.
So what, give up? NO!
Because an ideal home made generator is possible, it just looks really different to what you had in mind. It is basically a disk, imagine a breifcase, only round and about the same thickness. That si what your ideal generator will look like, and at that point it is practical and efficent and can be water proofed (it is so efficient that waste heat is greatly reduced as an issue, and cooling is simplified because of the increased speed around the rim of the much broader winding plate (which also becomes your flywheel and a good place to attach a brake...oh yes, a brake is a good idea for a wind generator, unless you enjoy that kind of gladitorial combat with six spinning blades).
So then we went looking to find if anyone was building gens along these lines...and wouldn't you know it, they were.
http://www.otherpower.com/Check around their wind turbine pages and get the pics of their winding plates and stuff. They build on a much bigger scale then what works for boats...but they have got the efficiency thing down, and it can be scaled down nicely.
Some of the other stuff on their page is pretty out there, though the wind anemometer made from a bicycle speedometer is either a work of madness or genius!
So what else can I drop into this that might be of help?
More blades is better. Lots of shorter blades is quieter and more efficient then two or three big long blades for wind gens.
Mounting higher up is better as there is more wind the higher above water level you get.
Do not expect too much from your wind generator....The sheltered anchorage you want for yourself at night is going to be the same place that blocks all wind to the wind turbine. Anchoring in an exposed 30 knots of breeze to feed your batteries seems a bit of a waste in terms of comfort and enjoyment (Trying to run the aft-rail charcoal grill in 30 knots is a special way to make friends with the boat owners downwind).
The added advantage of the broad disk type generator is that you get rid of the drag of one of the bearings(the whole thing runs on one), which is nice)
I think I have more or less emptied my brain on the subject for a bit. I am going to go back to parusing the posts.
Alex.
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