Commercial Units: Why DC Output Instead of AC?

[zangort]

Hi Folks;

Let me begin by thanking everyone for the wealth of knowledge they share here!

So here goes...

I'm looking to put in a ~1000w+ wind generator for a cabin near Silverton, CO, and have been mainly focused on a "commercial" unit to alleviate the construction and warranty issues.

My problem is that the optimal tower site is about 400ft total (includes 80ft tower) from the cabin, and because of very low winter temps I need to keep the batteries at the cabin.

At 400ft the traditional 24v DC outputs from many of these units would require ridiculous wire sizes, so my preference is for AC outputs.

So the question: if almost all of these units are wild 3-phase AC generators anyway, why do they all seem to automatically drop it to DC at the head unit, instead of rectifying and/or stepping the voltage down at their controllers (or allow for such after the wire run)?  Is it just to save running an extra wire?  Isn't running the "raw" AC more efficient (less losses) for even modest runs?

(ASIDE: I've looked at Bergeys, but they don't seem to offer this option in their smaller unit; the Whisper 200 looks like the best option with a 220AC output; The TLG-500 (although smaller) looks interesting, but I'm concerned about it's so-called furling approach.  I flatly refuse to consider units without any furling method.  I just want a really reliable system that I don't have to worry about much in the winter [when winds can really climb and temps can hit -15F at night].)

Thanks for your help!

David

[Flux]

for a given voltage there is no virtue in running ac. In fact it uses less copper to run dc for a given loss.

To benefit from ac you would to have to generate at a high voltage and use a transformer at the far end. For this to be significantly saving you would need quite an expensive transformer and the saving on cable cost would be offset to some extent by the transformer cost. At very long distances this becomes your only practical option but at 400 ft at 24v the difference would be very marginal.

It is a little difficult to judge with commercial machines but with a home built axial machine over that distance if you designed it right the direct connection would most likely be a cheaper option than using a transformer.

If you start with a low voltage ac machine and transform up at one end and down at the other you would never get transformer costs below cable cost and still keep the same efficiency especially if you can build your inefficiency into the cable rather than into the transformers or alternator.

Considering a 1000W turbine only averages a few hundred watts I don't think a 400ft cable run at 24v is such a disaster as you imagine but the issue is clouded by the fact that the manufacturers will already built the necessary inefficiency into the alternator already to cut costs. This makes the cable issue significantly worse than with a home build.

Flux

[Ungrounded Lightning Rod]

Note that if you DO homebrew a genny for a long distance run you can wind it for high voltage in the first place and only need transformers at the load end.  That saves you both cost and transformer losses.  The capacity and losses in the generator are essentially dependent on the total cross-section of copper in the coils, regardless of whether they're a lot of turns of fine wire or a few of fatter stuff.

You can go up to several hundred volts before you start needing extra insulation on the wire.  (There's a reduction in eddy current losses with more turns of finer wire.  But eddy current loss in the coils is a tiny fraction of the losses so this isn't a significant help.  And you can get the same at lower voltages by going N-in-hand.)

[electronbaby]

You also could calculate the line resistance and therefore its loss, and wind your axial machine to compensate and therefore be closer upon cut in speed. There are many games you can play with the balancing act, that you will never be able to do with a commercially available machine and not void the warranties.

Flux is right, the machine will average low output most of the time (site dependent of course) so the losses will be relatively low during "normal" winds.

I think the main reason for manufacturers putting the rectifiers on board the turbine, is to cut down on install cost (wiring) and to possibly save one slip ring connection. It also makes it so that the average installer will not need to know what a diode is. :-)

[zangort]

Hi Flux;

I understand... I guess my "limitations" were based on the Bergey unit (24VDC only) - that unit would be running #2s at a bare minimum, which is pretty hefty by any standard.  The Whisper 200 gives me options for 3-phase 48VAC (which drops requirements to #8s) or 230VAC (which is just about anything short of telephone wire!)

I know a lot of folks here would encourage me to build my own, but I don't have the time for design.  Assembly (of even the generator) is pretty easy, but I don't have the room and all of the equipment to construct the alternator, blades, etc.

It's a tough call sometimes with deciding on pre-engineered construction vs. cost of unit, but I'm willing to spend the money if it means less hassle.  I've seriously thought about the units at otherpower.com because of the cost savings and solid construction, but I'm still not certain if something goes wrong, and the 5 year warranties from the others are a nice peace-of-mind...

Thanks!

David

[electronbaby]

Solar Converters Inc. makes a boost circuit that is supposed to be able to be used with the Bergey XL.1 to allow it to function at 48v nominal. A good friend of mine purchased one, however is not going to be able to try it till late spring time. This might be up your alley, but unfortunately I can not attest to how well it works as of yet. :-/