when to use AC vs DC

[jkermath]

I am contemplating a 12' wind generator which should get close to 1kw and am wondering at what point do you begin using an AC motor versus DC to be able to tie into a grid? This is for our home and it is fairly windy here.

Jeff

[snake]

in fact a battery need dc current to be charged,it all depends if you build the wind turbine using magnets or its a ready made dc motor.if it is the 1st option,then it will be definitely an AC.i think ur setup should be an ac one.clarify me if i am wrong please.

[bob g]

i guess the answer might be hinged on:

1. economic factors, cost per kwatt from the grid?
   
2. offgrid vs getting a line brought in? and how far?
   
3. what your needs in kwatt/hrs/month are?
   
4. what your "real" wind resource is?
   
5. etc.
   
   

just about anything is possible with enough engenuity (sp), time or money

and what makes sense for one might be insanity for others?

i have seen grid tie inverters down to the 100-200watt capacity, which is quite small in the scheme of things, and might be perfect for someone with enough wind to keep it busy? the level of busy is a subjective term as well.

not sure there is a good answer to your question with the info provided, even with more info there are other factors that might sway you one way or the other?

some folks are very practical and want a return on investment, while others are content with feeling "green", and could care the less if it ever pays out.

maybe a bit more info would help?

bob g

[Flux]

The big problem with grid tie is the legal issue. Unless you use a certified inverter you have real problems.

Whether you can ever convince anyone to let you run an induction generator would depend on your local supply authorities. If they have had reasonable dealings with smaller commercial induction grid tie they may approve it, otherwise the answer is likely no.

The common schemes use an alternator rectified to dc and fed to an approved grid tie inverter either with or without batteries. Once the inverter is approved they aren't much interested in the turbine.

Trying to directly grid tie and induction machine will probably be rejected by the authorities unless you have the backing that the bigger commercial people have with approved control gear.

Flux

[Ungrounded Lightning Rod]

In addition to the legal issues with grid tying an induction generator you have other practical issues with a grid-tie only machine:

1.  You have no power when the grid is down.
   
2.  You have no load on the rotor when the grid is down.
   
   

The second one is a really bad thing.  The load on the rotor is what keeps the mill RPM from running away and the mill from self-destructing in a high wind. (Even if you have offset/hinged-tail furling the yaw forces that drive it depend on the drag of the turbine - which drops drastically when it's unloaded.  So the furling system won't prevent runaway without electrical load.)

Thus a grid-tie-only machine needs some other furling mechanism to provide safety during power outages.  Something like BIG mechanical brakes that come on when the power drops (and are thus a continuous load when you're trying to generate), or motorized furling that is powered by an uninterruptable backup supply (in which case you have all the DC and battery-maintenance issues of a make-DC system, plus extra control systems and mechanicals to be points of failure).

High winds are associated with storms, which tend to cause power outages.  So expect to be without power when you need to furl.

= = = =

So IMHO it makes far more sense for a home system to be:  Turbine charges battery, dump load controller prevents overcharge, inverter powers house and (optionally) backfeeds the grid and/or pulls extra power from the grid when needed.

As was pointed out above, if the grid-tie inverter is approved the power company won't care very much about the rest of the system.  You'll have your approvals, system safety, power during outages, a time-proven design, and simplicity (especailly where it comes to failure modes).

Now if you're a wind-farm with megawatt-level turbines you both need the extra control complexity and can amortize it across a lot more kilowatts of capacity and cost.

[Ungrounded Lightning Rod]

So IMHO it makes far more sense for a home system to be:  Turbine charges battery, dump load controller prevents overcharge, inverter powers house and (optionally) backfeeds the grid and/or pulls extra power from the grid when needed.

Think "Big UPS with a renewable energy charging system that can backfeed the grid when it has a power surplus."

[ADMIN]

Even if you do grid-tie with no battery bank (non-islanding) you need to convert the wild frequency 3-phase AC into DC for a grid-tie inverter to function.

Your only other option is a grid-synchronous wind turbine. Not ALL that difficult to design and build, but good luck getting the utility to let you hook it up if you built it yourself!

ADMIN

[jkermath]

thank you! I guess I'm trying to determine if this project is worthwhile... I have a 3,000 ft home, live in a windy area and want to have no more than 5k invested, can have a 35' tower so wanted a 12' generator.   3 kids we consume alot of power! Noone has done it in our area of Ann Arbor (probably due to commercial cost).  I know a worthwhile battery bank will be substantial and in addition need a place for it so wondered if I tied it into the grid if it made sense... Really interested in it but won't spend a fortune. I wanted to have some solar too later but really interested in the wind thing.  Your expertise is appreciated.