grid tie inverter concept with battery back up

[Alejandro luna]

Just wondering if this concept would work. Do not want to destroy any equipment if I am completely wrong for thinking this would work. Please share your thoughts.

[Rob Beckers]

Hi Alejandro,

There are a couple of issues with the concept as you drew it: You have a battery-based inverter connected to the grid. Some can do that (some Outback and Conext inverters for example, they are pricey though), most can't. The off-grid inverter creates its own grid, and that will clash with the utility grid. The two need to synchronize, or fireworks ensue.

The wind turbine you're showing is connected to a grid-tie inverter. Those inverters do exist (though they are getting rarer, as no regular inverter maker is producing them any more). Most grid-tie inverters are meant for solar PV though, and will not work well with wind as their input.

You have the same wind turbine also connected to a charge controller (as well as the grid-tie inverter). That won't work. There can be only one thing that is a load for the wind turbine, either a charge controller, or a wind inverter. Not both at the same time.

Hope this helps!

-RoB-

[Alejandro luna]

Thank you for the quick response rob!

I noticed the picture is poor quality but when the grid would go down I would manually cut off power from the grid and then and only then switch on the power of the battery based inverter.

I think this is probably a dumb question but could you explain further why the wind turbine couldn't have load from two sources?

Again thanks for the quick response! Can't wait to start putting everything together.

[Rob Beckers]

Alejandro, there are two ways power is extracted from wind turbines: The really simple way is to simply rectify the AC coming from the turbine's alternator, and connecting that straight to a battery bank. If the turbine was designed properly that works remarkably well. Some mechanism is needed to prevent overcharging the batteries in this case.

The other method (and that's what all wind grid-tie inverters use), is to use electronics to put an exact load on the turbine that is based on a lookup table linking output power and Voltage. If that table is done correctly, it will make the turbine run at its best lift-to-drag ratio, with the most power output for each wind speed. Some off-grid charge controllers can do this too, such as the MidNite Solar Classic.

Both mechanisms try to at least approximate the right load vs. the wind speed, even the more primitive one listed first. Getting the load on a turbine wrong means that either the turbine spins too slow, stalls, and never produces power worth a dime, or when it goes the other way and the load is too light it will overspeed and self-destruct.

The long and short is that the load on a turbine has to be (reasonably) right or bad things happen. So, connecting multiple loads to a single turbine won't work (unless it was designed that way, and both add up to something that is right for the turbine, but I've never seen that setup).

-RoB-

[Dave Schwartz]

A manual switchover will probably be illegal (or very expensive to have certified by the electrical authority) in any jurisdiction. What happens if you forget to disconnect the grid when it goes down and you start up your inverter? You try to feed electricity to the grid which either a) shorts out your equipment or b) electrocutes a lineman. Grid tie switching must be certified to standards to guarantee there are no circumstances where it tries to energize a dead grid. Not much tolerance for homemade systems in safety-critical applications.