Geothermal Inverter

[Colin McGonagill]

So I am in the midst of building a home based geothermal generator. I have just stumbled upon several inverters that meet my needs but wanted your opinion on some.

Grid vs Off Grid:

What is the main design difference?

From my homework it seems that the grid inverters are not ment to be used in the event of a power outage, they automatically shut off in order to not electrify the grid. Is there a way to "trick" the inverter to apply power. (Say apply an ac current simulating grid power)

The Off Grid versions seemed tailored to battery banks with low voltages and a different design philosophy which I dont yet understand.

My application needs the ability to have a grid tie, but in the event of a power outage, disconnect from the grid (say manually; thats easy) then apply power to the home, without a battery bank. I could add a small one as a buffer but mainly the system is way oversized so it shouldn't have a problem keeping up.

I have been looking at sunny boys and auroras with my preference being the aurora since I can specify the on trip voltage.

Will an offgrid inverter work best for my application (320V to 360V input) or should I stick to an on grid and retrofit a small 12Vto230V(splitphase) inverter to act as a trick for the other inverters to apply power?

There will be more questions to come but that will do for now.

[Rob Beckers]

Hi Colin,

First, welcome to Green Power Talk!

The fundamental difference between a battery-based inverter and a grid-tie inverter is that the former is a voltage source, while the latter is a current source. The effect is that a battery-based inverter will set a fixed voltage on its output, say 120V, while a grid-tie inverter will increase its output voltage until it reaches the current/power level that it wants to feed back into the grid, or runs out of voltage-room (and switches off with an error message). This difference is fairly fundamental, and I know of no ways to turn a grid-tie inverter into a voltage source, to make it work in the absence of an underlying grid (ie. so it would continue to create a local grid and provide power).

It is possible to use grid-tie inverters in combination with a battery-based inverter: The battery-based inverter creates a house-grid, this makes the grid-tie inverter happy and it will supply power to it. However, if there are no local loads to absorb that power the grid-tie inverter will increase its voltage. Most battery-based inverters have a topology that causes them to funnel power back from the house-grid side into the batteries (ie. you would be running the battery-based inverter in reverse). Charging batteries is good, but the downside is that there is no charge control and you have to make provisions to avoid overcharging the batteries. It would kill them in short order if allowed to happen regularly. That can all be done, and has been done. You can find more information by looking up "AC coupling", that is the term inverter manufacturers have giving to this combination of grid-tie and off-grid inverters.

-RoB-

[Colin McGonagill]

That helps alot. I however wonder why the design of the grid tie is current based. It makes sense from a controls perspective but it would seem running two voltage sources in parallel would be more benificial. For my app would you recommend an off grid inverter or Grid Tie? I can get buy with out having backup. But would like it if possible. I could design a transistor resistor bank if necessary but more than likely i would design the control on the generator to de throttle.