Help to design 20 KW grid tie system

The utility grid, to a GT Inverter, looks like a "giant AC Battery Bank"... So any power not used locally, simply goes "backwards" through the utility meter (depending on meter and your utility, the meter should actually turn backwards).

In the case of your AC genset, say the school only runs 5-6 days a week, the last day, the school is mostly shut down and the excess power tries to run backwards through the genset. Usually, this is not a good thing.

Not to say that this is not impossible to do (use solar assisted power generation)--But it is something that would have to be coordinated with the generator manufacturer and see what they support, and what vendors they support for solar power.

Many different ways to address the question, but I do not know of any off the shelf solutions myself (I am not in the solar/generator business, so there is a lot I do not know).

-Bill

While I can think of a couple of ways one could combine a genset+GT inverters--I don't know of any off the shelf solutions.

I would suggest you talk with somebody in-country that does make AC inverters, GT Inverters, or Gensets (inverter gensets would be very interesting).

It is very much a control theory type of problem. One way to understand the problem with an analogy:

You have a truck with a relatively inefficient gasoline (petrol) motor. You have ignition+gas peddle+transmission+brakes. Very understandable how to "run" the truck.

Then you add a second, very efficient diesel motor (almost "free" energy for this example). All you (as the driver) can do is turn the motor on/off and use your brakes. And, the advanced assist motor is run by your kids who play with the throttle--They can run it at full power, 1/2 power, or zero output at any time--The peak available power is based on the amount of sun hitting the roof of the truck (more sun, more peak power).

Here you go, driving down the road (up/down hill, crawling through traffic, highway speeds, etc.). And the kids are in the back seat playing with the throttle on the diesel. And you can pass a note back to them once in a while saying 20% power, 100% power, 80% power, oh my g*d--turn the thing off--As you are driving. Quite a problem, is it not?

I can see some interesting solutions--One being feed back based on fuel flow (say you want the genset to have a minimum fuel flow of 2 liters per hour). When the fuel flow exceeds 2 liters per hour, the GT controller is commanded to slowly bring up the solar power until the fuel flow drops to 2 liters per hour (and conversely, as the fuel flow drops below 2 liters per hour, the fuel flow is reduced). And you have a rapid response. If the voltage goes over 240 VAC, the GT inverter cuts back to zero output and waits for the fuel flow to stabilize again.

Note that this does require the genset to run 7 hours per day, and the GT solar would only shave fuel costs, not eliminate them.

SMA makes a GT Inverter that has a "throttle function"--Might be interesting.

That is one example of a "simple" feedback loop the recognizes that the genset need to run with a minimum output power.

Does anyone have something like that today? Not that I know of (again, I am not in the business), but it could be something that does not require a lot of new design/manufacturing efforts.

If you want something that can run your school without the genset running full time--You look for an "off grid" AC inverter that runs the school. And you get a high performance battery bank (high surge current like AGM, Lithium, or some other exoctic like liquid sodium). The battery can short term (say 15-30 minutes) run the AC inverter+school loads, and the solar "charges the Battery bank. And if the loads exceed solar capability, then you start up the genset + AC battery charger to keep the battery bank between 50 to 80% (depending on chemistry). Set the genset for 50% state of charge and the solar for 90% state of charge... The genset charger will minimize fuel usage and when genset demand drops to zero (say battery>80% State of charge), the solar is then capable of supplying 100% of the load without fuel usage.

You might even be able to do with a with an Inverter-charger that has generator support. Need a fair amount of program-ability for the inverter-charger system. Also, the batteries would require a fair amount of investigation as to how large a battery bank you need and how long the batteries would last.

The only difference here is the smaller battery bank (try to keep battery costs down). Otherwise this is how many folks here run their off grid/hybrid power systems today--We just have a battery bank large enough for ~2 days of loads, and 50% maximum discharge. Flooded Cell Lead Acid batteries sort of high internal resistance when compared to AGM/Li Ion/etc. batteries, so a battery bank large enough to sustain high surge loads (well pump for our example, an elevator for your school if I remember correctly). A flooded cell battery bank large enough to support high surge currents tends to also to be large enough to support 1-2 days of "no sun" power (to 50% discharge).

In the end, is this better than a simple AC Backup genst + Utility Power--I am not sure I could make a financial case for it. Batteries are the weakness for any off grid/hybrid power system (expensive, limited life).

Anyway, some quick thoughts on the subject--I am sure that others can think of other solutions.

-Bill