How do you force feed current to the grid from a Solar system

Re: How do you force feed current to the grid from a Solar system

SUNUP wrote: »

The AC GT Inverter cannot support AC loads without an AC power source present.


So, if I have a 3000 watt Grid tie inverter and the grid is off I can't use the 3000 watts to supply current to any loads in the house.

That is pretty much correct... The typical or standard Grid Tied AC inverter (or also called Grid Interactive Inverter) just measures the AC line voltage and frequency. If they are within specifications, the GT inverter "injects" current into the AC mains and the energy flows to where ever it is needed. Just like your car's alternator will inject current into the cars wiring... The loads will consume power, and the battery will either absorb or supply current to keep the voltage at ~12.7 volts or so.

But, to confuse the issue a bit more, SMA now has a new GT inverter that has a smallish (something like 10 amps at 120 VAC?) outlet that can run small AC appliances from sunlight. Intended to supply AC power in an emergency. It does have its limitations (cannot supply power unless the sun is up and shining ~9am to 3pm) and if a bird flies overhead or clouds come by, the AC power will "go away" until the sun returns (GT inverter outlet may need to be reset too--You usually don't want your AC appliances to have AC power cycling to them or "browning out" from voltage sags).

Thanks ggunn and BB I think the picture is starting to come into focus. Where could one get a Schematic of a GT Inverter?

You will not usually find a schematic for the Inverters--The companies want to keep that stuff relatively controlled... They make money from servicing their products and don't want a cheap mfg to make knockoffs of their products.

A GT Inverter is going to look sort of like a typical AC off grid inverter. Much of the way a GT inverter behaves is based on the control/feedback circuitry. And that is not going to be "obvious" from just having a schematic (control theory in engineering is a huge/complex subject with lots of math--it is also very interesting).

To throw even more into the mix, there are newer GT Inverters that do not use isolation transformers. This cuts down on costs and weight (less iron and copper in the GT inverters). The variations and solutions out there and how engineers/companies choose to address those markets is quite amazing (at least to me).

Do you have specific question/application you are looking to address?

We have a small 2 wheel enclosed trailer with the following items installed and used for a portable power source. 4 130 watt solar panels, outback FX 60 charge controller, xantrex pro sine 3 inverter, 8-6 volt wet cell batteries and a 4 KW Genset. We also just purchased 10, 235 watt solar panels and not sure if we want to do the grid tie or off grid system using the 10 panels which will determine what additional Equipment I will need to buy. We have an all Electric home 3ooo sq ft electric bill average about $300 per month. I am leaning toward the off grid side.

Flat out, for costs and maintenance issues, Grid Tied is the way to go. Your true cost of power ($$$/kWH) with a GT System can meet or even beat the cost of electricity in states with high electrical costs (in summer afternoons, people that use >~1,000 kWH per month can pay upwards of $0.50 per kWH, a GT system can "make electricity" for ~$0.15 to $0.30 per kWH).

HOWEVER--There can be issues with the electric utilities... Many smaller utilities will not allow GT solar (illegal to install). And some (becoming more and more) are changing their rate structures... For me, I pay ~$4.50 per month and $0.10 to $0.30 per kWH, depending on time of day and season. And I get "full credit" if I generate kWH to the grid (i.e., I usually generate power at $0.30 per kWH and buy power at night at $0.10 per kWH--Makes my array look ~3x larger on summer weekday afternoons at peak time of use rates).

Now, utilities are starting to see losses from distributed solar power--And some have now changed the rates to something like $40 to $100 per month fixed charges and $0.06 per kWH for energy usage. In those cases, solar GT and even conservation may not make much sense (i.e., I conserve, I save $0.10-$0.30 or more per kWH--The new rate plan only saves $0.6 per kWH and leaves the fixed charged dominating the billing costs).

Which then makes you look at off grid power.... Normally, if you include installation+capital costs+battery/electric replacements (every ~10+ years or so), the cost of off grid power is in the range of ~$1-$2+ per kWH--Still 10x what utility power costs. Some folks have gotten their off grid power costs down to ~$0.50 or so per kWH--but that takes work+shopping around to get those prices).

Normally, I would suggest conservation first... It is almost always cheaper to conserve than to generate power. Insulation, double pane windows, modern A/C and/or Heat pump systems (even heat pump based water heating) can be very cost effective and even make the home a bit quieter/less drafty to live in.

And look at your utility's rate plans... Understand how they bill for power (low per month fixed charges, high per kWH charges; or the opposite) and if you can even connect grid tied solar.

In my region (California), it is actually illegal to install Off Grid Solar power for my home and leave the grid unless I pay the utility for the right to leave the grid (stranding charges--Utility took out 40 year loads "in my name" for distribution/transformer/power plants/ etc. and I need to "reimburse" the utility for their long term costs--at least that is how I understand the justification).

In general, I don't see GT or off grid solar power systems as an "investment"... The are just one of those things that you have to pay for if you want the power (and/or emergency backup power)--Just like buying a car is not an investment--Just a cost of living the lifestyle we choose.

-Bill