Hi all, I get asked this question frequently, as I am sure many others do, and of course the first answer is "it cannot be done". But can it? I have a customer who is very keen on trying to make this happen. After a few lengthy conversations, he still wants to try it, even after understanding the limitations (i.e. a cloud passes by and now you cannot power loads...)
I have some thoughts on how to make this work, and thought I would post it here to get others comments who I am sure have more experience in something like this than I do. -PV system would have to be backfed, so that they can turn off the main breaker to disconnect from the utility entirely (and then have to look at the neighbors, or guess when the grid comes back) -Have a fairly small 12V off grid inverter/charger than can be powered by a single 12v battery. This inverter would have to be manually turned off until after the grid is down and the main breaker is disconnected. -Would also need an auto transformer to bump up the voltage to 240V. Something like the Outback PSX240. -Once the main breaker is off and the off grid inverter is on, would it be able to wake up the grid tied inverters? On this project, there are two SMA SB-40 inverters. -There would need to be some kind of dump/diversion load to deal with excess power coming from the GT inverters. I have not used these in the past, but are the controllers smart enough to just deal with any excess power in the system? -Have a trickle charger on the battery to keep it topped off most of the time. -Another thought would be to DC couple. This would involve having a DC switch in one of the strings going to the GT inverter, such that when the house is islanded from the grid, they can flip the switch and have that one string go to a high voltage CC. Then they could just run some small loads off the power from that one string, or maybe we could allow it to try to wake up the GT inverters as well. This particular homeowner specifically does not want much in the way of batteries and has a smaller budget than will allow something sensible like a Sunny Island. At least in upstate NY, there are many people who would like to be able to take advantage of their solar when the sun is shining, but who just do not have the budget for a legit battery system. I am somewhat surprised that there is no simple industry solution for this yet, although maybe it is more complicated than I am making it out to be. And of course, you need a very hands on homeowner who very well understands the limitations of this kind of setup. I would certainly appreciate any feedback or comments on this, and also would like to hear if anyone has tried this kind of setup before. Thanks much! -Dave *Dave Tedeyan, PE* Senior Engineer | Taitem Engineering, PC 110 South Albany Street | Ithaca, NY 14850 o. *607.277.1118 x121* f. 607.277.2119 www.taitem.com Solar • Sustainability • Energy • Design B-Corporation Best for the World 2018 Honoree
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