grid tie to grid tie with battery backup.

davidwillis · October 2013

Sorry, I have a lot of questions here. But first let me explain what I have.

I have 12 225W (60 cell) panels connected to a grid tie inverter. This is working great, but I have always wanted to also set it up for a small battery backup system so that when the power goes out I can keep essential things running. This has not happened yet, but I want it just in case. For example I have a wood boiler that needs to run a blower, and some water pumps (this would only be in the winter). I also have a fridge and a freezer that I need to keep cold if the power is out for very long.

So here is my plan. I have it wired so all these panels are in a series, however for every two panels I run the wires into my garage, and tie them together. I did it this way so that I can setup some switches to disconnect my grid tie inverter, and run 6 lines (each line is two panels in series) in parallel to a 48v battery bank. Basically this will reduce the voltage down to what would work well charging a 48v battery bank. It actually would not go directly to the battery bank, but I would use a charger like this: http://www.colemanair.us/vp_asp/scripts/shopexd.asp?bc=no&ccode=C440-HVM

Next I would just need to connect an inverter up. I am thinking I could just use a 110v true sine wave inverter, and hook the 110 up to each leg (since I would not use anything that uses 220v power off the inverter). Ohh, I almost forgot, I do have a shutoff switch to the grid that i would turn off before I connected my inverter.

So here are my questions.

1- Can I use 15A 3-way switches used for house lighighting (found at home depot) for switching over from the grid tie to the battery charger? They would have less than 8A DC running through them, and I would not switch them while under a load.

2- What is a good inverter to use. I am thinking it should be a low frequency one since it will have to start a freezer pump, etc. But I want one that I could run electronics as well. This one is tempting, but I am affraid it will not be as good as they say: http://www.ebay.com/itm/3000W-5000W-6000W-8000W-15000W-Low-Frequency-Power-Inverter-12-24-48VDC-Charger-/161042505900?_trksid=p2054897.l4275

I am also looking at this, but I don't know if it is a low frequency one. http://www.ecodirect.com/ProductDetails.asp?ProductCode=Samlex-SK3000-148&gclid=CIKygOnnrboCFaU5QgodiUEAyw

This one looks good too, but the THD is a little high. http://invertersupply.com/index.php?main_page=product_info&cPath=21_129&products_id=487

3- Is there a better way of going at this than I am trying to do?

Thanks

jcheil · October 2013

Re: grid tie to grid tie with battery backup.

Just a couple of points, from my experience:

Never use AC (only) rated equipment on DC unless you are trying to burn your house down.
Someone posted an awesome video a few days ago of a real world test of sending DC thru an AC breaker.
You would likely see the same result with a "switch" which, btw, I am not sure how you would be able to "only turn on/off when it is NOT under load" unless you were only going to flip those switches at night time when the sun isn't out? Kinda defeats the purpose of the switch then

And in my opinion, that Coleman "Charge Controller" is a piece of junk. Nothing but a huge relay that cuts in and out as needed to prevent overcharging of your batteries. Get a "real" charge controller.

And I won't even begin to comment on those china inverters that claim 15,000w at 12v YET only have standard 20-amp receptacle on them and no hard-wiring option. ugh...

Are you sure your current GT inverter cannot also support a battery bank?

BB. · October 2013

Re: grid tie to grid tie with battery backup.

I posted this link to 290 VDC on a standard AC breaker:

https://www.youtube.com/watch?v=HLX9cdB4TFQ

And here is what 300 amps of 12 VDC does to a nail/screw/bolt:

https://www.youtube.com/watch?v=Avg5mJw0qHc

Just to give an idea of what can happen with PV Power Systems--Very different than wiring your house for 120/240 VAC.

-Bill

bill von novak · October 2013

Re: grid tie to grid tie with battery backup.

1) Running all those wires will work but will be lossy. The closer you can do it to the panels the better. Another option is to just split it once, in the middle, then use a high voltage MPPT charge controller (like the higher voltage Midnight Solar CC's) to drop the voltage to 48V.

2) Inverter is your choice. I would avoid the Chinese types. The one you linked to claims that their 12V inverter can do 15kW. This is certainly a lie; it would be 1250 amps and those 5/16" studs aren't going to handle that. So you start out with them lying to you. There are plenty of good inverters out there from more reputable manufacturers like Prosine, Magnum, Exceltech, Outback etc.

3) Home Depot switches will work for the output but you can get circuit-by-circuit transfer switches (also at Home Depot) designed to do this for about $250. They're worth the reduction in hassle.

4) For DC switching use DC rated switches/relays. Digi-Key carries some good ones. Make sure you have a good DC disconnect for the string rated for 600 volts; that guarantees you can break the circuit before yoy do any switching.

Other notes -

You can switch to a 'real' hybrid inverter (like the Outback GTFX) that will solve all your problems and give you full time backup (protects your house when you're not there.)

For short term backup, cheap Best Buy UPSes can work without any rewiring.

The SMA "Secure Power System" option (on their grid tie inverters) gives you AC output even when the grid goes out, as long as you have sunlight.

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

Thanks for the comments.

My instincts were telling me the same thing on those cheap inverters.

Can you tell me more about this inverter

You can switch to a 'real' hybrid inverter (like the Outback GTFX) that will solve all your problems and give you full time backup (protects your house when you're not there.)

I am a little confused about what it does, does it work as a grid tie and a regular inverter? Also does it include the charge controller? It sounds like it does it all, but I suspect it is just an inverter with a switch built in to connect to your grid tie inverter, or your charge controller.

verdigo · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

Thanks for the comments.

My instincts were telling me the same thing on those cheap inverters.

Can you tell me more about this inverter

I am a little confused about what it does, does it work as a grid tie and a regular inverter? Also does it include the charge controller? It sounds like it does it all, but I suspect it is just an inverter with a switch built in to connect to your grid tie inverter, or your charge controller.

Outback's "G" inverters are actually what is referred to as a grid interactive inverter. Check out this http://www.solar-electric.com/oupogrsiwain.html for a good description and a place to link to the various models.

verdigo · October 2013

Re: grid tie to grid tie with battery backup.

I think magnum is offering inverters that allow A/C coupling which I'm sure someone is going to do a much better job of describing that process pretty soon than me.

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

Reading the description, in that link, it sounds like exactly what I need, however I don't see in the details (the downloaded pdf) what the solar panel aray voltage should be? It gives DC input voltage 42 to 68 VDC , but is that the battery bank, or the solar panel array, or both?

mike95490 · October 2013

Re: grid tie to grid tie with battery backup.

The Hybrid inverters only charge from the grid, or possibly a generator. For solar charging, you will need a separate Charge Controller (for the PV array).
the Xantrex XW & Conex inverters have up to 2 AC source inputs

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

The Hybrid inverters only charge from the grid, or possibly a generator. For solar charging, you will need a separate Charge Controller (for the PV array).
the Xantrex XW & Conex inverters have up to 2 AC source inputs

Thanks for clearing that up. So does the hybrid inverter work as a grid tie inverter, or do you need a separate grid tie inverter?

ArmandoEvans · October 2013

Re: grid tie to grid tie with battery backup.

mike95490 wrote: »

The Hybrid inverters only charge from the grid, or possibly a generator. For solar panel, you will need a separate Charge Controller (for the PV array).
the Xantrex XW & Conex inverters have up to 2 AC source inputs

Ok thanks for sharing information..I have got solar charging system and will now get seperate charger controller

ggunn · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

Sorry, I have a lot of questions here. But first let me explain what I have.

I have 12 225W (60 cell) panels connected to a grid tie inverter. This is working great, but I have always wanted to also set it up for a small battery backup system so that when the power goes out I can keep essential things running.

Some questions that you should ask yourself is what kind of incident you are trying to hedge against, how long is it likely to last, and how much you are willing to spend on it.

For example, if power outages are rare and short lived, IMO you should consider forgoing the functionality of your PV system during such an event and spending your time and money setting up a generator or a simple grid-charged battery backup system. A GT PV system is meant to offset your electric bill over the long term, and if grid outages are fleeting, the economic detriment you will experience from your PV system not operating during outages will be minuscule.

OTOH if outages are frequent and/or protracted, it may be advantageous for you to have a sustainable backup system which incorporates PV to keep your batteries charged, even though it will be a lot more expensive.

Your solution should fit the problem.

BB. · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

Thanks for clearing that up. So does the hybrid inverter work as a grid tie inverter, or do you need a separate grid tie inverter?

A hybrid inverter does both GT and Off Grid. In GT mode, the inverter takes excess charging current (from solar panels, wind turbines, etc.) from the battery bank and dumps it to the grid. Basically a "dump controller" with the grid being the dump load.

-Bill

bill von novak · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

Thanks for clearing that up. So does the hybrid inverter work as a grid tie inverter, or do you need a separate grid tie inverter?

You can do it both ways. You can do DC-linked (most common.) In this configuration it goes solar -> medium voltage DC -> charge controller -> low voltage batteries -> inverter -> grid. The inverter runs to pump power back out to the grid, converting it up from DC.

You can also do it via AC link. In this configuration you keep your grid tie system which delivers power to the grid via an internal switch inside the inverter. The inverter doesn't do anything normally other than keep the batteries topped off. When power goes out the inverter opens the switch and starts delivering power to the load. The grid tie inverter also delivers power to the load. In general you need either a grid-tie disconnect relay or a large dump load to make this work. There are several papers out there on how to do this. It's not intuitively obvious.

mike95490 · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

...does the hybrid inverter work as a grid tie inverter, or do you need a separate grid tie inverter?

it functions as a grid tie inverter & as a off grid inverter. hybrid.

ggunn · October 2013

Re: grid tie to grid tie with battery backup.

bill von novak wrote: »

.

You can also do it via AC link. In this configuration you keep your grid tie system which delivers power to the grid via an internal switch inside the inverter. The inverter doesn't do anything normally other than keep the batteries topped off. When power goes out the inverter opens the switch and starts delivering power to the load. The grid tie inverter also delivers power to the load. In general you need either a grid-tie disconnect relay or a large dump load to make this work. There are several papers out there on how to do this. It's not intuitively obvious.

An important distinction is that an AC coupled system uses two inverters - one to invert DC from PV and the other to invert DC from batteries.

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

ggunn wrote: »

An important distinction is that an AC coupled system uses two inverters - one to invert DC from PV and the other to invert DC from batteries.

This is something else I would like to learn more about. Could I get just a small sine wave inverter, and plug it into the house (of course make sure the grid is shut off first). This would give the signal to make the grid tie inverter start working. Then I could just charge the batteries with a 110v charger.... but what if my grid tie inverter puts out 2kw of power, but I am only using 500W? Will it blow up my small sign wave inverter by trying to feed the power to it?

bill von novak · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

Could I get just a small sine wave inverter, and plug it into the house (of course make sure the grid is shut off first). This would give the signal to make the grid tie inverter start working.

Simple answer - no. The grid tie inverter (GTI) would quickly exceed the loads, go overvoltage and shut down. You need an inverter that

1) can absorb the full output from the GTI
2) can handle bidirectional power transfer
3) has the battery capacity (primarily low enough impedance) to support rapid changes in supply and load (due to clouds, compressor starts etc)

BB. · October 2013

Re: grid tie to grid tie with battery backup.

Yes, it will "fault" the small sine wave inverter.

The GT inverter (as normally designed) will output whatever energy is avialable from the solar array... Up to 100% of its rated output.

So--In general, the Off Grid inverter needs to be at least as large, or larger, than the GT inverter (or maximum array power, which is highly variable between edge of cloud events, cold temperatures, etc.) that will be backfeeding it.

What will happen to the "too small" off grid inverter--Many will probably shut down as the over current circuitry (or battery over voltage limits) kicks in.

However--since they were never designed for such an operation, there is no guarantee that they will shut down safely every time.

-Bill

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

bill von novak wrote: »

Simple answer - no. The grid tie inverter (GTI) would quickly exceed the loads, go overvoltage and shut down. You need an inverter that

1) can absorb the full output from the GTI
2) can handle bidirectional power transfer
3) has the battery capacity (primarily low enough impedance) to support rapid changes in supply and load (due to clouds, compressor starts etc)

Are there any inverters made to do that?

bill von novak · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

Are there any inverters made to do that?

Trace SW series (old) and Outback GT series (modern) can do that. There's a Sunnyboy system out there as well.

ggunn · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

This is something else I would like to learn more about. Could I get just a small sine wave inverter, and plug it into the house (of course make sure the grid is shut off first). This would give the signal to make the grid tie inverter start working. Then I could just charge the batteries with a 110v charger.... but what if my grid tie inverter puts out 2kw of power, but I am only using 500W? Will it blow up my small sign wave inverter by trying to feed the power to it?

A grid tied inverter puts out all the power it can from the PV that is connected to it all the time, and if that power has nowhere to go, its output voltage goes out of spec and it shuts down. The SMA Sunny Boy/Sunny Island system has a slick way of dealing with that that lets the SI (battery inverter) throttle back the SB (GT inverter) gradually as demand diminishes.

DanS26 · October 2013

Re: grid tie to grid tie with battery backup.

David, you mentioned in your first post that you want to run blower motors, well pumps, plus refrig and freezer compressors. You better do a load study, especially start up loads, before you invest any money in this project.

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

Thanks everyone for the comments, I am learning a lot. However I am still a little fuzzy on how the ac coupled system would work.

Could someone explain how you would set that up with any of these

Trace SW series (old) and Outback GT series (modern) can do that. There's a Sunnyboy system out there as well.

A grid tied inverter puts out all the power it can from the PV that is connected to it all the time, and if that power has nowhere to go, its output voltage goes out of spec and it shuts down. The SMA Sunny Boy/Sunny Island system has a slick way of dealing with that that lets the SI (battery inverter) throttle back the SB (GT inverter) gradually as demand diminishes.

Also is there a battery inverter that would would make the batteries work kindof like a grid...

1) can absorb the full output from the GTI
2) can handle bidirectional power transfer
3) has the battery capacity (primarily low enough impedance) to support rapid changes in supply and load (due to clouds, compressor starts etc)

I just looked at the outback gt inverter, and it looks more like it uses one single inverter that does both grid tie and backup, but is there a small inverter that will do those three things to make the battery bank look like the grid to the already installed grid tie inverter (as long as you had a way to dump the load if the batteries got too full)?

bill von novak · October 2013

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

I just looked at the outback gt inverter, and it looks more like it uses one single inverter that does both grid tie and backup

Yes. In a DC coupled system the GTFX can provide both grid tie and backup. In an AC coupled system it provides only backup (and technically the GT interface) - a separate GT string inverter feeds back power.

but is there a small inverter that will do those three things to make the battery bank look like the grid to the already installed grid tie inverter (as long as you had a way to dump the load if the batteries got too full)?

No. Since both solar and your loads are unreliable (i.e. they vary depending on what you use and what the sun is doing) the inverter has to be able to both sink the maximum power generated by the GTI inverter and source the maximum you can use. Say you have a 3kW GT string inverter and a maximum of 2kW of loads. It would need to be at least a 3kW inverter.

(Now your next comment based on the direction you are going might be "but what if I just use a 100 watt inverter to 'trick' the grid tie system, then have loads come on when the power starts going up?" Yes, that is technically feasible - but no one has built such a device, nor would it be easy to build. It would have to stabilize the output of a grid tie inverter - and such an output is inherently unstable, since that inverter is trying very hard to detect that condition and shut down. The name for that process is "anti-islanding.")

davidwillis · October 2013

Re: grid tie to grid tie with battery backup.

Thanks everyone. I think I now have a much better understanding, and know all the options, and possibilities.

davidwillis · January 2014

Re: grid tie to grid tie with battery backup.

So I think I am going to change out my inverter for a GTFX (actually two of them). I am also going to increase my array, but I would like to see if this will work, and exactly what equipment I need.

I currently have 12 225 watt panels, and plan on adding 10 more 230 watt panels

since the panels are not all the same, I will need to use two charge controllers to keep the mppt working best. I would like to run two lines from each to each charge controller...

example: parallel two series of 6 panels (voc = 220v total watt = 2700 ) - feed this to one charge controller
Parallel two series of 5 panels (voc = 185, total watts = 2300) - feed this to the second charge controller

Now, since the voc is fairly high (over 150v, I would need to use two midnight 250 charge controllers (or on 250, and one 200). Then I guess I could parallel these into my 48v battery bank (can I do this?).

Now my next question is that I plan to parallel two GTFX3048 inverters (to get 220V split phase). Will these work with the midnight charge controller, or does it need to use outback charge controllers?

Does this plan make any sense? Do I need anything else?

mike95490 · January 2014

Re: grid tie to grid tie with battery backup.

davidwillis wrote: »

..... Now my next question is that I plan to parallel two GTFX3048 inverters (to get 220V split phase). Will these work with the midnight charge controller, or does it need to use outback charge controllers?..

Since you are getting new gear, why not a XW6048 and a midnight epanel to tie it all together with ? Or are you looking for the redundancy of dual inverters (questionable, because parts count just doubled.)

davidwillis · January 2014

Re: grid tie to grid tie with battery backup.

Thanks. I had not even looked at the XW6048, but that looks like it may be a better option. Does it include the chage controller, or does it need seperate charge controllers (like the GTFX does)?

mike95490 · January 2014

Re: grid tie to grid tie with battery backup.

6048 has internal 240VAC powered charger, but for the PV you will need charge controllers. I really wish Midnight went with a large passive heatsink instead of the thermostat fan. I hope my planned 2Kw array will be fine with it, but I don't want to be replacing fans and cleaning internal fins every couple years.

vtmaps · January 2014

Re: grid tie to grid tie with battery backup.

mike95490 wrote: »

I really wish Midnight went with a large passive heatsink instead of the thermostat fan. I hope my planned 2Kw array will be fine with it, but I don't want to be replacing fans and cleaning internal fins every couple years.

Apparently, the Classics do run hot when continuously pushed to their limits. But 2 kw into a 48 volt system is far within the limits of the classic, and it should run cool at that power level. You can keep it cooler by configuring your new array to have the lowest string Vmp that will work with 48 volt battery.

--vtMaps

grid tie to grid tie with battery backup.

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