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Rob Sigg
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Posted this on the TSP forums, looking for some solutions.

Hi guys, skip to the second paragraph if you arent interested in the backstory

My system has been running really well. I have 3 240 watt panels in series, a 60amp TSMPPT CC, 4 105 Ah batteries, 24 volts. I keep a small radon fan on a timer to run during the day since it is tied into my battery ventilation. I generally don't draw down past 90% DOD/SOC even without a day or so of sun, but when I dont have sun and I want to run more loads like my freezers it obviously drains too far. Now I can just keep the system as is for emergencies and basic ventilation which was my primary goal, but I would really like to put it to work everyday instead of just emergencies.

Is there anyway to connect my larger loads to my solar system and the grid so that when there is enough voltage and power the loads will use the solar system and when there isnt enough it will switch back to grid tie? I think there are relay options, but are there any other DIY type options? Thanks!

 
Marcos Buenijo
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Hey Rob, I know this is not the answer you were hoping for, but I can't help but to suggest that you should be using a grid tie system if you have grid electricity available. A battery system can be put on a float charge to be ready when/if you need it. There are inexpensive grid tie inverters on the market that allow you to connect the array directly to the inverter, then plug the inverter into a wall socket. These inverters match the frequency and phase of the ac power to the grid, then boost the voltage to feed all the power from the array to the home circuit. They also use MPPT tracking. I was considering this same configuration for myself, and it makes sense as long as grid power is available. I just can't justify beating up a battery if it's not necessary.
 
R Scott
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Marcos is right, there are simple grid-tie inverters out there--they backfeed through an outlet just like a suicide cord for a generator(!) but have GFI-like circuitry to shut down if grid power is lost.

Google grid-tie micro inverter.
 
Rob Sigg
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Sounds like an option, does anyone have any recommendations on brands etc? I assume the inverter can also be used as a standard inverter without grid tie? IM asking because it would be a good back up and would allow me to use my current inverter for other applicaitons too.
 
R Scott
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Nope. That's a downside. The safety circuit that keeps you from backfeeding the grid also means it NEEDS grid power to work.

There is a way to trick them into working using a whole house UPS, but that is getting complicated and beyond what I know how to do for sure. That can also get you into a grey area for legality--power companies take backfeeding pretty seriously.

Enphase is the well-known brand, but there are others.
 
Marcos Buenijo
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Rob Sigg wrote:Sounds like an option, does anyone have any recommendations on brands etc? I assume the inverter can also be used as a standard inverter without grid tie? IM asking because it would be a good back up and would allow me to use my current inverter for other applicaitons too.


R Scott is right. The inverters need grid power to function. However, I have wondered that it might be possible to use a small pure sine inverter on a battery system as a proxy for grid power to get these to function. Of course, there are other problems associated with that approach. If I had a solar system installed, then I would be curious enough to do some testing (wink wink, nudge nudge, .
 
Rob Sigg
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I wonder if I could hook up a programmable trickle charger to the battery bank so that when the panels arent charging the batteries the trickle charger will keep it above and below a certain voltage? Im only asking because the grid tie may not be an option with my provider. IM still waiting to hear from them. Besides, with only 720 watts I really wouldnt be putting too much into the grid.
 
Marcos Buenijo
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Rob Sigg wrote:I wonder if I could hook up a programmable trickle charger to the battery bank so that when the panels arent charging the batteries the trickle charger will keep it above and below a certain voltage? Im only asking because the grid tie may not be an option with my provider. IM still waiting to hear from them. Besides, with only 720 watts I really wouldnt be putting too much into the grid.


What I understand is that the battery is charged by two sources: (1) a charger powered by the grid, and (2) a controller connected to the solar array. I think this would work. However, whatever circuits are powered off the battery (and solar array by extension) would have to be isolated from the circuits powered by the grid.
 
Rob Sigg
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Right, so you got the panels going to a CC, then wiring from the CC to the batteries. Then a grid powered CC hooked directly onto the battery terminals at the same spot where the solar CC is connected. Although I think my inverter would then technically be hooked up to the grid CC as well and you are saying that wont work? Thanks!
 
Marcos Buenijo
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Rob Sigg wrote:Right, so you got the panels going to a CC, then wiring from the CC to the batteries. Then a grid powered CC hooked directly onto the battery terminals at the same spot where the solar CC is connected. Although I think my inverter would then technically be hooked up to the grid CC as well and you are saying that wont work? Thanks!


Just to be clear, you can connect a grid powered "CC" to a battery, connect the solar powered "CC" to the battery, then connect the battery to the inverter. However, you can't connect the AC side of the inverter to a circuit that is simultaneously powered by grid AC as that would be a "grid tie" system that requires a grid tie inverter.
 
Rob Sigg
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Ive attached a pic to show the idea. The CC that is run off of AC/Grid would need to be programmable so that its only charging when the voltage drops below a certain point and the array cant charge the batteries. Make sense? Is there such a CC?

 
Rob Sigg
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Had trouble with pic, see below.
 
Rob Sigg
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pic
system.jpg
[Thumbnail for system.jpg]
 
Marcos Buenijo
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Rob Sigg wrote:Ive attached a pic to show the idea. The CC that is run off of AC/Grid would need to be programmable so that its only charging when the voltage drops below a certain point and the array cant charge the batteries. Make sense? Is there such a CC?


That's pretty much what I was thinking. I don't know about the programmable controller, but perhaps a timer can be used to simply cut out the Grid CC during maximum solar exposure to the panels. That seems a simple solution. However, it seems there is a bigger problem. If the battery is charged by the grid, then it should be fully charged by the time the panels are producing. So, it seems there is little opportunity to use the energy delivered by the panel array. There would have to be loads operating while the array is producing and at or greater than the rate at which this electricity is generated in order to put the energy to full use. This is exactly the same problem presented by a grid tie system without net metering. Batteries really only make sense in the off grid setting. However, keeping a battery system on float for back up power seems reasonable.

It seems the only practical configurations include (1) grid tie with net metering, (2) off grid only when grid power is not available. I think there are ways to use thermal masses as "batteries" in lieu of the lead acid kind, but that's unconventional. For example, a large solar array could use a modest battery system by diverting the excess energy into loads that store energy in the form of ice or heated water for food preservation, air conditioning, space heating, and water heating.





 
Rob Sigg
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This wont work with a programmable feature based on voltage, the idea is to have the loads run off panels and batteries until the batteries get drained below a certain voltage, in my case probably 25.4v.

As for the storage idea, Im basically trying to do that with freezers, the problem is that even if I have them set to run during the day it wont work if its cloudy.
 
Marcos Buenijo
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Rob Sigg wrote:This wont work with a programmable feature based on voltage, the idea is to have the loads run off panels and batteries until the batteries get drained below a certain voltage, in my case probably 25.4v.

As for the storage idea, Im basically trying to do that with freezers, the problem is that even if I have them set to run during the day it wont work if its cloudy.


OK, I wasn't seeing the same purpose for the desired configuration. I thought perhaps you wished to avoid almost any discharge on the battery. I think the reason I misunderstood is because I can't see a good reason to use a battery when grid power is available. However, it seems in your case you are trying to put as much energy from the solar panels to use as you can while tapping the grid to (1) keep the loads energized, and (2) prevent excessive discharge on the battery. It seems you want some kind of automatic transfer switch. After I considered this I did a google search for the term "automatic transfer switch solar battery 12 volt" and came up with this (I know you've got a 24 volt system, these are just for ideas):

http://www.wind-sun.com/ForumVB/showthread.php?18406-Manual-to-Automatic-Transfer-Switch-Setup

I don't know if this will help as I didn't read the specs too much, but I'm just hoping this can lead you to a solution.
 
Rob Sigg
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you got it! Ive attached the response I got from Morningstar on the subject. Im not a smart person especially when it comes to electricity so most of this is over my head lol.
 
Rob Sigg
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Having problems attaching the pic again....grrrr!
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