Interactive PV systems and batteries

[George Stolz]

If a single-family PV system is interactive with the utility, would you include batteries in the installation, if you were designing the system?

Would you consider trading a generator for a battery bank for backup, if optional standby power was desired?

 

[Lcdrwalker]

From what I know about PV systems and batteries there would be several variables based on the reliability of POCO services and money. Batteries would be the cheapest for ride through power during short outages but would be ineffective for outages lasting more than a day. Batteries would have to be replaced every ten ten years or so. With proper maintenance, a generator will last forever.

 

[rabtrfld]

Generator!!! Batteries would deteriorate before they were ever needed.

 

[Billy_Bob]

That is the beauty of Net Metering and Grid-Tie PV systems - the utility acts as the "battery"!

-During the day excess generated electricity goes to the utility.
-At night the utility provides all the electricity.

No need for batteries. Batteries are expensive to replace!

For standby, I would prefer a generator. If power must not be lost to something until the generator kicks on, then a computer type UPS system (battery) could do the trick.

I once took a tour of a telco central exchange. They had a large bank of batteries just to provide power until their dual "jet engine" powered generators could ramp up to speed.

 

[davedottcom]

Hey George!

I've done a bunch of the Grid-Tied (Interactive Systems). The Interactive systems are designed to shut down in a power outage, making it impossible for it to back feed to the POCO. I don't recommend the batteries at all. Very expensive to maintain/replace. If you want to have emergency power, a generator is the way to go. It will usually be a completely seperate system... however you must be careful not to feed the inverters from the generator!
What will happen you say?
Well, in a power outage the Inverters will shut down, the generator will start... the inverters will start back up, and once they start producing power it will actually back feed the generator causing the Gen. Breaker to trip. Then the generator will just sit there running, but nothing will be energized!
So just make sure the inverters are not fed from the generator. This requires both a transfer switch for the Gen. of course & an isolating contactor between the Inverter AC output & the Solar "Back feed" breaker.

Dave

 

[iwire]

If a single-family PV system is interactive with the utility, would you include batteries in the installation, if you were designing the system?

No, I would not. The utility is the battery.

Would you consider trading a generator for a battery bank for backup, if optional standby power was desired?

No I would not.

It will cost you a lot more to provide batteries then a generator if you need any reasonable amount of current / run time.

 

[iwire]

however you must be careful not to feed the inverters from the generator!
What will happen you say?
Well, in a power outage the Inverters will shut down, the generator will start... the inverters will start back up, and once they start producing power it will actually back feed the generator causing the Gen. Breaker to trip. Then the generator will just sit there running, but nothing will be energized!

There is no reason that problem can not be worked around.

Back feeding the generator is no different then back feeding the utility.

 

[George Stolz]

Thanks for the replies!

My buddy is considering taking on a job with ICF walls and a SIP roof, and the interactive solar was just one more headache in a laundry list.

Off the cuff, I had suggested scrapping the battery idea and installing a generator, and then decided that I'd better get some input before throwing out potentially bad free advice.

I will keep the transfer switch in mind, many thanks.

 

[davedottcom]

There is no reason that problem can not be worked around.

Back feeding the generator is no different then back feeding the utility.

Bob, the poco doesn't usually have a tiny little 50 amp breaker on it! :grin:

Seriously, what would you suggest? The inverters need a 240 V AC circuit to even turn on. That same circuit is also the backfeed.

If there is a way, I'de be very interested in hearing it.

 

[iwire]

Bob, the poco doesn't usually have a tiny little 50 amp breaker on it! :grin:

As odd as it sounds I am in the middle of designing a 600 amp to 800 amp service upgrade because that little 600 amp breaker could be tripped by the 520 amps our inverters are capable of.:smile:

Seriously, what would you suggest? The inverters need a 240 V AC circuit to even turn on. That same circuit is also the back feed.

If there is a way, I'de be very interested in hearing it.

How large an array are you taking about?

If the array outsizes the gen set the simplest solution might be to break the array up between two inverters, one being sized same as the generator output.

 

[davedottcom]

That is the beauty of Net Metering and Grid-Tie PV systems - the utility acts as the "battery"!

-During the day excess generated electricity goes to the utility.
-At night the utility provides all the electricity.
.

It's not that clean-cut! It acts like a battery that charges you every time you draw power from it!
The poco gives credit (at a set rate) for any power that is backfed into the grid, however it charges a higher rate for the power it supplies. (Fuel costs, taxes...)

That's how they getcha! :smile:

 

[davedottcom]

As odd as it sounds I am in the middle of designing a 600 amp to 800 amp service upgrade because that little 600 amp breaker could be tripped by the 520 amps our inverters are capable of.:smile:



How large an array are you taking about?

If the array outsizes the gen set the simplest solution might be to break the array up between two inverters, one being sized same as the generator output.

Most are 5KW, 1 inverter.
Currently I'm doing a 26KW, with 5 inverters into a 400 Amp 3-phase service.

Do you mean only powering 1 of the 2 inverters via the generator? (5KW job)

 

[iwire]

Do you mean only powering 1 of the 2 inverters via the generator? (5KW job)

Yes, but I would also not be surprised if someone does not have a way to deal with this directly with hardware.

For example Outback Power systems has a nice set up that can deal with both a turbine and a solar array.

 

[davedottcom]

Bob, having only 1/2 of the 5KW system seems hardly even worth the effort, to me. You still need a constant 240V power source (Generator Running). Then again, it would take some of the load off the Gen. which increases the gas mileage!:smile:

I have it! A second, much smaller PV system that only provides the 240V power to the inverters! A transfer switch would isolate it from poco. No generator needed!