Grid-tied PV system with battery backup

[greenleaf]

Just curious if anyone has such a system?

I just started looking into this and will consider one if the cost is not too prohibitive.

Thank you!

 

[TomT]

As soon as you start adding batteries, the cost and maintenance skyrockets.

Just curious if anyone has such a system?

I just started looking into this and will consider one if the cost is not too prohibitive.

Thank you!

 

[planet4ever]

I don't know, I was thinking maybe seven years or so down the road when my Gen 1 LEAF starts looking and feeling like a dinosaur I might park it permanently in the yard and connect it to my PV system. :lol:

Ray

 

[greenleaf]

Let me add some color to the battery backup. With battery backup, your house basically will not have an electricity outage if the grid is down. When the grid is down, PV system can continue to function in the day to provide electricity to the house and to charge the batteries. At night, power can be drawn from the batteries.

The batteries need to be sized to requirements. By my very rough calculations, a battery system that supplies 5 kWh per day for three days would cost around $5k. To that, you need to add an inverter that ties the battery and grid together, probably in the range of $2k. And also add installation costs and permits.

Batteries would require maintenance (or replacement?) in 5 to 7 years. Probably longer if they are hardly used.

 

[Uccello]

I'd check with the local power company first to see what if any restrictions there are. I know when we installed our PV they made sure we understood that if the grid goes down, our PV will automatically shut down so it doesn't backfeed into the system unexpectedly.

I seem to have a vague memory when the Leaf was first announced that Nissan was planning on using the spent batteries for power backup in large buildings.

-u

 

[patrick0101]

Unless you have unreliable electricity service, battery backup is not worth the cost. You would be better off buying a bigger PV system with that money.

 

[greenleaf]

I'd check with the local power company first to see what if any restrictions there are. I know when we installed our PV they made sure we understood that if the grid goes down, our PV will automatically shut down so it doesn't backfeed into the system unexpectedly.

Yes, this is true. All grid-tied inverters detect "islanding" to prevent backfeed when the grid is down. The inverter for grid-tied system with battery backup would do the same.

 

[AndyH]

Just curious if anyone has such a system?

I just started looking into this and will consider one if the cost is not too prohibitive.

Thank you!

http://www.mynissanleaf.com/viewtopic.php?p=58234#p58234

There are a couple of ideas here, greenleaf. The "midnight special" from Homepower magazine gives details of essentially a 'whole house UPS' - and Hill's system with a 240V backup and a battery bank is another - and check out his link to the PriUPS project - the guy uses his Prius battery for backup power.

Enjoy!

 

[Ingineer]

I'd check with the local power company first to see what if any restrictions there are. I know when we installed our PV they made sure we understood that if the grid goes down, our PV will automatically shut down so it doesn't backfeed into the system unexpectedly.

Yes, this is true. All grid-tied inverters detect "islanding" to prevent backfeed when the grid is down. The inverter for grid-tied system with battery backup would do the same.

This is not true! There are combination inverters that do grid-tie as well as battery backup. For instance the Xantrex XW series can do both battery backup and grid tie (sell mode). It is true that there is anti-islanding protection in the grid-tie function, but once the grid goes down in a combination inverter, it simply switches modes to power the backup output.

There are other ways to do this, some are hacks, and some are limited in function. I've engineered a grid-tie/off grid solar system with a 192kWh battery back-up system. It produced 3-phase power at up to 33kW in backup mode, and could sell to the grid at up to 30kW (limited by panel performance). It was portable (everything but the panels) installed in a 20' shipping container. Just drop it and hook up the connections!

I installed it at Burning Man 2006 as a demonstration project with a 30kW array, and is now owned by a private entity.

-Phil

 

[greenleaf]

I'd check with the local power company first to see what if any restrictions there are. I know when we installed our PV they made sure we understood that if the grid goes down, our PV will automatically shut down so it doesn't backfeed into the system unexpectedly.

Yes, this is true. All grid-tied inverters detect "islanding" to prevent backfeed when the grid is down. The inverter for grid-tied system with battery backup would do the same.

This is not true! There are combination inverters that do grid-tie as well as battery backup. For instance the Xantrex XW series can do both battery backup and grid tie (sell mode). It is true that there is anti-islanding protection in the grid-tie function, but once the grid goes down in a combination inverter, it simply switches modes to power the backup output.

There are other ways to do this, some are hacks, and some are limited in function. I've engineered a grid-tie/off grid solar system with a 192kWh battery back-up system. It produced 3-phase power at up to 33kW in backup mode, and could sell to the grid at up to 30kW (limited by panel performance). It was portable (everything but the panels) installed in a 20' shipping container. Just drop it and hook up the connections!

I installed it at Burning Man 2006 as a demonstration project with a 30kW array, and is now owned by a private entity.

-Phil

I meant to say that it is true that grid-tied systems will prevent backfeed during grid power outage.

I am investigating the possibility to continue to have electricity for the home during a grid outage. I definitely want PV to continue to run.

 

[Herm]

The batteries need to be sized to requirements. By my very rough calculations, a battery system that supplies 5 kWh per day for three days would cost around $5k. To that, you need to add an inverter that ties the battery and grid together, probably in the range of $2k. And also add installation costs and permits.

Note that if you use lithium cells you should not keep them fully charged if you want them to last.. most of these systems still use lead-acid batteries.

 

[AndyH]

The batteries need to be sized to requirements. By my very rough calculations, a battery system that supplies 5 kWh per day for three days would cost around $5k. To that, you need to add an inverter that ties the battery and grid together, probably in the range of $2k. And also add installation costs and permits.

Note that if you use lithium cells you should not keep them fully charged if you want them to last.. most of these systems still use lead-acid batteries.

Most of the available large-format lithium is LiFePo4. They can be used as a direct replacement for lead-acid. Four LiFePO4 cells replaces six 2V lead acid cells. LiFePO4 is lighter, doesn't need ventilation, is easier to swap-out to replace a cell, and is more efficient than lead.

Look for one of the battery charge controllers that has programmable settings. Charge controllers designed for lead-acid have a programmed 'overcharge' designed to equalize the cells. You want to turn that equalize period off for lithium.

This voltage example is for a 12V battery. The voltage ranges and SOC corresponds if one has a 24 or 48V battery system.

The normal drop-out (low battery voltage) setting for a 12V inverter is 10V. That's 1.67V per cell for a 6-cell lead acid battery, but it's a perfect 2.5V per cell for LiFePO4. This is about 5-10% SOC.

On the end of charge side, 12V charge controllers end their charge around 14.2V - that's 3.55V per cell for a 4S lithium pack and about 80-82% SOC.

The 2.5V - 3.55V range from a standard lead-acid charge controller is perfect for a very long life LiFePO4 battery and will very likely provide a 2500-3000 cycle life. If the battery's sized correctly, it'll never completely charge or discharge - and that narrower DOD range provides more life.

I'm using a 1kWh LiFePO4 pack, portable inverter, a pair of 85W panels, and a custom management unit to recharge all my portable devices and provide a small back-up power box.

Andy

 

[AndyH]

Another option...

[youtube]http://www.youtube.com/watch?v=WCaVIhAtEJE[/youtube]

 

[klapauzius]

I had a grid-tied PV system with battery backup installed in 2010.
The main reason was the incentive structure at my location, not any necessity.
The batteries I have are sealed lead acid, they are not terribly expensive (~$200/kWH).
Since I dont really need them, I have just 4 kWH backup.
Maintenance is not an issue really. I guess ~8-10 years from now I need to get new
batteries, but since they are are hardly used at all, probably not. They are really just a buffer
for the PV system.

Installation was slightly more expensive, I would put the extra cost for the whole system at maybe
$2k -$3k, which was small compared to the total cost and more importantly, more than tripled my local production incentives (because at the time the only in-sate manufactured inverter was a GT battery backup system). Compared to a non-battery backed up system , I already recovered my extra cost in the 2 years since the system was installed.

Other than the particular incentive structure, I dont see any need for such a system, if you live in a place with an established infrastructure....out in the woods with no grid available, this might be different....OR if you cant have black/brown outs for whatever reason. But then you might be cheaper and better off with a local back up for the critical system in question (typically a computer).

Also note that a battery backed up system is slightly less efficient (~5%) due to losses in the battery charge controller.

 

[RegGuheert]

Also note that a battery backed up system is slightly less efficient (~5%) due to losses in the battery charge controller.

Efficiency would be an issue. Charge controller efficiency is about 98%, which is not bad, but the efficiency of the inverter can make a huge difference. IIRC, the efficiency of the 6 kW XW inverter at full load is about 89% while the new Enphase M215 CEC weighted efficiency is 96%. At 10 kW, that's a 700 W difference to the grid, even ignoring charge controller, battery, copper and shading loss differences. That equates to about a 1-MWh/year difference around here.

The other big difference is in system life/redundancy. If I use the following MTBFs: XW - 10 years (generous?), M215 - 230 years, PV - 600 years, I can expect the following for a 10-kW system:

XW battery-backed system: After 5 years, on inverter will have failed, taking down half the system. After 10 years, the other one will have failed.

M215 system: After 25 years, the warranty for the inverters and the panels finally expire. By that time 2 of the inverters and one of the panels, out of 42 each, should have failed. Even if I did not make any repairs to the system during that time, 93% of my system is still available. Of course, the failed inverters would have been replaced under warranty and might have been easy to replace, depending on their location in the array.

During a 30-year period, the microinverter system should produce an additional $3,000-worth of electricity (at $0.10/kWh!) and should save over $24,000 in repairs. I have ignored failures in the batteries, charge controllers and other components, which also likely will fail.

The bottom line is that central inverters have not achieved a level of reliability anywhere close to what can be achieved in a microinverter system. This is primarily due to the inability to derate the capacitors to a level which would give them a long life.

 

[klapauzius]

During a 30-year period, the microinverter system should produce an additional $3,000-worth of electricity (at $0.10/kWh!) and should save over $24,000 in repairs.

$24,000 seems a lot of money, given that the components (inverter ~ $2500, charge controller ~$500, battery $200/kWH) are not that expensive. How did you arrive at that number?

 

[RegGuheert]

$24,000 seems a lot of money, given that the components (inverter ~ $2500, charge controller ~$500, battery $200/kWH) are not that expensive. How did you arrive at that number?

I used $4000 per XW6048 times 6 (each one replaced three times, including at the end of 30 years). I can see that these can be had for $3000, so perhaps that is high. But if you include CC X6 @ $750 and battery replacement, then I guess you come out with a similar number.

The idea of what happens at 30 years is a question mark. I'm pretty sure the PV panels will mostly be O.K. since I think their design life is probably more like 40 or 50 years. The M215 microinverters probably don't have a design life much longer than 30 years, so it's possible that they would all start to fail around then. Hard to say...

BTW, regarding MTBF number reliability, I will say that Enphase *knows* the real in-situ MTBF for their inverters while it will be hard to pin down good numbers on anything else. If you want to verify the Enphase MTBF, it is pretty straightforward to do. Many people on this site and others have their systems linked in their signatures, including me. If you simply compare the lifetime production for each panel to the recent production, you can tell if any have been replaced. I have had one replaced, so it shows up. Do this for enough systems and you can start to get a feel for MTBF. I think my estimate of 10 years for larger inverters is pretty accurate given that the manufacturers claim they are having problems getting past that number.

Of course you can use other assumptions in a calculation, but the conclusion remains that any system with a central inverter will require much more maintenance and expense while producing less AC.

 

[davewill]

... Also note that a battery backed up system is slightly less efficient (~5%) due to losses in the battery charge controller.

Wouldn't the solar output normally be sent to the grid or used directly? I would have thought you'd only experience the overhead of the batteries in conjunction with an outage.

 

[klapauzius]

... Also note that a battery backed up system is slightly less efficient (~5%) due to losses in the battery charge controller.

Wouldn't the solar output normally be sent to the grid or used directly? I would have thought you'd only experience the overhead of the batteries in conjunction with an outage.

Yes, you would think, but in a battery backed up system the solar output goes through a charge controller (CC) first, which connects to the battery and the inverter.
1) The CC has an efficiency <1 (Maybe 95%-98%)
2) Even though the batteries dont get used at all, as long as you are grid tied, they have to be kept floating and I guess there are some small losses in that. My empirical observation is that the whole combination of CC and battery results in ~ 5% less than the stated efficiency of the inverter alone.

 

[klapauzius]

used $4000 per XW6048 times 6 (each one replaced three times, including at the end of 30 years). I can see that these can be had for $3000, so perhaps that is high. But if you include CC X6 @ $750 and battery replacement, then I guess you come out with a similar number.

The idea of what happens at 30 years is a question mark. I'm pretty sure the PV panels will mostly be O.K. since I think their design life is probably more like 40 or 50 years. The M215 microinverters probably don't have a design life much longer than 30 years, so it's possible that they would all start to fail around then

So that number would depend on the system size obviously...I have a single Outback inverter, which so far has worked flawlessly (but it is only 25 months old), so I have no idea how it will do 8 years later. But I have been told they are quite robust. I would also assume that if the inverter fails, it is probably due to some single component, that can be replaced, rather than to change the whole device?

Also, if you dont use the batteries much, which is typically the case in a grid tied system (which of course begs the question, why one would want a battery backed up system), I dont think you have to change them every ten years.

Does anyone have actual 10+ year experience with central inverters?