# Solar with storage and "generator" backup



## semipro (Oct 7, 2014)

I'm hoping to get some ideas here on how best to fill the green box below.  There seem to be quite a few posters here with PV experience.

The diagram below shows conceptually how I'd like to set up our household power supply for solar, onsite storage, and backup generation using our Toyota Highlander hybrid (HEV).  As far as I can tell, what's below is pretty much a standard grid-tied PV system with backup if you ignore the HEV.  

Despite a lot of reading I haven't yet figured out how to best integrate the HEV for backup generation.   There are now some PV inverter/controllers that can accommodate a backup generator input but those are 120/240 VAC.  It seems like you could integrate the HEV as just another PV string array but I just can't find the info to support that.  Many inverters now us MPPT to optimize PV array output but I don't think a MPPT-enabled inverter input would handle the HEV traction battery 288 VDC input well.  

For those not familiar with how the Toyota hybrid system works, if you leave the key in the run position and turn off all the accessories, the engine will start/stop as needed to keep the traction battery charged.  This makes it potentially useful as a quiet, low emission, and fairly efficient backup generator.  Of course, you have to tap into the 288 VDC from the traction battery somehow which is what I"m struggling with.  You can use a large data center style UPS to produce 120/240 VAC but I don't see how to integrate that with the PV and onsite battery storage.


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## woodgeek (Oct 7, 2014)

A lot of hybrids charge the 12V battery off the traction battery, using a 12V DC-DC converter, rather than having an alternator.  Then you can just get a 120V inverter to run some loads from the onboard DC-DC, in turn powered by the traction battery.  You would prob need to check out some prius forums.

If the 12v system is as described, you just have to figure out its max safe power rating (on my Leaf it is 1.7 kW), and get a charger for your battery bank that runs at that wattage max (and which will cycle off when the storage batt is full) and run it off a 120V inveter tied to the 12V system.  In this configuration, the storage abttery is really just a buffer, allowing you to make larger medium duration loads (above the DC-DC HEV rating), but the HEV will keep up so long as average loads are not too high.

You will think this kludgey....288V to 12V to 120VAC to 48V and back to 120V.


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## semipro (Oct 7, 2014)

woodgeek said:


> A lot of hybrids charge the 12V battery off the traction battery, using a 12V DC-DC converter, rather than having an alternator.  Then you can just get a 120V inverter to run some loads from the onboard DC-DC, in turn powered by the traction battery.  You would prob need to check out some prius forums.
> 
> If the 12v system is as described, you just have to figure out its max safe power rating (on my Leaf it is 1.7 kW), and get a charger for your battery bank that runs at that wattage max (and which will cycle off when the storage batt is full) and run it off a 120V inveter tied to the 12V system.  In this configuration, the storage abttery is really just a buffer, allowing you to make larger medium duration loads (above the DC-DC HEV rating), but the HEV will keep up so long as average loads are not too high.
> 
> You will think this kludgey....288V to 12V to 120VAC to 48V and back to 120V.


Kludgey yes but I like the alternative thinking of providing backup through the battery bank side of things.  
If I recall correctly there is also a 288 VDC to about 50 VDC DC-DC converter on our HEV that runs the AC compressor.   I wonder how the power rating on it compares with the 288VDC to 12 VDC converter. 

It still seems to me though that I should be able to connect the 288VDC from the HEV directly to a solar string type inverter which would be less kludgey and provide more power from the HEV.


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## woodgeek (Oct 7, 2014)

I think folks worry about the HVDC thing from a safety point of view....not only is the voltage high enough to be a little dangerous, any sort of short would be very dangerous (like the proverbial short thru the wedding ring).  Of course, you would 'fuse', but still, the batteries need to be respected....like a hot woodstove it is 'always on'.

I don't think your green box exists.  I think with existing off the shelf tech, your 'center' will be a big battery bank and everything will pour into that and take from it, and they will have to not buck each other.

I realize we're blue-skying a little here, but keep in mind that the battery basically doubles your PV cost/kWh (offgrid solar is ~2X grid tie) AND that gasoline ICE-derived kWh are also quite expensive.

Reading between your lines, it sounds like you want a grid-tie PV system and backup power w/o a genny.  Just read about the HEV, and get a big inverter for it and presto....backup power without a noisy genny.  IF you also want a PV system, go get a grid-tie system.  Grid tie PV inverters with backup functionality are very $$, and not popular.

I am thinking of retiring my 700W genny in favor of a 1.5 kW inverter on the LEAF (I would need to drive over to a Nissan dealership for a 20 min quickcharge every 2-3 days to top off the traction battery, or fall back to the 700W genny for longer/more severe outages)


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## semipro (Oct 7, 2014)

woodgeek said:


> I think folks worry about the HVDC thing from a safety point of view....not only is the voltage high enough to be a little dangerous, any sort of short would be very dangerous (like the proverbial short thru the wedding ring). Of course, you would 'fuse', but still, the batteries need to be respected....like a hot woodstove it is 'always on'.


Agreed, though I think it can be done safely.  The PV string arrays can run up to 600 VDC (but of course they're hardwired)



woodgeek said:


> I don't think your green box exists. I think with existing off the shelf tech, your 'center' will be a big battery bank and everything will pour into that and take from it, and they will have to not buck each other.


  I realize the green box doesn't exist as such but am trying to minimize the number of components and complexity.  As EV's become more common this concept of vehicle to grid (V2G) will become more important, and a solution for some of the load leveling issues that going to renewables creates.



woodgeek said:


> I realize we're blue-skying a little here, but keep in mind that the battery basically doubles your PV cost/kWh (offgrid solar is ~2X grid tie) AND that gasoline ICE-derived kWh are also quite expensive.


Point well taken but blue sky as you say.



woodgeek said:


> Reading between your lines, it sounds like you want a grid-tie PV system and backup power w/o a genny. Just read about the HEV, and get a big inverter for it and presto....backup power without a noisy genny. IF you also want a PV system, go get a grid-tie system. Grid tie PV inverters with backup functionality are very $$, and not popular.


We have a generator that we've had to use for up to 8 days at a time.  I hate running that thing. The noise, the smell, the pollution, the inefficiency.  Yeah, admittedly I want it all.  I want to start with our "dream" system and then pare it down from there once sanity kicks in.


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## peakbagger (Oct 7, 2014)

There is a frequent poster on solarpaneltalk that used a prius for backup power; he has a particular brand of UPS that matches up with the prius battery . I expect the concept woulbe to charge the UPS battery with a series string of panels that adds up to the UPS voltage. I expect you may need to build a custom charge controller.


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## midwestcoast (Oct 7, 2014)

No comments on the PV side, but:
If I were you I wouldn't sink too much $ into the Highlander/generator part of it since it's just a back-up to the backup.  Would do as wood geek suggests & go with a cheap 12v to 120 inverter. Check what the 12v system can handle (probably 1000 Watts) & size the investor accordingly, add an inline fuse & you're in business. Your batteries will handle the surge loads nicely. 
Only use is: The grid is down AND it's cloudy AND you've drained your storage batts AND you really need more juice.
Efficiency doesn't matter much in that role IMO.


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## semipro (Oct 8, 2014)

peakbagger said:


> There is a frequent poster on solarpaneltalk that used a prius for backup power; he has a particular brand of UPS that matches up with the prius battery . I expect the concept woulbe to charge the UPS battery with a series string of panels that adds up to the UPS voltage. I expect you may need to build a custom charge controller.


I'm pretty sure I know who you're talking about.  I've also located a data center UPS (10KVA) that seems compatible with my HEV but am concerned:
1). That its not built for the duty cycle that I need and that it won't hold up. 
2) That if I try to integrate it as a component within my "green box" above it won't operate correctly as it would only work when it sees a dead input (utility side) connection. If I'm trying to integrate it with a solar inverter it would only contribute power when the PV side of things was completely dead. 

I hadn't thought of using the PV with a charge controller to charge the UPS batteries.  That's really interesting. Of course I could only do this if I addressed issue 1 above which I might be able to with some additional cooling, etc.


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