High voltage solar array into 48 volt bank

[tomtmook]

A friend of mine has a sellback solar system.  It's got 7kW in panels to a Sunnyboy inverter.  Of course he's disappointed that it's useless if the power company isn't supplying power.  He wants to set up a battery backup system to supply power in the event that the utility power goes out.  It would include a additional low voltage inverter that would automatically switch in when utility power went away.  The capacity of the battery bank would be fairly small so he'd like to use his solar panels to charge it if he's experiencing an extended outage.  

We've been discussing it and he understands that 48 volts is the highest voltage battery bank size that most inverters will handle and he's willing to work with 48 volts.  It's the panel to battery connection that has me scratching my head.  

The panels are set up with a nominal output of over 300 volts.  I'd like to design a DC to DC converter to drop that down to 60 volts or so to feed an off the shelf charger.  

He would like to dump all 300 volts into the batteries.  

So my question is what will happen to the panels and the batteries if we did that?  He figures the internal resistance of the batteries and/or the panels will limit current and drop the voltage so the batteries won't see too high a potential at the plates.  I'm not so sure.  With a mismatch like that, something's going to heat up.  

Has anyone tried something like this?  What can we expect to see?  

Tom

[joestue]

My advice is search electric car forums and post this there if you are looking for a solution cheaper than 10-50 cents a watt.

"solid state" 240 vac chargers will accept up to 338 volts dc in without encroaching on any head room. the DC link on any power supply that advertises 99% power factor is a little higher than this, (typically 350-400) volts for 240 vac models, so basically you can just hook it up. (If you can find an appropriate battery charger)

What size charger do you need? You might try to contact SunnyBoy or whomever and find out if their 48 volt grid tie inverter can be used to charge batteries when fed from 300 vdc directly into the dc bus, rather than from the 240 Vac. I doubt they will help you with that but might as well try.

If I were to do this tomorrow I would open up the feedback loop on any generic surplus 48 volt power supply, stick a 12 volt unit in series with it and dial in the current by adjusting the voltage it thinks its seeing. This is the cheapest method, and the most likely to fail. But it will give you time to find a power supply company that has already built something for this application. I'm absolutely sure one has, it may take some searching, there are about ten thousand separate companies building power supplies.

[dnix71]

I have a coworker with 5KW of panels, and a battery bank for the times the grid is down (like after a major hurricane). It was all part of the package. It doesn't cost that much more for the battery backup part. >15% of the total

The grid-tie inverter they have can run the whole house (except for a/c) and has a charging feature so if the grid is down, it just runs off the batteries while charging them if the sun shines. My coworkers batteries are sized for 3 days with no sun.

This isn't do it yourself. If you are grid-tied, the electric company has a legitimate interest in their lineman's safety, and your homeowners insurance doesn't want you burning down the house either.

http://www.altenergymag.com/emagazine.php?issue_number=06.10.01&article=outback

[bob g]

Exeltech makes inverters up to 108vdc input, and advertize they will custom build

to whatever you want.

an option of course

bob g

[DamonHD]

Hi,

SMA (who makes the SunnyBoy range) already have a product or two including the 'blackout blocker' that will safely island your system if the grid goes down, and will use whatever downstream grid-tie is available to supply loads and charge batteries efficiently and safely.

Rgds

Damon

[DamonHD]

In more direct answer to your question, because the PV acts as a constant current source AFAIK and you'll be far from the maximum power point:

a) You'll get maybe 20% of the possible energy our of the panels, which is a terrible waste.

b) You may well heat the panels up, though I'm not convinced that you'll end up dumping the other 80%.  But since PV converts most incident energy to heat anyway then I doubt that of itself it would be a problem unless you cause hotspots.

Rgds

Damon

[richhagen]

The battery voltage would hold the array current down to the battery voltage in manual operation until the batteries could no longer absorb the current.  You would have to monitor it closely to prevent overcharging, and switch the panels to open circuit, or divert the current to a dump load when the batteries are near full.  I would think there might be some interesting arcing on those switches at 300VDC.  (Electron Baby's photo of the arcing array current from an HV array comes to mind)

I am not aware of any commercial charge controller for charging a 48V bank that could handle 300VDC input.  That said, I would not be surprised if there were some.  Obviously MPPT would be beneficial because of the extreme voltage mismatch, but even of not MPPT charge controllers, I am not currently aware of any suitable for a 48V bank that could handle even half of the 300VDC open circuit voltage from his array.  Rich

[wooferhound]

how are the solar panels connected together ?

could you rewire them for a lower voltage ?

[hydrosun]

At first I thought maybe the Sunny Island would work, but it would only charge the batteries when the grid was up. And then take power out of the batteries when the grid went down, without being able to recharge directly from the solar panels.

 The only way I know how to do what you want is to reconfigure the solar panels at a voltage below 120 volts go through a MX-60 to drop the voltage to the 48 volt battery and use a 48 volt grid tie inverter like the Outback VGTFX3648.

Chris

[Ungrounded Lightning Rod]

You'll dump the other 80% into heating the panels.  Which is also what happens if you don't pull the power from them while they're sitting in the sun (though the distribution of where it's dumped is slightly different).

Similarly, when battery charge is being regulated by shorting the panels you dump 100% of the generated power into the panels as heat (and this time the distribution IS the same as when you're dumping 80% while charging due to voltage mismatch).

The panels should survive it just fine.  It's only an extra 20% or so of the heating they experience under normal load and less than under short-circuit charge regulation.

He can hook 'em up directly to the batteries (with a suitable blocking diode if the array doesn't already have them included) to get started, despite the wild voltage mismatch.  (This is also what people can do for an emergency measure if their down-converting or MPPT solar charge controller fails and they have loads they still need to power - at battery voltage or using an inverter that's not part of the charge controller - while waiting for the replacement to arrive.)

Later he can multiply the charging power he's capturing from the array by a factor of five just by adding a suitable charge controller, such as those others are describing here.

[ghurd]

I will step forward with the obvious.

If his grid is absolutely horribly terrible, maybe it is down an hour a week?

About 1/2 of 1% of the time?

About 1/5 of 1/2 of 1% of the time when solar is available?

Not sure how efficient it would be maintaining the circuitry and a battery bank suitable to accept the input from a 7KW array.

Even if the battery bank were capable of accepting the 20% potential watts as direct amps, it is still a sizable system to maintain.  Economically and electrically.

Then he is thinking about an UPS sized to suit the entire household, and all the goodies that go with it.

Not an inexpensive proposition.

Seems to me like a gas or diesel back up gennie and a few HD extension cords would be cheaper to buy and maintain.

The grid here is like 100 years old, and I am lucky if it goes down long enough for me to break out and connect the back up stuff quick enough to impress the neighbors.

Might want to have a look at dlenox's posts.

His windmill can top out 'round about there, and he has a grid tied, battery backup, system.

G-

[tomtmook]

As far as the grid going down, my friend has been watching some of the older trees along his road lose their health.  Earlier this spring one took out a transformer and pole and resulted in a ten hour outage.  There are plenty more trees like that and the power company just hasn't cleared or won't clear them before they fall.  

Unfortunately, we can't reconfigure the panels to a lower voltage or the sellback Sunnyboy won't work.  He doesn't want to change what he has in place already.  

.

I never thought to try an AC charger with DC.  I'd assume that not all of them are made to handle that, especially the transformer input units.  If they use solid state switching circuits they just might work.  Anybody know of any particular units like that?  

.

So the mismatch will result in heating the panels.  That makes sense, as the internal resistance of the batteries is much lower than that of the panels.  This may work after all, as his intent is to keep the batteries charged while the power is out.  The panels will be much more efficient under their normal usage with the sellback inverter.  

.

Lastly, somebody mentioned arcing.  There are few mechanical relays that have long lifetimes switching voltages above 300 volts DC.  Those that are out there are quite expensive.  So we were looking at using solid state relays to avoid arcing and lifetime issues.  Does anybody have any experience using these?  good or bad.  

Thanks for all your responses.  

Tom