Grid Tie Daytime UPS w/o Battery

[rhgray]

Has anyone ever found or created a grid tied PV system that can provide local power when the grid is down during the daytime WITHOUT a battery backup? For a number of reasons, battery banks cannot be used in several applications I am interested in, and all that is desired is the ability to use daylight PV if the grid goes down. I have taken to calling this an "enron" system, as it would have been useful during the one hour rolling blackouts we suffered each afternoon in California -- and which are expected next year even without that company . . .

Roger

Pasadena California USA

[Jeff7]

I'm also definitely interested in this idea. Granted, grid-tie inverters are expensive, but that would be very nice to have.

One problem with running directly off the panels though - whatever you're running will of course shut down if you get a little bit of cloud cover. Be nice if there was a sort of ultra-capacitor, that could provide several amps of current for only about a minute or two - sort of a buffer, but not quite an all-out battery.

[edy252]

wouldn't a battery backup system be more convenient? i mean what would u do when there is no sun ? (storms....nighttime..etc.)

[troy]

There is no commercially available inverter that can give you back-up capabilities without a battery bank.  The reason is because it's extremely unlikely that your solar panels will make exactly the amount of electricity you need for your given load at the moment.  Solar production will either be too high, or more likely, too low.  So how does an inverter make up the difference?  It can't without batteries.

Sorry,

troy

[Jeff7]

If solar production is too high, wouldn't the inverter simply draw less current than is available? - much as a light bulb only draws what it needs, not the full available current of the power plant.

[Ungrounded Lightning Rod]

If solar production is too high, wouldn't the inverter simply draw less current than is available? - much as a light bulb only draws what it needs, not the full available current of the power plant.

If you had even a momentary surplus of load over power - like from a motor start or a bird flying past the panel - the inverter would shut down.  Then it would be a restart situation when the sun came back - and it might not restart if you now have stopped motors.

But the big issue is that we're talking a grid-tie system.  Babysitting that so you don't electrocute the linemen is a job for a computer.  Those things don't take well to having their power bouncing around while they're thinking.

The main reason for skipping batteries is a small reduction in cost.  But when you're paying the big bucks for a grid-tie system leaving off the batteries makes no sense at all.

[Drives]

http://www.homepower.com/files/GS_9.pdf

[srnoth]

Hey there,

You didn't mention specific reasons why you can't use batteries, but I presume space and funding might be some of the reaons. Anyway, it seems that basically it is almost impossible to us the inverter without a battery bank, for all the reasons others mentioned. So, why not simply get a very small battery bank, say less than 100 amp hours, so that way you could still use the inverter when the sun was out, and the battery would handle the extra current needed. I think it would solve quite a lot of problems, and shouldn't be too much trouble.

Why was it that you wanted to go without batteries anyway?

Cheers,

Stephen.

[nothing to lose]

No sence repeating what others said, I agree.

Also I would like to point out though that if your grid tied system could run you fine when the grid goes down (without using batteries), then WHY would you even need the grid to begin with. Think about that for a minute

What I mean is you never know when it's going down, could be anytime, any weather. If your solar/wind systems could handle that without relying on batteries for stored power then you could just as easily run directly from them all the time without even being tied to the grid.

If you knew "ABOUT" when the grid was going down each day and your solar sytems could handle it without batteries, just swap over to that everyday before that time period. Like if from 9AM-2pm it's likely to go down, if your system could handle the load direct don't even bother with the grid at all.

The only advantage of grid tied without batteries is if your selling back the power you make, running the metter backwards, or using it as you make it along with the grid usage to reduce the grid costs. In all those cases, when the grid goes down you still gonna go down too most likely if you don't have batteries to fall back on for stored power.

Solar is expensive and doesn't produce that much power for the large area it takes. So what kinda loads you planning to run off the solar??

Since you didn't give any real idea of what you are actaully wanting to do, specific answers or sugestions are imposible.

One suggestion though that might work to help you out is use a Computer UPS system. Those little sealed batteries inside are practically no batteries anyway, I mean they don't leak, give any fumes, take much space, or anything else. You never even know they are there till the powers off and your using them

And they last forever (almost) when taken care of. I have a UPS that is about 5-6yrs old I geuss and still working fine. This one is just normal UPS type use and the batteries are normally fully charged and we shut down that system when the power is off longer than 5 minutes.

You could get a UPS that handles your expected loads, I have a good 600watt one I use for an inverter. Connect diodes to the wires running to the batteries to prevent the UPS from charging them, charge the batteries instead with the solar pannels. Durring any grid outage your running from those little batteries, those are being kept charged as your using them by the solar pannels. If you have a few minutes of no sun you still stay up and running. This way you can sort of have a grid tied solar system

 Get a really good UPS that warns of low batteries before shutting down and you can also pull the wall plug and sort of run directly from the solar instead of using the grid. Solar charges the batteries in the UPS, UPS runs on the batteries, batteries get low plug back into the grid and run on it till the batteries recharge (like on rainy cloudy days)

Or something you could do if you have enough solar and the inverter to run direct is run the UPS off the inverter which is being power directly from solar. Then if the solar is out the UPS is still running from the batteries and you can connect to the grid by just unplugging the cord and pugging it into the grid outlet. I really dought you'd be running that much solar to run directly off off this way though.

Nothing I know of is gonna keep you running Grid tied without batteries when the grid goes down, but the above ideas would help you have your solar and use it too

Now I have read about those Solar Gorrilas installing 100watt grid tied inverters and pannels to feed the grid back power which is not using any battieries, but I consider that a joke in itself. I checked into that stuff. Was well over $300 for just the 100watt grid tied inverter not inculding any pannels, add another few hundred for 100watts of pannels and your close to $1,000 for 100watts for a few hours on some days!! What a joke, something for kicks for over paid bored people! Interesting reading, not a worthwhile project. If your interested in something like that do a search for Solar Gorrilas I think to find it, might be homepower magazine or something like that I read it. Closest thing to what you asked about that I can think of, maybe you'd get more ideas or maybe they found something alittle more worthwhile by now than 100watts!

[rhgray]

Actually, the several thousand for the batteries can be the tipping point . . . also, there are circumstances where the batteries are highly undesireable, even not allowed -- such as on a school site, where the caustic material requires special handling, and the replacement and upkeep costs keep most schools away from it.  

So far, I am really liking the big capacitor as a load leveler, as one possible.

R. Gray

Pasadena

[rhgray]

Fun !  

Of course at the moment we have a legal grid tie system.  What we need is that hour or two or three of daytime grace.  Long enough to evac a school, or keep my refrigerator and VCR clocks on during an rolling blackout .. .

R.

[rhgray]

Two things -- cost, and caustics.  And until I looked into it seemed silly to have to have all or nothing . . . basically what you have suggested is a modest whole system UPS, which might be lower upkeep and do the 250-300 W needed to keep the house up with no one home . . .

r.

[rhgray]

Pretty much what I had concluded, but much better put.  The UPS is looking like the way to go.  It just bugged me to have all that power there but unavailable because some trader or faceless executive decrees that my grid is going down for an hour today.

And yes, I'd thought of using a second inverter and small string to trick the primary inverter into thinking the grid was up, but as you noted, that gets expensive and, for other reasons, might not work.

Thanks!

R.

[srnoth]

Hey, I was doing some more thinking about this.

If you built a releatively large capacitor bank, and you had enough pannels to produce a good bit more power than you would use, it could work.

If you got a switching regulator that turned the panels on only when you needed to, say for a 12v inverter, the could come on when the capacitor bank reached 13.5v, and turned of at 14v, say several times a second. This would keep the inverrter with a constant voltage that it would like.

Once the pannels were big enough, say a leaf covered one, even if you lost that one, the others could still keep up the voltage. If there are clouds thought, you would loose all power, but I guess you expected that.

Anyway,

I think I will try something like this soon,

Cheers,

Stephen.