How to add solar to an existing backup power system

[chux0r]

Hi, folks.  A friend of mine lives in an area with rather unreliable grid power.  He has a backup system consisting of a all-in-one 2.5kW inverter slash charger controller and a 24V bank of 4 6V deep cycle batteries.  The inverter device, in UPS fashion, automatically charges the batteries when grid power is available, and switches over when the grid fails.  All in all, a really nice setup that I wish I had... not because I need it, but just because it's cool.

The system runs the bare essentials: night-time lighting (or occasional daytime windowless bathroom lighting), the stereo (the family can't live without music!), a ceiling fan or two (it gets hot!) and rarely, a computer.  The lights are all 120V CFLs.

Well, the grid has become quite a lot more unreliable of late, often disappearing for 12 hours or more each day.  In these cases, the batteries don't hold out for the entire blackout, and there's sometimes not enough time with grid power to charge them fully before the next blackout.

Because the climate is tropical (18 degrees latitude...), and 90% of the days are sunny, I'd like to help them augment their system with some kind of solar array.  However, because the current inverter/charger is so "smart," I worry about a way to do it that doesn't screw up its normal behavior.  That is, I'd like it to work as it does now, but in the absence of grid power, the solar should extend the battery life if the system is under load, or charge the batteries otherwise.

I assume the harder solution is to try to have the solar always "working."  That is, even when there is grid power, to have the solar helping charge the batteries, or if the batteries are charged, providing some energy to reduce grid usage.  This sounds difficult to me because I know if the panels are hooked right up, the charger will "see" the current, or resistance, or something but will in general not be able to charge the batteries correctly.

If there is a way to overcome this, it would be ideal.  However, since grid power (when it's there) is cheap as free, it would be sufficient to simply connect the solar only when grid power has failed.  For this, I assume something simple, such as a normally-closed relay powered on the grid side, would work.  Grid power would hold the relay open (disconnecting the solar from the system), but once it fails, the relay would close, and connect the solar to the batteries, which should either charge the batteries, or provide some extending power to the inverter.

Would this simpler setup work as I imagine it?  It certainly wastes the solar energy when the grid power is present, but probably requires less electronics, as the goal here is to simply improve availability, and not necessarily save any money.

Possibly I overestimate the difficulty of keeping the solar from confusing the charger.  If that's the case, a pointer or two would be highly appreciated.  I'll refer to existing threads on how to obtain solar panels, and such...

Thanks!

 -Chuck

[chux0r]

Oh, and I should probably add... the batteries have taken quite a beating lately, given the massive blackouts.  I've heard of conditioning them to restore some of their lost flavor, but I don't know of a resource that gives a procedure on exactly how to do this.

Clearly, if this beating keeps up, we should consider adding more capacity, so we don't  drain them to zero as often.  But given that the damage has already occurred, I'd like to know what we can do about it.  Thanks again...

[DanB]

Welcome to the board!

Im not an expert on this stuff.. hopefully someone that knows more than I will chime in.  Soloar is a good idea in this case probably (though overall gas or diesel might be cheaper)

To do everything you describe, you'd basicly want to install a small grid tie solar power system.  A 'grid tie' inverter would automaticly sell power back (and how that works depends on local laws and power company policies) - they can be programmed to do so only during peak hours if you like.  The same inverter can also serve as a battery charger.  You'd want a solar array, and I 'believe' the inverter/charger would automaticly sell back power only when you had it spare, and then it would charge your batteries (from the grid) only as needed.

I think the Trace/Xantrex SW series of inverters is probably well setup for this.  

[juiced]

Heres a link: http://www.juiced.ca/phpbb/viewforum.php?f=28

 I would consider solar cookers and everything else possible for the immdiate needs.

[kurt]

hmmm not knocking solar but if all you want is power when the grid does down a generator would be your best/most cost effective option. perhaps a propane / natural gas one. i know here a natural gas powered generator is the cat's meow for localized power outages because the gas company delivers the fuel to you so you don't have to store it or go out hunting it (can be a pain because without grid power the gas pumps don't work) the natural gas co. figures they have enough stored to last 3 days with no power to there pipeline....

you can just run the Gen as needed/when convenient to recharge the  ups battery bank just by plugging the ups into the gennset simple easy and much more cost effective than solar for your situation...

[JW]

I have heard that an OUTBACK invertor can hanble incoming AC from a gen-set, this differs from mill outputs since load freq is requlated. This came up when a user here on the board was complaining about having two separate ac circuits due to phase variances that can occur between grid systems AC gen-sets and the like, also a redundant 12v backup system came up(for lighting etc), but seemed impractical because of the advanced commonly avaliable inverters. This amazed the hell out of me, since I quickly figuired inverter power and gen-set never could be combined, in the same distrobution circuit. very tricky indeed.

JW

[chux0r]

Hmm... well, the SW series doesn't appear to be grid-tie, but I think what you're saying is put a grid-tie on the grid-side of the inverter/charger?  I guess that would be nice because it would provide power to the charger when it's needed (and not confuse the charging situation), and ease the load on the batteries when the grid is offline?

Sounds like it would work, but it sounds like an awful lot of hardware, and a couple stages of conversion losses.  After all, the inverter/charger is already there, as is a 24V battery bank.  If the solar is generating on the order of 28-36V or whatever, it just seems more efficient to just connect this somehow to the battery side of the equation and charge the batteries directly, or provide supplemental low voltage to the inverter.  It seems like a clever arrangement of diodes should be able to make this possible without upsetting the charger too much.  But I suppose that's my first question again.

After all, putting down $1500 - $2k on another inverter (on top of the $1-$2k in panels that would probably be needed) seems silly when there's already an inverter that's waaaaay more than sufficient right now.  (2.5kW is excessive, I think.  0.5kW would probably do it.)

On the flip side, maybe if I'm willing to spend the $1k-$2k at all, it would be best spent on beefing up the battery supply, and forget about the solar altogether.

[chux0r]

Exactly why I want to combine on the DC end of things, as it should be quite a lot simpler.  Is there no existing system that uses a UPS-style inverter with a secondary charging source?  It seems rather simple and elegant to me, if I can work out the details.

[chux0r]

You're right!  A nat-gas genny would be more economical (actually, a diesel generator is already available), however I imagine the solar to be less hands-on (it should just work automatically whenever the sun is out, which is all the time), and certainly be quieter.  Economy can take a back seat to these quality-of-life concerns, for now.  Our goal is actually to improve the availability of electricity, not necessarily make it cheaper.

When the batteries die, we could start up the generator, but we're usually not interested... we just live without electricity for a while, unless there's some dire emergency.  But if there was a little extra juice, we'd have that extra touch of convenience.

You also didn't mention, but are probably thinking that a generator will also just plain supply more power.  That's true.  I imagine $1-1.5k of panels would bring... what, say 150-200W max?  That's rather poor... We'd have to conserve, but it could reduce the strain on the batteries quite a bit, and probably close the gap on a majority of blackouts.  And again, if it takes little effort after it's set up, it's worth it.

[DanB]

I believe the SW is "grid tie" - it has a built in transfer switch, if you have excess power to your batteries, it will be able to sell power back to the power grid.  or it can take power from the grid to charge the batteries when needed.

A bit expensive but I think that unit, by itself - might be about all you'd need.  Im not sure if you'd require a charge controller for the panels or not in that situation.

[Flux]

I think you can probably add solar to charge the batteries as long as you have a controller to disconnect the panels at a voltage just below the voltage the internal charger floats the batteries at.

It may not be wise to take the batteries higher than the internal charger does as it will not have dump capability.

Can't be sure without detailed knowledge of the inverter, but up to float voltage I don't see how the internal charger can know the difference.

Flux

[Ungrounded Lightning Rod]

I just visited the company site last night.  The SW series has a grid-tie add-on unit available for some models.

[Ungrounded Lightning Rod]

A nat-gas genny would be more economical (actually, a diesel generator is already available), however I imagine the solar to be less hands-on (it should just work automatically whenever the sun is out, which is all the time),

Some inverters (example:  The trace SW series again) have automatic startup for a backup genny.  The Trace can also be set for "quiet time" so it won't turn the genny on when you're asleep unless the batteries are close to their discharge limit.

and certainly be quieter.

That's for sure (especially if the genny is a diesel  B-) ).

[ghurd]

I'd go solar. Its easy. Theres a genny there if it's needed.

Most of the larger UPS set ups I've messed with used a current limited switching relulator based charging system. Add more battery and it takes longer to charge them, and the grid there often isn't up long enough to charge the batteries all ready there.

I can't imagine a UPS type inverter being blown up if the batteries are at 28.2v instead of 27.4V. That would be some very poor design! (example: what if you replace the batteries with new ones that were just charged at the dealer to 29.2v?)

So, connect the solar straight to the batteries. Let solar and grid both charge it. It would reduce the grid use, no fancy control or sensing circuits, no wasted solar, no noise, all automatic, no expensive inverter.

If there is still concern about ruining the UPS, a Morningstar SunSaver could be set to 'Sealed' control voltage of 28.2v. That is probably just a tad over or under the UPS.

160. w of panel, and a charge controller can be had for $750.
     
320. w and a controller for maybe $1250.
     
     

If you are where I think, thats a lot of power. Maybe more than they need.

Whats the amp hours of these batteries?

CFLs and a stereo don't use much. 4 6V T-105 type batteries should hold more power than a couple CFLs and a stereo for 12 hours. Are these little 10 pound things?

If the batteries are not sized to the solar, 75w may be the right size for the batteries. Bigger panels can't 'make' more than the batteries can store.

G-

[chux0r]

and the grid there often isn't up long enough to charge the batteries all ready there.

That's an excellent point... yes, buying more batteries would be silly.  Thanks for pointing that out!

I can't imagine a UPS type inverter being blown up if the batteries are at 28.2v instead of 27.4V.

No, I'm not worried about the UPS blowing up.  I'm worried that if the UPS sees 28.2V coming from the solar, it won't bother helping charge the batteries.  However, now that I think of it, the batteries will pull down the voltage to their own voltage anyway, right?  This should mean the UPS should behave as if the solar is not there when we're in charging mode.  Is that a correct analysis?

Whats the amp hours of these batteries?

I believe they're 225Ah.  I think the issue with the batteries dying is just that the grid isn't up long enough to top them off.

So it sounds like the best option (ie. simple and cheap) would be to connect the panels directly to the batteries, but use a simple controller that will cut-off the panels when the batteries are above a specified voltage?  This should allow the panels to assist charging in charging mode, and provide additional power to the inverter when the grid is offline.  And the cutoff will prevent the panels from overcharging the batteries.

Does that sound about right, so far?

[chux0r]

Wow, yes, that FAQ is awesome... thank you.

[chux0r]

BTW, thanks to everyone for being so helpful to a total newbie.  I hope soon I can contribute as much knowledge as I take away.

[troy]

Also,

Depending on what kind of lead acid batteries we are talking about, if they get discharged to "dead" routinely, you can bet that their lifespan will be cut to far less than half of their "normal" lifespan.

In golf cart batteries for example (which is what I use), they recommend not discharging below 80% capacity on a regular basis.  ie, you can use 20% of capacity lots of times, but you can use 100% of capacity only a few times before the batteries are damaged beyond recovery.

Good luck and have fun!

troy

[ghurd]

Exactly the correct analysis.

Both of them will work on the charging. Each doing what it can, when it can.

Panels to the controller to the batteries.

Don't bother messing with any of the UPS stuff.

225AH? Man, they sound in bad shape. A stereo uses maybe 10 or 20w (no 16 year old kids), and say 3 @ 20w CFLs is 75w, or around 3 amp draw. Thats around 75 hours of use from a 225ah bank. Throw in a fan at 15w () and some computor, there should still be 50 hours.

Could part of the problem be the existing charger is too small? A 50w charger at best would put in about 2 amps, taking 5 days to recharge. A small UPS would have a small charger.

BTW- Some compainies make a single 12v panel that can be wired for 24V in it's juntion box. 1 80w panel would generally be eaiser and cheaper than 2 40w panels.

G-

[chux0r]

Yes, I've told them that running the batteries out is BAAAAD, but I can't seem to communicate how insanely bad.  I'll have to find out the charging rate... my guess is that if the system is designed for emergency use, it might be rated to take a day or more to charge.  Not enough in the current extreme conditions it's experiencing, when the grid's up for 16 hours at most.  I might recommend they just shut off power for a couple of days when the grid goes down, just to allow the batteries to come up to 100% for a change.  80w of solar would probably help out big time.  I also don't know how long these batteries have been in service... they could have been tortured like this for years, for all I know.  It might be time for a fresh set.

[ghurd]

80w 'rated' is about 2.35A at 24v. Boo-Koo days to solar recharge 225ah. Even where I figure they are.

Have them look harder their real usage when the grid is down. The batteries were garabage last year, or somebody thinks 4 girls and a hair drier use no power! Somebody isn't counting something.

Water pump? Septic stirer? Something else forgotten?

Ball park.

They should look into around a 300~360w system. About 5 amps @ 48v of solar... Thats a lot of power for back-up. I don't really see how they use that much with the numbers they give.

I would think 160w would be enough with the numbers I have, and the grid tie.

Funny story.

An Amishman calls. His setup isn't working right. I go look. Seems OK. I ask "Whats this wire for?". He answers "I have no idea." We follow it outside, up the downspout, across the gutter, into his oldest son's window, to a radio! True story.

Moral of the story.

Maybe a kid up stiars is using the playstation and 52" big screen tv. "The power is out, there isn't anything else to do."

[Wolvenar]

I would say that these batts are toast, for the little load you've got there. They should last a LOT longer, look into replacing them b4 getting solar , or do both, but at least 300 watts of solar or it wont really be much help.

Just my opinion

Wolv