Can I use higher voltage for a long wire run to 48v battery?

[danda]

I'm putting together components for a mid-to-large PV system.  Goal is to be fully off grid.

So far I have about 3KW in panels, but I'm planning to continue increasing that, perhaps up to 10Kw someday.

For best sun, my wire run will be about 120 feet.  Basically, the further I go, the better sun I can get (fewer trees / shadows ).

I've calculated that at 3KW, I am already going to need pretty heavy wire for that distance.  1/0 or even better 2/0.  That gets epensive real quick.   Also, 3Kw @ 48v is already 62.5 amps, so I am approaching the limits of most charge controllers.

What I would love to be able to do is to string my panels together for a higher nominal voltage, say 240v DC and then down-convert that to the maximum-power-point at the battery.   That way I could use much smaller wire.

The problem is that I don't know of any MPPT controller that can handle that high of voltage.

Does such a beast exist?   Any other recommendations?

What do you people with large arrays and long runs typically do?    Multiple wire runs and parallel charge controllers can really add up.

[DamonHD]

A grid interactive MPPT solar controller that you happen not to connect to the outside grid should be able to do it I'd have thought, such as one of the SMA models (Sunny Island?).  That will give you battery control and A/C for use in the house with appliances too.

Rgds

Damon

[SteveCH]

You can definitely do it, save on wire size and $$ by what you propose. My arrays are fifty feet from the battery bank so I am in a different situation, and I run it 36 volts from the array and have 12 v. battery bank. However, it is ultimately the same idea.

I don't have tables/formulas at hand, so I don't know what you are seeing for a 120' run as far as losses. That is a pretty good long way for lower voltages, of course. I don't know of any controllers that deal with 240 v. but since it is far from my situation I have never concerned myself with it. A lower voltage will open more options, but as I said, I don't know what losses you'd be looking at.

With 10 Kw, no matter what you find in controllers, etc., you are going to end up with multiple controllers and possibly wire runs [at 36 v. and 2 Kw and 50' run, I have three wire-pair runs from the array going to three controllers].

But you are on the right track.

[ghurd]

Yea, you will end up with multiple MPPT controllers no matter what.

Can save a big pile of money using 'USE' type aluminum wire.
G-

[TomW]

Let me think here a minute and pose a couple questions:

You will be spending several thousand dollars on panels?

You are balking at buying a few hundred bucks of copper to connect them?

What is wrong with this picture?

Look around for some used aluminum triplex. it is indestructible and commonly used as "drop" cable from the transformer to the entrance panel.

Looks like this:





This roll is about 120 feet long and cost < $100 used. It is #00 aluminum about as thick as my pinky 2 jacketed conductors and one bare usually used to support it over head. Probably good for a couple hundred amps with acceptable voltage drop over your intended distance.

I prefer copper but for the price I will suffer some resistance losses from Aluminum.

Spend some time looking and you can find used cable cheap.

I use it to bring 24 volts from my turbines 300 feet away overhead.

It just seems silly to spend a wad of cash on panels then get put off by the cost of proper cable to connect it all up.

I totally disagree with the poster who runs 36 volts to a 12 volt battery bank. Funny way to waste power. But if he is happy that is his option.  Paralleled up they would likely shove a LOT more bottom line power to the batteries. In series the amps is the amps from the weakest panel and the voltsage is fixed at battery volts. paralleled the amps would be panel number X amps at battery voltage. That part is simple math. Panels for 12 volts should put out 17 to 21 volts. Plenty of room for some voltage drop.

Do it right and do it once and be happy with it.

My 850 watts of 24 volt panels run through 75 feet or so of cable and it is #2 copper. Also bought used.

Tom

[ghurd]

Look around for some used aluminum triplex. it is indestructible and commonly used as "drop" cable from the transformer to the entrance panel.

I prefer copper but for the price I will suffer some resistance losses from Aluminum.

That's the stuff!
Underground Service Entrance / USE.  (USE-2?)
http://www.cerrowire.com/default.aspx?id=30

Even new it is cheap.
Go up a gauge or 2 and the loss is less than copper while still being a lot cheaper.
It is readily available.
It does not require conduit to be buried.
It is sunlight/UV resistant.

G-

[klsmurf]

Glad to here these comments about aluminum wire. One of my projects around here was to strip out 3 old unused grain bins. They all are wired with #4 aluminum. Was thinking of taking it to the recycler, but now I just might hang onto it. Over 300 feet. What I read is just make sure to use the proper connectors, especially if using copper also.

[rossw]

I'm putting together components for a mid-to-large PV system.  Goal is to be fully off grid.

So far I have about 3KW in panels, but I'm planning to continue increasing that, perhaps up to 10Kw someday.

I'm running a similar system - 3.5KW PV, 48V batteries.

For best sun, my wire run will be about 120 feet.  Basically, the further I go, the better sun I can get (fewer trees / shadows ).

I've calculated that at 3KW, I am already going to need pretty heavy wire for that distance.  1/0 or even better 2/0.  That gets epensive real quick.   Also, 3Kw @ 48v is already 62.5 amps, so I am approaching the limits of most charge controllers.

I have runs of around 70 feet. I ended up using 4 sq mm cable (about 10 or 12 AWG). I did it like this.... I broke the arrays down into 6 seperate arrays, each nominally 100VDC. I've run the cable from each array inside seperately. It might sound inefficient, but it lets me meter each seperately, isolate one if I'm going to be working on it, and use substantially smaller cables. Also, if one (cable) gets damaged, its just one - not the lot.

Here's the "inside" part: (click for larger image)
[img width= height=]http://house.albury.net.au/23may2010/thumb.100_4463.JPG[/img]

The circuit breakers (DC rated of course) and metering board are here:
[img width= height=]http://house.albury.net.au/16may2010/thumb.100_4434.JPG[/img]

I think the FlexMax 80 runs out of legs at about 4KW, so I'm nearly there. If I end up with more panels, I'll peel off panels and run two charge controllers through seperately connected bundles of panels.

Oh, I should also point out, I got some 150V/15A schottky diodes which are inside the metering board that isolates each bank from the others. Not strictly required, but prevents a shaded array from being backfed from the rest of them.

[DamonHD]

I'm definitely on for running separate cables from different sub-arrays.  In my case not because they had to run very far, but because of different lighting/shading and the ability to swap between inverters if I have to, ie because of a failure, to keep most of it running rather than have it all fail for one small component losing the magic smoke...

So I have 4 strings of 6 panels, each string being 1.29kWp, each on its own SB1100 (or SB1200 for the last string) inverter.

Rgds

Damon

[thirteen]

As a wiring note when using Al wire be sure that you go back and tightn the lugs used on the wire. Some Al wire will expand and contract and can losen up the connection thus causeing a hot spot or burnt wires. I work with four distribution panels that were wired with Al and every 6-8 months I go in and tighten the connections. Just a safty issue. I wish my systems could be closer but mine is going to be a combonation micro hydro / solar system. 700ft is the closest I can get with my hydro and for solar it will be 225 ft. Two different directions. Not enough sun in the winter, not enough water in the summer, not enough wind. I will be using transformers on each end. It is just alot of $$$ either way. Retirement 2 years away or 649 days estimated time. My days change some from time to time. Just an idea to ponder or toss.  thirteen

[SteveCH]

Let me think here a minute and pose a couple questions:




I totally disagree with the poster who runs 36 volts to a 12 volt battery bank. Funny way to waste power. But if he is happy that is his option.  Paralleled up they would likely shove a LOT more bottom line power to the batteries. In series the amps is the amps from the weakest panel and the voltsage is fixed at battery volts. paralleled the amps would be panel number X amps at battery voltage. That part is simple math. Panels for 12 volts should put out 17 to 21 volts. Plenty of room for some voltage drop.



Tom

Well, gee, the 36 volts does not go to a battery bank but to a pair of MX 60s which receive the 36 v and send 12 v. to the batteries. Which is what they are supposed to do. I could have chosen 48 v. or some other voltage. The three arrays of panels I have set up work out best for 36 v. to make the best use of the controllers, considering the mixed 2k worth of panels and the number of panels and the distance of transmission of the power and the limits of the MX 60s. Geeeeeeeez.  Actually, the voltage-in is greater than 36 because the panels, yes, put out much more than 12 v. However, most of us speak of 12 v. panels or 24 v. panels or whatever even though the panels actually put out more than what we call them. My neighbor runs 48 v. in to his Outback controllers and charges a 24 v. battery bank. Same deal. The controllers are designed to receive one of several voltages and output one of several voltages. I don't know what the big losses are supposed to be. If I ran the 12 v. [17-21 v.] down the hillside instead of the 36++v., I would have far greater losses.

[danda]

I'd very much like to hear more about your transformer scheme.  Do DC transformers exist?  brand/model/price?  Anyone have experience with using them for this purpose?

I had thought of using transformers also.  But all I read about is AC transformers.   Please educate me.

In my case, it seems like ideally it would work like this:

N solar arrays, each wired serially to put out 48v nominal.
Wire them all into a DC combiner box near the panels.  So each array has its own breaker.
feed that into a transformer that converts to say 600v DC
run small-ish wire 100+ feet to the house  ( need to calc it, perhaps 10 awg )
transform back down to 60v
feed into charge controller

Issues:

 1) over 4kw, we are over capacity for most charge controllers.  So it seems we'd need an additional transformer on each end every 4kw.  Or is there some way to "split" the current at the destination and feed into multiple charge controllers?

 2) Could perhaps avoid a transformer at the source just by serializing more panels, but I've been purchasing in matched sets of 2 or 4 panels to achieve 48v nominal, so I prefer to stick with that voltage for the arrays if possible.

If the transformer approach is not possible, or too expensive, then it is back to the drawing board.


An alternative would be to avoid

As a wiring note when using Al wire be sure that you go back and tightn the lugs used on the wire. Some Al wire will expand and contract and can losen up the connection thus causeing a hot spot or burnt wires. I work with four distribution panels that were wired with Al and every 6-8 months I go in and tighten the connections. Just a safty issue. I wish my systems could be closer but mine is going to be a combonation micro hydro / solar system. 700ft is the closest I can get with my hydro and for solar it will be 225 ft. Two different directions. Not enough sun in the winter, not enough water in the summer, not enough wind. I will be using transformers on each end. It is just alot of $$$ either way. Retirement 2 years away or 649 days estimated time. My days change some from time to time. Just an idea to ponder or toss.  thirteen

[danda]

thanks Damon for bringing this to my attention.  I haven't been looking much at the grid-tie stuff, as I have no plans in that direction.   In fact, I thought the newer grid-tie systems usually avoided a battery altogether.

You are saying that there is a sunny boy mppt inverter/charger that accepts high DC voltages and puts out both 48v and A/C?

I already have a couple trace sw4048s on hand, but this sounds like it merits some investigation.

A grid interactive MPPT solar controller that you happen not to connect to the outside grid should be able to do it I'd have thought, such as one of the SMA models (Sunny Island?).  That will give you battery control and A/C for use in the house with appliances too.

Rgds

Damon

[danda]

thank-you ghurd and tomw for the aluminum wire suggestions.   I'm not convinced yet, but will check prices and factor it into my thinking as a possibility.

btw, the cost is not just a couple hundred bucks.  I don't remember the exact figures, but a while back I calculated that if I run 2/0 copper, it would be over $500, perhaps even as high as $1000.  ( I just remember it was a big number. )  And that's for 3-4kw.  If I wanted to go to 8kw, it doubles.  bah.

And also, I have been getting my panels secondhand on the cheap, so its not as tiny a fraction of overall expense as one might first think.

Knowing that the power company is able to run large amounts of power over comparatively small wires makes me think I should be able to do it myself.  :-)

Yea, you will end up with multiple MPPT controllers no matter what.

Can save a big pile of money using 'USE' type aluminum wire.
G-

[danda]

Thanks for idea rossw.  I hadn't seriously considered using multiple runs of smaller wire.  Good to hear its working well for you, and I'll run some pricing calcs with that scenario.

I guess my biggest concern with it is scaling up though.  I'd like to be able to add new panels at-will.  I am running the wiring in underground 2" conduit.  so it seems like I would need to pre-wire up-front for as many arrays I contemplate ever having.

Perhaps that is true no matter what, but I'm hoping there is a way around that, such as the transformer high voltage scheme.

I'm putting together components for a mid-to-large PV system.  Goal is to be fully off grid.

So far I have about 3KW in panels, but I'm planning to continue increasing that, perhaps up to 10Kw someday.

I'm running a similar system - 3.5KW PV, 48V batteries.

For best sun, my wire run will be about 120 feet.  Basically, the further I go, the better sun I can get (fewer trees / shadows ).

I've calculated that at 3KW, I am already going to need pretty heavy wire for that distance.  1/0 or even better 2/0.  That gets epensive real quick.   Also, 3Kw @ 48v is already 62.5 amps, so I am approaching the limits of most charge controllers.

I have runs of around 70 feet. I ended up using 4 sq mm cable (about 10 or 12 AWG). I did it like this.... I broke the arrays down into 6 seperate arrays, each nominally 100VDC. I've run the cable from each array inside seperately. It might sound inefficient, but it lets me meter each seperately, isolate one if I'm going to be working on it, and use substantially smaller cables. Also, if one (cable) gets damaged, its just one - not the lot.

Here's the "inside" part: (click for larger image)
[img width= height=]http://house.albury.net.au/23may2010/thumb.100_4463.JPG[/img]

The circuit breakers (DC rated of course) and metering board are here:
[img width= height=]http://house.albury.net.au/16may2010/thumb.100_4434.JPG[/img]

I think the FlexMax 80 runs out of legs at about 4KW, so I'm nearly there. If I end up with more panels, I'll peel off panels and run two charge controllers through seperately connected bundles of panels.

Oh, I should also point out, I got some 150V/15A schottky diodes which are inside the metering board that isolates each bank from the others. Not strictly required, but prevents a shaded array from being backfed from the rest of them.

[DamonHD]

There are SMA systems that accept high-voltage DC inputs from panels and can interact with the grid and internally supply AC, and use a battery pack of for example 12V or 48V.  Nothing to stop you putting your own loads directly on those battery packs too.

Rgds

Damon

[ghurd]

thank-you ghurd and tomw for the aluminum wire suggestions.   I'm not convinced yet, but will check prices and factor it into my thinking as a possibility.

 I don't remember the exact figures, but a while back I calculated that if I run 2/0 copper, it would be over $500, perhaps even as high as $1000.  ( I just remember it was a big number. )  And that's for 3-4kw.  If I wanted to go to 8kw, it doubles.  bah.

Yea, you will end up with multiple MPPT controllers no matter what.

Can save a big pile of money using 'USE' type aluminum wire.
G-

Prepare to be pleasantly shocked.
It was a couple years ago.  Installing a decent sized PV array.
#1 triplex USE AL was Cheaper than out&back with #10 CU THHN, not including conduit for THHN, just the cost of the wire.
And #10 THHN was Not big enough, so it would have required multiple runs.  Plus conduit.

There are some issues with AL.
Need to get connectors rated for AL and CU.  Cheap.  I superstitiously prefer the type that do not have a hole that goes all the way through, meaning the AL and CU are physically seperate.  (Home Depot carries one type, Lowe's carries the other, can't recall who has what)
I use CU on both ends.  AL only for the long run.
Need to use "Owl Sh*t" on the connections.  The dude that cuts the wire will know the stuff by that name.
Need to re-tighten them for a few days in a row, then maybe once a week for a month.  I personally never had issues with being loose after that.

G-

[hydrosun]

I've seen too many cases of aluminum wire corroding to white dust when water gets into an underground wire. I'd never put an aluminum wire without conduit underground again. It's a real pain in the .... to find  the break.   The ends can be treated with antiox to keep it from corroding, and the wire put into conduit and then it's a viable option for large wire.
The MX-80 gives you the option of running at a higher voltage and smaller wire. It also increases the power output when the panels are cooller in the winter when an off grid system needs to compensate for less light. It also gives a lot of data for tracking the performance of the system. You will need to have some charge controller on the system. So the difference in price of the MX- 80 to another controller and the lower cost of wire, plus more info  may tip the balance.
You may want to run two separate wires now because there will need to be two controllers to handle 8kw of solar, unless you control the surplus with a diversion load.
If I understand the Sunny Island correctly , it couples with a grid tie Sunnyboy on the AC side to be able to mppt.  The Sunny Island just runs off the battery. So you need two inverters to do mppt and battery backup.
AC is needed to run transformers. DC voltage is changed by DC-DC converters that internally start and stop to create AC to run through small high frequency transformers. That is what is in the MX-60 and similar mppt controllers.
Chris

[rossw]

Thanks for idea rossw.  I hadn't seriously considered using multiple runs of smaller wire.  Good to hear its working well for you, and I'll run some pricing calcs with that scenario.

I guess my biggest concern with it is scaling up though.  I'd like to be able to add new panels at-will.  I am running the wiring in underground 2" conduit.  so it seems like I would need to pre-wire up-front for as many arrays I contemplate ever having.

Perhaps that is true no matter what, but I'm hoping there is a way around that, such as the transformer high voltage scheme.

Couple of possible hints. Go to your local electrical wholesaler and see what's available on the day. I got 80 metres of 6mm orange circular (quite a heavy 3 conductor cable) cheap because they'd cut it for someone who then changed their mind and needed a longer bit. For the rest of mine, I used 4 sq mm "TPS" (building wire). Because my arrays are not all in the one location, I ran each run in its own conduit run - but only used 20 or 25mm (can't recall). If I were using a larger conduit as you may be, just pull through an extra draw wire for later use. Any time you add another run, pull the new wire *and a replacement draw wire* through. I was able to get the 4 sq mm wire cheap too, because they were "end of run" drums. I got 98 metres (a short 100m roll) for half the normal price, which I think all up cost me about $200 for the cable, conduit, bends etc.

[tecker]

I'll kick in here with some  circuit details / your in the phase of your project that you need to be realistic . The solar needs to be close to the loads that's a fact you can't get around 50 feet is a100 foot circuit 100 feet is a 200 foot circuit etc . Your interconnect will be load over 25 feet so bite the bullet and get the solar in close