solar hooked strait to the battery question

[gww]

Is 90 to 100 volts going to work for charging if it is hooked directly to a 48 volt battery bank.  When the batteries clamp the voltage what will be the end result.  90 volts 20 amps.
Thanks
gww

[mab]

They will charge the battery but you will be running them at battery voltage and getting near Isc.

If 90-100v is Vpm of your panels, then you're only getting ~50% of your power to the batteries, the rest is lost as heat in the panels. If it's Voc then you may be a bit better (~60% maybe).

[gww]

So basically out of 1800 watts posible I would be getting maby 1000 watts.  So I need to wire them from between 68 volts to 72 volts and then get around 28 amps.  Then I would get about 1600 watts out of around 2000 watts worth of panels.  So the lower voltage would get me about 80% as opposed to 55% with the higher voltage?  Does it seem like I have it?  I was hoping to only wire them once in the hopes of getting anouther mppt charge controller in some future time.
Thanks
gww

[ghurd]

So basically out of 1800 watts posible I would be getting maby 1000 watts.  So I need to wire them from between 68 volts to 72 volts and then get around 28 amps.  Then I would get about 1600 watts out of around 2000 watts worth of panels.  So the lower voltage would get me about 80% as opposed to 55% with the higher voltage?  Does it seem like I have it?  I was hoping to only wire them once in the hopes of getting anouther mppt charge controller in some future time.
Thanks
gww

With normal panels, ~70 Vmp would be about right for a 48V bank.
Rated Imp would be around 28.5A, but it willl go above that under good conditions.

It would be worth a rewire later unless you plan on having the MPPT in the mail soon.
G-

[gww]

G
Thanks
gww

[mab]

So basically out of 1800 watts posible I would be getting maby 1000 watts.  So I need to wire them from between 68 volts to 72 volts and then get around 28 amps.  Then I would get about 1600 watts out of around 2000 watts worth of panels.  So the lower voltage would get me about 80% as opposed to 55% with the higher voltage?  Does it seem like I have it?  I was hoping to only wire them once in the hopes of getting anouther mppt charge controller in some future time.
Thanks
gww

sounds about right. Personally I'd aim for a lower voltage for Vmp; 60-65v (assuming they can be configured for that) and do without a MPPT but that's just me. 

[gww]

mab,
My memory is going.  I will have to wait and see what I get when I hook them in series and check at the end of my 200' wire run.   65 volts might be what I get whether Ilike it or not.  THey are home made half panels about 10 volts each unloaded.
thanks
gww

[bob golding]

along the same lines but not directly related. i am doing the same. ATM i have 3 36 volt panels in parallel feeding a 24 volt battery bank with wind turbine back up. i am thinking of getting another panel to make the total up to 1 kw nominal. the panels have cracked glass so not expecting to get full output anyway.
would i be better off wiring them as 2 panels in parallel wired in series. this should bring the OC volts up to 72 volts. the most i have seen from the 3 panels in parallel so far is about 9 amps into 24 volts. each panel is giving 8 amps short circuit current individually.
 i am not paying much for them so not expecting miracles. £50 each which is around $35.
the main thing is between the panels and the turbine i am not  having to use the genny. only used it once in the last 3 months when we had 4 days of fog and no wind.

[ChrisOlson]

Is 90 to 100 volts going to work for charging if it is hooked directly to a 48 volt battery bank.  When the batteries clamp the voltage what will be the end result.  90 volts 20 amps.

Our array is wired for 90 Vmp with MPPT on a 48V system.  And it works good.  But it don't work very good at all without the MPPT.

On 48V I think hooking direct is a bad idea.  You're dealing with an operating voltage range of 48 nominal up to 62 volts - a spread of 14 volts.  Solar panels don't like that kind of a voltage spread to work with.  On 24V the spread is only 24 nominal to 31, or 7 volts.  And on 12V the spread is 12 nominal to 15.5.  So 12V is the most unlikely candidate for MPPT because the voltage spread that the panels have to work with is really narrow.  24V is worse.  48V is worse yet.  Making MPPT almost necessary with 48V systems with solar - or give up a lot of panel output capacity without it.
--
Chris

[gww]

Bob and chris
Thanks.  I understand what you are saying about the 14 volt spread being a major loss.  280 watts at the 48 volt charging rate if putting out 20 amps.  That is about 210 kwh per year loss at a 4 hour average sun day and derated to around 52 percent actual usable power.  at $0.09 cents per kwh that adds up to $18.00 per year.  mppt charge controllers are $550.00.  I am afraid of the longevitie of my homemade panels but still want to use them.  I may buy the charge controller anyway as I have a feeling that during chrismas vacation I built my solar rack to hold exactly double of the 2820 watts worth of bought panels I have allready bought.  I am doing my best to hold off of more purchases untill I atleast install and use some of what I have.  I still have a $1000 dollar wire run.  I did want to understand how it really worked so I had an ideal where to go from here.

Chris
On a side note from a different post about tower higth,  I had a turbine took out by wind and so put up a forty foot tellephone pole.  fourty foot is nothing compared to medeam sized trees.  Must go higher.  The guy wires and fittings for just four at the top cost $400.00 at orslons and higher means multiple guy hights.  OUCH
Thanks
gww

[ChrisOlson]

Yeah, guyed towers aren't really any cheaper than self-supporting.  You need 25 tons of concrete in the foundation of a 80 foot self-supporting tower in most soils.  But that's a nice trade-off for the amount of real estate guyed towers take up, plus having guy wires strung all over tarnation.

I don't have any guyed towers left.  I had a 80 foot guyed tower on our lake property until the Lake Association went skitso over it.  I tore that tower down and put up a new free-standing tower.  The free-standers are a lot nicer - can lower and raise them with a 30 foot gin and 10,000 lb hydraulic winch.  And once they're up there's no messing around with guy wires to adjust or coming loose all the time.  Plus if you're in a wooded area (like our lake property is) you just need a narrow swath cleared to lay the tower down and don't have to worry about cables getting hung up in trees.

I put up a new 80 foot free-stander on our homestead property too last summer.  That tower is my test tower for new turbines and I got it set up so it when it's layed down the turbine is right in front of my shop door where I can grab it off the stub with a cherry picker.  Wouldn't be able to do that either with a guyed tower because the guy wires would be tangled up in building and trees.

I really don't care for guyed towers.
--
Chris

[gww]

Chris
Thanks for the post.  I saw the post where you made your own lattice tower.   I can look it up but if you feel like typing;
1.  how many yards of concreet is 250 tons of concreet?
2.  what aprox foot print are we talking for the concreet? length, width and depth?
3.  do your latice towers noodle at all and do you use wires from the top of tower to gen pole.

I am going to find your latice build and try to get some of these answers.

thanks
gww

[gww]

What I got done over x=mas.  Still needs more bracing.  I got the flue and didn't get it done.  Hope I don't have too much warping before I get to it again next fall.

Thanks
gww

[Crispy]

Your rack seems worthy of supporting the panels, but I am very concerned they will be able to withstand the wind load.

What is the square footage and what fasteners were used to attach the panel supports to the main structure?

[Mary B]

The 2x6? sloped pieces seem a bit light duty. I have 2 4x6's set on edge

[gww]

chrispy, maryalana

It is 12' x 40' on 4"x4" frame and maryalana is right abought the two by six sloped boards.  I attached each panel with 4, 1inch lagg bolts through the holes in the solar panal frame.  Yea I worry about wind sheer too.  I didn't have enough time and wanted to get something done over x=mas holiday.  I bought the expencive metal fixtures that they use on joist construction and just put it together then crossed my fingers.
gww

[Crispy]

The other thing is wood will warp as well as expand and contract depending on temperature and moisture content. Having rigid metal frames lagged to the wood will stress those frames as the wood moves. Using metal struts like MaryAlana did takes that stress off the panels.

At the least, you may want to consider some kind of heavy strap or fabricated metal bracket that goes around and over top the 2x6 and the 4x4 like they are doing for roof construction in hurricane country. Those joist hangers only catch the edge of the board so you do not have the strength of the full width of the board.

Wind load increases exponentially with speed. Some raw numbers for you to consider based on the dimensions you gave:

At half your planned final array ( 12 x 20 ) 240 sq ft. Double the Lbs of load below for the full array.

45 mph ( very likely at least once a year )   2500 Lbs
60 mph  4423 Lbs  WInd speed increase of 133%, load increase is 177%
90 mph which is close to what most designs are planned for.  9953 Lbs   200% increase in wind speed, 398% increase in load

Are your posts concreted in the ground? If so how deep and how much concrete? Even treated wood will degrade over time in the ground.

I hope others chime in to confirm or deny my worries. I'm still in the planning stages of my project and have read posts on this forum extensively. Considering the money invested in all those panels, a rack such as yours would make it very hard for me to sleep.

Crispy

[Mary B]

I went with concrete above grade and metal to the treated wood connectors for strength. All well above grade for longevity. And I was worried about the expansion and contraction issue until I went with the unistrut that is available at most Home Depot type stores. Also made a much stronger mounting surface.

[gww]

Crispy
The poles are about 18" deep.  aprox one bag of pre mix concreet pored in dry.
I probly should find it hard to sleep.  I have had some other bracing ideals also.

maryalana
I had some long metal heavy duty U-chanel steel but didn't actually know what was best.  I haven't had much luck with steel latly as while building this I had a turbine come down and the pipe bent by the wind.   This caused me to get sidetracked and I wanted the turbine up before my vacation was over and then the last three days I had to work on it I got the flue.  It will be several months before I can get back to it but when I do I will be retired and can adress it with out such time constraints.  I do mess up alot but find if I don't just jump in and do something I never get done so I do it and take my lumps as they come and try to improve my shortfalls without starting compleetly over.

I do like the critisizm except on my spelling,  ha ha.
Thanks for the help
gww

[Crispy]

gww,

Think of the rack as a big lever. If I counted correctly there are 5 posts in the back (high side ) of the rack.

The total weight of concrete is about 400 pounds in those 5 posts, so even if the 2x6's manage to stay attached to the 4x4 frame the first moderately strong wind that comes in on the back side of your panels  ( 45 mph = 2500 pounds of load ) will yank that whole thing out of the ground and send it sailing into the next county, especially if the ground is soggy.

Not trying to criticize in any way. Just trying to help prevent what appears to be an inevitable disaster that will cost you a lot more than the bent pipe on your windmill did.

Build yourself a nice deck with that wood and use steel and lots more concrete for your panels. Expensive, yes, but if all your panels get destroyed, starting over will cost way more.

Crispy

[Mary B]

Yeah I have 14,000 pounds of concrete under mine, a full yard per hole. The mounting steel adapters have a 3/4 inch steel L bolt the is 16 inches long. 4 pieces of rebar per hole that are tied in a cage and tied to the L bolt.

[gww]

Chrispy and Maryalana

Would it make any differance if I left a foot gap between the top row of panels and the bottom row?

If I inclosed the back with plywood to keep the wind from lifting the panels would this help?

I have a lean too that is 14'x14' on four utility poles with a tin roof that I have never had any issue with but I know it is not at a 45 degree angle.

Thanks for the responces
gww

[Crispy]

The wind load is based on the square footage of panels. I don't think a space between them will help much.

Attaching plywood to the structure to block the wind from the panels will only shift the point of attack for the wind to do it's dirty work.

Looking at MaryAlana's picture above and assuming 4 more panels were later added to the upper struts gives maybe 64 sq ft of panels (just a guess, hard to tell from a picture) so his wind load at 90 mph is around 2654 lbs with 14,000 lbs of concrete. Seems like overkill but maybe future expansion was planned for.

Your array will be almost 8 times the square footage with less than 1/30th the concrete. The math is not on your side here. It's understandable with all the time, hard work and money already invested that you would want to find a way to make this work, but I see twisted wreckage and broken dreams in the future if you proceed.

Just my opinion, but I think backed up by sound math. Perhaps others can chime in and give their opinion. Everyone here wants to see you succeed at this, I'm sure of that.

[ChrisOlson]

It is 12' x 40' on 4"x4" frame and maryalana is right abought the two by six sloped boards.  I attached each panel with 4, 1inch lagg bolts through the holes in the solar panal frame.  Yea I worry about wind sheer too.

I wouldn't worry about the 2 x 6's - that's as strong as any roof truss.  But solar panels do expand and contract which your wood frame won't like.  There's a really neat and easy way to fix that - use Unirac mounting rails on your wood frame to mount the panels:
http://www.ecodirect.com/UniRac-Solar-s/69.htm?utm_campaign=unirac&utm_content=main&gclid=CLK3_JDH5rQCFYs7MgodPgMALw

There's various places you can buy Unirac rails and that's just one example.  They are designed from the word "go" to allow for proper expansion and contraction of aluminum solar panel frames and if you bolt them to those 2 x 6 trusses it'll be just fine.

Your biggest problem with wind loading will not be from the southern facing side - it will be wind getting under it from the north.  Easy way to fix that is to close it in with either plywood sheeting or building steel.  Then you basically have a pole shed with a solar panel roof on it.
--
Chris

[Mary B]

Closer to 80sq feet, all 8 panels are mounted, each one is ~5'x2', I get 90+mph about once a year. Some years I get lucky and it comes from the west and town blocks it. I do see sustained winds of 60mph for 2-3 days at a time during bad snowstorms.

[gww]

Chris
Do I need to close the east west ends or will just finishing the north side be enough?
I have some heavy duty u chanel say inch and one half all sides.  It is steel though.  Would the panel frames dislike that.  Also the panel frames where only bolted from the bottom lip of the frame.  I did think of exspantion but thought that maby with the amount of flex in the panel frames and me not going from the top of the frame through the bottom but just through the bottom I might survive.  I only put up 5 panels cause I knew it would be 6 to 8 months before I could go back and check on them that it would be sort of a test.
Not discounting previos advice, just asking.
thanks
gww

[ChrisOlson]

I'd close in the ends too and put a door in one end so you can get in there to work on wiring.  To my way of thinking you didn't sink the posts far enough into the ground, but you have a lot of them.  A typical pole building, which will withstand winds no conventional stud type building on a slab will take, has green treat columns on 9 foot centers sunk into the ground 4 feet with a 80 lb bag of bag crete poured around each one at the bottom.

I think you have to look at it from the standpoint of a building.  But instead of shingles you got a solar panel roof on your building, and you only got a one way slope on the roof.  The wind load on the solar panel side can be high so you build inverted roof trusses to support that load.  You just need some treated 2 x 4 and plate nails to truss those 2 x 6's so they don't bend under load.  And you put in cross bracing in the building to strengthen it end to end and side to side.

Your span there is only 12 feet?  Drive around the countryside and find a farm someplace that has a pole frame building, stop in there and ask the farmer if you can look at how it's built.  You'll see right away what you have to do.  Pole frame buildings are incredibly strong because they're built like a bridge or lattice wind turbine tower - they will take 150 mph winds that will rip the steel off them, but the frame survives.  We have pole frame buildings here on our farm that have 60 foot clear span with 2 x 6 trusses on 9 foot centers.  But they're what is called a "W" type truss.  Our buildings here get 30+ tons of snow load on that 60 foot clear span in the winter and they don't give because the truss transfers the load to the columns.  And inverted W-type half truss is really easy to build with some 2 x 4's and plate nails.

Screw corrugated building steel to it on three sides, which is only about 30 bucks per 3' x 9' sheet, and it automatically braces the building sideways too.

Sure, you can throw big massive type beams at the problem.  I believe in using engineering instead of mass.

I would still consider those Unirac rails to mount your solar panels on the roof of your pole building.  They're designed for the job and they're not all that expensive considering the size of your array and the amount of panels you have to mount.
--
Chris

[gww]

Chris
Thanks for the responce.  I have actually built pole buildings when I was a teenager.  We built it with cut ceder poles and it is still standing today.  I am a little cheep as I have some home made panels on two racks in kokomo where I an now and when I take them apart I want to use the boards for bracing in MO.  I had to borrow my dads truck everytime I went to the lumberyard and just run out of time.  I knew it needed more but every step is one step closer if it last until I can improve it.  I actually only have twelve of those panels and the rest are framles home made ones.  I built the rack incase I get twelve more.  It should fit perfect.  I tried to buy the amout of stuff I needed to do it right but when I got that far I could see I needed more bracing.  I dug the holes with a three point hook auger and should have buried it but I lost a week due to a new grand baby and was really rushed and knew it was only a step towards the final product. 

I always have problims with the unirails when it comes to attachments to actually fasten the panels to them.  I have never worked with it and my normal proceedure is to walk through the whole store looking for things that might work for my application.  Many times what I end up with works but wasn't made for what I am using it for.  Got any suggestions for which rail and which fastiner to use?
Thanks
gww

[ChrisOlson]

I think on that setup I would use the channel rails that mount with L-brackets with T-bolts, center clips and end clips that attach the panels.  It makes installing the panels so easy - no holes to line up or drill and they all "just fit".  The rail also distributes loading evenly to all your trusses.
--
Chris

[gww]

Chris
My responces may be slow.  I am working 12 hours most every day so I can retire sooner. 
1.  If I run the rails, on panels that are five foot six inches long, do I put them across the 2x6 boards and not in line with the 2x6 boards? 
2.  With this size panel is two rails at one third the lenght of the panel (top third and bottom third) enough support? 
3.  Do I need the ground clips between the panels and frame or can I just ground the rails and each panel sepratly with one long wire?

I am dumb, as I looked at the link you posted and wasn't smart enough to take your suggestion and put part numbers to it.

Everytime I get a responce from you I seem to ask for more explination.  You would think english was not my first language.  Any way thanks for the suggestions and I will work on figuring it out.
 
I am really interested in the (is two rails per panel enough support) question.
Thanks
gww

[Mary B]

I ran my rails across, 2 rails per panel was plenty. Survived 75mph winds already.

[gww]

maryalana
How long are your panels? Are your rails aluminum?
Thanks
gww

[ChrisOlson]

gww,

Yes, the rails go across all the 2 x 6's, not inline with the 2 x 6's.  They got grounding clips for those Unirac mounts too so you can ground each panel to the rail, then ground the rail instead of running separate grounds to each panel.

Those Unirac rails are mounted so that the bolt holes under the panel frames approximately line up with where the rails are.

Believe me, if you saw them in person you'd go, "Why was I even thinking of messing with any other mounting method?"
--
Chris