# Help with Solar panel Design for 24 volt system

Solar Expert Posts: 107 ✭✭✭✭
I have finally figured out how to make a solar panel that will last!!! 
Now I have a few questions about the wiring, I have currently been building my panels w/38 cells for a 12 volt system putting me around 18 or 19 volts out put and round 3.5 amps. 3x6 cells
My question is regarding a 24 volts system… I have 2 sets of breakers in my house 1st is a 120 volts, which will be handled by the 12volts system and the 240 volts switches, which is where the 24 volts system comes in. My question is How should I wire the solar panels for the 24 volts system.. Should I ..
A.) continue to make 18,19 volt panels and wire them together in series or,
B.) make a complete panel out of 76 cells and come down at 38 volts??

What I’m thinking is:
I think that by wiring 2 panels together in series I’ll lose a lot of wattage vs just wiring the whole panel for 32 volts in from the start..
Example 1: 38 cells @ 1.5 watts =57 watts, @0.5 volts = 19 volts = if I wire 2 of those in series I have 38 volts but I’m still at 57 watts, losing the other 57 watts from the other panel.
Example 2: 76 cells @1.5 watts= 114 watts @0.5 volts = 38volts, don’t have to wire in series because I already have needed voltage.

So am I correct in thinking that I need to use example 2 when designing my solar panels for 24 volt system???

Now to the Good part, HOW TO MAKE A SOLAR PANEL THAT WILL LAST

1st step, tab the front of all solar cells

2nd step, go buy your encapsulating product, I found a cheap over the counter product that doesn’t seem to yellow that much, I’ve had my panel out in the elements for a month now and still no yellowing… this is the product I’m using: http://www.biosafe-inc.com/glazecoat.htm (you can get this product from Lowes or Home depot for \$22.00, it’s rated for indoor use, I’m in Atlanta GA and it’s been extremely hot and I’ve had no problems out of it, and it doesn’t yellow)

3rd step-PRIME the front of your solar cell, basically mix a small batch of Epoxy and paint it on the front of the solar cell (make sure paint brush strokes are in the same direction of the tabbing wire, don’t paint the epoxy on the front of it going against the tabbing wire, you might lift it up or break a soldered boned) After putting the epoxy on the front of the solar cell, put it on the glass face down where you want it to go. Do this for all the cells you plan to put in the panel, make sure you put the cells where you want them because once the Epoxy hardens it’s on there for good. (Wait a day for the epoxy to harden then go to step 4)

4th step- Solder the back of the solar cells together linking them in series. And wire all your bus bars, the cells are not moving while you work

5th step- Now that all the cells are linked together, out line the entire piece of glass with silicone. (What your trying to do is keep the Epoxy from drain off the glass onto your work area)

6th step – Mix more Epoxy and pour it all over the cells, don’t worry about bubbles, you fixed that problem when you painted the epoxy on the front of the cells in step 3 , like I said before I use this stuff. http://www.biosafe-inc.com/glazecoat.htm (wait a day for it to harden)

7th step- After it hardens, you’re pretty much done, get your miter saw and 2 L channels aluminum trim and cut your frame, actually, I do this step first; I mount my L channels to the glass to help keep the epoxy from spilling.

Now you have a long lasting solar panel.

• Banned Posts: 17,615 ✭✭
Re: Help with Solar panel Design for 24 volt system

It sounds like you are mixing 12 VDC and 24 VDC with 120 VAC and 240 VAC. This is a really bad idea. AC breakers aren't rated for DC Voltages, and you do not want to have AC and DC in the same panel box. One mis-wiring and things go up in flames.

As for a 24 VDC system, some manufacturers make higher Voltage panels. In terms of charging a 24 Volt system you can either use the higher Voltage panels or put two "12 Volt" panels in series.

A 185 Watt "24 Volt" panel may have specs like this:
Vmp 36.4, Voc 45.0, Imp 5.09, Isc 5.43, Length 62.2", Width 31.8"

A 135 Watt "12 Volt" panel may have specs like this:
Vmp 17.7, Voc 22.1, Imp 7.63, Isc 8.37, Length 59.1", Width 26.3"

As you can see the "24 Volt" panel is physically larger and the cells are configured for higher Voltage but lower current.

It doesn't matter to the batteries which way you do it: higher Voltage panel or two lower Voltage in series.
• Solar Expert Posts: 107 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system
As you can see the "24 Volt" panel is physically larger and the cells are configured for higher Voltage but lower current.

It doesn't matter to the batteries which way you do it: higher Voltage panel or two lower Voltage in series.

how do you config for Higher voltage but low current?
• Solar Expert Posts: 7,249 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system

Such a recipie for disaster
I'm just not going to say anything more
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 292 ✭✭✭✭✭✭
Re: Help with Solar panel Design for 24 volt system

I think it is great that you are building you own panels. I would like to add a suggestion.
You have only done a month of testing as far as the yellowing. you should really check the panel output the first day you make them and then in a month. Not yellowing does not mean that some of the suns rays are not being blocked, it just means that you cannot tell a difference by looking at them. Make one today and lay it side by side with the month old one and see if you can tell the difference. Also, a months time, in the effective 20 year lifespan of a panel, is just a drop in the bucket. What if a year from now the panel is so discolored that you cannot see through it any more? I am baseing(sp) this on years of automotive and remodeling work and pretty much everything I have ever used has a tinting problem with age, and alot of it is not in direct sunlight.

Just food for thought.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Help with Solar panel Design for 24 volt system
What I’m thinking is:
I think that by wiring 2 panels together in series I’ll lose a lot of wattage vs just wiring the whole panel for 32 volts in from the start..
Example 1: 38 cells @ 1.5 watts =57 watts, @0.5 volts = 19 volts = if I wire 2 of those in series I have 38 volts but I’m still at 57 watts, losing the other 57 watts from the other panel.

No. Whether you wire in series or parallel, the power (Watts) is additive. In series the voltage adds but the current is constant. In parallel the current adds but the voltage is constant. Power is the product of voltage and current, so the power is additive no matter how you connect the cells.
• Solar Expert Posts: 107 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system
ggunn wrote: »
No. Whether you wire in series or parallel, the power (Watts) is additive. In series the voltage adds but the current is constant. In parallel the current adds but the voltage is constant. Power is the product of voltage and current, so the power is additive no matter how you connect the cells.

you have 2 ..6volt, 225amp hour batteries wired in seires... you don't get 12volts at 450amp hours, you get 12 volts at 225amp hours, Solar panels work the same way from what i understand..

So wiring them together in series will give me 2x the amount of voltage but the wattage doesn't go up...this is what i thought, am i right?
• Banned Posts: 17,615 ✭✭
Re: Help with Solar panel Design for 24 volt system
you have 2 ..6volt, 225amp hour batteries wired in seires... you don't get 12volts at 450amp hours, you get 12 volts at 225amp hours, Solar panels work the same way from what i understand..

So wiring them together in series will give me 2x the amount of voltage but the wattage doesn't go up...this is what i thought, am i right?

225 Amp hours @ 6 Volts is 1350 Watt hours. 225 Amp hours @ 12 Volts is 2700 Watt hours.

Watts, or Watt hours, represent the total power: the Amps * the Volts (not including power factor which is why Watts sometimes doesn't equal Volt Amps - but that doesn't really apply here).

Wiring two panels in series the Voltage goes up, the Amps stays the same, the Watts go up.
Wiring two panels in parallel the Voltage stays the same, the Amps go up, the Watts go up.

Higher power panels typically have parallel strings of cells with by-pass diodes too. Gets complicated fast, doesn't it?
• Solar Expert Posts: 107 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system
Wiring two panels in series the Voltage goes up, the Amps stays the same, the Watts go up.
Wiring two panels in parallel the Voltage stays the same, the Amps go up, the Watts go up.

Higher power panels typically have parallel strings of cells with by-pass diodes too. Gets complicated fast, doesn't it?

Yes it does

What in the world, how can the wattage go up but not the amps? I thought the amps were dependent on the wattage? So if the wattage goes up then the amps will go up as well, right? OH, i get it.. never mind i figured it out.. Yes the wattage does go up, it has to go up, but so did the voltage, i'm no longer working at 12 volts i'm at 24 volts, in order for the amps to remain the same at 24 volts the wattage needs to double, So yes, the wattage does go up.. I stand corrected
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system

my comments and questions in bold.
I have finally figured out how to make a solar panel that will last!!! 
what is that at the end of those exclamation marks and what does it mean?

Now I have a few questions about the wiring, I have currently been building my panels w/38 cells for a 12 volt system putting me around 18 or 19 volts out put and round 3.5 amps. 3x6 cells
there is no need to go with 38 cells as the standard 36 cells would do just fine.

My question is regarding a 24 volts system… I have 2 sets of breakers in my house 1st is a 120 volts, which will be handled by the 12volts system and the 240 volts switches, which is where the 24 volts system comes in. My question is How should I wire the solar panels for the 24 volts system.. Should I ..
A.) continue to make 18,19 volt panels and wire them together in series or,
B.) make a complete panel out of 76 cells and come down at 38 volts??
you are scaring me. my question to you is, what are you doing as this can be misconstrued to mean something else unsafe here with your comments on mixing voltages at the breakers? btw, do you realize just how big 1 pv would be? i would just put 2 in series.

What I’m thinking is:
I think that by wiring 2 panels together in series I’ll lose a lot of wattage vs just wiring the whole panel for 32 volts in from the start..
Example 1: 38 cells @ 1.5 watts =57 watts, @0.5 volts = 19 volts = if I wire 2 of those in series I have 38 volts but I’m still at 57 watts, losing the other 57 watts from the other panel.
false for the current is the same and the voltage doubled making the power double.

Example 2: 76 cells @1.5 watts= 114 watts @0.5 volts = 38volts, don’t have to wire in series because I already have needed voltage.

So am I correct in thinking that I need to use example 2 when designing my solar panels for 24 volt system???
no, as you can get it by putting 2 pvs in series. it makes no difference if that same number of cells are in series in 1 pv or split between 2 pvs. only difference would be an external interconnecting wire instead of tabs and it would be easier mounting 2 smaller pvs that 1 extra large pv that may need a crane to get it up there.

Now to the Good part, HOW TO MAKE A SOLAR PANEL THAT WILL LAST

1st step, tab the front of all solar cells

2nd step, go buy your encapsulating product, I found a cheap over the counter product that doesn’t seem to yellow that much, I’ve had my panel out in the elements for a month now and still no yellowing… this is the product I’m using: http://www.biosafe-inc.com/glazecoat.htm (you can get this product from Lowes or Home depot for \$22.00, it’s rated for indoor use, I’m in Atlanta GA and it’s been extremely hot and I’ve had no problems out of it, and it doesn’t yellow)

3rd step-PRIME the front of your solar cell, basically mix a small batch of Epoxy and paint it on the front of the solar cell (make sure paint brush strokes are in the same direction of the tabbing wire, don’t paint the epoxy on the front of it going against the tabbing wire, you might lift it up or break a soldered boned) After putting the epoxy on the front of the solar cell, put it on the glass face down where you want it to go. Do this for all the cells you plan to put in the panel, make sure you put the cells where you want them because once the Epoxy hardens it’s on there for good. (Wait a day for the epoxy to harden then go to step 4)

4th step- Solder the back of the solar cells together linking them in series. And wire all your bus bars, the cells are not moving while you work

5th step- Now that all the cells are linked together, out line the entire piece of glass with silicone. (What your trying to do is keep the Epoxy from drain off the glass onto your work area)

6th step – Mix more Epoxy and pour it all over the cells, don’t worry about bubbles, you fixed that problem when you painted the epoxy on the front of the cells in step 3 , like I said before I use this stuff. http://www.biosafe-inc.com/glazecoat.htm (wait a day for it to harden)

7th step- After it hardens, you’re pretty much done, get your miter saw and 2 L channels aluminum trim and cut your frame, actually, I do this step first; I mount my L channels to the glass to help keep the epoxy from spilling.

Now you have a long lasting solar panel.

i won't comment on your building methods as i don't agree with building them for large home systems and it should be confined to the smaller diy solar projects that aren't critical.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Help with Solar panel Design for 24 volt system
you have 2 ..6volt, 225amp hour batteries wired in seires... you don't get 12volts at 450amp hours, you get 12 volts at 225amp hours, Solar panels work the same way from what i understand..

So wiring them together in series will give me 2x the amount of voltage but the wattage doesn't go up...this is what i thought, am i right?

No. It's like I told you before(copied from post #6):
Whether you wire [solar cells] in series or parallel, the power (Watts) is additive. In series the voltage adds but the current is constant. In parallel the current adds but the voltage is constant. Power is the product of voltage and current, so the power is additive no matter how you connect the cells.

Batteries are the same way. If you connect two in series you get the same current (amps) but twice the voltage. Two in parallel gives you twice the current at the same voltage. Either way it's twice the power (Watts) because Watts = Volts X Amps.

In your example above, if two 6V 225Ah batteries in series were to give you 450Ah at 12V, that wouldn't be 2 times the power (energy, actually, because its Amp-hours and Watt-hours), it would be 4 times. (6)(225) = 1350, (12)(450) = 5400, and 5400 = 4 X 1350.

No offense intended, but you should have a better understanding of basic electrical concepts before you go off designing components.
• Solar Expert Posts: 107 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system

Yeah Neil, i'm going to have to agree with you, 2 panels wired together will be easier to handle than one huge panel.

i use free glass that people are throwing out to make my solar panels w, some of the glass is pretty big and can hold 76 cells, I have one piece that can hold 114 cells.. This is the reason i asked...But after talking it over with you guys i'm taking your advice and going w/2 panels in series..

Far as building my own panels, i really really like the way my new panels are turning out.. I will continue to run my test, I also plan to post a video of my progress or lack there of, also plan to post a video of the wiring of the system so you guys can let me know where i'm going wrong and how to fix it... I have no clue what i'm doing here and might kill myself
• Solar Expert Posts: 630 ✭✭
Re: Help with Solar panel Design for 24 volt system

You need measurements, to validate your hard work. How much sunlight is being blocked with epoxy? Have you 'clear-coated' one cell, let it cure, and then measured its voltage and current against a non-coated cell? This would be MY first test before committing a whole module.
Yeah Neil, i'm going to have to agree with you, 2 panels wired together will be easier to handle than one huge panel.

i use free glass that people are throwing out to make my solar panels w, some of the glass is pretty big and can hold 76 cells, I have one piece that can hold 114 cells.. This is the reason i asked...But after talking it over with you guys i'm taking your advice and going w/2 panels in series..

Far as building my own panels, i really really like the way my new panels are turning out.. I will continue to run my test, I also plan to post a video of my progress or lack there of, also plan to post a video of the wiring of the system so you guys can let me know where i'm going wrong and how to fix it... I have no clue what i'm doing here and might kill myself
• Solar Expert Posts: 107 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system
bmet wrote: »
You need measurements, to validate your hard work. How much sunlight is being blocked with epoxy? .
Zero
bmet wrote: »
Have you 'clear-coated' one cell, let it cure, and then measured its voltage and current against a non-coated cell?
String together 5 cells and measured the voltage and wattage
Then strung together 5 covered them in the epoxy and stuck them to a piece of broken glass i use for my test and measured the Voltage and wattage, and didn't notice a difference between the one's not covered with epoxy vs the ones that weren't... So I moved forward to make a whole panel, Just finish my 1st month of test and haven't notice any drop in power since I first made it.... I have several different designs i've made and so far this is the best!!! I believe these panels will last as long as the glass doesn't break... If it does yellow then I'll probably lose alittle power, that's why I over kill on the number of cells i use, I try to get the voltage around 20-23 volts, in 5 years if it drops from 23 to 18 or 17, it won't be a big deal... However if I start off around 18 volts and drop down to 15 or 14 over the years then I'll have problems, because when the panels get hott, the voltage drops anyway regardless of who built it
• Super Moderators Posts: 26,758 admin
Re: Help with Solar panel Design for 24 volt system

For the most part, blocking light to cells will reduce their current, not their output voltage.

Quick test is to measure short circuit current to check for reduced output. 10 reduction in current is a 10% reduction in power (unless a cell or connection fails--then all bets are off).

Measuring Watts is a lot more difficult (you have to find the Pmp=Vmp*Imp operating point.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 107 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system
BB. wrote: »
For the most part, blocking light to cells will reduce their current, not their output voltage.

Quick test is to measure short circuit current to check for reduced output. 10 reduction in current is a 10% reduction in power (unless a cell or connection fails--then all bets are off).

Measuring Watts is a lot more difficult (you have to find the Pmp=Vmp*Imp operating point.

-Bill

Darn it bill, why you have to make everything so complicated? I was in-correct in saying i messured the wattage what i meant to say is i measured the amps with my panel amp meter and didn't notice any difference.

I keep it pretty simple, I measure the voltage, if it's 19 or more i'm happy and think to myself "that's good"
Then i measure the amps and if the amp meter goes up? i think to myself, "that's good, this panel is working" lol
Then i wait till 2pm-3 pm to measure amps again to get an ideal of what the panel's max amp is,(during all my test this is the time i've gotten the most amps out of the array) i get that number and mutiply it by 13.5 and claim that number as the wattage of the panel, so if i get 3 amps, i go 3 x 13.5 =40.5 ... this is a 40 watt panel

This is my math when it comes to rating my panels
• Super Moderators Posts: 26,758 admin
Re: Help with Solar panel Design for 24 volt system

Actually, you probably want Voc > 20.5 volts or so... And Vmp (more difficult to measure) at ~17.5 volts for use with a PWM controller (or the minimum input for a MPPT controller on a 12 volt battery bank).

Many panels are rated at Pmp=Vmp*Imp... However, if using a PWM controller, it looks more like:
• 5 amps * 17.5 volts = 87.5 watts marketing rating
• 5 amps * 14.5 volts battery charging = 72.5 watts useful to you rating
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Help with Solar panel Design for 24 volt system

It will probably be futile for me to suggest using a 2 Ohm resistor capable of handling 10+ Amps and feed the panel output to that, then measure the Voltage on the resistor and current going through the wire. That ol' I = V/R thing, y'know. Won't be perfect, but could approximate the Vmp & Imp points and at least give a fixed reference for comparing to other panels made the same way.

Just ignore me. My system components came from a factory. They also work. :roll:
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: Help with Solar panel Design for 24 volt system
It will probably be futile for me to suggest using a 2 Ohm resistor capable of handling 10+ Amps and feed the panel output to that, then measure the Voltage on the resistor and current going through the wire. That ol' I = V/R thing, y'know. Won't be perfect, but could approximate the Vmp & Imp points and at least give a fixed reference for comparing to other panels made the same way.

Just ignore me. My system components came from a factory. They also work. :roll:

coot,
resistors are rated in watts and feeding a pv's output to it at 12v, well you know the old p = v x i thingy after figuring the i with the old i = v/r from ohm's law.:roll:
• Banned Posts: 17,615 ✭✭
Re: Help with Solar panel Design for 24 volt system

Niel;

Yes, definitely not a one-size-fits-all testing procedure. I was just doodling with the idea of a fixed load to try the panels against to get some repeatable numbers vis V and A to be sure they're consistent. Since the panels in this case have a target of "12 Volt" specifications it would be approximately 2 Ohms. Probably 2.3 and 200 Watt, which could be a tad difficult to find (other than winding your own). Even so, you'd have the trouble of "is it as bright out today as it was yesterday?" to deal with.

Factories are wonderfully consistent in their production values. :roll:

I used to have some 8 Ohm, 300 Watt wire-wound (on big ceramic tubes) resistors for testing audio amplifiers. I used to have my sanity too; just another of many things lost or misplaced over the years.