24 volt array from six 135 watt panels.

Hairfarm
Hairfarm Solar Expert Posts: 225 ✭✭✭
Greetings,

What would be the best series/parallel wiring arrangement for a 24 volt system using six Kyocera KD135SX-LPU 135 watt solar modules? FYI, I would be using a Flexmax 80 CC in the system.

Also, I'm thinking I'll need eight Trojan T105 batts for a decent amp hours?

kind regards,

Hairfarm

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    Greetings,

    What would be the best series/parallel wiring arrangement for a 24 volt system using six Kyocera KD135SX-LPU 135 watt solar modules? FYI, I would be using a Flexmax 80 CC in the system.

    Also, I'm thinking I'll need eight Trojan T105 batts for a decent amp hours?

    kind regards,

    Hairfarm

    Easy.
    Two parallel strings of three in series.
    That will give you Vmp of 52.5 and Imp of 15.4. No need for fuses/breakers per string either.

    Reference thread: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    Eight T105's will give you 450 Amp hours @ 24 Volts. The array will be 810 Watts; too small for that much battery as you'd only get about 26 Amps from it (<6% peak charge rate).

    You also do not need an FM80 unless you are planning to expand. You could use a MidNite Kid here and one string of four batteries. Check my signature and you'll see my own system is similar in size.
  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Easy.
    Two parallel strings of three in series.
    That will give you Vmp of 52.5 and Imp of 15.4. No need for fuses/breakers per string either.

    Reference thread: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    Eight T105's will give you 450 Amp hours @ 24 Volts. The array will be 810 Watts; too small for that much battery as you'd only get about 26 Amps from it (<6% peak charge rate).




    Hi Cariboocoot,

    Thanks for the reference link.

    I should have mentioned in my post that I already had a 12 volt system in place that I want to switch to 24 volt. I already own the Flexmax 80 and six 6-volt 225ah batteries wired in three strings of 12v. And 810 watts seemed to charge my batts ok. But then again, I wasn't really taxing them too hard.

    My 12 volt inverter/charger failed and I was considering upgrading to a 24 volt inverter since I have to buy a new one anyway.

    In my previous 12V system I have 675 AH currently with a charge rate of C/12 or 8.3% of C/20:

    or - 675 / 12 = 56.25, 56.25 * 100 = 5625, 5625 / 675 = 8.3%. (* This also happens to be exactly a C/12 charge rate too. 56.25 *100 = 5625 / 675 = 8.3%.

    or factoring in the .77 efficiency in which case:
    810 Watts * 77% typical efficiency / 12 Volts system nominal = 52 Amps And so: 5200 / 675 = 7.7% charge rate. Probably a more realistic charge rate, huh?
    You also do not need an FM80 unless you are planning to expand. You could use a MidNite Kid here and one string of four batteries.

    I had already purchased the Flexmax 80 for my 12 volt system. So I'll continue to use that even though its overkill.
    No need for fuses/breakers per string either.

    I already have a combiner box in place for my three existing PV strings so the breakers were already in place. But that should be ok, I assume. As you suggested, I just need to change my array from three parallel strings of two 135W panels in series to two parallel strings of three 135W panels in series and get rid of the third breaker.

    Three questions for you:

    1) With my current six 6V batteries (@ 225ah each), I can't make a 24 volt system. I was previously using six batteries in my 12 system for 675ah. But now I either need four 6V batts or eight 6V batts, correct?

    One string of 6V batts would only 225ah. I don't know if that will be enough for my loads.

    2) If my 810W 12-volt system was able to keep up with my 675ah then why won't a 24 volt system keep up with 450 Amp hours @ 24 Volts? 450ah is less that 675ah.

    I have a 1069W 1/2 hp motor that will surge up to 4276W when starting and I'm thinking I need the 675 Ah.

    3) Will my charge rate of 7.7% still be the same with a 24 volt system?


    thanks for the info!

    Hairfarm
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    2) If my 810W 12-volt system was able to keep up with my 675ah then why won't a 24 volt system keep up with 450 Amp hours @ 24 Volts? 450ah is less that 675ah.

    12 volts X 675 ah = 8100 watthours
    24 volts X 450 ah = 10800 watthours

    You've got more pounds of lead and gallons of sulfuric acid to charge up in the 24 volt system. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    Three questions for you:

    1) With my current six 6V batteries (@ 225ah each), I can't make a 24 volt system. I was previously using six batteries in my 12 system for 675ah. But now I either need four 6V batts or eight 6V batts, correct?

    Yes; four times 6 Volts or eight times 6 Volts to get 24 Volts. Be wary of adding new batteries to old as the capacities may differ greatly even though they are the same make/model; it happens with age/cycling/abuse that capacity goes down.
    One string of 6V batts would only 225ah. I don't know if that will be enough for my loads.

    Well how much load have you got? Even at 25% DOD 225 Amp hours represents about 1kW hour AC (not including any 'bonus' power from PV after charging).
    2) If my 810W 12-volt system was able to keep up with my 675ah then why won't a 24 volt system keep up with 450 Amp hours @ 24 Volts? 450ah is less that 675ah.

    'Keep up with' is a dangerous term. 810 Watts end-to-end would be about 1.5 kW hours daily, in any configuration. You're looking at 8.1 kW hours capacity (total) vs. 10.8 kW hours capacity (total). And the fact is you could have been deficit charging all the while and not have noticed it yet.

    I'd expect 52 Amps peak from 810 Watts on 12 Volt system. That would be 7.7% charge rate, which should work providing DOD does not go over about 20%.

    Up to 24 Volts and you get 26 Amps, but on 450 Amp hour that is now 5.7% charge rate. See the difference? Lower the charge rate and you lose DOD 'room' because you can't replenish it as quickly in the same amount of sunlight hours.
    I have a 1069W 1/2 hp motor that will surge up to 4276W when starting and I'm thinking I need the 675 Ah.

    That is a big motor. That would be hard on a 4kW inverter, much less a 12 Volt system. Look in to improving its start surge if at all possible. You're looking at 178 Amps @ 24 Volts for that. Realistically it would be hard on 450 Amp hours (356 would be minimum). But do you want to invest in high battery capacity and the solar to charge it, or possibly put less money into reducing the load demand? Running Watts would be a challenge for that battery bank too, unless the demand is short-lived.

    I have a septic pump on my system. It pulls 1200 Watts running, which is a strain on 232 Amp hours @ 24 Volts (50 Amps or about 25% capacity). But it runs for 30 seconds (yes) and only midday when the panels can produce power otherwise unrealized. The start-up is not a problem because it has a soft-start mechanism on it.
    3) Will my charge rate of 7.7% still be the same with a 24 volt system?

    As above, because the actual battery bank capacity in Watt hours will change the rate drops 2%.

    Any chance of getting three more panels and adding a third string? You've got the combiner box already so that's good. Three KD140's are virtually identical to the 135's and would add 420 Watts/ 7.9 Amps to the mix bringing the array to 1230 Watts and charge current to 39 Amps thus boosting the rate for 450 Amp hours @ 24 Volts to 8.6%.
  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    Cariboocoot,

    Thanks for the very helpful response, but I'm getting a bit lost in the numeric soup.
    810 Watts end-to-end would be about 1.5 kW hours daily, in any configuration. You're looking at 8.1 kW hours capacity (total) vs. 10.8 kW hours capacity (total).

    I came up with the formula below awhile ago that gives me 4050 Kwh based on my previous 12 volt system. How did you arrive at 1.5kWh? Can you please tell me where I went wrong in my calculation below and show me the formula you used to arrive at 1.5 kW hours daily?:

    675 /2 = 337.5, 337.5 x 12v = 4050wh, 4050wh x .85 = 3442.5wh (3.4kwh)
    or: 12 volts * 675 AmpHours * 0.50 maximum battery discharge (long life) = 4,050 Watt*Hours = 4.05 kWH

    Up to 24 Volts and you get 26 Amps, but on 450 Amp hour that is now 5.7% charge rate. See the difference? Lower the charge rate and you lose DOD 'room' because you can't replenish it as quickly in the same amount of sunlight hours.

    Is this the formula you used to arrive at the above?
    "810 Watts * 77% typical efficiency / 24 Volts system nominal = 25.9 Amps
    2600/450 = 5.7"
    That right?
    That is a big motor. That would be hard on a 4kW inverter, much less a 12 Volt system. Look in to improving its start surge if at all possible. You're looking at 178 Amps @ 24 Volts for that. Realistically it would be hard on 450 Amp hours (356 would be minimum).

    Can you please show how you arrived at 178 amps @ 24 volts and the 356 minimum figure part. (To clarify, my inverter was 2000 watt with 4500k surge capability for 5 seconds. My 1/2hp pump uses 9.5amps (AC) * 115 = 1092 watts * 3 for a surge of 3277. It only comes on for about 4 minutes 5-6 times a day. Sorry for omitting this info in my previous post. Will that info affect the above calculation from you?)
    Any chance of getting three more panels and adding a third string? You've got the combiner box already so that's good. Three KD140's are virtually identical to the 135's and would add 420 Watts/ 7.9 Amps to the mix bringing the array to 1230 Watts and charge current to 39 Amps thus boosting the rate for 450 Amp hours @ 24 Volts to 8.6%.

    Well, I have enough room on the roof for three more panels. But then I would also need to buy two more batteries to get to 24 volts. Currently I have six 225ah batts (in my 12 volt system), so for 450ah @ 24V I would need two more batts to get to 450ah.

    Lastly, for a 24v system, I would need to buy three more panels (to get to 1230) and two more batteries, not too mention the Outback 24 volt inverter/ charger. I estimate these upgrades to be around:

    Option 1 - (24 volt system) $2611 for panels, batts and Outback Power VFX3024E 3000 Watt Inverter (24 volt)

    Option 2 - (Stay with my current 12v system) $1515 for the Outback Power VFX2612E 2600 Watt inverter (12 volt) by itself. Keep same panel and battery arrangement.

    A difference of roughly $1096.

    Considering the cost of three more panels and two more batteries I may just stick with a 12 volt system. I didn't realize it would cost me more to upgrade to 24 volts. I thought, wrongly, that it was just a matter of rearranging my PV panels. Doh!

    Thanks for keeping me pointed in the right direction.


    


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    Where the numbers come from:

    810 Watts * 4 hours equivalent good sun * 0.52 over-all system efficiency standard = 1.68 AC KWatt hours at the outlets. It doesn't matter if it's 12 Volt, 24 Volt, or 48 Volt based for this basic calculation.

    Your 675 Amp hours @ 12 Volts is 8100 Watt hours. 450 Amp hours @ 24 Volts is 10,800 Watt hours. 810 Watts on the first system gives you 52 Amps and 7.7% rate (810 * 0.77 / 12 = 52 Amps * 100 / 675). 810 Watts on the second gives you 26 Amps (double Voltage, halve current) and 5.7% rate because of the difference in actual Watt hour capacity of the batteries. The 24 Volt equivalent to 675 Amp hours on 12 Volts is 337.5 Amp hours.

    It is important to remember that when you are charging a battery bank the percentage is a spec of the whole capacity, not just the Amp hours used. Hence why we look at total Watt hours in comparing and based peak charge rate on the Amp hour capacity @ Voltage.
    Can you please show how you arrived at 178 amps @ 24 volts and the 356 minimum figure part. (To clarify, my inverter was 2000 watt with 4500k surge capability for 5 seconds. My 1/2hp pump uses 9.5amps (AC) * 115 = 1092 watts * 3 for a surge of 3277. It only comes on for about 4 minutes 5-6 times a day. Sorry for omitting this info in my previous post. Will that info affect the above calculation from you?)

    Watts of motor divided by system Voltage = current. So 4272 Watts will draw 178 Amps from 24 Volts. Sizing a battery bank to 'hold up' to this is a matter of percentage current draw. At 50% (178 * 2 = 356) your really stressing a battery. The V-drop under load can be significant, and actual battery capacity goes down depending on the load at the moment. So a battery rated for 220 Amp hours, for example, is only at that capacity with about 11 Amps being drawn. Real capacity literally changes moment to moment depending on how the battery is being used. We size them for a relatively small, averaged load over time. If there are large loads that must be run then they have to be accommodated. It can be a guessing game to do this too.

    Something else you should be aware of: the 24 Volt system will be more efficient. Same power delivered more as Voltage and less as current = less power going into heating wires. This can be particularly important for large loads like that motor. 3277 Watt surge is 273 Amps on 12 Volts. Let's call that 'massive' as it would be a significant load on 4/0 wire. On 24 Volts that current drops to half: 136.5 Amps which even 1/0 can manage for a bit. That does not include the efficiency increase.

    And then there's the general dictum to avoid 12 Volt systems unless you have no choice.
  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    Cariboocoot,
    The 24 Volt equivalent to 675 Amp hours on 12 Volts is 337.5 Amp hours.

    So your calculation for this is:

    810 / 24v = 33.75 (337.5)
    or for 12v system: 810 / 12v = 67.5 (675)
    Looks like you're moving the decimal over one space in your calculation above. What is the purpose of doing that?
    Something else you should be aware of: the 24 Volt system will be more efficient. Same power delivered more as Voltage and less as current = less power going into heating wires.

    This kind of statement is really an AHA moment for me - "power delivered more as Voltage and less as current". So simple.
    This can be particularly important for large loads like that motor. 3277 Watt surge is 273 Amps on 12 Volts. Let's call that 'massive' as it would be a significant load on 4/0 wire. On 24 Volts that current drops to half: 136.5 Amps which even 1/0 can manage for a bit. That does not include the efficiency increase.

    According to the Wind and Sun voltage drop calculator it looks like my wire will handle at least 300A @ 8 feet while under a 3% voltage drop using my 4/0 wire. In both pics I used the 273A "surge" load from your motor calculation above. (see attached pics)

    Attachment not found.

    This pic is calculated with a 24v system. Note the much greater length possible with 24 volts.
    Attachment not found.

    My previous 12v inverter was 7.5 feet from the batts using 4/0 wire. Just like you said, I'm was right near the edge with a 12V system. Also, it's really hot in the Mojave desert which would've further hampered my 12 volt system, I'm sure. Also, none of this takes into account the likelihood that there would've been other loads on the 12 volt system further reducing the available current when the shallow well pump decided to kick in.

    I probably need to seriously consider a 24v system and just take the financial hit. Maybe it was sign from the gods when my existing 12 inverter suddenly became an expensive paperweight.:cry:

    Thanks again.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    Cariboocoot,
    So your calculation for this is:

    810 / 24v = 33.75 (337.5)
    or for 12v system: 810 / 12v = 67.5 (675)
    Looks like you're moving the decimal over one space in your calculation above. What is the purpose of doing that?

    No, my formula is for Watt hour capacity: nothing to do with the 810 Watts of the array. That is a coincidence.
    12 Volts @ 675 Amp hours is 8100 Watt hours. 8100 Watt hours on 24 Volts is 337.5 Amp hours.
    According to the Wind and Sun voltage drop calculator it looks like my wire will handle at least 300A @ 8 feet while under a 3% voltage drop using my 4/0 wire. In both pics I used the 273A "surge" load from your motor calculation above. (see attached pics)

    4/0 wire is good for about 300 Amps maximum. If you have 8 feet of it loaded to 273 Amps the V-drop on a 12 Volt system is <2%. But as system Voltage goes down current goes up and the difference becomes more dramatic. It is possible to have the V at the inverter drop below LVD with such a sudden surge even though the numbers say it won't (deep cycle batteries aren't good at delivering fast, hard current; they prefer steady, even draw). Quite a number of people have experienced this just trying to get refrigerators to start on 2kW inverters; everything has to be in top condition to prevent the 'brown out' of the inverter.

    My previous 12v inverter was 7.5 feet from the batts using 4/0 wire. Just like you said, I'm was right near the edge with a 12V system. Also, it's really hot in the Mojave desert which would've further hampered my 12 volt system, I'm sure. Also, none of this takes into account the likelihood that there would've been other loads on the 12 volt system further reducing the available current when the shallow well pump decided to kick in.

    I probably need to seriously consider a 24v system and just take the financial hit. Maybe it was sign from the gods when my existing 12 inverter suddenly became an expensive paperweight.:cry:

    Thanks again.

    24 Volt system ease things drastically in current-handling. Probably pick up 2% efficiency for most loads.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    I probably need to seriously consider a 24v system and just take the financial hit. Maybe it was sign from the gods when my existing 12 inverter suddenly became an expensive paperweight.:cry:

    A 24 volt system is usually LESS expensive (and more efficient) than the equivalent 12 volt system.

    12 volt system:
    (11.6% charge rate)
    four t105 batteries in series/parallel
    six 135 watt modules
    controller
    12 volt inverter

    24 volt system:
    (11.6% charge rate)
    four t105 batteries in series
    six 135 watt modules
    controller
    24 volt inverter

    The 24 volt system costs less because you can use thinner wire and have less battery connections. The reason a 24 volt system seems so expensive to you is because you are pricing a larger system... more panels, more batteries.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    A big thanks to those on this thread for the helpful suggestions.
    Cariboocoot - Something else you should be aware of: the 24 Volt system will be more efficient. Same power delivered more as Voltage and less as current = less power going into heating wires. This can be particularly important for large loads like that motor. 3277 Watt surge is 273 Amps on 12 Volts. Let's call that 'massive' as it would be a significant load on 4/0 wire. On 24 Volts that current drops to half: 136.5 Amps which even 1/0 can manage for a bit. That does not include the efficiency increase.

    A question regarding AC surge power and DC amperage.

    As I've mentioned in this thread, I have a 9.5AAC 1/2hp shallow well pump. 9.5 * 120AC = 1140 watts. Now from what I understand, a pump surge rating is based on three times it's operating continuous wattage. So for my pump, it's operating wattage would be 1092.5 watts * 3 = 3277 watts. Cariboocoot estimated that my pump surge would pull 273ADC from the 12V battery bank.

    Now this surge will only happen for a split second before settling into its normal operating wattage of 1092 watts. I can't seem to wrap my head around what the minimum battery bank amp hours I need to power the split-second surge of 273ADC.

    If I use four Trojan T-105's @ 225ah in 24 volt system I would only have a max of 225ah. And 225ah is under the 273ADC that my pumps surge would need.

    My question is this:

    1) What is the minimum amount of 6v @ 225ah batteries I would need in a 24VDC system to make sure it can handle a 273ADC pump surge.
    2) What is the minimum amount of 6v @ 225ah batteries I would need in a 12VDC system to make sure it can handle a 273ADC pump surge.

    What about just needing 273 amps for under a second? I'm looking into an inverter with a 6000 watt surge for 5 seconds.

    Cariboocoot, - I want to look into a "soft start" device like the one you said you use in your current setup to reduce my surge wattage. Also because you said that batteries do not like sudden high surge demands made on them. Would you mind explaining how they work and where I can find one? It sounds like something I need. Will they work with any type of shallow well pump?

    Thanks!


    Cool, it just started to raining in the desert.:D
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    My question is this:

    1) What is the minimum amount of 6v @ 225ah batteries I would need in a 24VDC system to make sure it can handle a 273ADC pump surge.
    2) What is the minimum amount of 6v @ 225ah batteries I would need in a 12VDC system to make sure it can handle a 273ADC pump surge.

    What about just needing 273 amps for under a second? I'm looking into an inverter with a 6000 watt surge for 5 seconds.

    You're looking at two different things.
    3277 Watts on 12 Volts is 273 Amps. On 24 Volts it's half that: 136 Amps. A set of T10's can handle that much current momentarily. Supplying the power (Watts) with more Voltage and less current is a good thing.
    Cariboocoot, - I want to look into a "soft start" device like the one you said you use in your current setup to reduce my surge wattage. Also because you said that batteries do not like sudden high surge demands made on them. Would you mind explaining how they work and where I can find one? It sounds like something I need. Will they work with any type of shallow well pump?

    In the case of my septic pump it was built-in from the factory. Adapting such technology to existing motors is a bit more of a challenge as every motor is different. There are a lot of discussions of the forum about using capacitors (has to be the right one) and variable frequency drive (VFD) as well as some other techniques/devices to achieve this. You have to know what the motor is you're dealing with and then select the best choice for reducing that demand. Sounds like your pump doesn't even have a start cap! Maybe a better pump is in order? I replaced a 1/3 HP unit at the old house with a 1/2 HP one and the power consumption went down as a result: some pumps are better.
    Cool, it just started to raining in the desert.:D

    Raining here too. Making a mess as usual. :p
  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    Cariboocoot,

    Currently, I have six 6-volt 225ah T-105 Trojan batteries for 675ah @ 12 volts. I also have six PV panels @ 135-watt @ 810 watts.

    I was looking to upgrade to a 24 volt system, but to upgrade to a 24V system I would need to get two more batteries for a 24V @ 450Ah system and three more panels to power it at a sufficient charge rate.
    Any chance of getting three more panels and adding a third string? You've got the combiner box already so that's good. Three KD140's are virtually identical to the 135's and would add 420 Watts/ 7.9 Amps to the mix bringing the array to 1230 Watts and charge current to 39 Amps thus boosting the rate for 450 Amp hours @ 24 Volts to 8.6%.
    810 Watts * 4 hours equivalent good sun * 0.52 over-all system efficiency standard = 1.68 AC KWatt hours at the outlets. It doesn't matter if it's 12 Volt, 24 Volt, or 48 Volt based for this basic calculation.

    Based on your above calculation of 4 hours equivalent good sun * 0.52 over-all system efficiency standard placed my Kw hours at 1.68. The above equation shows that I need to eight T-105 @ 225ah to make a 450ah system.

    Ok, so there's no getting around having to purchase eight 6-volt batts for my 24 volt system. That's a necessity, but can I get away with maybe two more additional 135 panels instead of three more additional panels since my solar zone adds two more hours of sun than the four hours you used to calculate?

    Thing is, I live in an area that has 6 hours (not four) of average sun. Please look at the ZONE 1 area. The solar zone I live in is 6 hours of average sun and not the 4 hours that you used in your equation. Please see image.
    Attachment not found.




    1) What wiring configuration would I need to power 450ah @ 24 volts using eight 135 watt panels with 6 hours of sun?

    Up to 24 Volts and you get 26 Amps, but on 450 Amp hour that is now 5.7% charge rate. See the difference? Lower the charge rate and you lose DOD 'room' because you can't replenish it as quickly in the same amount of sunlight hours.

    Eight T105's will give you 450 Amp hours @ 24 Volts. The array will be 810 Watts; too small for that much battery as you'd only get about 26 Amps from it (<6% peak charge rate).

    I'd expect 52 Amps peak from 810 Watts on 12 Volt system. That would be 7.7% charge rate, which should work providing DOD does not go over about 20%.

    2) What would my charge rate be with 6 hours of sun instead of the four in your original calc? Could I get away with eight 135-watt panels instead of three strings of three?


    Is that that possible?

    thanks Cariboocoot,

    Just trying to save a bit without having to go back to a 12 volt system!

    Hairfarm
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    Okay the first problem is that 6 hours of average sun is not the same as 4 hours equivalent good sun. In a 6 hour average sun day you likely only get 4 hours of good sun, maybe less. Without using a tracker the panels are not pointed directly at the sun through the whole day, so there is a fairly steep bell curve of output from the panels. The hours of lesser output before and after peak output average into a shorter equivalent of peak output.

    Up here we have Summer days 16 hours long: they don't produce significantly greater amounts of solar power than days 8 hours long because the sun is not directly on the panels for any longer period of time just because the day is longer. And when figuring for charging batteries you want to look at the "worst case scenario" in sunny days; the shortest one. Go to PV Watts and check out the hourly daylight near where you are.

    What can you get out of eight 135 Watt panels? 1080 Watt array should deliver slightly over 2kW hours per day. You can improve the over-all efficiency by timing usage so that you 'harvest' more of the power that would otherwise go unrealized after batteries get recharged.

    You can also expect about 35 Amps of peak current, or a 7.7% charge rate on 450 Amp hours @ 24 Volts. That should work providing you don't have heavy concurrent loads or deeper than 25% DOD (I've never correlated an exact relationship between varying peak charge current and DOD but 10%/25% is balanced and as the charge rate goes down the DOD must also to maintain balance otherwise you end up with deficit charging).

    Your best array configuration with the eight panels is probably four parallel strings of two in series (these being 17-18 Vmp panels; two in series is minimum for a 25 Volt system).

    Again 6 hours of equivalent good sun is not that easy to achieve. Most fixed arrays would need a 10 hour day to do that good.
  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    Re: 24 volt array from six 135 watt panels.

    Hi Cariboocoot,

    Thanks again for the the suggestions. I went to PVWatts like you advised and here are the screen shots from that site:

    Attachment not found.Attachment not found.

    It says that I'm getting 6.37 ANNUAL hours per year (average?) in the desert based on a .810kW solar array facing south. Am I interpreting this correctly? And is this calculation for 6 hours of "good" sun? Maybe this is the reason there are so many utility-grade solar farms everywhere around me. My ZIP code is 92252.

    I've definitely decided on a 24 volt system since my 12 volt inverter died. But I have a converted evaporative (swamp) cooler that runs on DC. I have to have this cooler at the cabin. It's one of those non-optional items. It uses about 9ADC. Do you think a 24-12V step down inverter like this will work? Any downsides? How do you run your 12V item/s of off your 24 volt system?

    http://www.amazon.com/dp/B003P165NM/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=1RO50FNM3L52P&coliid=INSUXJQ9RMQC0

    Btw, I realize this isn't an item that AWS sells, but I couldn't find anything comparable for the same price on their website.

    thanks so much!
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    Do you think a 24-12V step down inverter like this will work? Any downsides? How do you run your 12V item/s of off your 24 volt system?

    http://www.amazon.com/dp/B003P165NM/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=1RO50FNM3L52P&coliid=INSUXJQ9RMQC0

    By specifications, it looks like it will work. I have no idea if that is a high quality unit. If you want a name brand with a good reputation consider Solar Converters. Solar Converters make a large number of products and NAWS does carry some of them, but NAWS does not show all the products on their website.

    http://www.solarconverters.com/index.php/products/84-dc-dc-step-down-controllers-ppt/161-ppt-12-24-20-rxx

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24 volt array from six 135 watt panels.
    Hairfarm wrote: »
    Hi Cariboocoot,

    Thanks again for the the suggestions. I went to PVWatts like you advised and here are the screen shots from that site:

    Attachment not found.Attachment not found.

    It says that I'm getting 6.37 ANNUAL hours per year (average?) in the desert based on a .810kW solar array facing south. Am I interpreting this correctly? And is this calculation for 6 hours of "good" sun? Maybe this is the reason there are so many utility-grade solar farms everywhere around me. My ZIP code is 92252.

    I've definitely decided on a 24 volt system since my 12 volt inverter died. But I have a converted evaporative (swamp) cooler that runs on DC. I have to have this cooler at the cabin. It's one of those non-optional items. It uses about 9ADC. Do you think a 24-12V step down inverter like this will work? Any downsides? How do you run your 12V item/s of off your 24 volt system?

    http://www.amazon.com/dp/B003P165NM/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=1RO50FNM3L52P&coliid=INSUXJQ9RMQC0

    Btw, I realize this isn't an item that AWS sells, but I couldn't find anything comparable for the same price on their website.

    thanks so much!

    The PV Watts site is geared for grid-tie installs but can be interpreted for off-grid. In this case you need to look at the lowest number of hours in December: 4.9. That's better than a lot of places do!

    Yes a DC to DC converter sized to handle the load requirement is usually the most practical way of powering a 12 Volt load from a 24 Volt battery bank.