Help with cabin system

System
System Posts: 2,511 admin
The PV system at our cabin in the mountains of SW Utah (10,000 feet elevation) has worked well since it was installed in 2003, but I need to move the ground-mounted panels to a pole-mounted system so that I can tilt them for greater efficiency and to keep them above snow level.

Main system components:

3 Shell (Siemans) BP SM110-12P panels (110W, 12V, 6.3A)
ProStar 30 Amp 12/24V charge controller
4 Surrette S-460 460 amp batteries, wired in series/parallel to produce 12V
Prosine 2500W inverter, 12V, 100A charger

The 3 PV modules are wired in parallel to provide max 330W, 12V (max 27.5 amps?). The issue is that the cable run from the pole-mounted location to the charge controller will be quite long; approximately 40 to 45 feet. The system currently uses 6 AWG cable from the modules to the controller, but the run is only about 25 feet long.

The question is, will this longer run exceed the maximum for 6 AWG cable so that I'll have to move the controller closer to the panels? The controller/batteries/inverter are currently located in the crawl space where winter temperatures do not normally fall below freezing. If I have to move these components closer to the array, I'll either have to move them up into the living space (which is not heated) or into a utility room that also is routinely below freezing in the winter. Moving them closer to the panels will also result in a longer run to the DC pump that pumps water from an underground tank into the pressure tank.

Another question: will the 4" steel pole (for an IronRidge or equivalent top of pole mount) provide an adequate natural ground for the system?

I appreciate your input!
Karl

Comments

  • System2
    System2 Posts: 6,290 admin
    Addendum: Help with cabin system

    The other part of the first question is whether I can somehow rewire the 3 panels in parallel/series to increase the voltage to 24 volts and thereby drop the current? But this is not compatible with the inverter voltage spec (12V).

    Thanks.
    Karl
  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Re: Help with cabin system

    For "code", they will require dedicated ground rod(s) for safety/lightning protection. Steel rusts, and concrete may not make for a good "ground path".

    Regarding cable resistance, you can use the Excel spread sheet calculator to see how much power you will loose (am for 3% max--typically most aim at 1.5% max--and remember for the calculator, runs are x2 -- round trip length).

    Your installation would do nicely with a MPPT type solar charge controller... The Morning Star is too small (15 amps max), so you are kind of stuck with the 60+ amp units. You can put all three panels in series, cut the the current down to 6 amps, and have the MPPT Charge Controller with Battery Temp Sensor in the cabin--where it really belongs (keep battery side wiring short).

    Outback and Xantrex are a couple controllers to look at. Either one will allow you to take any voltage (2 volts above max battery bank voltage) and efficiently down convert to your bank. You can start with 12 volts--and later on go to 24/48v with the same controller (just reprogram).

    -Bill

    PS: If your site is pretty much on bedrock/dry soil--ground will be a big problem anyway. You can search the web for various articles from HAM radio sites on how they have solved their various grounding issues (like this one).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Help with cabin system

    Worst case follows. You may be luckier.

    That pole will be a natural lighting rod. Be prepared to face the consequences of that.

    You will need good ground, at least 8' deep, near the pole mount.

    If panels can be protected from a strike 6" away on the pole, I don't know, the induced surge may kill them, even with a well grounded pole. The impedance in the wire run to the charge controller may save the controller, but I would advise to get some sort of arc gap lighting arrester for the DC input, mounted at the house wall. It gets a ground rod too.

    ----

    Now you also have to watch for the panels accumulating snow buildup on them and the rack. A steep enough angle to shed snow, may not be a good solar charging angle. Life's full of tradeoffs
    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 ,

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Help with cabin system

    Bill beat me to it,

    Clearly a few $$ invested in a MPPT controller would pay off big time in your situation. Wire the panels up in series, (36vdc nom), ~10 amps would be a piece of cake for the controller. Do a calc, but 45' at 36 vdc should be that much drop to worry about)

    One other question, from snow country. I have found that even sub zero my panels will melt off quite quickly after a snow. I assume your problem is the quantity that builds up over the winter, not on your panels per sey, but enough on the ground to cover the panels. If the panels just get covered with snow and then melt off, perhaps you are just as well to let sleeping dogs lie if you only use the cabin occasionally in the winter.

    Tony
  • System2
    System2 Posts: 6,290 admin
    Re: Help with cabin system

    Thanks for the input so far.

    We also have found that snow melts/slides off the modules very quickly. However, with over 360" of snowfall per year the snow and ice accumulates over the panels and takes some work to remove (when I'd rather be skiing!). I'll look further into MPPT controllers, but the initial results are a little disappointing since as Bill pointed out the Morning Star is a bit small and the others are way beyond our needs.

    Grounding is another issue that I will further investigate. If others have had experience with these types of installations please share your thoughts.

    Thanks,
    Karl
  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Re: Help with cabin system

    Do you need the 2,500 watt inverter? Seems a bit large for 330 watts of panels (maybe at 10-15 minutes at full 2,500 watt load)...

    If you wanted a smaller pure sine inverter (or even a cheap, smaller, mod-sine) at 24 VDC--then your panels would be OK with the Morning Star 15 amp MPPT charging a 24 VDC bank.

    Probably won't save you any money though (spending on new MPPT controller and inverter--plus if you are currently using 12 VDC in your cabin--you would loose that ability with a 24 VDC bank)... Might get a bit more power with less losses from a smaller inverter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Help with cabin system

    Yeah, the inverter is probably overkill for this system. We had planned on adding panels as our AC loads increased, but through better load management it turned out that we didn't need the extra capacity. We are still considering using AC for the refrigerator which is switchable propane/electric (now using propane), so we'll see.

    Also, our water booster pump is 12V.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Help with cabin system

    FYI,

    Using a 2 way or 3 way fridge on electricity is very ineffceint. If you are considering using 120vac for a fridge, consider buying a hi-ef 120vac only unit, and trading out the Propane. I have used LP fridges for years, and recently built a new building complete with a new LP fridge. If I had done my homework I would have discovered that it would have been cheaper to buy a good energy star fridge, plus a couple more panels, and used the solar.

    Tony
  • System2
    System2 Posts: 6,290 admin
    Re: Help with cabin system

    Yeah, from reading these forums (and others) I'm definitely getting the impression that use of our existing fridge on 120AC just isn't as viable as a new efficient model. When I get a chance I'll have to switch it over to 120AC and measure the actual consumption with a Kill-A-Watt or something similar.

    I'm still struggling a bit with the grounding solution; though I think I'm approaching a solution that will work for our system. I now realize that there is *some* disagreement and confusion regarding the proper way to ground PV systems. A lot of information to wade through...
  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Re: Help with cabin system

    Grounding is a "big ol' can o' worms"...

    For the most part, you are trying to prevent near by strikes from getting into your system... And two major areas of entry--wiring acting like antennas. The higher they are off the ground, the more "loopy" they are, and parallel runs that take separate routes (instead of being bundled tied--and twisted can help), bundling of Solar DC + AC inverter/DC house wiring together, multiple ground points driven into the ground, and mixing grounding theories together all add up to a very lightning susceptible system.

    Interestingly, a while ago the Wind-Sun administrator had said that the most lightning damage they see is on the AC inverter output--so properly running your AC wiring and adding lightning suppression there seems like a good place to start.

    I agree with Solar Guppy that grounding one leg of anything (such as solar panel DC, and even neutral bonding) is generally a bad idea for equipment survivability from lightning... Generally, the lightning energy induced into a system is "common mode"--i.e., all of the wires go up and down in voltage at the same time... If you put a voltmeter (or appliance) across the two wires, you will never measure any current or voltage...

    If you ground one lead (per NEC requirements), then one lead is now at ground potential and the other is at "lightning" potential... And this energy will find any path to ground/grounded lead it can--typically through your very expensive equipment.

    I like the idea of lighting suppressors--You mount two on every circuit (+/- leads, Hot/Neutral leads, etc.) to earth/safety ground. If the common mode voltage gets high enough--the energy flows from both leads to safety ground--preventing the creation of "differential voltages" between the hot and return leads.

    Because this violates NEC--and requires other actions to be taken to reduce overall safety hazards--doing this on your own without help/a bunch of studying can make things worse/unsafe.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Help with cabin system
    KarlP wrote: »
    Yeah, from reading these forums (and others) I'm definitely getting the impression that use of our existing fridge on 120AC just isn't as viable as a new efficient model. When I get a chance I'll have to switch it over to 120AC and measure the actual consumption with a Kill-A-Watt or something similar.

    I'm still struggling a bit with the grounding solution; though I think I'm approaching a solution that will work for our system. I now realize that there is *some* disagreement and confusion regarding the proper way to ground PV systems. A lot of information to wade through...



    Karl, Just saw this thread. Curious if you can update us on how you've solved your mount and grounding issues?

    Tim
  • System2
    System2 Posts: 6,290 admin
    Re: Help with cabin system
    KarlP wrote: »
    .....Main system components:

    3 Shell (Siemans) BP SM110-12P panels (110W, 12V, 6.3A)
    ProStar 30 Amp 12/24V charge controller

    The 3 PV modules are wired in parallel to provide max 330W, 12V (max 27.5 amps?)......
    I appreciate your input!
    Karl

    Karl, I have the same CC and actually if my math is correct then you should be getting max of 18.9 amps if wired as you say (3 x 6.3 = 18.9..(27.5???)... So, your 30amp ProStar should be plenty big.