Advice on new installation required

andy t
andy t Registered Users Posts: 12 ✭✭
in Solar Beginners Corner #1
Hi , I am looking for advice on my planned installation. We are completely off grid in the south west of England, and wish to remain this way. I have purchased 12 x 170w panels but none of the other components yet.


The panels I have are as follows
Sun earth Ningbo solar
TDB125X125-72-P
PMAX 170WP IMP 4.86A
VMP 35V ISC 5.36A
VOC 45V MAX SYSTEM VOLTAGE 750V
SPECIFICATIONS at
AM1.5 1000W/M2 25DEG C

I am planning on a 24volt system using an outback fm80 charge controller, with the panels in 6 strings of2. I had thought of using 4 strings of 3 but was worried that during a cold winter the voltage could rise above the max voltage for the fm80. Thoughts please?

I intend on using either a midnite solar mnpv6 or an outback pv8 combiner box. Thoughts on either of these combiner boxes appreciated. Would 5amp dc circuit breakers be appropriate for each string? I assume that I will need a larger circuit breaker between the combiner box and the charge controller. What would people recommend for this and best way of mounting? Also what recommendations for circuit breaker between charge controller and the battery bank?

All cable runs will be kept to a minimum, with the panels being mounted on a shed roof and the controller and batteries within the shed.

I have the 12 panels but have not yet purchased any of the other items, so I’m open to all advice on the system and the choice of components. Outback fm80 or midnite classic 150?

I have lived without electricity for over 10 years now so calculating my energy consumption is difficult. I would like to run a fridge freezer, energy efficient lighting, music and charging of small devices, eg phone and laptop. I do not have a tv or lots of electronic gadgets.

I apologise for any mistakes in the above post, I have written it on my phone with a young baby in my other hand, which was very difficult. Just as I posted it the connection crashed and I had to write it all again. It has taken a very long time just to post this.

Many thanks in advance for any help

Andy
Off grid,
12 x 170w in 6 strings, outback fm80, 600ah 24v in 2v cells, 
Outback gfx1424e. Victron 350w inverter.
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Advice on new installation required

    Welcome to the forum Andy.

    You've got "true 24 Volt" panels there! They are sometimes hard to find. I would not go more than two in series, not for the reason you mention (although Voc could be quite high - perhaps 117) but because the charge controller becomes less efficient as array Vmp goes higher in relation to system Voltage. I've tried three "true 24" panels in series here and over Winter it did touch shutdown for the controller. You probably don't get as cold as Canada in southern England.

    Either the MidNite or the Outback box should work fine. You want a higher breaker rating though; the panels usually have a maximum series fuse rating and it is usually 15 Amps. This is what you are protecting with panel breakers: their use as a conductor should they short out and receive the full current of the other panels. At 5 Amps you may get false trips on any day that pushes the current high (lots of sun especially with reflection off snow or water). You'd want a 10 Amp breaker per string if no rating is given.

    Since you have no electric consumption figure to go by you're sort of putting the cart before the horse buying anything. If we work from the array size we can determine what it is likely to support based on our habitual rules-of-thumb:

    Twelve 170 Watt panels = 2040 Watts.
    On MPPT controller for 24 Volt system: 65 Amps peak output current.
    650 Amp hour 24 Volt battery bank at 50% DOD: 7.8 kW hours stored power.
    Typical system AC power available @ 25% DOD: 3kW hours AC.

    That may seem like a lot, but you'll be amazed how fast your use goes up once you have the power. It will certainly run a refrigerator.

    Caveats: your actual production will vary with local insolation conditions. You may get bouts of bad weather necessitating generator use for days on end. You may get lots of sunshine enabling you to use opportunity loads during the day and increase your system's over-all efficiency. You will have to adjust component selection (i.e. battery bank size) for what is available where you are.

    The MidNite Classic is certainly considered the top in charge controllers these days, but that doesn't mean it's the one everyone should buy. The Outback FM80 works fine and costs less because it doesn't have all the features, some of which you may never need. Another company you'll want to look at the offerings from is Victron; they are well-regarded outside of North America (very small presence here so not much feedback).

    You'll also need some idea of what your maximum loads all at once might be so that you can size the inverter. As it is your system is working out to be slightly larger than my own, and you'd be surprised what all I manage to run from mine.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #3
    Re: Advice on new installation required

    I think those are the same panels I had on my cabin system, I think the Ningbo panels I had had the best framing I have ever seen on a solar panel, very robust!

    Here is the info on the panel, 'Coot hit it, it takes a 10 amp breaker.

    Always worry about the amount of sun in a traditionally overcast country, might consider a smaller battery bank (or over paneling) if you have poor solar isolation (lots of overcast).

    I spent most of a year camping with a small 10watt panel and a 5amp battery bank, you'll find the transition easier than most.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Advice on new installation required
    Photowhit wrote: »
    Always worry about the amount of sun in a traditionally overcast country, might consider a smaller battery bank (or over paneling) if you have poor solar isolation (lots of overcast).

    Completely agree. If you consider the 65 Amp potential to be the high end of charge rate (13%-15% - above that acceptance becomes an issue) you'd have maybe a 450 Amp hour battery bank (two parallel strings of T105's for example). You lose some stored power (5.4 kW hours vs 7.8 kW hours) but gain charging ability on less-than-perfect days.
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  • andy t
    andy t Registered Users Posts: 12 ✭✭
    #5
    Re: Advice on new installation required

    Thanks guys for your quick response much appreciated. Also good to hear from someone who is using the same panels, they do feel tough and thanks for the data sheet as I did not have this before.
    Im just a little confused by the potential 65amp, I thought the fm80 could potentially produce 80amp although I doubt this would be achieved through various looses and the lovely English weather. I was planning on around 600amp bank of 2volt cells but see your reasoning for the 450amp.

    What size circuit breaker would you put between the combiner box and fm80 and also between the fm80 and battery bank?



    Still figuring out what size inverter to use, i have to do some more calculations as to how much I might draw. I need to replace the roof before the installation so plan to be up and running in a couple of months, so have tome to decide on the inverter
    Off grid,
    12 x 170w in 6 strings, outback fm80, 600ah 24v in 2v cells, 
    Outback gfx1424e. Victron 350w inverter.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Advice on new installation required

    An FM80 can handle 80 Amps output, but there has to be enough panel on the input to do it. For a 24 Volt system it looks like this: 80 * 24 / 0.77 (typical efficiency) = 2493 Watt array. Outback gives different maximum array size because they use different derating.

    You don't actually need a breaker between the combiner and charge controller: PV's are self-limiting in current output so as long as the wiring is sized right there can't be any over-current problem. When breakers are used in that circuit they serve only as a disconnect. They must be rated for the maximum current (Isc of panels * number of parallel strings).

    Output of the controller should definitely have a fuse or breaker on it. Best to size the wiring and over-current protection for the maximum the controller can handle, that way if you add more panel later (providing you can; it can be difficult to find matching panels later) you're already wired for it.
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