Help! - in a pickle designing my off grid solar set-up

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greenbean
greenbean Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
Hello everyone,

I wish i had found this site... or maybe i did and should have posted weeks ago!!

so, I'm in the middle of setting up an off-grid 12 volt system. Our house is unusually wired for 12volt DC, never having been on the grid. We are upgrading the battery and want to charge from solar. New led technology means we can consider using the 12volt for all our lighting needs, plus my wife and I use laptops and wireless router and etc etc
oh goodness, i have so many questions... I have been trying to work this all out in between school work. I'm desperate to stop putting diesel in the generator!!

We figure on using about 60-80 ah per day (is this a good way of measuring how much we want from a battery?)
based on this I purchased a 6x2volt deep cell bank of batteries rated at 580ah, thinking battery life is extended by using less than 10-20%. this is now installed and running a few 12volt devices, recharging with a regular car battery charger (12amp). I think i need to employ a meatier charger.. perhaps 30amp for backup?

so, i want to set up some solar panels to charge the battery. How many panels? i saw this http://www.eiwellspring.org/offgrid/Living_on_12_volt_part2.htm which suggests that we need 1/20th power of the battery in panels... so 580/20 = 29 x 12volt =348watts.
We live up a mountain in west wales, blue skies are not that common.. so i am thinking about 2x250 watt panels (i've found a supply of these for £100 which seems great, thanks to the FIT tariffs being cut in the UK and market saturation???)

Am i way off the mark here?

i'll stop before i ask any more questions.. about cables and charge controllers and monitoring meters so we know what is happening!!

Hope somebody can help, thanks in advance,

tom and lara
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Help! - in a pickle designing my off grid solar set-up

    Welcome to the forum.

    Whoops.
    Normally the power consumed is measured in Watt hours, not Amp hours. But yours is an all 12 VDC system, yes? So that can be forgiven. :D
    Whoops again.
    You use 80 Amp hours (960 Watt hours) a day, but bought 580 Amp hour battery bank? That would store enough for three-and-a-half days. If these are true deep cycle batteries your automotive charger isn't going to do them any good because it isn't going to give them the right charging profile. Most notably a lack of Absorb stage. It also probably can't provide the 58 Amps of current they'd want as peak. Trying to charge them at the 12 Amp rate seen from this charger is going to take a long, long time and not be very efficient. Let's hope the batteries haven't started sulphating already. Got a hydrometer? Take some specific gravity readings of the cells.

    How many 'whoopses' are we up to? :p

    Charge rate. 5% (C/20) is a very minimum rate. 10% (the 58 Amps I mentioned before) peak current is going to see you better. Remember that rate is net after loads, so anything drawing while charging will detract from it. What we'd normally expect for that much battery:

    58 Amps * 12 Volts = 696 Watts, less typical 77% derating = 904 Watt array. Make it an even 1000 Watts if you like.

    What you could expect from 500 Watts on an MPPT controller: 32 Amps, or about 5.5%.

    Any chance you could trade that battery bank off for a smaller capacity? Say around (4*80) 320 Amp hours? That would give you your 80 Amp hours @ 25% DOD and would have a 10% charge rate from 500 Watts of panel.

    Your other choices would be double the panel or run the generator.

    Oh and don't forget the charge controller.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
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    Re: Help! - in a pickle designing my off grid solar set-up

    maybe you will expand on your loads later and decide to keep the large battery bank, but as coot said you need a good deal in pv to charge this large battery bank with. if the conditions are that cloudy then you may find you could want to go higher with the amount of pv watts.

    that does seem quite low for the pvs. what brand are they and what does the vmp, voc, imp look like on them?
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
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    Re: Help! - in a pickle designing my off grid solar set-up
    greenbean wrote: »
    ...We live up a mountain in west wales, blue skies are not that common.. so i am thinking about 2x250 watt panels.....

    Not going to be happy about that. I have a 3,000 watt array, on a cloudy day I get 200, maybe 300 watts out of it. If you don't have bright sunlight, you don't have power. PV will generate VOLTAGE on the light from a flashlight, but to get amps out, you need intense light. Cold days (sunlight reflecting off snow) generate more power than a hot day, panels are more efficient in the cold, but if you don't have bright sun & crisp shadows on the ground, you are not getting good power.
    The exception is bright days with slight haze, you don't produce peak power, but you can harvest higher than normal for the entire day.
    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 ,

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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #5
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    Re: Help! - in a pickle designing my off grid solar set-up

    Hi,

    You can use this estimator to see how much energy you'd produce in your location: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#
    Picking a random spot in Wales, shows that 1kW of PV will produce 1kWh of energy in December. So you'd need a bit more than that if you wanted to meet all your demand from solar... but of course you don't have to meet everything with solar if you've been living with a generator. So you can easily start off with the 500W array and then expand whenever you have the funds.

    One thing to watch out for is the panel voltages, if you're getting panels at that price they're mostly likely grid-tie panels that don't have the correct voltage to charge a 12V battery. So you would need to buy an "MPPT" charge controller to use them with your 12V battery. This type of charge is more efficient in cold weather but it's also more expensive than a normal PWM controller. Basically, if the panels have a Vmp rating of 17V (or thereabouts) then you can use a PWM controller, but if it's significantly more than that, then you'd need an MPPT.
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  • greenbean
    greenbean Registered Users Posts: 5 ✭
    #6
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    Re: Help! - in a pickle designing my off grid solar set-up
    niel wrote: »
    maybe you will expand on your loads later and decide to keep the large battery bank, but as coot said you need a good deal in pv to charge this large battery bank with. if the conditions are that cloudy then you may find you could want to go higher with the amount of pv watts.

    that does seem quite low for the pvs. what brand are they and what does the vmp, voc, imp look like on them?

    these are the panels i have seen:

    http://www.sharp.co.uk/cps/rde/xchg/gb/hs.xsl/-/html/polycrystalline.htm

    MM, thanks folks for your expertise, it seems as though i need to go back to my battery man and see if i can trade for something more suitably sized. Annoyed because i had priced up some trojan105s to do the job but was talked into a meatier set of batteries. back to plan a!!
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  • greenbean
    greenbean Registered Users Posts: 5 ✭
    #7
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    Re: Help! - in a pickle designing my off grid solar set-up
    mike95490 wrote: »
    Not going to be happy about that. I have a 3,000 watt array, on a cloudy day I get 200, maybe 300 watts out of it. If you don't have bright sunlight, you don't have power. PV will generate VOLTAGE on the light from a flashlight, but to get amps out, you need intense light. Cold days (sunlight reflecting off snow) generate more power than a hot day, panels are more efficient in the cold, but if you don't have bright sun & crisp shadows on the ground, you are not getting good power.
    The exception is bright days with slight haze, you don't produce peak power, but you can harvest higher than normal for the entire day.

    yes, i'm pretty certain that when we can afford, we will feed in a wind turbine to this system as well to try and match the local weather patterns and give us a more reliable supply. for now going with solar that will work some/most of the time and top-up from the generator.
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  • greenbean
    greenbean Registered Users Posts: 5 ✭
    #8
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    Re: Help! - in a pickle designing my off grid solar set-up

    So, I've gone back to my battery guy (an old-school guy been dealing with traction batteres for years and recently supplying to "off grid" people like you (sic))

    He has offered to replace them with a smaller set, come from an electric ambulance, 6*2v cells rated at 320 ah

    I'm happy with that arrangement, its going to be a slightly costly lesson but one learned in any case.

    Do you clever chaps think that i should go for what he is offering or should i try and extract myself and go for something else all togethr?

    cheers
    t
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: Help! - in a pickle designing my off grid solar set-up

    It comes down to getting enough battery capacity to supply the loads and enough solar capacity to recharge the batteries.

    I already did the math for you.
    320 Amp hours would provide your 80 Amp hour load need @ 25% DOD. This gives you 2 days autonomy before you hit 50% DOD. It would recharge on a good day with the 500 Watts of panel.
    580 Amp hours would provide your 80 Amp hour load need for 3.5 days before you hit 50% DOD. But you would have to have double the amount of panel to recharge even on a good day.

    Wales tends to have a lot of cloudy days. Something to do with the Atlantic Ocean dropping in vertically from time to time. You would probably be better off with the smaller battery bank and the larger solar array. Otherwise you will be running the generator to make up for the lack of sun.

    Those Sharp 250 panels are Vmp 30.9 and would have to be used with an MPPT type controller on any system Voltage.
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  • greenbean
    greenbean Registered Users Posts: 5 ✭
    #10
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    Re: Help! - in a pickle designing my off grid solar set-up

    Ok, so i'm back on track i think, 6*2v (320ah rated at c5) deep cell batteries on the way.

    now i need to get all the other equipment right

    assuming i go with my 2* sharp ND-R250A5 http://www.sharp.co.uk/cps/rde/xchg/gb/hs.xsl/-/html/polycrystalline.htm
    and reading that max voltage = 30.9 v and amps 8.1 what kind of mppt charge controller do i need? lets assume i might want to add another panel when i have the money to do so.

    am i missing something looking at controllers rated 20amp or 30 amp (for3 panels)? eg:
    http://www.photonicuniverse.com/en/catalog/full/54-High-efficiency-20A-MPPT-solar-charge-controller-%252Fregulator-with-LCD-display-for-up-to-260W-solar-panel-input.html
    http://www.swithenbanks.co.uk/Solar_Photovoltaic_Equipment/1130303/Provista_30A_MPPT_Solar_PV_Charge_Controller.html

    what do you guys think of these controllers?

    many thanks in advance

    tom


    Cariboocoot wrote: »
    It comes down to getting enough battery capacity to supply the loads and enough solar capacity to recharge the batteries.

    I already did the math for you.
    320 Amp hours would provide your 80 Amp hour load need @ 25% DOD. This gives you 2 days autonomy before you hit 50% DOD. It would recharge on a good day with the 500 Watts of panel.
    580 Amp hours would provide your 80 Amp hour load need for 3.5 days before you hit 50% DOD. But you would have to have double the amount of panel to recharge even on a good day.

    Wales tends to have a lot of cloudy days. Something to do with the Atlantic Ocean dropping in vertically from time to time. You would probably be better off with the smaller battery bank and the larger solar array. Otherwise you will be running the generator to make up for the lack of sun.

    Those Sharp 250 panels are Vmp 30.9 and would have to be used with an MPPT type controller on any system Voltage.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #11
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    Re: Help! - in a pickle designing my off grid solar set-up
    greenbean wrote: »
    yes, i'm pretty certain that when we can afford, we will feed in a wind turbine to this system as well to try and match the local weather patterns and give us a more reliable supply. for now going with solar that will work some/most of the time and top-up from the generator.
    Stock advice on wind:

    1) Be sure of your wind resource before you spend a dime on a wind turbine, and don't depend on "it feels windy out here".

    2) Do your homework on hardware; there is a lot of junk out there for sale that won't produce anywhere near the manufacturers' claims even if the wind resource is there. If there is not real data available, run like the, er... wind.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
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    Re: Help! - in a pickle designing my off grid solar set-up

    Likely peak charging current from 500 Watts of panel on an MPPT controller when feeding a 12 Volt system:

    500 * 0.77 / 12 = 32 Amps

    Therefor you need a controller capable of at least that many Amps output.
    The first one you link to says it is limited to 260 Watts on a 12 Volt system. It's out.
    The second one you link to is limited to 30 Amps output. Will not handle full current potential, no room for expansion (which you almost certainly will want to do).
    I don't see you getting out of this for less than the full price. A TriStar 45 MPPT would be minimal ($400+/-) and if you want that expansion room ... you're looking at a 60 or 80 Amp controller.
    That would be like a MidNite Classic 150 Lite or Outback FM60/FM80 - Not cheap or easy to get over there; better part of £300 I should think. Look for Victron equipment: http://www.victronenergy.com/solar/solar-charge-controllers/ Might be a better choice for you there.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #13
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    Re: Help! - in a pickle designing my off grid solar set-up

    NOt sure about getting stuff from 'the continent' but forum member Nigtomdaw is a Midnite products dealer and you could send him a personal email via the forum, just search for his name using advanced search to get to 'names'
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #14
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    Re: Help! - in a pickle designing my off grid solar set-up
    greenbean wrote: »
    ... 6*2v (320ah rated at c5) deep cell batteries ....

    320Ah at C5 might be double that at C20, the way we normally rate Off grid batteries in the States. You might be back where you were a monent ago. Are you familiar with the song "All my Life's a Circle"

    http://www.youtube.com/watch?v=1yjxWfyxpqY&feature=player_detailpage
    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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