Sun and wind

I would like to use a 1kw wind generator and 1 kw of solar panels. I also have a gas driven generator of 3 kw as backup. The system is 12V and I have 2 x 3 kw 12 v inverters and 3 x 900 ah batteries. I have no electricity from anywhere else and far from technical assistance. How would I marry this lot the best. I have more than enough wind and sun as it is close to the coast and open and high? The system would be used for between 3 and 4 hrs per day using about 2 to 3 kwh.


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sun and wind

    You would like to use it for what? It's essential to figure out what your end usage is before buying equipment.

    A basic off-grid solar electric system consists of the PV array, a charge controller (mppt type is best), battery bank, and inverter (if you're going for AC). A separate charging source such wind generator requires a separate charge controller. If you research the wind power section on this board you'll see that it probably isn't worth the investment; small scale wind rarely makes sense in terms of investment/return.

    Some other considerations: batteries require charge current of 5-10% of their amp hr rating. If you're charging 900 amp/hr battery, you need 45-90 amps of current. That's quite a lot, but 1000 watts of panels should manage. If your 3 batteries are in parallel that's 2700 amp/hrs and you're 'out of luck'.

    So, what are you going for in terms of usage and how have you got it connected now (if so)? Do you have any specifics as to brands/specs on your equipment?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
    Re: Sun and wind

    Hmmm... I am not quite sure how to help--I will assume that you are pretty comfortable around electricity and batteries because it sounds like already have something connected (batteries, inverters, and genset)?

    Basically, get your inverters+batteries+genset working the way you need.

    The batteries should be connected following one of the patterns in this link (thank you Mike90045). This will help ensure that all of the batteries are charged/discharged evenly (batteries are very sensitive if the resistance is not equal to all batteries when connected in parallel).

    Connecting the 12 volt input AC inverters... That is a lot of power at 12 volts--and I would suggest you get higher input voltage inverters (ideally 48 volt)--but you have what you have.

    Each inverter will require approximately:

    3,000 watts * 1/10.5 volts min batt * 1/0.85 worst case inv efficiency = 336 amps maximum

    So, you would require heavy enough copper wire from the battery common point to each inverter to handle over 340 amps of current (and, ideally a fuse--but you will have a difficult time finding one) and a switch to only turn on the inverter when you need it (hopefully, the inverter has a switch and fuse/breaker).

    For a 900 AH rated battery pack, I would suggest you limit your maximum continuous inverter load to C/5--any more than that and you will probably overheat the battery and damage it (not a hard rule--just my suggestion):

    900 amps / 5 = 180 amps

    180 amps * 12 volt battery voltage * 0.85 inverter eff = 1,836 watts max continuous load.

    In any case, you also do not want to discharge the battery by more than ~50% maximum for long life (900/2=450 AH at 12 volts usable power).

    Now, for choosing connecting your charging sources. In general, and good quality charge controller can be bolted to your battery bus/common point--and you can add any number of them as long as you follow some simple rules.

    The basic rule of thumb for flooded cell batteries is 5-13% of the battery banks 20 hour capacity (900 AH in your case) should be the charger's current. Below 5%, you will have trouble equalizing the battery--and take a long time to recharge (batteries below 75% state of charge, will have their sulfate begin to harden and quickly reduce battery life by years). Above 13% you may over heat the battery, and waste a bunch of energy (as heat).

    So, for your 900 AH battery bank--the basic charger should be (again, just recommendations--over or under can work--depending on your needs and money available):

    900AH * 0.05 = 45 amps minimum
    900AH * 0.13 = 117 amps maximum

    1,000 watts of solar panels will have a peak output current of:

    1,000 watts / ~17 volts (Vmp of panel) = 59 amps

    59 amps / 900 AH = 6.5% charging current (5%<6.5%<13%)

    You could add, up to, another 1,000 watts of solar panel if you need more power and still stay within the rule of thumb.

    So--you are close to using a 60 amp solar charge controller (several available) or getting one Outback FM-80 (80 amp) controller... Or splitting your solar panels with two smaller controllers (example 2x 45 amp controllers, etc.).

    Assuming you have a lot of sun (South Africa?) available--Say 6 hours a day (you have more sun, but we use "hours of sun" at Noon equivalent--adding all of the energy from 12 hours of sun into hours of full noon-time sun). Use a derating factor of 0.52 (basically, all of the losses from solar panels, wiring, charge controller losses, inverter losses, flooded cell battery losses, etc.):

    1,000 watts * 0.52 * 6 hours of sun = 3,120 Watt*Hours per day (3.1 kWh per day).

    This will just about meet your 2-3 kWhours per day requirement).

    Your average inverter loads would be (assuming 4 hours per day, 3 kWH of solar power available--just confirming you mean 3 kWatt*Hours per day of load--and not running 3kW of load for 4 hours per day which would be 3kW*4h=12kWhperday):

    3,000 WH * 1/4 hours = 750 Watt load (for 4 hours per day)

    The wind generator needs its own charge controller (many are "dump" type controllers--the wind turbine is always connected to the batteries and a dump controller just connects, typically, a heating element to the battery bank to "dump" excess energy to prevent the battery from overcharging). Because Wind Turbines typically need a load connected 100% of the time to prevent them from over-speeding in heavy wind--and because this is a "safety issue" -- in the US a second dump controller+heater is needed incase the first one fails.

    Wind tends to be pretty unreliable because of site conditions and less than ideal mechanical reliability of the wind turbine hardware. At least South Africa seems to make some decent wind turbines--you should have good product and support from them.

    But--the good news is you have just enough solar panels to meet your average power needs... So a little wind (with help from a fuel powered generator) during bad weather and/or with extra loads--you should be fine.

    Your battery bank is really a bit small for your expected loads... Normally we recommend a battery bank that is 6x your daily load (3 days of no sun, and 50% maximum discharge for best battery life vs battery cost). Your system's recommended maximum battery use:

    900 AH * 12 volts * 0.50 max discharge = 5,400 watt*hours (5.4 kWh) maximum usable energy (normal discharge)

    And your usage is 2-3kWhrs per day... This is only giving you 1 day worth of energy storage. So, you will need to watch your battery capacity so that you do not discharge it dead. Normally, a hydrometer (and thermometer) is the most accurate method for monitoring a battery bank's charge--but it is messy and a pain to check every day.

    I would suggest you install a battery monitor so you can watch the actual battery bank capacity in xx% or AH readings--will pay for itself the first time you don't kill your battery bank with over discharging. Trimetric is a good meter for the price. Xantrex is probably the high end (and includes programmable alarm contacts if you want a visual/audio warning). Of course, there are other brands and models that are probably a better deal/more available in your part of the world.

    What else is it your are looking for from us?

    Outback has a nice library of wiring plans. You may not need their equipment--but they still are useful for you to review a few and see exactly what is required for your type of system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Sun and wind

    Thanks everyone for the replies.

    The use is mostly houshold like TV, radio, Microwave oven etc maybe a kettle for a few minutes to boil and a hair dryer for 10 - 15 mins and a few lights. A PC for an hr or so. We use low energy lamps so those would be 150 watt - 200 watt when its dark for a few hrs. No ovens. It is on a farm where there are no grid electricity and we have no option but to go for such a system.

    Yes I have 3 x 900 Ah batteries @ 12V in parallel. Is this OK? They are made up from 6v units in series.

    I am a technical person (but this is a new field to me) and know that with this kind of thing there are more to it than just theory.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Sun and wind

    the 1kw of pv will not be enough to put a good charge into a battery bank of that size by itself. the same applies to the generator at 1kw. for a battery bank at 2700ah the minimum charge should be at least a 5% rate and that equates to 135a. 1000w of pv will be in the neighborhood of about 57a, give or take for an approximate charge rate of 2.1%. if you get something similar from the wind generator then you can see that both on at the same time is not going to equal that 5% minimum that you need.
    just throwing in my 2 cents, i say for you to get what you can in pv and if you can afford all of your needs in pvs, that would be great as wind isn't as reliable. odds are you won't get all of your needs in pv and you'll have to supplement with a generator, but one that can supply that minimum charge % or more. (figure 2.5kw-5kw) this would need to run for long times too so it may be advisable for you to buy a quality generator that operates at the lower 1800 rpm range.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sun and wind

    Some comment on your proposed loads:
    Microwave, electric kettle, and hair dryer = UH-OH!
    These things use over 1000 watts each, typically. That's 10 times a standard bulb or nearly 100 times a CFL. Even though you only use them for a few minutes (time hair-dryer use; I've had girls who could run the electric bill up significantly "just drying their hair") it adds up to the need for large storage capacity, large recharge capacity, and high-wattage inverter. (Some inverters "don't like" resistance loads like kettles and hair dryers and will interpret the sudden demand as a short, then shut down.)

    You should consider lightening all the loads as much as possible to conserve available power. As for heavy loads, a generator can be your friend. Especially if you can use it to re-charge while making toast. Or drying your hair.

    Is this off-grid or on?