Help with wind and solar hybrid system

ecoconsult

Hello,

I have been charged with the task of trying to figure out how long we can run a compressor on a solar/wind system we are building. We have a 600W wind turbine and an 85W (12V) solar panel charging a bank of 4, 12V batteries (122AH). I know this doesn't sound like enough but it is what we have. How long do you think it would take these to charge the bank of batteries?

We are trying to run a 460W (115V, 4A) air compressor off of it (going though a Whistler Pro-2500W 2,500 Watt Power Inverter). I am guessing we won't be able to run it for more than a couple hours a week, if that, but I don't know how to calculate it. I know I need to add in a factor of safety for cloudy/windless days.

We are in Connecticut and are only looking to run this in the summer months.

I am not sure what else to include but hopefully someone can help at least get me started.

Thanks!!!

BB.

Welcome to the forum EC!

OK, down to specifics... Is that 4x 122 AH batteries, or a 122 AH @ 12 volt battery bank?

Next, how have you measured the air compressor's power usage... With induction motors, there can be a fairly large difference between Watts and VA.... Real quick, with AC voltage, there is a "phase shift" between the voltage and the current--One way to write the power equations is:

Power = Voltage * Current == Accurate for DC Circuits
Power = Voltage * Current * Power Factor = Voltage * Current * Cos (voltage to current phase angle)
VA rating = Voltage * Current (used for planning wire sizes, fuses/breakers, inverter & generator sizing)

Cos (angle) and PF range from 0.0 to 1.0 with 0.0 being no power being used to 1.0 being AC power is used "perfectly". Induction motors typically run around 0.6 to 0.8 Power Factor.

If you can, get a Kill-a-Watt type meter and measure the Volts, Amps, PF, and kWH per day used.

So--Using some assumptions (you can plug in your own numbers if I am wrong):

• 4x 12 volt @ 122 AH batteries in parallel = 12 volt @ 488 AH battery bank
• Compressor is VA=115v * 4 amps = 460 VA
• Compressor is Watts = 115 V * 4 amps * 0.8 PF = 368 Watts
• 85 Watt solar panel
• 600 Watt turbine

First, assume the turbine contributes zero Watts. If you have data that shows (for example) consistent afternoon winds and you have measured X.X amps from the turbine, we can certainly throw that number in.

Assuming the battery bank is going to supply most of the power (i.e., no sun, no wind, worst case). Normally, would recommend daily cycling of 25% to 50% of battery capacity... Going down often below 50% will make battery life shorter (maybe a few hundred cycles for a deep cycle battery), and going below 20% state of charge will probably "kill" the battery bank.

• 12 volts * 488 AH * 0.50 max typical discharge * 1/(460 VA * 0.8 pf) * 0.85 inverter efficiency = 3.98 hours

Note that the 122 AH per battery is at (typically) a C/20 discharge rate (20 Hours to discharge a battery to "dead". The discharge rate above (based on 50% discharge) would be ~8 hour discharge rate -- So battery capacity may be round 100 AH (look up battery spec. if you have it), which changes to:

• 12 volts * 400 AH * 0.50 max typical discharge * 1/(460 VA * 0.8 pf) * 0.85 inverter efficiency = 3.3 hours

Next, how much sun do you get. Using PV Watts for Hartford Connecticut, with fixed array mounted to 27 degrees (laid flat by ~15 degrees for summer sun) we get:

Month
Solar Radiation
(kWh/m²/day)


1
2.92


2
3.79


3
4.34


4
5.17


5
5.33


6
5.86


7
5.67


8
5.39


9
4.72


10
3.71


11
2.65


12
2.37


Year
4.33


Call it ~5.0 hours of sun minimum for summer. The energy collected would be:

• 85 watts * 0.52 typical end to end solar system efficiency * 5.0 hours of sun per day = 221 Watt*Hours per day (long term average)

And that would power the Air Compressor:

221 WH per day / (460 VA * 0.8 PF) = 0.60 hours per day

Now--Some other design issues... For a battery bank, you should have between 5% and 13% rate of charge... And if you want to cycle the battery bank daily, you should probably look at 10% or more rate of charge:

• 488 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 459 Watt array minimum
• 488 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 919 Watt array nominal
• 488 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,195 Watt array "cost effective" maximum

So--For 1-2 day a week system, the 459 Watt minimum array would be OK. For a daily use system, I would suggest a 919 Watt array or larger.

If your battery bank is really 122 AH, then we need to look at some other issues (i.e., the battery bank is really not large enough to power a 460 Watt compressor).

I personally have not seen/read about very many small wind systems that generate much in the way of useful energy... If you have measured output over time (summer use), then we can take that into account.

Hope this helps.

-Bill