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nweeks

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Joined: 22/01/2007
Location: Australia
Posts: 36

Posted: 10:34pm 24 Mar 2007

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I had an idea last night in bed for a simple MPPT for wind turbines. The details are still fuzzy, but here goes:

Consider:
A turbine that can produce 40V before furling
A bank of 12V batteries, heavily discharged
A reasonably long cable run between the two

Analogy: A car stuck in third gear.

What happens if all your MPPT did was act like the clutch between the flywheel and the gearbox? By slipping the clutch, you can keep the engine running at a speed where more torque and power is available, and therefore provide more power to the wheels than just letting out the clutch.

...And, you can do it all without any current measurement!
This then feeds your standard battery charge controllers.

Circuitry:
Bank of large capacitors directly connected to turbine output
PixAXE reading capacitor voltage via voltage divider
PixAXE PWM output connect to high-current MosFETs, choppping power from capacitor bank to downstream loads(charge controllers, pumps, etc)
The capacitor acts as the flywheel on the engine. The mosfets act as the clutch/automatic gearbox.

By keeping the voltage of the capacitors at ~80% of available voltage from the current wind velocity, the output power from the caps will be greater than power if the turbine was directly connected, as the directly connected turbine's speed will be pulled down by the load, thereby reducing vMax available.

Pseudo code:
:TakeSteadySample
// Turn PWM Mosfet output off to remove load from turbine
// Read the turbine voltage 5 times with a second pause between each readying
// This allows the turbine to spin up to the maximum speed available in the current wind
// (Compare the vTurbine to vOverspeed each time to stop popping components like 7805 vregulators(32v max))
vMaxSpeed = vTurbine

for(a=0;a<100;a++){
// We do the following 100 times
:RampLoadFETS
    // TakeRunningSample
    if(vTurbine > vMaxSpeed*0.8){
      // Output voltage is greater than 80% of MaxSpeed Voltage
      // Increase load on turbine by increasing duty cycle of PWM
      pvmout += 10
    }

    // We leave a deadband here for perfect load matching

    if(vTurbine < vMaxSpeed*0.82){
      // Output voltage is now less than 82% of MaxSpeed Voltage
      // Decrease load on turbine by decreasing duty cycle of PWM
      pvmout -= 5
    }

    // Drive the PWM output
    pwmout(oMosfet,pvmout)
} // End of the 100 times sweetspot tracker

// Go back to the beginning to handle different wind speeds
goto TakeSteadySample

My turbine in progress:
HAWT Barrel TurbineEdited by nweeks 2007-03-26

Nigel Weeks
nweeks at karbonit dot com

davef
Guru

Joined: 14/05/2006
Location: New Zealand
Posts: 499

Posted: 11:06am 25 Mar 2007

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>This then feeds your standard battery charge controllers.

>PixAXE PWM output connect to high-current MosFETs, >choppping power from capacitor bank to downstream >loads(charge controllers, pumps, etc)

How do standard battery charge controllers behave with chopped input? I suspect the whole system needs to be designed with MPPT in mind.

I don't understand how a MPPT system could function without measuring both voltage and current. As far as I understand you want to adjust the load to extract max power, not just maintain a "high voltage".

I think the challenge is making a generic MPPT system that could handle many different generators and storage system voltages and make it for a reasonable price.

Keep working on it!

nweeks

Newbie

Joined: 22/01/2007
Location: Australia
Posts: 36

Posted: 11:06pm 25 Mar 2007

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The output of the Mosfets also has a capacitor to help smooth out ripples. The frequency of this will be several kHz, so chargers shouldn't have hassles with it.

Voltage without Current? Easy.
By simply maintaining the turbine side's capacitors at a nearly charged state(~80%), you can draw whatever current you like from it. It'll simply ramp up and down the duty cycle to keep the turbine running at it's sweet spot.

It may not be perfect, but with only a handful of components, it'll pay for itself in an afternoon.

N

Nigel Weeks
nweeks at karbonit dot com

domwild
Guru

Joined: 16/12/2005
Location: Australia
Posts: 873

Posted: 04:05am 27 Mar 2007

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Nigel,

Thanks for that. I will put this to a retired "Mr Analogue" for his consideration as I know nothing about electronics but have been teaching programming for 27 years.

Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up.

Winston Churchill

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