I guess the part that I am stuck on is this: IF the maximum power point is tied to the max output in voltage, then whatever it is does not really matter. Our buck/step down converter should already be variable, and in this case it should take as broad of a range in voltage as possible. Unless this really is not possible, and we need to tell the input side of this converter "hey, x amount of voltage coming in, prepare . . .". On the output stage, we need only track/sample voltage of the batteries to make sure we are giving them their maximum power point.
Does this make sense ? What am I missing ?
Yes, I am looking at this from a programmers view point where it seems very simple. Maybe it is, and maybe it is not, but if I try and tackle this, I will be using a programmable device with an onboard PWM.
By the way, thanks for all the links guys(and gal?), I had not found Flux's discussion previous to his final diary entry. A good bit of the rest is over my head, but that is not your fault :) I will re-read all until it becomes clear(or my brain melts).[ Parent ]
Instantaneous mill voltage varies as a function of rpm and load current. Added to that, increasing the load current will slow the mill's rpm. In and of itself, instantaneous mill voltage tells you precious little.
In my analog circuit, I have mill voltage and current, and rpm. Mill voltage times mill current gives me power. Differentiating this I can tell whether the power increased or decreased as a function of changing the pwm. I also modulate the pwm as a function of rpm, to try and track rapidly changing wind conditions. It probably needs a non-linear (exponential) transfer function on the rpm input to more closely track the cubic nature of power vs wind velocity.
Alternatively, you could use an anemometer to give an input of present wind velocity, and alter the pwm to load the mill down to a given rpm for that wind velocity.
Clear as mud, I know. But hope it helps.
Amanda[ Parent ]
However you do it digitally the problem is the same. I think you have got hung up on the concept of a voltage regulator. This is not a buck switching voltage regulator. Your requirement for tracking maximum power doesn't want you to maintain the dc output voltage ( that would be useful with batteries fully charged in place of another charge controller).
However you do it, you need to measure the power going into the battery, alter the pwm and see if the power increases or decreases then change pwm to get a better result.
Amanda seems to be using input volts and current to get power and that should be good enough but the power you are interested in is to the battery and efficiency may vary.
For very precise results you may consider the true battery voltage but it will be near enough to assume battery volts are constant and that the output power is proportional to output current. Surely any tracking scheme must include output current or at least input power if you opt for that route.( you need input volts and current in that case)
Input volts are not much use as they change with loading. You need emf for speed and unless you try the solar trick of open circuiting you can't get it ( best not try that trick with wind). You can get rpm from frequency ( or the time interval if you want to work backwards, possibly easier with a processor). Speed is not needed for a true power point tracker but it is useful as when you are right the speed will rise linearly and the power follows its cube. When things are going crazy and you have trouble tracking fast enough you can set the current roughly from speed cubed. When conditions are stable then you can modulate the pwm and find the peak power point .
In the perfect world you can measure wind speed with an anemometer and if you track the prop speed to follow wind speed by altering the loading you will be very close and that would be a simple way. It probably works well enough on clean sites and with large machines with their high inertias, tracking average power against average wind speed may be as good as you can get.
With small machines with fast response and the severe difficulty of measuring the same wind as the prop sees it may be much more tricky and it is far from convenient having to move the anemometer with wind direction or have a bank of them. It would at best only give you a ball park figure and you would still need the pwm modulating method for a final track.
Flux[ Parent ]
However you say above that we are going to cube the output power........ where is this coming from....if the ideal alt is following a square.... "Speed is not needed for a true power point tracker but it is useful as when you are right the speed will rise linearly and the power follows its cube. When things are going crazy and you have trouble tracking fast enough you can set the current roughly from speed cubed. When conditions are stable then you can modulate the pwm and find the peak power point ."
sigh.......What did I miss this time.
lost again.....oztulesFlinders Island Australia[ Parent ]
Normally, we would size the alternator to produce 100% of available wind power at the furling point, then run the alternator at less than what it is capable of at a given rpm to match the power available from the blades.
I think Flux presented some good graphs back in "Matching the Load".
Going back to battery charging, with direct connection and a highly efficient alternator the speed will not be able to rise significantly, if you were 100% efficient the speed couldn't rise and power would just try to shoot up if the prop could manage it.
With a tracker and converter you need to start phasing the converter back right above cut in to avoid stall. The load needs to come on in such a way as to keep the prop speed rising with wind speed. If you have an ideal prop and you stay on top of its power curve then the power absorbed from the prop should rise as speed cubed. Because of losses the power into the battery will not quite follow speed cubed but should be aiming for near it. With the sort of losses I have about and the characteristics of my props which seem to want to run at lower tsr on higher loads the thing seems to work out to tracking power with speed squared but the actual figure will depend on the prop and alternator and converter loss. If you aim for a curve rather than a true seeking mppt you probably won't get perfection but so far I haven't seen a true wind mppt tracker actually working to the point where I could say it is a commercial proposition.
It seems almost there and when it comes it will be simple to use and will work with any machine without set up but for now my approximate scheme has been working for about a year. Sadly I haven't been able to collect reliable data from it for reasons I didn't expect. Without accurate data I am reluctant to say much except that it is a 6ft machine and manages over 30A into a fully charged 24v battery in winds that are claimed to be about 25mph from the local airport and that is higher than the mill site.
It seems to start producing at about 6 mph.
