Post by thread:[EE]:: Maximum Power Point Tracking

MPPT (Maximum Power point Tracking) is a well known and basically simple concept which dynamically matches the effective resistance of a load to a power source to obtain maximum power transfer at any given time. The path from "basically simple concept" to "practical implementation" takes you via the minefield of "riddled with patents" and along other entertaining paths. The system is useful with power sources whose characteristics fluctuate widely. Typically wind turbines and solar panels are prime candidates. It's basically an "electronic automatic gearbox" that uses an energy converter to match load and source impedances. I am about to implement MPPT for a number of solar powered products. One simple method is to insert a buck converter between the panel and the battery or load and alter the buck PWM ratio dynamically to maximise delivered load power at any moment. For a battery a good enough result is usually obtained by maximising current. One method is to "dither" the output signal and note the power slope. If charge current increases with increasing load then the load is increased until next iteration. Processor based and discrete implementations exist. Processor based allow of great elegance (or stupidity) but discrete may be less patent littered. Question; Has anyone any practical experience with MPPT either as an end user or as a developer? Any comments, advice, warnings? Realised gains = ? Notionally you can get 30%+ extra from a solar system in low sun periods and less at high sun rates. Efficiency of your converter is obviously a factor. Where the solar panel is not initially well matched to its load even greater gains may be possible. Russell

I helped a group of college students on their solar powered car project. The body of the car was covered with solar panels. Each had its own MPPT converter. The MPPT converter was a simple boost converter run by a PIC. The PIC would dither the PWM based on measured current leaving the converter and heading towards the battery. If the last duty cycle step gave us increased current into the battery from this converter, we'd adjust the duty cycle in that direction again, then measure the current again. If the measured current was lower than the last one, we'd adjust the duty cycle the other way. "Every now and then" the PIC would sweep the entire acceptable duty cycle range to find the maximum. This insured we did not get stuck on a relative maximum somewhere (though relative maxima should not exist, just an absolute maximum). The converter per panel worked out nicely since each panel was illuminated differently. This project was 5 to 10 years ago. Harold -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available!

> Wouldn't they interact? I assume that relates to using many MPPT controllers to drive a single battery. I'd hazard: To some extent, but as a battery tends to be a low impedance source for small delta I_chg the interactions probably don't produce a wrong result - just lots of work for the trackers. And, as individual panels may produce different source impedances at any selected moment it's probably somewhat unavoidable. Panel shadowing which is something which should be avoided at all costs, causes a dramatic change in cell output. Even partially shadow part of a single cell and a raw panel's output will plummet as the resistance of the shadowed cell rises and affects the whole panel. Up market modern panels now often have FORWARD diodes across each cell. As long as cell resistance is low the diode does not conduct as Vcell < V_diode_forward. If a cell is shadowed or faulty the forward current from the other cells is conducted via the diode. A partially shadowed panel would lose voltage and MPPT would adjust its effective source impedance to match. Russell

Damn thats a nice solution! Is the circuit and program available anywhere? On Nov 28, 2007 7:43 AM, Harold Hallikainen <spam_OUTharoldTakeThisOuThallikainen.org> wrote: {Quote hidden}> I helped a group of college students on their solar powered car project. > The body of the car was covered with solar panels. Each had its own MPPT > converter. The MPPT converter was a simple boost converter run by a PIC. > The PIC would dither the PWM based on measured current leaving the > converter and heading towards the battery. If the last duty cycle step > gave us increased current into the battery from this converter, we'd > adjust the duty cycle in that direction again, then measure the current > again. If the measured current was lower than the last one, we'd adjust > the duty cycle the other way. "Every now and then" the PIC would sweep the > entire acceptable duty cycle range to find the maximum. This insured we > did not get stuck on a relative maximum somewhere (though relative maxima > should not exist, just an absolute maximum). > > The converter per panel worked out nicely since each panel was illuminated > differently. This project was 5 to 10 years ago. > > Harold > > > -- > FCC Rules Updated Daily at http://www.hallikainen.com - Advertising > opportunities available! > -

> I assume that relates to using many MPPT controllers to > drive a single battery. Same assumption.. I'd thought of something like this before, but it seemed that the controllers could interact and basically go nuts as each interferes with it's neighbors. I guess if you limit the excursions to some reasonable value, and limit how often you change, it could be stable. Or maybe I'm overly paranoid. :) Similar to windmills, where the power adjustment can cause oscillations in the blade structure, which feed back till something breaks. More scary that way though.

On Nov 28, 2007 10:58 PM, Apptech <.....apptechKILLspam@spam@paradise.net.nz> wrote: > Question; Has anyone any practical experience with MPPT > either as an end user or as a developer? > Any comments, advice, warnings? > Realised gains = ? I believe there will be quite some gains. http://www.iet.aau.dk/~des/papers/PID253435-1_EPE-PEMC2006.pdf Last time I saw an implementation of MPPT using a non-linear control method (cited in the above paper) and it seems to be effective.

>> Question; Has anyone any practical experience with >> MPPT >> either as an end user or as a developer? >> Any comments, advice, warnings? >> Realised gains = ? > I believe there will be quite some gains. That's why the world does it ! :-) I was hoping for some comments from people who had tried it in practice, but papers like that one are a good start :-). > http://www.iet.aau.dk/~des/papers/PID253435-1_EPE-PEMC2006.pdf Interesting. Some good references too. This paper makes the interesting, and retrospectively obvious when the diagram is examined, point that a rapid change in delta insolation may cause a perturb and observe algorithm to initially wander off in the wrong direction. Doing it is easy. Handling the implications and the patents may not be. Russell