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'[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.
|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.
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Wouldn't they interact?
> 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.
Damn thats a nice solution!
Is the circuit and program available anywhere?
On Nov 28, 2007 7:43 AM, Harold Hallikainen <hallikainen.org> wrote: harold
> 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
More scary that way though.
On Nov 28, 2007 10:58 PM, Apptech <paradise.net.nz> wrote: apptech
> 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.
Last time I saw an implementation of MPPT using a non-linear
control method (cited in the above paper) and it seems to
>> Question; Has anyone any practical experience with
>> 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 :-).
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.
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