> There are "maximum power point trackers" that adjust the load on the 
> array
> to get the maximum output power. I worked on one of these for a 
> solar
> powered car project. This car had a bunch of solar panels, each 
> getting
> different illumination. We did a pic controlled boost converter on 
> each
> panel. The PIC watched the output current (the current into the 
> battery
> from this particular converter) and adjusted the switch duty cycle 
> to
> maximize that current.

Don't confuse panel output and converter output when (if :-) )reading 
the following.

It's probably not intuitive that what Harold is saying is correct- but 
it essentially is. For a given insolation (= sunlight ) level a panel 
has an optimum voltage x current = power point and the overall panel 
plus converter has a point where power output will be a maximum. These 
may be slightly different points due to converter efficiency shifting 
with input voltage. If you are at maximum energy point and increase 
the load you may well increase the current BUT the voltage will drop 
proportionately more and energy output will fall.

The panel should be operated at the point where maximum energy is 
delivered to the load. For practical purposes at any given time 
maximum energy is probably at maximum converter current output into 
the battery. [[ie battery chemistry may play small games occasionally 
but probably well enough ignored]].

With a single converter and single panel, if you want true optimum 
power point you have to either

- use a separate insolation (sunlight) level sensor and model the 
panel performance or

- load the panel to a given current and measure voltage or vice versa 
and hereby determine insolation and then switch to optimum load or

- swing the converter load backwards and forwards across the area 
where energy maximum is (believed to be :-)) and select the best 
point. You could lock on a given point for a set period and then 
occasionally hunt either side of it to see if the optimum has moved. 
In full sunlight the changes will be gradual. Clouds or any other 
transient obstruction will move the optimum point.

The last of these 3 is easy to do under program control and makes no 
assumptions about panel performance. It has the disadvantage of being 
SLIGHTLY sub optimum in output as you have to sometimes have the panel 
off peak to determine where the peak is. Almost easier to write the 
program than to describe it :-)



-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist