Monday, May 12, 2014

Big Bertha #1 Joins Her Little Sisters

There she is, up there on the roof of my shop.  I had to mount her far enough from the edge of the roof to allow a foot path to get at the mounts (unlike her little sisters, whose mounts are only on the tops and bottoms).  In fact, you might ask why the panels are tilted slightly but at the same slant as the roof...  It is purely a concession to the high winds we get here - when they are strongest they come from the left in the picture.  By keeping the panels parallel to the roof and tilted slightly into the wind I prevent the wind from getting under them and lifting them - I hope.  The little ones have survived a winter, so that is some validation anyway.  I'd get more power from the panels if I tilted them more vertical...  until the first big windstorm.

And the crew is really producing power now!

But I fell into a trap when I last reported on the project.  I simply multiplied the short circuit current by the open circuit voltage to get a power output.  But that's wrong.  You'd never get the open circuit voltage when the output is shorted, and the current is zero when measuring the open circuit voltage.  The reality is that both the current and voltage will be lower at maximum power output.

In fact, any solar panel power collector should have MPPT circuitry (my grid-tie inverter does).  MPPT stands for Maximum Power Point Tracking - it means that the collector continuously varies the current being drawn while watching the voltage and looking for that magical balance point where the maximum power is being produced.  And it is not a fixed point, which is why you need a controller with active circuitry.  It changes with the amount of sunlight falling on the panel, with temperature, and whether any of the cells are partially or completely shaded at the moment.

So, as things stand right now, Big Bertha is producing about 110 watts in bright sunlight, and her sisters produce about 75 watts.  I checked our electric meter when everything in the house and shop was quiescent.  Indeed, it was turning backward, ever so slowly.  I can't wait to get more panels made!  I should have enough cells for another 4 or 5 Big Berthas, depending on how many I break in the fabrication process.

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