Optimize solar PV generation


So you figured out the very basics of Solar PV and you've seen a Solar PV schematic/diagram and you want to move onto the next step and do a little more research to see how well Solar PV will work on your home.  This was the exact topic that Scientific American blogger (and soon-to-be consumer of Solar PV electricity from panels on his roof) George Musser covered on his latest blog, Invert your thinking: Squeezing more power out of your solar panels.

George's overview of the very important details of optimizing solar PV generation is one of the simplest, yet most informative descriptions that I have seen on the web.  With George's permission, I have copied some of his blog which can be seen below in dark blue, but I insist that you read his entire blog for the full effect.

"The need for this equipment arises from how a solar photovoltaic cell works. Light shining on the cell knocks electrons off the silicon atoms, and an electrical voltage built into the semiconductor material pulls the electrons in one direction, creating an electrical current. What happens then depends on what you connect to the cell.

If you don’t connect anything and just leave the wires dangling, the current has nowhere to go, electrons pile up on one side of the cell, and the voltage across the cell increases until it reaches the built-in voltage -- typically 0.6 volts for silicon. The BP SX3400b panels that are going up on my house each consist of 50 cells connected in electrical series, for about 30 volts if you don’t connect an electrical load. Twelve of these panels are strung together for a total of about 360 volts.

When you attach a load and start to draw power from the cell, the voltage drops -- gradually at first, then precipitously as the electrons flow out too quickly for a voltage to develop across the cell. This behavior is captured in a graph known as the current-voltage, or I-V, curve. When the voltage reaches zero, the cell delivers its maximum current -- which is about 9 amps for my BP panel in full-on sunlight and less when it’s twilight or overcast. Because the cells in a panel and the panels in a string are wired in series, the amperage of one determines the amperage of all. If you need more current, you have to wire strings of panels in parallel. My solar array consists of two 12-panel strings, doubling the current.

Because power equals volts times amps, a panel doesn’t do a whole lot of good if it generates 30 volts at 0 amps or 9 amps at 0 volts. In between these extremes, it produces useful power, and there’s a sweet spot in the middle where the power is maximized -- for my panels, 8.16 amps at 24.5 volts, giving 200 watts of power. If you hit this sweet spot and point this panel straight at the sun, it will convert 16 percent of the incoming solar energy to electricity. When most people talk about efficiency, this is the number they’re referring to, but it presumes you've hit the sweet spot, and that's easier said than done." (continue reading)

George continues the blog to highlight how important the inverter is (for grid-connected systems) for optimization and lists some of the issues that result in poor PV performance (say that three times fast).  These issues include: electrical mismatches, partial shading, temperature fluctuations, damage or theft.

So there you have it.   Converting the sun's photons to electricity is fairly simple; optimizing that electric generation is the important part!

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If you are looking to find the exact sweet spot for any panel or collector, you should look into using the Solar Pathfinder instrument with the Assistant software. It has a feature called the "optimal orientation" tool which will tell you the exact azimuth and altitude which will give you the maximum output for the selected panel. See Side note: Also the best deal for the price! Its competitor is 5x the price and not as accurate.
That's a sweet looking home. That's some really good info. I didn't know that voltage would build up inside a solar cell if nothing was attached. Makes sense.

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