My recent post about clearing dust off panels elicited a very insightful comment from a reader. Kevin@unkilo wondered “if the dip caused by you cleaning the panels takes away the benefit gained by cleaning the panels?” I looked into the data and he was right to question it. My shadow caused a reduction in production almost exactly equal to the boost I received after the panels were dust-free. So when I went up there today to give them a hose-down (my new Saturday morning ritual) I was careful not to shade the panels with my body. Here is the resulting power profile:
The green line is solar power readings every two minutes. The red line at the bottom of the chart is our consumption (which I’ll save for another discussion.) But do you see that funny spike at 11am? Obviously, that’s when I was out washing them off. But why does the power spike and then drop back down? What’s going on there?
Well, I’m glad you asked because it’s an important effect to understand. The solar photovoltaic process has a significant temperature component. As I have described before, The typical solar cell is a silicon semiconductor, just like the CPU chips in our computers, and they operate more efficiently when they are cool. Here’s a very visual and obvious demonstration of that effect. the spike you see in the chart above happened when I sprayed the panels down with nice cold tap water from the garden hose. The panels had been in full sun all morning long and were quite hot at that point. The ambient temperature was close to 70F and I could actually see steam rise off the panels when the cold water hit them. Washing them actually had a multiplying effect, at least temporarily, because they were exposed to more light (dust and pollen had been removed) AND they were operating at a higher voltage thanks to the cold shower. Of course the temperature effect only lasted 10 or 15 minutes until the panels warmed back up again from the intense mid-day sun.
Here’s the plot of both power and voltage for one of the modules:
Power & Voltage (Module #1.1.11 )
You can see, there is a voltage spike that corresponds to power spike just after the 11am cleaning. The highest voltage readings are first thing in the morning as the sun is just coming up and the panels are nice and cool. Then the voltage drops off all day as ambient and cell temperatures climb. Then at the end of the day, the sun went behind the trees and power dropped off but the voltage spiked again.
The moral of this story is, temperature matters. Ground mount solar arrays will consistently out-perform roof mount arrays because of this effect. There are even some innovators out there working on hybrid PV & solar thermal collectors to keep the PV cool AND provide pre-heated water at the same time. Seeing this effect today definitely made me think again about that possibility.
Until next time, Keep Cool and Power On!
Energy Freak Show 