Two PV Effects: Temperature Dependence and Cloud Enhancement
Rinsing Semi-Dirty Panels
Cloud Enhancement Effect
The top panel shows a 24-hour time series of PV production in Watts on a totally cloudless day (blue). The bottom panel shows several temperature measurements for the same time period, one of which is the temperature of the solar panels (green). The blip in PV production at 10:30am is when I hosed down the panels in order to see if production increased when the panels were cleaner. At first I was very happy to see an immediate increase in production of about 11% (the blip), but the increase wasn't sustained, and it leveled out at a less exciting 2-3% increase in production from having cleaner panels and lower "soiling losses" (that's what they call it). The second figure shows that hosing down the panels also cooled them from 150°F to 100°F, which is an important reminder that the output of solar panels increases with decreasing temperature, so the cooler panels are more efficient at converting sunlight to electricity and the production goes up, but only as long as the panels remain cool. My secret entrepreneurial idea is to join solar PV and solar thermal by having circulating water in contact with the back of the PV panels in order to cool them down and increase production, while also capturing that heat like regular solar thermal panels.
These are two examples of an interesting cloud effect that can cause solar panels to briefly produce more power than they do when the sky is clear. The smooth underlying shape of the blue curve is the production when there are no clouds near the sun. On a clear day the panels receive and respond to both "direct" and "diffuse" light, where the direct component is from the sun, and the diffuse component is the much lower intensity of light coming from the blue sky in all directions. There is also a third source of light which is "reflected", and it might come from snow cover, a building, or clouds. The drops in production in the figures are when a cloud moves in front of the sun and blocks part of the direct light. However, if the cloud is near but off to the side of the sun, the panels see the full direct component plus the combination of diffuse light from blue areas of the sky and reflected light from the clouds. The reflected component is brighter than the blue sky it blocks, so the total non-direct light reaching the panels is greater than if it was all blue sky. Solar panels can briefly become super-charged when a bright cloud is near the sun to reflect light onto the panels. However, when the cloud arrives in front of the sun it has a much greater effect on reducing production, so overall clouds are bad for production, but a little less bad. I should mention that it's really more complicated than just "white is brighter than blue", because not all of the photons in white light have enough energy to produce electricity when they're absorbed by the solar panel (low-energy red photons make heat instead), whereas all of the high-energy blue photons can produce electricity.