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blinded with (solar panel) science! - Accretions

Fata Morgana
2012-04-03 02:49
blinded with (solar panel) science!
Public
Our solar panel system has been running for about a year and a quarter now. One awesome thing about using Enphase microinverters (other than the fact that they can activate each panel independently and generate power with even very low amounts of light) is that Enphase keeps track of your production and gives you access to it through their website and via an API (which I haven't taken advantage of, what with my nonexistent copious free time and all). For example, here's our daily production to date. If you integrate under the messy sine wave, you'd get 6.32MWh total.




The messiness of the graph is mainly due to cloudy days. For a closer look at just how big this effect can be, here's the last week's production with weather annotations. March 27 was a pretty sucky production day because the clouds remained thick all day long. March 31 started out similarly -- it was raining heavily all morning -- but then the sun came out in the afternoon. And it was sunny on April 1 pretty much all day.



But even if we smoothed the cloudiness away, there's a huge difference between peak production days near the summer solstice and peak production days near the winter solstice: there's more than a factor of four difference between our best day on June 10 and the local maximum in December.

What accounts for this big difference? We could think of a bunch of possibilities. There's the solar angle (the sun's maximum height in the sky) and the amount of atmosphere that sunbeams have to travel through both vary with the season, both governed by the Earth's tilted axis relative to its plane of orbit. There are also potential nonlinearities introduced by the angle of the solar panels and by tree shadows: ten of our 18 panels are on an eastward-tilted part of our roof, the other eight are on a southward-tilted part, and all get a bit of shade from nearby trees at certain times of the day and certain times of year. (We optimized for the best angles and as little shade as possible, of course.) And then there's varying atmospheric absorption the solar radiation, the abilities of our solar panels to absorb the radiation that reaches them (which varies by the part of the spectrum and declines over time), and even variation in solar radiation levels (we just *think* the sun is a constant source of energy up there, but it totally isn't!).

One of the easiest effects to account for is the effect of seasonal solar angle, which Wikipedia tells me is summed up by the awesome-sounding word "insolation." Based on this graph from the aforementioned Wikipedia page that I annotated, the solar angle alone (without accounting for atmospheric thickness -- basically, if our house were in space) accounts for a factor of 2.6 change between summer and winter, more or less:



So the solar angle alone accounts for amost two-thirds of the seasonal variation.

Though we played around with them for a while, the other factors proved harder to approximate. Do you have any thoughts on how to do it?
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The Water Seeker
plymouth
2012-04-03 11:54 (UTC)
(no subject)
Have you played around with PVWatts at all? It has a lot of those things in it. Insolation and sun angle definitely. There's a spot for shading coefficient though it's sorta fuzzy on how you'd estimate it. Dust collection on the panels is also a factor though unless you let them get really grungy that only accounts for a few percent loss. And will all the rain they should be cleaner in the winter!

http://www.nrel.gov/rredc/pvwatts/
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Fata Morgana
chimerically
2012-04-03 23:10 (UTC)
(no subject)
Very cool! It looks like it's having trouble connecting to the mapserver right now, but from what I can see, it looks so much like the tool the solar company used when we were planning the placement of the panels that it may well be the same thing.

I've read that an even bigger factor than dust is bird poop, because that's more opaque and is less likely to wash away with rain, but when I was last up there cleaning out the gutters I also sprayed off the panels and noted that birds don't appear to be perching on or over them at all.
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namazu
namazu
2012-04-03 16:51 (UTC)
temp effects?
Is there any difference in the efficiency of the panel due only to temperature -- i.e., do whatever chemical reactions occur in the panel itself have optimum temperatures (at 30C vs. 15C, e.g.)? (I doubt this would explain much of the residual seasonal difference, given the relatively limited seasonal temperature fluctuations in the Bay Area, and it would be difficult to separate out temperature from light effects a lot of the time, but I'm curious.)
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Fata Morgana
chimerically
2012-04-03 23:13 (UTC)
Re: temp effects?
We were wondering this too, but couldn't find much data on that. We did find graphs of what parts of the *spectrum* different kinds of solar panels absorb (crystalline silicon panels are one of the most broad-spectrum panels around), but Adam found it and I neglected to get the link from him.
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Jeff
lbchewie
2012-04-15 02:13 (UTC)
(no subject)
What percentage of your household energy needs are provided by your solar panel configuration, under optimal and average conditions?

While I'm not too familiar with the average energy consumption, the overall energy production looks very encouraging.
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