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Thursday, September 7th, 2006

solar shading, redux

I think photovoltaic array owners have a love/hate relationship with trees. On the one hand, trees are a solid part of any program to reduce carbon in the atmosphere — in other words, planting trees is something you have to do more of if you want to reduce global warming.

On the other hand, shade is the enemy of solar electricity generation. I quoted a surprising statistic when I wrote about PV shading in 2004: One completely shaded cell can reduce a solar panel’s output by as much as 75%. So, sure, I want to plant trees… just Not In My Back Yard.

Our PV generation is off by about 12% this year, as compared to 2005. I’m working with the vendor to identify possible explanations. So far, the most likely explanation is shade cover: it turns out trees get taller every year. Bastards.

4:10 PM … no shade

5:10 PM … no sun!

So, by 5pm (in late summer, anyway), 2/3 of my PV array is fully covered by the shadow of a huge Eucalyptus tree on the edge of our property. The problem with this is that the PV array’s output drops to nothing (literally, about 75 watts at the time of this photo) during 1/6 the peak-generation period, which is the time that PG&E pays us ~3x what they charge to sell us power during off-peak hours. Not only are we not earning the necessary big credits from 5pm-6pm to offset heavier winter usage, we’re also paying nearly 3x market rate for electricity during this time.

It could be worse. The tree could be in the neighbor’s yard.

Our acrobatic arborist is coming by next Tuesday to discuss the feasibility of beheading the Eucalyptus. We won’t chop it down, but we’ll take a dozen feet off the top.

I want to plant some more trees around the property, too (short ones, of course, below the roofline). Carbon offsets begin at home.

Update: the great big hole in the treeline

Tags: photovoltaic, solar, pv
posted to channel: Solar Blog
updated: 2007-05-08 13:21:41

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