Saturday, March 21, 2009

Solar Power Update

While the most interesting thing here is still the new generator in the basement (I'll get to it, I promise!), here is an update on the solar panels. We are coming up on the 3rd year in May, 2009, and there are enough data to get a good sense of how the system is performing.

The above graph shows the solar generation results. This energy issue has many moving parts, and the figure attempts to document the main changes associated with the solar panels. The black line is the actual amount of kilowatt-hours generated for each month from May, 2006 to Feb, 2009. The yellow filled area shows the expected amount of power production as calculated by the installers, Sunlight Solar in Milford, CT. The first month's value is low because the system wasn't put in until the first week or so had passed, but otherwise, the first year's worth of data fit the expectation pretty well. The differences between the expected and the actual can be from two reasons: (1) weather, and (2) model errors such as estimating the amount of shading from trees each month.

Continuing chronologically, there is a vertical red line in May, 2007 that marks the point where we cut down a tree in the front yard on the Eastern side and relatively close to the house. Notice how the amount of power generated increased after doing so. This fits with the notion that shading is a really big issue. Also note that the increase in power seems to be dominated by the summer months (May-Sep). The tree that was taken down affected the morning light when the sun was at higher angles. The Oct-Apr time was not affected (probably) because there are other trees that still shade the roof in the morning during those months.

Next up is the replacement of the older panels and the addition of 2 more panels in September, 2008 (as told in a prior blog entry). The light blue filled in area is the expected power using the original advertised expected generation multiplied by 1.24. The 24% increase is a quick estimate that comes from the addition of 2 panels plus the increase in panel efficiency. Also of note is the new power inverter that takes the direct current from the panels and converts it to the alternating current in sync with the power from the power company. I'm guessing that the actual generated power will exceed this expectation, but by how much we will have to wait and see.

There are a couple of other things worth noting. Even with the new panels, it appears that the array under performs in the winter months. This is probably a consequence of the shading model underestimating the amount of shade in the winter months. The winter is when the sun angle is lowest, and its light spends more time in the neighbor's trees then. This segues well to the next obvious point which is that more power is generated in the summer than winter. Actual power demand also peaks in the summer, and solar fits this pattern well. Solar power is peak power!

There are important economic consequences of the peak power fact, because peak power is much more expensive than base-line power. Power use increases during the hot summer months, primarily because of air conditioning. Solar power can meet a significant part of that demand while wind almost certainly won't. Wind is erratic and peaks in the Fall and Spring when the wind is the strongest. Wind will do nothing for you on those hot still summer days. Solar, however, will.

In terms of return on capital, the solar panels have saved over $1600 so far from a cost base of $10,878 (net cost after the CT 50% rebate but not including the federal tax credit, because we used that money to cut down another tree!). On an annualized basis, the panels have returned 5.4% (that is an after-tax equivalent number), and the rate of return is growing because of the newer efficient panels, the increased exposure due to the removed tree, and the increased power cost. Last year's return was an even 6%, and this year should be better, because power prices are still a high $0.18/KWH here in Southern CT. Sure has beaten the stock market over the last 3years!



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