Adding Solar, Part 1: Location, Orientation, Expectations

Joan and I have always had a general interest in alternative, or "green," energy. When a neighbor lost several immense ash trees to the emerald ash borer our view to the south improved, and I began to speculate about putting photovoltaic panels on our roof.

In October 2014 we had a chance to investigate further through the Green Ohio Tour. In earlier years we had been out of town or had other conflicts preventing checking out such events. One of the houses on this tour was only a few miles from our house, with a recently installed a photovoltaic system, and at 2pm the installer, Kevin Eigel of EcoHouse Solar, would be there to answer questions. This was a prime opportunity to find out more.

The homeowners were thrilled with their system, and said the installation process had been painless. Several other people were there to pepper Kevin with questions, and our interest was piqued. A few days later we invited EcoHouse Solar to come out and evaluate the suitability of our house for solar. That visit, and others, and follow-up research on the web, revealed a lot. This blog post will focus on the elements related our house's location, orientation, and shade, and thus our prospects for power generation.

Roof Pitch
The south-facing portion of our roof is at a 45º pitch, which is steeper than ideal.

This chart, originally from the tilt page of pveducation.org, shows the insolation (solar radiation) through the year with a 30º roof. Note that the green line, the module power, closely follows the ideal ("incident") blue line during the middle of the year, but doesn't capture all the possible energy during the winter, because the panel is aiming "above" the sun's lower track.

Here are the curves with our 45º roof. The output is "capped" at the middle of the year, due to the panel aiming "below" the sun's higher track, but there's almost no loss due to the angle during the winter months.

The two curves overlaid, with the 45º having an advantage shown in orange and the 30º advantage in yellow.

Azimuth
Azimuth refers to the compass orientation of the panel. The rooftop holding the solar panels faces 188º, or 8º west of due south. This is not a significant deviation from ideal. Using the Solmetric Insolation Tool, it would appear that the overall loss from both tilt and azimuth isn't so bad, with 97.8% of maximum insolation -- a figure almost too good to believe. 

This, however, doesn't take our location-specific shade into account. There is a large locust that's going to block a significant amount of morning sunlight, and trees to the west that will block the late afternoon sun. This picture was taken when there weren't any leaves on the trees. As you can see, the south-facing portion of our roof isn't huge, either.

The online PVWatts calcuator from the National Renewable Energy Laboratory helped me estimate the impact of shade. With this calculator, after specifying our location (I used the Port Columbus airport), I supplied several specifics about the proposed installation (all three companies that made proposals suggested a twelve-panel system).

The 'loss calculator' in the above image allows you to specify percentages of loss due to such factors as dust, soiling, snow cover, wiring mismatches, and so forth.  With the default value of 3% shade, the PVWatts estimate is 4,410 kilowatt-hours; back-calculating, that's 1,205 hours of full-sun equivalent per year hitting the 3.66 kwh array. For our case, I bumped the shade losses up to 20%. The results?

PVWatts predicts a typical year's output would be 3,694 kilowatt-hours, or only 1,009 hours of full-sun equivalent on the 3.66 kwh solar array. Interestingly, the proposals from three companies, including EcoHouse, all predicted an annual full-sun equivalent of only 900 hours; to match this the shade losses would need to be 29%. With judicious tree trimming I hope we can harvest 1,000 hours, but that's too complicated to predict. The shading effect will be worst when the leaves have popped out on the large, tall trees, and less when the leaves are gone -- five to sometimes six months of the year around here. The angle of the sun, and hence the spots where the shade falls, will vary throughout the year.

Clearly, comparing results to predictions will be a long-term effort.

In the next post I'll discuss the proposals and their cost and economics.