A reality based independent journal of observation & analysis, serving the Flathead Valley & Montana since 2006. © James Conner.

31 January 2017

The lower the sun’s altitude, the lower the sunbeam’s intensity

The sun is shining today, which means that the Stillwater Station community solar photovoltaic array is generating some electricity. But it’s not generating as much as it can on a sunny day in June because the winter sun is low in the sky and the sunbeam must travel through more air than in summer.

At meridian transit on the summer solstice, a sunbeam perpendicular to a photovoltaic panel delivers approximately 1,000 watts per square meter to that panel. At meridian transit today, the sunbeam delivers approximately 760 W/M^2. The graph below displays the altitude and azimuth of the sun today at half-hour intervals, and the power of the sunbeam at those intervals.

Here’s a table of the data displayed by the graph:

The nameplate rating for a PV panel assumes a solar intensity of 1,000 W/M^2. Atmospheric extinction has the effect of reducing the nameplate rating of the panel.

That’s one constraint nature imposes on a PV array. Another is the angle at which the sunlight shines on the panel.

A PV panels produces its nameplate rating only when the solar rays are perpendicular (normal) to the panel; when the angle of incidence is zero. As the angle of incidence increases, the panel’s output decreases as a function of the AOI’s cosine. At the fixed (non-tracking) Stillwater Station PV array, cosine losses are significant shortly after sunrise, and before sunset.

A PV panel that tracks in altitude and azimuth, keeping the sunbeam normal to the panel, reduces cosine losses to next to zero; but because of atmospheric extinction, the output still declines when the sun is low in the sky.

The Flathead Electric Cooperative has not, to my knowledge, released a month by month summary of the output of the Stillwater Station community array, let alone a summary that compares the actual output with the modeled output. That’s a shame because the information would be useful to Flathead Valley residents who are contemplating installing a PV array. FEC’s habit of not releasing such information may simply reflect the culture of secrecy that permeates the world of electric utilities, but it could also result from a policy of not wanting to encourage more community and rooftop solar.