Insolation

After yesterday’s heavy rains, this morning’s sun is low and slow to warm the air. Right now it’s 53º but that’s not a lot more than the 45º we had when I got up at 0500. Among other things I decided I might say a bit about:

Insolation — the a measure of solar radiation energy received on a given surface area in a given time.

Imagine all the things that affect this value.

  • The Sun’s output
  •  

  • Its distance from Earth as a function of season
  •  

  • The tilt of the Earth as a function of season
  •  

  • The Earth’s wobble
  •  

  • The Earth’s spin (time of day)
  •  

  • Particulates in the atmosphere
  •  

  • Water vapor in the atmosphere (clouds or not)
  •  

  • Animals in the atmosphere (flocks of Passenger Pigeons once “darkened the Sun”)
  •  

  • Thickness of the atmosphere (elevation)
  •  

  • Obstacles between the Sun and the “given area” (in this case our panels)
  •  

  • Angle of incidence of the panels (tilt and azimuth)
  •  

  • And a few more besides….

Insolation maps are available that take most of these factors into account.

This one shows an average.  Clearly there will be more in summer and less in winter.  It is the Sun’s time and elevation above the horizon that dictates the “gross insolation”.  The curves on the map also account for the interference of weather throughout the year.  For our home port that time and elevation picture looks like this:

Generally there is a reasonable correlation between the area under one of these Sun curves and the gross insolation.  So if one establishes the Summer Solstice as the benchmark of “1” (Close but not exact.), then today (15 Oct) we may receive 35% of that and on the Winter Solstice (same inexactitude) 16%.

With an annual average of 4-5 kW/sqm/day in our location this then means we might expect 6-7 in June and as little as 1 in December.  [Remember this discussion is about solar energy, not weather.  On any given day the temperatures may or may not correlate with insolation.]

Now the solar panels are rated at 405 Watts ( a performance they will seldom deliver).  Collectively, they are 3m square so over an average annual day they will receive ~12kW of insolation — wrong.  Those average figures assume the panel is tilted to the South at an angle that matches the latitude.  Well, they could be, but right now they aren’t.  So for flat panels this number is actually more on the order of 62% of 12 or let’s say 7.5 kW/day — and they are rated at 405W.

If I was selling electricity on Grid, this would matter.  I’d have panels that tracked the Sun and a staff of cleaners (ok, me)….

But I just want to recover the energy I used the prior day and keep battery draw on any given day to the lowest the sun will allow. In the winter, in Deale, this means floating the batteries to keep them healthy. That 16% of June’s insolation is plenty with panels this size. Whether winter insolation much farther south is sufficient for cruising energy demand is something we hope to be evaluating this time next year…

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