Sloping Off

The clear sky performance of solar devices is reasonably easy to predict.  Man has had a good working knowledge of sun-earth geometry since the dawn of time and until scientists starting messing with decaying atoms, the sun was time.  The equations may have gained some extra terms and decimal places, but an understanding of the principles is apparent in England's Stonehenge, Mayan Temples and many other ancient world heritage sites and sundials.  Over the past few weeks I have been attempting to clean up our "Simple Clear Sky" model.  This small lump of Python code trades accuracy for simplicity, my research interest in the the effect of clouds on solar devices, whilst a solar irradiance under a clear sky follows a smooth curve, soon as a cloud gets between the sun and the earth, the curve looks like the teeth of a halloween lantern, so high accuracy is not that important.  There will be more on this model in a later post.  The code clean-up is part of a large decluttering of spreadsheets and code fragments.

When I first became interested in wind and solar energy resources, I attempted to educate myself with crude wooden constructions in the backyard.  Having gathered the photos together, I realise must have caused my neighbours some embarrassment.  Having amassed some photographic evidence of things I wish I had never started, I have none for experiments which were vaguely useful.  One of these was a 1.5 watt amorphous PV panel which was south facing, but the angle relative to the ground could be varied from zero to ninety degrees.  The objective was to see how the output of a solar panel changed with slope under an overcast sky.  A summary of the results is shown in the graph below:

There are two sets of data, one for a clear day and the other for low overcast skies.  The clear sky curve has a maxima somewhere around 50 degrees.  A reasonable estimate of the clear sky results could have been made by the Python model.  The curves of the overcast sky are more interesting.  First they "peak" when the panel is horizontal, suggesting that the irradiance is more or less evenly distributed over the hemisphere of the sky, secondly, the relationship between slope and irradiance could be approximated by a linear relationship.

Whilst my backyard is generally sheltered from the wind, a stray gust ended this experiment, the mountings of which were later used as firewood.

Later this year, I hope to have a small 20 - 30 watt solar installation working in the back yard, which hopefully will have some data logging capability which will allow a cloud sky model to be firmed-up.