Solar Panel Optimization

Under a clear sky, the optimum mounting for a solar panel is tracking device which ensures that rays of the Sun are normal to the surface of the panel between sunrise and sunset.  The cost and complexity of this arrangement is often prohibitive and common practice is to mount them facing south (in the northern hemisphere) at an angle close to the latitute of the location.

For off-grid applications under a cloud sky (like the English one I'm writing thus under), the solar installation has to be sized for the weather during the grim months of December and January when the air mass as solar noon is close to 4.0 and sky is frequently overcast.

Under an overcast sky there is no direct normal component to solar irradiance, it is all diffuse and is more or less uniform around the hemisphere that is the sky.  This suggests the hypothesis the yield of a solar panel under an overcast sky will be higher during the winter months if it is mounted horizontally rather than on a south facing slope.

To test this hypothesis a series of occassional measurements of solar irradiance are being made around solar noon on horizontal and sloping surfaces.  The measure being used to analyse the results is the ratio of Horizontal Irradiance to Sloping Irradiance.  When the irradiance of the horizontal surface is higher than that on the slope, this ratio is greater than one.  The graph below summarizes the results to date, broken down by the extent of cloud cover:

This graph suggests the there is an advantage in mounting a solar panel horizontally when the prevailing cloud cover is broken or overcast.  During an English winter, the sky is typically full of broken cloud or overcast, during summer, then potential irradiance is much higher, this gives way to few and scattered clouds.  Under this conditions the horizontally mounted panel is a at a disadvantage and would not be an option for a grid-tied system  which is required to maximise it output over the course of a year.

These comments are more relevant to photovoltaic devices which will produce energy at low levels of irradiance rather than solar thermal devices which tend to be more effective when the direct beam element of the irradiance is high.