Wednesday 27 November 2013

Optical Properties of Solar Panels


All you ever wanted to know about Incidence Angle Modifiers…

When light strikes a transparent material, some of the light is reflected, some is absorbed on the way through and the rest is transmitted.  For low iron glass of the type used in solar panels with the light arriving ‘straight on’ (perpendicular to the surface) the figures are typically 8% reflected, 1.5% absorbed and 90.5% transmitted.

Light incident on a the clear cover for a solar panel is mostly transmitted, but some is absorbed or reflected
 Most of the light incident on a solar panel does not arrive perpendicular to the surface, but at some other angle as the sun moves across the sky over the course of the day and year.  The incidence angle is defined as the number of degrees between a ray of light and the line perpendicular to the surface.  As the angle of incidence increases so does the proportion of light reflected at a glass surface.  Since the light is not travelling straight through but at an angle with a longer path through the glass the proportion of light absorbed by the glass also increases.

As the light arrives at shallower angles, the proportion reflected increases

 The effect  light angle has on solar panel performance is quantified by a property called the Incidence Angle Modifier (IAM).  The IAM is measured and reported as part of the standard test procedure for solar panels and is defined as the efficiency of the solar panel at a given incidence angle divided by the efficiency when the light arrives perpendicular.  It takes into account not only the changes in transmission at the glass, but also any changes in reflection at the absorber.
For a flat plate solar collector, the IAM starts at a value of 1.0 when incidence angle is 0 and decreases as shown in the diagram as the incidence angle increases.

The incidence angle modifier captures the loss of efficiency as the angle of light changes from straight on

Evacuated tube type solar collectors have a different geometrical relationship with the light from the sun.  Only the light arriving on the centreline of the tube will arrive at the glass with an incidence angle of zero, as you move away from the centreline, the incidence angle increases and the so the proportion of light reflected and absorbed also increases.  This is partly explains why evacuated tube collectors often have a ‘zero loss efficiency’ (or optical efficiency) lower than that for flat plate solar collectors (the other factor being the gaps between the glass outer and the light absorbing surface inside).
Since a tube has circular symmetry, the proportion of light transmitted is unaffected by the direction the light is coming from.  The light passing through the outer glass wall for a single tube would be unaffected by the angle the light arrives from in the transverse direction (around the tube).

The geometry of a tubular solar collector gives higher reflection losses for light arriving straight on
but has circular symmetry, so the reflection doesn't change as the light angle changes
An evacuated tube solar collector is made up from a number of tubes installed side by side.  A gap is left between the tubes so they don’t touch one another and there is a gap between the outer glass and the inner light absorbing surface.  Consequently, a proportion of the light is not collected and passes between the tubes.

Tube collectors have gaps between each tube which cannot collect light

Some evacuated tube collectors have a tubular shaped absorber (so called Sydney Tubes or tube-in-tube collectors).  A feature of the geometry of these is that as the angle of incidence increases in the transverse direction (around the tubes), the absorber area increases compared to the effective area of a planar absorber – less light passes through the gaps. 

Tube collectors with tubular shaped light absorbing surfaces 'close the gap' as the light arrives at shallower angle

The transverse IAM for these products rises higher than 1.0 before falling to zero as the angle approaches 90 degrees. 

The IAM for a Sydney tube collector rises above 1.0 as the light arrives at shallower angles

All this is just another reason why you can't compare solar thermal panels by their simple gross efficiency.