Clouds, Attenuation and Diffusion

Under a clear sky, the irradiance of a solar device is largely determined, by sun-earth geometry and atmospheric conditions such a precipitate water content, gas, particles etc.  Under a cloud sky, the irradiance is attenuated and the direct beam becomes diffused and the nature of the spectrum changes.  The attenuation can be described by the Clear Sky Factor (CSF for short), this is the observed global/total horizontal irradiance expressed as a fraction of the estimated clear sky irradiance:

The diffuse fraction is the ratio of the diffuse irradiance to the total

The diagrams below show how these to variables change with the state of the sky.  These diagrams are idealised, even a casual glance at a clouded sky suggests that there is strong random element in the behaviour of clouds, for this reason it might be desirable to model CSF and DF as probability distributions.

Clear Sky

Under Pure Clear Sky conditions, there is no attenuation, the value of CSF is 1.00 and the diffuse fraction in the range 0.05 – 0.15.

Overcast Sky

At the other extreme, the modal value of CSF for the overcast sky is around 0.2 with a relatively narrow variation, say in the range 0.1 to 0.3.  No direct sunlight reaches the ground and the diffuse fraction is constant at 1.00

The Cloudy Sky

The cloudy Sky is an intermediate state between clear and overcast.  A common scenario is for cloud masses to move across the sky propelled by wind.  When a sensor on the ground sees clear sky, the instantaneous value of CSF is around one and the diffuse fraction will be greater than under a clear sky.  It is possible in the case of FEW clouds or high level clouds that CSF will exceed one due to the additional diffuse irradiance. When the Sun is obscured by cloud, there will be little or no direct irradiance and the diffuse fraction will approach one.  Over a time interval, say 15 minutes, the time average of CSF will be proportional to the amount of cloud cover.

Precipitation and Obscuration

Rain, snow and fog are frequently associated with low, dense cloud, added to which is a layer of water.  The effect, depending on the intensity of the weather is to lower the value of CSF below that expected from overcast cloud, sometimes to around 0.05.

A special case of the effects of weather on solar devices is when the receiving surface becomes covered in snow, in this case the CSF falls to zero, even though pure clear sky conditions may prevail.

Example

The graph show a series of measurements of illuminance taken just after solar noon on 08-Aug-2010.  The sky state could be described as scattered cloud.

The troughs indicate when the sensor was obscured by cloud, and the peaks are the intervals of clear sky.  The output of a solar device would fluctuate under these conditions, depending on the nature of the load, some form of buffer storage might be needed to average out the fluctuations.  The duration of the troughs gives some indication of the extent of the cloud cover.