Solar Power » Radiation

What the effects of solar radiation mean to solar power

Taking into account the various effects of solar radiation (e.g. intermittence, absorption, scattering, tilting, eccentricity), solar power converters, installations and yield forecasts can be optimised

Effect

Impact on Intensity

Insight

Variations in solar activity

+/- 1%

Negligible effect, unavoidable

Eccentricity in solar orbit

+/-3% over the year

Summer on the Southern hemisphere is more pronounced than on Northern hemisphere.

Planetary movements: Rotation and declination

Significant but predictable impact – lower intensity per m2 when sun is not in zenith.

There are two cycles:

  • 24h cycle of day and night
  • Annual cycle of summer and winter

Although all locations on earth enjoy the same number of daylight, differences depending on latitude are significant, while effects of longitude cancel each other out.

Large variations, but entirely predictable.

Absorption in atmosphere

Further attenuation through atmosphere.

This happens even on a clear day.

The longer the path through the atmosphere the larger the degree of attenuation. I.e. further away from the equator or around sunrise / sunset.

Subject to short-term random behaviour (e.g. clouds moving).

Scattering in atmosphere

Splits light into direct and diffuse light.

There is more diffusion further away from the equator and in humid climates.

Diffuse light can not be focused (concentrated), but can still be used in the photovoltaic effect.

Diffuse light has a larger proportion of long waves (infrared), which may not be sufficient to cause the photovoltaic effect in Silicon.

Tilting of surface

Also receive reflection off the ground

By tilting a surface towards the sun, the ratio of direct light that strikes the surface increases significantly, also helped by reflection off the ground. In the Northern hemisphere, the optimal tilt angle for a fixed surface is 10% less than its latitude.

Intermittence

Average intensity is between 10 – 30% of peak intensity

Solar power converters have to be sized such that they can handle peak intensity, though on average they can only use 10 – 30% of that peak.

 

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