Solar Spectrum

The energy in solar irradiation comes in the form of electromagnetic waves of a wide spectrum. Longer wavelengths have less energy (for instance infrared) than shorter ones such as visible light or UV.

The spectrum can be depicted in a graph, the spectral distribution, which shows the relative weights of individual wavelengths plotted over all wavelenghts, measured in W / m (wavelength).

The diagram displays the spectrum of a sun ray just outside the entry into the earth’s atmosphere. The peak of the spectrum is within the visible spectrum, but there are still significant amounts of shorter and longer wavelengths present.

Intensity and Energy

For the purpose of solar power, the most significant measures are the intensity and energy delivered – one measure at a point in time, the other over a period of time.

At a point in time

Irradiance [W/m²]: The intensity of solar radiation hitting a surface, which is the sum of the contributions of all wavelengths within the spectrum, expressed in units of Watts per m² of a surface.

Power [W]: Momentary total irradiance incident on a particular area.

Over a period of Time

Energy per unit area [kWh/m²]: Energy per unit area is a measure of irradiance incident on a surface over a period of time. It is often expressed

Surface Orientation

As sunlight is smoothly distributed over whole areas, a mere figure for intensity is never sufficient without knowledge of the orientation of the surface in question. Typically, the orientation of a surface is described by the zenith angle, the angle between the sunbeam and the normal of the area. If the surface area is not perpendicular to the sunbeam (i.e. zenith angle is not zero), a larger area is required to catch the same flow as the cross section of the sunbeam.

If I0 denotes the intensity on a surface with the sun in its zenith, the intensity, I, on an area where the sun is observed under the zenith angle  θ (see figure) the intensity is reduced to

Values for θ range from 0° to 90°. Turning the face of the area away from the sun means less energy is flowing through that area.