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The sun's radiaton delivers energy in abundance. Yet, technologies to harness that energy have been costly. However, with costs coming down and attractive incentives in many countries, solar provides great opportunities for developers, investors and consumers.

Is Solar the right solution for your needs?

Sunset Tanzania

Green Rhino Energy assists solar project developers and investors over the entire lifecycle. We provide advice on regulatory frameworks, revenue forecast, site potential, pricing and project due diligence.

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

Cosine Law
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.

Horizontal Surface Surface that lies flat on the ground of the earth
South facing surface The projection of the normal of the surface onto the ground points to South.
Perpendicular surface Surface that is perpendicular to the sunbeam with sun in zenith at θ = 0°.


Extraterrestrial Irradiation: The Solar Constant

The intensity of solar irradiation directly outside the earth’s atmosphere on a horizontal surface is almost constant at around 1,350 W/m2., the so-called “Solar Constant”.

Solar Constant
This entry point into the atmosphere is called “Air Mass – 0” or “AM-0”, where “0” points out that there is no air mass.There is a variation of solar intensity of about 1%, but this is a slow cycle. It is so small that it is negligible for the purpose of solar power.

Modelling Solar Irradiation

With planetary movements, processes in the atmosphere and other effects, solar radiation on earth is an intermittent source of energy.

On the earth’s surface the peak solar intensity hovers around 1 kW/m² on a horizontal surface at sea level with the sun in its apex on a clear day. In general, the value will depend on the position of the sun, the clearness of the sky and the geometry of the surface.

Due to the complex nature of some of the processes, no theoretical calculations for irradiance is entirely accurate. Nevertheless, these models are helpful in understanding the main drivers as well as:

  • Assisting in sizing of systems
  • Aiding in choice of technology, as some technologies are more appropriate in certain locations than others.
  • Forecasting energy generation
  • Use before or in lieu of detailed site survey.
  • Optimizing the design of devices and operations - especially predicting short-term variations of irradiance in the 1 - 10 minutes forecasting timeframe.