Andy Schroder

Explanation of Solar Energy Concepts

Energy and Power

In physics, energy is usually defined as "the ability to do work", typically measured in joules (J).
In the application of solar energy it is customary to measure energy in the units of kW*hours rather than Joules, where 1kW*hour=3.6*10⁶Joules.
Energy can exist in many different forms with varied levels of usefulness.
The goal of solar collectors is to convert solar energy into a useful form of energy. In the case of solar thermal collectors, the useful form of energy is heat.
Power is defined as the rate at which energy is supplied over a given amount of time. In physics, power is typically measured in Joules/second.
In the application of solar power, it is customary to measure power in the units of Watts, where 1Watt=1Joule/1second.

Solar radiation or insolation

Solar radiation is energy transmitted from the sun to the earth in the form of electromagnetic waves. These waves transmit energy at a certain rate and density. The solar flux defines the rate and density in which the energy is transmitted.
Solar flux is typically measured in W/m².
It is important to note that solar energy is more than the light humans can see with their eyes. "About half of the radiation is in the visible short-wave part of the electromagnetic spectrum [light you can see]. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum" (http://www.grida.no/climate/ipcc_tar/wg1/041.htm).
Direct solar radiation is the solar radiation in the form of a collimated beam with a known orientation.
Diffuse solar radiation is solar radiation in an uncollimated form. Diffuse radiation becomes uncollimated due to scattering in the atmosphere and reflection off of objects on the earth's surface (ground, buildings, etc.).
Total solar radiation is the sum of the direct and diffuse solar radiation.
For more information see http://en.wikipedia.org/wiki/Solar_radiation and http://en.wikipedia.org/wiki/Insolation

Sun Position

Although the earth actually revolves around the sun, for the sake of solar energy analysis, in the reference frame of the earth we can consider the sun to be moving about the earth. For example, the sun rises in the east and sets in the west.
The position of the sun is defined using spherical coordinates.

Azimuth and altitude angles are a function of geographic location and time.
The azimuth and altitude angles are defined assuming that the earth is effectively flat.

Azimuth angle: The angle of the sun about the plane of the earth's surface (the same direction that a compass would measure measure).
Altitude angle: The angle of the sun formed between the earths surface and the sun (i.e. how high the sun is in the sky when facing the sun).