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Solar Geometry Concepts
Solar Access Principles Solar geometry is used to assure a site has “solar access” to the Sun's energy. A site that has proper solar access will not be shaded by neighboring buildings or trees. This web page explains solar geometry concepts. These concepts are important when using the Solar Altitude and Azimuth Angles The Sun's position in the sky is defined by two angles. The Solar Altitude angle is the vertical angle up from the horizon. The Azimuth is the horizontal rotation angle from Due South (in the Northern Hemisphere) or from Due North (in the Southern Hemisphere).
The Altitude is zero when the Sun is on the horizon. It is 90 degrees when the Sun is directly overhead. In most places the Sun will never be directly overhead. That only happens in the tropics. When the Sun's altitude is lower, its path through the atmosphere is longer, causing some reflection and dispersal (weakening of the solar beam energy that arrives to earth). Solar Incidence Angle The Solar Incidence Angle is the angle between the Sun's rays and an imaginary line perpendicular to a collector's surface (this imaginary line is called the surface normal). An incidence angle of 0 means the Sun's rays are perpendicular to the collector surface, and an incidence angle of 90 means the Sun's rays are parallel to the collector surface. Maximum solar gain for a given intensity of sunlight is achieved when the incidence angle is zero, that is when the Sun's rays are perpendicular to the surface. This is because the cross section of light is not spread out, and also because surfaces reflect more light when the light rays are not perpendicular to the surface. The light beam cross section can be visualized with a flashlight:
While a smaller incidence angle means more gain for a given sunlight intensity, remember that at low solar altitudes the sunlight's path through the atmosphere greatly diminishes the sunlight intensity. Therefore, a low incidence angle (Sun rays perpendicular to surface) does not guarantee high gain. For example, a vertical window could have much more solar gain with a high incidence angle than when the Sun is lower in the sky. At a given time, similar surfaces may have different solar gain depending on the solar incidence angle of each surface:
Shadows are also smaller on this slope, allowing more area to receive solar energy:
Solar Skyspace HUD defined the solar skyspace as follows:
The skyspace is measured in azimuth and altitude. Depending on the application, a general recommendation for latitudes of 40 degrees or less can be a skyspace of 45 degrees azimuth east, and 45 degrees azimuth west:
The altitude depends on the seasonal use. Here are some general recommendations:
Use Visual Sun Chart to make sure proposed buildings and structures do not interfere with the collector's required skyspace. |
