Solmetric SunEye 210, User Manual

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4-1
Solar Access and Interpretation of Data
Introduction
4
Solar Access and Interpretation of Data
Introduction
This chapter gives an overview of the different variables involved in solar access and details
about how the SunEye calculates solar access. Solar access is defined as the amount of available
solar energy or insolation in a particular location.
The reported solar access percentages are the amount of site-specific solar insolation available at
the location where the data was captured given the shade-causing obstructions, divided by the
solar insolation if there were no shading.
• If all trees, buildings, and other obstructions were theoretically removed, the solar access percent-
ages would be 100%.
• If the measured site were completely shaded all year, the solar access percentages would be 0%.
In order to have an accurate estimate of the amount of insolation that will strike a specific sur-
face over time, four variables need to be considered.
• Solar irradiance
• Shading
• Air quality and clouds
• Surface orientation
Solar Irradiance
The sun is a tremendous source of energy for the earth and is, in fact, the only significant net
input of energy the earth has. Irradiance is the solar power incident on a surface, and is usually
expressed in watts per square meter. Irradiance multiplied by time equals insolation. The aver-
age irradiance arriving on the planet after passing through the atmosphere is commonly approx-
imated to be 1000 W/m
2
. The irradiance above the atmosphere is 1367 W/m
2
.
Due to the tilt of the earth’s axis, the sun is higher above the horizon in the summer months and
lower in the winter months. The sun will also appear higher in the sky when the observer’s lati-
tude is closer to the equator (0° latitude) and it will appear lower in the horizon as the observer’s
latitude is closer to the north or south poles (±90 ° latitude). The height of the sun in the sky is
not affected by the longitude or the east-west location of the observer on earth.