4-3
Solar Access and Interpretation of Data
Weather, Air quality and Clouds
Weather, Air quality and Clouds
Moisture, pollution, clouds, and particles in the atmosphere block a portion of the sun’s energy
from reaching the earth. This can impact the insolation and solar access calculations provided by
the SunEye. Two weather models are incorporated into the SunEye: NASA and (in the US only)
NREL – TMY3.
NASA Weather Model. The Clearness Index (K
T
) is a number from 0 to 1 that describes the
amount of solar radiation that passes through the atmosphere.
Historical clearness index data by region and solar insolation data is available from NASA at
http://earth-www.larc.nasa.gov/cgi-bin/cgiwrap/solar/sse.cgi
.
When the NASA weather model is selected, the SunEye uses the NASA Clearness Index data.
The SunEye contains an internal database of clearness indices by latitude and longitude for the
entire world. The SunEye software automatically accounts for the approximate average clear-
ness index for the region you specify for your session when calculating the solar access.
NREL – TMY3 Weather Model. The United States National Renewable Energy Laboratory
(NREL) publishes weather data for 1,020 locations in the United States. Each station has a
Weather Bureau Army Navy (WBAN) identification numbers. This data shows “Typical Meteo-
rological Year” (TMY3) data, and it contains insolation during all hours of the year. For more
information, please see
http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/
. The Sun-
Eye uses TMY3 hourly insolation data when the NREL – TMY3 weather model is selected.
When using this model, the NREL weather station must be selected, normally the one closest to
where the SunEye session is being created.
In addition, when the NREL – TMY3 weather model is used, the SunEye has precomputed val-
ues for the optimal panel orientation (tilt and azimuth) for maximum annual insolation for all of
the selected weather stations. The optimal orientation is dependent on weather, and allows cal-
culation of TOF and TSRF which are defined as follows:
• Tilt and Orientation Factor (TOF). TOF is the solar insolation at the actual tilt and orientation di-
vided by the insolation at the optimal fixed tilt and orientation, expressed in percent. Note that for
1 axis and 2 axis tracking TOF may be greater than 100%, since insolation may be more than with
the fixed panel.
• Total Solar Resource Fraction (TSRF). TSRF is the ratio of insolation available accounting for
both shading and the specific tilt and orientation of the surface, divided by the total insolation
available assuming no shade and assuming the surface is at the optimal tilt and orientation. TRSF
is expressed in percent, according to the following equation: TSRF = Solar Access x TOF. Note
that with 1 axis and 2 axis tracking TSRF may be greater than 100% because it is compared to a
fixed panel.
When the NREL – TMY3 weather model is selected, the SunEye can compute and display TOF/
TSRF for each skyline. The average TSRF for all selected skylines in a session is also displayed
in the exported reports.
To obtain Wh/m
2
or peak-sun-hours, take the solar access percentages from the SunEye and
multiply them by the average solar insolation for your region.
