Appendices - GSA

Solar Resources
The solar resource available outside the Earth’s atmosphere is immense and
always available. The solar radiation that gets filtered through the atmosphere
with its accompanying clouds, moisture, pollution, etc. is still enormous, though
availability follows daily and seasonal patterns occasionally interrupted by
weather events. The challenge in working with the solar resource for electricity
generating applications is that the resource is relatively diffuse and
unconcentrated.
For making electricity, only bands of the solar spectrum within the visible light
wavelengths are utilized and the objective for PV manufacturers is to utilize as
much of that available energy as possible. For heating applications, whether for
space (air) or materials (mass), the light energy is transformed into heat energy
as it strikes a surface and is absorbed.
In the continental U.S., the intensity of solar radiation during the middle 6-8 hours
of the day is usually in the 500-1000 W/m 2 range. A daily pattern of insolation
intensity can be seen in the graph below.
Figure A-9: Daily insolation pattern integrated into a yearly average of 5.5
sun-hours/day
Source: http://www.solarexpert.com/grid-tie/system-performance-factors.html
To facilitate climate comparison and predict system performance, the amount of
solar radiation that falls on a collector throughout the day has been integrated to
determine the area under the curve. The conventional level of intensity is 1,000
W/m2 (1kW/m2) termed peak sun-hours. The solar resource at a site is often
reported in sun-hours/day which equates to kWh/ m 2 /day. There have been
extensive weather data collection efforts throughout the U.S. for number of years
and with a wide variety of data collectors, end users and end uses in mind.
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