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Issue #216 July 2008
Intelligent Energy Solutions
Living And Working Off The Grid
Part 1: Planning And Design
by George Martin
Start | Power Usage | Estimating Solar Radiation | Equipment | Solar Panels | Inverters | Charge Controller | Battery | Miscellaneous Items | System Cost | Sources & PDF
ESTIMATING SOLAR RADIATION
The University of Western New Mexico—which is located in “Silver,” as the locals call it—has a great web site for weather (http://weather.wnmu.edu). It displays solar radiation in watts per square meter. If you look at any of the solar radiation charts on the Internet, you will find that the southwestern United States is great for producing solar power. It is even better than islands like Hawaii. The University of Oregon has a great sun chart program (http://solardat.uoregon.edu/SunChartProgram.html). Figure 1 shows the sun path for our location in New Mexico.
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| Figure 1—This is a six-month chart of the sun’s path for our latitude and longitude. Note that July to December is the mirror of December to June. |
Next, let’s assume that we have no buildings, trees, or mountains to block the sun. This is a good assumption because we are in the middle of nowhere and cloudy days should be our only issue. I also used the U.S. solar radiation resource maps on the Renewable Resource Data Center’s web site (http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas) to estimate the solar radiation for our location using a fixed mount tilted at the latitude for the month of December. The result was 5 to 6 kWh/m2/day (typical) and 3 to 4 kWh/m2/day (minimum). I figured December with its minimal sunlight should be the worst-case design. There are plenty of reports on the Internet for solar planning. I’ve just mentioned a few that I found useful.
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