Photovoltaic Tutorial:

Optimum Solar PV Array Placement

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How to determine true south and an array site's proximity to it

The term "true south" lets anyone with a compass know that magnetic south is not what counts here. True south (or due south) equates to geographical south. And to get that particular bearing with a compass needle (which is, after all, guided by magnetism) you may have to add or subtract some degrees to account for magnetic declination.

It turns out that the magnetic North Pole lies several hundred miles from the geographical North Pole. It's currently hovering off the coast of Greenland and continues to drift about a half a degree every year. Right now a compass needle in the Unites States may point away from true north up to about 20 degrees. Hence the need to adjust your compass readings for declination.

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An isogonic chart (left) tells you how far from true north a compass will point anywhere along an isogonic line. You can also look up your declination online. You'll have to add or subtract some degrees from your compass readings to get accurate results. (If you live along the line with a zero declination, no adjustment is necessary.) To avoid the need to repeat the calculation each time you take a bearing, you can also set your compass for declination once by loosening the azimuth dial screw and resetting the alignment the correct number of degrees to the left or right. (See photo on the right.) Check here for more info on how to use a compass and factor in declination. You can also look up the declination value for a city at the NOAA website.

Google Earth offers an alternative method of determining the compass direction of your rooftops or other array location. Like any map, this one is aligned to geographical north, so declination is never an issue.

Google Earth
Use the free Google Earth to check the roof of your house and check its orientation to true south. The satellite photo will also identify ductwork and other features that factor into mounting a solar array. Naturally, a shading assessment must be done on site. (Note: Shading is discussed at length on the next page of this section.)

Once you have the compass bearing for your potential array site, you'll have to do some math to see how far off it is from true south. For example, if the bearing is 210 degrees, the math would be 180 - 210 = - 30 degrees. This signifies an array orientation that's 30 degrees west of true south. Below, you can see how this info is used to estimate insolation (i.e. the standard measuring unit of solar radiation). It's combined here with an array's tilt or the roof slope to predict an accurate percentage of the sun's available insolation that can be converted by the modules into electricity.

According to this diagram, in the region of Edwards AFB (Southern Cal) you can still capture 90 percent of available radiation even at a wide variance from true south (i.e. 180 degrees). Array tilt is also accounted for with the roof pitch and degrees of tilt shown on either side. Click here to generate a diagram for another location, courtesy of Solmetric.com. You can also download a handy lookup feature that operates inside Google Earth.

As stated earlier, the array azimuth angle (compass direction) can be 45 degrees or more away from true south and still generate most of the power output you would get with a true south orientation. But in cities above 50 degrees of latitude (Canada, Alaska, etc.), the margin of variance shrinks considerably. In fact, the farther away you live from the equator, the greater the efficiency loss for arrays turned away from true south. In this case, you'll need to spend extra time analyzing the sun hour data set, the shading assessment, and diagrams like the one above before deciding on the best orientation and tilt angle for your modules.

How to project an array's power output with PV Watts

Thanks to the internet, you can quickly estimate the kilowatt hours produced by a proposed solar array by logging on to an NREL site called PV Watts. This app walks you through the steps of generating a report based on the data you enter, including your zip code and the average rate you pay your utility company for grid electricity.

This is the data generated by PV Watts for a Seattle solar array whose proposed size is 10 kilowatts, facing true south and tilted at the same angle as the city's latitude. You can press the back button on your browser and try different tilt and azimuth combinations to see if you can increase the annual output. The results above, for example, can be improved by lowering the array tilt to 35 degrees. Seattle. of course, is at a higher latitude than most American cities. At lower latitudes between 20 and 40 degrees, the numbers will change very little. Incidentally, DC-to-AC derate factor is a catch-all setting that accounts for power losses due to DC to AC electricity conversion, shading, wire losses, and system downtime.

Continued on Page 3... (How to Measure Shading)

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