Fig 4 Rack Geometry

the Fall increase the PV output by about 5 to 8%. This is really not a very great increase in performance, but the success or failure of an AE system depends on attention to detail. We personally consider that a 5% increase in our PVs performance is well worth the twice yearly expenditure of 15 minutes of our time to adjust the rack.

On roofs that are not horizontal (and most aren't), the legs get shorter as the roof gets steeper. A good overall, nonadjustable, mounting angle is your latitude. If you live at 40° latitude, then mount the rack so that the angle between the rack's face and horizontal is 40°. The table shows the proper leg lengths for South facing roofs and a variety of latitudes. This table assumes the use of 6 foot rack rails and skids. The top of the table contains roof angles from 0 degrees (flat) to 60 degrees from the horizontal. The left side to the table shows latitude in 5 degree increments. The actual leg lengths in feet are in the body of the table.

Consider someone living at 38° latitude with a 25° slant on his roof. The table shows a leg length of 1.36 feet. Note that this table shows leg length decreasing as the roof's angle approaches the latitude. Once the roof's angle becomes greater than the latitude, the legs are attached to the bottom of the rack rather than the top. Instead of raising the top of the rack to face the Sun, we raise it's bottom.




If you're into math, the formula used to generate this table is based on the Cosine Law. Here is a solved and generalized equation that will give leg lengths for all situations regardless of rack or skid dimensions, latitude or roof angle. L= length of the Leg in feet R= length of the Rack in feet S= length of the Skid in feet

P= the angle of the roof's plane to the horizontal in degrees A= your latitude in degrees

The geometry is much the same for wall mounting, but the skids are vertical. In any case, don't be afraid to mount the skids however you must, adjust the rack's elevation, and cut the legs to fit. This approach while, low tech, gets the job done

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