Thursday, April 29, 2010

Solar John: Mounting Solar Panels on Your Roof

From guest blogger Solar John.....

To operate at peak efficiency, solar panels should be aligned perpendicular to the sun. But since the sun’s position in the sky is constantly changing, frequent manual alignment is not practical. While solar trackers, which automatically align the panels for the sun’s daily east to west transition, are available, they are expensive and prone to failure. A less-expensive option is to mount the panels facing south, and to include an easy way to adjust the tilt in order to compensate for the position of the sun in the sky as the seasons change.

While the summer sun appears almost directly overhead at mid-day, winter sun appears to the south. The drawing below shows the sun’s position, and the approximate best angle for roof-mounted panels for the summer and winter seasons.

To determine the optimum angle for your solar panel for each season, you’ll first need to know the latitude for your location. You can find that information here:

The website listed below allows you to enter the latitude for your location, and then calculates the optimum angle for each of the twelve months of the year:

While you probably don’t want to change the angle every month, changing it two to four times per year will improve the performance of your array. Using data for my location, I’ve decided to use a 25 degree angle for spring and summer, and 50 degrees for fall and winter. Since my roof is already at a 17 degree angle, my panel mounts need to make up the difference, or an additional 8 degrees for the spring and summer position, and an additional 33 degrees for fall and winter. To measure the angles, I’ve purchased the tool shown below from my local hardware store:

I made my panel mounts using 1” X 1” X 1/8” angle aluminum. I start by cutting two long pieces, one for each side of each panel. I securely mount those to the frame of each solar panel. I then cut the top and bottom legs, and the feet. I’ve decided to make the bottom legs three inches in length. Elevating the panels three inches above the roof allows plenty of air to circulate under the panels, keeping them from overheating. This is an important design consideration because solar panels loose efficiency at high temperatures. The upper legs, as shown in the photo below, are 8” in length. The difference between the length of the lower and upper legs results in the additional 8 degrees of tilt that I need for my summer configuration. Longer upper legs will be used in the fall and winter months. While only two panels are shown here, additional panels can easily be added along side the others. To do that, the legs between the panels need to be modified slightly.

Legs are attached to 3” feet, made from the same aluminum angle stock. I used 2 ½” stainless steel lag screws to securely mount the assembly to the roof. To prevent leaks, I’ve used a generous amount of clear silicon sealer under the feet as I attached them to the roof. To eliminate the possibility of rust, I used stainless steel bolts, nuts, and washers to hold everything together.

I’m fortunate to have a south-facing roof on which to install my solar panels. Array mounting would be more complicated, and expensive, if my roof were east-west oriented. Shading from the array in the spring and summer helps to reduce heat in the attic. Notice in the photo that the panel's junction box is at the top, and therefore easily accessible.

An advantage to the steep winter configuration is that snow quickly falls off of the panels.

I'm confidant that this mounting structure will withstand high winds, and I've trimmed nearby trees to prevent damage from blowing and falling limbs. I should not experience shading from nearby trees execpt in the very early morning and again very late in the day. I look forward to the day when six more panels are added to this array.

Solar John

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