Thursday, February 15, 2007

Mounting Solar Panels on Your Roof

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:

http://www.weatherunderground.com/

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:

http://www.wattsun.com/resources/calculators/photovoltaic_tilt.html

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

10 comments:

jj3000 said...

I read your blog on the aluminum hardware, I think sure save money to get manufacterers standoffs, what is required to modify this to mount a panel adjacent to it?

So is the idea that you will go up the roof and swap out the legs every 6 months to get a different angle? What about use 2 vertical angle pieces with holes aliged at the angles you want, to change the height.

I just wanted to say, might want to be careful with the SS nuts on SS bolts as they tend to gall and weld together. Here are some tips

http://www.estainlesssteel.com/gallingofstainless.html

Solar John said...

Thanks jj3000 for your suggestions.

1. The center legs need to be notched since the panels are mounted side-by-side. This is easier to picture when you're installing the panels, than it is to explain here. Picture a flat piece of 1/8" aluminum for the inside legs instead of an angle piece.

2. An adjustable vertical angle piece might cast a shadow on the panels which is something I want to avoid.

3. It might be a good idea to pre-lubricate the stainless steel mounting hardware to avoid the problem you've mentioned. So far I've had no problems.

John

Dugdale said...

John,

Good write up, I am planning on doing something similar with my solar blog.

I am curious about the heat build up and when it gets too hot it losses efficiency - can you tell me more about that?

SolarDave

Anonymous said...

HI, Like what you've done... here's a suggestion to your mounting dilemma.

Create an external frame to go around all your panels. For larger runs include two intermediate rails with a gap between them. Have a bolt stretch the gap in the centre.
Drill hole in the middle of each end and place a bolt with a lock nut, have the bolt protrude somewhat through the end to create a spigot. This is for those that can't just weld a spigot on.
Now create to uprights at each end to mount onto the roof and drill a hole in the top of each one pole. this mount can also be made as a triangle to spread the load.
Next make another triangle shape bracket (think car alternator bracket) with a curved bolt hole.

So what you end up with is a row of panels that can pivot to any angle. With a locking system that will not cast any shadows as it is mounted to the frame and moves with it when it tilts. You would drill one more hole in the mounting post to accommodate the locking bolt.
regards,
Stuart W
Melbourne Australia

Earl said...

Seems like a cheap and efficient way to mount your panels to your roof. Nicely done. How do you handle the power cables going from the panels to your batteries? Did you drill a hole in the roof for the cables to enter the house?

John said...

At the present time the wires run down the side of the house parallel to the gutter. I need to improve on that in the near future, with advice from an electrician. I want to comply with the electrical codes.

Anonymous said...

Hi John,

I'm working on a solar charger for Burning Man & saw your mounting info. Great idea! The best deal I've found for a mount is $75 w/shipping, but I can get enough aluminum at Home Depot for about $30. Problem is cutting and drilling holes - Home Depot can't do it & a local sheet metal shop charges $160/hr...yikes! Did you do that part yourself? Unfortunately, I don't have the means & don't know anyone who can do it. Oh well, $75 isn't that bad.

Karl
San Francisco

John said...

To do it yourself you'll need a hack saw, an electric drill, and an assortment of drill bits. I use a center punch to more accurately establish the location for the holes. I use a file to remove the sharp edges created by the cutting and drilling. Good luck! sj

ayan said...

Cool, solar power from solar panels is way to go! This is one cool way to maximize the use of the renewable sources of energy and green way to consume energy.

ZenHomeEnergy said...

Installing rooftop solar panels is one of the best ways to increase the value of a home or building.

Solar Panels