Note: The information contained in this periodical weblog may be outdated. This was my personal weblog published before January 1, 2015. Since then I have been the co-owner and Principal Solar Designer at Florida Solar Design Group.

Beat the Heat with Expansion Joints in Solar Rails

If your solar photovoltaic module installation in Southwest Florida has rails longer than 30 feet and there is no expansion joint, you are asking for trouble. We have about a 75 degree Fahrenheit temperature swing around here, and aluminum rails can expand and contract substantially. The temperature of the rail can exceed the ambient temperature substantially. Let’s look at the numbers…

Aluminum has a thermal expansion coefficient of approximately 13 microns per inch per degree Fahrenheit. A 30 foot rail section is 360 inches. Let’s assume the aluminum will experience a 100 degree temperature range from 30 – 130 degrees Fahrenheit.

0.000013 x 360 x 100 = 0.468 inches

That means a 30 foot rail section can expand/contract by almost 1/2 inch! The purpose of the expansion joint is to prevent the rails from buckling. Thermal expansion also puts a lot of stress on the roof attachments and the hardware used to fasten modules to the rail. it is critical that these expansion joints are installed, and installed correctly. Because the joint breaks electrical c0ntinuity, it is important to make sure your grounding system bridges the gap.

A proper expansion joint with WEEB bonding strap.
A proper expansion joint.

The picture above shows a Unirac Solarmount rail at the Lee County Public Safety Building in Fort Myers. The Unirac splice bar has only two screws in it on one side of the joint. A WEEB bonding strap is used to provide electrical continuity for proper grounding. This is not the cheapest option, but I’m not about doing it cheaply. I want it done right!

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