Thursday, February 18, 2010

6 things ballasted PV racking vendors don't want you to know

Racking (mounting) systems for photovoltaic and solar thermal systems fall into two basic categories: ballasted and penetrating. Here is a brief explanation of each:
  • Ballasted - The word ballast simply means "weight added for stability". The concept is that a mounting rack, usually some type of metal frame, is arranged on the rooftop and instead of fastening it to the roof structure, it is held down by weights. The ballast (commonly concrete blocks) need to be heavy enough to resist wind, snow and seismic forces.
  • Penetrating - This term applies to any type of system where the waterproofing membrane of the roof system is penetrated to make the attachment to the structure. Wind, snow and seismic forces are resisted by the mechanical attachment to the building structure.
Here's what the guys selling ballasted systems don't want you to know:
  1. When they say "non penetrating", they really mean "mostly non penetrating" or "sometimes non penetrating. Ballasted systems do require mechanical attachment to the structure to comply with code if they are located in a seismic zone.
  2. Future roof repairs and maintenance will be outrageously expensive. It would be great if roofs never needed to be worked on, but the reality is that every roof needs some maintenance and, unfortunately, occasional repair work. Roof work is expensive enough with a wide open roof. Now imagine paying someone to remove thousands of pounds of cinder blocks and disassembling solar arrays just to figure out where a roof leak is originating. Elevated racking systems can be easily worked around to keep the roof clean and well maintained.
  3. Future re-roofing? Forget about it. You can't replace the roofing with a ballasted PV system in place. Well, actually you can, but nobody would pay the price. The labor to remove and replace the system and all the ballast blocks would be prohibitive. With a properly installed elevated racking system, re-roofing is very feasible.
  4. Roof penetrations are a good thing. Every roof has penetrations. Every skylight, air conditioner, plumbing vent, electrical conduit, fan unit, etc. is a roof penetration. The roof penetration just has to be done right. Roofers know how to do it. The reason for the bad feeling in the industry about roof penetrations is because of all the poorly installed (and leaking) penetrations installed by people with no roofing expertise. Don't be afraid of roof penetrations, just do them right.
  5. Ballasted PV systems block water flow. Good drainage is critical for a healthy and long lasting roof. The way ballasted racking systems sit directly on the roof with the cinder blocks spread out to hold them down, water cannot flow in the direction and velocity intended in the original design. Dirt and debris will build up around the obstructions and accelerate the deterioration of the roof membrane.
  6. Elevated PV racking systems give you higher power output per square foot of roof. Ballasted systems need unobstructed roof areas because they can't span over pipes, vents, skylights and other rooftop equipment the way a racking system can. This can radically limit the number of modules you can fit on a given roof. Also, being so close to the roof and at a low angle, the modules will build up more heat which lowers efficiency. Elevated racks allow air flow keeping the modules cooler.

1 comment:

  1. It was nice coming across the post. Thanks for sharing such informative stuff.