How High Above The Roof

sub3marathonman · December 2008

I am trying to figure out how important panel height above the roof is for panel efficiency in a relatively hot area like Central Florida.

Has anybody done research to show the efficiencies at say 1", 2", 3", etc.? I guess the main problem would be during the three or four hottest months.

Also, by raising the height further above the roof, aren't you also altering the wind resistance of the panels and putting more stress on the mounting hardware?

I am thinking that Northern Arizona Wind & Sun should know something about blazing heat.

Solar Guppy · December 2008

Re: How High Above The Roof

I have panels on both 2" and 4" off the roof, there a definite difference having more airspace, probably close to 10C on my roof anyways

As for wind loading, it makes no difference, wind loading is the lift effect ( think airplane wing ) and your PE will do the calculations for the permit based on mounting hardware

Windsun · December 2008

Re: How High Above The Roof

We recommend 4-6" for large arrays, and an absolute 2" minimum.

In large arrays even with 4" it can get to be like an oven under the panels in sunbelt areas with little breeze.

sub3marathonman · December 2008

Re: How High Above The Roof

Solar Guppy wrote: »

I have panels on both 2" and 4" off the roof, there a definite difference having more airspace, probably close to 10C on my roof anyways

That is what I was trying to figure out. So that 10°C would work out to almost a 5% difference for 2" less height (Pmax = -0.5%/°C). At least in the summer.

If it was always a 5% difference that would work out to 400 kwh/year if the array was producing 8000 kwh/year. At $0.15/kwh, that would be $60/year, or over 20 years $1200 dollars.

To me that is an incredible difference in return for such a seemingly small difference in mounting height.

niel · December 2008

Re: How High Above The Roof

it would be interesting to see the results of any testing along this line, be it scientific or not. also for different roofing materials, differing ambient temperatures and humidities, and maybe even roof pitch and pv inclination as to how they effect the temperature of the pvs.

crewzer · December 2008

Re: How High Above The Roof

I haven’t seen inch-by-inch incremental performance data. However, a six inch (15 mm) air gap seems to be the “magic” number for roof-top mounted arrays. For example, here’s a link to SMA’s discussion on “Mounting Style”: http://www.sma-america.com/en_US/service/glossary.html?0=

Home Power somewhat addressed this topic in their recent article “Module Mounts and PV Performance” (HP #124; April/May 2008, pg 61). Modules mounted at a 30 degree angle to a flat roof operated ~12 C cooler and performed ~6% better than modules mounted 4.25” above and parallel to a sloped roof.

HTH,
Jim / crewzer

sub3marathonman · December 2008

Re: How High Above The Roof

The SMA link was interesting, especially that their calculations say that a pole mounted array will only operate 10°C cooler than an array mounted less than 6" above a sloped roof.

I am wondering, since there is a cost to getting the additional 2", at what point does it become not worth it to do? Also, how often are the panels so overheated during the day and during the year?

You would think that somebody somewhere would have data for this.

mike95490 · December 2008

Re: How High Above The Roof

sub3marathonman wrote: »

You would think that somebody somewhere would have data for this.

I think there are so many variables, that a "standard" would apply to nobody.

"Gut" feeling says 6" is a good goal, with shorter heights causing more loss over the life of the system.

How High Above The Roof

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