Unirac SOLARMOUNT options

[jpoet]

Being in Albuquerque, it seems obvious that my installer is using Unirac. Since I just have a typical asphalt shingle roof, it looks like they can just lag the L-brackets right to the roof.

I have read that there is a significant benefit to having the panels "off" the roof by at least 5 inches. This week, temps here are topping 100^oF, and it will likely be at least that hot in July and August.

My installer was planing on the SOLARMOUNT product with the 'light' rails, but based on http://unirac.com/wp-content/uploads...etails_104.pdf the maximum height with that configuration would only be 3 ¹¹/₁₆ inches. If I make them use the "standard" rails, then that would get me up to 4 ¹/₂ inches. Another option would be to make them use stand-offs, but that significantly increases the cost.

If anyone has any thoughts or opinions, I would love to read them.

Thanks,

John

[emartin00]

Stick with the standard L brackets. The extra inch isn't going to make or break your efficiency, and it's going to cost more to install.

[sensij]

The NOCT temperature model used in NREL's System Advisor Model increases the NOCT by 2 deg C for 2.5"-3.5" of roof clearance. More clearance than that, and the model leaves NOCT alone, making no distinction between roof mounted and open rack ground mounted in this respect. So, the benefit of increasing the height beyond the 3-11/16 may be small.

That said, I'm not sure that bolting the L-foot directly to the shingles is considered a best practice. You might want to look into using something like Quickmount PV's QBlock, or Iron Ridge's Flash Foot. It could add $15 or so per penetration, but will give you more height and should be better waterproofing.

[J.P.M.]

Flush mounted (zero clearance) to a roof is not advisable for a lot of reasons besides array temp. considerations. For most situations, 4" is about a reasonable minimum clearance for roof mounted systems with respect to air circulation lowering array temperatures. For air circulation, more clearance is better, but the benefits to lowered array temperatures by increasing that clearance beyond about maybe 6" or so for a moderately sloped roof of, say, 20 deg. slope or thereabouts is probably not as much as increasing the clearance from, say, 3" to 6".

FWIW, my array's clearance is somewhere between 9" and 11.5 ", depending on where/how its measured, but that was by design for purposes other than, and in addition to air circulation and array temp. considerations.

More FWIW, and this is little more than anecdotal because I don't have a hand held anemometer (yet), but the wind velocity at the downwind side of my array on the underside seems to be something like about 1/2 as great as the wind velocity at the same location but on the front (top) surface of the array. Seems a matter of some common sense that the racking and restricted area under the array is most responsible for those observations. If I get more curious about it, I'll look in the open literature for air flow under an array relative to free stream flow over a flat plate.

Depending on your roof shingle/tile material and color, orientation, wind vector and other things, expect a max. (array-roof) temperature difference of, depending on roof color, slope, orientation and wind vector of something like 25 C. to 35.C. above the ground level air temperature. The higher temp. diff. for a dark concrete tile roof in relatively still air, the lower temp. for lighter colored roof, and moderate to higher wind vectors. Since 04/10/17 through 06/18/17, on clear days (with N= 38 measurement days), at the time of minimum daily incidence angle, my array's average max. daily temp. diff. (array to ambient air temp. on the roof) has been 28.7 C., with the amb. temp. on the roof being ~ 3-5 C. or so above ground temp., depending on conditions. Average coincidental wind velocity was 1.6 m/sec., average P.O.A irradiance was 1,016 W/m^2. You can probably expect a bit higher temp. difference due to greater irradiance as a result of being about a mile above sea level, but I'm unaware of your other particulars, or what the design may turn out to be.

On mountings: NOMB or property, but if it was me, I'd use double flashed standoffs. Roof penetrations are the last place to cheap out, especially with so many of them.

[jpoet]

As far as mounting to the roof, I just found http://unirac.com/wp-content/uploads...LLATION_45.pdf
As long as they follow that, I should be okay, right? The diagram showing it would be 3 ¹¹/₁₆ for the light rail did not include that "Square Aluminum Piece", but I don't know how thick that is. I will ask Unirac.

Going with stand-offs seems to greatly complicate it: http://unirac.com/wp-content/uploads...LLATION_47.pdf
Using Uniracs design tool, that also seems to add about a thousand dollars to the cost, depending on the configuration.

I leave near the foot hills of the Sandia Mountains and there is an empty lot on that side of my house. The result is that I can get some pretty significant wind from that direction. One of my arrays will be on that side of the house and I have around 23 degrees of pitch to the roof. My roof is only about 5 years old.

Thanks guys.

[jpoet]

Unirac says that "Square Aluminum Piece" is 0.25" thick.

[sensij]

As far as mounting to the roof, I just found http://unirac.com/wp-content/uploads...LLATION_45.pdf
As long as they follow that, I should be okay, right? The diagram showing it would be 3 ¹¹/₁₆ for the light rail did not include that "Square Aluminum Piece", but I don't know how thick that is. I will ask Unirac.

Yes, if they are using that flat flashing, I'd just go with that.

[jpoet]

Thank you, Sensij. I have verified with them, that that is what they will be using.