Inter-Row Spacing for roof mounted solar

[Animag771]

I'm planning out 3 rows of panels on my roof, adjusting twice a year. I've tried doing the trigonometry to figure everything out, but I'm getting confused. I'm not normally someone who likes to be spoon fed information, but I'm stumped here. All of these angles are throwing me off and I'm not sure if the angles are based on the sun's angle or the horizon for some of these calcs. To add to the confusion, all of the info I have found is for ground mounting and I'm mounting on a sloped roof.

I used this site for my math:

https://www.civicsolar.com/support/installer/articles/determining-module-inter-row-spacing

The panels are 65" in length and my coordinates are 33.16, -97.76 and my roof slope is 14 degrees.

So what I did was take my winter angle of 33.41 degrees on December 21 and subtracted my roof angle, which gave me my new angle of 19.41 degrees.

Using this information I got a height difference of 22" which gives me a module row spacing of about 64". Finally I account for the difference in azimuth angle of 43 degrees and I end up with a minimum module spacing of about 47".

I've done the math, but it doesn't seem right. With all of the confusion going on between my wife (the math teacher) and me I think we messed something up.


Will someone who actually knows what they are doing PLEASE take a look at this and check if it's correct? I think I've given enough information to plug the numbers in and figure out what the correct winter angle and minimum panel spacing should be.

[Animag771]

OMG, half of my post disappeared when I submitted it... Fixing now.

Edit: Fixed.. Apparently it doesn't like the degree symbol.

[Mike90250]

Generally, roof mounts follow the slope of the roof. It's easier, cheaper and generally more robust, to add panels, rather then try to tilt rows on a roof. Snow dams, wind issues and shadows are all problems caused by tilting roof mounts.

[Mike90250]

OMG, half of my post disappeared when I submitted it... Fixing now.

Edit: Fixed.. Apparently it doesn't like the degree symbol.

Sorry about that, the forum software has a anti-spam feature that looks at the font and char set to try to decide if a post is legit or not. spam often includes weird fonts and mixed character sets.

[Animag771]

Generally, roof mounts follow the slope of the roof. It's easier, cheaper and generally more robust, to add panels, rather then try to tilt rows on a roof. Snow dams, wind issues and shadows are all problems caused by tilting roof mounts.

I appreciate the response, though it kind of ignores most of my post. I do understand that adding panels would be easier than all of this math and the effort of having to go up on the roof to change the tilt angle multiple times a year. Snow dams aren't an issue in TX and the only shadow problems I'll run into is from the panels themselves. For me however I would rather optimize the panels instead of just adding more and I don't mind the additional work involved. I also think retrofitting my own adjustable tilt system would be much cheaper than adding panels.

[J.P.M.]

I'm planning out 3 rows of panels on my roof, adjusting twice a year. I've tried doing the trigonometry to figure everything out, but I'm getting confused. I'm not normally someone who likes to be spoon fed information, but I'm stumped here. All of these angles are throwing me off and I'm not sure if the angles are based on the sun's angle or the horizon for some of these calcs. To add to the confusion, all of the info I have found is for ground mounting and I'm mounting on a sloped roof.

I used this site for my math:
https://www.civicsolar.com/support/i...er-row-spacing

The panels are 65" in length and my coordinates are 33.16, -97.76 and my roof slope is 14 degrees.

So what I did was take my winter angle of 33.41 degrees on December 21 and subtracted my roof angle, which gave me my new angle of 19.41 degrees.

Using this information I got a height difference of 22" which gives me a module row spacing of about 64". Finally I account for the difference in azimuth angle of 43 degrees and I end up with a minimum module spacing of about 47".

I've done the math, but it doesn't seem right. With all of the confusion going on between my wife (the math teacher) and me I think we messed something up.


Will someone who actually knows what they are doing PLEASE take a look at this and check if it's correct? I think I've given enough information to plug the numbers in and figure out what the correct winter angle and minimum panel spacing should be.

I think I might know a little about it but my rate was $250/hr. before I retired (but dumb looks were always free). I can however, offer a point in a direction or two that may aid self help, but that's because I'm a lousy enabler.

Since you're not someone who likes to be spoon fed, get yourself a copy of Duffie & Beckman for accurate, fundamental and informative information. D.& B. will get you the answers you're looking for but will not spoon feed you. This is not a one line calc, but it is mostly chap. 1 stuff. Besides some persistence (but not too much), high school math is all you'll need.

Dirty/cheap trick: Sometimes it also helps to "move" a sloped roof to a different latitude (but the same longitude) while keeping the same absolute roof slope (and azimuth). The "new" latitude will be = your latitude - the roof slope. That will put your roof at a horizontal orientation at the new location. Then, put the array on it. Your new latitude will be 33.16 - 14 = 19.16 degrees. Then use D & B. to get lowest useful solar altitude and az. on Dec. 21 or whenever time/date you want to consider at that new latitude. Also calc something called the solar profile angle at a chosen time ( like, say 1600 hrs. solar time on Dec. 21) and see what happens to it at various array tilts, again, all on that horizontal surface and get solar angles as calc'ed' at that adjusted latitude.

Second trick/rough guess/more SWAG/rule of thumb (with the latter mostly for calculating thumbs, but maybe worth noting): Sawtooth array pitch and/or tilt adjustment has a goal of avoiding (mostly) winter shading or seeing what the possibilities are by those adjustments. On a horizontal roof (which you now have at the "new" latitude) get the annual output for a horizontal array the same size as your (horizontal) roof for your actual location for the winter solstice month, or clear sky 12/21 output. Either way, that's about what the output will be for a sawtooth array on that (horizontal) roof with a row pitch that's wide enough to just avoid most shading on 12/21. Sometimes useful as a 2d check.

Not exact, but closer than you might think.

All that aside, I second what Mike writes. Not trying to be a suck up here, but he probably forgot more than most who come here looking for answers will know for some time. I lived in the sun now but spent most of the first half century of my life in/around central western NY state. Suit yourself, and I appreciate your location and climate, but tilting an array on a roof will probably not come to a good end in any climate without planning for the consequences of wind and it's effects. And for all that, most every time, the output won't increase all that much. If nothing else, taking an array off it off a parallel to the roof orientation or changing it at all will change the wind loadings and most likely cause some uplift problems or other considerations.

I'm about the biggest fan of R.E. I know of and I want it to work, but there are limits.

[DanS26]

Another calculation you might want to make is the damage to your roof by walking around vs the extra value of production.

If you shorten the life of your roof by ten years you have gained nothing.

[bcroe]

Having just built a tilting array support, I think you will find building and maintaining one will be
a lot bigger project than first appearances. Given its on a roof, its a good candidate for electric
activators so you don't need to go up there. As for calculating the angle/spacing, sometimes I
just draw up an accurate graphical solution which will do better than one decimal place.

I have been in TX when there was snow. good luck, Bruce Roe

[DanS26]

If you don't like math......build a scale model on a piece of cardboard, then using a spotlight as the sun.......watch the shadows.

[ImInPhxAZ]

Put what you're trying to accomplish into PV watts vs a flat mount. You'll find interesting results, especially for being Texas. Here in AZ, PV watts for summer showed less than 5% difference being 290 degrees vs 200 degrees. For winter, it was more like 25%. However, when looked at annually, it was something like 17% reduction by not having optimal tilt and orientation. And that's a big orientation difference.

When I did the math, the extra hardware to accommodate tilting, the extra expense for a structural engineer to certify wind load, (panel = wind sails), the higher wind noise the higher chance for unsightly bird nesting underneath (harder to keep birds out), and the general appearance of them sticking up, simply wasn't worth it. Opted to upsize array 20% extra cover needs and keep it simple and flush. Need more power in summer anyway, so the curve works better this way too.


Model both your potential layouts in pvwatts, you might be surprised how little you lose with orientation down here in South, and how that 20% more array will be more helpful in summer than the 25% extra power in winter.

[ButchDeal]

The inter row spacing on tilted modules is also going to drastically reduce the number of modules yu can fit on the roof due to shadowing from each other. When you count that you can fit more modules as well as all the other benefits others have already pointed out, installing without a tilt on the roof is far better.

[J.P.M.]

Put what you're trying to accomplish into PV watts vs a flat mount. You'll find interesting results, especially for being Texas. Here in AZ, PV watts for summer showed less than 5% difference being 290 degrees vs 200 degrees. For winter, it was more like 25%. However, when looked at annually, it was something like 17% reduction by not having optimal tilt and orientation. And that's a big orientation difference.

When I did the math, the extra hardware to accommodate tilting, the extra expense for a structural engineer to certify wind load, (panel = wind sails), the higher wind noise the higher chance for unsightly bird nesting underneath (harder to keep birds out), and the general appearance of them sticking up, simply wasn't worth it. Opted to upsize array 20% extra cover needs and keep it simple and flush. Need more power in summer anyway, so the curve works better this way too.


Model both your potential layouts in pvwatts, you might be surprised how little you lose with orientation down here in South, and how that 20% more array will be more helpful in summer than the 25% extra power in winter.

Just keep in mind that low tilts will foul quicker and so suffer more from reduced output due to soiling/dust with very low/horizontal tilts turning into mud pans.

[peakbagger]

Not sure if you are in area with snow but realize snow and adjustable tilt collectors is a bad mix. Snow can slide down the upper row and end up behind the lower row and once there it takes quite awhile to melt, adds a lot of snow load to the roof and also can add to potential ice damming along the lower edge of the roof. Standard PV array design tools dont work for tilted panels as the wind loading is significantly higher with tilted panels. A PE can do the math and charge you accordingly to put a stamp on it.

I do have 2 KW pole mount that is manually adjustable. I designed it and didnt need a PE stamp design when I installed it. I set it up counterbalanced around a central pivot point so I can adjust the tilt in about 3 minutes with two tools by myself. My wall mount is also adjustable but I need a hydraulic floor jack and a long 2x8 that is adjustable length to adjust that array. Its a small array, I mostly adjust it as it shades an angle bay window in the summer and cuts my AC load.

[Wy_White_Wolf]

You did a good job of figuring the distance for solar noon on Dec 21st. But is that the only time you want full power that day? Might want to rerun the calculations using the suns elevation at say 9am and 3 pm to give them a window that they could have full sun instead of just a few minutes. There are a few sites that will give you the suns elevation based on time of day.

WWW

[Animag771]

Well I'll admit it... Everyone is right. Doesn't make sense to tilt the panels for the roof. I just don't have the room for it. I've done some more math (I hate trigonometry) and I've figured out that in order to tilt the panels it would take up the majority of my roof space and there would be no room left for additional panels due to the spacing. So I guess I'll have to mount them along the slope. I built my place with a single roof slope to the south and it's only 22x32, so for now I'm going with 3 rows of 5 panels in portrait. This will give me 15 panels now and if they aren't producing enough for my needs at least I'll have enough room left over to mount up to 10 more in the future. I suppose I could still find a way seasonally tilt the highest 5 panels to eek out just a bit more power, but it may not be worth it, who knows.


Kind of sucks though... I'm big on trying to maximize the efficiency of what I've got, versus just getting more stuff. Heck, that's why I'm building my house as small as it is. I guess in this case though, the most efficient way to do it is by leaving enough room now to add more later.