Planning new install - 5 degree metal roof aimed north, need advice

[snowmonster]

First post. Have enjoyed the reading so far here.

I've got new construction with a standing seam metal roof that I want to put solar on. It was designed to support a solar array 45' deep x 42' wide. My latitude is 40.4259 in Indiana. The key problem here is that the roof here has a 5 degree slope to the north and I do have to worry about snow in the winter.

I could post all the details of electric bills and system size, etc. But is my best option to simply install more panels on a fixed/flat roof mount and simply deal with less production? I know I would have to worry about dirt/etc and it would be more difficult to deal with if a panel in the middle goes out. Or am I better of slightly tilting (5 degrees using 10 degree tilt legs) the panels in the opposite direction (toward the south) and deal with shading from neighboring rows and snowbanking in the winter? I've looked at the row spacing calculators and they don't take into account the fact that each row farther north is lower than the previous because of the roof level. It seems like shadowing is a major issue with this as the tilt legs get taller.

PVwatts isn't really that bad for a 5 degree roof aimed north so I'm guessing I'm best off with a flat roof mount and more panels? I know it would be massively better with a 25 degree southern facing array but that's not realistic for this application.

Thanks in advance!

[Mike90250]

Do you have space for a pole mount or a ground mount ?

[snowmonster]

Do you have space for a pole mount or a ground mount ?

Unfortunately no. It's actually office space and we maximized the footprint. I guess pole mount is theoretically possible but I'm looking at a 20kW+ system and that would be crazy. The roof is the only option and something is better than nothing in my mind.

[Mike90250]

Does the power harvest projection pencil out? How many years to break even ? And you understand there would be no power from the array in case of a Grid Outage?

Will array setback rules apply in your case (3 foot walkways for firefighter access) Do you have pre-apprval from the utility for a project over 10KW ? Will that turn you into a commercial power producer with resulting low selling rates ?

[snowmonster]

Based on PVwatts, it doesn't look that bad to me even with a 5 degrees north sloped roof. I figure 5 years tops to break even. Power outages in my area aren't common and I'm fine with an occasional day of being closed. We've had 2 days of that in the past 14 years. An appropriately sized battery bank would be cost prohibitive. The dimensions provided did account for a 3-4' setback for firefighter access. I'm in Indiana with potentially bad net metering laws going forward. I'm looking to cover expenses but do not depend on reimbursement for overproduction.

[J.P.M.]

1.) You mention some things you'll need to address, but how do you plan to deal with them ?

2.) If the roof was designed for a large solar array, why was it built facing north ?

3.) As a first approx. to address the row pitch vs. tilt vs. shading for tools that don't allow north facing roofs/support surfaces, use a horizontal roof and a (5 + 5) = 10 deg. tilt angle for the rows for a 5 deg. southerly tilt.

(Note to Ampster: This is an example of why the distinction between flat and horizontal might be useful to help avoid confusion)

4.) As a 1st est., an approximate fact of life: Before losses to snow cover and greater fouling, annual production out of any array with rows in a sawtooth pattern on horizontal surfaces such as a roof will be about the same as an array covering the whole roof with no gaps that has a horizontal orientation parallel to the roof. The same approx. actually applies to sawtooth arrays on surfaces of most any angle.

5.) More 1st approx. stuff: Your optimum tilt (from the horizontal - not the roof) will be ~ 40 deg., maybe a bit less. Using #4 above, set the row pitch for no self shading from the array with the panels at a 40 deg. tilt (from the horizontal - not from the roof). Or, better yet, use PVWatts and get an optimum tilt and azimuth and then set the row pitch for either minimum shading or most cost effective row pitch - the two are probably not the same - using a shading tool of your choice.

6.)The optimum tilt will minimize the array size and maybe save a few bucks in material. The 40 deg. or so tilt will mean a beefier support structure, a fair amount of engineering and quite possibly a roof/structure that you might find inadequate to the task with respect to applied/imposed loads from wind forces.

7.) A lower tilt will mean more array area required to meet whatever duty you're designing for and if a low enough tilt to get the wind loads low enough for the supports and building to handle, you may not have enough array production to meet the duty you've designed for

8.) Whatever you do, and if you plan on getting a reasonable amount of production out of the array during snow season, I'd suggest you get some advice from folks like Bruce here with respect to living with an array in snow country and ignore what he writes at your peril - at least with respect to designing arrays for snow.

9.) If you haven't done so already, I'd suggest you download a slightly dated version of "Solar Power Your Home for Dummies", or get a revised (newer) version for ~ 20 bucks at bookstores or Amazon and then get familiar with PVWatts. If you had done those two things first, your plans might well have been different. As it looks to me (only), from what you've written so far, you'll have a less than fit for purpose design. Maybe it can be salvaged with some education.

10.) If it was me (and it ain't) and I still lived in snow country (I don't), if I considered doing PV at all in snow country, particularly of that size, I'd first reconsider doing it, and then, if I continued, I'd first consider mounting panels on the south wall of the building in adjustable leanto fashion and consider summer shading from eaves/gutters and use about 3-5 ft. of ground clearance for snow and then maybe mount a row or two of panels sawtooth fashion on the roof with the same 3-5 ft. elevation clearance for snow. I'd then see how it goes and then perhaps consider adding rows as you see fit until you meet your required duty or you run out of roof.

Good luck.

Welcome to the neighborhood.

[solarix]

Panels mounted at less than 15 degrees of tilt get dirty and stay dirty. Your 5 degree roof should be treated as a flat roof and put tilt racks on a 10ft spacing. We use 20 degree tilt here at 34 deg latitude, but you'll want more tilt up north and maybe a lot of tilt to deal with snow.

[littleharbor]

Would not a reverse tilt in snow country be a big snow trap?

[snowmonster]

All great points I knew I would get from this site.

Agreed that the roof wasn't completely designed for solar as it has a 5 degree northern slope. I meant to say that structurally it can support the weight of an array.

I sat down and did some math/trig/geometry this AM and determined I can use 20 degree tilt legs (for an effective 15 degree tilt) and 5.5' row spacing with zero shadows from neighboring rows and still get 59 panels on the roof. This at least keeps the panels aimed in the right direction and reduced fouling. I know a 40 degree tilt is optimal but I would only have 2 rows of panels if that were the case. It is interesting to note that Renvu's row spacing calculator says I can get away with 27" spacing with landscape orientation but clearly there will be shading issues. It is an office building and the front of the building faces south - the appearance of panels on the front of the building isn't what we are going for.

I did mention the snow thing but we don't really get the crazy lake effect snow like you'll see in Michigan. I don't expect to have all that much production in Dec/Jan anyway so I'm not terribly worried about that.

PVwatts info:
Flat system : 24.0 kW system mounted flat on the 5 degree north facing roof = 27,567 kWh annually but fouling is certainly an issue and difficult to clean
15 degree southern tilt: 20.4 kW system with an effective 15 degree southern tilt (using 20 degree tilt legs) = 27,682 annually

It was interesting to me that there is only 384 kW difference in December between the two but I know a 15 degree tilt doesn't accomplish much.

Working on different configurations/row spacing/tilt angles now.

[J.P.M.]

Would not a reverse tilt in snow country be a big snow trap?

More than a horizontal roof and among other factors, probably as some f(angle between roof slope and array slope, roof slope). A somewhat absurd example: A 45 deg. slope single row array elevated, say, 2m off a 45 deg. north tilting roof for example, but common sense hopefully intervenes at some point.

[Ampster]

(Note to Ampster: This is an example of why the distinction between flat and horizontal might be useful to help avoid confusion)
....

Confusion?
There has only been one person on this forum who has been insisting that most roofs are flat. Perhaps that person is still confused. It is hoped that the following attachment, which was posted by a moderator some time ago, will clear up any confusion. I presume the red circle was added by the moderator for emphasis.

Hopefully that ends the need for further discussion. Sometimes one's beliefs do not correspond with generally held understanding of nomenclature in the construction industry.

[Ampster]

.....

Agreed that the roof wasn't completely designed for solar as it has a 5 degree northern slope. I meant to say that structurally it can support the weight of an array......

I thought that was what you meant by "support". One of the advantages of attaching solar to a standing seam roof is that you don't need to penetrate the roof to attach the racking. Weight is only one design factor. Uplift is another that will increase as you increase the tilt.

[snowmonster]

I thought that was what you meant by "support". One of the advantages of attaching solar to a standing seam roof is that you don't need to penetrate the roof. to attach the racking. Weight is only one design factor. Uplift is another that will increase as you increase the tilt.

That is why I chose that roof style. The uplift issue is in the works as well now that I'm looking at a 30 degree tilt relative to the roofline.

[solarix]

Some building departments will want you to prove that your standing seam roof has adequate fasteners (as in - lots more than usual) to withstand the additional uplift caused by a solar wing attached to the seams.

[J.P.M.]

All great points I knew I would get from this site.

Agreed that the roof wasn't completely designed for solar as it has a 5 degree northern slope. I meant to say that structurally it can support the weight of an array.

I sat down and did some math/trig/geometry this AM and determined I can use 20 degree tilt legs (for an effective 15 degree tilt) and 5.5' row spacing with zero shadows from neighboring rows and still get 59 panels on the roof. This at least keeps the panels aimed in the right direction and reduced fouling. I know a 40 degree tilt is optimal but I would only have 2 rows of panels if that were the case. It is interesting to note that Renvu's row spacing calculator says I can get away with 27" spacing with landscape orientation but clearly there will be shading issues. It is an office building and the front of the building faces south - the appearance of panels on the front of the building isn't what we are going for.

I did mention the snow thing but we don't really get the crazy lake effect snow like you'll see in Michigan. I don't expect to have all that much production in Dec/Jan anyway so I'm not terribly worried about that.

PVwatts info:
Flat system : 24.0 kW system mounted flat on the 5 degree north facing roof = 27,567 kWh annually but fouling is certainly an issue and difficult to clean
15 degree southern tilt: 20.4 kW system with an effective 15 degree southern tilt (using 20 degree tilt legs) = 27,682 annually

It was interesting to me that there is only 384 kW difference in December between the two but I know a 15 degree tilt doesn't accomplish much.

Working on different configurations/row spacing/tilt angles now.

1.) The dead weight of most any array by itself (probably < ~ 5lbm/ft^2 of array or so) is not much of a problem for most any roof (often/usually a roof has a design dead load of ~ 40 lbm/ft^2 or more), but all required load combinations still need checking. The uplift and downward loads from wind on a well designed roof need checking and design consideration(s). Structurally, the common weak points or at least points of concern in a rooftop PV array design are the array supports, their attachment method(s) to the roof, and the resultant point loads and their effect(s) on the roof at those attachment points and on the roof in toto. If your jurisdiction requires permitting for PV, you may well run into a requirement to provide calcs/drawings to show that the design is fit for purpose with respect to those and possibly other loading conditions and combinations.

2.) If you already know this, ignore it, but using a 15 deg. off horizontal tilt on a 5 deg. north sloped roof, if you're designing for a row pitch that, for example, avoids ALL (self) array shading from the more southerly rows, between 0800 and 1600 hrs. solar time on the winter solstice, for the Renvu tool, you'll need to use a "sun angle" as Renvu defines it of about 8 degrees or so - that's your approx. solar elevation angle at 0800 or 1600 hrs. on ~ 12/21 at ~ 40.2 deg. N. lat.
That'll give a row pitch of ~ 220 " w/panels in portrait or ~ 139" in landscape. So, landscape orientation gets you a tighter row pitch but fewer panels per row in about inverse proportion to the ratio of panel L X W ratio and thus an approx. wash in max. allowable array area per roof area. I'd note that the landscape orientation may result in a lower design wind loading for about the same area.
Other stuff I've got more/less agrees w/the Renvu tool. One product/model is something called "SAM" from NREL. Given good input, it does a fair/good job on shjading, but it's a somewhat long learning curve.

3.) If you can live with some shading, using a 16 deg. "sun angle" will get you shade free from 0900 to 1500 on the winter solstice at your approx. location and doing so may not cost too much in terms of annual output, particularly if you're not counting or expecting much winter production. Looks like that row pitch drops to ~ 140" for portrait orientation and 86 " or so for landscape panel orientation. Again, the lower wind loading in landscape may be a consideration.

4.) FWIW, my numbers for annual output pretty much agree with what you report. I'd note those #'s seem in line w/my comment that, to a 1st approx., the max. output of an unshaded sawtooth array on a horizontal roof is about the same as the output of a horizontal array on that roof.