Elevation angle question?

icarus

This is a dumb question, that I know the empirical answer to, but I ask it because it might be a source of confusion.

When we build houses, we use a roof angle stated as a ratio to express a pitch,, say 4/12, where 4=rise in inches, and 12=run in inches. We can then convert this to degrees, in this case 4/12=18.43 degrees. This relationship is 18.43 degrees FROM the HORIZONTAL.

In solar siting angles, we talk about elevation tilt. For fixed tilt we consider latitude, +-say 15 degrees for seasonal changes in sun elevation. So in this example, if we lived at 45degrees N, we would tilt our array at 45 degrees +- (30-60 degrees). But this number must be from the VERTICAL axis.

In other words my above roof (4/12) would be 71.57 degrees from the Vertical. Am I right in this assumption, and if so, people need to pay attention to this discrepancy in nomenclature.

If I am wrong in my assumptions, I will be mildly embarrassed, but I hope someone will correct me even so!

Tony.

All the small installations I have done, we have done using our eyes and our real world to guide us. Some panels have been on fixed roofs that are of about the right angle. Others are on wall mounts with hinges to adjust the angles for the seasons.

BB.

Re: Elevation angle question?

Tony,

I am not sure I understand your question... And, unfortunately, you picked 45 degrees--so it does not matter if measured from horizontal or vertical--the answer is still 45 degrees.

Tan^-1 (rise/run) = degrees from Horizontal.

The +/- 15 degrees is relative to whatever your mounting angle.

For example, I am at N37 degrees. Assume my roof is 4/12 then, the angles for a fixed array should be:

Roof Angle is 18 degrees

Mount angle above roof would be 37-( 18 )=+15 degrees higher than roof (for year round)

Mount angle for summer would be 37-(18+15)=+4 degrees higher than roof

Mount angle for winter would be 37-(18-15)=34 degrees higher than roof

Referenced to a vertical reference plane (vs a horizontal reference angle):

90-(horz. ref. angle)=vert. ref. angle

90-(37)=53 degrees from vertical (year round collection)
90-(37-15)=68 degrees from vertical (summer optimum)
90-(37+15)=38 degrees from vertical (winter optimum)

Notice that I used parentheses to make the problem a bit more clear--It keeps the sign of the tilt flatter (-15 degrees) for summer and tilt higher (+15 degrees) for winter.

the whole "90-(x)" is the conversion from horizontal reference frame to the vertical reference frame--It is the sign change of the winter/summer offset of 15 degrees that gets confusing if I wrote the equations without parens as:

90-37=53 degrees from vertical (year round collection)
90-37+15=68 degrees from vertical (summer optimum)
90-37-15=38 degrees from vertical (winter optimum)

Then it makes the whole winter/summer offsets "look like" they are backwards/wrong.

To keep this straight in my head--I think of the extremes... Equator (panel flat to ground) and North Pole (panel vertical), then think of the angles I want to measure/reference.

Is this what you where asking?

-Bill

icarus

Re: Elevation angle question?

Bill,

Unwittingly by choosing 45 degrees Lat. I made the problem less easy to see! Math has never been my long suit,,, and I stand embarrassed! Playing around with the PV watts calculator illustrates it quite well.

So for a location of 45 degrees north, say Portland OR, a 12/12 roof angle would be the ideal year round elevation,,, 45 degrees!

Seattle would need a steeper pitch, say 47 degrees, or about 13/12! (I have built a house with a 17/12 pitch once)!

I understand your example.

Your suggestion to think of the equator and the pole helps keep it clear in ones mind!

Tony

BB.

Re: Elevation angle question?

Tony,

I thought in the land of snow, you guys either had steeply pitched roofs or had to get up there with a snow shovel every once in a while.

-Bill

icarus

Re: Elevation angle question?

Snow is both an enemy and an ally.

My roof is 8/12 on the front, and a fairly flat 4/12 on the back. This was due to some design considerations. The 8/12 holds snow because of it's grip on the asphalt, but when I replace it with standing seam it will slide quite quickly on the front. The back will hold snow all winter, and does get quite deep. I over engineered the roof details that 3-5' of snow, even with some rain on it should be fine. Even with standing seam it probably won't slide until spring. One advantage of the snow, is a significant insulation factor on the roof! Also, the snow shoveled up tight against the foundation skirting keeps the floor much warmer. In winters with little snow, we burn way more wood, even if it is warmer net/net. Also the snow sliding on the steep side presents it's own hazards. Large snow slides will rip off gutters, damage decks etc.

I grew up having to shovel several building roofs several times a winter. I swore whenI built this new place, I was going to make it so that I never had to go on the roof in the winter. There are two small PV panels on the roof, just over the peak. They clear themselves after a part day if we have only had a few inches. A foot requires me to jump up and give them a sweep. I am considering moving them down in the winter to another portable rack on the ice,,,,maybe next year.

Tony

Moe

A Visual Answer...

using my 40ºN latitude

crewzer

Re: Elevation angle question?

Tony,

I got a bit lost in the discussion above... But, just to clarify, a PV array's "tilt" angle is measured from the horizontal.

Example 1: A PV array on the Equator (0 degrees lat.). Placing the array at 0 degree tilt, i.e., leaving it horizontal, will position the array perpendicular to the Sun's rays on ~Mar 21 and ~ Sep. 21.

Example #2: My PV array (39 degrees N lat.). Placing the array at 39 degree tilt from the horizontal will position the array perpendicular to the Sun's rays on ~Mar 21 and ~ Sep. 21.

HTH,
Jim / crewzer

icarus

Re: Elevation angle question?

Jim,,

For some reason, I was reading something yesterday, and had a very senior moment!

I would suggest to the MODs that they delete this entire thread, except your answer "Solar array 'tilt angle' is measured from the HORIZONTAL!" Just like roof angle is measured from the HORIZONTAL!.

Which was the question!

Tony

PS Sorry for all the confusion!