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PV roof mounts

PV roof mounts

PV roof mounts

thread626-261703: PV Mounting Systems

My roof has a 52 panel PV array with a DC rating of 11.7 kW.

The standard installation of U-tracks are bolted to the roof, which has a 30 degree angle. My peak power production occurs in late March and September is 10 to 10.5 kW for the solar noon hour - my house is in New Jersey facing magnetic south (~178).

I feel like the SOLAR industry knowledge is based on yield tables and existing hardware developed for Florida and Arizona without any real understanding of the principles involved.

Do they make a roof mounting system that can be adjusted seasonly?

Can somebody explain to me how much I can increase my production if my roof angle was 40 degrees and how much more if I made seasonal adjustments?

Thanks in advance,

RE: PV roof mounts

As I recall, basic adjustablility could increase your yearly production by 15 to 25%, depending on the number of adjustments you make each year.

Continuous tracking can get you up to around a max of 38%.

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

RE: PV roof mounts

Thanks GregLocock for the link, but it didn't really answer the question.

Here is my annual production:
Date    total energy produced(kWh)
4/3/2010    1686.71
5/1/2010    1625.23
6/1/2010    1633.85
7/1/2010    1674.91
8/1/2010    1491.92
9/1/2010    1430.93
10/1/2010    1172.9
11/1/2010    951.5
12/1/2010    757.09
1/1/2011    808.44
2/1/2011    1121.64
3/1/2011    1500.35
4/3/2011    161.31
total    16016.78

Here is the table for Latitude = 40 Degrees North Month Sun
JAN     30 60 South
FEB     39 51 South
MAR     50 40 South
APR     62 28 South
MAY     70 20 South
JUN     73 17 South
JUL     70 20 South
AUG     62 28 South
SEP     50 40 South
OCT     38 52 South
NOV     30 60 South
DEC     27 63 South

Question : if I lowered the array tilt from 30 to 20 for the summer (May, June, July), what kind of production increase will I get?
1.6 MW-hrs to between 1.8 and 2.0?

I guess what I don't understand is you only get the best (normal) angle for the noon hour - if my roof was at 40 degrees instead of 30 degrees would my annual production really be better? or just better for a different set of months (more in March and less in April)

Thanks again for your responses


RE: PV roof mounts

You have to pay attention to the time that the sun is at each angle.  Yes you can make your tilt for optimum at 12:00, but you must also realize that it is only there for 1 second, hence it is better to optimize for the optimum power production angle for each day, the tilt angle for that day that produces the maximum daily power output, but then that changes day to day too, so it turns out better that you only change tilt about 4 times per year, but not every 3 months as you might expect.  You have to make more changes during the spring and the fall when then Sun is changing faster.  And you don't optimize the tilt for the maximum height of the sun in June either.  Turns out that its better to set it for May and leave it there till you change it again in late July, then make another change in early Sept another in lae October, then leave it there until April, or whatever is the optimum schedule for your latitude.

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

RE: PV roof mounts


I suggest going to NREL's PV Watts calculator http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

You can enter your system size, tilt angle, and orientation. PV Watts will give you estimated monthly energy production, based on 20 year historical irradiance and temperature data.  You can then go back and repeat the simulation with a different tilt and see how much your energy harvest changes.  

30 degree tilt is pretty good for a grid tied system like yours.  I suspect that even with NJ's lucrative SREC program, it won't be worth your time to make the angle of the racking system adjustable and then go out and change it on a regular basis.  Keep in mind that as you raise the array off the roof you will need to account for additional wind loading in your structural design.


RE: PV roof mounts


Thanks to all who responded!

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