PV roof mounts

[mauner]

thread626-261703

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,

 

[GregLocock]

I could but it isn't very interesting. here's a calculator

http://www.wattsun.com/misc/photovoltaic_tilt.html

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[IRstuff]

It should be pretty clear that given the Earth's tilt, the angle change has to be 46º during the course of the year.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[BigInch]

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

 

[IRstuff]

If you split the difference, i.e., line up the panel in the middle of the range,, cos(23deg) = .921, so the view factor change is less than 8%. Most antireflection coatings are angle sensitive, so you might lose a few more percent at the extremes.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[mauner]

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
MONTH SUN ARRAY TILT
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

 

[IRstuff]

cos(23deg) = .921

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[BigInch]

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

 

[jfpe]

Mauner,

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.

-John

 

[mauner]

Thanks to all who responded!