Direction of panels to sun

scheek · October 2013

I have 8 panels approx. 120 watts each and in series. I have mounted the panels on poles but have not locked them into place. Is it good to have half in a more SE direction for morning and noon times and some favoring a SW direction for afternoon.

I live in Mississippi where sun is lower ( more southern) in winter and higher in the summer. I have them on about a 45 degree angle.

Thanks.

BB. · October 2013

Re: Direction of panels to sun

Not a hard question... Use PV Watts and pick a city near you. Say Meridian Mississippi. Fixed Array, and play with different angles/directions and see what you get.

Here is the PV defaults with 1 kWatt array (smallest PV Watts will let you enter) setup for 77% derating (Grid Tied system). You can use 0.52 if this is an off grid system.

"Station Identification"
"City:","Meridian"
"State:","Mississippi"
"Lat (deg N):", 32.33
"Long (deg W):", 88.75
"Elev (m): ", 94
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 0.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 32.3"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.55, 82, 6.72
2, 4.86, 101, 8.28
3, 4.92, 110, 9.02
4, 5.69, 120, 9.84
5, 5.63, 119, 9.76
6, 5.60, 112, 9.18
7, 5.68, 117, 9.59
8, 5.48, 113, 9.27
9, 5.11, 104, 8.53
10, 5.24, 114, 9.35
11, 4.27, 92, 7.54
12, 3.85, 90, 7.38
"Year", 4.99, 1273, 104.39

Now, lets tilt the panel to 45 degrees (+17.3 degrees to above settings):

"Station Identification"
"City:","Meridian"
"State:","Mississippi"
"Lat (deg N):", 32.33
"Long (deg W):", 88.75
"Elev (m): ", 94
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 0.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 45.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.71, 85, 6.97
2, 5.00, 104, 8.53
3, 4.84, 108, 8.86
4, 5.36, 113, 9.27
5, 5.12, 107, 8.77
6, 5.00, 99, 8.12
7, 5.11, 104, 8.53
8, 5.10, 105, 8.61
9, 4.94, 101, 8.28
10, 5.33, 116, 9.51
11, 4.48, 97, 7.95
12, 4.11, 96, 7.87
"Year", 4.84, 1236, 101.35

Vs a full 2-axis tracking system (the best you can ever do):

"Station Identification"
"City:","Meridian"
"State:","Mississippi"
"Lat (deg N):", 32.33
"Long (deg W):", 88.75
"Elev (m): ", 94
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 0.8 kW"
"Array Type: 2-Axis Tracking"
"Array Tilt:","N/A"
"Array Azimuth:","N/A"

"Energy Specifications"
"Cost of Electricity:"," 8.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.22, 97, 7.95
2, 6.08, 128, 10.50
3, 6.10, 138, 11.32
4, 7.45, 160, 13.12
5, 7.54, 162, 13.28
6, 7.72, 158, 12.96
7, 7.82, 164, 13.45
8, 7.04, 148, 12.14
9, 6.29, 130, 10.66
10, 6.51, 143, 11.73
11, 5.30, 116, 9.51
12, 4.84, 113, 9.27
"Year", 6.41, 1656, 135.79

As you can see, more vertical hurts in summer and helps in winter. And you can compare with a full 2-axis tracking system to see the "ideal".

And you can play the same game with SE/SW orientation too.

Many people will lay a system at -15 degrees (more flat) in summer and +15 degrees for the winter. Some people, after a few years will not bother with manual tilt any more.

In general--It ends up being easier and not that much more expensive to add 10-20% more panel rather than going through a manual oreintation twice a day or 2-4 times a year. The one joker in the deck is for folks up north. Vertical panels mounted high shed snow and pick up reflections from snow fields in front of the array.

Closer to the equator, people can mount their panels "Flat"--However, having 5-10 degree minimum tilt makes the panels better at "self cleaning" (dust, pollen, leaves).

Your choice.

-Bill

NorthGuy · October 2013

Re: Direction of panels to sun

It depends on your goal.

If you grid-tied and only worry about overall production, point them all south.

If you're off-grid and ready to sacrifice some production to make your production period longer, you can split them.

If you're in doubts, you may consider to make them adjustable.

RobW · October 2013

Re: Direction of panels to sun

Hi, fellow Mississippian!

I found this somewhere on the web, but I can't find the reference or URL right now. It's a formula for figuring array tilt. Some of the pros here can tell us if it seems valid or not. The formula already has my latitude included. Substitute yours.

Adjust to summer angle on April 18
Adjust to autumn angle on August 24
Adjust to winter angle on October 7
Adjust to spring angle on March 5

My latitude: 33.907

For summer, take the latitude, multiply by 0.92, and subtract 24.3 degrees.

33.907 x 0.92 - 24.3 = 6.89 degrees

For spring and autumn, take the latitude, multiply by 0.98, and subtract 2.3 degrees.

33.907 x 0.98 - 2.3 = 30.92 degrees

For winter, take the latitude, multiply by 0.89, and add 24 degrees.

33.907 x .89 + 24 = 54.18 degrees

EDIT: Found the reference: http://www.macslab.com/optsolar.html

Gives you formulas for fixed mount, adjust twice a year, and four times a year. What I included above is for adjusting tilt four times a year.

BB. · October 2013

Re: Direction of panels to sun

PV Watts is based on actual measurements over ~20 years--So it takes local weather conditions into account.

We can try PV Watts for Meridian Mississippi, use defaults and then use your two calculations:

"Station Identification"
"City:","Meridian"
"State:","Mississippi"
"Lat (deg N):", 32.33
"Long (deg W):", 88.75
"Elev (m): ", 94
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 32.3"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.55, 327, 26.81
2, 4.86, 406, 33.29
3, 4.92, 441, 36.16
4, 5.69, 480, 39.36
5, 5.63, 475, 38.95
6, 5.60, 448, 36.74
7, 5.68, 466, 38.21
8, 5.48, 452, 37.06
9, 5.11, 416, 34.11
10, 5.24, 456, 37.39
11, 4.27, 370, 30.34
12, 3.85, 358, 29.36
"Year", 4.99, 5094, 417.71

"Station Identification"
"City:","Meridian"
"State:","Mississippi"
"Lat (deg N):", 32.33
"Long (deg W):", 88.75
"Elev (m): ", 94
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 6.9"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.84, 257, 21.07
2, 4.01, 333, 27.31
3, 4.54, 409, 33.54
4, 5.69, 483, 39.61
5, 6.02, 513, 42.07
6, 6.18, 502, 41.16
7, 6.17, 515, 42.23
8, 5.61, 468, 38.38
9, 4.86, 397, 32.55
10, 4.44, 386, 31.65
11, 3.34, 286, 23.45
12, 2.89, 262, 21.48
"Year", 4.72, 4810, 394.42

"Station Identification"
"City:","Meridian"
"State:","Mississippi"
"Lat (deg N):", 32.33
"Long (deg W):", 88.75
"Elev (m): ", 94
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 54.2"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.73, 343, 28.13
2, 4.98, 416, 34.11
3, 4.67, 417, 34.19
4, 4.99, 420, 34.44
5, 4.65, 388, 31.82
6, 4.47, 353, 28.95
7, 4.59, 373, 30.59
8, 4.71, 388, 31.82
9, 4.71, 383, 31.41
10, 5.26, 457, 37.47
11, 4.53, 392, 32.14
12, 4.20, 390, 31.98
"Year", 4.62, 4721, 387.12

From the numbers, the two tilt calculations do increase the amount of sun for a few months (summer/winter depending on tilt)... So, if you need more power, perhaps it is worth moving summer/winter tilt for you...

On the other hand add 10% more panels and you would "break even" on the months of interest, and collect more power overall.

10% differences are just about in the "noise"... Just year over year variations are 5-10% in amount of sunlight.

-Bill

zoneblue · October 2013

Re: Direction of panels to sun

In general it works out best to point the panels at the best sun, ie summer sun, equinox to equinox. Optimising for winter only if you have critical winter loads, and loads of money.

The exceptions are:
- grid tie with max, or time of day tarif constraints.
- high daytime loads, eg air con.
- problem battery to pv ratios, such that excessive charge currents may be a problem.

For those virtual tracker setups are good. For all other mortals fixed at latitiude works best.

RobW · October 2013

Re: Direction of panels to sun

I put together a spreadsheet to play with the tilt angles and dates I previously referred to in post #4 above. I took my figures from PVWatts.

I compared the solar radiation for a fixed tilt at latitude angle vs. those tilt angles suggested above for four positions a year. The fixed tilt is 100% for my spreadsheet. All other figures are in relation to the 100% fixed tilt value. Here's the results.

                 Solar Radiation@           Solar Radiation@    Solar Radiation@   Solar Radiation@  Solar Radiation w/
                 32.3 Degree Fixed Tilt     6.9 Degree Tilt      31 Degree Tilt    54 Degree Tilt    Two Axis Tracking
----------------------------------------------------------------------------------------------------------------
January               100.00%	                80.17%	          99.15%	     104.82%          119.55%
February              100.00%	                82.99%	          99.59%	     102.07%          126.14%
March                 100.00%	                92.45%	         100.00%	      94.90%          124.49%
April                 100.00%	                99.82%	         100.35%	      88.44%          130.47%
May                   100.00%	               106.93%	         100.71%	      83.48%          133.93%
June                  100.00%	               110.16%	         100.89%	      80.93%          137.61%
July                  100.00%	               109.03%	         100.88%	      81.59%          138.41%
August                100.00%	               102.37%	         100.36%	      86.68%          128.47%
September             100.00%	                96.04%	         100.20%	      91.88%          124.55%
October               100.00%	                85.52%	          99.61%	      99.81%          125.68%
November              100.00%	                78.91%	          99.29%	     105.69%          125.59%
December              100.00%	                75.79%	          99.21%	     108.16%          127.38%

As you can see, there's only one month, June, at the summer 6.9 degree tilt angle, that improves at least 10% (barely) over the fixed tilt angle. I think that's equivalent to adding a 100W panel to a 1kW array. BB, you were right, as usual. There's not much advantage to adjusting the tilt angles four times a year over a fixed tilt angle at latitude the entire year. I don't know if these figures will extrapolate to other latitudes other than my own, but I would suspect so.

BB. · October 2013

Re: Direction of panels to sun

Rob, nice work. Neat idea to call the fixed/recommended tilt at 100%.

If you still have the spread sheet, put the last column as 2-axis--That will give you the maximum improvement you can see.

-Bill

PS: One thing to add--From what I have seen, it is hard to add 10%, but it is easy to lose 20%+ when playing with tilt.

RobW · October 2013

Re: Direction of panels to sun

Edited the table above to include figures for two axis tracking.

BB. · October 2013

Re: Direction of panels to sun

Thank you Rob--Very nice presentation of the data.

-Bill

ggunn · October 2013

Re: Direction of panels to sun

scheek wrote: »

I have 8 panels approx. 120 watts each and in series. I have mounted the panels on poles but have not locked them into place. Is it good to have half in a more SE direction for morning and noon times and some favoring a SW direction for afternoon.

I live in Mississippi where sun is lower ( more southern) in winter and higher in the summer. I have them on about a 45 degree angle.

Thanks.

Do not split a string. If you have a string with some modules pointing in one direction and some pointing in another, the current through the string will always be that of the lowest performing module, e.g, it will do the opposite of what you are trying to accomplish.

Direction of panels to sun

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