Virtual tracking

mountainmanmountainman Registered Users Posts: 439 ✭✭
Looking at Pv watts comparing 
Panels tilt set at latitude 34°
  180° vs 135°. (S vs SE)
for my area Summer production is better at 135°.
Winter production is better at 180°  Yearly average is only down about 5%. at 135°.
My question is there a formula for 
Reduction in amps or watts for 
Panels that are say 20-30-45 degrees off from 180° at solar noon?
I think its called  cosine angle?




2kw array 6 345 q cells  make sky blue 60 cc
 6 230ah GC @36 volts 
18 amp accusense charger. 3650 champion 

Comments

  • mountainmanmountainman Registered Users Posts: 439 ✭✭
    edited September 2020 #2
    As an experiment I'm gonna build an adjustable rack. Then set the  panel at 
    10° increments from 130° to 180° at solar noon.
    With a 230 ah battery use a 100 amp battery tester for my load. 
    Is there a more accurate way?
    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    More or less, get a small solar panel (even a single cell) crystalline type, and put a low Ohm resistor on the output and log the (few 1/10ths of a volt drop across the resistor) current from the "shorted" panel/cell--And that current is proportional to the Watts/Sq solar energy... I.e., Isc=1,000 W/sqM.

    If you don't have or want to use a logging meter, there are some USB data loggers that will do the same thing for many months (or longer)--And you just download the USB memory to your spread sheet.

    Otherwise, I would suggest using PV Watts or similar and plug in the direction/tilt and bracket your "best overall" harvest (or summer or winter waited, depending on your specific power needs).

    https://pvwatts.nrel.gov/pvwatts.php

    For example, using Charlotte NC, 225 degrees (south west), and 35 vs 25 degree tilt--Using defaults for everything else, I got 4.97 vs 4.99 hours of sun average over 1 year.

    Vs 180 degrees and 35 degree tilt (latitude--typical for best year round harvest) and got 5.29 hours of sun (average for year).

    Seems pretty easy to build a little spread sheet (and you can download CSV file for monthly numbers) and see what the differences are seasonally or monthly vs tilt/direction.

    Think you will get "close enough" results without having to actually log daily/monthly/yearly with different array configurations. The difference from day to day will probably swamp 10 degree tilt changes.

    And, while pointing other than south and within 10 degrees of sun angle is always going to cost you harvest... "minor" changes do not really have very large effects.

    The formula for figuring out sun angle to panel vs harvest--The basic equation is:

    "loss of harvest" to flat panel = Cosine of the sun angle to panel (0 degrees is facing sun, 10 degrees is a bit off axis, 60 degrees is way off axis):

    Cosine 0 degrees = 1.0 (100% harvest)
    Cosine 10 degrees = 0.985 harvest (1.5% loss of harvest)
    Cosine 60 degrees = 0.5 harvest (50% loss of harvest)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,976 ✭✭✭✭✭
    As an experiment I'm gonna build an adjustable rack. Then set the  panel at 
    10° increments from 130° to 180° at solar noon.
    With a 230 ah battery use a 100 amp battery tester for my load. 
    Is there a more accurate way?
    Sounds like you want to spend a lot of money to squeeze a penny more out of your system. 
    In the end an off grid solar electric system MUST be wasteful to ensure a long battery life.

    Indeed to get a good measure and properly test, you will need multiple systems in case a single system has a single problem, so you can take a median reading. You will also need multiple systems for each angle so that you can run the tests concurrent, so weather patterns don't skew the results. All so your system can be a couple % smaller so you can over produce 25% rather than 35% most days? 
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • mountainmanmountainman Registered Users Posts: 439 ✭✭
    edited September 2020 #5
    Bill yes that is what I'm interested in knowing loss of harvest vs panel
     angles. 
    As an example a 50 ah battery would need roughly a 100 watt panel 5 amps set at 180° for a 10% charge rate.
    But If at 120° 60° off from 180= 50%.
    Then I would need double the array
    2 100 watt panels 
    1 at 120° and 1 at 240° 
    I have researched pv watts and understand monthly/yearly production 

    But my interest is more of what 
    The loss of peak amperage be at solar noon on a given day if the panel was 45° or 30° off from 180°.
    Currently I'm getting 19 peak amps 
    Aimed due south with a 345 watt panel @12 volts.
    So with a 60° angle 50% loss  I should get~9.5. 
    What percentage of loss would it be if the panel was at a 45°or 30° angle from 180°?
    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
  • BB.BB. Super Moderators, Administrators Posts: 32,924 admin
    Using a calculator that came with my laptop OS, set to degrees (rather than radians== Where Pi is 180 degrees)

    To give you a simple table:

    Cosine 0 degrees = 1.00
    Cosine 10 degrees = 0.98
    Cosine 30 degrees = 0.87
    Cosine 45 degrees = 0.71
    Cosine 60 degrees = 0.50
    Cosine 90 degrees = 0.00

    Roughly, the sun varies its track in the sky by +/- 30 degrees. It you tilt the array to your latitude, then it would see 30 degrees off angle (we typically vary fixed array angles by -15 for optimum summer, and +15 degrees for winter).
    • winter/summer solstice is 30 degrees off; Cosine 30 degrees = 0.87
    • spring/fall equinox angle to noon sun is 0 degrees if array facing facing south
    If you were facing the array south east (example), the sun will not be as high in the sky, so you till the panel more for peak harvest. Cosine of 0 will give you 1.00, however, you have more atmosphere, so there will be some degradation of sunlight.

    You can use a website like this to figure out sun position for SW and SE (or whatever you choose) for "optimum" solstice tilt (scroll down for tracks of sun through sky).

    https://www.sunearthtools.com/dp/tools/pos_sun.php

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mountainmanmountainman Registered Users Posts: 439 ✭✭
    Thanks exactly what I was looking for.
    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
  • mountainmanmountainman Registered Users Posts: 439 ✭✭
    I just want to say you guys are awesome. I'm glad your forum exists 
    All that knowledge you share for free.
    You need to be paid!!
    Thanks for all your help.

    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
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