pvwatts doesn't make sense to me

rollandelliott
rollandelliott Solar Expert Posts: 834 ✭✭
in General Solar Power Topics #1
So I'm submitting the paperwork for a utility rebate and running various scenarios through PVwatts. The thing that puzzles me is that there is not a huge difference on what orientation I place the panels.
For example if I place the array at due East I get 4.66(kWh/m 2/day)
if I place the array at due South I get around 5.3 (kWh/m 2/day)
this is for a fixed mount 10KW system near Daytona Beach, FL.

Seems like common sense would dictate you get half as much energy when you face the panels at 90 degrees away from the sun.

What am I missing?!?
Link below if you want to play around with it.
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Florida/Daytona_Beach.html
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: pvwatts doesn't make sense to me

    Afternoon clouds and thunder showers?

    Facing west gives you 4.48 hours per day (1 year average).

    Solar Guppy (also from Florida) has found arrays that face east of south have better performance at his place too because of weather (and panel temperature too?)... (I don't remember the exact offset from south).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
    #3
    Re: pvwatts doesn't make sense to me

    It's not just a Florida thing.
    Plugged in Detroit , MI and Charlotte, NC 
    E          S         W
Detroit , MI    3.43    4.28    3.31
Charlotte, NC  4.12    5.07    4.07

    Seems like system performance is dropped about 20% when arrays are at 90 degrees to the sun. I would of guessed 50%, guess orientation is not that big of a deal.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: pvwatts doesn't make sense to me

    there is still a great deal of energy to be had in the east and the west. during the winter solstice there would be a bit of a problem with not very much power produced when it's aimed east or west, but it won't be 0w. now in the case of the east it would produce nearly after sunrise, but solar intensity is still low. as the sun goes towards the southern horizon it gets stronger in intensity, but production will drop off some due to an increase in the angle to the pvs. a similar thing is in the west. for obvious reasons neither will be as strong as to the south is and is especially true during the winter solstice and mattering less during the summer solstice. aiming to the south is the best overall direction unless there are other influences such as morning fogs or evening storms and this can vary somewhat per location. upwards of around 20-25% losses is significant for aiming due east or west on average, but winter losses can be around 50%.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #5
    Re: pvwatts doesn't make sense to me

    Without tracking, being due east or due west is only about a 10% hit in harvest. The most energy of the day is when the panels are close to perpendicular to the sun, about a 4 hour window. The Range from East to West gives close to the same 4 hour window most seasons, hence the harvest isn't that different.
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #6
    Re: pvwatts doesn't make sense to me
    BB. wrote: »
    Afternoon clouds and thunder showers? ...

    I believe PVWatts' insolation numbers are reflective of weather conditions (clouds, humidity, etc.) specific to the location chosen. I don't believe, though, the insolation numbers are reflective of temperature changes.

    I believe PVWatts' kWh numbers are reflective of both weather conditions and temperature changes for the location specified.

    Best regards,

    Bill
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: pvwatts doesn't make sense to me

    Actually, as far as I can tell, PV Watts does not take ambient/solar panel temperatures into account.

    -Bill "I could be wrong" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #8
    Re: pvwatts doesn't make sense to me
    BB. wrote: »
    Actually, as far as I can tell, PV Watts does not take ambient/solar panel temperatures into account.

    -Bill "I could be wrong" B.


    I think it takes average location temps into account. if it didn't then my longer days June production should be higher than my April and May production and it is not. In fact June and July are both a far amount less. The only explanation I can come up with is the temps are higher in June.


    "Station Identification"
    "City:","Phoenix"
    "State:","Arizona"
    "Lat (deg N):", 33.43
    "Long (deg W):", 112.02
    "Elev (m): ", 339
    "PV System Specifications"
    "DC Rating:"," 12.5 kW"
    "DC to AC Derate Factor:"," 0.770"
    "AC Rating:"," 9.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 24.5"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.5 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 4.72, 1312, 111.52
    2, 5.74, 1442, 122.57
    3, 6.49, 1740, 147.90
    4, 7.67, 1946, 165.41
    5, 7.91, 2024, 172.04
    6, 7.77, 1856, 157.76
    7, 7.52, 1868, 158.78
    8, 7.37, 1848, 157.08
    9, 7.10, 1728, 146.88
    10, 6.45, 1692, 143.82
    11, 5.21, 1367, 116.20
    12, 4.48, 1256, 106.76
    "Year", 6.54, 20078, 1706.63
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Re: pvwatts doesn't make sense to me

    Bill and Ted,

    From here:
    Note: Because the PVWatts overall DC-to-AC derate factor is determined for STC, a component derate factor for temperature is not part of its determination. Power corrections for PV module operating temperature are performed for each hour of the year as the PVWatts calculator reads the meteorological data for the location and computes performance. A power correction of -0.5% per degree Celsius for crystalline silicon PV modules is used.

    -Bill "it looks like I am wrong" B. :blush:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #10
    Re: pvwatts doesn't make sense to me

    the good of this is you can plug in the derate factor, seems my system is running about 0.80 - 0.81. ;)
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #11
    Re: pvwatts doesn't make sense to me
    BB. wrote: »
    Actually, as far as I can tell, PV Watts does not take ambient/solar panel temperatures into account.

    -Bill "I could be wrong" B.

    Here's my understanding of 3 key pieces of using PVWatts (but I too could be wrong):

    First:

    PVWatts' Solar Insolation numbers reflect actual conditions for a location without regard to temperature or derating factor (shading, orientation, system design, equipment performance).

    I.e., (for my location - Ft. Worth is closet using Version 1):
    From PVWatts: 5.46 kWh/m2/day average for the entire year
    My system: 225 W/panel @ 25° C STC

    5.46 kWh/m2/day * 365 days/yr * 225 W/panel * 36 panels = 16,128 kWh/year

    This is what my system would produce annually, as estimated using PVWatts' solar insolation average for the year, at a standard test condition temperature of 25° C, and assuming a derating factor of 1.0. I believe this 16,128 kWh/year is refered to as my 'Nameplate Performance.' It reflects estimated weather conditions minus temperature changes for my location. It also does not include derating considerations.


    Second:

    From PVWatts: 14,782 kWh/yr at a derating factor of 1.0

    This is what my system would produce, annually, as estimated by PVWatts. Now, in addition to estimated weather conditions for my location, it includes estimated temperature changes. It's still assuming a derating factor of 1.0.

    Note that this number, 14,782, is actually a calculation from the number you look up from PVWatts.

    11,367 kWh/yr (from PVWatts) / 0.769 derating factor = 14,782 kWh/yr


    Third:

    From PVWatts: 11,367 kWh/year at a derating factor of 0.77

    This is what my system is estimated to produce, annually, now taking into account shading, orientation, system design, and component performances at a default level, in addition to weather and temperature considerations.

    This number is a direct lookup from PVWatts.


    I'd value comment that either affirms my understanding above, or, shows that the interpretation is not correct yet.

    Best regards,

    Bill
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #12
    Re: pvwatts doesn't make sense to me
    solar_dave wrote: »
    "Station Identification"
    "City:","Phoenix"
    "State:","Arizona"

    I believe, for this example:

    1.) 'Nameplate Performance' = 29,839 kWh/yr at 25° C STC (An esitmate that reflects weather conditions for the Phoenix area, but not temperature changes, and does not reflect considerations associated with shading, orientation, system design or equipment performance either).

    2.) Theoretical Performance = 26,109 kWh/yr at Derating of 1.0 (An estimate that reflects weather conditions and temperature changes. Does not reflect considerations for shading, orientation, system design, or equipment performance).

    3.) Default Performance = 20,078 kWh/yr at Derating of 77% (an estimate that reflects everything: weather coniditions, temperature changes, and all factors that go into the derating number).

    Best regards,

    Bill
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #13
    Re: pvwatts doesn't make sense to me
    solar_dave wrote: »
    the good of this is you can plug in the derate factor, seems my system is running about 0.80 - 0.81. ;)

    Yes, agreed.

    In fact I directly measure this for my system, comparing actual output to PVWatts' supplied numbers. On a 30 day rolling average basis, my Derate Factor gets as high as 0.88 (late June) and as low as 0.41 (mid-Jan.) (see chart below). And on a 12 month rolling avg basis, it runs at at 0.70.

    SolarPVefficiency.png

    Best regards,

    Bill
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #14
    Re: pvwatts doesn't make sense to me
    a0128958 wrote: »
    Yes, agreed.

    In fact I directly measure this for my system, comparing actual output to PVWatts' supplied numbers. On a 30 day rolling average basis, my Derate Factor gets as high as 0.88 (late June) and as low as 0.41 (mid-Jan.) (see chart below). And on a 12 month rolling avg basis, it runs at at 0.70.


    Bill

    You have to remember that it is using historical data for most of the offsets. I suspect much of your low is weather related, more cloudy days perhaps than the average year. Seems my systems beats the PV watts number consistently over the year when using a 0.77 derate.
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #15
    Re: pvwatts doesn't make sense to me
    solar_dave wrote: »
    You have to remember that it is using historical data for most of the offsets. I suspect much of your low is weather related, more cloudy days perhaps than the average year. Seems my systems beats the PV watts number consistently over the year when using a 0.77 derate.

    Agreed. Same for my 30-day high (0.88) this past late June, during a long drought period of nothing but clear skys and low humidity.

    I'm not surprised your system is, long term, multi-year at about 0.80 - 0.81. It looks to be designed and implemented very well.

    In Mar. 2012 I'll have two years to compute my actual Derating Factor for the lifetime of the system.

    Best regards,

    Bill
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