PV watts 2 and real world mounting

SolarLurker
SolarLurker Solar Expert Posts: 122 ✭✭
in Solar Beginners Corner #1
I have read that a PV system should face true south, however whenever I plug my true south into PV watts to it shows yearly production declining. Is this a fluke with pv watts or am I missing something? Should I just point south?

In additional could some one please explaing why lattitiude in the default pitch, when 31 degrees produce more kwh per year.

Since my system will be ground mounted I want to ensure the system is as efficent as possible.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: PV watts 2 and real world mounting

    Actually "true South" isn't "solar South" : http://www.builditsolar.com/SiteSurvey/FindingSouth.htm

    A bit like true North & magnetic North.

    Another good thing to check out is your optimum angle: http://www.macslab.com/optsolar.html

    But what's worse, for anyone's particular site the "best angle" is whatever provides the most sun on the panels during the course of the day. This may not be the technical "optimum orientation" if that has obstacles in the way. And there's more! If you're in a hot climate you can sometimes increase yield by pointing a bit towards East, causing most of your solar to be collected in the cooler morning hours before the heat starts to eat up panel efficiency.

    I hope that's helpful. Or at least amusing. :D
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: PV watts 2 and real world mounting

    now i don't use pv watts myself and i can't say one way or the other of what the situation is with it for you, but if by true south you mean geographically and not magnetically then that is essentially correct. some circumstances would sway that into other directions some, but i don't think the program is calculating for the heat of the day, fog in the mornings, shading for you.
    now as to your latitude, that is the average point you will face the sun best overall mathematically. at equinox when the day and night is equal in time length is when this occurs. during summer it has more light than darkness and is higher in the sky with winter being less light and lower in the sky. winter is the biggest problem because of the narrower angles and window of time for exposure, but it is this time when a south facing exposure is the most critical geographically. to get a better power average many will opt for better harvest during the winter months and may be as much as latitude +15°. i might add that this is the angle in relation to the ground with 0° as flat and at the equator and 90° as vertical at the geographic poles (all facing geographic south when applicable). of course this latitude figure is also for the solar noon or highest point above the horizon and all other times it will be lower in the sky meaning a steeper slope for the pvs.
    the program itself would be a rough guide i would think as each individual circumstance can vary widely and to know exactly what is best means a professional solar survey be done.
    does this help?
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #4
    Re: PV watts 2 and real world mounting
    SolarLurker wrote: »
    I have read that a PV system should face true south, however whenever I plug my true south into PV watts to it shows yearly production declining. Is this a fluke with pv watts or am I missing something? Should I just point south?

    In additional could some one please explaing why lattitiude in the default pitch, when 31 degrees produce more kwh per year. ...

    Could you provide some more information on how you're using PVWatts? While I have some understanding of PVWatts ver 1, I wasn't able to replicate your results.

    For example, if I vary azimuth away from the default 180°, in either direction, the annual kWh forecast declines as I would expect.

    Same for if I vary pitch away from my default longitute, again in either direction. The annual kWh forecast declines.

    I'm interested to understand this phenomenon you've found. Thanks.

    Best regards,

    Bill
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  • SolarLurker
    SolarLurker Solar Expert Posts: 122 ✭✭
    #5
    Re: PV watts 2 and real world mounting

    I am using PV watts 2 beta, below are my results from 180 at 35 degrees

    Station Identification
    Cell ID: 0263368
    State: Pennsylvania
    Latitude: 41.8 ° N
    Longitude: 76.7 ° W
    PV System Specifications
    DC Rating: 11.6 kW
    DC to AC Derate Factor: 0.840
    AC Rating: 9.74 kW
    Array Type: Fixed Tilt
    Array Tilt: 31.0 °
    Array Azimuth: 180.0 °
    Energy Specifications
    Cost of Electricity: 10.0 ¢/kWh

    Results

    Y 4.09 13803 1380.30

    and now default

    Station Identification
    Cell ID: 0263368
    State: Pennsylvania
    Latitude: 41.8 ° N
    Longitude: 76.7 ° W
    PV System Specifications
    DC Rating: 11.6 kW
    DC to AC Derate Factor: 0.840
    AC Rating: 9.74 kW
    Array Type: Fixed Tilt
    Array Tilt: 41.0 °
    Array Azimuth: 180.0 °
    Energy Specifications
    Cost of Electricity: 10.0 ¢/kWh

    Results


    Y 4.04 13621 1362.10
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #6
    Re: PV watts 2 and real world mounting

    PV Watts 1 says:

    PV Watts Help Page:
    Azimuth Angle For a fixed PV array, the azimuth angle is the angle clockwise from true north that the PV array faces. For a sun-tracking PV array with one axis of rotation, the azimuth angle is the angle clockwise from true north of the axis of rotation. The azimuth angle is not applicable for sun-tracking PV arrays with two axes of rotation.
    The default value is an azimuth angle of 180° (south-facing) for locations in the northern hemisphere and 0° (north-facing) for locations in the southern hemisphere. This normally maximizes energy production. For the northern hemisphere, increasing the azimuth angle favors afternoon energy production, and decreasing the azimuth angle favors morning energy production. The opposite is true for the southern hemisphere.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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