Re: Solar flux calculation

Discussion in 'Photovoltaics' started by Bill Kaszeta / Photovoltaic Resources, Aug 4, 2008.

  1. On Sat, 2 Aug 2008 16:00:10 -0700 (PDT), mundt <> wrote:

    >I wish to calculate the solar flux falling on my panels (Ip) for
    >modeling purposes.
    >
    >I have:
    >1. the insolation from a local meterology station (at local solar
    >noon) = Is
    >2. the solar declination (at solar noon) = Td
    >3. my latitude = Tl
    >4. my panel tilt (azimuth) =Tp
    >5. my panels face due south
    >
    >To correct the insolation for the declination of the sun, I believe
    >that I need to divide the measured insolation (Is) by the cosine of
    >the actual sun angle (Td-Tl). Thus the solar flux on a surface NORMAL
    >to the sun is Is/cos(Td-Tl).
    >
    >To correct for the tilt angle of my panels, I believe I need to
    >multiply the NORMAL flux by the cosine of the angle between the
    >normal surface and the actual panel surface, i.e. cos(Td-Tl-Tp).
    >
    >The flux my panels then see is thus: Ip = Is *cos(Td-Tl-Tp)/cos(Td-
    >Tl).
    >
    >Is this a correct formulation?
    >
    >Thank you in advance for your comments.


    Not so simple.

    There are two major measurements made by many observing stations,
    global irradiation on a horizontal surface (using a pyranometer) and direct
    normal irradiation (pyrheliometer or pyranometer with a shadow band).

    Sunshine on a tilted surface has three components, direct normal (the
    component that can be concentrated with a magnifying glass), diffuse
    (the blue sky and/or reflections from clouds), and reflected from the
    ground (albedo).

    There is some good information at
    http://www.nrel.gov/rredc/models_tools.html
    and the related site.

    Your calculations apply to the direct beam component on a clear day.
    You need to make sure you understand the origin of the data you are using
    before applying calculations.

    There are computer programs that do these calculations, but
    the old links I have no longer function. There are solar radiation
    handbooks with full details.


    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
    Bill Kaszeta / Photovoltaic Resources, Aug 4, 2008
    #1
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