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How do you measure insolation?

Discussion in 'Photovoltaics' started by mike, Feb 22, 2008.

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  1. mike

    mike Guest

    I'd like to do some experiments with solar collectors, both PV and non-PV.
    So how do I measure the energy coming out of the sky to calibrate
    the measurements?
    The obvious solution is to point a photographic light meter at the sky.
    Where do I point it? Does pointing directly at the sun overstate
    the average energy? And what about frequency response? Does the
    measurement intended for photography correlate with the energy spectrum
    that concerns absorbing light/heat? Don't clouds distort the energy
    spectrum? In the northwest, overcast is the predominant
    weather pattern.

    The objective is to make measurements under different conditions and
    correlate the results to assess relative performance of collection
  2. Al Forster

    Al Forster Guest

    Usually with a pyranometer (
    or a pyrheliometer (

    For frequency distributions, you could measure with a referenced solar
    cell and a selection of appropriate colour filters and normalize to
    your pyranometer readings.

    about $195

    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
  4. mike

    mike Guest

    Quoting the site

    The spectral response of the LI-200 does not include the entire solar
    spectrum, so it must be used in the same lighting conditions as those
    under which it was calibrated. Therefore, the LI-200 should only be used
    to measure unobstructed daylight. It should not be used under
    vegetation, artificial lights, in a greenhouse, or for reflected solar

    end quote

    This is the whole reason I asked the question. I'd like to make
    comparative measurements of different configurations in different
    lighting situations and get meaningful comparisons of which is better.
    The first experiments are scheduled to be hot-box heat collectors.
    Visible light calibration may or may not bear any relationship to
    wavelengths used for heating air/liquid.

    And $195 is out of the question. Cheap/free/homebrew is what I desire.

    Painting a thermocouple black is the current front runner.
    Thanks, mike
  5. Guest

    Omigod, serious money :)
    Steve Baer paints an aluminum plate black and measures the temperature rise
    over time with an IR thermometer. In 250 Btu/h-ft^2 full sun, 1 ft^2 would
    absorb 250x0.5/60 = 2.08 Btu in 30 seconds. A 1/8" 1.76 pound plate with
    1.76x0.214 = 0.377 Btu/F would warm 2.08/0.377 = 5.5 F without much heat
    loss to the surroundings, if it starts at the same temperature.

  6. mike

    mike Guest

    NOw, that's what I'm talkin' about. cheap, simple, results supported by
    math. I like it.
    Thanks, mike
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