Hello,

in my opinion, you can't split the total radiance to direct and diffused components if you don't have extra sources of information. For any given location, the situation changes on an hourly basis so the model won't work without aerosol and cloud inputs.

However, I believe you can estimate the ratio of direct and diffused radiances by using two (or more) radiometers coupled with telescopes (by definition, a pyranometer collects the radiation in a hemisphere, so you can't use it for this purpose). One radiometer must be attached to a solar tracker and the second one must be pointed elsewhere (the best offset angle is to be modeled).

If the apertures and the detectors of both radiometers are the same then you'll get the ratio from the ratio of their signals and the geometric ratio of solar disk solid angle and 2xPI. To achieve higher accuracy, I would use more than one radiometer pointed off the sun (or some combination of a 2Pi radiometer and a solar tracker, which masks the solar disk).

Hi,

you'll need to take into account the following components:

*) for direct solar irradiance, you need to calculate the radiative transfer with the molecular absorption at each wavelength (depending on the wavelength, you will need the molecular gase profiles up to the thermosphere and the spectral information coming from HITRAN or GEISA databases).

*) the indirect part is related to diffusion by molecules and particles. The latter part is the most difficult since it involves multiple scattering effects. The inputs for this part are the aerosols and clouds (both come from satellite observations on a day-to-day basis). You will need the densities and particle type and size distribution at each altitude.

*) some of the light reflected from the clouds will never reach the surface - technically this is related to a diffusion, but this radiance is lost to space, and the direct/diffusive ratio is calculated for the radiance reaching the surface, so one has to take these losses into account.

For all parts you will need to know the solar zenith angle.

Unfortunately, I can't suggest something simpler - maybe, there's some parameterization, but the clouds need to be set up per type, per layer, so I can't imagine that one can simplify this to a single line of code.

Hi Artem

Thanks a lot for your insightful response, and you are right there is nothing simple about nature, least of all light!

Could I please ask :

If the pyranometer is located on top of a building and measuring the total irradiance at (essentially) the earth surface level, then surely we don’t need to worry about atmospheric particles and losses of diffused component within clouds, since those losses are already accounted for by the fact that they never reach our pyranometer?

And if we are measuring the total global horizontal irradiance at ground level, are there any models that enable us to break the total down to direct and diffused?

I will be using these outputs for building energy modeling, and won't need spectral level information, only the overall direct and diffused element on a horizontal plane...

Thanks again

M

Hello,

in my opinion, you can't split the total radiance to direct and diffused components if you don't have extra sources of information. For any given location, the situation changes on an hourly basis so the model won't work without aerosol and cloud inputs.

However, I believe you can estimate the ratio of direct and diffused radiances by using two (or more) radiometers coupled with telescopes (by definition, a pyranometer collects the radiation in a hemisphere, so you can't use it for this purpose). One radiometer must be attached to a solar tracker and the second one must be pointed elsewhere (the best offset angle is to be modeled).

If the apertures and the detectors of both radiometers are the same then you'll get the ratio from the ratio of their signals and the geometric ratio of solar disk solid angle and 2xPI. To achieve higher accuracy, I would use more than one radiometer pointed off the sun (or some combination of a 2Pi radiometer and a solar tracker, which masks the solar disk).

Hi Mohammad,

Have you considered exploiting the fact that diffuse light, unlike direct light, is polarised? https://en.wikipedia.org/wiki/Rayleigh_sky_model

Thanks again Artem for your insight.

Thanks Alastair for your input too, I think we just need to get a pyranometer that generates both direct and diffused readings!

Mohammad,

I don't think a pyranometer that reports both direct and diffuse radiation exists. You need to invent one.

You can buy computer controlled tracking devices for telescopes so they can fix on stars. If you replaced the telescope with a pyranometer and pointed it at the sun you could measure direct radiation. If you measured total radiation with another pyranometer then the difference would give you the diffuse radiation.

Here's what you should be doing :-)

https://www.raspberrypi.org/magpi/tracking-telescope-mount/

I would go even further and simplify the setup to two calibrated pyranometers and a (lightweight) solar tracking mask driven by two stepper motors and one Arduino Mega. The first pyranometer will measure the total radiation and the second one will give the diffused component, and both of them will remain fixed.

P.S. Another possibility is to use just one pyranometer - in this case you don't need to bother about the cross-calibration, but the mechanical system will be more sophisticated - the mask needs to track the sun and rotate by 90 deg from time to time to let measure the full flux.

Thanks Alastair and Artem

Your advice has been most helpful.

M

There is a web page here which discusses your problem. https://www.e-education.psu.edu/eme812/node/644

What you need is a Pyrheliometer and a Pyranometer