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# Optical depths for the ASHRAE Clear Sky Model

I'm implementing the revised ASHRAE (Tau) clear sky model and am trying to find a source for the beam and diffuse optical depths described in the engineering reference: https://www.energyplus.net/sites/defa...

The depths appear to be based on the Beer-Lambert law but I can't find any documentation on how the diffuse and beam are calculated in the ASHRAE material. The goal is to produce the solar radiation estimates for use in a EPW file.

Question: Can someone provide documentation for calculating beam/diffuse optical depths? Or is it just better to use the Zhang-Huang model?

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A more up to date link, which includes a section on revised coefficients (ASHRAETau2017): https://bigladdersoftware.com/epx/doc...

( 2020-01-28 15:01:51 -0600 )edit

ASHRAE Handbook - Fundamentals, 2017, Chapter 14. Perhaps that includes a link to the ASRHAE Research project that developed the new coefficients and would show the methodology?

( 2020-01-28 15:04:40 -0600 )edit

Do you need to calculate your own coefficients (from measurements) or you just do not know where to find them?

( 2020-01-29 07:56:12 -0600 )edit
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@Gio I'm trying to calculate the optical depths; I have the ASHRAE design conditions data

( 2020-01-29 13:32:44 -0600 )edit
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That's a good point @Julien Marrec - My copy of fundamentals 2009 mentions ASHRAE RP-1453...I'll try and track that down and post an answer

( 2020-01-29 13:37:40 -0600 )edit

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The optical depth estimates in ASHRAE climatic data were calculated using a high performance clear sky radiation model called REST2, developed by Chris Gueymard. The REST2 model was used to calculate radiation data for the various locations in the ASHRAE climatic data and then a 2 parameter regression model was fitted to the results to derive the revised clear sky model.

The ASHRAE research project notes that the optical depths are more appropriately called "pseudo" optical depths (because optical depth is typically employed when the air mass = 1). Gueymard further states that aerosol optical depths are very sensitive to water vapor content and are not recommended for use; rather he suggests the use of the Angstrom coefficients, which it appears is what ASHRAE has tabulated as the beam and diffuse optical depths.

Gueymard, Chris. (2004). High performance model for clear-sky irradiance and illuminance.

Thevenard, D & Gueymard, Chris. (2010). Updating the ASHRAE climatic data for design and standards. ASHRAE Trans.. 116(2). 444-459.

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