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How can I find the sky clearness index for ASHRAE clear sky model?

asked 2022-03-07 15:11:06 -0600

Elin's avatar

updated 2022-03-08 05:11:36 -0600

Hello, I want to use ASHRAE clear sky model in DesignBuilder(EnergyPlus) which requires sky clearness as an input. As I understood this index is the ratio of daily global radiation on horizontal surface to daily extraterrestrial radiation and it doesn't exceed 1. However, DesignBuilder help says that this index can vary between 0-1.2. Besides, the fig. 5 in chapter 33 of ASHRAE handbook 2007 shows the clearness number for the US which varies between 0.85-1.2. In other sources, this number is often between 0.5-0.8. I don't really understand why this happens. Am I missing something? I would be grateful if you could help me.

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answered 2022-03-16 12:08:20 -0600

Keigo's avatar

Why do you need to find sky clearness? Normally weather data (ddy file) includes sky clearness. If not, normally 0 is used for WinterDesignDay and 1 is used for SummerDesignDay as a conservative side.

I/O Reference describes under what circumstance sky clearness can exceed 1. "1.0 represents a clear sky at sea level. Values greater than 1.0 may be used for high altitude locations."

Solar radiation increases with altitude due to smaller optical depth of the atmosphere at higher altitudes.

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Asked: 2022-03-07 15:11:06 -0600

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Last updated: Mar 16 '22