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Which method does Elements use to split global solar radiation into direct and diffuse?

asked 2019-07-03 07:01:17 -0600

Bambo gravatar image

updated 2019-07-03 15:50:55 -0600

Hello everyone! I'm Giandomenico, and I am a master student in Building Engineering at Politecnico di Bari. For my master thesis project I need to develop an EPW file of Bari. I have already gathered weather data but unfortunately, I have only global solar radiation. I've tried to split it, using Watanabe method but I've found some issues. So I have seen that Elements has its own equation to split global solar into direct and diffuse. Does anyone know what these equations are? Thank you in advance for answering me!

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answered 2019-07-03 10:15:12 -0600

updated 2019-07-03 10:16:15 -0600

Elements does not split global solar radiation into direct (normal) and diffuse, it only relates those three metrics. The method that Elements uses to relate Global, Normal (Beam), and Diffuse Solar Insolation is as follows:

The factors that affect the solar radiation are: the location (latitude and longitude which determines the solar altitude angle or zenith angle) and timezone (affects the interpretation of the date/time field vs solar time) and the three solar metrics: Global Solar, Normal Solar, and Diffuse Solar. The metrics are related as follows:

GlobalSolar = NormalSolar * CosZenithAngle(time, lat, long) + DiffuseSolar

  • GlobalSolar = Global Solar Irradiation on a Horizontal Surface
  • NormalSolar = Beam Solar Irradiation Normal to the Rays of the Sun
  • DiffuseSolar = Diffuse Solar Irradiation on a Horizontal Surface

The zenith angle in Elements results from your location and your time-of-day. It is automatically calculated for you but you could also determine it from any good book on solar engineering or by searching the web. In Elements, you can add it to the spreadsheet view by clicking "Add" in the bottom left corner of Elements: see "Cosine of Zenith" and "Solar Zenith Angle". Note that, in Elements, the cosine of the zenith angle is averaged over the given hour of insolation.

So, Elements does not determine NormalSolar or DiffuseSolar other than to ensure that the above relationship holds.

In order to estimate the normal and diffuse components when you only have global solar (for example), you may want to check out the literature for that. This below set of presentations might be a good start:

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I have just read your email, thank you a lot for your answer and for giving me a tip on how to estimate components of solar radiation

Bambo gravatar image Bambo  ( 2019-07-03 10:36:14 -0600 )edit

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Asked: 2019-07-03 06:59:14 -0600

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Last updated: Jul 03 '19