Question on Beam irradiance (in TMY) and Albedo

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Aika Kamei
Topic Author

11 Mar 2013 07:52 #1403 by Aika Kamei

Question on Beam irradiance (in TMY) and Albedo was created by Aika Kamei

I have two questions, and sorry for my broken English.

First,
I would like to know why 'Beam irradiance' in TMY data is so high value.
I thought that the this equality holds;

Global horizontal irradiance = Beam irradiance + Diffuse irradiance

However, in TMY data, the beam irradiance often exceeds the global irradiace.
For excample,
on SAM/AK Anchorage.tm2, Time:2265 Global irradiance = 295 W/m2, Beam irradiance = 482 W/m2
Would you tell me why this happen?

Second,
I need the hourly albedo data from Best Weather Data for international locations (in EPW format).
How can I copy the data to excel?
When possible, I would like to get the data with other results such as absolute air mass, ambient temperature.

Waiting for your kind reply.

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Paul Gilman

11 Mar 2013 17:11 #1404 by Paul Gilman

Replied by Paul Gilman on topic Question on Beam irradiance (in TMY) and Albedo

Dear Aika Kamel,

The sum Global horizontal irradiance = Beam irradiance + Diffuse irradiance is valid for a surface oriented toward the sun, such as a PV array with 2-axis tracking. You can test that in SAM's flat plate PV model by simulating a system with 2-axis tracking and comparing the following values in the hourly results:

Subarray 1 incident total irradiance
Subarray 1 incident diffuse irradiance
Subarray 1 incident beam irradiance

The EPW files contain comma-separated data, so you can easily open them in a spreadsheet program and copy columns of data. If you use Excel to do this, be sure to work with a copy of the EPW file, not the same file that you use with SAM. If you save the file in Excel will add commas to the header rows it may change number formatting in a way that makes the file unreadable by SAM.

Best regards,
Paul.

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Aika Kamei
Topic Author

12 Mar 2013 18:27 #1405 by Aika Kamei

Replied by Aika Kamei on topic Question on Beam irradiance (in TMY) and Albedo

Thank you very much, Paul.

Okay, I got it!
I had a wrong idea about the beam normal irradiance for the irradiance on the horizontal surface.
The beam(direct) normal irradiance is equal to the irradiance on surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky, right?

and,
I was able to open the EPW, however it was sometimes hard to read.
Is there any way to refer the albedo data in SAM?

Best regards,
Aika.

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Paul Gilman

12 Mar 2013 19:00 #1406 by Paul Gilman

Replied by Paul Gilman on topic Question on Beam irradiance (in TMY) and Albedo

Dear Aika,

You are right about the relationship between total, beam, and normal irradiance. The sum is only valid for a surface oriented toward the sun.

There is not a way in SAM to extract the albedo data from an EPW file.

Best regards,
Paul.

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Aika Kamei
Topic Author

12 Mar 2013 20:38 #1407 by Aika Kamei

Replied by Aika Kamei on topic Question on Beam irradiance (in TMY) and Albedo

Dear Paul,

I really appreciate your answer to such newbie questions.
I will try to be skilled in the use of SAM.

Thanks a million!

Best regards,
Aika

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Paul Gilman

19 Jul 2013 23:05 #1408 by Paul Gilman

Replied by Paul Gilman on topic Question on Beam irradiance (in TMY) and Albedo

As of SAM Beta 2013.7.12, SAM shows the albedo value from the weather file when you click View hourly data on the Location and Resource page. You can right-click graphs in the data viewer to export the data from graphs, which means there is now a way to extract the albedo values from a weather file using SAM.

Best regards,
Paul.

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