DNI confusion

[Steve]

Hello,

I am using SAM version 2011.5.4, and a preloaded TMY file for the
climate (Phoenix, AZ) of a PV system.

Under the climate tab, where annual weather data is reported the DNI
reads 2518.6 kWh/m^2.
Dividing this by 365 yields 6.9 kWh/m^2 daily. This number seems
correct.

However, when I export the data to excel and select the monthly data
sheet -
I sum the "Incident Beam Radiation (kWh/m2), Monthly" column and
divide by 365 to get the avg. daily DNI and it calculates 4.76 kWh/
m^2.

I notice a similar problem in the hourly data sheet as well.

Is Incident Beam Radiation (kWh/m2) not the DNI?

Thanks,
Steve

[Paul Gilman]

Dear Steve,

The incident beam radiation values in the results are probably lower
than the direct normal irradiance values in the weather file because
you modeled an array with a fixed array (no tracking) or with single-
axis tracking. If you run the model with two-axis tracking, the two
values should be very similar.

Best regards,
Paul.

[Steve]

Paul,

Thanks for the prompt response.

According to the literature in the SAM help:

"Global Horizontal (kW/m2), Hourly

Global horizontal radiation value from the weather file, which
represents the radiation over the hour at the end of the hour.

Incident Beam (kW/m2), Hourly

Incident beam radiation value from the weather file, which represents
the radiation over the hour at the end of the hour.

Incident Diffuse (kW/m2), Hourly

Incident diffuse radiation value from the weather file, which
represents the radiation over the hour at the end of the hour.

Radiation Total (kW/m2), Hourly

Incident total radiation calculated by the weather processor based on
solar angles determined from the latitude and longitude values in the
weather file and array orientation options specified on the Array
page.

Incident Total (kWh), Hourly

The total average hourly incident global radiation absorbed by the
array, the product of the incident radiation in kilowatts per square
meter and the array area in square meters from the Array page."


I would expect Global Horizontal, Incident Beam, and Incident Diffuse
to be direct transfers from the Weather File. It does not sound like
incident angles are taken into consideration until the Radiation Total
column.

Thanks,
Steve

On Jul 26, 6:41 pm, Paul Gilman <solar.advisor.supp...@nrel.gov>
wrote:

> > Steve- Hide quoted text -
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