Re: [EnergyPlus_Support] diffuse and direct solar

[Linda Lawrie <linda@xxxxxxxxxx>]

At 07:34 AM 11/18/2002, Adriana Angelotti wrote:
> I'm checking Energy Plus results by
> comparing meteorological data provided as input in the epw file
> with the output obtained from the simulation.
>
> I found that the hourly output "Diffuse Solar" does not match
> the input "Diffuse Horizontal Radiation".
>
> About the output "Direct Solar", I wonder wether it is
> horizontal direct solar radiation, calculated by Energy Plus, or Direct
> Normal Radiation, in which case it should match the input in the epw
> file.

As shown in the .csv of the weather files, the two solar radiation values
used in EnergyPlus are:

Direct Normal Radiation {Wh/m2}
and
Diffuse Horizontal Radiation {Wh/m2}

These are called Direct Solar and Diffuse Solar in the EnergyPlus reporting.  (eplusout.eso)

If you use an hourly time step and reporting, you should see the same values reported.  If you are using less than hourly time steps in your EnergyPlus input, things will get a bit confusing due to EnergyPlus Weather Interpolation and the "summing" of these values when reported.

Solar radiation values have a particular problem in EnergyPlus reporting and the interpolation scheme as they are not "continuous" values (like dry bulb where not much variation happens during the course of a day) and the interpretation of the data coming from weather sources is also a bit confusing. 

For example, the TMY2 manual says for the Solar Radiation value:
"Amount of solar radiation in Wh/m2 received
within a 5.7° field of view centered on the sun
during the 60 minutes preceding the hour
indicated"

In practice, virtually all energy analysis programs treat the incoming solar radiation as an "instantaneous" value.  This works well for hourly programs which only have one value to deal with.  EnergyPlus has a compounding problem here -- because it must determine how to break up that hourly value into several time steps (unless the time step input is hourly).

Linda

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