Global radiation [W/m2] as output

asked 2016-09-01 06:47:43 -0500

Elbaek
53 ●4

updated 2017-05-04 13:56:07 -0500

How do you extract the Global radiation from Eplus? I've been looking into the weatherfiles and the data entries for Global radiation does not seem to correspond with those reported by EnergyPlus under following two outputs:

Site Diffuse Solar Radiation Rate per Area

Diffuse solar is the amount of solar radiation in W/m2 received from the sky (excluding the solar disk) on a horizontal surface.

Site Direct Solar Radiation Rate per Area

Site Direct Solar Radiation Rate per Area is amount of solar radiation in W/m2 received within a 5.7° field of view centered on the sun. This is also known as Beam Solar.

I'm aware that you have to adjust the direct solar with respect to solar angle, but even then i do not seem to get the same results. I've been using the following formula for the conversion:

G_global = G_direct*cos(90-SolarAltitude)+G_Diffuse.

Thanks in advance for any assistance.

EDIT: By the way, the specific values i see in the weatherfile and from EnergyPlus output is as follows:

At 12:00, the weather file reads: Global radiation: 103 W/m2 Diffuse radiation: 63 W/m2 Direct radiation: 264 W/m2

Energy plus outputs (also 12:00) read: Solar Altitude: 11.2 degrees Site diffuse: 50.5 W/m2 Site direct: 188 W/m2

EDIT 2: Correct answer.

Archmages comment gave the answer to my question. It reads: The weather file value is interpolated down to work for individual EnergyPlus timesteps. Solar data in the weather file is also shifted a half hour compared to the other weather data, making the interpolation tricky. Values inserted into your equation need to be from, and compared to, outputs at the shortest reporting frequency or the time averaging will introduce errors. I am afraid it is all much more complicated that it appears.

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answered 2016-09-01 08:42:00 -0500

Archmage

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There is also an output variable called Surface Outside Face Incident Solar Radiation Rate per Area. Assuming you want global horizontal, then look at this output for a horizontal surface.

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Hi there. Thanks for the reply. I have also tried this output, and i'm afraid i can't make sense of that value either. I get the following values using only numbers from EnergyPlus outputs.

G_global = G_direct*cos(90-SolarAltitude)+G_Diffuse Inserting the number give: 188.*cosd((90-11.2))+50.5 = 87.02 W/m2

The number reported by EnergyPlus is 89,6 W/m2 - so fairly close to the above calculation. However, the weatherfile still read 103 for the hour. I'm unsure where this inconsistensy comes from.

Thank you

Elbaek ( 2016-09-02 01:57:45 -0500 )edit

The weather file value is interpolated down to work for individual EnergyPlus timesteps. Solar data in the weather file is also shifted a half hour compared to the other weather data, making the interpolation tricky. Values inserted into your equation need to be from, and compared to, outputs at the shortest reporting frequency or the time averaging will introduce errors. I am afraid it is all much more complicated that it appears.

Archmage ( 2016-09-02 07:30:55 -0500 )edit

Hi again. I was aware of the interpolation, which is why i compared only values at exact timesteps (12:00), at which all other values (temperatures etc.) was perfectly aligned. I did not know about the 30 minute shift in solar data! This is probably the culprit.

Thank you so much for your competent assistance. I'll look into this next week and try to remember to confirm if this was in fact the case.

Elbaek ( 2016-09-02 08:18:30 -0500 )edit

The solar radiation in the data file is not shifted by half an hour. It is given as the sum of the incident radiation over the previous hour. Some programs do treat this as the instantaneous value at the half hour, but this does not return the same sum as from the data file. I suspect that this is the difference between the values that you are seeing.

Tim McDowell ( 2016-09-02 09:14:36 -0500 )edit

Hi Tim. Thanks for your input.

I'm running Eplus on a 1 minute timestep at the moment. I would have expected the readings to be either average over the whole past timestep, or as Archmage suggested pointsvalues from somewhere in the timestep - possibly at the half-minute mark. I'm fairly sure the unit on all of the solar measurements are in rates (W/m2). So i'm guessing by sum you mean just the average rate over the timestep?

Elbaek ( 2016-09-05 01:17:57 -0500 )edit

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answered 2016-09-06 09:13:02 -0500

Tim McDowell
651 ●3 ●2

My comment was for the data in the weather file and not for EnergyPlus' specific interpretation of the weather file. If you look at the definition of the global horizontal solar radiation in TMY3 it is "Total amount of direct and diffuse solar radiation received on a horizontal surface during the 60-minute period ending at the timestamp" and the units are Watt-hour per square meter. So if you are comparing back to the weather file this is what you are comparing to.

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Oh. Okay. I'm talking about comparing output from EnergyPlus to the weatherfile. The weatherfiles are typically given in hourly values, meaning that Wh/m2 effectively gives you the average W/m2 throughout the hour. So i guess both ways of look at it are right.

I can confirm that Archmage was right in his answer in relation to the question i (meant) to ask.

Thank you for the response anyway. :)

Elbaek ( 2016-09-07 04:57:31 -0500 )edit

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