Elbaek's profile - activity

I can't remember if you can get that output directly, but you could always calculate it using the direct and diffuse solar radiation on horizontal (this you can get), and the various solar-angles.

I agree with David. Furthermore, the distribution of windows on different facades will also affect your result. If i understand the question right, you should by no means expect the same answer from different WWRs.

I agree with David. Furthermore, the distribution of windows on different facades will also affect your result. If i understand the question right, you should by no means expect the same answer from different WWRs.

Hi Archmage.

I'm aware that adiabatic walls still contribute to thermal mass (as they should). What i'm not convinced entirely of is that you can be sure no heat is 'lost' through these walls when they are not symmetric.

The same boundary conditions on a symmetric wall would create an adiabatic layer directly in the middle of the wall, where heat from each zone would never diffuse through. For assymetrical walls (eg. 100 mm insulation followed by 100 mm concrete) in buildings where temperatures are fluctuating - can you be certain of the creation a truly adiabatic layer?

Thank you, Rasmus

Hi.

I've been reading up on how Eplus handles the adiabatic boundary condition. Documentation says: "EnergyPlus will apply the same boundary conditions to each side of the construction so that there is no temperature difference across the surface."

I get why this would work well for symmetrical walls, but what if you have a typical concrete slab floor with insulation and wood on top? As far as i can tell, wouldn't that cause potential heat exchange/loss to that construction element?

Hope that was clear. :)

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. :)

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?

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.

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

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.