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Help With Constructing the Simplest Load Balance with EnergyPlus

asked 2023-12-08 13:32:43 -0600

chriswmackey's avatar

updated 2023-12-10 08:29:48 -0600

Hello Unmethours Community,

I have been struggling to construct the simplest load balance that I can from a single-zone unconditioned box EnergyPlus model. There are no internal loads and all gains and losses are purely from the climate (solar and conduction). My reasoning is that, if I add up the values of the following two EnergyPlus outputs, the result should be zero if EnergyPlus is truly running a balanced calculation:

Surface Window Net Heat Transfer Energy

Surface Inside Face Conduction Heat Transfer Energy

When I do this for a box model without any windows, I essentially get what I'm after where the sum of these values gives me the following result for each month in kWh, which is very close to zero:

  • Jan: -1.738775
  • Feb: -1.122338
  • Mar: -0.431097
  • Apr: -1.676013
  • May: -0.037371
  • Jun: -0.344535
  • Jul: -1.520733
  • Aug: -1.758476
  • Sep: -1.89579
  • Oct: -2.799482
  • Nov: -2.171337
  • Dec: -1.434317

However, as soon as I add windows, things start becoming imbalanced such that, by the time my box model is 90% glazed, I get the following monthly kWh totals by summing those two E+ outputs across the model:

  • Jan: 317.94952
  • Feb: 346.767665
  • Mar: 361.9911
  • Apr: 338.261545
  • May: 390.638818
  • Jun: 378.609665
  • Jul: 418.09426
  • Aug: 382.281574
  • Sep: 365.228805
  • Oct: 356.298138
  • Nov: 300.882507
  • Dec: 327.779518

The full load balance of the model can be visualized with the following chart, where the "Storage" term represents the remainder after summing those two E+ outputs:

image description

Granted, I am calling it storage because I just don't know what it is but it's not an accurate description because the average air temperature in the room is not heating up over the whole simulation.

So my question is what is it that I am missing when I use those two EnergyPlus outputs to try and construct the load balance of the room? Where are those 300-400 kWh of thermal energy going in each month if they are not leaving through the windows or the opaque surfaces? And is there a combination of E+ outputs that I could request for this model, which would actually produce a result that sums to zero for a complete balance.

Here is the IDF of the single zone box with 90% glazed walls in case anyone wants to reconstruct this experiment for themselves:

Thank you, as always.

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answered 2023-12-12 11:25:53 -0600

chriswmackey's avatar

I have an update after some more tests. I think I have narrowed down the issue to the fact that the Surface Window Net Heat Transfer Energy output does not include a full balance of energy transfer across the windows of the Zone and there is a significant amount of energy transfer across the window to the outside of the zone that is excluded in this output. First, I can say pretty confidently that it's not a fluke of my particular model since I get these large remainders using a wide range of different climate files and they persist whether the building is conditioned or unconditioned. As long as the model is very glassy, I cannot get the balance to sum to zero and there is consistently a net positive load balance.

Insufficiencies in the Surface Window Net Heat Transfer Energy output would explain why, when there are no windows in the Zone and I simply sum the Surface Inside Face Conduction Heat Transfer Energy output, I get something close to zero (since there are no values for the Surface Window Net Heat Transfer Energy in this case). However, when I do the inverse of this situation and I set the floors and roofs to be adiabatic and ensure all of the walls have 100% glazing, summing all of the Surface Window Net Heat Transfer Energy outputs does not give me zero. In fact, I get monthly totals of kWh that are even worse than the 90% glazed case that I looked at earlier:

  • Jan: 443.939339
  • Feb: 500.495785
  • Mar: 543.489841
  • Apr: 523.035294
  • May: 604.308156
  • Jun: 593.26514
  • Jul: 650.61024
  • Aug: 599.868727
  • Sep: 548.410587
  • Oct: 524.485894
  • Nov: 431.45041
  • Dec: 446.534646

This is what the load balance looks like for this case: image description

Where the storage term is, again, this remainder that is trying to account for the fact that Surface Window Net Heat Transfer Energy does not give us the full heat transfer across the window.

Given the way that this Window Net Transfer output is described in the E+ docs, I'm inclined to think that there is either a bug in EnergyPlus or the docs are wrong and one of these terms is not accounted for in the calculation of Surface Window Net Heat Transfer Energy:

  • Surface Window Transmitted Solar Radiation Rate
  • Convective heat flow from the zone side of the glazing
  • Net IR heat flow from the zone side of the glazing
  • Short-wave radiation from zone transmitted back out the window
  • Conduction through window frame and divider, if present

I should also clarify that there is no frame and divider in any of these tests that I am running and the window is not shaded. So Just the first 4 terms should describe the full full load balance of my most recent test.

It's also possible that there is still some term other than those accounted for in Surface Window Net Heat Transfer Energy, which I ... (more)

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@chriswmackey nice work Chris this question was interesting to read I learnt something ; 0!

antonszilasi's avatar antonszilasi  ( 2023-12-14 00:38:22 -0600 )edit

FYI, I have opened an issue on the EnergyPlus GitHub after being unable to conclude why the Surface Window Net Heat Transfer Energy is more positive than it should be:

chriswmackey's avatar chriswmackey  ( 2023-12-19 11:20:41 -0600 )edit

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Asked: 2023-12-08 13:32:43 -0600

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Last updated: Dec 12 '23