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Heat fluxes through wall not matching, leading to cumulative heat gain over annual simulation

asked 2020-07-07 14:02:32 -0600

DerricksRoss gravatar image

updated 2020-08-25 10:59:45 -0600

mldichter gravatar image

Hello all, I’m performing some modeling to look at the heat transfer through wall sections as well as determine the temperatures of the internal and external wall surfaces. I am currently having the model output the internal and external surface temperatures and conduction heat transfer rates. I initially noticed that the fluxes do not match and assumed it was some sort of thermal storage created by the wall materials warming/cooling. I then began inspecting the surface heat storage rate per area to see if they would all add up. The good news is that for each hour, the fluxes and the heat gain do all sum to zero (Figure 1: Heat flux table JPEG in drive folder). The bad news is that when I look at the heat gain of the wall over the course of the annual simulation, the heat gain in the wall climbs over time, where I would expect it to bounce around 0 as it stores/dissipates over time (Figure 2: Cumulative Heat Gain JPEG in drive folder). This could potentially make sense if the wall was slowly warming up over time but the surface temperatures do not suggest this. Is there something I am missing to explain how the wall is storing energy over time but the temperature is not increasing.

Notes: All analysis was performed on the Occupied Zone East Wall. Files are located in the linked drive folder(Energy plus files, Weather file, Excel file with representative days for each season (winter, shoulder, summer), figures)

Google Drive: https://drive.google.com/drive/folder...

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answered 2020-08-25 15:21:56 -0600

mldichter gravatar image

updated 2020-08-25 15:40:50 -0600

Looks like the culprit is an EnergyPlus bug. Unfortunately for @DerricksRoss, MaterialProperty:PhaseChange objects must use the HeatBalanceAlorithm = ConductionFiniteDifference to simulate, which appears to be causing the bug.

Here is a google drive link to the files.

Here is a link to the github issue I posted.

I took the CondFD1ZonePurchAirAutoSizeWithPCM.idf example file and made a few changes to it to simplify the model, the most important deleting the MaterialProperty:PhaseChange object in the IDF.

The model showcasing the bug has HeatBalanceAlgorithm = ConductionFiniteDifference. I made a sum of the heat flux into and out of the wall using

Output:Variable,*,Surface Outside Face Conduction Heat Transfer Rate per Area,Hourly;
Output:Variable,*,Surface Inside Face Conduction Heat Transfer Rate per Area,Hourly;

which results in the graph below in a five year simulation. As you can see, there is a trend of accumulation of heat in the wall. Keep in mind that this wall has NO PCM in it.

image description

The bug disappears by changing to HeatBalanceAlgorithm = ConductionTransferFunction, which is the default choice in the IDFEditor. The graph is below and heat storage in the wall is cyclical, as it should be.

image description

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Asked: 2020-07-07 14:02:32 -0600

Seen: 367 times

Last updated: Aug 25 '20