Area and volume calculations in EnergyPlus

asked 2018-04-23 01:22:50 -0500

Tobias
181 ●5

updated 2018-04-23 13:32:24 -0500

2208 ●6 ●10 https://github.com/jasondegraw

I have a question regarding a discrepancy I have found between the EnergyPlus tips and tricks (https://bigladdersoftware.com/epx/doc...) and a study I am performing.

To be more specific the tips and tricks document states that: “A suggested approach is to use outside dimensions for exterior surfaces, and centerline dimensions for interior surfaces. This produces fully connected geometry with an appropriate amount of floor area, zone volume, and thermal mass.” However in my study I have found that EnergyPlus uses that total distance between surfaces to calculate floor area. This can be seen when calculating equipment load and people load (since that is inputtet pr. floor area). Does anybody have an explanation for this?

Cheers

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answered 2018-04-23 13:48:13 -0500

Jason DeGraw

2208 ●6 ●10 https://github.com/jasondegraw

The EnergyPlus geometry model represents walls as zero-thickness surfaces, and the linked document is providing guidance on how to get from a building description that includes wall thickness (e.g. architectural drawings) to one that does not include wall thickness(the E+ input). If done badly, this process could end up distributing thermal mass etc. improperly. In many cases, the discrepancy introduced is not worth worrying about, but it is possible to override volume and floor area in the zone object to get what you need when the E+ calculated values aren't good enough.

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Thank you for your answer! So to make sure that I have understood your explanation of the tips-and-tricks-document correctly:

1) Using an architectural model (with wall-thickness) as base, one should assign the E+ zero-thickness surfaces to the outside dimension for exterior surfaces and centerline dimension for interior surfaces?

2) If the above is understood correctly then I'm a little confused as to why E+ then calculates the floor area as the total distance between its zero-thickness surfaces, without subtracting given wall thicknesses? (shown from the result of my study)

Tobias ( 2018-04-24 14:08:51 -0500 )edit

Yes, that's generally speaking a good approach. It is not always the best approach (the last sentence of the document you linked to describes an example: old buildings with really thick walls).
E+ doesn't do that because there are many other possible ways to do the subdivision, and how it is done isn't available to the engine. Even if it was, in many situations there are complications (wall thickness isn't uniform, etc.), so it makes more sense to provide an override.

Jason DeGraw ( 2018-04-24 14:58:47 -0500 )edit

Thank you Jason!

1) Since my exterior walls are very thick, I will follow the recommendation in the last sentence and use centerline dimension all around, in order to produce the correct amount of thermal mass. -Do you happen to know how the amount of thermal mass is calculated exactly (couldn't seem to find the information in the documentation)? -And secondly do you know if this approach also will produce the correct amount of surface areas for the heat loss calculation?

Using centerline dimension I will then use your recommended override to input the correct floor area and zone volumen

Tobias ( 2018-04-28 04:39:47 -0500 )edit

The documentation probably wouldn't cover that, you'd need to look at the code to be sure. But I think I wouldn't worry about that too much if you feel like your approach is sound and you don't get strange results. If you encounter issues, then worry about it. The same goes for the surface areas. The differences from where you choose to draw the surfaces will hopefully be pretty small. You could always do both and see what the differences are. That'll take making two models, but would establish what the possible variation is.

Jason DeGraw ( 2018-04-28 12:42:34 -0500 )edit

Makes sence - thank you

Tobias ( 2018-05-12 06:48:20 -0500 )edit

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