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High Plenum Temperature

asked 2015-08-27 16:46:35 -0500

updated 2015-11-23 08:43:35 -0500

I am modeling in EnergyPlus an existing DOE-2 model. The layout of building is pretty simple: 2 stories, 10 zones (8 perimeters and two cores). There is an unconditioned plenum above each zone. Here is the problem: I am noticing big discrepancies in zone temperatures in the summer between the two models, especially in the unconditioned plenum above each zone.

I've been trying to model everything identically :

  • The thermal performance of the opaque assembly is similar in both cases,
  • The thermal performance of the windows is similar (generated using WINDOW),
  • For the simulation, I am using a DOE-2 BIN weather file that I converted to EPW,
  • Efficiency and capacity of the HVAC equipment is the same,
  • The performance curves used in the DOE-2 model have been converted,
  • The internal gains and schedules are identical, etc ...

I am doing this exercise to learn more about EnergyPlus (I'm coming from DOE-2) but I cannot pinpoint what could be the issue here. I am probably missing something here that I may or may not know about EnergyPlus. Any input would be really appreciated!

Here is an example of what I am noticing. This example is for the perimeter zone facing East on the top floor and the plenum zone above it:

image description

image description

We can see that for the EnergyPlus model the HVAC system meets the loads during the day and at night, when it is off, the zone temperature goes up by almost 10 deg. F. This seems to be due to the fact that the temperature in the plenum is really high during the day, way higher than in the DOE-2 model.

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answered 2015-08-31 08:20:47 -0500

kwalkerman gravatar image

Just because there is a difference between EnergyPlus and DOE-2 doesn't make one right and the other wrong. With limited information, it's hard to pinpoint exactly what the difference is, but here are some ideas:

  1. Lighting Return Air Fraction - is the amount of heat from the lighting that is returned to the plenum modeled identically in the two models?

  2. Thermal mass of construction assemblies - it's my understanding that DOE-2 and EnergyPlus handle the thermal mass of construction assemblies fairly differently. Try running various models with both lightweight and heavyweight assemblies.

  3. Minimize variables - it seems like your example zone has too many variables. Run models with a core zone - no exterior surfaces at all - and see what the difference is. This would help identify whether the difference in the models is due to the plenum, or if there are larger differences in the building envelope.

  4. HVAC Systems - if you're getting started with EnergyPlus, setting up HVAC systems can take some getting used to. It's likely that the HVAC systems are not operating identically.

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Asked: 2015-08-27 16:46:35 -0500

Seen: 98 times

Last updated: Aug 31 '15