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Contradiction in simulation results: eQUEST VS OpenStudio

asked 2016-07-11 17:50:26 -0500

Morteza gravatar image

updated 2016-07-12 09:16:54 -0500

I am studying annual energy costs of two baseline models for a LEED residential building, PTAC VS PTHP. This is a 9 story building in Washington, DC and to simplify the process just created a bulk model per ASHRAE 90.1-2007 energy modeling requirements.

Keeping everything the same in two models except the HVAC system, simulation results per eQUEST version 3.65, built 7163 indicate changing PTAC to PTHP will increase the annual energy cost from $219,597 to $236,476 (-8.1% annual energy cost savings). This was the results that I was expecting based on my previous experience.

Surprisingly, when I use OpenStudio-1.12.0.ef50b89958-Win64, simulation results indicate changing PTAC to PTHP will decrease the annual energy cost from $102,130 to $117,449 (13% annual energy cost savings).

In short:

  • In eQUEST, changing PTAC to PTHP = 8.1% annual energy cost increase
  • In OpenStudio, changing PTAC to PTHP = 13% annual energy cost savings

following are links to files of this experience. I would appreciate it is someone would review these models and let me know the reason(s) of this contradiction.

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I would start by looking at a single HVAC type (PTAC or PTHP) in between the two models trying to figure out why you have half the consumption in OpenStudio compared to eQuest to begin with...

Julien Marrec gravatar image Julien Marrec  ( 2016-07-12 09:20:35 -0500 )edit

@Julien Marrec, I think as @ljbrackney pointed out it is the result of different approaches between the two calculation engines. I am struggling to find out why with EnergyPlus simulation engine PTHP system is more energy cost efficient than PTAC but in DOE-2 engine it is the opposite.

Morteza gravatar image Morteza  ( 2016-07-12 12:27:55 -0500 )edit

I seriously doubt that a factor of 2 is an acceptable range of deviation due to the calculation engine only. Otherwise there's either something really wrong about DOE 2 or about EnergyPlus, or both, as the purpose of both are seemingly to simulate actual real-world systems.

Julien Marrec gravatar image Julien Marrec  ( 2016-07-12 23:10:46 -0500 )edit

I tend to agree with Julien - the engines will contribute to some disagreement, but not all - certainly not as much as you're seeing. Did you follow up on @pflaumingo's observation below?

ljbrackney gravatar image ljbrackney  ( 2016-07-13 08:41:52 -0500 )edit

Yes I did, please refer to the answers part

Morteza gravatar image Morteza  ( 2016-07-14 10:53:03 -0500 )edit

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answered 2016-07-12 11:17:49 -0500

pflaumingo gravatar image

Taking a very brief look at the files it can be seen that there are many differences between your OpenStudio and eQuest models. For example, your EIR inputs in your eQuest PTHP model for heating is very different to 1 / COP of your OpenStudio PTHP. I would look further into aligning your inputs.

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answered 2016-07-14 11:28:45 -0500

Changing from a PTAC (with some form of heating at COP <=1) to PTHP where heating COP > 3 should always reduce energy consumption. Also, there should be no differences in cooling energy if the cooling system performance is the same.

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This is true and in this experience the simulation results in both of the engines approved it too. However, in heating dominated climate zones (i. e. Zone 4A) and for heating dominated building types (i. e. residential buildings) changing PTAC to PTHP may increase the annual energy cost to a certain level, which depends on heating efficiency of the two systems. This is the effect of the difference between costs of different sources of energy. PTAC uses gas for heating, which is cheaper than the electricity, but PTHP uses electricity.

Morteza gravatar image Morteza  ( 2016-07-14 12:56:39 -0500 )edit

answered 2016-07-12 09:06:28 -0500

updated 2016-07-12 09:24:09 -0500

The differences may be attributed to the underlying engines (DOE-2 and EnergyPlus) rather than the interfaces you're using. The engines take very different approaches to how they structure the problem and model HVAC equipment. As one example: a number of advanced HVAC systems can only be approximated in DOE-2 through clever configuration of input parameters, whereas EnergyPlus allows for explicit modeling of more complex systems. The list of differences between the engines is fairly long, and there are a number of papers, articles, etc. on the internet that attempt to dissect them in some detail. Here is a fairly extensive one with additional citations that may be helpful in explaining some of them.

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@ljbrackney, thank you for your comment and the provided information, finding in this thesis paper is very helpful; I will study the whole paper.

Morteza gravatar image Morteza  ( 2016-07-12 12:08:46 -0500 )edit

@pflaumingo, thank you for your comment, I will check all the entries in both of the models one more time and will wait to hear more from you.

Morteza gravatar image Morteza  ( 2016-07-12 12:13:36 -0500 )edit

answered 2016-07-14 11:00:04 -0500

Morteza gravatar image

updated 2016-07-14 14:46:35 -0500

I revised all the data entries in all of the models. Now all entries including system efficiencies of PTAC and PTHP systems comply with ASHRAE 90.1-2007. The default rated cooling COP for PTHP was 5 in OSM and 3.2 in eQUEST, revising this value increased the annual energy consumption of the OSM model. In addition, the energy cost related to DHW was not included in the OpenStudio models. With these changes, the differences between the annual energy consumptions of the identical models in different engines are reasonable, but my original question is still there; in EnergyPlus engine PTHP is more energy cost efficient than the PTAC but in DOE-2 engine it is the opposite. I think EnergyPlus engine (the default values for the system) assumes PTHP much more energy efficient system than the PTAC. Following is the summary table of the simulation results.

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Have you compared the curves for the equipment between the two simulation engines? Have you compared fan inputs? The PTHP fan in OpenStudio consumes far less energy, which is fairly suspicious. Interior lighting is off by a fair amount, which is really shouldn't because ideally it would just be LPD x schedule fraction in both engines. Equipment loads are rather far off.

pflaumingo gravatar image pflaumingo  ( 2016-07-14 11:08:56 -0500 )edit

As you have mentioned, the PTHP fan in OpenStudio consumes far less energy. This is the main reason for the energy consumption difference of the PTHP in two engines. As you can see in the following screen shots above entry for both of the models are the same, but the annual energy consumptions are very different!

Morteza gravatar image Morteza  ( 2016-07-14 14:46:06 -0500 )edit

What about fan control? Constant fan vs cycling fan operating modes. Check the PTHP Supply Air Fan Operating Mode Schedule Name input field. Add a constant fan schedule if the input is blank. I'm not sure how EQuest handles fan operating mode.

Schedule:Compact, OnSched, Fraction, Through: 12/31, For: AllDays, Until: 24:00, 1.0;

rraustad gravatar image rraustad  ( 2016-07-14 15:09:37 -0500 )edit

Thank you for your suggestion, Supply Air Fan Operating Mode Schedule Name was “Always On Discrete” in PTAC and was blank in PTHP so I added the same schedule for PTHP. Now the fan energy consumption is the same in both of the OSM models. Thank you all for your time and your help in this issue.

Morteza gravatar image Morteza  ( 2016-07-14 17:28:17 -0500 )edit

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Asked: 2016-07-11 17:50:26 -0500

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Last updated: Jul 14 '16