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EIRfPLR vs EIRfPLR&dT

asked 2018-09-17 07:48:04 -0500

updated 2018-09-17 18:09:21 -0500

For the chiller electric input ratio as a function of part load ratio chiller curve, IES-VE and eQuest both use EIRfPLR&dT, a bi-quadratic curve which requires 6 coefficients (a-f) and has two variables, PLR and lift (dT, ECWT and LCHWT).

But for Chiller:Electric:EIR EnergyPlus seems to only allow "EIRfPLR" as a quadratic curve, with only one variable, PLR, and 3 coefficients (a-c).

It then doesn't appear that its possible to use use the same curve fits in IES-VE or eQuest as it is in EnergyPlus. There is a third option using Chiller:Electric:ReforumlatedEIRbut that equation is either bi-cubic, and depending on LCWT (not ECWT) and PLR, or a custom curve dependent on dT* and T*dev.

I found this because I created custom chiller curves for use in IES-VE, and wanted to run some tests in EnergyPlus, but found it wouldn't accept my bi-quadratic curve.

Can someone explain the theory behind why one of these curve fits should be used over another and which one is 'better', and confirm that its not possible to use the same curve in all softwares? Unless you are using, the EIRfPLR quadratic curve only since you can always use fewer coefficients, but not more.

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Which object are you refering to? Chiller:Electric:EIR for eg uses 3 curves: CapfT, EIRfT, EIRfPLR (Engineering ref). It's the DOE 2.1 formulation, plus a couple new features to model stuff like evaporative condensers.

Julien Marrec gravatar imageJulien Marrec ( 2018-09-17 08:01:06 -0500 )edit

@Julien Marrec, I've added more detail to my question to better explain what I am looking for. I'm specifically interested in the EIRfPLR curve, not CAPfT or EIRfT.

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2018-09-17 18:03:59 -0500 )edit

Do you have a link to the IES reference manual perhaps? Because reading the DOE2.1 Engineers manual (see here around page 575 of the PDF), eQuest uses the exact same formulation as E+ at least in DOE21 (I don't have windows so I don't have the latest eQuest to check)

Julien Marrec gravatar imageJulien Marrec ( 2018-09-18 02:57:08 -0500 )edit

Specifically, I'm interested to see the entire set of curves IES VE uses. Because if they use two curves, CapFT and EIRfPLR&dT, and E+ uses CapFT, EIRfPLR and EIRfdT, then it's just a different way of representing the same space.

Julien Marrec gravatar imageJulien Marrec ( 2018-09-18 02:58:42 -0500 )edit

IESVE uses the same the general 3 curve model from the old DOE2, see the link below around figure 3-107 (looks like the screen shot is a mistake and a copy of the previous curve, but the text below is right). The function of each curve seems to be the same as the DOE2 method, with the exception to the complexity of the EIRfPLR one that Anna mentioned, that she and I can’t figure out. Seems like mybe they’ve just adjusted the model slightly?

https://help.iesve.com/ve2018/electri...

bbrannon4 gravatar imagebbrannon4 ( 2018-09-18 07:31:05 -0500 )edit

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answered 2018-09-18 13:33:51 -0500

Erik Kolderup gravatar image

updated 2018-09-18 13:39:47 -0500

Hi, Anna.

The Chiller:Electric:EIR model in EnergyPlus is the same as the DOE2.1 model, which is also available in eQUEST. The form of the model is the same in both cases, but the coefficients would be different for the same chiller because EnergyPlus expects SI units and DOE2/eQUEST expect IP units (there's an EnergyPlus "coeffconv" utility that can help with converting DOE to E+). The three curves are CAP-FT, EIR-FT and EIR-FPLR.

The Chiller:Electric:ReforumlatedEIR model in EnergyPlus is similar to the eQUEST model that has the added "dT" term, and the goal is to more accurately represent the impact of condenser water temperature reset. But the EnergyPlus model uses a different form compared to the eQUEST model. The main differences are 1) the EnergyPlus model takes condenser water leaving temperature as an input rather than entering temperature, and 2) the EIR-FPLR curve is a function of both PLR and condenser water temperature.

Here's the section in the Engineering Reference: https://bigladdersoftware.com/epx/doc...

I believe that it's better to use the reformulated curve if you have the appropriate performance data to be able to create one, especially if you are evaluating the impact of condenser water control. But the other models are pretty good too.

I can't speak to IES-VE curves.

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Asked: 2018-09-17 07:48:04 -0500

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Last updated: Sep 18 '18