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Chiller performance curve adjustment for ASHRAE90.1 Baseline case

asked 2022-10-04 11:39:13 -0600

Keigo's avatar

updated 2022-10-04 11:44:58 -0600

This question is related to my previous post, but I separated.

I want to model ASHRAE90.1 Baseline cases. Specific COP and IPLV need to be input. COP is easy to be input. For IPLV, Chiller:Electric:EIR has three performance curves that directly affect the IPLV calculation.

  1. Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve
  2. Cooling Capacity Function of Temperature Curve
  3. Electric Input to Cooling Output Ratio Function of Temperature Curve

Each curve has many Coefficients. Which Coefficient should be adjusted to get the target IPLV?

ANSI/ASHRAE/IES Standard 90.1-2016 Performance Rating Method Reference Manual shows two curves: Chiller Cooling Capacity Adjustment Curve and Electric Chiller Cooling Efficiency Adjustment Curves. And the Manual says that the default curves should be used for the Baseline case. Actually, the coefficients of the two curves shown in the Manual are different from the defalut coefficients which are generated from HVACTemplate:Plant:Chiller. It may be because EnergyPlus uses SI unit and the Manual is written with IP unit. I'm not sure. But if these two curves should not be changed, the only curve we can change for the Baseline case is Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve. This curve is Curve:Quadratic and it has only three coefficients. I think any of the three coefficients can be changed. Is my interpretation correct?

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answered 2022-10-05 09:37:38 -0600

Jim Dirkes's avatar

ASHRAE 90.1's IP version is definitely NOT applicable for use with E+. All E+ curves must use SI units (and the coefficients will differ for IP and SI).

I'm not sure whether the PNNL Reference Guide has any "Authority" to define Baseline curves - and thus can be considered authoritative "suggestions". In part that's because most certification bodies are not using 90.1-2016 and in part because the "Reference Manual" is not actually part of any code - it's a supporting document. (I was not aware of the PNNL document previously - thanks for making it known to me.)

The PNNL document has curve coefficients for both EIR f(T) and EIR f(PLR) on p.261, so there are three curves in total shown in that document. For each of the curves, all coefficients act in concert to produce a characteristic performance. For a Quadratic curve, that is also true, so you can change any of them, but should do so with an eye toward the overall impact.

One aspect of these curves is that they need to produce a "normalized" result of exactly 1.0 at the AHRI design conditions. I normally calculate the result at AHRI conditions, then adjust it proportionally to achieve 1.0.

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Thank you for your answer. I overlooked the EIR f(PLR). Then, right, all three curves are shown in the PNNL Reference.

you can change any of them

OK, but so many options...

Keigo's avatar Keigo  ( 2022-10-06 02:05:59 -0600 )edit

One aspect of these curves is that they need to produce a "normalized" result of exactly 1.0 at the AHRI design conditions.

Do you mean that the COP@100%load corrected by the performance curves should tally with the Reference COP? If that's the case, I agree, but the problem is that EnergyPlus does not use the AHRI condition as I mentioned in my previous post. If I prioritise the reported IPLV in the html file, I need to use the same conditions that EnergyPlus uses.

Keigo's avatar Keigo  ( 2022-10-06 02:14:43 -0600 )edit

Please report the apparent error to E+ Issues page at

Since the program bug affects only part load conditions, the COP should calculate correctly (at 100%)

Regardless, do the best within the constraints. For DX coils, I edit only the EIR f(T) curve and leave the f(PLR unchanged - it's a "fictitious" calculation, after all, unless you're using the actual performance data from a manufacturer (which is difficult to obtain and better addressed using one of the E+ Table objects).

ps, You are doing what few modelers do - working to make the IPLV correct!

Jim Dirkes's avatar Jim Dirkes  ( 2022-10-06 06:13:53 -0600 )edit

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Asked: 2022-10-04 11:39:13 -0600

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Last updated: Oct 05 '22