The PlantLoop:EIR heat pump power calculation is incorrect

[rraustad]

Issue overview

The equation:

this->powerUsage =
    (this->loadSideHeatTransfer / this->referenceCOP) * eirModifierFuncPLR * eirModifierFuncTemp * InputPowerMultiplier * this->cyclingRatio;

uses loadSideHeatTransfer which is already multiplied by PLR a few lines up. This means the eirModifierFuncPLR curve output must be near unity to give appropriate results. The EIRfPLR curve typically accounts for a change in power as a function of PLR. Of course, the definition of this curve could be discussed.

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[jmarrec]

The EIRfPLR curve typically accounts for a change in power as a function of PLR. Of course, the definition of this curve could be discussed.

Right, that's the gist of the problem here.

The testfiles are using these coefficients:

EnergyPlus/testfiles/PlantLoopHeatPump_EIR_WaterSource.idf

Lines 281 to 323 in cb9e70f

	Curve:Biquadratic,
	CapCurveFuncTemp, !- Name
	1.0, !- Coefficient1 Constant
	0.0, !- Coefficient2 x
	0.0, !- Coefficient3 x**2
	0.0, !- Coefficient4 y
	0.0, !- Coefficient5 y**2
	0.0, !- Coefficient6 x*y
	5.0, !- Minimum Value of x
	10.0, !- Maximum Value of x
	24.0, !- Minimum Value of y
	35.0, !- Maximum Value of y
	, !- Minimum Curve Output
	, !- Maximum Curve Output
	Temperature, !- Input Unit Type for X
	Temperature, !- Input Unit Type for Y
	Dimensionless; !- Output Unit Type
	Curve:Biquadratic,
	EIRCurveFuncTemp, !- Name
	1.0, !- Coefficient1 Constant
	0.0, !- Coefficient2 x
	0.0, !- Coefficient3 x**2
	0.0, !- Coefficient4 y
	0.0, !- Coefficient5 y**2
	0.0, !- Coefficient6 x*y
	5.0, !- Minimum Value of x
	10.0, !- Maximum Value of x
	24.0, !- Minimum Value of y
	35.0, !- Maximum Value of y
	, !- Minimum Curve Output
	, !- Maximum Curve Output
	Temperature, !- Input Unit Type for X
	Temperature, !- Input Unit Type for Y
	Dimensionless; !- Output Unit Type
	Curve:Quadratic,
	EIRCurveFuncPLR, !- Name
	1.0, !- Coefficient1 Constant
	0.0, !- Coefficient2 x
	0.0, !- Coefficient3 x**2
	0.0, !- Minimum Value of x
	1.0; !- Maximum Value of x

I can't think of another object were the EIRFPLR for a constant COP machine would be EIRFLPR(PLR) = 1. All other objects I can think of, (eg Chiller:Electric:EIR) would expect EIRFPLR(PLR) = PLR

It's not clear to me reading the I/O reference that the HeatPump:PlantLoop:EIR:XXX objects behave that way either.

https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-plant-equipment.html#plhp_eir_cooling_inputs_eirft

https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-plant-equipment.html#field-electric-input-to-cooling-output-ratio-function-of-temperature-curve-name

[jmarrec]

eg ChillerELectricEIR, taking ElectricEIRChiller McQuay WSC 471kW/5.89COP/Vanes EIRFPLR as an example

https://github.com/NREL/EnergyPlus/blob/cb9e70f16540f0dca5c979f106d985dd8d301d6d/datasets/Chillers.idf#L256-L262

[rraustad]

To be consistent with other models the equation of power would use full capacity/COP and use an EIR curve proportional to PLR. There has always been a question of whether EIRfPLR should show proportionality of power to PLR or if the PartLoadRatio variable should account for that adjustment (i.e., the equation below could be written different ways). The existing equation uses this->loadSideHeatTransfer/COP which already accounts for PLR, which is why the EIRfPLR curve for this model is unity. There was an attempt to correct this in #9979 but that change was reverted here because of diffs and this issue opened to discuss/correct the power calc in this model.

this->powerUsage =
(availableCapacity / this->referenceCOP) * eirModifierFuncPLR * eirModifierFuncTemp * InputPowerMultiplier * this->cyclingRatio;

[mjwitte]

As @rraustad explains above, it all depends on what EIR means. From the IDD

Chiller:Electric:EIR

  A3 , \field Electric Input to Cooling Output Ratio Function of Temperature Curve Name
       \note Electric Input Ratio (EIR) as a function of temperature
       \note EIR = 1/COP
       \required-field
       \type object-list
       \object-list BivariateFunctions
       \note curve = a + b*CWS + c*CWS**2 + d*ECT + e*ECT**2 + f*CWS*ECT
       \note CWS = supply (leaving) chilled water temperature(C)
       \note ECT = entering condenser fluid temperature(C)
  A4 , \field Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve Name
       \note Electric Input Ratio (EIR) as a function of Part Load Ratio (PLR)
       \note EIR = 1/COP
       \required-field
       \type object-list
       \object-list UnivariateFunctions
       \note quadratic curve = a + b*PLR + c*PLR**2 is typical, other univariate curves may be used
       \note PLR = part load ratio (cooling load/steady state capacity)

If EIR=1/COP and we apply modifier curves to that, then flat curves of 1 would represent constant COP.
Then CurrentPowerIn = CurrentLoad*CurrentEIR.

But if only one model thinks this way, and the rest assume CurrentPowerIn = AvailableCapacity*CurrentEIR then we should change this one to be consistent and attempt to add a laugh test for the EIRfPLR curve (here and everywhere) to warn if the user (or the curve library) is understanding this curve differently.

Do any other tools have a similar curve for chillers? If so, how is it used?

[mjwitte]

I looked at the ASHRAE Handbook of Fundamentals 2021, Chapter 19 for some guidance. Based on the equations in the text (esp the first one below), it appeared to me that EIRfPLR (f2 below) should be flat=1 for constant efficiency. From page 19.16:

But then a little further down on p. 19.17, the example curves clearly show the other assumption, so go figure. I would argue that the first line of eq 6 above should be P=PIR x Cavail.

[rraustad]

DOE 2.1E Eng. doc https://doe2.com/Download/DOE-21E/

[jmarrec]

@Myoldmopar I just realized you implemented the WWHP model in #6766. Do you have any thoughts?

Is this just a case of fixing the documentation, or should we also change the definition of the EIRFPLR / EIRFT curve?

[jmarrec]

Ping @Myoldmopar