VRF performance curve: EIR modifier for PLR > 1

asked 2022-06-28 08:27:19 -0500

171 ●1 ●6

The VRF curve system model in energyplus has a specific performance curve to modify the EIR as a function of the part-load ratio (PLR) when the PLR is greater than 1.

However, actually the PLR will never be greater than 1 during the simulation, because the algorithm limits the indoor units capacity to match PLR=1 (as described in Engineering reference v9.5, page 1183). In fact, the outdoor unit can't provide more capacity than its maximum available capacity. So in practice, this performance curve is never used.

I thought that this performance curve was useful to introduce power input data for a Combination Ratio (CR) greater than 100%. But in this situation the PLR isn't greater than 1 as well, since the maximum available capacity of the outdoor unit already includes the Combination Ratio correction (page 1179, equation 16.449). However, the cooling power calculation (page 1186, equation 16.461) modifier curves doesn't include any correction as a function of CR.

Am I right about the PLR calculation? And if so, is there any specific input that I'm missing for taking into account the change in the reference power input when CR>100%? Something similar to the Combination Ratio correction factor, but applied to the power input calculation.

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answered 2022-06-28 09:01:19 -0500

rraustad

13599 ●72 ●12 http://www.fsec.ucf.edu/

As your description of the issue suggests it is hard to tell if that specific performance curve is used correctly given the interactions of the TUs and outdoor unit. Figure 8 of this document shows the capacity and power as a function of combination ratio (CR) for 2 specific VRF models. The inflection point of each of these models shows where CR=1. There is certainly a difference in both capacity and power at CRs > 1. To be sure this curve is never used, a user would have to set up a building model where the load (and TU total rated capacity) exceeds the outdoor unit capacity such that the TUs would provide more cooling capacity then what is provided by the "operating" capacity of the outdoor unit. If I recall correctly, I have seen PLR exceed 1, but only rarely. Which may mean the issue you describe is real or that the model I was testing was not set up correctly.

In the equations below, CoolEIRFPLR2 is the performance curve specified by the AirConditioner:VariableRefrigerantFlow objects input field for Cooling Energy Input Ratio Modifier Function of High Part-Load Ratio Curve Name (heating equation also shown). The question is, does it ever get used? To prove or disprove that this curve is used would require an input file that exercises this VRF model with loads greater than what is available. There are also 2 report variables that describe when this happens: VRF Heat Pump Maximum Capacity Cooling Rate and VRF Heat Pump Maximum Capacity Heating Rate. If either of these report variables are NOT 1E+20, the outdoor unit has exceeded it's "operating" capacity. Where "operating" means the rated capacity adjusted for current operating conditions (e.g., current OAT and Twb,indoor).

if (state.dataHVACVarRefFlow->CoolingLoad(VRFCond) && CoolingPLR > 0.0) {
    vrf.OperatingMode = ModeCoolingOnly;
    if (CoolingPLR > 1.0) {
        if (vrf.CoolEIRFPLR2 > 0) EIRFPLRModFac = CurveValue(state, vrf.CoolEIRFPLR2, max(vrf.MinPLR, CoolingPLR));
    } else {
        if (vrf.CoolEIRFPLR1 > 0) EIRFPLRModFac = CurveValue(state, vrf.CoolEIRFPLR1, max(vrf.MinPLR, CoolingPLR));
    }

if (state.dataHVACVarRefFlow->HeatingLoad(VRFCond) && HeatingPLR > 0.0) {
    vrf.OperatingMode = ModeHeatingOnly;
    if (HeatingPLR > 1.0) {
        if (vrf.HeatEIRFPLR2 > 0) EIRFPLRModFac = CurveValue(state, vrf.HeatEIRFPLR2, max(vrf.MinPLR, HeatingPLR));
    } else {
        if (vrf.HeatEIRFPLR1 > 0) EIRFPLRModFac = CurveValue(state, vrf.HeatEIRFPLR1, max(vrf.MinPLR, HeatingPLR));
    }

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Thank you very much for your answer. I have the same kind of graph shown in Figure 8, but in my case the power consumption for CR>1 doesn't remain flat, as remarked in the paper.

My point is that the right hand side of this graph doesn't represent the system performance at PLR>1. It represents the system performance at PLR<=1 when CR>1. Because the total outdoor unit capacity (the denominator of PLR) is calculated taking into account the correction factor for CR>1. Then, I think that the EIRFPLR curves should include somehow the CR as an independent variable as well.