# Where are the default chiller curve coefficient coming from in OS?

As the name implies, I'm trying to understand what's the rationale behind the default curve that come with the chiller

Let's take for example a `Chiller:Electric:EIR`

and look specifically at the `EIRFPLR`

curve, that is the Electric Input Ratio as a function of the Part Load Ratio.

This curve is quadratic: $$EIR = a + b\times PLR + c \times PLR^2$$

OpenStudio has the following default coefficients: - $a = 0.06369119$, - $b = 0.58488832$ - $c = 0.35280274$.

It seems that it leads to a pretty efficient chiller, unless I'm missing something. I've compared it t:

the Comnet modeling guidelines here (Table 6.8.2-9: "Default Efficiency EIR-FPLR Coefficients – Water-Cooled Chillers" > Centrifugal)

DOE-2 Centrifugal/5.50COP from the E+ dataset (EnergyPlusV8-3-0\DataSets\Chillers.idf)

a | b | c | |
---|---|---|---|

Comnet | 0.171493 | 0.588202 | 0.237373 |

DOE-2 Centrifugal COP 5.50 | 0.222903 | 0.313387 | 0.463710 |

OpenStudio | 0.063691 | 0.584888 | 0.352803 |

I end up with the following curves where I'm displaying COP (SI units) instead of EIR since it's more intuitive in my opinion.

Anyone can explain why it seems so much more efficient than the rest?

How are you arriving at your COP number 'in SI units'? COP is a dimensionless value, it's a measure of energy produced divided by energy consumed.

COP in W/W. I just specified SI Units because I've seen confusion about it before, where people would use the word COP when it was in fact an EER ($BTU_{out}/W_{in}$)

Bonus point: which curve (from the E+ dataset for example) one would use for a centrifugal water cooled chiller? Let's say per ASHRAE 90.1-2010. I think this might warrant a question of its own, since they only give you a rated COP and an IPLV to match.