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Which EnergyPlus VRF Model?

asked 2017-06-16 14:16:09 -0500

Mike Kennedy's avatar

Does anybody have guidance on choosing which EnergyPlus VRF model to use?

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The answer I was looking for was not what is in the manual but what peoples experience with the models are. After spending time on the new VRF-FluidTCtrl model I found manufacturers will not provide data to support the model, the only typical data that is in the E+ example file. Also, if you use the fluidTctrl model, in version 8.7 the terminal objects do not work with the Fan:SystemModel object (github issue 6218). I have one other unexplained issue with the VRF-FluidTCtrl-HR object that could be user or program error but have not had time to explore further.

Mike Kennedy's avatar Mike Kennedy  ( 2017-08-09 12:59:50 -0500 )edit

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answered 2017-06-18 09:46:39 -0500

From the Input-Output Reference for the group of VRF objects in EnergyPlus:

There are two alternative VRF models in EnergyPlus:

System curve based model (VRF-SysCurve). In this model, a number of system level curves are used to describe the VRF system performance. This model corresponds to the AirConditioner:VariableRefrigerantFlow object.

Physics based model (VRF-FluidTCtrl). This model is able to consider the dynamics of more operational parameters and is applicable for fluid temperature control. This model corresponds to the AirConditioner:VariableRefrigerantFlow:FluidTemperatureControl object.

(Please refer to the engineering reference for more technical details of the two models).

From the Engineering Reference for those two VRF objects in EnergyPlus:

The System Curve based VRF Model (VRF-SysCurve) ... can address both the heat pump and heat recovery operating modes and support air-, evaporatively-, or water-cooled condenser equipment.

Compared with the empirical system curves based VRF-SysCurve model, the VRF-FluidTCtrl model adheres to a physics-based development providing the ability to simulate the refrigerant loop performance and consider the dynamics of more operational parameters. The main features of the VRF-FluidTCtrl model include:

  • Introduction of separate curves for capacities and power inputs of indoor and outdoor units instead of overall curves for the entire system.
  • Allowing of variable evaporating and condensing temperatures in the indoor and outdoor units under various operational conditions.
  • Allowing of variable fan speed based on the temperature and zone load in the indoor unit.
  • Allowing of further modifications of operational parameters (e.g., evaporating temperature, superheating degrees, and supply air flow rate) during low load conditions.
  • Development of a physics-based model to calculate thermal loss in the refrigerant piping network (varies with refrigerant flow rate, operational conditions, pipe length, and pipe and insulation materials) instead of a constant correction factor.

These features enhance the simulation accuracy of the VRF system performance in both heating and cooling modes, especially during low load operations.

Refer to other Unmet Hours questions HERE and HERE for some discussion on performance curve inputs.

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Thank you for this information. I had already read manual, Input-Output and Engineering. Perhaps my question is better stated, in your or others experience in applying the two models, which would you (they) use again and for what reason? If the physics model is more accurate, why use the curve method at all? I've run into significant issues (bugs) with other new energyplus models that resulted in common equipment behavior not being able to be modeled and I would prefer to have some smooth sailing. Thanks a bunch

Mike Kennedy's avatar Mike Kennedy  ( 2017-06-21 09:15:00 -0500 )edit

Hello. I would like to know if VRF-FluidTCtrl calculates the coil sensible base on the refrigerant temperature. Thanks.

JLS's avatar JLS  ( 2017-08-25 01:59:54 -0500 )edit

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Asked: 2017-06-16 14:16:09 -0500

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Last updated: Jun 18 '17