How chiller inputs work in Sefaira

Overview

Various chiller inputs can be edited in Sefaira. This article describes:

  • how we simulate chillers in Sefaira
  • what we've observed for air-cooled and water-cooled chillers as results to expect.
  • how EnergyPlus uses those changes 

How we simulate chillers

In Sefaira, for both Water-cooled and Air-cooled chillers, we make some key assumptions and simplifications to keep the workflow simple and easy to use. These are as follows:

  • Chillers are not staged. They are assumed to be sized as a single unit for the full project capacity. This means we don't account for different efficiencies that may be possible to be realised from very large chillers, or the efficiency limitations of very small chillers.
  • Chillers applied are real chiller models with real part load performance curves. They are based on real chillers that are middle-of-the-range in terms of the size they're designed to serve.
  • The air-cooled chiller used is a “Carrier 30XA220 743.7kW/3.1COP” chiller.
  • The water-cooled chiller used is a Carrier 19XR 742kW/5.42COP/VSD” chiller.
  • The district chiller....
  • Note the chillers have default COPs but these are still adjusted by the full-load COP value entered into Sefaira.

How we adjust the COP of the Chiller

During the simulation, the actual COP of a chiller at the given working condition in the given moment is calculated based on the nominal COP value from our web-app that is then corrected with the EIR (Energy Input Ratio) modifier. EIR modifier is a function of a supply chilled water temperature and the outside air temperature for air-cooled chillers or cooling tower return temperature (or chiller condenser inlet temperature) for water-cooled chillers. EIR has the following relationship to the COP: COP = 1 / EIR

You'll see the EIR modifier referred to below.

Dynamic performance of the Air-cooled chillers used

Following graph shows how EIR changes based on the different OAT (outside air temperature) and CHW (chilled water) supply temperatures.

carrier.PNG

 

Note that supply water temperature for air-cooled chillers does not have a substantial impact on the efficiency of the chiller. Indeed if the outside air temperatures that are lower than approx. 34C, having a colder chilled water supply temperature is more efficient with this particular product.

Dynamic performance of the Water-cooled chillers used

Following graph shows how EIR changes based on the different OAT (outside air temperature) and CHW (chilled water) supply temperatures.

 image__26_.png

Note performance changes a lot more with variations in chilled water temperature for the water-cooled chiller.

Dynamic performance of the District chillers used

Following graph shows how EIR changes based on the different OAT (outside air temperature) and CHW (chilled water) supply temperatures.

 image__27_.png

Note that district chillers are set up with the assumption that there is no way of managing the efficiency of the system dynamically.

How we modify chilled water temperature in EnergyPlus 

There are 3 objects affected by the chilled water loop in EnergyPlus:

  • HVACTemplate:Plant:ChilledWaterLoop
  • HVACTemplate:Plant:Chiller:ObjectReference
  • Chiller:Electric:EIR

We use the chilled water temperatures entered into Sefaira to edit inputs in each of these parts of the idf file. 

 

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