[BLDG-SIM] eQuest Default f(PLR) Skepticism

[Kevin Madison]

Perhaps it would help to clarify how DOE-2.2 (the simulation engine 
behind eQUEST) calculates hourly energy input for boilers and chillers.

For boilers, the hourly energy input is:
  Hourly Energy = Cap(hour) * HIR * HIRf(plr)

So while the HIRf(plr) may increase as part load decreases, which is not 
uncommon for standard atmospheric boilers, the energy use will certainly 
decrease with plr because the required output of the boiler for the hour 
decreases.

For chillers, DOE-2 uses the following relationship to calculate the 
electricity input to the chiller each hour:

  Caphour  = Capacity * CAPf(t1,t2)
  PLR      = Load / Caphour
  dT       = t2 -- t1
  Elechour = Caphour * EIR * EIRf(t1,t2) * EIRf(PLR,dT) / 3413 Btu/kW

where

  Caphour      hourly capacity, Btuh (this is dependent on condenser and 
evaporator conditions for that hour)
  Capacity     rated capacity, Btuh
  CAPf(t1,t2)  correction to capacity for temperatures, curve CAP-FT
  t1           leaving chilled-water temperature, °F
  t2           condenser temperature, °F
  PLR          Part load ratio
  Load         Hourly load, Btuh
  dT           Temperature differential across chiller, °F
  Elechour     electric input to the chiller, kW
  EIR          rated electric input ratio
  EIRf(t1,t2)  correction to EIR for temperatures, curve EIR-FT
  EIRf(PLR,dT) correction to EIR for part-load ratio and dT, curve EIR-FPLR

Again, the primary factor affecting chiller energy use is the cooling 
capacity needed for that hour. Just because you don't have access to the 
dual function information doesn't mean you shouldn't be accounting for 
it in the simulation. Chiller performance is dependent on all operating 
conditions including load, condenser conditions and evaporator conditions.

For a more complete discussion on these simulation concepts, refer to 
the DOE-2 documentation included with the eQUEST installation. Look in 
Dictionary:HVAC Components:Boiler:Boiler Energy Consumption and 
Dictionary:HVAC Components:Chiller:Chiller Energy Consumption.

Kevin Madison
Madison Engineering PS
Seattle WA
USA

Taylor Keep wrote:
>
> eQuest models boiler and chiller plants with default part load curves 
> that I think may be incorrect. As I understand it, the f(PLR) curves 
> are used as a direct multiplier on the HIR for boilers and EIR for 
> chillers, with full load (1.0 PLR) corresponding to a 1.0 multiplier. 
> If this is true, the f(PLR) curve should increase at part load for 
> atmospheric boilers (atmospheric boilers become somewhat less 
> efficient at part load). The default atmospheric boiler curve 
> decreases almost linearly down to zero! I am having a tough time 
> wrapping my head around this.
>
> On the chiller side, the default f(PLR) is a bi-quadratic function 
> using both dT and PLR as variables, so it is f(PLR,dT). Since I never 
> have this dual function information in my general chiller selections, 
> I have been using a standard f(PLR) function quoted at a fixed dT from 
> the manufacturer. The curve I get from a McQuay 400-ton chiller 
> selection is quadratic, with decreasing EIR down to 50% load and 
> increasing EIR below 50% load. I seriously doubt that the eQuest 
> default corresponds with this entry because changing the function 
> produces a huge change in performance.
>
> Do any of you have any thoughts or suggestions about the accuracy of 
> default f(PLR) curves? Should I scrap my "improved," real curves - 
> they are drastically changing the model performance?!?!
>
> Taylor
>
>
> ________________________________________________________
> Taylor Keep
> Mechanical, LEED® AP
> _                          _
> Arup
> 901 Market Street Suite 260
> San Francisco, CA 94103
> tel: 415 946 0279
> fax: 415 957 9096
> taylor.keep at arup.com
> _www.arup.com_ <file://www.arup.com>
>
>
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