[Bldg-sim] VRF modeling in Visual DOE (& other software) --Software Limitations & LEED Rating

[Andrew Craig]

Ravi---

To my knowledge,a VRF system can not be explicitly modeled in eQUEST or any other "DOE" modeling software at this time.  The only software I know of that has this system selection is Trace 700 and EnergyPro.  

There has been some discussion of the best way to model this system in eQUEST as it is showing up more and more on projects.  One way that would give you some benefit of "load sharing" is by modeling it as a central WSHP system.  You would have to adjust the compressor efficiency and take out most if not all of the pump energy, but it might be a better approximation than a PTAC or PSZ.  Modeling the VRF condenser as a cooling tower with a VSD might be closer to the truth as well.  The biggest weakness I see is in heating mode.  The WSHP could use an electric boiler, but it would max out with an HIR of 1.  I'm sure the VRF condenser operates with a higher COP.  It's all an educated guess at this point.  

Here's looking forward to further development of eQUEST and other modeling software.

Regards,
Andrew Craig


-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org on behalf of Ravi Srinivasan
Sent: Fri 2/15/2008 7:04 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] VRF modeling in Visual DOE (& other software) --Software Limitations & LEED Rating
 
Dear Building Simulationists!

We have come across several projects with VRF systems. Although several
publications, including the ASHRAE Journal (April 2007) issue's article
"Variable Refrigerant Flow Systems" by William Goetzler discusses in detail
VRF systems and its efficiency, the limitations of energy modeling software
does not permit to accurately model such systems.

Any insight into the following is much appreciated:

(1) Has anyone had luck modeling a VRF system in Visual DOE? (or EnergyPlus
or eQuest); I "heard" Trace can model VRF system -- can anyone confirm?
Currently, the only way to model a VRF system (in VDoE) is to select "PTAC"
or "PSZ" and modify EER values of the cooling unit. But, this is not even
close to the savings one can achieve in reality. This does not take into
consideration the capacity (heat) of the refrigerant (R-410a, for example).
Is there a procedure developed by ArchEnergy or others energy software
developers to accommodate such systems?

(2) Buildings (or owners / developers) should not be penalized due to the
limitations of energy modeling software!
If ASHRAE 90.1-2004 can stipulate a 10% reduction in LPD for introducing
"Occupancy sensors" in the building, why not introducing a reduction (as a
percentage) for VRF integration until the software are "upgraded"? Why
cannot USGBC accept a flat rate (say, 20% savings) if a VRF system is
integrated in the building?

Need your insight & inputs!

Good day!

RAVI SRINIVASAN (PhD), CEM, LEED AP, Assoc AIA
CEO & Head, Building Energy & Sustainability
SILPA INC
s i m p l e  .  s c i e n t i f i c   .   s u s t a i n a b l e
www.silpainc.com


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