[Bldg-sim] PTHP Baseline outperforming VAV

James V Dirkes II, PE jim at buildingperformanceteam.com
Mon May 23 15:18:52 PDT 2011


Dear Marcus,

.         It seems that your Baseline system, at >5 floors, should be VAV
with a chiller instead of a heat pump (using LEED 2009).

.         As modeled, your baseline DOES have a notably higher COP, so it
may well use less energy.  Part load performance will have a significant
effect, so you may want to check and compare  those curves.

.         Also check pump  delta T and total pressure drop.  If you continue
with a non-chiller system, pump energy is  a significant factor.  If your
Baseline becomes a chiller system, pump energy will probably become similar.

.         Check fan total pressure drop for each system since your fan power
is significantly larger in the Proposed system.

.         In the US, it is common to apply occupancy sensors on a LEED
project and ASHRAE 90.1 allows a 10% power reduction when these are used.
You are not taking advantage of that and may want to do so.

 

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Marcus
Sent: Monday, May 23, 2011 4:24 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] PTHP Baseline outperforming VAV

 

Hi all, 

Having some difficulty in an App. G model for a 6-floor 14,000 m2 GFA Hotel
in zone 1B (Abu Dhabi). Assuming standard design for each, should a proposed
VAV system out-perform window AC units (the baseline)? 

Proposed: 
VAV with 2 Air-cooled centrifugal chillers, chiller COP = 3.1 (2.8 if
condenser fan energy included)
Envelope - Better windows decrease heating demand by 8% rest of envelope is
minimum required for compliance

Baseline: 
System Packaged terminal heat pump, COP = 3.52

Using Energyplus v6.0 the results are;

Site Energy in End Uses                  BL   Proposed   COP5
                                         MWh     MWh     MWh
                       Space Cooling    747.5  1052.0   715.6 
                      Heat Rejection      0.0     0.0     0.0 
                       Space Heating      1.3     0.1     0.1 
                               Pumps      0.0    60.0    60.0 
                     Fans - Interior     42.2   137.7   137.7 
                     Fans - Car park      0.0     0.0     0.0 
                   Interior Lighting    530.5   530.5   530.5 
                   Exterior Lighting      0.0     0.0     0.0 
               Service Water Heating      0.0     0.0     0.0 
        Receptacle/Process Equipment    146.3   146.3   146.3 
               Data Centre Equipment      0.0     0.0     0.0 
            Elevators and Escalators      0.0     0.0     0.0 
                   Total Site Energy   1467.9  1926.6  1590.2

I have the energy for the baseline, the proposed with COP = 3.1, and then
trying COP = 5. It seems that even if we drastically increase the COP of the
chiller plant in the proposed VAV system, we still can't get any energy
savings. Is this realistic, an artifact of the simulation, or just an error
on my part? 

Any guidance would be appreciated, 

Marcus
-- 
Marcus Jones, LEED AP, M.Sc. 
Freelance energy consultant
Vienna, Austria



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