[Bldg-sim] PTHP Baseline outperforming VAV

Bishop, Bill wbishop at pathfinder-ea.com
Tue May 24 07:56:42 PDT 2011


Hi Marcus,

 

I agree with your selection of PTHP for the baseline, since your project
is a hotel, which is specifically listed in the notes to Table G3.1.1A
as being a residential building type.

I am not surprised that you are showing an energy penalty versus an
ASHRAE 90.1 PTHP - the cooling and fan energy is relatively low for
those units.

 

As James said, make sure you are accurately modeling your fan and
pumping energy in the proposed system. Unfortunately, there are no
pressure drop adjustment allowances for Systems 1 & 2.

The maximum allowable SHGC (0.25) is already pretty low for Abu Dhabi,
so your "better windows" might not have a lower SHGC than the value
required for the baseline, and you will not get a solar load reduction
in the proposed model.

 

Since your baseline cooling efficiency is dependent on the system
capacities, make sure you are not using too large of a baseline cooling
COP. (It should actually be calculated separately for each baseline
system.) Using the efficiency equation from Table 6.8.1D, assuming 400
cfm/ton air flow rate and removing the fan power energy from the
efficiency, I calculate COP = 2.94 for a 15,000 Btu/hr unit (for
example). There is a separate heating COP but your heating load is
negligible per your report below.

 

Regards,

Bill

 

 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Cheney
Sent: Monday, May 23, 2011 9:55 PM
To: James V Dirkes II, PE
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] PTHP Baseline outperforming VAV

 

Hi Marcus, 

 

Also pay attention to better glass in the proposed design. In a
cooling-dominated building, better window means the heat can not easily
escape from the building. You will not easily get credit from improving
your window here. 

 


Regards, 

 

Cheney

 

LinkedIN @ http://ca.linkedin.com/pub/yu-cheney-chen/27/637/72b
<http://ca.linkedin.com/pub/yu-cheney-chen/27/637/72b>  





On Mon, May 23, 2011 at 3:18 PM, James V Dirkes II, PE <
jim at buildingperformanceteam.com> wrote:

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 <tel:616%20450%208653> 

 

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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