[Bldg-sim] PTHP Baseline outperforming VAV

Marcus jones.0bj3 at gmail.com
Thu May 26 04:38:49 PDT 2011


HI all, thanks for the excellent feedback, just wanted to report back for
the archive;

Yes, since hotel is considered residential, I am applying PTHP.

In ASHRAE 90.1-2007 SI, table 6.8.1D, the COP equation is 3.60 - (.213 x
Cap/1000) where Capacity is in units of [kW]. This must be an error? Cap
units should be [W] or equation should be 3.60 - (.213 x Cap). Using this
equation, this limits the COP of a PTHP to between 3.15 for 2.1 kW units and
below and 2.66 for 4.4 kW and above (with COP inclusive of fan energy).
Thanks for drawing my attention back to this equation, this significantly
increases the Baseline cooling.

I agree, my pump and fan energy also needs to be correctly sized according
to pressure drop, I just used the defaults as a place-holder until detailed
design.

Generally the point of chilled water vs. Dx having no clear advantage is a
good one, chilled water introduces greater heat transfer approach as pointed
out and requires pump energy. However, we have learned that the client is
actually only using VAV in certain zones, the guest rooms will be served by
zone terminal variable flow chilled water coils (fancoils) and AHU's with
enthalpy recovery. Unfortunately modeling this system is proving quite
challenging in EnergyPlus so I don't have any updated results yet.

Sincere thanks again for the comments!

Cheers,

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



On Tue, May 24, 2011 at 5:40 PM, Paul Riemer <Paul.Riemer at dunhameng.com>wrote:

> Marcus,
>
> Compared to packaged DX equipment, an air cooled chiller has two
> fundamental efficiency disadvantages: extra heat transfer approach and the
> pumping energy of the water.
>
>
>
> Load reductions always help, but to hone in a comparison of the
> efficiencies of these mechanical systems, I also suggest you look closely
> at:
>
> -          the COP values and their corresponding rated ambient
> temperatures in the referenced standards and on your equipment literature
>
> -          compressor types, sizing, and curves
>
> -          maybe, thermal storage
>
>
>
> Good luck,
>
>
>
> *Paul Riemer, PE, LEED AP*
>
> Associate / Mechanical
>
> *DUNHAM*
>
>
>
> *From:* bldg-sim-bounces at lists.onebuilding.org [mailto:
> bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Bishop, Bill
> *Sent:* Tuesday, May 24, 2011 9:57 AM
> *To:* Marcus
>
> *Cc:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] PTHP Baseline outperforming VAV
>
>
>
> 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
>
>
>
> [image: Signature in jpg form]
>
>
>
> *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
>
>
>
> 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
>
>
>
> *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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