Dear all,
This is a multifamily mid-rise building, 9 floors above grade. Per ASHRAE
Table G3.1.1A the Baseline HVAC system is system 1 (PTAC with hot-water
fossil fuel boiler) 9.3 to 11 EER.
Selected HVAC system for the proposed design is WLHP (3 Ton High efficiency
HP by Daikin, 15.9 EER and 4.83 COP) with 2 cell cooling tower, DOAS unit
(9.8 EER) and 2 Boilers (0.92 Eff.).
Although the efficiency of the selected HVAC systems is much higher than
the efficiency of the baseline model, the simulation results indicate
annual energy consumption of the proposed design is much higher than the
baseline!
Am I missing something or did something wrong in the models? Please see the
attached files of the both models
.
I would greatly appreciate it if anyone kindly gives some feedback on this,
Morteza
Hi Morteza,
I have not reviewed the results or detailed inputs of your model, but
one thing to be aware is that depending on the rating conditions of your
efficiency data, you may have to adjust the EIRs to be consistent with
DOE-2 performance curves. The default DOE-2 curves are normalized to
the GSHP rating condition, so if using these and if your efficiency data
is @ the WSHP rating condition, you will need to apply these adjustments.
Here is the table from the DOE-2 help provided with eQUEST. There is a
library of DOE-2 WSHP curves available from ClimateMaster that do not
need these adjustments.
- David
Dear eQUEST experts,
I am still struggling to find out the main reason(s) for high-energy
consumption of WLHP system comparing to PTAC system and what can be done to
make this system more efficient that the baseline system. In other words
for complying with EAp2 requirements, for a mid rise residential building
what types of HVAC systems (other than split system HP) can be selected
which consume less annual energy than the PTAC.
I would appreciate it if you would share your experience on this and make
some recommendation.
Morteza
Morteza,
I took a quick look at your models and the one thing that sticks out to me is your pump energy. The difference between PTAC and WSHP is huge. I would expect some difference since the WSHP needs to run the pumps in cooling mode as well as heating while the PTAC only needs pumps in heating, but the difference in energy seems too big. Also keep in mind that the heating and cooling efficiencies of the WSHPs are only for that piece of equipment, they don?t include the heat rejection required in cooling mode, the boiler input in heating mode, or the pumps. For those reasons, I wouldn?t expect the WSHP system to be a lot more efficient than the PTAC.
Tim
Tim Johnson
Mechanical Engineer
o 208.336.4900 | d 208.577.5645
[CTA_email_graphic]
Morteza and Tim,
Attached is a nice article from ASHRAE Journal discussing HVAC system selection for envelope-dominated buildings. It compares modeled performance of VRF, GSHP, boiler+chiller, and WSHP in a generic 9-story multifamily building in a variety of climates, and concludes that WSHP is the worst option.
Maria
--
Maria Karpman LEED AP, BEMP, CEM
________________
Karpman Consulting
Phone 860.430.1909
41C New London Turnpike
Glastonbury, CT 06033
Tim and Maria
Thank you for sharing your experience on this issue.
Morteza
Some of the advantages of a WLHP system are related to maintenance and service life compared to systems with air-cooled condensers and requiring envelope penetrations. These concerns won?t show up in an ASHRAE 90.1 appendix G energy analysis. (Higher infiltration from PTACs?) Also I?d expect some efficiencies of scale from central equipment if your building is large, compared to the PTACs, this may not apply here.
But regarding your EAP2 questions?WLHP offers an energy savings when parts of the building are in heating while others are in cooling ? depending on your building?s form and location, as well as modeling assumptions about setpoints, this may not be taking place. For a residential building it might be reasonable to assume that some portion of the residential units are set to different temperature ranges.
Conversely, if your location is cooling dominated all year, some of the advantages of WLHP go away. One thing to check here is the end-use demand components in your model. Is the WLHP using less at full load than the air-cooled system? If so, then you need to investigate the modeling assumptions about the controls of the WLHP system when operating at part load. Make sure that what you gained in peak efficinecy isn?t given back when the system is only partially loaded.
Verify that the WLHP control for the condenser loop is variable flow (if the proposed heat pump units have two-way control valves?if not, talk to your engineer about that!) and that there is a an adequate temperature range for the loop so that the towers and boilers don?t come on immediately. WLHP condenser loop which should be in the range where a condensing boiler will operate successfully, verify that the proposed case takes advantage of this compared to the baseline. Make sure that the condenser loop is operating on call for demand so that the model isn?t assuming continuous operation.
Regarding the DOAS, make sure that the baseline system has the same OA requirements at the PTAC units. Lastly, verify that the fan power for the WLHP units is entered based on the design pressure drop and not eQUEST defaults. Double-check that the fan run-times for the equipment are the same?which for LEED would be fans operating continuously. You probably have this in place already.
I see an interesting paper forwarded by Maria came through while I was typing. Note that the graphics in Figure 1 and Figure 2 are for CO2 emissions, and have non-zero axes, so there could be some variance for your specific case depending on the relative cost of utilities.
It is very interesting ? if we take a proposed high-efficiency WLHP system from Figure 2 in Atlanta producing about 625 MTCDE compared to the Figure 1 ASHRAE 90.1 baseline air-source heat pump it is just under 650 MTCDE. In your case, by adding the DOAS and some other efficiencies and depending on your specific utility rates, it should be possible for a high-tech WLHP system to at least reach equivalency with PTACs if not surpass it. Additional savings should result if you are able to add efficiency from upgraded envelope, lighting, and DHW measures in the proposed design. Check the LS reports to verify that building heating and cooling loads are reduced in the proposed case compared to the baseline.
David
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates
Morteza,
If you are looking for a relatively simple upgrade from PTACs (with electric resistance heat), consider packaged terminal heat pumps.
Keith Swartz, PE
Senior Energy Engineer | Seventhwave | Madison.Chicago.Minneapolis
(formerly Energy Center of Wisconsin)
608.210.7123 | www.seventhwave.org
Dear Keith,
Thank you so much for all the clarifications and recommendations. I will
follow your instruction and coordinate with the mechanical engineer who
thinks it is possible to achieve a considerable energy savings with the
WLHP system.
I truly appreciate your help,
Morteza