[Equest-users] Modeling Active Chilled Beams in eQUEST v3.64

[Bishop, Bill]

Dan,

What do you mean "my airflow increased by quite a bit"? Are you letting eQUEST autosize the DOAS design airflow? Going from memory, the instructions from my workaround write-up were appropriate if you manually enter the DOAS supply flow rate, which should be easy to do once the total OA requirement is determined, and assuming a 100% OA unit. My instruction for the DESIGN-COOL-T and HEAT-T values was to ensure that system/zone flow rates and capacities for the occupied zones below would be autosized correctly, since there is a high rate of heat transfer with the plenum zones due to the air wall ceilings. Also, while the zone type for the plenum zones should be set to "conditioned", no thermostat schedules should be assigned, assuming the DOAS will not be sized to meet space sensible loads. Taking into account your issue, I would recommend the DESIGN-COOL-T and DESIGN-HEAT-T for the plenum zones be set to the range of expected return air temperatures to the DOAS, which should be closer to the range of room temperatures maintained by the systems serving the sensible loads of the lower occupied zones. Again, I don't think it will matter much once you manually enter the DOAS airflow rate (and plenum zone airflow, and OA) and remove plenum zone temperature schedules.

Regards,
Bill

From: Daniel Caporizzo [mailto:DanielMC at BVHis.com]
Sent: Thursday, October 30, 2014 10:17 AM
To: Bishop, Bill; equest-users at lists.onebuilding.org
Subject: RE: Modeling Active Chilled Beams in eQUEST v3.64

I've adjusted my model so that the DOAS picks up the OA/people from the plenum zones, and kept the single plenum per floor. I did notice that you get errors unless you change the ceiling construction to something with U-value input, as opposed to layers input, once you change the ceilings themselves to air walls.

I am noticing that the DOAS airflow is sensitive to the temperature difference between the DESIGN-COOL-T and DESIGN-HEAT-T in the plenum zone. The DOAS workaround describes "Pick DESIGN-COOL-T and DESIGN-HEAT-T values for the plenum zones that match the likely DOAS supply air temperature range" but if I get the temperature difference too close to the DOAS MIN-SUPPLY-T and MAX-SUPPLY-T I notice my airflow increases by quite a bit. Did you have a "standard" temperature difference between the plenum zone and the DOAS supply that you used? Also would this setup account for situations where you would not want to supply room neutral air through the DOAS?

Thanks,
Dan

From: Bishop, Bill [mailto:bbishop at pathfinder-ea.com]
Sent: Tuesday, October 28, 2014 10:50 AM
To: Daniel Caporizzo; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: RE: Modeling Active Chilled Beams in eQUEST v3.64

Dan,

Good questions regarding the plenum method for modeling DOAS.

Plenum per zone vs. common plenum per floor: I haven't tried doing it with a common plenum per floor, but I think it could work in many circumstances. A couple situations where it wouldn't work are if you wanted to model 1.) more than one DOAS per floor, or 2.) DCV using the "DCV-ZONE-SENSORS" MIN-OA-METHOD (because you'd only have one zone). In a situation where the ventilation for some zones on the floor is served by a different system, you just wouldn't model the ceilings for those spaces as air walls.

No need for OA-FROM-SYSTEM. Just assign all OA loads served by the DOAS to the plenum zones.

You don't have to assign any interior loads to the plenum zones other than OA. The DOAS return air will get sensible heat transfer across the air walls (ceilings) from the zones below. You can assign people to the plenum zones instead of the zones below, especially if you want to model DCV, but also if you want space latent loads to be handled by the DOAS system (as people are often the only latent load). Be aware that assigning people to the plenum zones instead of the lower zone(s) will affect the autosizing of the system that serves the lower zone(s).

Regards,
Bill

From: Daniel Caporizzo [mailto:DanielMC at BVHis.com]
Sent: Tuesday, October 28, 2014 10:12 AM
To: Bishop, Bill; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: RE: Modeling Active Chilled Beams in eQUEST v3.64

A few questions on this approach:

Do you need to have a separate plenum zone for each "regular" zone? Currently my model is set up with one plenum zone for each floor. If it's the only way to get it to work, then I will probably have to backtrack to the wizard edit mode. Not ideal but with this model I tried to minimize how much work I did in detailed edit mode.

Did you set up the OA-FROM-SYSTEM for the zones' system or just set the OA requirement for each of the zones to 0 so only the plenums have OA? Do you need to assign the people to the plenum zones as well? I assume equipment and lighting loads would stay within the "regular" zone - correct me if this is not the case.

Thank you for the catch on the static, I thought I had picked that up but I must have missed it.

Thanks,
Dan

From: Bishop, Bill [mailto:bbishop at pathfinder-ea.com]
Sent: Friday, October 24, 2014 11:47 AM
To: Daniel Caporizzo; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: RE: Modeling Active Chilled Beams in eQUEST v3.64

Dan,

As an alternative to using dummy zones, I suggest trying an alternative workaround ("plenum method") I have advocated for, which may be appropriate depending on how you would design your active chilled beam system. (If your system would be constant volume, without a separate DOAS, you could just model it with the Induction Unit system type. If you want to allow for variable flow, and/or separate DOAS unit, read on.) I have had luck modeling DOAS by creating separate systems (as they are designed) and assigning them to the plenum spaces above the occupied spaces. The description of how to do this, and the merits and potential drawbacks, can be found in the "NEW (?) eQUEST DOAS workaround using plenum spaces" thread in his archive: http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/2013-November/thread.html. It would be an easy alternative for you to try modeling in your schematic model, as you have few zones and they all have plenum zones above them. Also, note that while you said your static is the same in both models, you have 1" static on supply and exhaust for the heat recovery portion of the DOAS systems, which you did not include in the VAV model. This is about 90,000 kWh as shown on the ERV reports.

Regards,
Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Daniel Caporizzo
Sent: Thursday, October 23, 2014 2:19 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Modeling Active Chilled Beams in eQUEST v3.64

Hi everyone,

I am working on a project where I have to compare the energy usage between a primarily VAV system and an active Chilled Beam system. I started with the VAV system and modeled the building that way, and then made a copy to utilize active Chilled Beams. The building is a 5 story, new 400,000 SF office building.

I have heard different opinions on how to properly model this in eQuest. So far I have tried using induction units with no separate DOAS but my fan energy consumption came out at least 3x what I had for VAV. I then tried induction units with a DOAS set as their "outside air from system" but this didn't help as much. Another option, and what I am currently using is the following: Fan coil units for each zone and a VAV unit for the DOAS.


-        Each zone is set to its own FCU system

-        Building is split down the middle for main AHUs, so each DOAS serves half the building's fan coils.

-        Each DOAS system is set up with a dummy zone whose zone has the OA requirement of its system's FCUs. The dummy zone has no cooling or heating schedule (including these causes a lot of unmet hours) either.

-        FCUs are set to a secondary CHW loop with a 6 degree delta and 58 deg F supply temp, the DOAS are set to a 44 deg F and 12 degree delta on the main CHW loop. I have set my FCU with 64 deg F LAT for cooling and 83 deg F LAT for heating; these are numbers I have seen on other chilled beam designs but it was not for an office application.

-        DOAS systems each have an enthalpy wheel due to amount of OA

-        FCU air-side economizer is default at Fixed-Fraction - I noticed that changing this will drastically increase the amount of space heating energy consumption. I have tried to match it to OA enthalpy or OA temperature (what the AHU will likely be) and it is at least a 2x increase in heating energy.

-        Zoning is purely based on perimeter and core since the project is in early schematic phase.

I am very new to designing chilled beams so I am unsure if my numbers are coming out correctly. In comparison to the VAV model, my heating energy is very low (it is low in both models, which is another issue). I also noticed that my fan energy does not drastically change between systems, when the chilled beam should have a much smaller air handler. (Although my static is set the same in both models)

Has anyone had any experience with modeling chilled beams that they are willing to share?

Thanks,
Dan
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