[Bldg-sim] Modelling groundwater cooling via concrete slab

Timothy Moore timothy.moore at iesve.com
Wed Jun 10 22:16:51 PDT 2015


Claire,

Cory provided a good description of the best approach and made a good point that, for an early study, it will probably be best and very fast to use the “chilled ceiling” panel mode in IES-VE ApacheHVAC---especially considering you’re modeling a hydronic ceiling slab, rather than an exposed floor.

As you’re attempting to represent a massive hydronic concrete slab, you can enter an appropriate value for the mass of the slab in the “Weight of chilled panel (excluding water)” parameter in the “chilled ceiling” type dialog. This will provide representation of thermal inertia (capacitance) suitable for a quick early study. It will also provide modeling of radiant exchange between other room surfaces and this massive ceiling “panel”.

You can quickly set up a generic “chilled ceiling” panel type with a suitably conservative maximum cooling capacity. For use in zones of different floor areas, and thus differing cooling capacities, you can multiply the number of panels within the controller dialog as needed to represent realistic cooling capacities for each floor area. The cooling capacity will also be constrained during simulation by the flow rate you set in the controller dialog and the moderate supply water temperature from the CHWL representing your ground water supply.

The main benefit of setting up the full hydronic slab model, as detailed in the ApacheHVAC user guide appendix on this, is that you can very accurately model the effects of slab mass and u-value of the average path between the water in the hydronic tubes and the surface of the slab, response time, radiant exchanges with other surfaces in the zone, and direct-solar gains received by the floor slab surface if this were exposed to the space above in any part of the building (may not be important in your case, if the cooled slab is only every exposed as a ceiling, and not a floor).

As the quick panel-based model will limit cooling capacity as a function of water flow rate and temperature, but does not have a way to actually model the limiting effects of the tube spacing and depth within the concrete, it will be important to appropriately constrain the cooling capacity either by the flow rate control setting and/or the maximum capacity entered in the panel type dialog. There are numerous papers published on the topic, and the CBE paper at the following link summarizes the conclusions from many of them including (on page 3) cooling capacity on the order of 77 W/m2 for chilled concrete slabs (excluding any contribution from ventilation systems) with moderate SWT and surface temperature held more or less constant:

http://escholarship.org/uc/item/3j52t8vz#page-3

That and the many publications it cites might give you a better idea of appropriate maximum cooling capacity of your particular hydronic slab design and water temperature.

Cheers,
Timothy


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Timothy Moore
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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Duggin, Cory
Sent: Wednesday, June 10, 2015 10:37 AM
To: Claire Das Bhaumik; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Modelling groundwater cooling via concrete slab

I would use the chilled ceiling room units and connect them chilled water loop supplying the constant temperature, but set the chiller fuel code to none and zero out any heat rejection energy such as cooling tower or condenser fans.  System 9g would be a good start since it can be used for ventilation and has the room units already assigned.  The only thing you won’t see is the thermal mass effects of the slab.  The chilled ceiling is really to model just a panel.  For an early study, I wouldn’t go through the trouble of trying to get the slab right as it requires much more effort.

Cory Duggin, PE, LEED AP BD+C
Associate/Energy Engineer
TLC Engineering for Architecture
direct:

615-346-1939



From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Claire Das Bhaumik
Sent: Wednesday, June 10, 2015 9:02 AM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] Modelling groundwater cooling via concrete slab

I'm looking for the most suitable approach for modelling groundwater cooling in a thermal model. It's proposed to have pipes embedded in a concrete ceiling with groundwater flowing through them. The groundwater supply temp will be fixed at around 12degC.

I'm using IES software but I expect the possible approaches would be similar with most packages.

I could either use a simplified approach calculating the cooling capacity from the flow rate and temperature difference, or use a detailed systems modelling approach. I want something fairly quick but reasonably accurate (don't we all?!) as it's a preliminary study.

Thanks,


Dr Claire Das Bhaumik CEng FCIBSE

Partner - Inkling LLP

 e: claire at inklingllp.com<mailto:claire at inklingllp.com>

t:  07950 282800

w: www.inklingllp.com<http://www.inklingllp.com/>

Follow us on Twitter: @DasInkling

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