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
t: 07950 282800
w: www.inklingllp.com
Follow us on Twitter: @DasInkling
Partnership No. OC367619
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
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
[IES]
Timothy Moore
Senior Product Manager
T: +1 (415) 983-0603
T: +44 (0) 141 945 8500
www.iesve.com
Integrated Environmental Solutions Limited. Scotland registration SC151456
Helix Building, West Of Scotland Science Park, Glasgow G20 0SP
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Claire,
It sounds like you are using IES but you mention "other packages" as well so I thought I'd outline the ways that ground water cooling through hydronic slabs can be modelled in DesignBuilder EnergyPlus.
1. In DesignBuilder v4.5 there is an HVAC template which allows you to directly model this system with water from a ground borehole flowing through a HX component which cools water flowing through a second circuit into ceiling surfaces. You get the cooling injected into exactly the right part of the surface and slab thermal mass is included. You just have to load the template and set the position of the pipes in your ceiling slab construction. You can select a template with the number of boreholes or use water from a lake. For later more detailed design work you can refine the ground loop design using a third party tool such as GLD or GLHEPRO.
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Other simpler options include:
2. As suggested by others you can use a dummy chiller setting a fuel code to ignore the fuel consumption.
3. Use district cooling.
Options 2 and 3 would allow you to set a fixed 12degC for supply temperature. Option 1 would calculate the correct supply temperature based on the capacity of the ground water supply.
I hope it helps.
Andy
Andy Tindale
Managing Director I Tel: +44 (0)1453 755500 I andy.tindale at designbuilder.co.uk
DesignBuilder Software Ltd, 1st Floor, Clarendon Court, 54-56 London Rd, Stroud, GL5 2AD
www.designbuilder.co.uk
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