Hi group,
I have a building with 8-foot concrete floor slab (50 feet below grade), 3- to 6-foot below-grade exterior walls, and 3- to 8-foot interior partitions in the sub-grade shells.
I wondered if anyone had a quick hint they could give me, particularly in light of LEED 2009 review that will happen. The calculus is done, and the transfer function time constant is so huge that the inner surface temperature of the walls just isn?t going to vary much. Ground temperature is 60?F all year and doesn?t vary. After they are poured, it?ll take 8 to 10 months for the 8-foot walls to cool down to indoor (conditioned) ambient temperature just from the heat of curing.
Any comments or experience will be most appreciated.
Thanks,
Dave
David R. Weigel, PE
Hi Dave,
From a LEED perspective, I think you have a lot of liberties here to model or not model specific elements. The critical thing is for documentation to be clear & open regarding the decisions you make and be sure to apply those decisions uniformly between the models.
Heat of curing is something you could opt to not model at all ? reasonably choosing to model the eventual ?steady state? as that should best represent the building?s long term annual internal load profile. If you should choose to include this internal heat load for the model - intuitively I would expect to see this applied identically to both models as a uniform space equipment load (assigned to a ?free? meter) for all affected spaces (perhaps with an annual fractional profile that reduces/eliminates the load over time, per your referenced calculations).
How to model that future ?steady state? after heat of curing probably poses the more interesting conundrum ? I would defer to your mechanical designers to take an approach to envelope loads to match their plans for sizing the heating equipment. Some might consider the huge thermal lag of the earth/concrete masses to render heat transfer negligible over time (which would bend you towards modeling thermally massive but adiabatic partitions). Others might consider the surrounding earth a constant heat drain and insulate or bump heating equipment capacities accordingly. From a LEED modeling perspective, best advice is to not make assumptions that disagree with the rest of the design team ? communicate and move forward with a consensus.
~Nick
[cid:489575314 at 22072009-0ABB]
NICK CATON, P.E.
I'd recommend going with Nick's suggestion of a steady-state model with
input from the design team as far as their load calculations go (assuming
you aren't also the mechanical designer). From Section 3 Definitions in
ASHRAE 90.1-2007, both C-Factor and R-Value are defined by ASHRAE to be
steady-state values and while U-Value is not, it would be safe to assume
the intention of the standard is a steady-state model for Appendix G.
Also, EER values are defined as steady-state ratings for equipment.
Genreally, the energy modeling from Appendix G is not intended to be an
exact prediction of energy use of a building the year it opens, or a year
after. This is explicitly stated in G1.2(2):
"Neither the proposed building performance nor the baseline building
preformance are predictions of actual energy consumption or costs for the
proposed design after construction. Actual experience will differ from
these calculations due to variations such as occupancy, building operation
and maintenance, weather, energyuse not covered by this procedure, changes
in energy rats between the design of the building and occupacy, and the
precision of the calculation tool."
Now, I don't bring up that paragraph to dredge up long conversations of the
accuracy of energy models, but if the concern is really how to get an
accurate model out of this unique circumstance, I'd suggest citing this
paragraph directly, in additon to the references before from Section 3,
when explaining why you used a steady-state model.
The bigger curiosity for me is...what exactly are you building here? :)
Jeremy R. Poling, PE, LEED AP+BDC
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Good points Nick and Jeremy (as usual).
The design team totally ignored these walls and floor for sizing equipment ? treated them as adiabatic -- with good reason that I did not state. The internal process loads include (which will answer Jeremy?s curiosity) the accelerator, cyclotron, power supply, and high-energy beam containment mazes for a proton therapy machine. The walls and slab are so thick to shield the outside areas from stray radiation, and also keep the machine from experiencing any movement (really tight tolerance). The process loads are really big, over 1200 kW electrical demand but with a reasonable load factor. Process loads in the building are about 40% of total energy.
I?ve learned a lot, it?s wild.
Since all the math was done, I did simulate the spaces by themselves with the thickest heavy-weight concrete the program would allow, and again with adiabatic and massy walls. The overall difference in site energy use was truly negligible.
My main concerns were in choosing the route that most closely meets the rules in App G, and explaining it in my LEED documentation.
You have both been quite helpful. I?ll post back when I have come to a conclusion, and will follow up if review comments arise.
Best to all of you,
Dave