Chris-
One thing to keep in mind regarding eQUEST/DOE-2's "natural ventilation"
model, is that it is based on the Sherman-Grimsrud method, which was
specifically developed for estimating envelope infiltration in smaller
(1-3 story), single-family structures. Below is a link to a copy of the
original report outlining the method. The report gives some good
background on other infiltration models, and the context in which the
model was developed (incl. comparison to measurements). As described in
the report and the DOE-2 help manuals, you will find that many of the
model inputs are specifically relevant to small houses. Finally, in the
authors own words (p. 4), "this report introduces an infiltration model
that sacrifices accuracy for versatility and simplicity." Therefore, in
my opinion, using this model, especially for larger structures, has a
good potential for giving misleading results.
gundog.lbl.gov/dirpubs/10852_ShermanGrimsrud.pdf
Unfortunately, DOE-2's "S-G_NV" method is the only NV model _readily_
available in eQUEST. By readily available, I mean you can select it and
enter inputs within the detailed or other BDL interface, and many of the
inputs have defaults. However, just based on my experience in
estimating the inputs for three of the DOE-2 "nonresidential"
infiltration methods (AIR-CHANGE+AIR-CHANGES/HR,
AIR-CHANGE+INF-FLOW/AREA, and CRACK), these models can be quite
sensitive to inputs, including those in the SITE-PARAMETERS, and finding
estimates of these inputs for large buildings in literature is
difficult, if they even exist at all. I have not had the opportunity
to compare the S-G_NV model with another NV model for larger buildings
(such as CFD), but I would guess that if they are close, it would be
mostly luck that you picked the correct S-G_NV inputs.
I have modeled a couple nonresidential buildings that attempt to use a
NV strategy to offset mechanical cooling needs. However, to be
conservative, I often only accounted for an expanded comfort range
(similar to what Timothy described) rather than trying to use the
DOE-2's S-G_NV method. Although I think some buildings are
well-designed to take advantage of NV, in my mind, unless you have some
sort of mechanical controls to open windows, all bets are off. Not that
there aren't some real potential energy savings with well-designed NV
strategy, its just that the methods for estimating the impacts w/
eQUEST/DOE-2 are not sophisticated enough. Anyone out there have any
evidence or comparative modeling that indicates otherwise?
After doing a lot of thinking about modeling NV, one idea for a
"work-around" that accounts for NV using eQUEST, is using custom LOADS
(infiltration) / SYSTEMS (availability) schedules and other inputs based
on pre-processing of weather and building geometry data (may require
some iterative runs...). I would guess that the majority of NV impacts
are from wind-driven infiltration through windows. Along these lines,
below is a link
PNNL paper that describes how to develop E+ "wind-driven" infiltration
model inputs (basically the same as DOE-2 "AIR-CHANGE" method) using
local wind pressure coefficients and a design infiltration rate (leakage
@ 75Pa). For combination wind+stack driven air flow through windows,
the model would likely have to get more complicated, but pre-processing
to calculate DOE-2 inputs gives you a lot of flexibility in what you
account for...
I have not actually had the opportunity to develop this kind of method
yet, but I hope to get a project where the design (and budget) justifies
this level of analysis. With any type of work-around, I would plan to
do some sort of validation with another model, such as CFD, or a
calibration exercise, but these approaches for validating NV open other
cans of worms...
Anyone have any experience with validating the NV model (or your own)
that exists in the one of the available building energy simulation
programs? Any other ideas for "work-arounds"? Has anyone gotten
feedback from reviewing bodies (GCBI, utilities, code reviewers) on the
use of DOE-2's "S-G_NV" method?
www.pnl.gov/main/publications/external/...reports/PNNL-18898.pdf
David Reddy
