All,
we regularly design and model buildings which rely in part on natural
ventilation as a source of free cooling. The issue we are often faced with
is how to capture this benefit when performing a LEED Appendix G analysis.
Appendix G states that all conditioned spaces must have BOTH heating and
cooling. We would like to remove mechanical coolig in the proposed design
building and thus gain benefit when comparing to the mechanical cooling
system in the reference baseline building.
Has anyone tackled this issue and come to a satisfactory resolution?
Thanks for your help.
Chris Flood
I am currently doing a naturally ventilated building for LEED and I
believe that if the space is achieving indoor temperatures between the
given set point using natural ventilation than the mechanical system
energy is 0 for the proposed design. Any hours where natural ventilation
can't achieve indoor temperatures within the set points, a mechanical
system needs to be modeled even if there isn't one in the proposed
building. This helps keep the unmet hours within the requirements for
LEED.
Cheers,
And how did you model the natural ventilation?
In summer when external temperatures are above the room setpoint natural
ventilation during the day would not provide any cooling.
Chris Flood
eQuest, TAS and IES, I know can model natural ventilation. In IES and
TAS you can actually model the operation of the windows by defining
limits on indoor temperature, wind speeds etc. eQuest is a little more
basic where you can use a S-G function for infiltration which calculates
higher infiltration rates based on outside wind conditions. eQuest then
calculates if indoor temperatures will be kept between the indoor
temperatures based on the outside conditions. If yes than it will shut
off the mechanical system and calculate 0 energy for that hour.
Therefore in programs like IES and TAS you may be able to keep
conditions below the set point even if the outdoor temperatures are
hotter due to air movement etc. You may also want to come to an
agreement with the client to allow a slightly higher set point, however
you will need to also apply this to the baseline model. Otherwise for
LEED, you can't take account for the hours were naturally ventilation
doesn't provide comfort. Also note that the unmet load hours can be a
maximum of 300 hours so potentially you could take credit for 300 hours
for natural ventilation even if it is above the set point.
Chris,
While you'll not be permitted to remove the cooling system for the
proposed building in the PRM analysis, you can design and operate the
building and systems as mixed-mode such that the cooling system never
runs. The catch is that you must use the same setpoints at any given
simulation time step for both proposed and baseline buildings. Thus, if
your mixed-mode control strategy would raise the cooling setpoint when
the outdoor ambient temperatures are elevated-e.g., to allow the
occupants to get whatever benefit they can from opening the windows,
stored cooling from thermal mass, etc.-you would need to use the same
reset strategy (i.e., tracking the OA temperature) for the cooling
control set points in the baseline building. Ultimately, the building
owner will then have to approve of the number of hours your model
indicates the building will operate with elevated cooling set points.
Best,
Timothy Moore