Re: [EnergyPlus_Support] Venting Wall Cavities

["Richard J. Liesen" <rliesen@xxxxxxxx>]

>Note also that moisture transfer
>estimates for this (or any) construction in EnergyPlus are not as
>refined as those calculated by a program such as WUFI.  When the brick
>is wet by rain and then the sun shines on it, there is a very strong
>water vapor flow toward both the inside and outside.  Also probably
>significant energy dissipation in evaporation.  EnergyPlus does not
>calculate how wet the brick may get for this transfer process because
>the necessary calculation detail would likely dramatically slow down a
>calculation process for building energy use.  Just one on the
>compromises of life until the next generation of PC's duplicate super
>computer power for everyone.

Hello,
I wanted to update some folks on the current and the near future moisture 
capabilities of EnergyPlus.  The current capabilities of moisture modeling 
in EnergyPlus are first and foremost the moisture interactions inside the 
zone.  Using the EMPD model option in EnergyPlus will allow a user to have 
the interior surface of the wall element interact with moisture for 
adsorption/desorption ie. night venting.  Another current capability of 
EnergyPlus is the MTF or Moisture Transfer Function.  The MTF option will 
allow for a first order approximation of the linear material property 
moisture effects for both adsorption/desorption and transmission of 
moisture through the wall element.  The key words are "first order 
approximation" and "linear material properties" as the limitations for the 
evaporation-condensation theory.

As Ned Lyon was pointing out there have been limitations when a user wanted 
to simulate a whole building due to execution time for these large 
problems.  Programs like WUFI and MOIST from NIST do a much more detailed 
analysis, but are usually limited to a wall element or a zone.

In the next release there will be a couple of solution algorithms included 
which will use a Finite Difference solution technique.  This implementation 
will be classified as unsupported due to the fact that future development 
is necessary to make it robust enough for general usage.  It will be 
available for researchers and for stout-hearted users to look at who have 
an understanding of Finite Difference and the stability requirements 
necessary.  Not every wall element will run for the next release due to 
lack of stability and the solution technique.  But we are starting to 
bridge the gap to detailed high computation type of solution techniques.

In the development of the newer solution techniques, my guess is proving 
out correctly that the MoistFD (Moisture Finite Difference) with non-linear 
properties runs faster or about the same as the MTF.  So for non-linear 
properties this appears to be the most appropriate implementation 
method.  Not only does it provide non-linear moisture properties for the 
simulation it does so with a quicker simulation time then the MTF 
solution.  It also provides the ability to look at Temp and Moisture 
profiles through the building surface.  As a by product the Conduction 
Finite Difference (CondFD) algorithms are a trivial addition.  Some timings 
are shown below for this implementation.

Timings for a annual one zone model with 7 surfaces (including one ground 
connected surface) using an auto sized purchased air using the Chicago, IL 
weather file are:
     CTF             30secs
     CondFD      59secs
     MoistFD     3min 27secs
     MTF             7min 32secs

Future time and effort in development will be necessary to make this more 
robust and a general solution technique option for all user.  At the same 
time we all hope as Ned does that computers continue growing faster and faster.


Thanks
Dr. Richard J. Liesen  Ph.D. M.E.
Associate Director, Building Systems Laboratory
Department of Mechanical and Industrial Engineering
University of Illinois at Urbana-Champaign
www.bso.uiuc.edu
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