Re: [EnergyPlus_Support] report variable: surface conduction

["Michael J. Witte" <mjwitte@xxxxxxxx>]

There are three relevant report objects in StandardReports.idf:

  Space Gains
  Peak Space Gains
  Space Gain Components at Cooling Peak

These are suggested reports to give an overview of gains from internal loads 
and infiltration.  The last of these reports "Space Gain Components at Cooling 
Peak" must be used with caution, because it is reporing heat *gains* at the 
time of the peak cooling *load*.  The convective portions of these gains 
contribute instantely to load, but the radiant portions of these gains become 
load at some later time, so this is not a "peak load components" report.

The heat balance method in EnergyPlus does not track every Watt of cooling (or 
heating) load back to it's original heat gain source.  And within the surface 
heat balance, surface conduction from the outside is mixed with the impact of 
radiant internal gains and solar fluxes on the interior of the surface.  Some 
modelers feel that "Zone Opaque Surface Inside Face Conduction Gain" is 
relevant when summed over a long period of time to give an indication of the 
net gain/loss throught the building envelope, but it is definitely not valid as 
a peak load component.

For 100% convective HVAC systems, using a thermostat controlled on the zone air 
dry bulb temperature, the following peak cooling load components can be 
reported accurately:

  Zone People Convective Heat Gain [J]
  Zone Lights Convective Heat Gain [J]
  Zone Electric Equipment Convective Heat Gain [J]
  Zone <Gas, Other, etc.> Equipment Convective Heat Gain [J]
  Zone Infiltration Sensible Heat Loss [J]
  Zone Infiltration Sensible Heat Gain [J]

This covers all of the convective heat gains which instantly impact the cooling 
loads (I may have forgotten an item here, but this should give the general 
idea).  The rest of the zone cooling load comes from

  Surface Int Convection Heat Gain to Air [J]

This represents the current impact on cooling *load* from the time history of 
the following types of *gains*:  envelope conduction, solar gains, radiant 
components of internal gains, and I may be forgetting something here.  I see 
that we do not have a zone-level report variable for this which sums the 
convective gain from all surfaces in the zone - that might be a useful new 
report variable.

The HVAC system ventilation load is not part of "Zone/Sys Sensible Cooling 
Energy[J]".  For cooling, it can be reported with:

  Zone Mechanical Ventilation Cooling Load Increase [J]
  Zone Mech Ventilation Cooling Load Increase: OverHeating [J
  Zone Mechanical Ventilation Cooling Load Decrease [J]

Complicated?  Yes.  More discussion follows in my reply to the next post on 
"Report Variables - Gain/Load?".

Mike


On 22 May 2007 at 9:27, Ery Djunaedy wrote:

> All,
> 
> I want to have the peak load components report a la eQuest, ie:
> - overall heat transfer through window
> - overall heat transfer through opaque surfaces
> - people gains
> - lights gains
> - equipment
> - infiltration
> - ventilation
> 
> What variables are relevant for that? I looked at the
> StandardReport.idf in DataSets folder, but in the peak cooling
> components, the first two of the above list is not reported.
> 
> For window heat transfer, I use these two variables:
> Zone Window Heat Gain
> Zone Window Heat Loss
> 
> What about the heat transfer through opaque surfaces? Is this where
> the "Zone Opaque Surface Inside Face Conduction Gain" would be
> relevant? I want to double check about this because the I/O Reference
> is full of warning about using this variable.
> 
> Thanks,
> 
> Ery
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
> 
> EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the
> Documentation link and press the "search" button.
> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 


========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx





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