[EnergyPlus_Support] Re: WindAC output

["don_shirey <shirey@xxxxxxxxxxxxxxxxxx>" <shirey@xxxxxxxxxxxxxxxxxx>]

Ed:

You bring up some very good points, and we are well aware of the 
latent performance degradation issues that you discuss below.  Nearly 
all building energy simulation programs ignore this phenomena
although 
it is very important for predicting indoor humidity levels.

A model for latent performance degradation will be added to
EnergyPlus 
later this year.

Henderson and I are currently working on a research project for DOE
to 
look at this issue in further detail, with the goal of improving 
Henderson and Rengarajan's existing latent degradation model.


--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Edward Lyon"
<eglyon@s...> 
wrote:
> You should be aware of one other issue I have observed with the 
current DXACCoil model.  The SHR is adjusted for coil performance at 
model input conditions, but there is apparently no degradation of SHR 
in relation to PLR.  
> 
> I performed some AC total capacity change calculations for the same 
load conditions.  I got different zone RH results depending on the
fan 
cycle.  I found that for my base model, my 1x system capacity was 
greatly oversized.  A 0.1x system would run full time through most of 
my test period while maintaining close to set point temperature.  I 
had 50 to 55% RH for a model that goes to 100% when there is no 
mechanical dehumidification is available.  My assumption is that 
EnergyPlus reasonably models these PLR=1 conditions with the current 
equations. A 0.5x system and all larger systems (1x, 5x and 10x) had
a 
PLR less than 0.1 for the same time period.  With a cycle fan 
operation I found that all oversize systems degraded to exactly the 
same zone RH curve which was slightly higher than the constant run 
0.1x system but less than 60% RH.  When the 0.1x system did run less 
than PLR=1, it approached the same RH limits as all the other
systems. 
 The PLR for the 0.1x  system did not go below about 0.7.
> 
> The constant fan models did not degrade to the same RH curve as the 
cycle fan or as other systems in the rest group.  The RH curve of 
larger units typically crisscrossed the RH for the 0.1x system. 
However, the lowest zone RH curve belonged to the 10x system and it 
maintained an RH consistently below the 0.1x system.  This
performance 
is completely contrary to field observations.  It is highly likely 
that short cycling, constant fan systems provide 0 dehumidification 
because coil evaporation will return all condensed moisture back to 
the zone space.
> 
> The EnergyPlus results in this study seem to be inconsistent with 
Henderson's observations (IAQ and Energy 98) that for residential and 
small commercial DXAC systems, SHR goes from the rated value to 1 as 
PLR goes from 1 to about 0.5.  The existing EnergyPlus model makes 
some adjustments for SHR based on calculated time step total
capacity, 
but there seems to be a need to have another PLR degradation variable 
to properly model these conditions.  I suppose that it is possible 
that the current EnergyPlus model uses the PLF for this degradation 
which is limited to a minimum of 0.7, but the documentation does not 
describe the specific SHR degradation with PLR that Henderson 
recounts.
> 
> Consequently, any system using a DXACCoil is likely to have 
significant periods of questionable zone humidity prediction whenever 
systems operate at part load.  This probably impacts comfort 
calculations and humidity control reheat calculations since these 
systems seldom reached a humidity control set point.
> 
> I suspect that a larger latent load would cause some of the systems 
to predict higher zone RH conditions because of reduction in total 
latent capacity due to PLF operation. However, this degradation is
not 
the same as the SHR loss related to PLR that is found in oversized 
equipment.
> 
> Ned Lyon
> 
> Senior Staff Engineer
> Simpson Gumpertz & Heger Inc.
> 
> 41 Seyon Street
> Building #1, Suite 500
> Waltham, MA  02453
> 
> Phone:  781-907-9000
> Fax:  781-907-9009
> 
> 
> >>> alexander.beisteiner@f... 02/24/03 10:12AM >>>


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