Energy+ Thermal Model in Grasshopper

[David Birge]

Dear DIVA team... these plugins are great. Thanks for doing this
work.

I was wondering if you could tell me what variables the plug-in gives
to the Energy Plus program as defaults OTHER THAN the ones we control
with the grasshopper viper module. That is to say, does it feed Energy
Plus with any air-exchange information?

I'm using your module to do a thermal analysis for a house design in
Vermont and the heat loss from infil/exfiltration and the ventilation
system will be most of the heat losses in the winter. I just need to
know if I should add my own calculations or what, if any, assumptions
your viper module tells the Energy Plus program.

Thanks so much,

David Birge

[J. Alstan Jakubiec]

Hi David,

Viper does assume some default value for infiltration, 0.000302 m^3/s per
m^2 of exterior wall area for every hour in the year. You can see and edit
this in the file,
c:\DIVA\Thermal\Grasshopper\IDF_components\00_base_file.idf
Unfortunately (or fortunately if you know E+), it is all in EnergyPlus
file format.

For reference, the part of the file concerning infiltration reads,
...
! ********************************************************************
! INFILTRATION

ZoneInfiltration:DesignFlowRate,
ZONE1 Infiltration, !- Name
ZONE1, !- Zone or ZoneList Name
INFIL_SCH, !- Schedule Name
Flow/ExteriorArea, !- Design Flow Rate Calculation Method
, !- Design Flow Rate {m3/s}
, !- Flow per Zone Floor Area {m3/s-m2}
0.000302, !- Flow per Exterior Surface Area {m3/s-m2}
, !- Air Changes per Hour
1.0000, !- Constant Term Coefficient
0.0000, !- Temperature Term Coefficient
0.0000, !- Velocity Term Coefficient
0.0000; !- Velocity Squared Term Coefficient
...

Best,
Alstan

[David Birge]

Thanks for the answers. I did in fact open up the idf file with the
editing program and could see the assumptions about infiltration and
mechanical systems.

Another question arises for me. The energy heating and cooling load
can be used as a proxy for energy lost or gained correct? That is to
say, it assumes 0 internal mass so that the modeled heating/cooling
system load is essentially a direct relationship to how much heat is
gained or lost, as every BTU or kW that is added to the system to keep
the temperature even, is to make up for that same BTU or kW lost or
gained through infiltration and heat losses gains through walls, etc.

Is this a correct assumption?

-- David

[Max D]

I would assume that assumption not to be really spot-on under all circumstances, as at certain points in time the building will be free-running and hence the cooling / heating loads would not be part of the total; losses and gains per surface are best investigated directly on the idf level. E+ does consider structural mass.

Best wishes,

Max

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[David Birge]

Max,

Thanks... I understand that people, lights, mechanical will all
produce heat and thus skew the heating/cooling loads outputed in
viper. If, however, these are brought to 0 where possible, and there
is no thermal mass added (perhaps assume a wall made only of
insulation, or inputting R-value directly), then the assumption would
be pretty close, say 5% error or less?

I appreciate the heads up on the losses per surface, etc. I'll look
into that...

The issue I'm dealing with though is trying to set up as rigorous a
model as I can to test the cost/benefit of different thermal
strategies (insulation, infiltration, passive solar, etc.). I know
this work has been done before, but it becomes outdated immediately
and cost of materials, etc. changes constantly. The hope is to make
the model dynamic and as short a feedback loop as possible for a
specific location. Anyway... thanks for all your work on this. You
guys are doing a huge service to the profession.

-- David

[J. Alstan Jakubiec]

Hi David,

I am not sure it is easy to get the envelope gains/losses per component
directly from Viper right now -- Jonnie can speak better to that. One
convenient thing EnergyPlus offers are summary reports on a variety of
useful things. If you want to edit the IDF files again in
c:\DIVA\Thermal\Grasshopper\IDF_components\, you can output some useful
summary reports that lead themselves to your goal,

OutputControl:Table:Style,
HTML,
JtoKWH;

Output:Table:SummaryReports,
InputVerificationandResultsSummary,
ClimacticDataSummary,
ZoneCoolingSummaryMonthly,
ZoneHeatingSummaryMonthly,
WindowReportMonthly,
EnvelopeSummary,
SensibleHeatGainSummary,
SpaceGainsMonthly,
PeakSpaceGainsMonthly,
SpaceGainComponentsAtCoolingPeakMonthly;

There is a complete list of the possible summary reports in the
EnergyPlus-7.0 Input/Output documentation.

You'll have to find the resulting html file in the
c:\DIVA\Thermal\Grasshopper\Projects\ folder, which is unfortunately not
an automated process as one would like.

Best,
Alstan