Re: [EnergyPlus_Support] What is the difference ?

[Linda Lawrie <linda@xxxxxxxxxx>]

Q: [Time Step] What is the difference between the TimeStep indicated in the 
TimeStep in Hour object and the "variable" timestep indicated in the HVAC 
simulation (ref. Engineering Documentation)?

A: The TimeStep In Hour object specifies the "basic" time step for the 
simulation.  This is used in the Heat Balance calculation as the driving 
time step.  When the HVAC portion of the simulation begins it's solution 
for the current time step, it uses the basic time step as its maximum but 
then can reduce the time step, as necessary, to reach the solution.

The technical details of the approach are explained in the 
<file:///EngineeringDoc.pdf>Engineering Documentation, under "Integrated 
Solution Manager".

Users can see the effect/actual HVAC time step used if they select the 
"detailed" option on an HVAC report variable (e.g. Zone/Sys Air Temp).  To 
contrast, the "Zone" variables will only be reported on the Heat Balance 
time step (e.g. Mean Air Temperature).

Q: [Time Step] Time steps recommended in E+ is 4 for non-HVAC and 6 for 
HVAC calculations. Does it mean using time step of 1 will cause E+ to 
diverge or sacrifice the accuracy?

A: Actually, 4 or 6 Time steps per hour are necessary for the HVAC 
calculations.  The heat balance simulation will give reasonable results at 
1, 4, or 6 Time steps per hour.

Note that the Time Step value must be evenly divisible into 60 -- therefore 
is limited to:

1,2,3,4,5,6,10,12,15,30,60

Q: [Time Step] How do the Loads and Systems modules handle time step settings?

A: This scheme is discussed in detail in the provided EnergyPlus 
<file:///EngineeringDoc.pdf>Engineering Documentation (Integrated Solution 
Manager and following sections) and the papers that are referenced 
there.  The entire Predictor-Corrector is based on this zone-system 
connection and needs pages to describe its connection.  In a nutshell, the 
system tries to simulate lock step with the heat balance time step.  When 
temperature instabilities are determined, the HVAC systems reduce their 
simulation Time steps to something shorter than the heat balance time step, 
currently with a one minute minimum.  The systems stimulate at this shorter 
time step until the end of the heat balance time step, then they update and 
the entire simulation moves forward in time.


Linda

EnergyPlus WebSite: http://www.energyplus.gov
Owner: EnergyPlus_Support list
Member: EnergyPlus Development Team



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