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[EnergyPlus_Support] Re: Outside temperature limit

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Outside temperature limit
From: "bsiengineer" <arch_engel@xxxxxxxxxxx>
Date: Tue, 13 Jul 2004 17:44:25 -0000
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Zhen,

What would be unusual is if your return air humidity is higher than 
the outdoor humidity, on average.  Normally, the important factor to 
consider for economizer energy savings is the enthalpy.  For example, 
it takes more energy to cool 20 C air at 80% RH than to cool 24 C air 
at 50% RH.  

I like to think of "free cooling" as the condition when the outdoor 
air temperature is less than or equal to the supply air temperature--
typically 12.8 C in the US.  When the temperature is above this, but 
still quite cool I would call it "economizer" range, where the 
cooling coil is used, and 100% outdoor air is used because the 
outdoor air has lower enthalpy than the return air, but not low 
enough to supply without cooling it.  Not everybody uses those 
definitions for "free cooling" and "economizer".

Where I live, if we assume the return air to be 24 C and 50% RH, the 
best economizer lockout temperature is 20 C.  Setting it higher uses 
more energy.

I hope this helps.

David


--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, zhen tian <tianzhen9@y...> 
wrote:
> Hi, David:
>  
> About the outside temperature limit, I have a question. For air 
system, the supply air temperature is around 13 to 16 degree C, the 
return air temperature is around 25 is the indoor temperature is 
around 24.  To my understading, if the outside temperature is under 
return air temperature, you can use "free cooling".  If the outside 
temperature is between supply and air temperature, say 13 to 25 
degree C, you can use 100 percent outside air for cooling, it will 
save energy for mixing air.
>  
> So could you tell me what is unusual? 
>  
> Thanks.
>  
> Zhen
>  
> 
> bsiengineer <arch_engel@h...> wrote:
> Zheng,
> 
> Thank you for your response.
> 
> My model has only two zones.  I have previously run this model many 
> times with a constant volume system, and encountered no errors.
> 
> I have manipulated the convergence tolerances with no effect on the 
> iterations.
> 
> My model has no economizer, so the outside temperature limit will 
not 
> affect it.  I think that in many US buildings the value of 19 is a 
> good choice.  The best temperature limit for a dry-bulb economizer 
> depends on the typical outdoor conditions and the typical return 
air 
> conditions.  If your return air is 25 C and is normally more humid 
> than the outdoor air then 25 may be a good choice, but I think this 
> would be very unusual.
> 
> Regards, David
> 
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, zhen tian 
<tianzhen9@y...> 
> wrote:
> > I had this iteration problem tried this method before. 
> >  
> > I found this problem was related to the zone number and zone 
> equipment.  The more zones, and the more equipement in the same 
zone, 
> the more possibilty of this iteration problem. 
> >  
> > I found that if you set the convege tolerance to 0.00005, and 
this 
> still appear, even you set minor values, this method will not work.
> >  
> > Try to change the ousiside temperature limit from 19 to 20 or 18, 
> this warning may go away. The example file uses 19 degree C, but I 
do 
> not think it is correct. It should be around 24 to 26. You can give 
> me a response if it works.
> >  
> > From my perepective view, it will be better for the uses if 
> Energyplus can focus on solve and improve the prolems in the 
existing 
> models rather than to incorporate new models right now.
> >  
> > By the way, I want to ask if somebody or some company has 
succeedly 
> used Energyplus for simulating of real big building projects. 
> >  
> > Regards, Zhen
> > 
> > bsiengineer <arch_engel@h...> wrote:
> > Rahul,
> > 
> > Thank you for your help.  I have made several simulation runs 
with 
> > different convergence tolerances (both increased and decreased 
> > values) on the cooling coil controller and reheat controllers but 
> > these changes have had no effect on the number of iterations 
> reported 
> > in the error file.  Does anybody have any other suggestions?
> > 
> > Thanks,  David
> > 
> > 
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Rahul Chillar 
> > <chillarrahul@y...> wrote:
> > > Hello
> > > This same problem had occured with a user earlier , ........... 
> Max
> > > Iteration errors can be eliminated by increasing the 
convergence 
> > tolerance
> > > on your controllers
> > >  
> > > For example look at the controller information below and you 
will 
> > easily figure out how to solve that. If it does not kindly let us 
> > know so that we can look at your file which you uploaded.
> > >  
> > > EARLIER CASE with problems:
> > >  
> > > CONTROLLER:SIMPLE,
> > > Central Cooling Coil Contoller 1, !- Name
> > > TEMP, !- Control variable
> > > Reverse, !- Action
> > > FLOW, !- Actuator variable
> > > VAV Sys 1 Outlet Node, !- Control_Node
> > > Cooling Coil Water In, !- Actuator_Node
> > > 0.002, !- Controller Convergence Tolerance: delta
> > > temp from setpoint temp {deltaC}
> > > autosize, !- Max Actuated Flow {m3/s}
> > > 0.0000001; !- Min Actuated Flow {m3/s}
> > >  
> > >  
> > > CONTROLLER:SIMPLE,
> > > Central Heating Coil Contoller 1, !- Name
> > > TEMP, !- Control variable
> > > Normal, !- Action
> > > FLOW, !- Actuator variable
> > > VAV Sys 1 Outlet Node, !- Control_Node
> > > Main Heating Coil 1 Water Inlet Node, !- Actuator_Node
> > > 0.002, !- Controller Convergence Tolerance: delta
> > > temp from setpoint temp {deltaC}
> > > autosize, !- Max Actuated Flow {m3/s}
> > > 0.0000001; !- Min Actuated Flow {m3/s}
> > >  
> > > These values created over 350 Max Iteration warnings. Then the 
> > values were
> > > changed to:
> > > CONTROLLER:SIMPLE,
> > > Central Cooling Coil Contoller 1, !- Name
> > > TEMP, !- Control variable
> > > Reverse, !- Action
> > > FLOW, !- Actuator variable
> > > VAV Sys 1 Outlet Node, !- Control_Node
> > > Cooling Coil Water In, !- Actuator_Node
> > > 0.001, !- Controller Convergence Tolerance: delta
> > > temp from setpoint temp {deltaC}
> > > autosize, !- Max Actuated Flow {m3/s}
> > > 0.0000001; !- Min Actuated Flow {m3/s}
> > >  
> > >  
> > > CONTROLLER:SIMPLE,
> > > Central Heating Coil Contoller 1, !- Name
> > > TEMP, !- Control variable
> > > Normal, !- Action
> > > FLOW, !- Actuator variable
> > > VAV Sys 1 Outlet Node, !- Control_Node
> > > Main Heating Coil 1 Water Inlet Node, !- Actuator_Node
> > > 0.001, !- Controller Convergence Tolerance: delta
> > > temp from setpoint temp {deltaC}
> > > autosize, !- Max Actuated Flow {m3/s}
> > > 0.0000001; !- Min Actuated Flow {m3/s}
> > >  
> > > The errors were reduced to over 50 Max iteration warnings.
> > > 
> > > When Values Changed to 
> > > CONTROLLER:SIMPLE,
> > > Central Cooling Coil Contoller 1, !- Name
> > > TEMP, !- Control variable
> > > Reverse, !- Action
> > > FLOW, !- Actuator variable
> > > VAV Sys 1 Outlet Node, !- Control_Node
> > > Cooling Coil Water In, !- Actuator_Node
> > > 0.0001, !- Controller Convergence Tolerance: delta
> > > temp from setpoint temp {deltaC}
> > > autosize, !- Max Actuated Flow {m3/s}
> > > 0.0; !- Min Actuated Flow {m3/s}
> > >  
> > >  
> > > CONTROLLER:SIMPLE,
> > > Central Heating Coil Contoller 1, !- Name
> > > TEMP, !- Control variable
> > > Normal, !- Action
> > > FLOW, !- Actuator variable
> > > VAV Sys 1 Outlet Node, !- Control_Node
> > > Main Heating Coil 1 Water Inlet Node, !- Actuator_Node
> > > 0.0001, !- Controller Convergence Tolerance: delta
> > > temp from setpoint temp {deltaC}
> > > autosize, !- Max Actuated Flow {m3/s}
> > > 0.0; !- Min Actuated Flow {m3/s}
> > >  
> > >  
> > > ALL MAX ITERATION ERROR ELIMINATED.
> > > Try the same procedure in your file , i am sure they will go 
> away. 
> > These occur because outer loop has higher tolerance set than 
inner 
> > loop . Hence it is unable to converge. 
> > > If they go away with above change you need not worry about the 
> > results even if you loosen up the tolerance.
> > >  
> > > Thanks
> > > Rahul Chillar
> > > Graduate Research Assistant
> > > Building Systems Laboratory
> > > Univerisity of Illinois UC
> > >  
> > >  
> > >  
> > > 
> > > bsiengineer <arch_engel@h...> wrote:
> > > My simulation is having a problem of exceeding the maximum 
> > iterations 
> > > on the HVAC system.  I am simulating two interior rooms served 
by 
> a 
> > > single-duct VAV reheat system.  This model has previously been 
> run 
> > > successfully with a single-duct constant volume system.  The 
> system 
> > > is electric preheat and reheat, and purchased chilled water 
> > cooling, 
> > > so it's fairly simple.
> > > 
> > > I will upload my file, Iterations.idf, to the 
> > > folder "Problem_Submittals".  Can anyone help me figure out 
what 
> is 
> > > going wrong?
> > > 
> > > Thanks!
> 
> 
> 
> 
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