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Does E+ take into account the fact that specific heat varies with temperature ?

asked 2020-06-04 08:08:41 -0600

Cochabambi's avatar

updated 2020-06-04 08:50:39 -0600

Hello everyone,

I am quite new to EnergyPlus and I would like to know if EnergyPlus takes into account the fact that the specific heat of air changes with temperature ?

I am currently using the Conduction Transfer Function algorithm for my simulations. I have looked in the Engineering Reference and what I found is that it was possible to define Phase Change Materials and in that case, the specific heat varies with temperature.

So I just wanted to be sure that, if I define some specific heat in the "Material" object, it will be considered constant with temperature ? What about the air specific heat ?

Thank you for your help

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When you ask "if the specific heat of air changes with temperature", what are you referring to? Outside air, air moving around an HVAC system, air gaps within window or opaque surface construction assemblies, or something else?

Aaron Boranian's avatar Aaron Boranian  ( 2020-06-04 08:51:57 -0600 )edit

So what I am interested in is to predict the indoor air temperature. But in my question, I am referring to outside air and indoor air mainly. And concerning materials specific heat, I am referring to the user-defined properties in the "Material" object.

Cochabambi's avatar Cochabambi  ( 2020-06-04 10:27:24 -0600 )edit

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answered 2020-06-04 11:14:48 -0600

Yes, the specific heat of outside air and indoor air of zones change over the course of EnergyPlus simulations. If you are modeling HVAC systems, EnergyPlus can provide results for the zone air using the following output variable:

  • Output:Variable,*,System Node Specific Heat,hourly; !- HVAC Average [J/kg-K]

You will need to find the name of the zone air node that you are interested in, then replace the "*" in the two examples above with that zone air node name (or whatever specific node in the HVAC system you are interested in). If you are not modeling HVAC systems, then this output variable is not available.

If you don't see a node name in your model for outdoor air, you could use output variables for other outdoor air properties and calculate specific heat using psychrometric equations.

You are correct that if you apply phase change materials you can specify how their specific heat changes with temperature, but that needs the Conduction Finite Difference algorithm. With the default Conduction Transfer Function algorithm the specific heat set in Material objects stays constant over the simulation.

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Ok thank you for your clear and complete answer !

Cochabambi's avatar Cochabambi  ( 2020-06-04 11:30:53 -0600 )edit

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Last updated: Jun 04 '20