Default air density and air heat capacity

asked 2020-03-05 07:09:29 -0600

arnespandau
171 ●4

updated 2020-03-05 10:39:10 -0600

shorowit

11891 ●201 ●9 https://www.nrel.gov/b...

Hello users, I need to compare TRNSYS 16 and EnergyPlus 9.1. I use the IDF Editor for modelling my building. Now I want to figure out why the results of infiltration are different in both programs.

In TRNSYS air density is 1.204 kg/m³ and the specific heat of air is 1.012 kJ/kg K

What is the default air density in EnergyPlus? Or is EnergyPlus using the "Site Outdoor Air Density" for the calculation of the infiltration energy?

And where can I define the heat of air capacity? Is it correct, if I use the "ZoneCapacitanceMultiplier:ResearchSpecial" and set the Temperature Capacity Multiplier to 1,012?

Thank you.

Kind regards Arne

answered 2020-06-04 11:01:05 -0600

Aaron Boranian

14155 ●83 ●24 http://bigladdersoftware.com/

updated 2020-06-04 11:14:45 -0600

If you have an HVAC system defined in EnergyPlus, you should have access to the following output variables:

Output:Variable,*,System Node Current Density,hourly; !- HVAC Average [kg/m3]
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).

The "Site Outdoor Air Density" will tell you the air density of outdoor air from the EPW weather file.

The ZoneCapacitanceMultiplier:ResearchSpecial object is used to multiply the total sensible heat storage capacity of a zone, not to set the zone air's specific heat.