Sara,
what is happening in EnergyPlus is:
Energyplus, uses inside and outside surface heat balance to determine the thermal interaction of the building surfaces with inside and outside environments. The heat balance equations for inside and outside surfaces are solved simultaneously and the resulting surface temperatures reported are key value that reflect each surface is thermally balanced.
Let us look at the outside surface:
The outside surface interacts or exchanges heat with outside air by convection, long-wave radiation exchange with the sky, ground surface, and the outside air mass. There is also shortwave solar radiation striking on the outside surface depending the time of the day if the surface is not shaded. There is also
conduction heat flux (into or out of) the outside surface. The conduction is the link to the inside surface heat balance and hence that is why we need to solve both surface heat balance equations simultaneously. The net output of the outside surface heat balance is the outside surface temperature that balances the various heat transfer components for each exterior surface at each time step.
The mechanisms or the modes of heat transfer by which heat loss or gain occurs from or to the outside surface (with the environment) are RADIATION and CONVECTION only. We do not have as such conduction heat transfer from the surface to either ENVIRONMENT (of course we are neglecting conduction through the air). Back to your questions:
Answer (1): heat loss or gain mechanisms
Therefore, when you say heat loss or gain (to or from the ENVIRONMENT in this case outside environment) it occurs via convection and radiation modes only. Having said that
the impact of the conduction is already included in these two terms as the conduction is already part of the surface heat balance equations (for details see Engineering Reference v5, Pages 40-43).
Answer (2): to get estimate of the heat loss or heat gain, the following equation can be used:
Q = A * [hconv * (TsurfaceOutside - Tairoutside) ! convection loss or gain from or to the outside air
+ (hrad1 * (TsurfaceOutside - Tsky) ! radiation exchange with the sky at the sky temperature
+ (hrad2 * (TsurfaceOutside - Tground) ! radiation exchange with the outside ground surface
+ (hrad3 * (TsurfaceOutside - Tairoutside)] ! radiation exchange with the outside
air node
Depending on the value of TsurfaceOutside, which is the outcome of the surface heat balances, the three linearized radiation coefficients, and the convection coefficient, the net Q for each exterior surface at any time step can be estimated as heat loss or gain.
But you have to specify output variables for the various coefficients and temperatures in the IDF file so that EnergyPlus report the linearized radiation coefficients and the temperatures for each surface of interest to the epluout.csv output file. For all variables that you would like EnergyPlus report to the csv file look into the eplusout.rdd fle. Here are sample report variables request for surface coefficients:
Output:Variable,*, Surface Ext Convection Coeff, timestep;
Output:Variable,*, Surface Ext Rad to Air Coeff, timestep;
Output:Variable,*, Surface Ext Rad to Sky Coeff, timestep;
Output:Variable,*, Surface Ext Rad to Ground Coeff,
timestep;
I hope this helps,
Bereket Nigusse
Florida Solar Energy Center
From: sara <my_sunny_night@xxxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Wed, July 7, 2010 12:00:08 AM
Subject: [EnergyPlus_Support] Re: Heat Loss through Exterior Walls
Dears,
I still cannot calculate the amount of heat loss through the exterior walls that includes both conduction and convection losses. Can anyone tell me how to?
Sara
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "sara" <my_sunny_night@...> wrote:
>
>
> Liam,
>
> Let's suppose it's steady state. Then I want to calculate the amount of heat loss through the exterior walls in cold climates. I need to mention that this heat loss includes both conduction and convection losses through the walls.
>
> Sara
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Liam O'Brien" <liambrn@> wrote:
> >
> > Sara,
> >
> > It's not that simple because of transient effects. For example, it would be possible for heat to be "entering" a wall from both sides because of storage effects. As Karen said, your best bet it to pick one surface in the wall and try to understand the heat loss at that boundary. For instance, this output variable:
> > "Zone,Average,Surface Int Convection Heat Rate [W]"
> >
> > Liam O'Brien
> >
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "sara" <my_sunny_night@> wrote:
> > >
> > >
> > > Karen,
> > >
> > > I mean that like the variables for window heat loss/gain, such as "Zone,Average,Window Heat Gain", "Zone,Sum,Window Heat Gain Energy", and "Zone,Average,Zone Window Heat Gain", I've searched in the RDD file such variables for walls. But, I didn't find anything.
> > >
> > > Thanks,
> > > Sara
> > >
> > >
> > >
> > >
> > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Karen Walkerman <kwalkerman@> wrote:
> > > >
> > > > Sara,
> > > >
> > > > I think that the variable you want is the convective heat loss between the
> > > > exterior wall and the interior. From my understanding, heat loss through
> >
> > exterior walls in EnergyPlus is calculated as convective losses between the
> > > > interior air space and the wall. The wall interior surface temperature is
> > > > calculated based on the wall construction and interior and exterior
> > > > conditions.
> > > >
> > > > --
> > > > Karen
> > > >
> > > > On Tue, Jul 6, 2010 at 4:36 AM, sara <my_sunny_night@> wrote:
> > > >
> > > > >
> > > > >
> > > > > Hi Dears,
> > > > >
> > > > > I have 5 walls exposed to outside. I want to know the amount of heat loss
> > > > > through them in total. But, in Output:Variable there isn't any variable for
> > > > > calculating the mentioned amount.
> > > > >
> > > > > Thanks,
> > > > > Sara
> > > > >
> > > > >
> > > > >
> > > >
> > >
> >
>