Area handling in UFH PanelHeating and PanelHeatingSegment

[Daniel12449]

Hi,
I noticed odd behavior in the BESMod.Systems.Hydraulical.Components.UFH.PanelHeatingSegment model:

This model implements two surface area parameters, A and A_floor, which are set equal inside. Based on the parametrization in the parent model BESMod.Systems.Hydraulical.Components.UFH.PanelHeating A is supposed to be the surface area of the pipe, while A_floor is the floor surface area:

PanelHeatingSegment panelHeatingSegment[dis](
    redeclare package Medium = Medium,
    each final A=tubeLength*floorHeatingType.diameter*Modelica.Constants.pi/dis/
        2,
   ...
    each final A_floor=A) annotation (Placement(transformation(extent={{-58,1},{-8,51}})));

from BESMod.Systems.Hydraulical.Components.UFH.PanelHeating.

This raises multiple points:

• If they are supposed to be different areas, setting them equal inside the PanelHeatingSegment is not logical, therefore the parameter should be changed to remove the assignment on this level: Code
  parameter Modelica.Units.SI.Area A_floor=A "Area of surface of wall / Floor";
• In both panel_Segment[1-2] the area should be changed from A to A_Floor, since those seem to be modeling the floor based on the other parameters. Code
• Another resistance could be added that models the heat transfer from the pipe to the floor, this would then use A
• A_Floor assignment in PanelHeating should be divided by discretization layers, otherwise all layers receive the full floor area.

What are your opionions on this topic?

Best regards,
Daniel

[FelixStege]

Hi,
thanks for opening this Issue!
I agree, that the UFH model has some flaws we need to adress.

• Setting A_floor=A in the PanelHeatingSegment keeps the model behavior the same to the original model from the AixLib (AixLib.Fluid.HeatExchangers.ActiveWalls.BaseClasses.PanelHeatingSegment). The extra parameter A_floor enables setting different values for the effective area for the heat exchange between the pipe and the concrete and the concrete and the zone air in the top level model (PanelHeating).
• The panel_Segment represents the actual concrete (or whatever material the floor consists of) of the floor. Therefore the first heat resistance represnets the heat flow from the pipe into the floor (via the area of the pipes A). The second resistance, however, represents the heat flow from the core of the floor to its surface (from which radiative and convective heat transfer with the air occurs). So for this, you are correct. The second resistance should use A_floor here.
• This extra resistance is already in place (the HeatConductionSegement uses two resitances). One could argue, that introducin another temperature state for the actual pipe woud be more accurate (with models like AixLib.Utilities.HeatTransfer.CylindricHeatTransfer), however, this would also increase complexity for potential minimal accuracy gain.
• I agree, we should change this 👍🏼

An addtional, maybe even bigger problem is the general behavior of the UFH model. If you execute the simulate and plot script for its test (BESMod.Systems.Hydraulical.Transfer.Tests.TestUnderfloorHeating), you see that it does not reach its nominal Q_flow. Addiiotnally, the spacing seems really small (3.5 cm). So we should generally critically reflect all the parameters used.

If you have suggestions or have already tried some fixes, please let me know!

[Daniel12449]

Hi, thanks for the detailed answer!

First of all, keeping the same behavior as the original model seems like the right choice to me.

Right now each panelHeatingSegment gets the full A_floor assigned, is this intended behavior or should this be divided by the amount of discretization elements?

Looking at the assignments of the area inside the panelHeatingSegments, i can see that the Area for twoStart_RadEx and HeatConv gets assigned to A_floor (100m²), but the panel_Segments still use A instead ( kA = 71.55 -> A=16m² instead of 100m² with kTop = 4.47 in the TestUnderfloorHeating model). This looks like setting A=A_floor does not work as intended or is this the expected behavior?

Regarding your 2nd and 3rd point, in the currently used model only a single combined kA value can be assigned, so to assign two different kA values for both thermalConductors, this model would need to be updated to allow independent assignments?

[FWuellhorst]

You are looking at the defaults in panel heating segment, but the panel heating overrides those @Daniel12449 :

  PanelHeatingSegment panelHeatingSegment[dis](
    redeclare package Medium = Medium,
    each final A=tubeLength*floorHeatingType.diameter*Modelica.Constants.pi/dis/
        2,
    each final eps=eps,
    each final T0=T0,
    each final VWater=VWater/dis,
    each final kTop=kTop_nominal,
    each final kDown=kDown_nominal,
    each final cTop=cTop,
    each final cDown=cDown,
    each final isFloor=isFloor,
    each final calcMethod=calcMethod,
    each final hCon_const=hCon_const,
    each final A_floor=A)
```
So there is a difference between A and A_floor, but indeed, A_floor should be `A_floor = A/dis`.
This is the main change from the AixLib model @FelixStege, so that the k values are actually valid for the pipe area. 
But if this is better or worse depends on the k values one assumes. 

[Daniel12449]

I might just have misinterpreted the intended behavior of the model.

If i now understand the model correctly, the pipe surface area is used for all areas as long as no A_floor is provided to the model. If A_floor is set, use A_pipe for the heat conduction (panel_Segment1/2) and A_floor for radiation and convection?

I previously assumed since the panel_Segments model the concrete/ceiling itself, it should use A_floor and not A_pipe.

The default behavior in the test model BESMod.Systems.Hydraulical.Transfer.Tests.TestUnderfloorHeating (without the A_floor=A/dis) is this:

• transfer.ufh[1].panelHeatingSegment[1].A: 16 m²
• transfer.ufh[1].panelHeatingSegment[1].A_floor: 100 m²
• transfer.ufh[1].panelHeatingSegment[1].panel_Segment1.kA: 71.55 W/K
• transfer.ufh[1].panelHeatingSegment[1].kTop: 4.47 W/m²K
• transfer.ufh[1].panelHeatingSegment[1].HeatConv.A: 100 m²

This shows that HeatConv is using A_floor, while the panel_Segment1 & 2 use A.
-> 71.55 W/K / 4.47 W/m²K = 16 m²

This is with the PanelHeatingSegment declaration of the previous answer by @FWuellhorst.

Thank you both for taking the time to look into this!