Solver problems in design case for heat pump model

[manuel32128]

I would like to write a simulation for a real heat pump in order to then compare real and simulated values during operation. The heat pump is merely a demonstration model for students. The condenser and evaporator are both refrigerant-air heat exchangers and use ambient air as source and sink (as an explanation for the unusual parameters :) ).
After the final parameterization, I encounter problems with the solver. Before that, all the sub-steps worked.

In the final offdesign, only the ambient temperature and the compressor power are to be specified.

If anyone could help me with this, that would be great.
Thank you in advance.
HeatPump_tespy_R134a.py

[fwitte]

Hi @manuel32128,

thank you for reaching out. The issue is two issues:

• The specification of the units is incorrect, that needs to be done through nw.units.set_defaults, e.g. nw.units.set_defaults(power="kW"). You used the old way, which was only working for temperature, pressure, enthalpy, entropy and some others (only Connection class attributes). If you print the compressor power at the end, you will see: print(cp.P.val_with_unit) is 0.112 Watts.
• From that, the mass flow will be so small compared to pressures/enthalpies, that it will not be possible to find a solution.

As a side note: with the latest version of tespy, you can also directly impose saturation temperature instead of using CoolProp to calculate saturation pressure and impose that. Instead of

T_start = 273.15 + (50)
p_cond = CP.PropsSI("P", "Q", 1, "T", T_start, working_fluid) / 1e5
c2.set_attr(T=65, p=p_cond, fluid={working_fluid: 1})

you ca do:

c2.set_attr(T_dew=50)  # or T_bubble, no difference here

Best

[fwitte]

Edit to that above: It does not work with the change I suggested, there is actually a bug in the software! I'll open an issue, fix it and make a release.

[fwitte]

I just published version 0.9.9 where the issue has been fixed.

[manuel32128]

Thank you so much for your quick response and all your effort. I will try it out as soon as possible.

[fwitte]

One more addition: Is there an actual drum in your real system? If not, I'd advise you to look into the SectionedHeatExchanger or MovingBoundaryHeatExchanger, here you find a comparison on what the different heat exchangers do (and what the do not do): https://tespy.readthedocs.io/en/main/tutorials/heat_exchangers.html#tespy-tutorial-heat-exchanger. You can also exchange your Condenser with the same type of component, and for both evaporator and condenser impose pinch td_pinch.

Best

[manuel32128]

No, there isn't an actual drum in my real system. I just thought that this would be the best way to model the heat transfer, as this is how it is done in the tutorial “Build a Heat Pump Stepwise.” But since you have suggested it, I will take a look at the other options and adjust my model. Thank you very much.

[fwitte]

Main reason for that is, that the tutorial has been sitting there for quite a bit of time (one of the first things in the docs of tespy). The main goal of that tutorial was to conceive the concept of building up systems step by step to understand the parameters to be set etc.. At that time, the MovingBoundaryHeatExchanger and SectionedHeatExchanger were not available.