steam generating heat pump

[lennipapa]

Hi, I'm modeling a single-stage vapor compression heat pump for steam generation with the following cycle:

Heat source: 70°C → 60°C (water)
Heat sink: 90°C water → 115°C saturated steam (1 kg/s at ~1.69 bar)
Refrigerant: R601 (n-pentane)
Components: Compressor (η_s=0.35) → Condenser (with Drum) → Expansion Valve → Evaporator

(the isentropic efficiency is set very low to avoid compression into wet steam)

Issues I'm encountering:

Evaporator thermodynamic violation: The solver produces a refrigerant evaporation temperature (~74°C) that is higher than the heat source inlet (70°C) and outlet (60°C)

The error messages show:

Negative ttd_l (lower terminal temperature difference)
Negative heat transfer coefficient (kA < 0)
Negative effectiveness values

steam_generating_hp_rev01.py

[fwitte]

Hi @lennipapa,

thanks for reaching out. In your case you have simply connected the evaporator in the wrong way: The hot side is your warm water that get's cooled down from 70 to 60 °C. You had that one the cold side.

On a sidenote, depending on what you are going to do with your model, you might want to use a MovingBoundaryHeatExchanger (or SectionedHeatExchanger) instead of the Condenser. The stepwise tutorial in the docs needs to be updated to use that same component, I guess. You can see here what the difference between the components is: https://tespy.readthedocs.io/en/main/advanced_tutorials/heat_exchangers.html#tutorial-heat-exchanger.

Best

Francesco

[fwitte]

Three more additions:

• You do not need to specify the fluid list in the Network class anymore. I thought I had removed all the hints to do that in the docs, can you tell me, where did you get that information?
• If you change from Condenser (has an inbuilt x=0 at hot side outlet) to MovingBoundaryHeatExchanger make sure to specify x=0 at the hot side outlet connection.
• I updated the stepwise.py tutorial in the docs to use the MovingBoundaryHeatExchanger instead of the Condenser as well.

[lennipapa]

Hi @fwitte,

thanks for the fast reply!

For the network specification I used the copilot. I guess it used older informations.

I've changed the Condenser component to a MovingBoundaryHeatExchanger.

I'm encountering a CoolProp error when the sink outlet (steam side) tries to reach exactly the saturation temperature. To work around this, I set the sink outlet slightly above saturation on line 87.
The error occurs because CoolProp has numerical issues when calculating properties exactly at the saturation point for water. Is this the correct approach to handle this, or is there a better way?

Additionally, regarding the refrigerant outlet from the condenser . If I want to further subcool the refrigerant at the condenser outlet, do I need to add a separate subcooler component in this case, or is there another option using the moving boundary heat exchanger?

steam_generating_hp_rev02.py

[fwitte]

For the network specification I used the copilot. I guess it used older informations.

got it!

I'm encountering a CoolProp error when the sink outlet (steam side) tries to reach exactly the saturation temperature. To work around this, I set the sink outlet slightly above saturation on line 87. The error occurs because CoolProp has numerical issues when calculating properties exactly at the saturation point for water. Is this the correct approach to handle this, or is there a better way?

That is an issue of your setup: In the drum the water is in two-phase state. Since you have already defined the pressure there, the temperature is linear dependent on it (as it is the saturation temperature). You would usually specify the circulating mass flow with respect to the overall mass flow (e.g. m=Ref(otherconn, 2, 0) for making it double the mass flow of the water) or the vapor mass fraction (e.g. x=0.7) of the fluid returning from the condenser into the drum.

Additionally, regarding the refrigerant outlet from the condenser . If I want to further subcool the refrigerant at the condenser outlet, do I need to add a separate subcooler component in this case, or is there another option using the moving boundary heat exchanger?

You can just set td_bubble=5 (for 5 K subcooling) instead of x=0. Here is the list of parameters you can specify: https://tespy.readthedocs.io/en/main/building_blocks/connections.html#modules-connections-label