Why is the ground loop pump pressure decreasing over time in example "Residential Ground Source Heat Pump"?

I am working on the following example:

https://uk.mathworks.com/help/releases/R2022b/hydro/ug/residential-ground-source-heat-pump.html

I have noticed that the pressure of the Controlled Volumetric Flow Rate Source (TL) block ("Ground Environment/Ground loop pump") is decreasing overtime (while the differential pressure stays the same).

This does not seem representative of a real ground source heat pump.

Why is this pressure decreasing over time and is it possible to stabilise it?

Answer 1

[MathWorks Support Team]

The pressure decrease that you have observed is due to the large amount of heat being transferred from the thermal liquid network to the two-phase fluid network by the evaporator. To avoid this behaviour, you can increase the ground loop flow rate. 

Increasing the ground loop flow rate by a factor of about 50 stabilises the pressure. To do so, you can execute the following command in the MATLAB Command Window:

>> qGroundLoopPump= qGroundLoopPump * 50;

While a higher ground loop flow rate might be more realistic, in our model, this leads to an operating temperature of the two-phase fluid in the evaporator of -20 degC (rather than a more typical 2 degC).

If are working on sizing a system with a two-phase fluid network, we recommend the workflow on this doc page:

https://www.mathworks.com/help/releases/R2022b/hydro/ug/modeling-a-refrigeration-cycle.html