TL pressure Keep descending until the error is reported below the minimum effective value in matlab "official example of heat pump"

The problem can be repeated through the following steps：

• Open the official heat pump example of matlab（link）with the command "sscfluis_residential_ground_source_heat_pump"
• change simulink stop time to 3600*48*20
• run it, and wait error occure (e.g. At time 417579.726180, one or more assertions are triggered. See causes for specific information. Pressure at port A1 must be greater than or equal to Minimum valid pressure. The assertion comes from: Block path sscfluids_residential_ground_source_heat_pump_origin/Evaporator)

i add two sensor to watch pressure change of TL in-out,

and the scopr output just like this,

I don't quite understand why the pressure keeps falling?

This problem has troubled me for a long time. At the beginning, I thought it was a parameter setting error or incorrect module use. I realized that it was probably not my fault until I ran the official example!

This is an official example of matlab. It is unlikely to be a code or parameter problem, so is this a software bug?

Thanks in advance for your answer!

best wish~

Answer 1

[Yifeng Tang]

It's a model problem, in my opinion, meaning some important feature is missing from the model. In this case, some type of tank in the coolant loop for the ground loop, just like the tank in the model for the house loop. Adding a tank to the loop stablize the pressure in the coolant loop (see below). There is usually such a tank in the actual system. Its exposure to air helps stablize the overall pressure of the liquid, just as in the model.

In general, one needs to be very careful when using an ideal source block in a fluid loop (e.g. that ideal volumetric flow source named Groud loop pump). What I've found is that such blocks can be an infinite source or sink of energy in a fluid loop, and sometimes leads to pressure or temperature dropping or rising without bound. Adding stablizing components like tank or accumulators usually gets rid of such problems, as now you have a reference point of the pressure.

[X my]

Thanks your answer~

After I keep trying and searching，I find that the following ways can help stabliziing pressure

• Add tank did help stabliziing pressure
• Increasing the ground loop flow rate by a factor of about 50 (link)
• adding a local restrictions component also has some effect

these method can solve the problem, but I'm still confused about the simulation result.

According to my common sense, The pressure of a closed-loop TL in the real world will not decrease continuously, regardless of "the flow rate of the water pump" or "whether there is a water tank or pressure limiting component "

Moreover, this phenomenon greatly increases the difficulty of simulation debugging.

just like it adds an additional “pressure-stabliziing” task to me which dose not exist in real application

(I need to design the model parameters that meet the heat transfer efficiency in real application, and also must meet the pressure stability in matlab simulation.)

If I'm wrong, please point out～

[Yifeng Tang]

Usually pumps, the real device the ideal flowrate source trying to approximate, pulls liquid from a tank, or in many applications called a reservoir. So I would argue the tank very likely exists in real applications. The tank/reservoir may be open to atomsphere, or pressurized, or seal together with some volume of air (like a coolant tank in a car).

The pipe and heat exchanger blocks in Simscape has fixed volume and only one working fluid, or in other words, the volume has to be totally filled with the defined thermal liquid. And the whole system of liquid has to observe the conservation of mass. As an source block tries to move liquid through this closed system, any pressure change introduced by the source block needed to overcome the flow resistance will lead to change in liquid density, as defined in the liquid property block. The density change, together with the conservation of mass, means the system of equations now need to find a new equilibrium state so the liquid can expand or shrink to fill the same amount of volume. This can then lead to more pressure change. I believe this is what you observed in the original model.