[TRNSYS-users] Convergency problem - using type 534

[lucas.lira at scottish-southern.co.uk]

Dear All,

I am currently simulating a system where 2 different energy units are 
connected to the same storage tanks at different nodes. In a 20 nodes tank 
the first will be connected between nodes 3 and 7 and the second between 
nodes 9 and 20.  The first will be supplying water at around 55C and the 
second unit at around 40C, with a higher flowrate.  The load is connected 
between nodes 1(extracting from tank) and 18 (return).

The idea of the project is to use the tank stratification to keep the 
second energy unit running at as low temperature as possible. 
The problem that i am getting is that the system is not converging. 
Different units are listed with this problem but the storage tank is 
involved in all cases, and some times the convergence problem involves the 
tank alone. 

Trying to identify if the problem was related with control units operating 
in too large time steps i reduced its value down to 30s and later 
eliminated all control units. The convergence problem still happens 
specially at the storage tank.

The second guess is problems regarding the heat capacity in the tank, 
which I am not sure how to solve. The number of nodes were reduced from 
the initial 20 to 10, rising each node capacity, but without success. 

The inversion mixing flowrate ( type 534 tank) was originally set to -1 
but then changed to "amount of water in each tank/ per time step" ( In a 
8000 L tank with time step of 5 min and 10 nodes, the flow was set to 9600 
kg/h). Even although now inversion was being observed, type 534 was still 
presenting convergence problems.

Rising the tank size to 80 000L the problems disappears. 
Using the original configuration (20 nodes 8000L capacity and no 
inversion):
        - Depending of the connection points the convergence problem does 
not occur.
        - Depending of the first unit's rated power (when reduced), the 
convergence does not occur.
                - Depending of the losses through the tank (if high), the 
convergence problem seems to not occur.

I am not sure what I can do to get the required results. I am simulating a 
real case, therefore unable to change tank and energy unit sizes or flow 
rates. I do have a strong feeling that the problem is related with the 
heat capacity through the nodes in the tank but can't find or think of a 
solution (even though small time steps should also make difference, 
shouldn't it?). 

I would appreciate any idea of how to proceed or suggestion of what else 
can be wrong and I may be missing.

Kind regards

Lucas Lira

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