[TRNSYS-users] Convergency problem - using type 534

[lucas.lira@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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