[TRNSYS-users] Convergency problem - using type 534

[Michaël Kummert]

Lucas,

You have already tried two of the “usual suggestions” to get a simulation to
converge: reducing the time step and making sure tank nodes are not too
small compared to the volume of fluid that enters them during one time step.
I am afraid can only make generic recommendations, but here they are:

- Try re-arranging the components in a different order (“Component order”
tab of the control cards). I usually tend to place the tanks at the very end
of the calling order, the logic being that they have more thermal mass so
it’s no big deal if the other components use the value of previous time step
as a starting point. I also try to put the controllers after the components
they depend on for the control decision, and for the rest I try to use a
logical order. Ooviously you have to make a choice at some point as most
system have closed information loops so the order cannot be completely
logical. This sounds a bit like wishful thinking but it has worked in many
cases for me.

- Add pipes in the model if you haven’t done so until now. Usually anything
with thermal mass or time delay helps with convergence, although in your
case the storage tank seems to be at the heart of the problem.

- Make sure the flowrate controls are synchronized with the actual system
operation. I have seen some simulations where a device was set off (e.g. a
heat pump) without turning the pump off at the same time. If that loop is
connected to, say an immersed heat exchanger in a tank, you could end up
with a closed fluid loop with very small temperature differences that cause
non-convergences.

What usually works for me is a combination of all of the above (including
what you did already) – and lots of coffee
 

Best wishes,
Michaël

____________________________
Michaël Kummert
École Polytechnique Montréal



From: lucas.lira at scottish-southern.co.uk
[mailto:lucas.lira at scottish-southern.co.uk] 
Sent: 13 November 2009 09:08
To: trnsys-users at cae.wisc.edu
Subject: [TRNSYS-users] Convergency problem - using type 534


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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