[TRNSYS-users] Convergency problem - using type 534

Tue Nov 17 01:24:15 PST 2009

Dear All,

Thanks for the feedbacks, they were all very helpful. Answering back to 
thank and to clarify how I (hopefully) solved the problem, which may help 
future users in the same situation.

Moving the components solving order, especially placing the storage tank 
in the end, helped a lot to improve the convergence but did not solve the 
problem completely. But it did give me a good idea of where the main 
problem actually was (before I was being misled that it was focused in the 
storage tank).

The circuit leaving the load was the reason for the problem on 
convergence. In this circuit, part of the water returning from the load 
would be recirculated to keep the load feeding temperature around a given 
value. The combination between this configuration with the way the load 
demand is inserted (external file controlling the flowrate) was giving me 
some sort of "circular reference". Because of it, did not matter what I 
do, one component would always present at some point a convergence 
problem. The solution was to add a type 661 component before the load. 
This"opened" the loop, vanishing all the convergence problems.

Once again, thanks for all the feedbacks.

Kind regards,

Lucas Lira

Michaël Kummert <michael.kummert at gmail.com> 
13/11/2009 22:24

To
<trnsys-users at cae.wisc.edu>
cc
<lucas.lira at scottish-southern.co.uk>
Subject
RE: [TRNSYS-users] Convergency problem - using type 534

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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Scottish and Southern Energy plc, Inveralmond House, 200 Dunkeld Road, Perth, Perthshire, PH1 3AQ. Registered in Scotland Number. 117119
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