[TRNSYS-users] Powell's method solver

[David Bradley]

Kyle,
   The Powell solver is only really necessary for systems that contain very 
little capacitance and because your system has a storage tank in it, I 
would be very surprised if you needed to resort to the Powell solver. If 
you do use the Powell solver, you need to also make sure that all of the 
controller components in your system are also set up to use the Powell 
solver as well. In order to get your system to work with the Successive 
Substitution, you will need to use the NSTK parameter in your controller, 
or you may need to implement NSTK within your thermosyphon component. The 
idea of NSTK is that in formulating mathematical models of systems, we 
simplify the physics of the system to some extent. The unfortunate 
byproduct of this simplification is that the simulated system doesn't have 
as much inertia (robustness, stability, etc.) as the real system does. In 
your case, the real thermosyphon system would take some time to actually 
reverse flow directions while in the simulated system, it can change flow 
directions within a time step from one iteration to the next. NSTK is a 
limit that components often put on the number of times that they can come 
up with a different answer before they "stick" to a decision regardless of 
how many more times they are called within that time step. At the following 
time step, their decision is allowed to change again. Most often NSTK is 
implemented in controllers to prevent them from ON/OFF cycling in an 
infinite loop. In your case, you might implement the same in the regime 
where your flow is low and can change directions within from one time step 
to the next. The Type2 ON/OFF Differential Controller implements NSTK if 
you want to see one method for how it works.
   I hope that helps.
David


At 05:45 PM 6/22/2005, Benne, Kyle wrote:
>Hello TRNSYS group,
>
>Thank you Werner for helping with my previous question.  I am developing a 
>thermosyphon water heater model such as the type45, however it is desired 
>to have a model that will accommodate reverse thermosyphoning such as what 
>often happens at night.  I am using TESS Type604 pipe that can handle 
>reverse flow and TESS Type534, a stratified storage tank.  The flow rate 
>is calculated by a thermosyphon promoter that computes the flow rate based 
>on the temperature of the elements in the system.  This type previously 
>did not handle reverse flow, therefore I modified it to output a flag if 
>the flow was reversed.  A small piece of code interprets this flag and 
>directs the flow to the correct connections of the system.  For example If 
>the flow is in the assumed positive direction, the inlet of the pipe 
>designated inlet A is input with the magnitude of the flow calculated by 
>the thermosyphon promoter.  The opposite end of the pipe is given a flow 
>rate of 0.  This strategy successfully de!
>  termines forward and reverse flow and directs the inputs accordingly, 
> but fails to converge when the flow rate is switching between forward and 
> reverse direction.  I have read in the TRNSYS documentation that the 
> successive substitution method often has difficulty with 
> discontinuities.  I then decided that Powell's solution technique might 
> be more capable for this problem, however I am having difficulty getting 
> it to work.  When the Powell solution technique is chosen the program 
> errors saying the input file needed by the Type534 storage tank cannot be 
> read.  This is odd considering it reads the file fine using the 
> substitution method.  I believe the Powell method is more particular 
> about what initial values I choose for the inputs, and may require other 
> things, but I am not certain about what those might be.  If there is 
> someone who managed to read this entire message and can offer some 
> pointers I would appreciate the advice.
>
>         -Kyle
>
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