Re: [TRNSYS-users] (no subject)

[Jeff Thornton <thornton@tess-inc.com>]

Michael,

Trying to implement detailed control strategies in TRNSYS using just equations rarely works.  And it doesn't surprise me that your controls look good on the plotter before you hook up the pump - because as soon as you hook up the pump you get into a dynamic situation where the pump affects your control variables and your controller immediately affects your pump.  Things won't converge and you'll get errors - or you'll generate a bad control signal and cause a NAN condition.

That being said, there are several ways to control variable speed pumps in an SDHW application:

1) Use the PID controller
2) Use the iterative feedback controller
3) Use the useful collected energy from the last time step to calculate the flow rate you'll need at this time step to meet your goal.
4) Use one of the TESS collectors that have the variable speed option coded into the model.

Regardless of which method you choose, make sure you have pipes with capacitance and use a smaller time step - typically in the 1-5 minute range.  We do a ton of SDHW modeling and never implement detailed controls using just equations.

Jeff Thornton



On Nov 20, 2014, at 3:52 AM, Michael Stochniol <stochnio@mail.hs-ulm.de> wrote:

> Hallo everyone,
> 
> im trying to create a control function using equations with logic operators for the control of a variable speed pump, in order to adjust the massflow trough the pump. I wrote a control funktion and plotted it without connecting it to the pump and it seemed to work right. But if i am conecting it to the pump my simulation freezes after around 700 timesteps or less. I am trying to create solar domestic hot water system. The question is, is it possible to create this kind of control circuit with equations ? I have similar problems with every control loop i am trying to create this way. The math should work but implemented  in TRNSYS (TRNSYS 17) the simulation stick somewhere. 
> 
> I am no experienced user so maybe someone could give my some advise how to implement this kind of control functions right? 
> 
> i am very grateful for any help or ideas.
> 
> thanks in advance
> 
> Michael Stochniol       
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