Re: Controller TYPE2b

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Alberto,

> I have tryed your advices in the system but with only one controller to
> limit the error.
> Working with one controller it get "stuck", although it get rid of all
> oscillations using a small timestep (0.02h).
> How much sorter has to be a timestep than the time constant of my system to
> get rid of all the oscillations?
> How could I know and change the time constant of my system?

You cannot change the time constant of your real system without changing 
your system... However, if you neglected some thermal capacitance in the 
model (e.g. if you did not model pipes), adding it back can increase the 
"simulated time constant".
As I said before, oscillations in an ON/OFF controller does not alwys 
indicate that your results are not good, so you do not HAVE to reduce 
the time step until they go away.

> I didn´t understand "the tank has enough inercia to de-couple the
> controllers" (sorry, I am an inexpert)

I just meant that if you would have a very small tank modelled with one 
node only, setting the collector pump on or off would have an 
instantaneous effect on the tank temperature, hence on the on/off signal 
of the secondary loop. And vice-versa. But I assume that in your case 
turning one of the pumps on or off has a relatively small effect on the 
tank temperatures, so that it does not have an immediate effect on the 
second controller (the time step will have an influence again)

> On the other hand, I was checking the outputs and TRACE and a lot of times
> the controller don´t work well, and most of them are in the ON criteria. In
> adiction, sometimes the collector outlet temperature changes to a too much
> high value, for example from 30 to 70ºC instead the control output is OFF
> and the diference with tank temperature is bigger then the Upper input
> temperature (Monitoring =100ºC).
> It could be because the pump is OFF and the fluid temperature in the
> collector increases but I think it is too much.

I am not sure I understand you correctly. Usually the monitoring 
temperature is the tank temp., not the collector temp. This may explain 
some of your oscillations. also, it is normal that the collector 
temperature increases a lot if the pump is switched off when the sun is 
shining. Most solar collectors have stagnation temperatures well above 
100°C.
Kind regards,

Michaël Kummert

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_________________________________________________________

Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
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E-mail: kummert@xxxxxxxxxxxxx

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