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Re: [TRNSYS-users] (no subject)

Michael,
 The issue is most likely that there isn't enough thermal capacitance for the timestep you have chosen once you add feedback into your controlled loop. You should make sure that your timestep is at the very most about 5 minutes. Second, you should make sure that there is a model in every loop that accounts for the thermal mass of the fluid in the loop. This could be a tank or a pipe. The pipe or tank should be able to contain at least as much liquid as the pump is able to transfer during one timestep at its maximum flow rate.

 Lastly, trying to control the outlet temperature of the solar collector to maintain a setpoint is particularly tricky as the sun is coming up and going down. When flow is off and the sun starts to come up the collector temperature goes very high (stagnant liquid) so the pump turns on at which point the collector outlet temperature goes right to the inlet temperature because there isn't enough solar energy to support much temperature rise when there is flow. Then the pump shuts off because it can't attain the outlet temperature you want at which point the temperature goes up again because the liquid is stagnant. If you have access to the TESS Solar library there is a solar collector model in there (Type539) that builds the outlet temperature seeking controls directly into the collector model so that it can backsolve for the flow rate required to attain the outlet temperature without having to let the TRNSYS kernel solver try to find it by iteration.
Kind regards,
 David


On 11/20/2014 03:52, Michael Stochniol 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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