Ventana software support forum

Hi everyone, I have the following problem:

I have a electricity flow which is decribed as: capacity * "CHP on" ~J/S
capacity=5000 ~J/S
CHP on=IFTHENELSE(value X>0,1,0)

so the flow will only be switched on when value X>0 which is good. But at the moment the flow starts it needs to continue for 30minutes (which is de minimum runtime of the machine) regardless what value X will be after that.

Would you have any suggestions how to solve this problem?

thanks in advance,

best regards,

Wouter Maas

Will the PULSE function do what you need?

The problem I encountered with the pulse function is that you have to specify a starting time. I only know that the installation is switched on when x>0... Not at what time this will happen

The key is that the minimum run time constraint means that the machine's state is not memoryless. That means the machine's run status needs to be a level (state variable).

One approach is in the attached model.

You might also take a look at the discrete event models in the user guide, in C:\Program Files\Vensim\models\mguide\9DISCRET

Tom

Dear Tom,

thanks a lot for the help! The system like this works indeed. I am only wondering about one the device starting and stopping. YOu devide there with the time step which makes the unit 1/minute. That means that I have to change the unit into 1/second if I change my timestep to second?
And I assume that the last start time needs to be: HP starting >0,Time,Time?
thanks again,

gr Wouter

[Edited on 7-14-2010 by walin]

[Edited on 7-14-2010 by walin]

Correct on both points.

- units change to seconds (or whatever your time dimension is).
- my syntax of SAMPLE IF TRUE( starting, time, time ) works, because Vensim treats starting like a boolean variable, 0=false, >0 = true (I think) but your formulation, SAMPLE IF TRUE( starting>0, time, time) is probably clearer and more robust.

Tom