Controlling events in a hybrid Modelica model

Question

I am confused by the hybrid modelling paradigm in Modelica. On one hand, events are useful, on the other hand, they are to be avoided. Let me explain my case:

I have a large model consisting of multiple buildings in a neighborhood that is simulated over 1 year. Initially, the model ran very slow. Adding noEvent() around as many if-conditions as possible drastically improved the speed.

As the development continued, the control of the model got more complicated, and I have again many events, sometimes at very short intervals. To give an idea:

Number of (model) time events : 28170
Number of (U) time events     : 0
Number of state    events     : 22572
Number of step     events     : 0

These events blow up the output (for correct post-processing I need the variables at events) and slows the simulation. And moreover, I have the feeling that some of the noEvent(if...) lead to unexpected behavior.

I wonder if it would be a solution to force my events at certain time steps and prohibit them in between these time steps? Ideally, I would like to trigger these 'forced events' based on certain conditions. For example: during the day they should be every 15 minutes, at high solar radiation at every minute, during nights I don't want events at all.

Is this a good idea to do? I guess this will be faster as many of the state events will become time events? How can this be done with Modelica 3.2 (in Dymola)?

Thanks on beforehand for all answers. Roel

Answer 1

A few comments.

First, if you have a simulation with lots of events (relative to the total duration of the simulation), the first thing I would encourage you to do is use a lower order integrator. The point here is that higher-order integrators normally allow you to take longer time steps. But if those steps are constantly truncated by events, they just end up being really expensive.

Second, you could try fixed-step integrators. Depending on the tool, they may implement this kind of "pool events and fire them all at once" kind of approach in the context of fixed-time step integrators. But the specification doesn't really say anything on how tools should deal with events that occur between fixed time steps.

Third, another way to approach this would be to "pool" your events yourself. The simplest way I could imagine doing this would be to take all the statements that currently generate events and wrap them in a "when sample(...,...) then" statement. This way, you could make sure that the events were only triggered at specific intervals. This would be more portable then the fixed time step approach. I think this is what you were actually proposing in your question but it is important to point out that it should not be based on time steps (the model has no concept of a time step) but rather on a model specified sampling interval (which will, in practice, be completely independent of time steps).

As you point out, using "sample(...,...)" will turn these into time events and, yes, this should be faster.