Add constraint to force equal pumps to operate with the same operating point

Could very well increase computation speed when multiple (equivalent) pumps are used. It reduces the generality a little bit, so probably make it a flag (but default=True, if it really increases speed a lot).

If constraint is not possible, we should just combine equivalent pumps (assuming with the right pump switching matrix) into one. That may be trickier though, especially if e.g. pump 4 depends on the status of pump 3 (and not on the status of 1 & 2), with pump 1-3 equal and pump 4 different. Then we have to transform those constraints as well.

Also, we might end up with a Q * nr_pumps_on, which may or may not be convex (I'm inclined to say the latter).

For a situation with two equal pumps, I think the following constraints should be sufficient (subscripts indicate pump number):

Q_2 \ge Q_1 - M \cdot \left(1-S_2\right)

Q_2 \le Q_1

H_2 = H_1

with

S_2

the status of the second pump, and

M

the maximum possibly discharge of

Q_1

(which we know, and already have stored somewhere).

@Klaudia > Does this look alright to you?

assigned to @Klaudia

unassigned @Klaudia

From some initial testing on the two pumps example, it seems that using only the first two constraints is faster (in computational time). In number of nodes visited, using all three constraints is best, but IPOPT seems to have a harder time (more iterations) per call.

• Default (no additional constraints) is about ~130 sec, ~500 bonmin steps.
• All constraints is about ~180 sec, and ~230 bonmin steps.
• Constraints 1 & 2 is about ~100 sec, and ~350 bonmin steps.