[TRNSYS-users] Controlling heating/cooling equipment

[knut.erik.enerstvedt@niva.no]

Dear all,

I wish to simulate a particular cooling system, but find it difficult to
control it the way I want.

I use Type 56 in "temperature level control", as the main objective is to
simulate operation of the cooling equipment. The equipment delivers cooling
to the building through chilled water and a fan coil, and the amount of
cooling delivered is controlled by adjusting the water flow rate (variable
speed pump, control signal between 0 (min) and 1 (max)). Therefore I need a
controller with a continuous control signal, and not just an on/off signal.
I wish to have, as far as possible, a continuous and smooth operation of
the system during the period of day when cooling is needed.

Type 23 PID controller is not ideal to use for this task, as its control
signal is based on the difference between the zone temperature and a set
point temperature and goes to zero immediately when the zone is cooled to
below the set point. Then at the next time step the zone temperature is
above the set point and cooling is turned on again and so on. This results
in a lot of oscillating on/off operation of the system, and I wish to avoid
this.

I have tried Type 22 iterative controller, hoping to achieve just the right
amount of cooling at each time step, but have encountered some
difficulties. The pump flow rate can not be too low, so I need to use a
"threshold for non-zero value" of about 0.3. This causes the pump to never
start at all, even when its control signal obviously should be above 0.3. I
also find it difficult to get TRNSYS to converge properly when using Type
22.

I will be grateful if anyone has some advice on this.


Kind regards,
Knut Erik Enerstvedt





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