Design of an Automatic Control Algorithm for Energy-Efficient ...

7 The HVAC-system controller
The HVAC-controller receives its set-point from the optimiser. In the inner loop the task
is to realise those inlet air conditions as fast and as good as possible with the available
hardware. Here conventional automatic control objectives are of importance: speed,
precision and accuracy.
As already mentioned in Section 4.1, this control has to be adapted to the hardware.
Therefore, only general control structures will be shown in this chapter. If not stated
otherwise a “standard” car HVAC-system is assumed as it is presented in Section 3.2.
7.1 Temperature and humidity control
In car HVAC-system heat and humidity control are usually coupled since the air is first
cooled (and dehumidified) and then reheated. The humidity of the air after the evap-
orator is not measured, while its temperature has to be captured to avoid icing. This
measurement will be used as indicator for humidity.
The here presented control design is based on feed-forward signals to allow fast
reaction to set-point changes. To give accuracy and to compensate for model errors a
closed-loop PI-controller is added.
A scheme for doing this in a standard HVAC-system is shown in Figure 7.1. Equa-
tions for the calculations are found in Section 3.2 describing the HVAC-hardware func-
tioning.
The required temperature of the saturated air coming from the evaporator is cal-
culated on the basis of the given ���. If it is higher than or equal to than the ambient
temperature, the evaporator can be switched off. This is due to the fact that no humidi-
fication is possible. A lower limit for the temperature should be around 4 degrees Celsius