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

7 The HVAC-system controller 70
7.2 Heat pump control
For a heat pump the scheme described before can be used as well. The cooling and the
heating power have to be translated into hardware settings. The heat source or sink
can be taken directly from the optimiser output if a design variable for this has been
introduced (cf. Section 4.5).
The de-icing should be implemented by using a predefined program and by sus-
pending the inlet temperature control. Therby, no interference with other control signals
can happen during the heating and melting of the ice on the front heat exchanger. This
avoids too short de-icing which would lead to fast ice re-accumulation.
7.3 Fan control
Normally no sensor is used for the air flow, so open-loop control is the only option. It is
based on the fan characteristics as well as the system impedance of the HVAC-system.
This gives one operating point (= air mass flow) for a given fan voltage, as shown in
Figure 3.5 in Section 3.2.1. If these points are known, a fan voltage for a desired air flow
can be given. In this way the air flow is controlled with the help of a look-up table.
However, there are a lot of disturbances making this approach inaccurate. As
described in the system description in Section 3.2.1 countermeasures have to be realised
depending on the geometry and HVAC-system. Even if some relations can be given, for
example for the pressure rise at the air inlet, measurements (or simulations) are required
to quantify the resulting influence on the air flow.
7.4 Flap control
The air mass flow percentage to the outlets (or passing the flap) has to be translated
into a flap opening angle. This is done with look-up tables relating these two parameters.
To obtain these, experiments for each flap have to be performed. For a mass-production
vehicle this could also be implemented in hardware by a gear or a cam disc making the
control “see” a (nearly) linear relation.