Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 171

with minimum energy values or best performance identified from over the
full operating range of a system, and in addition can be used as a optimal
controller for optimal operation of the HVAC system.
8.6 Optimal control of air-side systems
To save energy consumption and improve an indoor environment, various
optimal control set-points, such as DCV-based fresh air set-point (which was
addressed in detail in Section 8.4), supply air temperature set-point reset,
VAV supply air static pressure set-point reset and so on can be used for air-
handling systems. In a typical VAV system, the local loop controls and the
supervisory (or optimal) control are listed as follows (Wang 1999, Wang
and Jin 2000).
Local loop controls include:
1 AHU supply air temperature control;
2 static pressure control;
3 zone temperature control (zone air flow control);
4 outdoor air flow rate control;
5 return fan control.
Supervisory (optimal) controls include:
1 outdoor air flow rate set-point reset;
2 static pressure set-point reset;
3 AHU supply air temperature set-point reset.
The local loop controls are essential for the system to operate properly to
provide a comfortable and healthy indoor environment, while the supervisory
controls allow the system to operate with a better or optimized performance.
The outdoor air flow rate set-point reset affects the energy consumption
(mainly coil load) and the indoor air quality. The static pressure reset affects
the energy consumption of the fan only. However, this resetting may affect
the indoor environment if it does go to an incorrect range. If it is set too low,
the critical zone may not be provided with sufficient air flow. If it is set too
high, noise problems may occur. The AHU supply air temperature set-point
reset will affect the thermal comfort and energy use of the system (mainly
fan consumption).
8.6.1 Online optimal reset of static pressure set-point
The control strategy aims at minimizing the VAV fan energy consumption by
minimizing static pressure. In order to supply sufficient air to every individual
zone as well as to minimize static pressure, the static pressure is controlled to
be just sufficient for the most heavily loaded zones. The online strategy makes
use of all the VAV damper positions represented by relevant VAV damper