Simulation of static pressure reset control in comfort ventilation

Variable air volume (VAV) ventilation systems reduce fan power consumption compared to constant air volume (CAV) systems because they supply air according to the airflow demand. However VAV ventilation systems do not take fully into account the potential energy savings as the control strategy operates the terminal boxes and the air handling unit (AHU) independently without pressure integration.

The pressure in the main duct is maintained at a constant static pressure (CSP) which corresponds to the pressure required under the design full load condition. Under part load conditions, the fan provides excessive static pressure which is dissipated via throttling at the terminal boxes. As a result significant fan power is wasted in mechanical energy losses.

The development of sophisticated direct digital controls (DDC) creates possibilities to integrate feedback from the dampers into the building management system. In this way the operation of central plant equipment is adjusted in real time according to the actual pressure demand; this control scheme can be implemented by the static pressure reset (SPR) method.

The SPR control method ensures that at least one damper remains fully opened; thus the fan generates only enough pressure to satisfy the airflow demand in the most critical zone. Consequently the airflow resistance of the ductwork is maintained at a minimum and the fan operation is optimized. There are various approaches to implement the control scheme of the SPR method; the state of the art is represented by the method of trim and respond based on pressure alarms.

This study investigates the operation of the SPR control method of trim and respond based on pressure alarms in a CO2 demand application where large air volumes are provided to three classrooms. The investigation was based on simulations performed with a fully dynamic model of a VAV ventilation system that was developed in the Simulink programming tool which is add-on software to MATLAB mathematical programming language.

The Simulink model was developed in previous research work and was built based on the International Building Physics Toolbox (IBPT), which is a library of blocks constructed for the thermal analysis in building physics. For the purpose of the current investigation the IBPT toolbox was remodelled to integrate the calculation of the airflow demand based on the CO2 concentration occurring in the zone.

The performance of the Simulink model was in previous work evaluated based on the experimental setup of a ventilation system. The investigation of the SPR control algorithm of trim and respond based on pressure alarms disclosed some issues that need to be addressed and optimized before the algorithm can effectively establish the pressure conditions that satisfy the pressure demand under high airflows.

In short the algorithm must be tuned to the application beforehand or, preferably, actively learn to perform from continuous feedback before it presents a real plug-and-play solution.