Improved energy efficiency in governmental institutions using decentralised fans in ducts in ventilation systems

The present research project has studied a novel VAV ventilation system in which terminal dampers are replaced with decentralized fans. The purpose of the study was to investigate two issues:

1. The possibility of making balance in such a ventilation system, i.e. to control the speed of the main fan and the decentralized fans in relation to each other such that the ventilation demands are satisfied in all zones.
2. The potential for energy saving in comparison with a VAV ventilation system with terminal dampers.

The conventional VAV ventilation system with terminal dampers was studied first as a reference for the comparison purpose. The outcomes are as follows:

– A practical method was proposed to apply the static pressure reset (SPR) strategy based on measuring the static pressure at the terminal dampers to energy efficient control of the conventional ventilation system. Implementing the proposed strategy is straightforward in practice. The pressure instruments can be added to an existing ventilation system without requiring any substantial change.

– The experimental results from the five separate experiments have shown a reduction of 48%, 36%, 27%, 21% and 9% in fan power use with pressure reset compared to constant static pressure in different total airflow rate. Thus, the amount of power reduction can vary in different operating points and also depends on the size of ventilation system.

– As shown in the experimental results, there is a risk for having starved zones with the proposed SPR strategy for the VAV ventilation system with dampers. A practical solution can be to consider a permanent safety factor in applying the pressure reset. Rather than subtracting pmin from the current pressure setpoint, a percentage of that can be subtracted. This reduces the risk for having starved zones, but also the amount of energy saving.

– The SPR control proposed for the VAV ventilation system with terminal dampers has been adapted to the VAV ventilation system with decentralized fans.

– The experimental results have indicated the ability of the proposed SPR control to make balance in the ventilation system with decentralized fans. It was even possible to have zero airflow rates in one zone without using a damper as shown in the laboratory experiments.

– The experimental results from four pair of experiments providing four different total airflow rates have shown almost the same power use for both ventilation systems, except in the lowest tested airflow rate (75l/s). This happened while the same main fan was used in both systems, meaning a less efficient main fan in the novel ventilation system at the tested operating points. However, calculations have shown the potential for energy savings.

– Calculations have shown an average reduction of 30% in power use with the ventilation system with decentralized fans when having the same efficiency for the main fan in both ventilation systems. Thus, a right choice for the main fan will lead to energy saving in the novel ventilation.

– In addition to the main fan efficiency, the efficiency of decentralized fans also affects the amount of energy saving in the fan system. Energy saving is not achieved in rather low airflow rates even with using the main fan with the same efficiency as the damper system (for example, for the total tested airflow rate of 75l/s in the present research). A minimum airflow rate is required to obtain energy saving in the fan system. This aspect needs to be further investigated in the future work.

– The SPR control of the VAV ventilation system with terminal dampers was implemented on a ventilation system serving three classrooms at a university campus (SDU) in Odense, Denmark. The measurements taken during two separate weeks, one week when the system was running based on the constant static pressure strategy and one week when the system was running based on the SPR control strategy, were studied. The measurement results indicated that both control strategies can provide an acceptable indoor air quality according to BR15 in terms of CO2-content in the indoor air while the SPR control strategy uses less energy for providing the required airflow rate on average. Although applying the SPR control strategy has reduced the average energy use, the measurements carried out was not sufficient to quantitatively compare the two control strategies from energy efficiency point of view. The measurements covered rather short periods (one week each) and the measurements were taken in two various times of a year. Experiment on the full-scale ventilation plant at SDU is still on-going. The future papers will cover long-term measurements in which the effect of seasonal change can be investigated.