**4. Control strategy**

144 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

**Figure 7.** Simulation model (block diagram) of the air handling unit model.

The ventilation system served four classrooms in a school. The ventilation system was designed for a maximum CO2 level of 800 ppm, which approximately gives the same flow rate as mandated by the Norwegian building regulations. Minimum flow rates were determined from 2 liters/s per m2 floor area. Design values rates for each room thus as

Ventilation was turned on at 07.00, and shut down at 18.00. The main ducts and AHU were designed to handle 4600 m3/h of air. Infiltration ratios were 0.1 air changes per hour. The maximum flow rate from the fan characteristic was 2.2 m3/s and the pressure peak was 2100 Pa at 0.6 m3/s. The HRU was sequentially controlled with water-to-air heating and cooling coils, using PID controllers. At design conditions, the heating coil was able to increase air temperature by 15°C, with supply and return water temperatures of 80 and 60° C. The cooling coil gave a maximum air temperature drop of -15°C at 7/13°C water temperatures.

Temperature efficiency of the HRU at design conditions was 60%. The HRU had a purge sector and the leakage factor was specified to 0.05 (Sørensen, Riise, 2010). It should be noted that the leakage was considered to be constant over the HRU, even though this may vary for

**3.4. Case study** 

• Maximum occupant load: 28 • Ventilation flow rate (m3/h): 1150 • Minimum flow rate (m3/h): 430

• Room volume (m3): 180

a VAV system (Sørensen, 2008).

follows:

Flow regulation was achieved through static pressure difference control of the fan and CO2 control of the room airflow via local dampers. To account for steady state offset and to avoid too aggressive control, a PI controller was used on the fan. Pure P control was used locally. Maximum set point for the fan pressure difference control was 450 Pa. At this point the fan provided the design flow rate (at which the system was balanced and all local dampers were fully opened). To reduce throttling, and to ensure sufficient minimum flow rates to all zones, minimum damper positions were set to 30% of maximum. The correction gain of the pressure set point (Kpath) was calculated from a minimum flow rate of 0.8 m3/s at design pressure difference, and from a minimum pressure difference of 200 Pa. These values were chosen from the fan characteristics. Hence, the steady state paths of control were as shown in Fig. 8.

Energy Efficient Control of Fans in Ventilation Systems 147

• Although minimum flow rates were sufficient for a Kpath of 1.0, the system was unable to provide enough air during occupancy. This shows in the CO2 responses. It can be explained by first looking at a condition where all dampers are closed and secondly a condition where dampers are not closed. In the first case the fan delivers the minimum flow rate at 200 Pa pressure difference. The dampers are in their minimum positions (30% open). Then some dampers start to open, and as a result, the fan pressure difference drops. To compensate for the pressure drop, the fan increases speed to maintain the 200 Pa set point. The new flow rate is not sufficiently large to change the set point to a larger value (2880 m3/h). Thus, no changes in set point, even while the

**Figure 9.** Simulation results of the system with pressure difference reset. The two upper diagrams show the SP and flow rate during a day. The remaining diagrams show CO2 concentrations of the rooms.

amount of air varied.

Room CO2 set points were 700 ppm and supply air temperature was controlled between 15 and 18°C, dependent on outdoor temperature.

**Figure 8.** Paths of the pressure difference set point of the fan as a function of the correction gain Kpath.
