Time-averaged ventilation for optimized control of variable-air-volume systems

Abstract

Typical Variable Air Volume (VAV) terminals spend the majority of time at their minimum airflow setpoints. These are often higher than the minimum ventilation requirements defined by code, resulting in excess energy use and a risk of over-cooling the spaces. We developed and tested a Time-Averaged Ventilation (TAV) control strategy in an institutional building on the UC Berkeley campus to address this issue. Whenever a zone does not require cooling, TAV alternates the VAV damper between partially open and fully closed so that the average airflow matches a predefined ventilation setpoint. Compared to the existing, base case scenario using single-max VAV logic, this strategy reduced the mean zone airflow fraction from 0.44 to 0.27 during the intervention period. The corresponding reductions in average heating, cooling, and fan power were 41%, 23%, and 15% respectively. In addition to being programmed directly in a native control system, TAV may be applied via sMAP as a low-cost retrofit strategy in any building that has a BACnet network and direct digital control (DDC) to each VAV terminal. • We tested it in a Variable Air Volume system building with single-max control logic and high minimum flow rates.• Hourly average airflow accurately met the required ventilation rates in each zone.• Results show a reduction in fan, reheat, and chilled water energy. Keywords

Evaluation of a cost-responsive supply air temperature reset strategy in an office building

“…Figure 10 shows that an example where this change incurred a significant total cost penalty because chilled water cost increased far more than fan costs decreased. This is likely due to the fact that many zones in the case study building spend the majority of their time operating at the minimum air flow setpoint [7], which is a relatively common pattern in office buildings [6,25]. The cost-responsive reset strategy recovered from the communication error and gradually converged back to the original setpoint of 18.3 °C (65 °F) over the course of the next 12 iterations (1 hour), with total cost decreasing with each increase in SAT.…”

“…1 If using the time-averaged ventilation approach described in [7], we recommend using the ventilation setpoint instead of the current zone minimum airflow setpoint. 2 Note that in the case study presented later, we implemented this equation slightly differently.…”

“…The zone level temperature setpoints are defined to maintain thermal comfort, according to standards such as ASHRAE 55 or ISO 7730 [1,2], and the most suitable setpoint values to use is a topic of ongoing research [3][4][5]. The airflow setpoints, both at the zone and the AHU level, are defined to maintain adequate ventilation and indoor air quality (IAQ) in the zone according to standards such as ASHRAE 62.1 or California Title 24, and again, the optimal values are a topic of ongoing research [6][7][8]. Similarly for the control of the airside economizer [9][10][11].…”

Optimization of BuildingsEnergy Consumption by Designing Sliding Mode Control for Multizone VAV Air Conditioning Systems

“…In [17], VAV optimized control was presented, which utilized ventilation based on average time. The proposed strategy replaced the position of the VAV damper between fully closed and partially open when no cooling was required in a zone.…”

Quantifying energy losses in hot water reheat systems

“…Lastly, recently developed control strategies used to improve the energy efficiency of VAV systems, such as those described and demonstrated in [16][17][18][19][20] and recently formalized in ASHRAE Guideline 36 [21], successfully avoid most unnecessary reheat energy. These new strategies reduce the minimum airflow setpoint at the VAV terminal unit to a more appropriate level.…”