Energy and ventilation performance analysis for CO₂-based demand-controlled ventilation in multiple-zone VAV systems with fan-powered terminal units (ASHRAE RP-1819)

Abstract

CO₂-based demand-controlled ventilation (DCV) has been proved to be energy-efficient by altering ventilation rates according to surrogated indications of CO₂ levels in single- and multiple-zone single-path variable air volume (VAV) systems. However, DCV for multiple-zone VAV systems with multiple recirculation paths is still untapped. Multiple-zone VAV systems with series and parallel fan-powered terminal units (FPTUs) are studied in this study. Energy and ventilation performance of the DCV strategies with dynamic resets are evaluated. The DCV control sequences for FPTUs are first summarized. A co-simulation approach combining EnergyPlus with CONTAM is adopted. The DCV controls are implemented in the EnergyPlus energy management system (EMS) module. New EnergyPlus EMS actuators are added and a customized EnergyPlus module is built to achieve the DCV control for each system. An office building is used to demonstrate the benefits from the proposed DCV strategies in four different U.S. climates zones. Two baselines for ventilation requirements (ASHRAE 62.1, California Title 24) are considered. The proposed DCV strategies resulted in 7–14% and 7–21% HVAC energy savings for each system, on a source energy basis, compared with the baseline of a simplified ASHRAE 62.1 approach. Both systems could achieve good compliance with the ventilation requirements in ASHRAE Standard 62.1.

Nomenclature

Cpr	=	concentration of CO2 in the primary air, ppm
Cret	=	concentration of CO2 in the return air at the air handler, ppm
Ep	=	primary air fraction, the fraction of primary air in the discharge air to the ventilation zone
Ez	=	zone air distribution effectiveness, a measure of the effectiveness of supply air distribution to the breathing zone
Fa	=	supply air fraction, the fraction of supply air in the ventilation zone, including sources of air from outside the zone: Fa = Ep + (1 – Ep) $\times$ Er
Fb	=	mixed-air fraction, the fraction of supply air in the ventilation zone from the fully mixed primary air: Fb = Ep
Fc	=	outdoor air fraction, the fraction of outdoor air in the ventilation zone, including sources of air from outside the zone: Fc = 1 – (1 – Ez)$\mathrm{}\times$ (1 – Er)$\mathrm{}\times$ (1 – Ep)
Vbz	=	breathing zone outdoor airflow, cfm
Vbz-P	=	population component of the breathing zone outdoor airflow
Vbz-A	=	area component of the breathing zone outdoor airflow
Vdz	=	zone discharge airflow, cfm
Vds	=	system discharge airflow, which is the sum of all the zone discharge airflow, cfm
Vpz	=	zone primary air
Vou	=	uncorrected outdoor air rate, cfm
Vot	=	outdoor air intake, cfm
Xs	=	system level average outdoor air fraction
Zdz	=	zone discharge air fraction, the fraction of the zone outdoor airflow in the zone cooling airflow
Zpz	=	zone primary air fraction, the fraction of the zone outdoor airflow in the zone primary airflow
Zd	=	system-level discharge air fraction
Zp	=	system-level primary air fraction

Acknowledgments

This work was funded by American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) through research project 1819: CO₂ Demand Controlled Ventilation in Multiple Zone VAV Systems with Multiple Recirculation Paths.

Additional information

Funding

This work was funded by American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) through research project 1819: CO₂ Demand Controlled Ventilation in Multiple Zone VAV Systems with Multiple Recirculation Paths.