The Advantages of Demand Controlled Ventilation (DCV)
Demand Controlled Ventilation (DCV) refers to ventilation strategy that employs an indicator as a control for modulating ventilation rate. The indicator in this case is carbon dioxide (CO2. ) There exist others such as temperature and humidity, which together with CO2 make up the primary indicators in HVAC systems. Other indicators include CO and inert gases like argon. The basic idea is pegging the ventilation rate of a HVAC system to the rate of emission of these pollutants, which reflect occupancy of a space and thereby optimizing on energy usage.
In simpler terms, DCV operates on the fact that providing the amount of ventilation air should be based on the changing occupancy of a building rather than on preset or design occupancy. This will save energy and at the same time help maintain Indoor Air Quality (IAQ) for health and improved productivity of occupants.
The indicator is usually detected and analyzed by a controller; a synthesized signal is then sent to the HVAC plant. The plant will either have a damper or a variable speed fan which will rump up/down the ventilation rate. In this paper the author will discuss CO2 based DCV. This DCV strategy has a longer and more colorful history than any other DCV strategy.
Early interventions were set back by the expensive sensors which rendered them cost-ineffective hence unnecessary. However later improvements have brought the costs down to around 250USD.Considering the sensor cost and retrofitting the system incase it is pneumatic controlled damper system in the expensive case scenario, the payback time has been considerably reduced making the CO2 DCV strategy not only feasible but also ASHRAE 62-1989 compliant.
Some studies, such as Davidge (1991) described DCV principles as well as having a demonstration project in a 30,000m3 office space.During summer; the airflow from damper was enough to control CO2. He also incorporated a system, which used light switch, motion sensor and CO2 controller. In the CO2 controller, the fan came on at 800ppm and shut of at 600ppm.The occupants rated the CO2-system very favorably.
Other studies such as Ogasawara et. al (1979) evaluated the potential energy savings for a DCV system in a 30,000m2 department store in Tokyo, Japan. Three ventilation strategies were compared; fixed outdoor air at design rate, manual control with maximum ventilation on Sunday (busiest day) and half of that on weekdays and DCV. The DCV algorithm was proportional control with closed damper at 800ppm and fully open at 1000ppm.Energy use was calculated for four cooling months and four heating months. The DCV system reduced energy usage by 40% for cooling months and by 30% for the heating season.
BENEFITS OF CO2-DCV
The potential of CO2 -based DCV for operational energy savings has been estimated in some cases to be from USD 0.05 to more than USD 1 per square foot annually. The highest payback will therefore accrue in high density spaces with highly fluctuating density e.g. schools, buildings, meeting rooms, retail establishments, auditoriums, movie theaters and in areas with high utility rates.
According to Lawrence Berkeley National Laboratories, five case studies were carried out in large office buildings with CO2 -based DCV, all of which reported energy savings that resulted in payback time of between 0.4 to 2.2 years.
More advantages of CO2-based DCV include:
1. Improved IAQ-Levels of CO2 are decreased to comfortable and acceptable levels.
2. Improved humidity control means that there is no unnecessary influx of humid outdoor air, which discourages growth of mold and mildew.
3. Records air quality data. The readings are logged to provide reliable record of proper ventilation in a building.
4. Reduced operation-running time for HVAC equipment.
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