DCV is becoming a popular measure for ventilation systems, due to its benefits. DCV can improve both energy efficiency and indoor air quality, and it can help buildings meet the requirements of certification programs like LEED and WELL. Since the outdoor airflow is optimized, DCV can also reduce the cost of heating or cooling that air.
The basic principle of DCV is increasing or decreasing the outdoor airflow according to occupancy sensors. There are many methods to count the number of occupants in a building, but a dedicted CO2 sensor normally offers the highest precision. Occupancy can be correlated with the concentration readings of a CO2 sensor, since it is one of the products of human metabolism.
Direct Energy savings of DCV
DCV reduces the operating cost of airing systems directly. An air handler that provides its rated cfm all the time will consume more energy, compared with a unit that optimizes its flow. There are many ways to modulate the cfm provided by a fan.
The conventional methods to control airflow are using a damper at the fan outlet, and operating the unit intermittently to reduce the average airflow. Both reduce the total energy consumption with respect to a fan providing its full airflow, but they have limitations:
- A damper can reduce airflow by closing its opening. However, this produces a drop of static pressure that wastes fan power.
- Intermittent operation stops the air supply periodically. ASHRAE standards and building codes only allow this in certain applications.
The most efficient way to control the air supplied by a fan is with a variable frequency drive, or VFD. By adjusting the voltage and frequency provided to a motor, a VFD reduces its rpm and and so the fan airflow. There is no need to restrict airflow with a damper, and there is no need to stop the circulation of air at intervals. Finally, a fan controlled by a VFD consumes less energy than fans with dampers or intermittent operation.
Indirect Energy savings of DCV
The savings in terms of energy that a DCV system can achieve extend beyond the fans and their motors. Consider that the outdoor air must be cooled during summer and heated during winter, in order to keep a suitable indoor temperature. When the outdoor air supply is adjusted by a DCV system, the associated heating and cooling loads are also reduced.
These indirect savings achieved by DCV change throughout the year. DCV achieves the highest air conditioning savings during summer, when the outdoor air is very hot. The same applies for heating savings on the coldest days of winter. At times of the year when the weather is more moderate, the heating and cooling savings from DCV are also smaller. However, the ventilation savings and IAQ benefits are constant throughout the year.
DCV can be deployed along with other measures to increase the overall HVAC efficiency. For example, an airside economizer can detect when the outdoor air is cool enough to provide “free air conditioning”.
- In this case, the cooling savings achieved with more outdoor air are higher than the ventilation savings from reduced airflow.
- The DCV system is overridden to increase the outside air supply and take advantage of free cooling.
Keep in mind that DCV can be applied anywhere, regardless of the weather. On the other hand, an economizer is restricted to moderate climate zones where the outdoor air can provide free cooling at certain times of the year.
Energy recovery ventilation also works along with DCV. The concept is very simple: heat and humidity are exchanged between the OA supply and the exhaust air, to reduce the workload on heating and cooling equipment. The exhaust air can precool the outdoor air during summer, and it can preheat it during winter. This further increases HVAC efficiency.