Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 161

and with minimum fresh air control) in a Hong Kong commercial building
from site monitoring. Compared to the case with minimum fresh air control,
the monthly coil energy consumption from November to March was reduced
by 3, 67, 77, 65, and 15 per cent respectively. The coil energy consumption
when using the economizer cycle was reduced by 41.7 per cent in the winter
period, which was estimated to be equivalent approximately to 12.1 per cent
of the annual energy consumption of the cooling coil.
The site monitoring also shows that better indoor air quality was achieved
when economizer control was used. Under partial free cooling modes, the
outdoor air flow rate was set to maximum and more fresh air was drawn in
to dilute the indoor pollutants. The mean and maximum CO 2 concentration
in this case were found to be about 60-150 ppm lower than that recorded at
the minimum fresh air mode. In total free cooling mode, CO 2 concentration
was about 50-90 ppm lower than that at the minimum fresh air mode.
8.4.2 Demand-controlled ventilation
Outdoor ventilation airflow is one of the key factors affecting air quality in
air-conditioned indoor spaces. DCV is one of the alternative strategies to
achieve acceptable indoor air quality with minimum energy consumption.
Current ASHRAE Standard 62-2004 requires that the minimum design out-
door ventilation air flow rate should be based on the occupancy of the space
and the area of the space.
Measuring directly the actual concentration of pollutants in indoor air is
ideally the best means of controlling indoor air quality. However, it is hard
to find one sensor that is able to accurately sense all the major pollutants at
the same time. Even for typical pollutants such as volatile organic compounds
(VOCs), it is hard to find an accurate and reliable sensor that is reasonably
simple and sufficiently low cost to allow it to be used in online control appli-
cations. Occupancy sensors are widely used in the control of lighting systems.
They are suitable in cases when only the on/off (occupied or not occupied)
status needs to be detected. Some other techniques, such as the computer
vision technique, can be used to detect the number of occupants in a space.
However, its application is obviously restricted by the geometrical layout of
the indoor space for wide commercial applications in building management
systems.
Use of CO 2 measurement for outdoor ventilation control is a well-developed
technology and widely used method in ventilation-control applications where
occupants are deemed to be the main source of pollutants. Although CO 2 in
indoor spaces should be controlled within an allowable range, CO 2 control
itself is generally not a great concern as it is normally well below the limit
in air-conditioning applications. In the control system, CO 2 concentration is
often used as the indicator of indoor pollutants as an indirect method as it is
related to the degree of occupancy in the indoor space and therefore related
to the pollutants generated by occupants. In other applications, such as car