Demand Controlled Systems With fuzzy Controllers to maintain Indoor Air Quality - An Energy Saving Approach

The term air conditioning not only prescribes comfort temperature and relative humidity, but also the quality of air inside the room. Indoor Air Quality (IAQ) has become a concomitant of air conditioning. The pollutants generated inside the room affect the quality of air inside the room. The major pollutants considered are occupant generated (carbon dioxide - CO2, odour), sulphur dioxide and toluene etc. The monitoring and controlling of all pollutants is cumbersome. Of all, CO2 is frequently considered as the surrogate index of pollutants provided that there are no other major sources of adverse pollutants present. ASHRAE standard 62-1999 recommends a ventilation rate of 8 L/s per person. Because carbon dioxide is a bioeffluent, the monitoring of CO2 concentration inside a space helps in ventilation control, thereby maintaining IAQ and, in turn, comfort conditions. Therefore, to achieve this in optimal system design and control, the instantaneous occupancy level is required. To estimate the instantaneous occupancy level, the instantaneous CO2 concentration and its generation rate must be known. The impasse with CO2 as an indicator of occupancy is the cost and the time response of the sensor. Furthermore, the system behaviour is non-linear. These factors call for more accurate design of controllers. Hence fuzzy controllers can be chosen as the best option. Not only the ventilation flow rate but also the supply flow rate can be varied to achieve optimum comfort control from the system. It has been found that the system settling time is very much improved when using a fuzzy controller as opposed to conventional controllers. This enables fast varying occupancy levels to be monitored for the optimum control of ventilation to maintain IAQ. The regulation of supply and ventilation flow rates is also shown to be able to reduce fresh air demand by over 50% when compared to an unregulated system. Apart from the resultant energy savings achieved in the provision of fresh air, the air conditioning load is also reduced.