Evaluation of indoor air quality and energy use in air conditioned office buildings

Poor indoor air quality is a common problem in air conditioned office buildings in particular where the room air distribution has not been fully considered and fresh air supply has been reduced for the purpose of energy saving. Recent advances in technology and better understanding of indoor air quality issues have however led to the development of air conditioning systems that can provide better indoor air quality in office buildings. This research has evaluated the energy consumption of two groups of air conditioning systems relative to different levels of indoor air quality. The two groups of air conditioning systems considered were all air and the displacement ventilation systems. Both are capable of providing indoor air quality that complies with the ASHRAE standard. Computer models of these two systems were established and APACHE simulation software was used to evaluate their dynamic thermal and energy performance when different percentages of air are re-circulated. The ventilation decay equation, using carbon dioxide as the key indoor air quality indicator, was then used to evaluate the level of air quality. The results indicated that all air systems with over 20% air re-circulation would not achieve the recommended level of air quality. Re-circulating air would therefore not be an acceptable means of energy saving although an alternative is to incorporate heat recovery as an integral part of the air conditioning system. Displacement ventilation was found to provide the best indoor air quality. Results showed that the operating energy of displacement ventilation with chilled ceiling is only slightly higher than a comparable variable air volume system and further consideration should be given to their more widespread use in offices.