Energy and Buildings 43 (2011) 2684–2695 Contents lists available at ScienceDirect Energy and Buildings j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n b u i l d Air quality in rooms conditioned by chilled ceiling and mixed displacement ventilation for energy saving W. Chakroun a,∗ , K. Ghali b , N. Ghaddar b a Department of Mechanical Engineering, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait b Department of Mechanical Engineering, American University of Beirut, P.O. Box 11-0236, Beirut 1107-2020, Lebanon a r t i c l e i n f o Article history: Received 3 June 2011 Accepted 19 June 2011 Keywords: Contaminant transport in plumes Indoor air quality with mixed return air Chilled ceiling displacement ventilation a b s t r a c t A transient-contaminant-transport model is developed for assessing IAQ in the breathing zone when introducing return air into rooms conditioned by CC/DV system to save energy. The steady state transport model of [1] is extended to transient conditions while accounting for significant wall plumes associated with external loads. Experiments are performed to validate the extended model predictions of IAQ expressed in the level of CO 2 concentration. Experiments are conducted in a chamber with two external walls in Kuwait Climate. Measurements are recorded in time of the air temperature and CO 2 concentration at selected locations in the room and compared with values predicted by the model. Experimental results agreed well with model predictions. The maximum errors in predicted CO 2 concentrations are less than ±25 ppm in presence of external load. 60% fresh air fraction resulted in 37% less energy consumption compared with 100% fresh air CC/DV system. The validated model is applied to a case study in Kuwait to evaluate energy saving over the cooling season for a typical office space while using mixed DV air. Energy savings of up to 20.6% can be realized using mixed supply air while maintaining IAQ compared with energy used for the 100% fresh air. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The continuing rise in energy demand, costs and the associ- ated environmental problems,notably climate change,is causing increased emphasis on the design of energy efficient air- conditioning systems for both industrial and comfort applications. Buildings in Kuwait as an example account for more than 50% of all energy use in the country [2]. In the summer, the HVAC systems represent 70% of peak load.Targeting energy efficiency in HVAC systems to provide thermal comfort and good indoor air quality is a strategic intervention to reduce energy consumption in the build- ing sector [3]. The chilled ceiling displacement ventilation (CC/DV) system is one HVAC system that is known to provide high indoor air quality by introducing 100% fresh supply close to the floor level dis- placing warm air into exhaust and creating a clean occupied lower zone in the space. The 100% fresh air exceeds the ASHRAE Stan- dard 62.1 [4] minimum acceptable ventilation requirements of the space. The energy consumption cost of conditioning ambient air to appropriate supply condition of the displacement ventilation sys- tem is substantial. The mixing of part of the return air with the ∗ Corresponding author. Tel.: +965 24985804; fax: +965 24847131. E-mail address: wchakroun@gmail.com (W. Chakroun). supply air may present an opportunity to reduce overall system energy consumption without violating indoor air quality standard. Several researchers [1,5,6] reported that for a 100% fresh air sup- ply condition of equal indoor air quality and thermal comfort level, the CC/DV system consumed 53% less cooling energy than the con- ventional system at 100% fresh air system over the cooling season. The initial cost of the CC/DV system was found by [3] to be higher, but the payback period based on transient operation is less than 3 years when used to replace a 100% fresh air conventional sys- tem. However, when the 30% fresh air case of the conventional system was compared by the 100% fresh air CC/DV system,the CC/DV system did not offer any advantage in terms of cost over the conventional system. It is clear that competiveness is directly tied with the amount of fresh air used in the CC/DV system which at 100% value exceed the minimum acceptable value for ventila- tion given that Kuwait climate is characterized by high outdoor temperature that exceeds indoor comfort conditions by 20–30 ◦ C. Thermal comfort is ensured in the CC/DV conditioned space when the vertical temperature gradient in the occupied zone is less than 2.5 ◦ C/m [7]. Many researchers have adopted the “stratifica- tion height” in the CC/DV-conditioned spaces to be the measure of the indoor air quality [8–10]. The stratification height is defined as the elevation at which the density gradients disappear in the rising air and its plume spreads horizontally. It is determined from mass 0378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2011.06.019