New airborne pathogen transport model for upper-room UVGI spaces conditioned by chilled ceiling and mixed displacement ventilation: Enhancing air quality and energy performance Mohamad Kanaan a , Nesreen Ghaddar a,⇑ , Kamel Ghali a , Georges Araj b a Department of Mechanical Engineering, American University of Beirut, Beirut, Lebanon b Department of Pathology & Laboratory Medicine, American University of Beirut, Beirut, Lebanon article info Article history: Received 12 April 2014 Accepted 19 May 2014 Keywords: Modeling bacteria transport UV germicidal irradiation Air disinfection Mixed displacement ventilation Energy efficiency abstract The maximum allowable return air ratio in chilled ceiling (CC) and mixed displacement ventilation (DV) system for good air quality is regulated by acceptable levels of CO 2 concentration not to exceed 700 ppm and airborne bacterial count to satisfy World Health Organization (WHO) requirement for bacterial count not to exceed 500 CFU/m 3 . Since the CC/DV system relies on buoyancy effects for driving the contami- nated air upwards, infectious particles will recirculate in the upper zone allowing effective utilization of upper-room ultraviolet germicidal irradiation (UVGI) to clean return air. The aim of this work is to develop a new airborne bacteria transport plume-multi-layer zonal model at low computational cost to predict bacteria concentration distribution in mixed CC/DV conditioned room without and with upper-room UVGI installed. The results of the simplified model were compared with layer-averaged con- centration predictions of a detailed and experimentally-validated 3-D computational fluid dynamics (CFD) model. The comparison showed good agreement between bacteria transport model results and CFD predictions of room air bacteria concentration with maximum error of ±10.4 CFU/m 3 in exhaust air. The simplified model captured the vertical bacteria concentration distribution in room air as well as the locking effect of highest concentration happening at the stratification level. The developed bacteria transport model was used in a case study to determine the return air mixing ratio that minimizes energy consumption and maintains acceptable IAQ with and without UVGI. Results showed that the use of upper-room UVGI resulted in 35% in energy saving, whereas the use of in-duct UVGI achieved no more than 12% energy saving, both compared to 100% fresh air case. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Heating ventilation and air conditioning (HVAC) systems is a major contributor to energy consumption in buildings and its contribution is expected to increase in the upcoming years [1]. Therefore, targeting energy cost associated with providing good indoor air quality (IAQ) in HVAC systems is a strategic intervention to reduce building energy consumption [2,3]. A widely used indicator of air quality is the CO 2 concentration level in an air- conditioned room. The CO 2 gas is recognized by ASHRAE (American Society for Heating, Refrigeration and Air conditioning Engineers) as the surrogate ventilation index since it is a good indicator of occupancy and ventilation rate within a space. Although CO 2 by itself is not considered an indoor air contaminant, but it is emitted by humans who also give off a wide range of ‘bioeffluents’ which can build up in space, due to poor ventilation [4]. However, in pres- ence of occupants with contagious infections releasing airborne viruses or bacteria, the ventilation requirements might be much higher beyond the requirements for CO 2 concentration to prevent spread of disease. The WHO (1988) [5] recommends not more than 50 CFU of fungi/m 3 and a maximum number of bacteria of 100 CFU/m 3 of air for hospital environments. In office spaces, the maximum allowable limit of bacterial and fungal count is 500 CFU/m 3 [3] although some researchers suggested that the counting of human normal flora bacteria above 200 CFU/m 3 air would be considered high [6,7]. This would pose prohibitive venti- lation requirement contributing to increased cooling load and energy consumption for hot humid climates. This has led to consid- eration of other methods that can disinfect air in the duct or in the space which include using ultraviolet germicidal irradiation (UVGI) [8,9] or using high efficient particulate air (HEPA) filters at the http://dx.doi.org/10.1016/j.enconman.2014.05.073 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +961 350000 x2513 E-mail address: farah@aub.edu.lb (N. Ghaddar). Energy Conversion and Management 85 (2014) 50–61 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman