Evaluation of thermal comfort, indoor air quality and energy saving of a Local Exhaust Ventilation system in an Office room (LEVO)

Transcription

1 Evaluation of thermal comfort, indoor air quality and energy saving of a Local Exhaust Ventilation system in an Office room (LEVO) Ahmed Qasim Ahmed 1, 2, *, Shian Gao 1, Ali Khaleel Kareem 1 1 Department of Engineering, University of Leicester, Leicester LE1 7RH, United Kingdom 2 Engineering Technical College, Middle Technical University, Baghdad, Iraq *Corresponding author: phone: +44(0) ; aqaa2@ le.ac.uk, en_ahmed82@yahoo.com. SUMMARY In this paper, the Local Exhaust Ventilation for an Office room (LEVO) is investigated numerically in details. In this system the contaminated warm air caused by the office heat sources (two occupants, two computes and two lamps) are extracted directly from the occupant area before mixing with the rest of the room air. Thermal comfort (predicted mean vote PMV and predicted percentage of dissatisfied PPD), local thermal discomfort (vertical temperature gradient), Indoor Air Quality (IAQ) (particles concentration in the breathing area and breathing zone) and energy saving are used as the evaluation index to assess the performance of using the LEVO system in an equipped office room served by displacement ventilation (DV). Experimental data from previous published work are used to validate the Computational Fluid Dynamic (CFD) model of this study. The new results showed that significant improvements to the human thermal comfort, IAQ and energy saving can be achieved in the room that used the LEVO system. PRACTICAL IMPLICATIONS A significant improvement of the indoor thermal environment and energy saving was achieved in room used the LEVO system. KEYWORDS Local exhaust ventilation, CFD, Thermal comfort, Indoor air quality, Energy saving 1 INTRODUCTION Most people around the world spend the majority of their time inside offices, schools and other buildings. Therefore, the indoor thermal environment, the Indoor Air Quality (IAQ) and human thermal comfort should be maintained at high level. In recent decades, a various configurations of the ventilation strategies and devices are developed to improve the indoor thermal conditions and provide a comfortable indoor environment for the occupants and to reduce the energy consumption. Local Exhaust Ventilation (LEV) system (also called Personalised Exhaust (PE) system) is one of the most important ventilation strategies and it is widely used in industrial sector to provide a healthy and comfortable environment for workers. The LEV system is a type of ventilation system which is used to control on contaminant

2 transmission in occupied zone (Dygert and Dang, 2010, Dygert and Dang, 2012, Melikov et al., 2010, Zítek et al., 2010), where the contaminated and warm air is extracted locally, which consequently improves the quality of the microenvironment around the occupants. Melikov et al. (2011) used the concept of the LEV system to develop a hospital bed. They investigated the reduction of the exposure for doctors and patients in the hospital ward with and without using the LEV system. They revealed that by employing the LEV system the exposure level has decreased significantly for persons who seat near to the patients. Dygert and Dang (2012) found that by employing a local exhaust suction device in an air plane, the reduction in exposure to contamination coming from other passengers was up to 90%. They also found this type of the LEV is suited to a high density occupation. Qian et al. (2008) studied the transmission contaminant in a hospital room using downward ventilation system. They revealed that the fine particles removal efficiency was enhanced by the located exhaust diffuser at ceiling level. However, the positioned exhaust diffuser at the floor level will enhance the particles removal efficiency for the large size of particles. Cheong and Phua (2006) studied the performance of the contaminant removal using different methods of the ventilation system in a hospital ward. They found that the performance of removal contaminant was enhanced significantly when the supply and exhaust diffuser was located at the wall behind patient bed. Yang et al. (2013) investigated the performance of three different types of the PE devices. They revealed that an enhancement of the inhaled air quality was achieved when the PE system is employed and located in level just above the shoulder. Junjing et al. (2014) studied the performance of contaminant removal effectiveness at 12 different positions for two background ventilation systems. In their investigation a novel ventilation system, Personalized Ventilation Personalized Exhaust (PE-PV) system was used and installed on the chair above shoulder level. They found that using this system can improve the human thermal comfort and quality of the inhaled air for the seating occupants. Most previous investigations were focused on the using of the LEV system in limited applications such as in hospital wards, air planes and other industrial applications. However, very limited studies or inadequate investigation was conducted for using the LEV system in a typical office room combined with Displacement Ventilation (DV). Thus, in this study a novel ventilation system called Local Exhaust Ventilation for Office room (LEVO) was investigated numerically to show its impact on the indoor thermal microenvironment around the occupants and energy consumption. In this study, a validated CFD model was used. Generally, most of the indoor pollutant in the office room comes from furniture, heat sources and human activity (Licina et al., 2015, Pereira et al., 2009). Thus, in this paper the particles were released from two contaminant sources to simulate pollutant rise from office equipment and human activity. 2 METHODS The main aim of using the proposed air distribution system, LEVO, is to control on the thermal environment in the occupied zone and reduce the pollutant concentration in a microenvironment around the occupants as well as improving the energy saving. Fig.1a and b show the simulated room details and schematic diagram of the LEVO system combined with the room workstation respectively. In this system, the reading lamps are combined with the exhaust diffuser in one unit and located above the heat sources such as monitors, computers

3 and occupants (see Fig.1 b). By using the LEVO system, the warm and pollutant air which is generated by the occupants and other office activities is extracted locally before reaching to the rest air of the room. Furthermore, in order to improve the human thermal comfort, the extracted warm air was directed towards the foot level, which contributed to reducing the temperature differences between the foot and head level. A small amount of heat will transfer through the heat exchangers, H.E_1 and H.E_2, provided with this system (see Fig.1 b) which subsequently helps to reduce the temperature in area near to foot and contributes to decreasing the temperature difference between the head and foot level. This will contribute to creating a healthy and comfortable environment for the occupants as well as enhancing the energy consumption by increasing the exhaust temperature. Two contaminant sources were used in this study to simulate the indoor contaminants generated by the office equipment and occupant s activities. A 0.7 µm of particle size with density of 912 kg/m3 was used in this study. The supply DV diffuser (1 m 0.6 m) was positioned at the floor. As recommended by Cheng et al. (2013), the return diffuser (0.8 m 1m) was installed at the upper boundary, 1.3 m from floor level, of the occupied area (see Fig.1 a). The room set temperature was 24 C. The total supply air flow rate was 84 L/s with the supply temperature of 19 C and the recirculated air flow rate was 29.4 L/s. The comparison study between the room that used the LEVO system and room without using this system (reference case) was performed to evaluate the performance of using the LEVO system regarding to the indoor thermal comfort, air quality and energy saving. Figure 1. a) Configuration of simulated room; 1- occupant_1; 2 occupant_2; 3 pc case; 4- pc monitor; 5- displacement ventilation (DV) inlet; 6- return inlet; 7- contaminant source_1 at occupant_1 8- contaminant source_2 at occupant_2, b) The LEVO system. 2.1 CFD modelling For accurate prediction of indoor air velocity and pollutants dispersion, a suitable turbulence model requires to be selected from the various existing turbulence models. In this study, the two equation renormalized group RNG k-ε turbulence model was employed to predict the indoor turbulence air flow. The CFD program ANSYS FLUENT was employed in this study to solve the Navier-Stokes equations and calculate the Lagrangian trajectories in a 3D computational model. The Boussinesq assumption was used to calculate the change in air density due to variations of air temperature. The SIMPLE algorithm was selected for pressure and velocity field coupling, and the second order upwind discretization scheme was used to solve all the variables in the simulation cases except pressure which was solved by a staggered

4 scheme named PRESTO!. The mesh independent test was performed to select the proper mesh size for the simulation work. Table 1 shows the heat emitted from each heat source. Table 1. Cooling load for the simulated office room. Heat sources. Occupants Pc_case Pc_monitor Lamps Q space heat load (W) W Air flow validation In order to evaluate the validity of the turbulence model which is used in this study, the simulation results for air temperature were compared with the experimental data of Cheng et al. (2013). In their study, the indoor thermal comfort and energy consumption were investigated at various locations of return diffuser. Fig. 2 shows a comparison between the experimental and simulated air temperature for case 1 and at three points (point A, B and C). A good agreement between the experimental and simulated results can be seen from these figures. All other details can be found in Cheng et al. s publication (Cheng et al., 2013). Figure 2. Comparison between the simulated and experimental temperature profiles at three points for case (1); (triangle symbol: experimental results; dash line: simulated results Particles distribution validation The experimental results of Chen et al. (2006) were used to validate the Lagrangian particle-tracking model (DPM). As shown in Fig. 3 a, the room dimensions were 0.8 m 0.4 m 0.4 m. The inlet and the outlet diffusers had the same dimensions (0.04 m 0.04 m) and were located at the centre of the room. The supply velocity was m sec and the particle diameter was 10 µ m with density of 1400 kg/m 3. Fig.3 b shows the comparison between the simulated and experimental results at one location, x= 0.4 m. From this figure, a reasonable agreement between the predicted and experimental results was found (see Fig.3 b). a) b) Figure 3. a): Schematic diagram of ventilated chamber, b) Comparison between the simulated (dash line) and experimental (square symbols) results.

5 3 RESULTS AND DISCUSSION 3.1 Indoor thermal comfort The indoor thermal comfort was evaluated using Fanger's comfort equations (Fanger, 1970). In this model, two indices, predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD), were used to assess the thermal balance for the whole human body. For the acceptable indoor thermal comfort requirement, the PMV and PPD values should be in the range of -0.5<PMV<0.5 and PPD> 10 percent respectively. A small PPD and PMV values are highly recommended for a good indoor thermal comfort. Table 2 shows the comparison between the PMV and PPD results for each case and for both occupants. The results revealed that, for both cases, the PMV and the PPD indices were approximately the same with a slightly increasing in the case that uses the LEVO system, which was due to that the HE_1 and HE_2 have an influence on the air velocity and temperature (see Fig.4 a) which subsequently influences the thermal environment in these regions. This was in line with the findings of Horikiri et al. (2014). Table 2. PMV-PPD indices for each case study and for both occupants Occupant_1 Occupant_2 Ref. LEVO REF LEVO PMV PPD PMV PPD PMV PPD PMV PPD a) b) Figure 4. a) Temperature distribution ( ) near foot zone for both occupants, b) Monitoring points positions. 3.2 Temperature distribution in the vertical direction According to the ASHRE Standard (2004), the temperature difference between the head level and foot level ( T head foot ) should not exceed 3. As shown in Fig. 5 (a), four locations (points 1, 2, 3 and 4) at two points for each occupant were employed in this study to assess the thermal comfort of the occupants. Fig. 5 b shows the temperature differences, ( T head foot ), for both occupants. Frome these figures, it is clear to note that the use of the LEVO system enhanced the vertical distribution of the air temperature in all positions compared with the reference case. This is because the LEVO system works to extract the warm air in the vicinity of the occupants and directs it towards the foot level. This process leads to decrease the air temperature at the head level and slightly increases air temperature at the foot level, which subsequently improves the human thermal comfort. Furthermore, the LEVO system is found to contribute to reducing the temperature differences between the upper and the lower parts of the room and create a homogenous temperature distribution in all room domains (see Fig. 6).

6 Figure 5. a) Monitoring points, b) Temperature gradient ( ) in vertical direction for both cases. Figure 6. Temperature distribution at plane x=2 m:(a) LEVO and (b) Reference case. 3.3 Evaluation of energy saving The performance of using the LEVO system regarding to energy saving was investigated for the current study. Based on the simulation results, Cheng et al. (2013) developed a new method to evaluate the energy saving in a room served by a STRAD system by calculating the reduction in cooling coil load. The term c p m e (T e T set ) represents the reduction amount of cooling coil load. T e, T set and m e refer to the exhaust temperature, room set temperature (24 ) and exhaust mass flow rate respectively. This term was employed in the current work to evaluate the performance of using the LEVO system in saving energy. As listed in Table 3, a significant enhancement of the energy saving was achieved in room using the LEVO system compared with the reference case. This was because the local extraction of the heat generated from the computers, lamps and occupants contributed to an increase in the exhaust air temperature, consequently improving the potential of energy saving. This is consistent with the findings reported by Ahmed and Gao (2015). In energy saving assessment, other factors such as thermal comfort, temperature distribution in vertical direction and indoor air quality should be considered carefully. Table 3. Energy saving for cooling coil for each case study. Ref. LEVO Exhaust air temperature T exhaust ( ) Return air temperature T return ( ) Q coil (W) = c p m e (T e T set ) (%) Q coil Q space 3.4 The quality of the indoor air in breathing and inhaled zones The IAQ plays a central role in the assessment for any air distribution system. For the current study, the quality of the indoor air was evaluated at two zones: inhaled zone and

7 breathing zone. The normalised contaminant concentration was defined as: C n = C p C e where C n is normalised concentration and C p and C e refer to the contaminant concentration in a specific zone and the concentration at exhaust respectively. Figs. 7 a and b show the normalised particle concentration in the breathing and inhaled zone respectively. From these figures, it can be noticed that the quality of the indoor air in both breathing and inhaled zones was enhanced significantly with the LEVO system compared with reference case. For breathing zone the LEVO system contributed to reducing the contaminant concentration to around 48 % (see Fig.7 a).this was because a large amount of the contaminant was extracted locally from the LEVO system before reaching to the breathing zone. Furthermore, thermal plumes of the heat sources bring additional amount of contaminant to be extracted from the novel system before dispersion in the breathing and the inhaled zones. While for the inhaled zone, it is clear that with the LEVO system, the inhaled air quality was significantly improved for both occupants. However, the inhaled air quality for occupant 1 was better than the quality for occupant 2 (see Fig.7 b). That was because the location of occupant 1 was near to the DV supply. This caused the velocity of the supply air to be able to drive the contaminant away from occupant 1 zone which helps to enhance the air quality in this area. This was in line with the findings of Sadrizadeh and Holmberg (2015). Figure 7. The quality of indoor air for a) breathing zone, b) inhaled zone. 4 CONCLUSION In this study the performance of using the concept of LEV for office applications was investigated numerically. The results show that significant improvements to the indoor thermal comfort, indoor air quality and energy consumption were achieved in the room used the LEVO system. With this system, around 48% of the contaminant concentration reduction was achieved in the breathing zone. In addition, the energy consumption by the cooling coil load was reduced by about 22.5 % compared with 5.0 % for the reference case. Therefore, the using of the LEVO system will contribute to enhancing the indoor thermal environment as well as improving the energy saving. ACKNOWLEDGEMENT The authors would like to thank the ministry of higher education of Iraq for the financial support.

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