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Thermal environment and air quality in office with personalized ventilation combined with chilled ceiling
Building and Environment 92 (2015) 603e614
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Thermal environment and air quality in office with personalized ventilation combined with chilled ceiling Aleksandra Lipczynska a, b, *, Jan Kaczmarczyk a, Arsen K. Melikov b a
Department of Heating, Ventilation and Dust Removal Technology, Silesian University of Technology, Konarskiego 20, 44-100 Gliwice, Poland International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Nils Koppels All e Byg. 402, 2800 Kgs. Lyngby, Denmark
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2015 Received in revised form 17 May 2015 Accepted 27 May 2015 Available online 28 May 2015
The thermal environment and air quality conditions provided with combined system of chilled ceiling and personalized ventilation (PV) were studied in a simulated office room for two occupants. The proposed system was compared with total volume HVAC solutions used today, namely mixing ventilation and chilled ceiling combined with mixing ventilation. The objective of the study was to evaluate whether PV can be the only ventilation system in the rooms equipped with chilled ceiling. The room air temperature was 26 C in cases with traditional systems and 28 C when PV was used. PV supplied air with the temperature of 25 C. PV improved thermal conditions and was up to nearly 10 times more efficient in delivering clean air at workstations than mixing ventilation systems, which resulted in strong protection of occupants from the cross-infection. In the room space outside workstations no substantial differences in thermal environment were found between studied systems. The room air mixing with PV working alone was at the same level as with mixing ventilation. No substantial differences in contaminants' concentration distribution and air-change effectiveness were found between the studied systems in the occupied zone outside workstations. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Personalized ventilation Radiant cooling Thermal comfort Contaminant distribution Personal exposure
1. Introduction Thermal environment and indoor air quality in offices affect health, comfort and performance of occupants [1e3]. HVAC systems often consume more than 40% of total building energy needs. Thus creating comfortable conditions in spaces at low energy use becomes an important challenge for HVAC engineers and requires development of new solutions. Centrally controlled total volume ventilation systems (e.g. mixing ventilation or displacement ventilation), which are typically used in offices, aim to create uniform indoor environment in the occupied zone of rooms which cannot satisfy all occupants. In the case of mixing ventilation air is supplied far from occupants and is mixed with polluted room air before reaching people's breathing zone. With the displacement ventilation cool and clean air is supplied at low level, near the floor and is moved upward by the buoyancy flows present in the
˛ ska, Katedra Ogrzewnictwa, Wentylacji i * Corresponding author. Politechnika Sla Techniki Odpylania, ul. Konarskiego 20, 44-100 Gliwice, Poland. E-mail address: [email protected] (A. Lipczynska). http://dx.doi.org/10.1016/j.buildenv.2015.05.035 0360-1323/© 2015 Elsevier Ltd. All rights reserved.
room. Thus air should be cleaner in the lower (occupied) zone. However, in many practical applications the air cleanness is lower than assumed during design process [4,5]. The air cleanness especially decreases when the pollution source is located close to the occupant. In such situation the human boundary layer is transporting contaminated air to the breathing zone. Also movements created by walking people disturb the pollution stratification and decrease the cleanness of the air [6]. Moreover due to risk of draught at the feet its' cooling capacity is limited. The performance of total volume air distribution systems is discussed in Ref. [7]. Promising air distribution is achieved by personalized ventilation. It delivers clean air directly to occupants' breathing zone and provides possibility for individual control of the microenvironment at each workstation [8]. Therefore personalized ventilation has potential to improve inhaled air quality compared to total volume ventilation [9e11]. Previous studies showed also that personalized ventilation decreases intensity of some Sick Building Syndrome symptoms in comparison with mixing ventilation [9,12]. However, due to smaller airflow rates and limitation of the lowest supply air temperature personalized ventilation
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Nomenclature ACE ADPI CC MV PV TV WS q ta tacn teq to tw
Air-Change Effectiveness Air Distribution Performance Index chilled ceiling mixing ventilation personalized ventilation total-volume ventilation workstation flow rate, L/s air temperature, C corrected time-averaged air temperature at n-point, C manikin based equivalent temperature, C operative temperature, C cooling water temperature, C
systems are not able to remove substantial amount of sensible heat load generated in spaces. In such cases additional cooling system is needed. The chilled ceiling is a popular hydronic radiant cooling solution used in different type of buildings. It has higher cooling efficiency than traditional total air systems and is easy to maintain [13]. Radiant ceiling cooling provides “cool head and warm feet” environment which is preferred to “warm head and cool feet” environment often created by total volume air systems. Nevertheless, the chilled ceiling requires installation of additional ventilation system to provide fresh air into spaces. Typically chilled ceiling is installed with mixing ventilation. However, it is suggested that displacement ventilation combined with chilled ceiling has increased cooling capacity and may provide environment with high air quality and acceptable thermal comfort conditions [14e18]. Loveday et al. [19] reported that the stratification flow, typical for displacement ventilation, can be disturbed because of chilled ceiling use. Air cooled under the ceiling creates downward convection flow, which increases the warm and contaminated upper region. Additionally chilled ceiling cools walls due to increased radiant heat exchange, which can increase downward convection flow near walls, causing transport of pollutants from the mixing contaminated region into the supply air and occupied zone [20]. Reduced chilled ceiling temperature increases the air velocity at height of 0.1 m and 1.1 m indicating mixing in the stratified zone, which can cause additional decrease of the air quality [19]. Combined system of chilled ceiling and displacements ventilation was also studied with integrated personalized evaporative cooler [21,22]. Personalized evaporative cooler is installed at workstations and explores the idea of “ductless” personalized ventilation [23,24]: air is drawn from the floor level and supplied toward occupant's face and upper body parts. Difference compared to “ductless” personalized ventilation is that air passes through saturated water sponge in the cooler and is cooled by 3e4 K. This combined system is proposed for hot and dry climate, where problematic are peak loads resulting in oversized HVAC systems. An aim of personalized evaporative cooler is to improve thermal conditions locally at the workstations during peak hours when chilled ceiling combined with displacement ventilation is not capable of maintaining design room temperature. Benefits of the personalized evaporative cooler for occupants are temporary and are focused only on the thermal comfort. Hitherto performed studies have not considered personalized ventilation as the only ventilation system operating in the room when combined with radiant cooling. Personalized ventilation
va εc
tr
ti fn
air velocity, m/s Contaminant Removal Effectiveness time-averaged air temperature in the reference point, C local age of air, min effective draft temperature, C
Subscripts E exhaust exh exhaled i in the i-location inh inhaled n in the n-point S supply
ensures convective cooling to the upper body parts of occupants, which can be controlled according to their individual preferences. Thus combining it with chilled ceiling should be in addition to improved perceived air quality also an effective way to improve thermal comfort in rooms at temperature higher than the upper limit of 26 C recommended in the standards [25]. It has been reported that increasing the maximum allowed air temperature in the room and implementing personalized ventilation system together with mixing ventilation may be an effective energy-saving strategy [26]. It may be expected that under certain operating conditions combining chilled ceiling with personalized ventilation instead of mixing ventilation will also lead to reduction of the energy consumption. Moreover, change in air distribution strategy from total volume air distribution to localized air distribution should bring benefits in improving perceived air quality at workstations. The proposed combined system of chilled ceiling and personalized ventilation is novel and its performance needs to be studied. The objective of the presented research was to identify the thermal environment and air quality conditions in an office room for two occupants equipped with personalized ventilation combined with chilled ceiling and to compare them with room conditions created by solutions used today such as mixing ventilation and combined system of chilled ceiling and mixing ventilation. An important part of the study was to evaluate whether mixing ventilation can be replaced by personalized ventilation as the only ventilation system in the rooms with chilled ceiling considering the environment at the workstation and in the occupied zone. 2. Methods a) Experimental facilities Experiments were performed in a climate chamber (L W H: 4.12 m 4.2 m 2.89 m) arranged as an office with 2 workstations (Fig. 1). The chamber was equipped with total volume mixing ventilation (MV), personalized ventilation (PV) on each desk and with a chilled ceiling (CC). The round movable panel was used as a PV diffuser because of high efficiency in delivering clean air to occupants at low turbulence intensity of the supplied personalised flow [27]. Three linear diffusers mounted at the centre of the ceiling were used for MV. Exhaust diffusers were located in the ceiling corners, as indicated in Fig. 1. Eighteen radiant cooling panels (1.19 m 0.59 m) were built-in the suspended ceiling and covered 75% of its area.
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Fig. 1. Lay-out of the room: plane view with measurement points and reference point (ref).
Two occupants were simulated by thermal manikins dressed in summer clothes (total insulation with chair was 0.59 clo) seated at the workstations (WS1 and WS2) with computers. The manikins were shaped as a 1.7-m tall women. Manikins' body consisted of several segments in order to study the local effect of non-uniform thermal environment. One of the manikins with 23 body segments was located at WS1 and the other manikin with 17 body segments at WS2. The surface temperature of individual body segments was controlled to be the same as the skin temperature of an average person in a state of thermal neutrality [28]. The manikin at WS1, referred in the following as “polluting manikin” was connected to artificial lungs to simulate breathing process. Five water heated panels (0.8 m 1.59 m) simulating heated window were installed on one of the room's walls and five electric foils placed on the floor were used to simulate solar heat gain in the room (Fig. 1). Lighting in the chamber was provided by four office lamps (40 W each). Three pollution sources were simulated in the room. The air exhaled by the “polluting manikin” was marked with sulphur hexafluoride (SF6) to represent infectious air exhaled by a sick person. The tracer gas simulates the transportation of exhaled droplet nuclei smaller than 5e10 mm, which include most of bacteria and viruses [29,30]. The pulmonary ventilation rate was 6 L/ min. The typical breathing frequency for a person in light activity was simulated (2.5 s e inhalation, 2.5 s e exhalation, 1 s e break) [31,32]. Tetrafluoroethane (R134a) was dosed at the armpits of the “polluting manikin” to simulate other human bioeffluents. Additionally a passive contaminant source generated from a surface
(0.8 m 1.2 m) located near to the floor on the wall opposite to the window was simulated. The surface released carbon dioxide (CO2). b) Experimental conditions The performance of four combinations of the installed systems: total volume mixing ventilation (TVMV), chilled ceiling combined with mixing ventilation (CCMV), chilled ceiling combined with mixing ventilation and personalized ventilation (CCMV/PV), and chilled ceiling combined with personalized ventilation (CC/PV) was studied under summer conditions. CCMV/PV and CC/PV systems were studied for three PV airflow configurations. Detailed information on the experimental conditions and system operation settings are listed in Table 1. The air temperature in the office was set to 26 C cases with traditional systems (TVMV and CCMV) and to 28 C when PV was used. Air temperature was used as a design parameter to define supply air temperatures for the MV and PV systems instead of operative temperature as recommended in EN 15251 [25] and EN ISO 7730 [33] because of its feasibility in heat balance calculations. Furthermore, traditional control systems are typically equipped with air temperature sensors, not with operative temperature sensors. The supply air temperature in the case of MV was 16 C. The PV supplied air at 25 C. In order to avoid draught discomfort the difference between the PV supplied air temperature and the room air temperature was set to 3 K. The designed total heat load in the room was 72 W/m2 at 26 C (occupants: 66 W/manikin; computers: 65 W/computer; solar heat
Table 1 Experimental conditions and designed operating parameters. Case
Conditions Design ta ( C)
Total sensible heat load (W/m2)
CC
MV
TVMV CCMV CCMV/PV_1 CCMV/PV_2 CCMV/PV_3 CC/PV_1 CC/PV_2 CC/PV_3
26 26 28 28 28 28 28 28
72 72 66 66 66 66 66 66
PV
qtot (L/s)
tw ( C) (inlet/outlet)
qMV (L/s)
qPV1 (L/s)
qPV2 (L/s)
e 17.2/18.7 20.8/21.9 21.5/22.5 21.1/22.2 18.7/20.3 19.9/21.3 19.4/20.9
82 26 12 12 12 e e e
e e 7 (qP) 15 (qP,inc) 15 (qP,inc) 13 (0.5qBþqP) 21 (0.5qBþqP,inc) 21 (0.5qBþqP,inc)
e e 7 (qP) 15 (qP,inc) 7 (qP) 13 (0.5qBþqP) 21 (0.5qBþqP,inc) 13 (0.5qBþqP)
(qBþqP) (qB) (qB) (qB)
82 26 26 42 34 26 42 34
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load: 654 W; lighting: 160 W) and 66 W/m2 at 28 C (occupants: 53 W/manikin; computers: 65 W/computer; solar heat load: 553 W; lighting: 160 W). The ventilation airflow rates supplied to the room were determined based on calculations of either heat balance or according to recommendations in the standard EN 15251 [25]. The total supply airflow rate, qtot, in the case of TVMV was selected according to thermal balance to keep designed air temperature in the room. Under the remaining conditions the supply airflow rate was calculated as a sum of flow rate needed for removal of lowpollution from building materials, qB (12 L/s e corresponding to 0.7 L/s per m2), and flow rate for removal of pollution from occupants, qP (7 L/s per person) according to the recommendations in EN 15251 [25] for category II. The personalized airflow rate, qPV, was set either at 7 L/s (equal to qP required by standards) or was increased to 15 L/s per person (qP,inc). In cases without MV, i.e. CC/ PV each PV diffuser supplied airflow which was a sum of 0.5qB rate (required by standards for building pollution removal) and airflow rate because of occupancy: qP or qP,inc. In the TVMV case all ceiling supply and exhaust diffusers were used. In the remaining cases only the exhaust 1 was used and the middle supply diffuser in the cases with MV (Fig. 1). The temperature of the cooling water, tw, circulating through CC panels depended on the heat load in each studied case and was set to maintain constant the air temperature in the room. The lowest supply water temperature was set based on heat and moisture balance calculations not to allow for a water vapour condensation at the CC panels surface. Two persons were taken into account as the moisture sources in the calculations. The highest relative humidity of room air did not exceed value of 50%.
evaluation in sedentary activity, but it needs to be used with caution to perform correct assessment [38]. The ADPI is based on air temperature and velocity measurements in the occupied zone, from which the effective draft temperature (fn) is calculated according to equation (2):
fn ¼ tacn tac 8:0ðvan 0:15Þ
(2)
Average test zone temperature (tac) is calculated from tacn values for all measurement points. The time-average value in the reference point (tr) was calculated to obtain the temperature correction factor at each measurement location for any changes in the reference temperature during measurements. ADPI is defined as a percentage of points where effective draft temperature meets criteria: 1.7 C fn 1.1 C. d) Air quality measurements and analyses The tracer gas concentration was measured with two sets of multi-gas sampler and analyser. Concentration measurement was based on the photoacoustic principle. Measurements were performed in 8 points throughout the room, in the air inhaled by the “polluting manikin” and above lips of the “exposed manikin” [39]. Furthermore measurements were also done in the supply and in the exhaust air. Five points (locations no. 2, 4, 14, 22 and 24 in Fig. 1) at height of 1.7 m were selected to represent possible presence of a standing person. Additionally at the location no. 14 gas concentration was measured at heights of 0.1 m, 0.6 m and 1.1 m to identify vertical gas concentration gradient in the room. In order to compare the contaminant distribution between the conditions the tracer gas concentration data were normalized according to the following equation (3):
c) Thermal environment measurements and analyses Thermal environment at the workstations was evaluated with the thermal manikins. Segmental and overall equivalent temperatures, teq, were calculated using the following equation (1):
teq ¼ tsk Qs =hcal
(1)
where tsk is the manikin skin surface temperature ( C), Qs is the manikin dry heat loss (W/m2), hcal is the heat transfer coefficient (W/m2K) obtained in the prior calibration. The equivalent temperature, teq, represents the effect of non-evaporative heat loss from the human body [34]. Measurements of air temperature (ta), globe temperature (tg), air velocity (va) and turbulence intensity were performed at 21 locations in the room, outside of the workstations, as indicated in Fig. 1 (locations 7, 11, 12 and 17 are excluded due to furniture arrangement). The globe temperature was measured by a grey sphere sensor. At each location temperature and air velocity sensors were located at the standardized heights of 0.1 m, 0.6 m, 1.1 m, and 1.7 m [35]. The globe temperature measured at heights of 0.6 and 1.1 m corresponded to the operative temperature (to) [36]. Air velocity and Tu were measured at additional heights of 0.05 m, 0.3 m, 2.0 m and 2.4 m. The temperatures were measured with an accuracy of ±0.2 K. The uncertainty of the air velocity measurements was 0.02 m/s ±1% of readings. All measured results were averaged of 5 min. The draught rate was calculated according to EN ISO 7730 [33]. This factor predicts percentage of people dissatisfied due to cooling effect of air movement. The Air Distribution Performance Index (ADPI) was used to evaluate level of mixing in the room based on thermal conditions. The ADPI is a method suggested by ASHRAE [37] for testing air diffusion performance of mixing ventilation systems under cooling mode. The index can be also an indication for occupants' comfort
Cn:i ¼ ðCi CS Þ=ðCE Cs Þ
(3)
where Ci is concentration measured in the i point of interest, CS is concentration in the supply air and CE is concentration in the exhaust air. The concentration values measured in air supplied by MV and PV were equal. In cases of CCMV/PV system the value obtained in PV supply was used for the calculations. The Contaminant Removal Effectiveness (εc) was calculated according to equation (4) [40] and was used to compare the influence of PV on the contaminants' concentration in the occupied zone.
. εc ¼ CE C i
(4)
where C i is average concentration in the room. The local values of Contaminant Removal Effectiveness (εcp;i ) are calculated by taking the ratio between the measured concentration in the exhaust (CE) and in the i point (Ci). The airborne infection transmission between occupants was determined based on the rebreathed fraction of infectious air [41] by equation (5):
f ¼ ðCinh CS Þ=Cexh
(5)
where f is the volume fraction of the contaminated exhaled air in inhalation, Cinh is concentration measured in the inhaled air, Cexh is concentration of the tracer gas in the exhaled air of the polluting manikin. Additional measurements for cases with total supply airflow of 26 L/s (CCMV, CCMV/PV_1 and CC/PV_1) were performed to investigate the Air-Change Effectiveness (ACE), which represents system's ability to exchange air in the room. ACE was measured by a tracer gas concentration decay test and calculated according to ASHRAE 129 [42]. The following equation (6) was used:
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Table 2 Average measured values of air temperature (bold font) and globe temperature (normal font) in occupied zone for sitting and standing occupancy. Case Avg temperature ( C) Max horizontal difference (K) Vertical difference 1.1 me0.1 m
Vertical difference 1.7 me0.1 m
ACE ¼ tE =ti
0.6 m 1.1 m 0.6 m 1.1 m avg (K) max (K) PD (%) avg (K) max (K) PD (%)
TVMV
CCMV
CCMV/PV_1
CC/PV_1
26.2 ± 0.4/26.4 ± 0.5 26.2 ± 0.4/26.3 ± 0.6 1.4/1.7 1.3/2.0 0.2/0.1 0.5/0.4 0.5/e ¡0.2/0.2 0.2/0.5 0.4/e
26.2 ± 0.3/26.3 ± 0.5 26.2 ± 0.3/26.3 ± 0.4 1.0/1.4 1.1/1.4 0.3/0.0 0.6/0.3 0.5/e 0.2/0.1 0.8/0.4 0.6/e
28.2 ± 0.3/28.2 ± 0.4 28.3 ± 0.3/28.2 ± 0.4 1.0/1.2 0.9/1.1 0.0/0.1 0.2/0.3 0.4/e ¡0.1/0.1 0.4/0.3 0.4/e
28.2 ± 0.3/28.2 ± 0.4 28.2 ± 0.2/28.2 ± 0.3 0.9/1.3 0.8/1.1 0.0/0.1 0.3/0.2 0.4/e 0.0/0.1 0.2/0.2 0.4/e
(6)
where tE is local age of air in the exhaust, ti is average local age of air at breathing level in the occupied zone. Local age of air, ti, is the average time in which air reaches the given i point since air entered the room [40]. Seven sampling points in the occupied zone were used for ACE calculations: two at WSs and five at 1.7 m height for standing person (Fig. 1).
3. Results a) Thermal environment Results of the measured thermal parameters in the occupied zone outside of WSs revealed only small differences within the studied system combinations. Therefore only results of selected cases for each system are presented in the following. The effect of the radiant cooling on the globe temperature was small. Differences between air temperature and globe temperature in most measured points were within the accuracy of sensors. Only with TVMV system at height of 1.7 m and with CCMV at 0.1 m the globe temperature was up to 0.9 K higher than the air temperature. Vertical air temperature differences between head and ankle levels for seated and standing persons are presented in Table 2. In all cases the differences were under the allowed maximum of 2 K for category A thermal environment [33]. The highest values, 0.5e0.6 K, for a seated person appeared with TVMV and CCMV. The corresponding maximum percentage of dissatisfied was 0.5% [33]. For a standing person the highest value of 0.8 K (0.6% dissatisfied) was recorded with the CCMV. In all measurement locations the vertical distribution of air and globe temperature met uniformity criteria according to ISO 7726 [35]. The deviations of the values measured in each height from the average value of the four heights at the location were within criteria of ±1.5 K regardless of studied system configuration. Based on this result the average room temperature values were calculated from the readings at the abdomen level (Table 2). Both for a seated and standing person the average room air temperature in the room was 0.2e0.3 K higher than the set point of 26 C or 28 C. Vertical profiles of average air velocity, calculated from the mean velocity measured at all locations at the same height, were similar in shape for all cases (Fig. 2). Differences between conditions with the same HVAC system configuration, CCMV/PV or CC/ PV, were within ±0.01 m/s. The highest values were characteristic for the level of 0.05e0.1 m. The velocity decreased with height to the level of 1.7 m where it started to increase. Among all studied systems, TVMV operating at the highest supply air flow of 82 L/s generated the highest air velocity values in the room. For remaining cases the supply flow ranged from 26 L/s to 42 L/s. In all studied cases the highest air velocity was typically
Fig. 2. Vertical profile of average air velocity (calculated based on readings at each height).
measured at the height of feet (0.05e0.1 m) in the area between WSs and wall with door. This was especially evident in the case with TVMV, as shown in Fig. 3. Smoke visualizations revealed a strong longitudinal-loop flow between walls with window and with door, which was generated by the buoyancy flow from the heat sources concentrated next to the windows. In the area between WSs and door no obstacles were located, e.g. partitions, furniture or lamps, which could block the loop flow and decrease the air velocity. The highest values of the draught rate (Fig. 4) in the occupied zone appeared in the reference cases TVMV and CCMV, where airflows supplied through MV were higher than when PV was in use (Table 1). In those cases draught rate was within range of 13.9e21.7%. In cases with CCMV/PV the risk of draught sensation was much lower, 5.1e10.7%, and with CC/PV 2.9e13.3%. Table 3 present results of ADPI analyses performed according to ASHRAE 113 [37]. The average temperature in the test zone was equal at TVMV and CCMV cases. The difference in tac between all cases with systems using PV, CCMV/PV and CC/PV, was 0.1 K, i.e. lower than the measurement accuracy. The differences between tr and tac did not exceed 0.2 K. For all analysed systems the ADPI was above 80%, i.e. above the minimum ADPI value recommended by ASHRAE for good air mixing in the room away from the workstations. The cases with CC/PV system were evaluated as the cases with the highest performance. ADPI in these cases was equal to 91e93%. At all analysed cases points outside requirements were mainly focused in the space between the window and WSs, where the highest temperature and the lowest air velocity values were measured. At TVMV and CCMV cases points at locations behind WSs (no. 2 and 22, Fig. 1) were also marked as “problematic”. The highest number of points exceeding comfort requirements for
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Fig. 3. Air velocity distribution at the level of feet (0.1 m) for the case TVMV. Colour scale presents distribution in 0.05-m/s. steps. Average (avg), minimum (min), maximum (max) and standard deviation (SD) values at this level are presented.
thermal manikins. The influence of the analysed systems on the teq determined for 23 body parts and for the whole body is presented in Fig. 6. Cases CCMV/PV_3 and CC/PV_3 are not presented as the obtained results are equal to the results of the cases CCMV/PV_2 and CC/PV_2 respectively. PV cools mostly the upper body parts (head, neck, chest and back) compared to the other systems. Depending on the supplied airflow rate through the PV the equivalent temperature recorded at the face was from 2.4 K to 14.0 K lower than design room air temperature at WS1 and from 5.0 K to 9.5 K at WS2. The influence on the remaining body parts is similar for all cases except TVMV system, where lower body parts were cooled. Use of the PV resulted in whole body cooling effect up to 0.8 K at the highest airflow from RMP of 21 L/s. Upper body parts are also cooled by the CC when it is in use because of increased radiant heat exchange. At all cases the asymmetry between body side exposed to the warm window and the side exposed to the room was observed. The face and upper arms were the most sensitive body parts for which the teq difference between left and right side ranged from 1.7 K up to 4.2 K. The highest asymmetry (from 1.1 K at lower body parts and forearms to 4.2 K at the face and upper arms) appeared at CCMV and TVMV cases, whilst the lowest (from 0.2 K at lower body parts and forearms to 1.7 K at the face and upper arms) in cases with CC/ PV system. Fig. 7 presents air velocity field at the face measured at 13, 15 and 21 L/s PV airflow rates. The measurements were performed at distance of 40 cm from the PV diffuser. At 13 L/s the air velocity at the central part of the face was at the level of 1.0e1.2 m/s and in the range 1.2e1.5 m/s and 1.7e2.0 m/s at supply airflow rate of 15 and 21 L/s respectively. The core of the jet with velocity above 1.0 m/s covered the whole facial area of the manikin. b) Air quality
Fig. 4. Average air velocity in the occupied space (with minimum and maximum mean values) and maximum draught rate.
effective draft temperature was registered at CCMV system. At this case the ADPI was 82%. Fig. 5 shows operative temperature distribution in the room at height of 0.6 m for a seated person (results at height of 1.1 m for a standing person are analogues). The distribution of air and operative temperature in the room was similar for the studied cases: the differences between air temperature and operative temperature were within measurement uncertainty, i.e. within ±0.2 K. It can be seen from the results in the figure that operative and air temperatures are higher in the part of the room near to the window than in the rest of the room. The maximal horizontal temperature difference was the lowest with the CCMV/PV (1.0e1.2 K at 0.6 m and 1.0e1.1 K at 1.1 m) and CC/PV systems (1.2e1.3 K at 0.6 m and 1.1e1.2 K at 1.1 m). The biggest differences of 1.7 K (0.6 m) and 2.0 K (1.1 m) were recorded for the TVMV. Thermal environment at the WSs was studied in details with Table 3 Air diffusion performance index of the analysed systems in occupied zone based on air velocity and temperature distribution. Case
TVMV CCMV CCMV/PV_1 CCMV/PV_2 CCMV/PV_3 CC/PV_1 CC/PV_2 CC/PV_3
tr ( C)
26.1 26.0 28.1 28.0 28.0 28.0 28.0 28.0
Occupied zone tac ( C)
ADPI (%)
26.1 26.1 28.2 28.1 28.1 28.2 28.2 28.2
86 82 86 89 88 91 93 91
The normalized concentration values at height of 1.7 m (breathing height of a standing person) measured outside WSs at selected locations are presented in Fig. 8a, c, e. The normalized concentration in the air inhaled by the exposed and polluting manikins as well as at point no. 14 at 1.1 m height obtained with the studied systems is compared in Fig. 8b, d, f. Contaminants' distribution patterns outside the WS were similar for all three pollution sources: passive from the wall, exhaled air and bioeffluents. For all three contaminants the highest concentration values were recorded at locations close to sources: at points no. 22 and 24 (Fig. 1). In conditions with PV the personalized air supplied towards the face carried the exhaled air backwards and thus the concentration of pollutant from exhaled air slightly increased behind the “polluting manikin” (point no. 22) compared to TVMV and CCMV systems. Nevertheless, compared to TVMV and CCMV systems the use of PV decreased the bioeffluents' concentration behind WS1 without increasing it in the remaining part of the room. Differences in normalized concentration between the studied systems were larger at WSs than at any other location in the room. The highest values of all pollutants' concentration in the air inhaled at WSs were measured at TVMV and CCMV cases, whereas the use of PV decreased the concentration in the inhaled air by 12.4e84.4%. The decrease in concentration depended mainly on location of the pollution source in the relation to the WS and on PV airflow rate. In CCMV/PV_1 case all three pollutants' concentration in inhaled air was higher at WS1 compared to other cases using PV because of close localizations of polluting sources and the lowest PV airflow. At higher PV airflows at CCMV/PV_2, CCMV/PV_3 and CC/PV cases concentration of all contaminants in the inhaled air was similar at both WSs.
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Fig. 5. Plane distribution of operative temperature at 0.6 m: a, b) design ta 26 C; c, d) design ta 28 C. Colour scale presents distribution in 0.5-K. steps. Average (avg), minimum (min), maximum (max) and standard deviation (SD) values at this level are presented.
Fig. 6. Equivalent temperature determined with thermal manikin at WS1 (results at WS2 are analogous to WS1) for studied cases.
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Fig. 7. Air velocity profiles at the face with the PV airflow rate: a) qPV ¼ 13 L/s, b) qPV ¼ 15 L/s, c) qPV ¼ 21 L/s.
Fig. 8. Normalized concentration of from: a, b e wall, c, d e air exhaled by “polluting manikin”, e, f e bioeffluents from “polluting manikin”.
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Table 4 Contaminant Removal Effectiveness for points outside workstations (εc) and for all points (εcall ) together with local air quality indices at workstations (εcp;i ). Case
TVMV CCMV CCMV/PV_1 CCMV/PV_2 CCMV/PV_3 CC/PV_1 CC/PV_2 CC/PV_3
Wall pollution
Exhaled air
Bioeffluents
εc
εcall
εcp;WS1
εcp;WS2
εc
εcall
εcp;WS1
εcp;WS2
εc
εcall
εcp;WS1
εcp;WS2
0.78 0.80 0.85 0.87 0.88 0.84 0.78 0.83
0.79 0.80 0.92 1.04 1.05 1.00 0.93 0.99
0.79 0.75 0.97 5.68 5.16 4.44 4.38 4.19
0.85 0.85 2.21 5.20 4.72 4.50 4.66 4.60
0.88 0.83 0.85 0.83 0.83 0.81 0.81 0.83
0.84 0.83 0.88 0.99 0.99 0.97 0.98 1.00
0.57 0.79 0.64 5.46 4.68 4.47 5.21 4.25
0.95 0.88 2.42 5.27 4.64 4.57 4.87 4.81
0.83 0.77 0.83 0.86 0.86 0.81 0.79 0.83
0.79 0.74 0.86 1.03 1.02 0.97 0.95 0.99
0.53 0.54 0.69 4.49 4.15 4.20 4.97 4.41
0.94 0.86 2.13 5.26 4.74 4.57 4.83 4.78
Table 4 presents results of Contaminant Removal Effectiveness (εc) and selected local air quality indices (εcp;i ) calculations based on steady state measurements showed in Fig. 8. From the definition these indices represents system's ability to remove contaminants from the space and are influenced by the location of pollution source. Thus, the highest values of εcp;i in the space outside WSs were recorded in points no. 2 and 4 (Fig. 1), which were at the opposite side of the room than contaminants sources, regardless the analysed ventilation system. In overall performance, εc based on values recorded outside WSs was changing in a range from 0.77 to 0.88. At all cases with PV the local air quality indices at WSs were higher than with TVMV and CCMV systems. PV was up to 9.6 times more effective in removing the contaminants from breathing zone than TVMV and CCMV. Such improvement at WSs resulted in higher overall effectiveness (εcall ) with CCMV/PV and CC/PV systems than with TVMV or CCMV. Air distribution is important for reduction of the airborne disease transmission. Fig. 9 presents the volume fraction of rebreathed infected air calculated for the “exposed manikin” at the WS2 and for “people standing” in the room. Results revealed higher εcp;i values at WSs when PV was used which indicate substantial decrease in airborne infection transmission at those locations. The fraction of infectious air decreases below value of 0.07% as a result of PV use. Compared to TVMV the probability to inhale infected air decreased by 35% with the lowest PV airflow rate of 7 L/s (case CCMV/PV_1). Higher PV airflow rates resulted in 60e80% improvement in occupants' protection from cross-infection. The data also show that reducing supplied airflow to the minimum hygienic levels at CCMV case due to use of separate cooling system results in increase of the probability to inhale infectious air by 65% compared to TVMV. Comparing cases with 26 L/s, increased concentration of exhaled contaminants behind “polluting manikin” was observed in cases with PV. Results confirm previous findings that PV air jet increases transportation of exhaled contaminants into the space [10,23,43]. Increase of concentration in points 22 and 24 resulted in up to 0.07% higher rebreathed fraction of infectious air and 0.04% increase in point 14 compared to CCMV. However, most of the time employees spend in the office at WSs, where rebreathed fraction dropped substantially when PV was used. Improvement in the cross-infection protection because of PV is much higher than increase of infectious contaminant concentration outside WSs. In the remaining part of the room differences are negligible. Table 5 presents results of ACE measurements performed for cases with total supply airflow chosen according to hygienic requirements (26 L/s at CCMV, CCMV/PV_1 and CC/PV_1). ACE describes how ventilation system is efficient in delivering fresh air into the breathing level. The higher air quality indices were achieved with PV at WSs than with CCMV. Use of PV resulted in shortening the local age of air at WSs up to 5 times with CC/PV system compared to CCMV. Also at the breathing level for a
Fig. 9. The volume fraction of rebreathed infectious air.
standing person and in the exhaust the local mean age was 2.5e3 times shorter with systems using PV than with CCMV system. All analysed systems provided good level of mixing in the room. The ACE calculated for points outside of WSs was at the similar level of 1.01e1.02 for all cases which correspond to value proper for wellmixed ventilation (ACE ¼ 1). 4. Discussion Air quality at the workstations with personalized ventilation, evaluated based on air quality indices (εcp;i , ACE and cross-infection risk) was substantially improved compared to conditions at workstation in the room with mixing ventilation (TVMV and CCMV systems). The size of the improvement depended on the personalized airflow rate. Records of contaminants' concentration in the inhaled air (Fig. 8b, d, e) show that there are no difference between cases with PV flow rate of 13 L/s and higher. This confirms previous findings [44] that air quality increases with the increase of the personalized airflow rate to a constant maximum value and is not affected by a further increase of the airflow. PV used at workstation improves also local thermal conditions in rooms with high temperature of 28 C, but due to localized air flow it needs to be used reasonably. The upper body parts were cooled down by up to 14 C at flow rate of 21 L/s. Such intensive cooling together with the pressure exerted by the high air velocity at the face (>1.7 m/s) created by this flow rate could be perceived as uncomfortable by high percentage of people. With PV flow rate of 13 L/s at CC/PV_1 case the equivalent temperature recorded at the face was around 18 C and air velocity of 1.0e1.3 m/s. Such conditions should be perceived more refreshing than unpleasant. Further research with human subjects' participation is needed to evaluate
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Table 5 Air-change effectiveness calculations [42]. Point
Exhaust WSs Standing positions 1.7 m
ACE1.7m
CCMV
Exposed manikin Polluting manikin 2 22 4 14 24
CCMV/PV_1
CC/PV_1
Ci,avg (ppm)
ti (min)
Ci,avg (ppm)
ti (min)
Ci,avg (ppm)
ti (min)
720.57 735.61 729.32 724.89 719.76 778.96 764.43 749.62 1.01
76.49 72.25 71.87 71.41 71.71 79.67 77.54 76.97
1820.64 1491.24 1283.59 1728.95 1757.25 1971.69 2087.47 2013.61 1.01
29.93 23.00 19.49 26.65 26.73 30.97 32.63 31.51
1258.38 694.42 638.33 1113.34 1158.41 1290.31 1434.26 1346.67 1.02
25.96 13.85 14.72 22.17 22.78 26.29 29.10 27.42
the acceptable range of operation for PV. Based on recorded contaminants' concentration and equivalent temperature values, the flow rate of 13 L/s may be considered for recommendation of maximum flow supplied by used type of PV diffuser. The quality of environment in the room outside workstations was not affected by the studied system combinations. The air and operative temperature (Table 2, Fig. 5) as well as air velocity patterns (Fig. 2) in the occupied zone out of WSs are similar in all cases. Achieved values of ADPI (Table 3) indicate good level of air mixing regardless of the used system. No substantial differences in air quality conditions were found between analysed ventilation solutions in the room outside WSs. The localization of the pollution source influenced the local increase of contaminants' concentration. It is estimated that in present study the biggest influence on the airflow pattern in the room had the convective plume from the heat sources. WSs (occupants with computers) and solar gains (warm window and heated by the solar radiation floor) were located at one side and created a longitudinal loop flow between walls with window and with door. It resulted not only in a similar distribution of contaminants' in the occupied space, but also of air velocity. The Contaminant Removal Effectiveness index in the occupied zone was at the similar level for all cases. Reduction of airflow rate supplied to the space to the minimum hygienic levels due to use of separate cooling system (CC) results in increase of the cross-infection possibility by 65% comparing to TVMV. Combining chilled ceiling with personalized ventilation system in that case performed better than combination with mixing ventilation. Depending on the type of office work occupants can spend from 40% to 70% of their working time at their desks [45,46]. If occupants spent most of the time at the workstation, the strong protection from airborne transmission thanks to PV use is much more substantial than the slight increase of possibility to inhale infectious air in the background. An important objective of this study was to evaluate whether PV is able to deliver fresh air efficiently not only at the WSs but also in the rest of the occupied zone when it operates as the single ventilation system. Besides the contaminants' distribution in the space an important index in ventilation systems' evaluation is their ability to distribute fresh air within the room. The obtained results demonstrated that CC/PV system distributed fresh air in the room space in a similar way to mixing ventilation system. No differences were found in ACE between CCMV, CCMV/PV and CC/PV outside workstations. It is shown that PV can work as the only ventilation system delivering fresh air to the office room with two workstations. The performance of CC/PV was comparable to results achieved with CC in conjunction with displacement ventilation. Schiavon et al. [15] studied ACE values for the CC combined with displacement ventilation in the office for two occupants depending on the cooling ratio, h, between these combined systems. Reported ACE values at
WSs where slightly higher than values achieved in current study for CC/PV. However, measurements were performed in the steadystate conditions which often are not the same in the real buildings. Halvonova and Melikov [6] showed that displacement ventilation used alone was more vulnerable to the disturbance caused by a walking person than when combined with “ductless” PV. Based on this and taking also into account the total supplied airflow of 26 L/s with CCMV/PV and CC/PV, much smaller than with combined system of CC and displacement ventilation, it may be concluded that CCMV/PV and CC/PV systems can be more effective than combination of CC with displacement ventilation. This study shows that PV is able to create uniform thermal environment and good air quality in the occupied zone when it is combined with CC and even may be superior to the traditional TVMV and CCMV. CC/PV performs the same way in the occupied zone as the remaining system configurations but at the WS environment of higher category than the category in the occupied zone can be achieved. The findings reported by this study were obtained for the office for two occupants with the certain geometry, furniture arrangement and location of heat and pollution sources and are applicable mainly for this type of office. The impact of the furniture arrangement on the performance of different radiant and convective cooling systems were performed by Bolashikov et al. [47] in a similar office as the one used in the presented work. Studied cases included also the TVMV and CCMV systems. It was concluded that the room set-up had negligible influence on the air temperature and velocity distribution in the room. Therefore it may be expected that the room arrangement would not affect the thermal conditions also in a room with CC/PV system. There is a lack of comprehensive data on the contaminant distribution and the cross-infection protection for different room arrangements. Some results were reported by Cermak et al. [48]. They analysed office lay-out where two WSs are localized behind each other. It was shown that the fraction of rebreathed infectious air increases for an occupant not using PV who is seated behind a sick person. Current results also reveal that PV increases the concentration of exhaled pollution at the close location behind the “polluting manikin”. It was also concluded [47] that the strong protection from the cross-infection provided with PV makes the possible increase of concentration in the room not relevant. It is expected that the results are applicable for single or double office rooms, but more research is needed to examine application of the CCPV in larger premises, e.g. open-space offices. An important feature of PV is possibility of individual control of preferred supply airflow. If PV is the only ventilation system in the room, the additional diffuser(s) should be mounted to fulfil the hygienic requirements for fresh air supplied to the room. This additional diffuser(s) should be fully integrated with PV system to control the proper amount of the air supplied to the space when
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occupants would decrease the flow delivered by PV at their workstations. It can be mounted as a part of room equipment, e.g. as a desk partition. The use of raised-floor plenum could be a promising installation solution [49]. It would give PV a flexibility in the room arrangement, especially in the open-space offices. At the same time it can be used by other building services like piping or electrical/ phone cables. However, it has to be noted that research on underfloor air distribution system and its application in practice showed that raised-floor plenum needs to be designed carefully to achieve proper control of supply air temperature [50]. If raised-floor plenum had been used, additional floor diffusers could also easily deliver air needed for hygienic requirement or for cooling purposes in the rooms with a low cooling demand, where conjunction of PV and CC is not needed. In that case PV with integrated floor diffusers, working as a total air system, could be more energy efficient solution for investment than CC/PV. Detailed energy analyses are required to study proposed system implementation. 5. Conclusions Air quality at the workstations is substantially improved and probability infectious air exhaled from sick occupant to be inhaled decreased when PV is used compared to the studied systems without PV. Local cooling provided with PV improves thermal conditions at workstations, especially in rooms with high temperatures. Human subject study is needed to evaluate the acceptable operation range for the personalized cooling. The room air mixing, contaminants' concentration distribution and air-change effectiveness in the occupied space apart of the workstations with PV working alone was at the same level as with MV. Thermal environment apart of the workstations was the same and uniform regardless the studied system. Chilled ceiling combined with PV alone may be applied successfully in practice and be superior to traditional total volume air systems or chilled ceiling combined with mixing/displacement ventilation. Acknowledgements This research was performed and funded at the International Centre for Indoor Environment and Energy. The stay of Aleksandra Lipczynska at the Centre was supported by a scholarship under the “DoktoRIS - Scholarship programme for innovative Silesia” cofinanced by the European Union under the European Social Fund covered by Human Capital Programme (POKL.08.02.02-24-001/13). References [1] O. Seppanen, W. Fisk, M. Mendell, Ventilation rates and health, ASHRAE J. 44 (8) (2002) 56e58. [2] O. Seppanen, W. Fisk, Some quantitative relations between indoor environmental quality and work performance or health, HVAC&R Res. 12 (4) (2006) 957e973. [3] D. Wyon, P. Wargocki, How indoor environment affects performance, ASHRAE J. 55 (3) (2013) 46e52. [4] H. Brohus, P.V. Nielsen, Personal exposure in displacement ventilated rooms, Indoor Air Int. J. Indoor Air Qual. Clim. 6 (3) (1996) 157e167. [5] A. Melikov, G. Pitchurov, K. Naydenov, G. Langkilde, Field study on occupant comfort and the office thermal environment in rooms with displacement ventilation, Indoor Air 15 (3) (2005) 205e214. [6] B. Halvonova, A.K. Melikov, Performance of “ductless” personalized ventilation in conjunction with displacement ventilation: Impact of disturbances due to walking person(s), Build. Environ. 45 (2) (2010) 427e436. [7] A.K. Melikov, Advanced air distribution, Ashrae J. 53 (11) (2011) 73. [8] A. Melikov, Personalized ventilation, Indoor Air 14 (2004) 157e167. [9] J. Kaczmarczyk, A. Melikov, P. Fanger, Human response to personalized ventilation and mixing ventilation, Indoor Air 14 (2004) 17e29.
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Vesely, Performance of ductless personalized ventilation in conjunction with displacement ventilation: physical environment and human response, Build. Environ. 81 (2014) 354e364. [25] EN 15251, Indoor Environmental Input Parameters for Design and Assessment of Energy Performance of Buildings Addressing Indoor Air Quality, Thermal Environment, Lighting and Acoustics, European Committee for Standardization, Brussels, 2007. [26] S. Schiavon, A. Melikov, C. Sekhar, Energy analysis of the personalized ventilation system in hot and humid climates, Energy Build. 42 (5) (2010) 699e707. [27] Z. Bolashikov, L. Nikolaev, A. Melikov, J. Kaczmarczyk, P.O. Fanger, New air terminal devices with high efficiency for personalized ventilation application, in: Proc of Healthy Buildings 2003, 2003 December 4-11, pp. 850e855. Singapore. [28] S. Tanabe, E. Arens, F. Bauman, H. Zhang, T. 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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Thermal environment and air quality in office with personalized ventilation combined with chilled ceiling Aleksandra Lipczynska a, b, *, Jan Kaczmarczyk a, Arsen K. Melikov b a
Department of Heating, Ventilation and Dust Removal Technology, Silesian University of Technology, Konarskiego 20, 44-100 Gliwice, Poland International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Nils Koppels All e Byg. 402, 2800 Kgs. Lyngby, Denmark
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2015 Received in revised form 17 May 2015 Accepted 27 May 2015 Available online 28 May 2015
The thermal environment and air quality conditions provided with combined system of chilled ceiling and personalized ventilation (PV) were studied in a simulated office room for two occupants. The proposed system was compared with total volume HVAC solutions used today, namely mixing ventilation and chilled ceiling combined with mixing ventilation. The objective of the study was to evaluate whether PV can be the only ventilation system in the rooms equipped with chilled ceiling. The room air temperature was 26 C in cases with traditional systems and 28 C when PV was used. PV supplied air with the temperature of 25 C. PV improved thermal conditions and was up to nearly 10 times more efficient in delivering clean air at workstations than mixing ventilation systems, which resulted in strong protection of occupants from the cross-infection. In the room space outside workstations no substantial differences in thermal environment were found between studied systems. The room air mixing with PV working alone was at the same level as with mixing ventilation. No substantial differences in contaminants' concentration distribution and air-change effectiveness were found between the studied systems in the occupied zone outside workstations. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Personalized ventilation Radiant cooling Thermal comfort Contaminant distribution Personal exposure
1. Introduction Thermal environment and indoor air quality in offices affect health, comfort and performance of occupants [1e3]. HVAC systems often consume more than 40% of total building energy needs. Thus creating comfortable conditions in spaces at low energy use becomes an important challenge for HVAC engineers and requires development of new solutions. Centrally controlled total volume ventilation systems (e.g. mixing ventilation or displacement ventilation), which are typically used in offices, aim to create uniform indoor environment in the occupied zone of rooms which cannot satisfy all occupants. In the case of mixing ventilation air is supplied far from occupants and is mixed with polluted room air before reaching people's breathing zone. With the displacement ventilation cool and clean air is supplied at low level, near the floor and is moved upward by the buoyancy flows present in the
˛ ska, Katedra Ogrzewnictwa, Wentylacji i * Corresponding author. Politechnika Sla Techniki Odpylania, ul. Konarskiego 20, 44-100 Gliwice, Poland. E-mail address: [email protected] (A. Lipczynska). http://dx.doi.org/10.1016/j.buildenv.2015.05.035 0360-1323/© 2015 Elsevier Ltd. All rights reserved.
room. Thus air should be cleaner in the lower (occupied) zone. However, in many practical applications the air cleanness is lower than assumed during design process [4,5]. The air cleanness especially decreases when the pollution source is located close to the occupant. In such situation the human boundary layer is transporting contaminated air to the breathing zone. Also movements created by walking people disturb the pollution stratification and decrease the cleanness of the air [6]. Moreover due to risk of draught at the feet its' cooling capacity is limited. The performance of total volume air distribution systems is discussed in Ref. [7]. Promising air distribution is achieved by personalized ventilation. It delivers clean air directly to occupants' breathing zone and provides possibility for individual control of the microenvironment at each workstation [8]. Therefore personalized ventilation has potential to improve inhaled air quality compared to total volume ventilation [9e11]. Previous studies showed also that personalized ventilation decreases intensity of some Sick Building Syndrome symptoms in comparison with mixing ventilation [9,12]. However, due to smaller airflow rates and limitation of the lowest supply air temperature personalized ventilation
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Nomenclature ACE ADPI CC MV PV TV WS q ta tacn teq to tw
Air-Change Effectiveness Air Distribution Performance Index chilled ceiling mixing ventilation personalized ventilation total-volume ventilation workstation flow rate, L/s air temperature, C corrected time-averaged air temperature at n-point, C manikin based equivalent temperature, C operative temperature, C cooling water temperature, C
systems are not able to remove substantial amount of sensible heat load generated in spaces. In such cases additional cooling system is needed. The chilled ceiling is a popular hydronic radiant cooling solution used in different type of buildings. It has higher cooling efficiency than traditional total air systems and is easy to maintain [13]. Radiant ceiling cooling provides “cool head and warm feet” environment which is preferred to “warm head and cool feet” environment often created by total volume air systems. Nevertheless, the chilled ceiling requires installation of additional ventilation system to provide fresh air into spaces. Typically chilled ceiling is installed with mixing ventilation. However, it is suggested that displacement ventilation combined with chilled ceiling has increased cooling capacity and may provide environment with high air quality and acceptable thermal comfort conditions [14e18]. Loveday et al. [19] reported that the stratification flow, typical for displacement ventilation, can be disturbed because of chilled ceiling use. Air cooled under the ceiling creates downward convection flow, which increases the warm and contaminated upper region. Additionally chilled ceiling cools walls due to increased radiant heat exchange, which can increase downward convection flow near walls, causing transport of pollutants from the mixing contaminated region into the supply air and occupied zone [20]. Reduced chilled ceiling temperature increases the air velocity at height of 0.1 m and 1.1 m indicating mixing in the stratified zone, which can cause additional decrease of the air quality [19]. Combined system of chilled ceiling and displacements ventilation was also studied with integrated personalized evaporative cooler [21,22]. Personalized evaporative cooler is installed at workstations and explores the idea of “ductless” personalized ventilation [23,24]: air is drawn from the floor level and supplied toward occupant's face and upper body parts. Difference compared to “ductless” personalized ventilation is that air passes through saturated water sponge in the cooler and is cooled by 3e4 K. This combined system is proposed for hot and dry climate, where problematic are peak loads resulting in oversized HVAC systems. An aim of personalized evaporative cooler is to improve thermal conditions locally at the workstations during peak hours when chilled ceiling combined with displacement ventilation is not capable of maintaining design room temperature. Benefits of the personalized evaporative cooler for occupants are temporary and are focused only on the thermal comfort. Hitherto performed studies have not considered personalized ventilation as the only ventilation system operating in the room when combined with radiant cooling. Personalized ventilation
va εc
tr
ti fn
air velocity, m/s Contaminant Removal Effectiveness time-averaged air temperature in the reference point, C local age of air, min effective draft temperature, C
Subscripts E exhaust exh exhaled i in the i-location inh inhaled n in the n-point S supply
ensures convective cooling to the upper body parts of occupants, which can be controlled according to their individual preferences. Thus combining it with chilled ceiling should be in addition to improved perceived air quality also an effective way to improve thermal comfort in rooms at temperature higher than the upper limit of 26 C recommended in the standards [25]. It has been reported that increasing the maximum allowed air temperature in the room and implementing personalized ventilation system together with mixing ventilation may be an effective energy-saving strategy [26]. It may be expected that under certain operating conditions combining chilled ceiling with personalized ventilation instead of mixing ventilation will also lead to reduction of the energy consumption. Moreover, change in air distribution strategy from total volume air distribution to localized air distribution should bring benefits in improving perceived air quality at workstations. The proposed combined system of chilled ceiling and personalized ventilation is novel and its performance needs to be studied. The objective of the presented research was to identify the thermal environment and air quality conditions in an office room for two occupants equipped with personalized ventilation combined with chilled ceiling and to compare them with room conditions created by solutions used today such as mixing ventilation and combined system of chilled ceiling and mixing ventilation. An important part of the study was to evaluate whether mixing ventilation can be replaced by personalized ventilation as the only ventilation system in the rooms with chilled ceiling considering the environment at the workstation and in the occupied zone. 2. Methods a) Experimental facilities Experiments were performed in a climate chamber (L W H: 4.12 m 4.2 m 2.89 m) arranged as an office with 2 workstations (Fig. 1). The chamber was equipped with total volume mixing ventilation (MV), personalized ventilation (PV) on each desk and with a chilled ceiling (CC). The round movable panel was used as a PV diffuser because of high efficiency in delivering clean air to occupants at low turbulence intensity of the supplied personalised flow [27]. Three linear diffusers mounted at the centre of the ceiling were used for MV. Exhaust diffusers were located in the ceiling corners, as indicated in Fig. 1. Eighteen radiant cooling panels (1.19 m 0.59 m) were built-in the suspended ceiling and covered 75% of its area.
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Fig. 1. Lay-out of the room: plane view with measurement points and reference point (ref).
Two occupants were simulated by thermal manikins dressed in summer clothes (total insulation with chair was 0.59 clo) seated at the workstations (WS1 and WS2) with computers. The manikins were shaped as a 1.7-m tall women. Manikins' body consisted of several segments in order to study the local effect of non-uniform thermal environment. One of the manikins with 23 body segments was located at WS1 and the other manikin with 17 body segments at WS2. The surface temperature of individual body segments was controlled to be the same as the skin temperature of an average person in a state of thermal neutrality [28]. The manikin at WS1, referred in the following as “polluting manikin” was connected to artificial lungs to simulate breathing process. Five water heated panels (0.8 m 1.59 m) simulating heated window were installed on one of the room's walls and five electric foils placed on the floor were used to simulate solar heat gain in the room (Fig. 1). Lighting in the chamber was provided by four office lamps (40 W each). Three pollution sources were simulated in the room. The air exhaled by the “polluting manikin” was marked with sulphur hexafluoride (SF6) to represent infectious air exhaled by a sick person. The tracer gas simulates the transportation of exhaled droplet nuclei smaller than 5e10 mm, which include most of bacteria and viruses [29,30]. The pulmonary ventilation rate was 6 L/ min. The typical breathing frequency for a person in light activity was simulated (2.5 s e inhalation, 2.5 s e exhalation, 1 s e break) [31,32]. Tetrafluoroethane (R134a) was dosed at the armpits of the “polluting manikin” to simulate other human bioeffluents. Additionally a passive contaminant source generated from a surface
(0.8 m 1.2 m) located near to the floor on the wall opposite to the window was simulated. The surface released carbon dioxide (CO2). b) Experimental conditions The performance of four combinations of the installed systems: total volume mixing ventilation (TVMV), chilled ceiling combined with mixing ventilation (CCMV), chilled ceiling combined with mixing ventilation and personalized ventilation (CCMV/PV), and chilled ceiling combined with personalized ventilation (CC/PV) was studied under summer conditions. CCMV/PV and CC/PV systems were studied for three PV airflow configurations. Detailed information on the experimental conditions and system operation settings are listed in Table 1. The air temperature in the office was set to 26 C cases with traditional systems (TVMV and CCMV) and to 28 C when PV was used. Air temperature was used as a design parameter to define supply air temperatures for the MV and PV systems instead of operative temperature as recommended in EN 15251 [25] and EN ISO 7730 [33] because of its feasibility in heat balance calculations. Furthermore, traditional control systems are typically equipped with air temperature sensors, not with operative temperature sensors. The supply air temperature in the case of MV was 16 C. The PV supplied air at 25 C. In order to avoid draught discomfort the difference between the PV supplied air temperature and the room air temperature was set to 3 K. The designed total heat load in the room was 72 W/m2 at 26 C (occupants: 66 W/manikin; computers: 65 W/computer; solar heat
Table 1 Experimental conditions and designed operating parameters. Case
Conditions Design ta ( C)
Total sensible heat load (W/m2)
CC
MV
TVMV CCMV CCMV/PV_1 CCMV/PV_2 CCMV/PV_3 CC/PV_1 CC/PV_2 CC/PV_3
26 26 28 28 28 28 28 28
72 72 66 66 66 66 66 66
PV
qtot (L/s)
tw ( C) (inlet/outlet)
qMV (L/s)
qPV1 (L/s)
qPV2 (L/s)
e 17.2/18.7 20.8/21.9 21.5/22.5 21.1/22.2 18.7/20.3 19.9/21.3 19.4/20.9
82 26 12 12 12 e e e
e e 7 (qP) 15 (qP,inc) 15 (qP,inc) 13 (0.5qBþqP) 21 (0.5qBþqP,inc) 21 (0.5qBþqP,inc)
e e 7 (qP) 15 (qP,inc) 7 (qP) 13 (0.5qBþqP) 21 (0.5qBþqP,inc) 13 (0.5qBþqP)
(qBþqP) (qB) (qB) (qB)
82 26 26 42 34 26 42 34
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load: 654 W; lighting: 160 W) and 66 W/m2 at 28 C (occupants: 53 W/manikin; computers: 65 W/computer; solar heat load: 553 W; lighting: 160 W). The ventilation airflow rates supplied to the room were determined based on calculations of either heat balance or according to recommendations in the standard EN 15251 [25]. The total supply airflow rate, qtot, in the case of TVMV was selected according to thermal balance to keep designed air temperature in the room. Under the remaining conditions the supply airflow rate was calculated as a sum of flow rate needed for removal of lowpollution from building materials, qB (12 L/s e corresponding to 0.7 L/s per m2), and flow rate for removal of pollution from occupants, qP (7 L/s per person) according to the recommendations in EN 15251 [25] for category II. The personalized airflow rate, qPV, was set either at 7 L/s (equal to qP required by standards) or was increased to 15 L/s per person (qP,inc). In cases without MV, i.e. CC/ PV each PV diffuser supplied airflow which was a sum of 0.5qB rate (required by standards for building pollution removal) and airflow rate because of occupancy: qP or qP,inc. In the TVMV case all ceiling supply and exhaust diffusers were used. In the remaining cases only the exhaust 1 was used and the middle supply diffuser in the cases with MV (Fig. 1). The temperature of the cooling water, tw, circulating through CC panels depended on the heat load in each studied case and was set to maintain constant the air temperature in the room. The lowest supply water temperature was set based on heat and moisture balance calculations not to allow for a water vapour condensation at the CC panels surface. Two persons were taken into account as the moisture sources in the calculations. The highest relative humidity of room air did not exceed value of 50%.
evaluation in sedentary activity, but it needs to be used with caution to perform correct assessment [38]. The ADPI is based on air temperature and velocity measurements in the occupied zone, from which the effective draft temperature (fn) is calculated according to equation (2):
fn ¼ tacn tac 8:0ðvan 0:15Þ
(2)
Average test zone temperature (tac) is calculated from tacn values for all measurement points. The time-average value in the reference point (tr) was calculated to obtain the temperature correction factor at each measurement location for any changes in the reference temperature during measurements. ADPI is defined as a percentage of points where effective draft temperature meets criteria: 1.7 C fn 1.1 C. d) Air quality measurements and analyses The tracer gas concentration was measured with two sets of multi-gas sampler and analyser. Concentration measurement was based on the photoacoustic principle. Measurements were performed in 8 points throughout the room, in the air inhaled by the “polluting manikin” and above lips of the “exposed manikin” [39]. Furthermore measurements were also done in the supply and in the exhaust air. Five points (locations no. 2, 4, 14, 22 and 24 in Fig. 1) at height of 1.7 m were selected to represent possible presence of a standing person. Additionally at the location no. 14 gas concentration was measured at heights of 0.1 m, 0.6 m and 1.1 m to identify vertical gas concentration gradient in the room. In order to compare the contaminant distribution between the conditions the tracer gas concentration data were normalized according to the following equation (3):
c) Thermal environment measurements and analyses Thermal environment at the workstations was evaluated with the thermal manikins. Segmental and overall equivalent temperatures, teq, were calculated using the following equation (1):
teq ¼ tsk Qs =hcal
(1)
where tsk is the manikin skin surface temperature ( C), Qs is the manikin dry heat loss (W/m2), hcal is the heat transfer coefficient (W/m2K) obtained in the prior calibration. The equivalent temperature, teq, represents the effect of non-evaporative heat loss from the human body [34]. Measurements of air temperature (ta), globe temperature (tg), air velocity (va) and turbulence intensity were performed at 21 locations in the room, outside of the workstations, as indicated in Fig. 1 (locations 7, 11, 12 and 17 are excluded due to furniture arrangement). The globe temperature was measured by a grey sphere sensor. At each location temperature and air velocity sensors were located at the standardized heights of 0.1 m, 0.6 m, 1.1 m, and 1.7 m [35]. The globe temperature measured at heights of 0.6 and 1.1 m corresponded to the operative temperature (to) [36]. Air velocity and Tu were measured at additional heights of 0.05 m, 0.3 m, 2.0 m and 2.4 m. The temperatures were measured with an accuracy of ±0.2 K. The uncertainty of the air velocity measurements was 0.02 m/s ±1% of readings. All measured results were averaged of 5 min. The draught rate was calculated according to EN ISO 7730 [33]. This factor predicts percentage of people dissatisfied due to cooling effect of air movement. The Air Distribution Performance Index (ADPI) was used to evaluate level of mixing in the room based on thermal conditions. The ADPI is a method suggested by ASHRAE [37] for testing air diffusion performance of mixing ventilation systems under cooling mode. The index can be also an indication for occupants' comfort
Cn:i ¼ ðCi CS Þ=ðCE Cs Þ
(3)
where Ci is concentration measured in the i point of interest, CS is concentration in the supply air and CE is concentration in the exhaust air. The concentration values measured in air supplied by MV and PV were equal. In cases of CCMV/PV system the value obtained in PV supply was used for the calculations. The Contaminant Removal Effectiveness (εc) was calculated according to equation (4) [40] and was used to compare the influence of PV on the contaminants' concentration in the occupied zone.
. εc ¼ CE C i
(4)
where C i is average concentration in the room. The local values of Contaminant Removal Effectiveness (εcp;i ) are calculated by taking the ratio between the measured concentration in the exhaust (CE) and in the i point (Ci). The airborne infection transmission between occupants was determined based on the rebreathed fraction of infectious air [41] by equation (5):
f ¼ ðCinh CS Þ=Cexh
(5)
where f is the volume fraction of the contaminated exhaled air in inhalation, Cinh is concentration measured in the inhaled air, Cexh is concentration of the tracer gas in the exhaled air of the polluting manikin. Additional measurements for cases with total supply airflow of 26 L/s (CCMV, CCMV/PV_1 and CC/PV_1) were performed to investigate the Air-Change Effectiveness (ACE), which represents system's ability to exchange air in the room. ACE was measured by a tracer gas concentration decay test and calculated according to ASHRAE 129 [42]. The following equation (6) was used:
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Table 2 Average measured values of air temperature (bold font) and globe temperature (normal font) in occupied zone for sitting and standing occupancy. Case Avg temperature ( C) Max horizontal difference (K) Vertical difference 1.1 me0.1 m
Vertical difference 1.7 me0.1 m
ACE ¼ tE =ti
0.6 m 1.1 m 0.6 m 1.1 m avg (K) max (K) PD (%) avg (K) max (K) PD (%)
TVMV
CCMV
CCMV/PV_1
CC/PV_1
26.2 ± 0.4/26.4 ± 0.5 26.2 ± 0.4/26.3 ± 0.6 1.4/1.7 1.3/2.0 0.2/0.1 0.5/0.4 0.5/e ¡0.2/0.2 0.2/0.5 0.4/e
26.2 ± 0.3/26.3 ± 0.5 26.2 ± 0.3/26.3 ± 0.4 1.0/1.4 1.1/1.4 0.3/0.0 0.6/0.3 0.5/e 0.2/0.1 0.8/0.4 0.6/e
28.2 ± 0.3/28.2 ± 0.4 28.3 ± 0.3/28.2 ± 0.4 1.0/1.2 0.9/1.1 0.0/0.1 0.2/0.3 0.4/e ¡0.1/0.1 0.4/0.3 0.4/e
28.2 ± 0.3/28.2 ± 0.4 28.2 ± 0.2/28.2 ± 0.3 0.9/1.3 0.8/1.1 0.0/0.1 0.3/0.2 0.4/e 0.0/0.1 0.2/0.2 0.4/e
(6)
where tE is local age of air in the exhaust, ti is average local age of air at breathing level in the occupied zone. Local age of air, ti, is the average time in which air reaches the given i point since air entered the room [40]. Seven sampling points in the occupied zone were used for ACE calculations: two at WSs and five at 1.7 m height for standing person (Fig. 1).
3. Results a) Thermal environment Results of the measured thermal parameters in the occupied zone outside of WSs revealed only small differences within the studied system combinations. Therefore only results of selected cases for each system are presented in the following. The effect of the radiant cooling on the globe temperature was small. Differences between air temperature and globe temperature in most measured points were within the accuracy of sensors. Only with TVMV system at height of 1.7 m and with CCMV at 0.1 m the globe temperature was up to 0.9 K higher than the air temperature. Vertical air temperature differences between head and ankle levels for seated and standing persons are presented in Table 2. In all cases the differences were under the allowed maximum of 2 K for category A thermal environment [33]. The highest values, 0.5e0.6 K, for a seated person appeared with TVMV and CCMV. The corresponding maximum percentage of dissatisfied was 0.5% [33]. For a standing person the highest value of 0.8 K (0.6% dissatisfied) was recorded with the CCMV. In all measurement locations the vertical distribution of air and globe temperature met uniformity criteria according to ISO 7726 [35]. The deviations of the values measured in each height from the average value of the four heights at the location were within criteria of ±1.5 K regardless of studied system configuration. Based on this result the average room temperature values were calculated from the readings at the abdomen level (Table 2). Both for a seated and standing person the average room air temperature in the room was 0.2e0.3 K higher than the set point of 26 C or 28 C. Vertical profiles of average air velocity, calculated from the mean velocity measured at all locations at the same height, were similar in shape for all cases (Fig. 2). Differences between conditions with the same HVAC system configuration, CCMV/PV or CC/ PV, were within ±0.01 m/s. The highest values were characteristic for the level of 0.05e0.1 m. The velocity decreased with height to the level of 1.7 m where it started to increase. Among all studied systems, TVMV operating at the highest supply air flow of 82 L/s generated the highest air velocity values in the room. For remaining cases the supply flow ranged from 26 L/s to 42 L/s. In all studied cases the highest air velocity was typically
Fig. 2. Vertical profile of average air velocity (calculated based on readings at each height).
measured at the height of feet (0.05e0.1 m) in the area between WSs and wall with door. This was especially evident in the case with TVMV, as shown in Fig. 3. Smoke visualizations revealed a strong longitudinal-loop flow between walls with window and with door, which was generated by the buoyancy flow from the heat sources concentrated next to the windows. In the area between WSs and door no obstacles were located, e.g. partitions, furniture or lamps, which could block the loop flow and decrease the air velocity. The highest values of the draught rate (Fig. 4) in the occupied zone appeared in the reference cases TVMV and CCMV, where airflows supplied through MV were higher than when PV was in use (Table 1). In those cases draught rate was within range of 13.9e21.7%. In cases with CCMV/PV the risk of draught sensation was much lower, 5.1e10.7%, and with CC/PV 2.9e13.3%. Table 3 present results of ADPI analyses performed according to ASHRAE 113 [37]. The average temperature in the test zone was equal at TVMV and CCMV cases. The difference in tac between all cases with systems using PV, CCMV/PV and CC/PV, was 0.1 K, i.e. lower than the measurement accuracy. The differences between tr and tac did not exceed 0.2 K. For all analysed systems the ADPI was above 80%, i.e. above the minimum ADPI value recommended by ASHRAE for good air mixing in the room away from the workstations. The cases with CC/PV system were evaluated as the cases with the highest performance. ADPI in these cases was equal to 91e93%. At all analysed cases points outside requirements were mainly focused in the space between the window and WSs, where the highest temperature and the lowest air velocity values were measured. At TVMV and CCMV cases points at locations behind WSs (no. 2 and 22, Fig. 1) were also marked as “problematic”. The highest number of points exceeding comfort requirements for
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Fig. 3. Air velocity distribution at the level of feet (0.1 m) for the case TVMV. Colour scale presents distribution in 0.05-m/s. steps. Average (avg), minimum (min), maximum (max) and standard deviation (SD) values at this level are presented.
thermal manikins. The influence of the analysed systems on the teq determined for 23 body parts and for the whole body is presented in Fig. 6. Cases CCMV/PV_3 and CC/PV_3 are not presented as the obtained results are equal to the results of the cases CCMV/PV_2 and CC/PV_2 respectively. PV cools mostly the upper body parts (head, neck, chest and back) compared to the other systems. Depending on the supplied airflow rate through the PV the equivalent temperature recorded at the face was from 2.4 K to 14.0 K lower than design room air temperature at WS1 and from 5.0 K to 9.5 K at WS2. The influence on the remaining body parts is similar for all cases except TVMV system, where lower body parts were cooled. Use of the PV resulted in whole body cooling effect up to 0.8 K at the highest airflow from RMP of 21 L/s. Upper body parts are also cooled by the CC when it is in use because of increased radiant heat exchange. At all cases the asymmetry between body side exposed to the warm window and the side exposed to the room was observed. The face and upper arms were the most sensitive body parts for which the teq difference between left and right side ranged from 1.7 K up to 4.2 K. The highest asymmetry (from 1.1 K at lower body parts and forearms to 4.2 K at the face and upper arms) appeared at CCMV and TVMV cases, whilst the lowest (from 0.2 K at lower body parts and forearms to 1.7 K at the face and upper arms) in cases with CC/ PV system. Fig. 7 presents air velocity field at the face measured at 13, 15 and 21 L/s PV airflow rates. The measurements were performed at distance of 40 cm from the PV diffuser. At 13 L/s the air velocity at the central part of the face was at the level of 1.0e1.2 m/s and in the range 1.2e1.5 m/s and 1.7e2.0 m/s at supply airflow rate of 15 and 21 L/s respectively. The core of the jet with velocity above 1.0 m/s covered the whole facial area of the manikin. b) Air quality
Fig. 4. Average air velocity in the occupied space (with minimum and maximum mean values) and maximum draught rate.
effective draft temperature was registered at CCMV system. At this case the ADPI was 82%. Fig. 5 shows operative temperature distribution in the room at height of 0.6 m for a seated person (results at height of 1.1 m for a standing person are analogues). The distribution of air and operative temperature in the room was similar for the studied cases: the differences between air temperature and operative temperature were within measurement uncertainty, i.e. within ±0.2 K. It can be seen from the results in the figure that operative and air temperatures are higher in the part of the room near to the window than in the rest of the room. The maximal horizontal temperature difference was the lowest with the CCMV/PV (1.0e1.2 K at 0.6 m and 1.0e1.1 K at 1.1 m) and CC/PV systems (1.2e1.3 K at 0.6 m and 1.1e1.2 K at 1.1 m). The biggest differences of 1.7 K (0.6 m) and 2.0 K (1.1 m) were recorded for the TVMV. Thermal environment at the WSs was studied in details with Table 3 Air diffusion performance index of the analysed systems in occupied zone based on air velocity and temperature distribution. Case
TVMV CCMV CCMV/PV_1 CCMV/PV_2 CCMV/PV_3 CC/PV_1 CC/PV_2 CC/PV_3
tr ( C)
26.1 26.0 28.1 28.0 28.0 28.0 28.0 28.0
Occupied zone tac ( C)
ADPI (%)
26.1 26.1 28.2 28.1 28.1 28.2 28.2 28.2
86 82 86 89 88 91 93 91
The normalized concentration values at height of 1.7 m (breathing height of a standing person) measured outside WSs at selected locations are presented in Fig. 8a, c, e. The normalized concentration in the air inhaled by the exposed and polluting manikins as well as at point no. 14 at 1.1 m height obtained with the studied systems is compared in Fig. 8b, d, f. Contaminants' distribution patterns outside the WS were similar for all three pollution sources: passive from the wall, exhaled air and bioeffluents. For all three contaminants the highest concentration values were recorded at locations close to sources: at points no. 22 and 24 (Fig. 1). In conditions with PV the personalized air supplied towards the face carried the exhaled air backwards and thus the concentration of pollutant from exhaled air slightly increased behind the “polluting manikin” (point no. 22) compared to TVMV and CCMV systems. Nevertheless, compared to TVMV and CCMV systems the use of PV decreased the bioeffluents' concentration behind WS1 without increasing it in the remaining part of the room. Differences in normalized concentration between the studied systems were larger at WSs than at any other location in the room. The highest values of all pollutants' concentration in the air inhaled at WSs were measured at TVMV and CCMV cases, whereas the use of PV decreased the concentration in the inhaled air by 12.4e84.4%. The decrease in concentration depended mainly on location of the pollution source in the relation to the WS and on PV airflow rate. In CCMV/PV_1 case all three pollutants' concentration in inhaled air was higher at WS1 compared to other cases using PV because of close localizations of polluting sources and the lowest PV airflow. At higher PV airflows at CCMV/PV_2, CCMV/PV_3 and CC/PV cases concentration of all contaminants in the inhaled air was similar at both WSs.
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Fig. 5. Plane distribution of operative temperature at 0.6 m: a, b) design ta 26 C; c, d) design ta 28 C. Colour scale presents distribution in 0.5-K. steps. Average (avg), minimum (min), maximum (max) and standard deviation (SD) values at this level are presented.
Fig. 6. Equivalent temperature determined with thermal manikin at WS1 (results at WS2 are analogous to WS1) for studied cases.
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Fig. 7. Air velocity profiles at the face with the PV airflow rate: a) qPV ¼ 13 L/s, b) qPV ¼ 15 L/s, c) qPV ¼ 21 L/s.
Fig. 8. Normalized concentration of from: a, b e wall, c, d e air exhaled by “polluting manikin”, e, f e bioeffluents from “polluting manikin”.
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Table 4 Contaminant Removal Effectiveness for points outside workstations (εc) and for all points (εcall ) together with local air quality indices at workstations (εcp;i ). Case
TVMV CCMV CCMV/PV_1 CCMV/PV_2 CCMV/PV_3 CC/PV_1 CC/PV_2 CC/PV_3
Wall pollution
Exhaled air
Bioeffluents
εc
εcall
εcp;WS1
εcp;WS2
εc
εcall
εcp;WS1
εcp;WS2
εc
εcall
εcp;WS1
εcp;WS2
0.78 0.80 0.85 0.87 0.88 0.84 0.78 0.83
0.79 0.80 0.92 1.04 1.05 1.00 0.93 0.99
0.79 0.75 0.97 5.68 5.16 4.44 4.38 4.19
0.85 0.85 2.21 5.20 4.72 4.50 4.66 4.60
0.88 0.83 0.85 0.83 0.83 0.81 0.81 0.83
0.84 0.83 0.88 0.99 0.99 0.97 0.98 1.00
0.57 0.79 0.64 5.46 4.68 4.47 5.21 4.25
0.95 0.88 2.42 5.27 4.64 4.57 4.87 4.81
0.83 0.77 0.83 0.86 0.86 0.81 0.79 0.83
0.79 0.74 0.86 1.03 1.02 0.97 0.95 0.99
0.53 0.54 0.69 4.49 4.15 4.20 4.97 4.41
0.94 0.86 2.13 5.26 4.74 4.57 4.83 4.78
Table 4 presents results of Contaminant Removal Effectiveness (εc) and selected local air quality indices (εcp;i ) calculations based on steady state measurements showed in Fig. 8. From the definition these indices represents system's ability to remove contaminants from the space and are influenced by the location of pollution source. Thus, the highest values of εcp;i in the space outside WSs were recorded in points no. 2 and 4 (Fig. 1), which were at the opposite side of the room than contaminants sources, regardless the analysed ventilation system. In overall performance, εc based on values recorded outside WSs was changing in a range from 0.77 to 0.88. At all cases with PV the local air quality indices at WSs were higher than with TVMV and CCMV systems. PV was up to 9.6 times more effective in removing the contaminants from breathing zone than TVMV and CCMV. Such improvement at WSs resulted in higher overall effectiveness (εcall ) with CCMV/PV and CC/PV systems than with TVMV or CCMV. Air distribution is important for reduction of the airborne disease transmission. Fig. 9 presents the volume fraction of rebreathed infected air calculated for the “exposed manikin” at the WS2 and for “people standing” in the room. Results revealed higher εcp;i values at WSs when PV was used which indicate substantial decrease in airborne infection transmission at those locations. The fraction of infectious air decreases below value of 0.07% as a result of PV use. Compared to TVMV the probability to inhale infected air decreased by 35% with the lowest PV airflow rate of 7 L/s (case CCMV/PV_1). Higher PV airflow rates resulted in 60e80% improvement in occupants' protection from cross-infection. The data also show that reducing supplied airflow to the minimum hygienic levels at CCMV case due to use of separate cooling system results in increase of the probability to inhale infectious air by 65% compared to TVMV. Comparing cases with 26 L/s, increased concentration of exhaled contaminants behind “polluting manikin” was observed in cases with PV. Results confirm previous findings that PV air jet increases transportation of exhaled contaminants into the space [10,23,43]. Increase of concentration in points 22 and 24 resulted in up to 0.07% higher rebreathed fraction of infectious air and 0.04% increase in point 14 compared to CCMV. However, most of the time employees spend in the office at WSs, where rebreathed fraction dropped substantially when PV was used. Improvement in the cross-infection protection because of PV is much higher than increase of infectious contaminant concentration outside WSs. In the remaining part of the room differences are negligible. Table 5 presents results of ACE measurements performed for cases with total supply airflow chosen according to hygienic requirements (26 L/s at CCMV, CCMV/PV_1 and CC/PV_1). ACE describes how ventilation system is efficient in delivering fresh air into the breathing level. The higher air quality indices were achieved with PV at WSs than with CCMV. Use of PV resulted in shortening the local age of air at WSs up to 5 times with CC/PV system compared to CCMV. Also at the breathing level for a
Fig. 9. The volume fraction of rebreathed infectious air.
standing person and in the exhaust the local mean age was 2.5e3 times shorter with systems using PV than with CCMV system. All analysed systems provided good level of mixing in the room. The ACE calculated for points outside of WSs was at the similar level of 1.01e1.02 for all cases which correspond to value proper for wellmixed ventilation (ACE ¼ 1). 4. Discussion Air quality at the workstations with personalized ventilation, evaluated based on air quality indices (εcp;i , ACE and cross-infection risk) was substantially improved compared to conditions at workstation in the room with mixing ventilation (TVMV and CCMV systems). The size of the improvement depended on the personalized airflow rate. Records of contaminants' concentration in the inhaled air (Fig. 8b, d, e) show that there are no difference between cases with PV flow rate of 13 L/s and higher. This confirms previous findings [44] that air quality increases with the increase of the personalized airflow rate to a constant maximum value and is not affected by a further increase of the airflow. PV used at workstation improves also local thermal conditions in rooms with high temperature of 28 C, but due to localized air flow it needs to be used reasonably. The upper body parts were cooled down by up to 14 C at flow rate of 21 L/s. Such intensive cooling together with the pressure exerted by the high air velocity at the face (>1.7 m/s) created by this flow rate could be perceived as uncomfortable by high percentage of people. With PV flow rate of 13 L/s at CC/PV_1 case the equivalent temperature recorded at the face was around 18 C and air velocity of 1.0e1.3 m/s. Such conditions should be perceived more refreshing than unpleasant. Further research with human subjects' participation is needed to evaluate
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Table 5 Air-change effectiveness calculations [42]. Point
Exhaust WSs Standing positions 1.7 m
ACE1.7m
CCMV
Exposed manikin Polluting manikin 2 22 4 14 24
CCMV/PV_1
CC/PV_1
Ci,avg (ppm)
ti (min)
Ci,avg (ppm)
ti (min)
Ci,avg (ppm)
ti (min)
720.57 735.61 729.32 724.89 719.76 778.96 764.43 749.62 1.01
76.49 72.25 71.87 71.41 71.71 79.67 77.54 76.97
1820.64 1491.24 1283.59 1728.95 1757.25 1971.69 2087.47 2013.61 1.01
29.93 23.00 19.49 26.65 26.73 30.97 32.63 31.51
1258.38 694.42 638.33 1113.34 1158.41 1290.31 1434.26 1346.67 1.02
25.96 13.85 14.72 22.17 22.78 26.29 29.10 27.42
the acceptable range of operation for PV. Based on recorded contaminants' concentration and equivalent temperature values, the flow rate of 13 L/s may be considered for recommendation of maximum flow supplied by used type of PV diffuser. The quality of environment in the room outside workstations was not affected by the studied system combinations. The air and operative temperature (Table 2, Fig. 5) as well as air velocity patterns (Fig. 2) in the occupied zone out of WSs are similar in all cases. Achieved values of ADPI (Table 3) indicate good level of air mixing regardless of the used system. No substantial differences in air quality conditions were found between analysed ventilation solutions in the room outside WSs. The localization of the pollution source influenced the local increase of contaminants' concentration. It is estimated that in present study the biggest influence on the airflow pattern in the room had the convective plume from the heat sources. WSs (occupants with computers) and solar gains (warm window and heated by the solar radiation floor) were located at one side and created a longitudinal loop flow between walls with window and with door. It resulted not only in a similar distribution of contaminants' in the occupied space, but also of air velocity. The Contaminant Removal Effectiveness index in the occupied zone was at the similar level for all cases. Reduction of airflow rate supplied to the space to the minimum hygienic levels due to use of separate cooling system (CC) results in increase of the cross-infection possibility by 65% comparing to TVMV. Combining chilled ceiling with personalized ventilation system in that case performed better than combination with mixing ventilation. Depending on the type of office work occupants can spend from 40% to 70% of their working time at their desks [45,46]. If occupants spent most of the time at the workstation, the strong protection from airborne transmission thanks to PV use is much more substantial than the slight increase of possibility to inhale infectious air in the background. An important objective of this study was to evaluate whether PV is able to deliver fresh air efficiently not only at the WSs but also in the rest of the occupied zone when it operates as the single ventilation system. Besides the contaminants' distribution in the space an important index in ventilation systems' evaluation is their ability to distribute fresh air within the room. The obtained results demonstrated that CC/PV system distributed fresh air in the room space in a similar way to mixing ventilation system. No differences were found in ACE between CCMV, CCMV/PV and CC/PV outside workstations. It is shown that PV can work as the only ventilation system delivering fresh air to the office room with two workstations. The performance of CC/PV was comparable to results achieved with CC in conjunction with displacement ventilation. Schiavon et al. [15] studied ACE values for the CC combined with displacement ventilation in the office for two occupants depending on the cooling ratio, h, between these combined systems. Reported ACE values at
WSs where slightly higher than values achieved in current study for CC/PV. However, measurements were performed in the steadystate conditions which often are not the same in the real buildings. Halvonova and Melikov [6] showed that displacement ventilation used alone was more vulnerable to the disturbance caused by a walking person than when combined with “ductless” PV. Based on this and taking also into account the total supplied airflow of 26 L/s with CCMV/PV and CC/PV, much smaller than with combined system of CC and displacement ventilation, it may be concluded that CCMV/PV and CC/PV systems can be more effective than combination of CC with displacement ventilation. This study shows that PV is able to create uniform thermal environment and good air quality in the occupied zone when it is combined with CC and even may be superior to the traditional TVMV and CCMV. CC/PV performs the same way in the occupied zone as the remaining system configurations but at the WS environment of higher category than the category in the occupied zone can be achieved. The findings reported by this study were obtained for the office for two occupants with the certain geometry, furniture arrangement and location of heat and pollution sources and are applicable mainly for this type of office. The impact of the furniture arrangement on the performance of different radiant and convective cooling systems were performed by Bolashikov et al. [47] in a similar office as the one used in the presented work. Studied cases included also the TVMV and CCMV systems. It was concluded that the room set-up had negligible influence on the air temperature and velocity distribution in the room. Therefore it may be expected that the room arrangement would not affect the thermal conditions also in a room with CC/PV system. There is a lack of comprehensive data on the contaminant distribution and the cross-infection protection for different room arrangements. Some results were reported by Cermak et al. [48]. They analysed office lay-out where two WSs are localized behind each other. It was shown that the fraction of rebreathed infectious air increases for an occupant not using PV who is seated behind a sick person. Current results also reveal that PV increases the concentration of exhaled pollution at the close location behind the “polluting manikin”. It was also concluded [47] that the strong protection from the cross-infection provided with PV makes the possible increase of concentration in the room not relevant. It is expected that the results are applicable for single or double office rooms, but more research is needed to examine application of the CCPV in larger premises, e.g. open-space offices. An important feature of PV is possibility of individual control of preferred supply airflow. If PV is the only ventilation system in the room, the additional diffuser(s) should be mounted to fulfil the hygienic requirements for fresh air supplied to the room. This additional diffuser(s) should be fully integrated with PV system to control the proper amount of the air supplied to the space when
A. Lipczynska et al. / Building and Environment 92 (2015) 603e614
occupants would decrease the flow delivered by PV at their workstations. It can be mounted as a part of room equipment, e.g. as a desk partition. The use of raised-floor plenum could be a promising installation solution [49]. It would give PV a flexibility in the room arrangement, especially in the open-space offices. At the same time it can be used by other building services like piping or electrical/ phone cables. However, it has to be noted that research on underfloor air distribution system and its application in practice showed that raised-floor plenum needs to be designed carefully to achieve proper control of supply air temperature [50]. If raised-floor plenum had been used, additional floor diffusers could also easily deliver air needed for hygienic requirement or for cooling purposes in the rooms with a low cooling demand, where conjunction of PV and CC is not needed. In that case PV with integrated floor diffusers, working as a total air system, could be more energy efficient solution for investment than CC/PV. Detailed energy analyses are required to study proposed system implementation. 5. Conclusions Air quality at the workstations is substantially improved and probability infectious air exhaled from sick occupant to be inhaled decreased when PV is used compared to the studied systems without PV. Local cooling provided with PV improves thermal conditions at workstations, especially in rooms with high temperatures. Human subject study is needed to evaluate the acceptable operation range for the personalized cooling. The room air mixing, contaminants' concentration distribution and air-change effectiveness in the occupied space apart of the workstations with PV working alone was at the same level as with MV. Thermal environment apart of the workstations was the same and uniform regardless the studied system. Chilled ceiling combined with PV alone may be applied successfully in practice and be superior to traditional total volume air systems or chilled ceiling combined with mixing/displacement ventilation. Acknowledgements This research was performed and funded at the International Centre for Indoor Environment and Energy. The stay of Aleksandra Lipczynska at the Centre was supported by a scholarship under the “DoktoRIS - Scholarship programme for innovative Silesia” cofinanced by the European Union under the European Social Fund covered by Human Capital Programme (POKL.08.02.02-24-001/13). References [1] O. Seppanen, W. Fisk, M. Mendell, Ventilation rates and health, ASHRAE J. 44 (8) (2002) 56e58. [2] O. Seppanen, W. Fisk, Some quantitative relations between indoor environmental quality and work performance or health, HVAC&R Res. 12 (4) (2006) 957e973. [3] D. Wyon, P. Wargocki, How indoor environment affects performance, ASHRAE J. 55 (3) (2013) 46e52. [4] H. Brohus, P.V. Nielsen, Personal exposure in displacement ventilated rooms, Indoor Air Int. J. Indoor Air Qual. Clim. 6 (3) (1996) 157e167. [5] A. Melikov, G. Pitchurov, K. Naydenov, G. Langkilde, Field study on occupant comfort and the office thermal environment in rooms with displacement ventilation, Indoor Air 15 (3) (2005) 205e214. [6] B. Halvonova, A.K. Melikov, Performance of “ductless” personalized ventilation in conjunction with displacement ventilation: Impact of disturbances due to walking person(s), Build. Environ. 45 (2) (2010) 427e436. [7] A.K. Melikov, Advanced air distribution, Ashrae J. 53 (11) (2011) 73. [8] A. Melikov, Personalized ventilation, Indoor Air 14 (2004) 157e167. [9] J. Kaczmarczyk, A. Melikov, P. Fanger, Human response to personalized ventilation and mixing ventilation, Indoor Air 14 (2004) 17e29.
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