1、 Harald Andersson is a PhD student in the Department of Building, Energy and Environmental Technologies, University of Gvle, Sweden. Mathias Cehlin is Assistant Professor in the Department of Building, Energy and Environmental Technologies, University of Gvle, Sweden. Bahram Moshfegh is Professor in
2、 the Department of Building, Energy and Environmental Technologies, University of Gvle, Sweden. Energy-Saving Measures in a Classroom Using Low Pressure Drop Ceiling Supply Device A Field Study Harald Andersson Mathias Cehlin, PhD Bahram Moshfegh, PhD ABSTRACT Between 1990 and 2006 the energy use by
3、 ventilation systems in Swedish schools doubled. This is explained by high airflows in schools because of the high occupant density. Studies show that 87% of Swedish schools use constant air volume (CAV), and it is estimated that a change to variable air volume (VAV) could save 0.12-0.33 TWh (4.1*10
4、12 - 1.1*1013 Btu) per year. Therefore the aim of this study is to investigate whether it is possible to replace displacement ventilation (DV) with mixing ventilation (MV) to create a comfortable indoor climate in a typical classroom and at the same time decrease the energy use by using VAV and Low
5、Pressure Drop Ceiling Supply Device (LPDCSD). The study used two LPDCSDs which consist of circular channels with 190/228 round jets placed in an interlocking pattern, with a horizontal one/two-way-direction. The field study was carried out in a school which is intended to be extensively renovated. T
6、he school currently has DV and CAV. The study was carried out by installing MV with LPDCSD in one of the typical classrooms. Several different air-flow rates were investigated using tracer-gas technology to measure the local mean age of the air in the occupied zone. Simultaneously, thermal comfort a
7、nd vertical temperature gradients were measured in the room. The results show nearly uniform distribution of the local mean age of air in the occupied zone, even in the cases of relatively low air-flow rates. Since the mixing of air is more or less the same in the entire occupied zone VAV can be use
8、d to reduce air-flow rate based on the desired CO2-level. Because of the number of students in each classroom and the fact that changes in air-flow rates have no significant effect on the degree of mixing, it is possible to reduce the air-flow rates for extended periods of time. Finally, since the L
9、PDCSD has a lower pressure-drop than the currently used supply devices and it is possible to use VAV to lower the airflows in cases with reduced heat loads, it is possible to significantly reduce the energy usage in the school while maintaining the IAQ, increasing the thermal comfort and the availab
10、le floor area of the occupied zone. INTRODUCTION The results from a government survey regarding energy use in Swedish public buildings show that energy use by ventilation systems in Swedish schools doubled from 1990 - 2006. The electrical power used by ventilation fans in Swedish schools has gone fr
11、om 11 kWh/m2 (3500 Btu/ft2) to 23 kWh/m2 (7300 Btu/ft2). This is on average 27% of the total electrical power usage in Swedish schools.This is explained by high airflows in schools because of the high occupant density. Most Swedish schools have mechanical ventilation (90%) and 75% of these systems h
12、ave heat recovery. The survey show that 87% of Swedish schools use constant air volume (CAV) and it is estimated that a change to variable air volume (VAV) could save 0.12-0.33 TWh (4.1*1012-1.1*1013 Btu) per year. (Energimyndigheten, 2010) The school in this case study is owned by the municipal hou
13、sing company and is about to be extensively renovated and retrofitted. The municipal housing company has the ambition to lower energy use and environmental impact of all their buildings. The school currently has wall-mounted displacement ventilation (DV) connected to a CAV system with heat recovery.
14、 Because of complaints from teachers and students about the thermal comfort in the occupied zone close to the supply devices, the housing company wants to change from wall-mounted DV to ceiling-mounted mixing ventilation (MV), even though DV normally has higher ventilation efficiency (Cao .et al 201
15、3). Therefore the aim of this study is to investigate experimentally whether it is possible to replace DV with MV to create a comfortable indoor climate in a typical classroom and at the same time decrease the energy use by using VAV and Low Pressure Drop Ceiling Supply Device (LPDCSD). EXPERIMENTAL
16、 SET-UP AND PROCEDURE This field measurement was performed in a classroom at a local school in the city of Gvle in Sweden. The school currently has DV and CAV in all of its classrooms. The classrooms dimensions are 8.0 7.5 3.0 m (26.2 24.6 9.8 ft) and are equipped with two wall-mounted DV supply dev
17、ices. The study was carried out by installing MV with LPDCSD in one of the typical classrooms and five test cases were conducted with different set-ups, see Table 1. Figure 1 LPDCSD and DV supply devices The different set-ups vary in supply devices, heat loads and airflow rates. Case 1 aims to simul
18、ate the current situation at school which consists of two wall-mounted DV supply devices with a total airflow of 300 l/s (636 cfm). Cases 2-5 use two LPDCSDs which consist of circular channels with 190/228 round jets placed in an interlocking pattern, with a horizontal one/two-way direction. Case 2
19、has similar airflow rate as Case 1 in order to make a comparison between the two systems, while Cases 3-5 use airflows based on 5 l/s (10.6 cfm) per person plus 0.6 l/s (1.3 cfm) per m2 floor area (ASHRAE, 2007). The set-ups also included person-simulators (manikins) with a thermal power of 95 W (32
20、0 Btu/h) each to simulate a person with a Met value of 1.0. Cases 1-3 used 28 manikins to simulate a full classroom, while Case 4 used 22 manikins and Case 5 used 16 manikins to simulate smaller classes. The classroom was illuminated by seven pairs of fluorescent lights, consuming a total of 14 36 W
21、 (14 120 Btu/h) electrical power and the test equipment inside the classroom is estimated to consume 150 W (510 Btu/h) of electrical power. During each case IAQ, vertical temperature gradient and thermal comfort were measured. Table 1. Case Set-up Case Supply device Manikins Airflow l/s (cfm) Heatlo
22、ad W (Btu/h) Ts C (F) Case 1 DV 28 299 (634) 3300 (11260) 20.8 (69.4) Case 2 LPDCSD 28 312 (660) 3300 (11260) 20.8 (69.4) Case 3 LPDCSD 28 181 (383) 3300 (11260) 20.6 (69.1) Case 4 LPDCSD 22 156 (330) 2700 (9212) 20.7 (69.3) Case 5 LPDCSD 16 133 (281) 2200 (7500) 20.4 (68.7) Tracer gas decay method
23、with SF6 was used to measure the total airflow as well as the variation of the local mean age of air (p) throughout the breathing zone (BZ) (Sandberg 1983). Figure 2 show the experimental set-up of the classroom where the red points indicate the measuring points for tracer gas. Tracer gas measuring
24、points (TGMP) 1-5 are all located at a height of 1.2 m and are in the BZ. TGMP6 is located at the outlet at a height of 2.6 m in order to measure the nominal time constant (n). Figure 2 Experimental set-up: eith TMPs A-K in blue, TGMPs 1-6 in red and manikins in black. The local mean age of air (p)
25、and nominal time constant (n) were used to calculate the local air change effectiveness (i): i = np (1) The CO2-concentraition (Cs) was estimated using the following equation (assuming steady-state and fully mixing condition): The airflow per person (V0) is based on the number of manikins and the to
26、tal airflow through the classroom as measured in TGMP6. It is assumed that the outside air has a CO2 concentration (Co) of 400 PPM and that each person would generate 0.0052 l/s (0.011 cfm) CO2 (N) at a Met value of 1.0 (ASHRAE, 2007). The vertical temperature gradient was measured using eight therm
27、ocouples placed at heights of 0.1, 0.5, 0.9, 1.3, 1.7, 2.1, 2.5 and 2.9 m (0.3, 1.6, 3.0, 4.3, 5.6, 6.9, 8.2 and 9.5 ft). The temperature was measured every second and logged for 20 minutes in each of the 11 thermal measure points (TMPs) A-K which are marked as blue points in Figure 2. The average v
28、alue of measurements from the occupied zone (0.1-1.7 m (0.3-5.6 ft) (Tp) was used to calculate the efficiency of heat removal (t) with the following equation: = (3) Where the temperature of the exhaust air (Te) was logged using a temperature logger and the supply temperature (Ts) was logged from the
29、 ventilation system. t is similar to a heat exchanger effectiveness and is a measure of the heat removing ability of the system. (Awbi, Gan, 1993) The thermal comfort was measured in the same TMPs A-K at four heights for each TMP. The heights 0.1, 0.6, 1.1 and 1.7 m (0.3, 2.0, 3.6, and 5.6 ft) were
30、chosen to represent the height of ankle, waist and neck for both sitting and standing persons. Temperature transducers and Constant Temperature Anemometer (CTA) probes were used to measure thermal comfort, air velocity air temperature and draught rating. The PMV and PPD values were measured directly
31、 based on dry heat loss and assuming a Met of 1.0 and a Clo of 0.8 to correspond to a sitting person with spring clothing. Measurements was taken every five seconds and logged for 10 minutes for each and every height for all 11 TMPs. The mean values for air temperature at heights 0.1 and 1.7 m (0.3
32、and 5.6 ft) were used to calculate the vertical air temperature difference between ankle and neck for a standing person (T). All measurements were started two hours after the ventilation and the manikins had been started in order to facilitate steady-state conditions. All measurements were taken dur
33、ing April and May 2015. The average outside temperature varied during the five different days from 8 to 11 C (46 to 52 F). = + 0(2) RESULTS Figure 3 shows the mean vertical temperature gradient through the measure points A-E, F and G-K for Case 1. It is clear that Case 1 with DV has much higher vert
34、ical temperature gradient compared with Cases 2-5 (Figure 4) which has LPDCSD. It has a high vertical temperature difference within the occupied zone (22.7-25.8 C) (72.9 -78.4 F) and almost no temperature difference between 1.7-2.9 m (5.6-9.5 ft). Figure 3 The vertical temperature gradient for Case
35、1 The vertical temperature gradient for Cases 2-5 has almost the opposite behavior as Case 1. There is a very small temperature different within the occupied zone (less than 1.5 C (2.7 F) and a decrease in temperature between 1.7-2.9 m (5.6-9.5 ft) with 1.6 C (2.9 F). The average temperature within
36、the occupied zone in Cases 2-5 varies between 24.2 25.1 C (75.6 -77.2 F). Figure 4 The vertical temperature gradient for Cases 2-5 Table 2. Results of Vertical Temperature Gradient Measurement Case Ts C Te C Tp C t % Case 1 20.8 (69.4) 25.2 (77.4) 24.5 (76.1) 118% Case 2 20.8 (69.4) 24.2 (75.6) 24.2
37、 (75.6) 99% Case 3 20.6 (69.1) 24.9 (76.8) 24.9 (76.8) 100% Case 4 20.7 (69.3) 25.0 (77.0) 25.1 (77.2) 98% Case 5 20.4 (68.7) 24.3 (75.7) 24.3 (75.7) 99% Due to stratified conditions DV has a higher heat removal efficiency than LPDCSD, which has average heat removal efficiency of 99%. Table 3. Resul
38、t of Thermal Comfort Measurement Case PPD % PMV - T C (F) T C(F) Air Velocity m/s (fpm) DR % Max Min Max Avg. Max Avg. Max Avg. Max Case 1 A-E 10.9 -0.52 0.24 24.3 (75.7) 3.1 (5.6) 0.05 (9,8) 0.15 (29,5) 2.3 16.4 F 6.9 -0.28 0.05 24.4 (75.9) 2.2 (4.0) 0.05 (9,8) 0.05 (9,8) 0.9 2.5 G-K 6.9 -0.15 0.30
39、 24.5 (76.1) 2.6 (4.7) 0.05 (9,8) 0.09 (17,7) 1.1 6.9 Case 2 A-E 8.9 -0.42 -0.09 24.3 (75.7) 0.4 (0.7) 0.12 (23,6) 0.22 (43,3) 9.6 19.9 F 9.6 -0.45 -0.37 24.0 (75.2) 0.4 (0.7) 0.14 (27,5) 0.15 (29,5) 1.2 12.9 G-K 10.9 -0.49 -0.22 24.0 (75.2) 0.4 (0.7) 0.15 (29,5) 0.20 (39,4) 1.2 23.7 Case 3 A-E 5.9
40、-0.14 0.20 25.5 (77.9) 0.5 (0.9) 0.06 (11,8) 0.10 (19,7) 3.3 6.1 F 5.9 -0.18 -007 25.0 (77.0) 0.3 (0.5) 0.07 (13,8) 0.10 (19,7) 4.5 6.1 G-K 5.8 -0.14 0.16 25.1 (77.2) 0.2 (0.4) 0.09 (17,7) 0.19 (37,4) 5.8 13.7 Case 4 A-E 5.2 -0.17 0.15 24.9 (76.8) 0.6 (1.1) 0.06 (11,8) 0.11 (21,7) 4.1 8.1 F 6.0 -0.1
41、9 -0.04 24.7 (76.5) 0.6 (1.1) 0.05 (9,8) 0.10 (19,7) 3.9 8.1 G-K 5.6 -0.09 0.11 25.2 (77.4) 0.3 (0.6) 0.09 (17,7) 0.15 (29,5) 7.1 12.1 Case 5 A-E 5.2 -0.09 0.06 24.7 (76.5) 0.4 (0.7) 0.05 (9,8) 0.07 (13,8) 1.7 3.7 F 7.0 -0.30 -0.18 24.4 (75.9) 0.4 (0.7) 0.06 (11,8) 0.09 (17,7) 3.2 5.3 G-K 7.7 -0.36
42、-0.12 24.4 (75.9) 0.3 (0.5) 0.07 (13,8) 0.12 (23,6) 4.3 8.4 The results from thermal comfort measurements show that Case 1 has some problems with low PMV values which results in a PPD of around 11 % see table 3. It also has some problems with draught and high T, all of which occurs at measurment poi
43、nts A-E close to the supply devices. Case 2 with LPDCSD has the same airflow as Case 1 (DV) but has slightly better PMV and PPD values although it has some problems with draught and high air velocities in the occupied zone. Cases 3-5 which all have lower air flows than cases 1-2 have all very low PP
44、D values and none of them has any problems with either high air velocities or draught (ANSI/ASHRAE 2010). Table 4 illustrates the results from the tracer gas measurement which clearly shows that the mean age of air differs very little throughout the BZ (TGMP 1-5) in all five cases. Table 4. Results
45、of Tracer Gas Measurement Case TGMP1 TGMP2 TGMP3 TGMP4 TGMP5 TGMP6 CO2 * p min i - p min i- p min i- p min i - p min i - p min ppm Case 1 9.9 1.01 10.0 1.01 9.9 1.01 10.5 0.96 10.5 0.95 10.0 n.a. Case 2 9.7 0.99 9.7 0.99 9.8 0.98 9.5 1.00 9.5 1.01 9.6 862 Case 3 16.7 0.99 16.6 1.00 16.7 0.99 16.8 0.
46、99 16.6 1.00 16.6 1198 Case 4 19.3 0.99 19.6 0.98 19.3 1.00 18.7 1.03 19.0 1.01 19.2 1127 Case 5 22.7 1.00 22.9 0.99 22.5 1.00 22.7 1.00 22.5 1.00 22.6 1024 * Calculated CO2-concentration in BZ according to equation (2). Results from the tracer gas measurements, as well as of the air temperature mea
47、surements indicate that LPDCSD is a well-functioning mixing ventilation system for this classroom. Therefore it is reasonable to assume that CO2-concentration in the BZ for the mixing ventilation cases can be estimated using equation (2). Table 4 shows that the CO2-concentration in the BZ in case 3
48、and 4 is slightly over the recommended value of 1100 PPM, since it is necessary to have at least 7.5 l/s (16 cfm) per person to reach an acceptable level of CO2-concentration 700 ppm over outside air concentration i.e. 1100 PPM CO2 (ASHRAE 2013). CONCLUSION The results from the tracer gas measuremen
49、ts show that LPDCSD is able to provide similar IAQ as DV when considering air change effectiveness as IAQ indicator. This coupled with the results from the thermal comfort measurements shows that using LPDCSD combined with VAV is beneficial for energy use and the thermal climate since it is possible to lower the airflow in order to keep an acceptable thermal comfort in cases with lower heat load. In th
