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Contribution of natural ventilation in a double skin envelope to heating load reduction in winter
Building and Environment 44 (2009) 2236–2244
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Contribution of natural ventilation in a double skin envelope to heating load reduction in winter Yu-Min Kim a,1, Soo-Young Kim b, *, Sung-Woo Shin c, 2, Jang-Yeul Sohn a,1 a
Department of Architectural Engineering, Hanyang University, Seoul, Republic of Korea Department of Housing and Interior Design, Yonsei University, Seoul, Republic of Korea c Department of Architectural Engineering, Hanyang University, Ansan, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2008 Received in revised form 22 January 2009 Accepted 23 February 2009
This study examined the contribution of a double skin envelope (DSE) to the heating energy savings brought about by natural ventilation in office buildings. A DSE was applied to the east- and west-facing walls on an actual three-floor building. Field measurements and computer simulations were performed in winter. The results implied that the DSE on the west-facing wall contributed to energy savings when natural ventilation was supplied from the cavity to the indoor space. The DSE facing east was not recommended for energy savings by natural ventilation because of its smaller exposure to solar irradiance. Multiple linear regression models were developed based on field measurements to predict the temperature variation in the cavities, and effective control logics will be discussed in a future study. Of all variables, the outdoor air temperature was the most significant factor influencing the air temperature in the cavity. Computer simulation indicated that the air in the cavity was heated to the required temperature without consuming additional energy when the ratio of the diffused irradiance to global irradiance was smaller than 0.69. The cavity in the DSE worked as a thermal buffer zone and contributed to reducing heating energy consumption by 14.71% in January. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Double skin envelope Natural ventilation Heating load Energy saving
1. Introduction Curtain wall structures covered with glazing are commonly used on high-rise buildings since they provide an aesthetically pleasing exterior. Due to the escalation of energy costs, the buildings are tightly sealed with glazing having a greater heat resistance. Although the buildings using the curtain wall structure are tightly sealed, they have an increased energy load since the higher ratio of glazing and its weak thermal resistance allow solar radiation to penetrate into the indoor space. This is directly related to energy consumption in buildings that emits chemical compounds and increases the environmental load, causing serious problems such as the green house effect and global warming. Moreover, a certain ventilation rate is required in buildings according to the guidelines suggested by the nations’ authorities so that indoor space can be healthy and comfortable [1–3].
* Corresponding author. Tel.: þ82 2 2123 3142; fax: þ82 2 313 3139. E-mail addresses: [email protected] (Y.-M. Kim), [email protected] (S.-Y. Kim), [email protected] (S.-W. Shin), [email protected] (J.-Y. Sohn). 1 Tel.: þ82 2 2220 0313. 2 Tel.: þ82 31 400 5132. 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.02.013
Tightly sealed buildings can cause various health symptoms in residents. Reducing energy consumption in buildings using natural ventilation is considered to be the preferable way to eliminate these negative effects. To mitigate the energy and health problems caused by the curtain wall structure, the double skin envelope (DSE) system has been applied to building design and construction [4]. This system is considered to be an environment-friendly design since it utilizes natural ventilation and daylight so that comfort can be achieved with less energy consumption [5]. However, the amount of airflow and its temperature should be carefully adjusted to avoid a negative influence on the indoor thermal environment and energy load when natural ventilation is provided from the cavity to the indoor space. Control of the cavity must be set up to accommodate the change in configuration of the double skin. This study seeks to provide fundamental data for use in the development of control logics to achieve better energy savings using natural ventilation in the double skin envelope. Field measurements were performed in a three-story building with double skins on its eastern and western facades. The data were validated using simulation software to propose necessary ventilation rates that can be achieved in the double skin envelope. Based on the validated results, the energy savings were determined using computer simulation.
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2. Research method 2.1. Building for measurement The field measurements were performed in a building located in Ansan, South Korea (Latitude: 37170, Longitude: 126 490 ). The building was completed in August of 2007, and has three floors. The first and second floors were used as experimental facilities for testing sustainable building components. The third floor was used as office space for research scientists and administration staff. The appearance and plan of the building with the double skin envelope are shown in Figs. 1 and 2. The long axis of the building is in the south to north direction, and was tilted counterclockwise by 26 from the north–south axis. The double skin envelopes were installed on the eastern and western facades of the building, and covered all three floors in the designated area shown in Fig. 2. The conceptual description of the double skin envelope used in this study is shown in Fig. 3.
Fig. 3. Description of double skin envelope.
Fig. 1. View of tested building.
The internal and external skins were covered with glazing from the first floor to the third floor. The dimension of the cavity was 5.7 m (width) by 0.5 m (depth) by 13.2 m (height). The side surfaces between the internal and external skins were also covered with glazing to separate the cavity from the adjacent double skin envelope. In the internal skin, a wall consisting of masonry and mortar stood at the top of each floor, and covered 1.2 m from the top. Venetian blinds were installed on the outside of the internal skin in the cavity. The depth and distance of each blind slat was 2.54 cm (100 ). The blind slats covered the whole internal skin with no tilt angle. Air inlet and outlet openings were installed at the bottom and top of the cavity, which were covered by transparent plastic panels. The dimensions of the air inlets and outlets were 0.60 m (width) by 0.35 m (depth). The air inlet was 0.3 m above the ground. Outdoor air was induced into the cavity and naturally ventilated through it by buoyancy. The roof was installed above the outlet at the top of the third floor to prevent rain or snow from penetrating into the cavity, but allowed air to naturally ventilate out. All windows and openings in the internal skin were closed. 2.2. Data monitoring
Fig. 2. Layout of tested building.
Data collection was performed daily starting in December of 2007 to evaluate the performance of the double skin envelope over the winter season. The daily monitoring periods were from 00:00 to 24:00, and the monitoring interval was 1 min. LI-COR photometric and irradiance sensors were used for monitoring outdoor illuminance and irradiance to examine the influence of irradiance on the cavity temperature. The sensors detected those variables as currents and converted them into standard units using calibration constants. The sensitivity deviation of the sensors was within 1% [6]. An automatic data collection system was used for data recording which detected signals in voltages from –5 V to þ5 V.
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Since the photometric and irradiance sensors generated signals in currents, and the data logger detected signals in voltages, resistors manufactured by LI-COR were installed between each sensor and data logger channel to covert the signals. This data monitoring method was identical to that used in a previous study [7]. The variation in air temperature inside and outside the cavity, and the variation in air velocity at the inlets and outlets of the cavity were measured to evaluate the performance of the double skin envelope. The temperature signal was generated as a voltage which fell into the detection range of the data monitoring system. However, the air velocity signal was generated as a current and then converted into a voltage signal. 2.3. Computer simulations for validation Computer simulation using TRNSYS was employed in this study to validate data monitored in field measurements. The validated results worked as a fundamental foundation to predict ventilation rates and energy savings using TRNSYS in this study. TRNSYS has been widely applied to energy analysis for buildings in practice [8–11]. It uses transfer function method which is commonly used in analyzing the influence of building parameter and thermal interaction between building envelopes [12,13]. TRNSYS is a transient system simulation program with a modular structure that was designed to solve complex energy system problems by breaking the problem down into a series of smaller component [14]. In this study, TRNSYS was primarily used as a simulation tool to predict ventilation rates and energy savings that occurred with use of the double skin envelope. To predict ventilation rates and energy savings using TRNSYS, validations between the simulated and monitored data were performed. Monitored weather data from the tested building were input for validation. Standard weather data for the city in which the tested building was located were also input to predict ventilation rates through cavity and energy savings. The standard weather data were provided by a governmental authority. They were the result of many years of monitoring, and represent the mean variation in temperature for that location. However, the data show variations from the experimental data because the temperature variation on a particular day often does not match that of standard weather data. Boundary conditions were set up for the computer simulations. It was assumed that the dimension of the office area in the first and second floors was 5.7 m wide, 7.68 m deep, and 3.6 m high. The office on the third floor was 5.7 m wide, 7.68 m deep, and 2.4 m high. The indoor office space was assumed to be adjacent to the cavity space, which was modeled as a separate space using a TRNSYS type defined by the software. The thermal properties of glass used for the internal and external skins are shown in Table 1. The U-values for external and internal walls were assumed to be 0.375 W/m2 K and 0.957 W/m2 K, respectively. It was assumed that the conditions for the Venetian blinds were equal to those of field measurements. Since the distance between each slat was 2.54 cm, the heat transfer conditions varied according to the solar altitude and azimuth. In this study, the incident angle of direct solar radiation, and the portion of solar radiation penetrating through the open space between the blind slats were calculated according to the geometrical conditions of the sun and blind slats. It
Table 1 Thermal properties of glass. Skin
U value [W/m2 K]
Solar heat gain coefficient
Absorption coefficient
Reflection coefficient
Internal External
2.83 5.68
0.755 0.855
0.101 0.095
0.126 0.075
Fig. 4. Ventilation progress from cavity to indoor space.
was assumed that 50% of the solar radiation that reached blind slats was absorbed by them, and the remaining amount was reflected toward the indoor space. These data were used as input data for the boundary conditions caused by the Venetian blinds. The indoor temperatures of the office space and corridor were set at 22 C and 15 C from 9:00 to 19:00. For the remaining time period, the temperatures of the office and corridor were both assumed to be 5 C. Outdoor air was assumed to fill the cavity and be heated primarily by solar radiation after being induced into the cavity through the opening at the bottom of the double skin envelope. For those boundary conditions, the time step set up in the TRNSYS was 1 min in this study. When the cavity temperature exceeded 22 C, it was assumed that the window on the internal skin was open and the air in the cavity was supplied to the indoor space. Opening options and airflow procedures are conceptually described in Fig. 4. The window was not closed until the cavity temperature dropped below 22 C. Based on these boundary conditions, the predictions generated by the TRNSYS are discussed in this study.
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3. Results 3.1. Variation of irradiance, temperature, and air velocity in cavity Data from January 16th and January 17th were selected for discussion since they appeared to be good examples of typical clear and overcast winter days. The solar altitude and azimuth were theoretically calculated for these two days. The maximum solar altitude and azimuth on January 16th were 31.72 and 62.88 , respectively. On January 17th, the maximum angles were 31.89 and 63.17. The values calculated for the two days were comparable. Figs. 5 and 6 show outdoor horizontal and indoor vertical irradiance on the two days. On the clear day (January 16th), the outdoor horizontal irradiance showed a pattern similar to the variation in the solar altitude, and the maximum irradiance was 540.12 W/m2. The maximum vertical irradiance on the east side was 199.38 W/m2. The outdoor vertical irradiance on the western side increased drastically to 640.84 W/m2 before solar noon, because the long axis of buildings was tilted 26 from the south. The vertical irradiance was stronger than the horizontal irradiance on the east side until 9:10 in the morning. The vertical irradiance was stronger than the horizontal irradiance on the western side from 2:00 to 5:00 in the afternoon due to the lower solar altitude. On the overcast day (January 17th), the horizontal irradiance was not significantly affected by the change in solar altitude. The
Fig. 7. Variation of cavity temperature (January 16, 2008, east-facing).
Irradiance [W/m^2]
1000 East West Horizontal
800
Fig. 8. Variation of cavity temperature (January 16, 2008, west-facing).
600
400
200
0 7:00
9:00
11:00
13:00
15:00
17:00
Time Fig. 5. Horizontal and vertical irradiances (January 16, 2008).
maximum irradiance was 122.66 W/m2 on the eastern side and 184.59 W/m2 on the western side, while the maximum horizontal irradiance was 325.59 W/m2 at solar noon. The temperature variations in the cavity are shown in Figs. 7–10. On the clear day, the temperature variations in the eastern and western cavities showed a pattern similar to the variation in the vertical irradiance. The cavity temperature was significantly influenced by direct solar radiation. It also appeared to be affected by vertical irradiance on the overcast sky day, but the temperature did not increase as much as it did on the clear day. The maximum difference between the temperatures at the air inlets
1000 East West Horizontal
Irradiance [W/m^2]
800
600
400
200
0 7:00
9:00
11:00
13:00
15:00
17:00
Time Fig. 6. Horizontal and vertical irradiances (January 17, 2008).
Fig. 9. Variation of cavity temperature (January 17, 2008, east-facing).
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7 Outlet Center Inlet
Velocity [m/s]
6 5 4 3 2 1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time
Fig. 10. Variation of cavity temperature (January 17, 2008, west-facing).
Fig. 13. Variation of air velocity in cavity (January 17, 2008, east-facing).
7
5 4 3 2
Outlet Center Inlet
6
Velocity [m/s]
Velocity [m/s]
7
Outlet Center Inlet
6
5 4 3 2
1 1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
0 0:00
Time
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time Fig. 11. Variation of air velocity in cavity (January 16, 2008, east-facing). Fig. 14. Variation of air velocity in cavity (January 17, 2008, west-facing).
7
1 Outlet Center Inlet
0.8
5
Flow Rate [kg/s]
Velocity [m/s]
6
0.9
4 3 2
East West
0.7 0.6 0.5 0.4 0.3 0.2
1
0.1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time Fig. 12. Variation of air velocity in cavity (January 16, 2008, west-facing).
and outlets was 8.58 C on the eastern side, and 9.87 C C on the western side. The temperature differences between the inlets and outlets generate buoyancy effects that cause natural ventilation in the cavity. The variations in air velocity are shown in Figs. 11–14. The air velocity in the middle of the cavity was very low since the cross-sectional area was much greater than that of the air inlet. The amount of air induced into the cavity was used as a source of natural ventilation that functioned as a thermal buffer zone in the cavity. This influences the heating load variation and energy
0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time Fig. 15. Mass flow rates of air in cavity (January 16. 2008).
savings. Figs. 15 and 16 show the mass flow rates converted from the velocity described in Figs. 11–14. 3.2. Prediction for cavity temperature variation The temperature variation in the cavity formed a thermal buffer zone between the outdoor and indoor spaces. Since the temperature influenced the heating load in the building, it was necessary to predict the temperature variation with changes in outdoor weather conditions. In this study, the temperature of the cavity was
Y.-M. Kim et al. / Building and Environment 44 (2009) 2236–2244
east- and west-facing walls under clear, partly cloudy, and overcast conditions. They were tested using ANOVA to determine if the models were acceptable. The results are shown in Tables 2–4. Overall, four independent variables considered in the model were effective in predicting cavity temperature under the three sky conditions. The ANOVA test indicated that all prediction models were acceptable at a very low significance level. For the sky conditions considered in this study, the coefficient of determination was greater than 0.93 for the east- and west-facing facades. The cavity temperature was strongly correlated with the four independent variables. In particular, the outdoor air temperature was the most significant contributor to the variation in cavity temperature under all sky conditions which implies that the cavity temperature was strongly influenced by the outdoor temperature. Under clear conditions, the coefficients of determination in the models were 0.949 and 0.971 for the east- and west-facing conditions, respectively. This means that the variation in cavity temperature for the east- and west-facing facades was reduced by 94.9% and 97.1% when the four independent variables were changed. The coefficients of the slope of the models were examined under a 5% significance level. All of the slopes for the east- and west-facing facades were acceptable within the tested significance level. Under partly cloudy conditions, the linear correlation between the cavity temperature and the four variables was slightly stronger
1 0.9
East West
Flow Rate [kg/s]
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
2241
21:00
Time Fig. 16. Mass flow rates of air in cavity (January 17, 2008).
predicted using a multiple linear regression method that minimized the Error Sum of Squares. The solar altitude and azimuth, irradiance, and outdoor temperature were used as independent variables in the prediction models. The data monitored between December and February were used for the prediction. Regression models were developed for the
Table 2 Prediction models for clear sky. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Altitude Azimuth Outdoor Air Irradiance ANOVA
t
Sig.
Unstandardized coefficients
Std. error
B
0.654 0.038 0.076 0.001 0.012 0.000 1.002 0.004 0.032 0.000 R2 ¼ 0.949, F(4,11910) ¼ 55521.3, Sig. ¼ 0.00
17.04 59.35 30.09 281.39 138.10
0.00 0.00 0.00 0.00 0.00
t
Sig.
26.18 70.68 50.14 382.13 171.25
0.00 0.00 0.00 0.00 0.00
t
Sig.
88.43 14.96 67.39 195.74 173.52
0.00 0.00 0.00 0.00 0.00
Std. error
4.682 0.053 0.034 0.002 0.051 0.001 1.075 0.005 0.020 0.000 R2 ¼ 0.971, F(4,11910) ¼ 99354.1, Sig. ¼ 0.00
Table 3 Prediction models for partly cloudy sky. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Altitude Azimuth Outdoor Air Irradiance ANOVA
Std. error
Unstandardized coefficients B
0.753 0.029 0.070 0.001 0.014 0.000 0.978 0.003 0.032 0.000 2 R ¼ 0.953, F(4,17184) ¼ 87213.1, Sig. ¼ 0.00
t
Sig.
98.94 15.12 85.34 249.75 210.82
0.00 0.00 0.00 0.00 0.00
Std. error
4.392 0.044 0.027 0.002 0.047 0.001 1.057 0.004 0.020 0.000 2 R ¼ 0.981, F(4,17184) ¼ 110928.1, Sig. ¼ 0.00
Table 4 Prediction models for overcast sky. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Altitude Azimuth Outdoor Air Irradiance ANOVA
t
Sig.
Std. error
0.407 0.025 0.083 0.001 0.017 0.000 0.895 0.002 0.033 0.000 R2 ¼ 0.931, F(4,24002) ¼ 81217.4, Sig. ¼ 0.00
Unstandardized coefficients B
16.36 86.04 67.40 382.75 178.36
0.00 0.00 0.00 0.00 0.00
t
Sig.
Std. error
3.607 0.034 107.58 0.019 0.001 13.99 0.037 0.000 91.94 1.024 0.003 313.59 0.023 0.000 294.32 R2 ¼ 0.959, F(4,24002) ¼ 139.376.8, Sig. ¼ 0.00
0.00 0.00 0.00 0.00 0.00
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than under a clear sky. The coefficients of determination were increased by 0.004 and 0.01 for the east- and west-facing facades, respectively. This means that the variation in cavity temperature was reduced by an additional 0.4% and 1% compared to the variation in cavity temperature under clear sky. Under overcast sky conditions, the correlation was weaker than under the other two sky conditions, but the coefficients of determination were still greater than 0.93. This means that the variation in cavity temperature can be reduced by at least 93% when the sun was completely covered by clouds causing insufficient solar irradiance. In summary, the outdoor air temperature was the strongest factor influencing the variation in cavity temperature. The influence of the outdoor air on cavity temperature became less as the sky conditions changed to partly cloudy and overcast. Solar altitude and irradiance were the next important contributors. Although the outdoor air was an important factor, it was related to the variation in irradiance from the sun. Hence, the irradiance should not be considered to be a minor factor in the model. In addition, the correlation between solar irradiance and outdoor air temperature needs to be examined further. 3.3. Validation for measured and simulated cavity temperatures The influence of natural ventilation occurring in the cavity of a double skin envelope on ventilation rates in the indoor space was analyzed. Since the data obtained from the field measurements were not enough to analyze the effect of natural ventilation, computer simulations were used to predict the influence of natural ventilation. Simulation software has its own computational algorithms and limitations, so the simulation results were not always similar to those from the measurements. To examine the range of variation between them, the measured data were validated using TRNSYS which has been widely applied to thermal analysis for buildings. For the east- and west-facing facades, the cavity temperature monitored from January 16 to 17 was compared with that generated by TRNSYS which used a portion of the monitored data as
Predicted Temperature [C]
30
West-facing y = 0.9299x - 0.9216 R2 = 0.9759
East-facing West-facing 20
10
0 -20
-10
0
10
20
30
East-facing y = 0.8037x - 1.1549 R2 = 0.9598
-10
-20
Measured Temerature [C] Fig. 17. Correlation between measured and predicted cavity temperatures.
input for the prediction. These two results were compared using linear regression analysis and its results are shown in Fig. 17. Overall, the correlation between them was strong and the model was acceptable at a low significance level as shown in the ANOVA test results in Table 5. The coefficients of determination for the eastand west-facing conditions were 0.9597 and 0.9758, respectively. This implies that the variation in the predicted cavity temperature was reduced by 95.97% and 97.58% when the monitored cavity temperature changed. In particular, the east-facing condition was exposed to more solar radiation since the building was rotated by 26 as shown in Fig. 2. The edge of the double skin on the eastfacing facade was adjacent to the south-facing wall. The heat that accumulated on the wall was transferred to the cavity space, and influenced the simulation results. In summary, the analysis’ results provide a strong foundation for additional simulations to predict the influence of natural ventilation in the cavity on ventilation rates and reduction of heating loads in spaces covered by double skin envelopes. 3.4. Prediction of ventilation rate and heating load reduction The variation of ventilation rates through the double skin envelope was predicted according to the conditions of air temperature in cavity. TRNSYS was used as a primary simulation tool for the prediction. Fig. 18 shows the variation in ventilation rates predicted for January. The air that filled the cavity could be supplied to the indoor space when the ratio of diffused solar radiation to global solar radiation was less than 0.69. This condition was generally considered to be a clear and partly cloudy day [15], and occurred only 17 times in January. On the 17 days when air could be supplied to the indoor space from the cavity, the total amount of air supplied by cavity was 67,866 m3. The maximum amount of air supplied to the indoor space was 8125.9 m3 per day, which replaced the indoor air 19.35 times. The indoor air could be replaced at least 0.81 times per day using the double skin envelope. It appears that the heating load was effectively reduced on the 17 days when the air from the cavity could be used. However, it does not appear that the double skin envelope was effective on the other days in January. This result implies that a clear and partly cloudy sky would be preferable when using a double skin envelope to reduce the heating load in winter. The double skin envelope could be used as a thermal buffer zone to mitigate the influence of outdoor air on the indoor space. The double skin envelope would contribute to a reduction in energy consumption by supplying air to the indoor space from the cavity at a desirable temperature, and would also function as insulation. The heating load of double and single skin envelopes was compared to each other using a TRNSYS simulation. It was assumed in the simulations that the internal skin of the double skin envelope was the single skin envelope, making the volume of the indoor space formed by the double skin envelope and that formed by the single skin envelop equal. The same building material properties were assumed for both cases.
Table 5 ANOVA test for correlation between predicted and measured cavity temperature. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Measured ANOVA
t
Sig.
Std. error
1.155 0.016 0.804 0.003 R2 ¼ 0.959, F(1,2880) ¼ 68708.9, Sig. ¼ 0.00
Unstandardized coefficients B
71.83 262.12
0.00 0.00
t
Sig.
41.62 341.32
0.00 0.00
Std. error
0.922 0.019 0.930 0.003 R2 ¼ 0.976, F(1,2880) ¼ 116497.2, Sig. ¼ 0.00
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Ventilation Rate [ X 1,000m3/day]
9 8
0.8
7 6
0.6
5 4
0.4
3 0.2 0.0
2
Diffused/Global Irradiance Ventilation rate
1
6
11
16
1 21
26
31
0
30 25
Temperature [C]
Diffused Irradiance / Global Irradiance
10 1.0
Cavity Outdoor
20 15 10 5 0 -5 9:00
For the double skin envelope, two scenarios were assumed. One was that natural ventilation supplied from the cavity to indoor space was available only when the cavity temperature was greater than 22 C. It was assumed that the air in the cavity was supplied to the indoor space without being sent to an HVAC system. The other had no ventilation from cavity even if the cavity temperature was greater than 22 C. In this case, the cavity space functioned as a thermal buffer zone between indoor and outdoor. For the single skin envelope, the outdoor air was directly supplied to an HVAC system, otherwise the air supply procedure was the same as that for the double skin envelope. The daily variations in heating load for those three cases in January are shown in Fig. 19. When ventilation was available from cavity to indoor space, the variation of cavity temperature for a selected day in January is shown in Fig. 20 Overall, heating load was reduced by the contribution of preheated air from the cavity and the thermal buffer zone of the double skin. When the air temperature in the cavity was greater than 22 C, it was supplied to the indoor space. The amount of air supplied from the cavity was 67,866 m3 as discussed above, and this contributed to the reduction in the HVAC system load. Overall, the heating load for single skin was the greatest among three cases. The load for double skin without ventilation from cavity showed the lowest value. The difference in heating load caused by double skin with ventilation and without ventilation was 1%. It means that the heating load can be effectively reduced when the cavity in double skin functioned as a thermal buffer zone.
11:00
13:00
15:00
17:00
19:00
Time
Time [day] Fig. 18. Variation of solar irradiance and natural ventilation in cavity.
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Fig. 20. Temperature variation in cavity under natural ventilation condition (January 26).
However, this case does not provide any natural ventilation through the cavity. When the single skin envelope was used the heating load was greater than that of double skin envelope by 14.71% in January. Single skin does not appear to contribute to reducing heating load in buildings. Since the double skin with ventilation reduces heating load providing natural ventilation from the cavity, it is considered to be an environment-friendly envelope system that can be applied to building designs. When the double skin was applied to the building envelope, the heating load was reduced by the thermal buffer zone which resulted in energy savings. The insulation formed by the cavity contributed to a 14.71 % reduction in heating energy consumption when compared to the single skin envelope. This occurred because of the reinforcement of thermal resistance by the building envelope. The preheated air layer between the internal and external skins acted as additional insulation that reduced heat transfer through the envelope. The thermal buffer zone that formed in the cavity had a temperature which was greater than the outdoor temperature, and reduced the heat transfer through the envelope and cavity of the double skin.
4. Conclusions Field measurements and computer simulations were performed on an actual building with a double skin envelope to examine the influence of the double skin on heating load reduction. A summary of general findings of this study is as follows.
350
Heating load [MJ/day]
300 250 200 150 100 Double+Cavity Ventilation Double+No Cavity Ventilation Single
50 0
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31
Time [day] Fig. 19. Heating load variation for double and single skin envelope.
1. The variations in cavity temperature correspond to the patterns of vertical irradiance on the external skin. The air temperature in the cavity was influenced by irradiance when the sky was clear and partly cloudy. This resulted in ventilation of air to the indoor space with a high enough temperature for heating. However, the double skin envelope did not function effectively under overcast skies since there was not enough radiation to heat up the air in the cavity. 2. The double skin envelope on the east-facing facade did not receive enough solar radiation in winter, and failed to achieve any effective ventilation rate. This implies that the east-facing double skin envelope would not function beneficially in practice. However, the west-facing skin received enough solar radiation and succeeded in generating ventilation for the indoor space. It appears that the double skin envelope would effectively reduce the heating load in winter.
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3. Multiple linear prediction models indicated that the cavity temperatures of east- and west-facing walls were most significantly influenced by the variations in outdoor air temperature. This appeared to occur because the outdoor temperature was primarily influenced by the variations in solar altitude and irradiance under clear and overcast skies. This study suggests that the variation in outdoor temperature is a useful factor for application to control strategies when the windows on the internal skin are controlled based on the variation in cavity temperature. 4. Computer simulations indicate that the air in the cavity was heated to the required temperature without consuming additional energy when the ratio of diffused irradiance to global irradiance was smaller than 0.69. This occurred for 17 days during January. On those days, the total amount of indoor air could be replaced a minimum of 0.81 times, and a maximum of 19.35 times per day. The double skin envelope considered in this study effectively increased thermal resistance and reduced heat transfer. The cavity worked as a thermal buffer zone and contributed to reducing heating energy consumption by 14.71% in January.
5. Limitations and future work This study was performed in an actual building with a double skin envelope during the winter season. The results discussed in this study are only applicable for winter conditions. Buildings are exposed to all seasons for the entire year, and the various weather conditions influence their energy consumption. The cavity space of the double skin was effective in winter since the air in the cavity works as a thermal buffer zone. However, this would work against the cooling load in the summer and result in significant energy consumption when the double skin envelope becomes overheated due to an accumulation of solar irradiance in the cavity space. This study lacks an analysis for the summer period when a cooling load exists. Further study during the summer would be useful, and the analysis is being studied by the authors. The building considered in this study had double skin envelopes that faced east and west. In general, the double skin is considered to be effective when it is installed on the south facade of a building, since the influence of solar irradiance on the cavity space would be beneficial to energy savings. Further analysis under various building orientations would be useful. More energy can be saved if
the indoor air is re-circulated through the cavity, but recirculation was not considered in this study. The double skin envelope appears to save more energy than the single skin, however, a greater initial cost is necessary. The energy saved by a double skin envelope pays back its investment cost for its entire lifetime. An economical analysis would be beneficial. Computer simulations that have particular computational algorithms were used in this study to predict the variation in ventilation rates and energy savings for the double skin. Additional simulations using different software with various computational algorithms would be useful in providing more reliable results. Acknowledgement This work was supported by the Sustainable Building Research Center of Hanyang University which was supported by the SRC/ERC program of MEST (R11-2005-056-02002-1). References [1] ASHRAE Standards 62-1999. Ventilation for acceptable indoor air quality. ASHRAE; 1999. [2] ANSI/ASHRAE/IESNA Standards 90-1-2001. Energy standard for buildings except low-rise residential buildings. ASHRAE; 2001. [3] National building code for Korea. 2005. [4] Compagno A. Intelligent glass façade, material practice design. Boston Berlin, Germany: Birkhauser Publisher for Architecture; 1999. [5] Kim S, Song K. Determining photosensor conditions of a daylight dimming control system using different double-skin envelope configuration. Indoor Built Environ 2008;16:411–25. [6] Li-Cor, Inc.. LI-COR sensor instruction manual. Li-Cor, Inc.; 1991. [7] Kim S, Kim J. The impact of daylight fluctuation on a daylight dimming control system in a small office. Energy Build 2007;39:935–44. [8] Eicker U, Huber M, Seeberger P, Vorschulze C. Limits and potentials of office building climatisation with ambient air. Energy Build 2006;38:574–81. [9] Ballestini G, Carli M, Masiero N, Tombola G. Possibilities and limitations of natural ventilation in restored industrial archaeology buildings with a doubleskin façade in Mediterranean climates. Build Environ 2005;40:983–95. [10] Collet F, Serres L, Miriel J, Bart M. Study of thermal behavior of clay wall facing south. Build Environ 2006;41:307–15. [11] Saelens D, Roels S, Hens H. Strategies to improve the energy performance of multiple-skin façades. Building and Environment 2008;43:638–50. [12] Chel A, Hayak J, Kaushik G. Energy conservation in honey storage building using Trombe wall. Energy Build 2008;40:1643–50. [13] Chen T. A method for the direct generation of comprehensive numerical solar building transfer functions. Sol Energy 2003;74:123–32. [14] Catalina T, Virgone J, Blanco E. Development and validation of regression models to predict monthly heating demand for residential building. Energy Build 2008;40:1825–32. [15] Rea M. IESNA handbook. 9th ed. The Illuminating Engineering Society of North America; 2002.
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Contribution of natural ventilation in a double skin envelope to heating load reduction in winter Yu-Min Kim a,1, Soo-Young Kim b, *, Sung-Woo Shin c, 2, Jang-Yeul Sohn a,1 a
Department of Architectural Engineering, Hanyang University, Seoul, Republic of Korea Department of Housing and Interior Design, Yonsei University, Seoul, Republic of Korea c Department of Architectural Engineering, Hanyang University, Ansan, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2008 Received in revised form 22 January 2009 Accepted 23 February 2009
This study examined the contribution of a double skin envelope (DSE) to the heating energy savings brought about by natural ventilation in office buildings. A DSE was applied to the east- and west-facing walls on an actual three-floor building. Field measurements and computer simulations were performed in winter. The results implied that the DSE on the west-facing wall contributed to energy savings when natural ventilation was supplied from the cavity to the indoor space. The DSE facing east was not recommended for energy savings by natural ventilation because of its smaller exposure to solar irradiance. Multiple linear regression models were developed based on field measurements to predict the temperature variation in the cavities, and effective control logics will be discussed in a future study. Of all variables, the outdoor air temperature was the most significant factor influencing the air temperature in the cavity. Computer simulation indicated that the air in the cavity was heated to the required temperature without consuming additional energy when the ratio of the diffused irradiance to global irradiance was smaller than 0.69. The cavity in the DSE worked as a thermal buffer zone and contributed to reducing heating energy consumption by 14.71% in January. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Double skin envelope Natural ventilation Heating load Energy saving
1. Introduction Curtain wall structures covered with glazing are commonly used on high-rise buildings since they provide an aesthetically pleasing exterior. Due to the escalation of energy costs, the buildings are tightly sealed with glazing having a greater heat resistance. Although the buildings using the curtain wall structure are tightly sealed, they have an increased energy load since the higher ratio of glazing and its weak thermal resistance allow solar radiation to penetrate into the indoor space. This is directly related to energy consumption in buildings that emits chemical compounds and increases the environmental load, causing serious problems such as the green house effect and global warming. Moreover, a certain ventilation rate is required in buildings according to the guidelines suggested by the nations’ authorities so that indoor space can be healthy and comfortable [1–3].
* Corresponding author. Tel.: þ82 2 2123 3142; fax: þ82 2 313 3139. E-mail addresses: [email protected] (Y.-M. Kim), [email protected] (S.-Y. Kim), [email protected] (S.-W. Shin), [email protected] (J.-Y. Sohn). 1 Tel.: þ82 2 2220 0313. 2 Tel.: þ82 31 400 5132. 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.02.013
Tightly sealed buildings can cause various health symptoms in residents. Reducing energy consumption in buildings using natural ventilation is considered to be the preferable way to eliminate these negative effects. To mitigate the energy and health problems caused by the curtain wall structure, the double skin envelope (DSE) system has been applied to building design and construction [4]. This system is considered to be an environment-friendly design since it utilizes natural ventilation and daylight so that comfort can be achieved with less energy consumption [5]. However, the amount of airflow and its temperature should be carefully adjusted to avoid a negative influence on the indoor thermal environment and energy load when natural ventilation is provided from the cavity to the indoor space. Control of the cavity must be set up to accommodate the change in configuration of the double skin. This study seeks to provide fundamental data for use in the development of control logics to achieve better energy savings using natural ventilation in the double skin envelope. Field measurements were performed in a three-story building with double skins on its eastern and western facades. The data were validated using simulation software to propose necessary ventilation rates that can be achieved in the double skin envelope. Based on the validated results, the energy savings were determined using computer simulation.
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2. Research method 2.1. Building for measurement The field measurements were performed in a building located in Ansan, South Korea (Latitude: 37170, Longitude: 126 490 ). The building was completed in August of 2007, and has three floors. The first and second floors were used as experimental facilities for testing sustainable building components. The third floor was used as office space for research scientists and administration staff. The appearance and plan of the building with the double skin envelope are shown in Figs. 1 and 2. The long axis of the building is in the south to north direction, and was tilted counterclockwise by 26 from the north–south axis. The double skin envelopes were installed on the eastern and western facades of the building, and covered all three floors in the designated area shown in Fig. 2. The conceptual description of the double skin envelope used in this study is shown in Fig. 3.
Fig. 3. Description of double skin envelope.
Fig. 1. View of tested building.
The internal and external skins were covered with glazing from the first floor to the third floor. The dimension of the cavity was 5.7 m (width) by 0.5 m (depth) by 13.2 m (height). The side surfaces between the internal and external skins were also covered with glazing to separate the cavity from the adjacent double skin envelope. In the internal skin, a wall consisting of masonry and mortar stood at the top of each floor, and covered 1.2 m from the top. Venetian blinds were installed on the outside of the internal skin in the cavity. The depth and distance of each blind slat was 2.54 cm (100 ). The blind slats covered the whole internal skin with no tilt angle. Air inlet and outlet openings were installed at the bottom and top of the cavity, which were covered by transparent plastic panels. The dimensions of the air inlets and outlets were 0.60 m (width) by 0.35 m (depth). The air inlet was 0.3 m above the ground. Outdoor air was induced into the cavity and naturally ventilated through it by buoyancy. The roof was installed above the outlet at the top of the third floor to prevent rain or snow from penetrating into the cavity, but allowed air to naturally ventilate out. All windows and openings in the internal skin were closed. 2.2. Data monitoring
Fig. 2. Layout of tested building.
Data collection was performed daily starting in December of 2007 to evaluate the performance of the double skin envelope over the winter season. The daily monitoring periods were from 00:00 to 24:00, and the monitoring interval was 1 min. LI-COR photometric and irradiance sensors were used for monitoring outdoor illuminance and irradiance to examine the influence of irradiance on the cavity temperature. The sensors detected those variables as currents and converted them into standard units using calibration constants. The sensitivity deviation of the sensors was within 1% [6]. An automatic data collection system was used for data recording which detected signals in voltages from –5 V to þ5 V.
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Since the photometric and irradiance sensors generated signals in currents, and the data logger detected signals in voltages, resistors manufactured by LI-COR were installed between each sensor and data logger channel to covert the signals. This data monitoring method was identical to that used in a previous study [7]. The variation in air temperature inside and outside the cavity, and the variation in air velocity at the inlets and outlets of the cavity were measured to evaluate the performance of the double skin envelope. The temperature signal was generated as a voltage which fell into the detection range of the data monitoring system. However, the air velocity signal was generated as a current and then converted into a voltage signal. 2.3. Computer simulations for validation Computer simulation using TRNSYS was employed in this study to validate data monitored in field measurements. The validated results worked as a fundamental foundation to predict ventilation rates and energy savings using TRNSYS in this study. TRNSYS has been widely applied to energy analysis for buildings in practice [8–11]. It uses transfer function method which is commonly used in analyzing the influence of building parameter and thermal interaction between building envelopes [12,13]. TRNSYS is a transient system simulation program with a modular structure that was designed to solve complex energy system problems by breaking the problem down into a series of smaller component [14]. In this study, TRNSYS was primarily used as a simulation tool to predict ventilation rates and energy savings that occurred with use of the double skin envelope. To predict ventilation rates and energy savings using TRNSYS, validations between the simulated and monitored data were performed. Monitored weather data from the tested building were input for validation. Standard weather data for the city in which the tested building was located were also input to predict ventilation rates through cavity and energy savings. The standard weather data were provided by a governmental authority. They were the result of many years of monitoring, and represent the mean variation in temperature for that location. However, the data show variations from the experimental data because the temperature variation on a particular day often does not match that of standard weather data. Boundary conditions were set up for the computer simulations. It was assumed that the dimension of the office area in the first and second floors was 5.7 m wide, 7.68 m deep, and 3.6 m high. The office on the third floor was 5.7 m wide, 7.68 m deep, and 2.4 m high. The indoor office space was assumed to be adjacent to the cavity space, which was modeled as a separate space using a TRNSYS type defined by the software. The thermal properties of glass used for the internal and external skins are shown in Table 1. The U-values for external and internal walls were assumed to be 0.375 W/m2 K and 0.957 W/m2 K, respectively. It was assumed that the conditions for the Venetian blinds were equal to those of field measurements. Since the distance between each slat was 2.54 cm, the heat transfer conditions varied according to the solar altitude and azimuth. In this study, the incident angle of direct solar radiation, and the portion of solar radiation penetrating through the open space between the blind slats were calculated according to the geometrical conditions of the sun and blind slats. It
Table 1 Thermal properties of glass. Skin
U value [W/m2 K]
Solar heat gain coefficient
Absorption coefficient
Reflection coefficient
Internal External
2.83 5.68
0.755 0.855
0.101 0.095
0.126 0.075
Fig. 4. Ventilation progress from cavity to indoor space.
was assumed that 50% of the solar radiation that reached blind slats was absorbed by them, and the remaining amount was reflected toward the indoor space. These data were used as input data for the boundary conditions caused by the Venetian blinds. The indoor temperatures of the office space and corridor were set at 22 C and 15 C from 9:00 to 19:00. For the remaining time period, the temperatures of the office and corridor were both assumed to be 5 C. Outdoor air was assumed to fill the cavity and be heated primarily by solar radiation after being induced into the cavity through the opening at the bottom of the double skin envelope. For those boundary conditions, the time step set up in the TRNSYS was 1 min in this study. When the cavity temperature exceeded 22 C, it was assumed that the window on the internal skin was open and the air in the cavity was supplied to the indoor space. Opening options and airflow procedures are conceptually described in Fig. 4. The window was not closed until the cavity temperature dropped below 22 C. Based on these boundary conditions, the predictions generated by the TRNSYS are discussed in this study.
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3. Results 3.1. Variation of irradiance, temperature, and air velocity in cavity Data from January 16th and January 17th were selected for discussion since they appeared to be good examples of typical clear and overcast winter days. The solar altitude and azimuth were theoretically calculated for these two days. The maximum solar altitude and azimuth on January 16th were 31.72 and 62.88 , respectively. On January 17th, the maximum angles were 31.89 and 63.17. The values calculated for the two days were comparable. Figs. 5 and 6 show outdoor horizontal and indoor vertical irradiance on the two days. On the clear day (January 16th), the outdoor horizontal irradiance showed a pattern similar to the variation in the solar altitude, and the maximum irradiance was 540.12 W/m2. The maximum vertical irradiance on the east side was 199.38 W/m2. The outdoor vertical irradiance on the western side increased drastically to 640.84 W/m2 before solar noon, because the long axis of buildings was tilted 26 from the south. The vertical irradiance was stronger than the horizontal irradiance on the east side until 9:10 in the morning. The vertical irradiance was stronger than the horizontal irradiance on the western side from 2:00 to 5:00 in the afternoon due to the lower solar altitude. On the overcast day (January 17th), the horizontal irradiance was not significantly affected by the change in solar altitude. The
Fig. 7. Variation of cavity temperature (January 16, 2008, east-facing).
Irradiance [W/m^2]
1000 East West Horizontal
800
Fig. 8. Variation of cavity temperature (January 16, 2008, west-facing).
600
400
200
0 7:00
9:00
11:00
13:00
15:00
17:00
Time Fig. 5. Horizontal and vertical irradiances (January 16, 2008).
maximum irradiance was 122.66 W/m2 on the eastern side and 184.59 W/m2 on the western side, while the maximum horizontal irradiance was 325.59 W/m2 at solar noon. The temperature variations in the cavity are shown in Figs. 7–10. On the clear day, the temperature variations in the eastern and western cavities showed a pattern similar to the variation in the vertical irradiance. The cavity temperature was significantly influenced by direct solar radiation. It also appeared to be affected by vertical irradiance on the overcast sky day, but the temperature did not increase as much as it did on the clear day. The maximum difference between the temperatures at the air inlets
1000 East West Horizontal
Irradiance [W/m^2]
800
600
400
200
0 7:00
9:00
11:00
13:00
15:00
17:00
Time Fig. 6. Horizontal and vertical irradiances (January 17, 2008).
Fig. 9. Variation of cavity temperature (January 17, 2008, east-facing).
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7 Outlet Center Inlet
Velocity [m/s]
6 5 4 3 2 1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time
Fig. 10. Variation of cavity temperature (January 17, 2008, west-facing).
Fig. 13. Variation of air velocity in cavity (January 17, 2008, east-facing).
7
5 4 3 2
Outlet Center Inlet
6
Velocity [m/s]
Velocity [m/s]
7
Outlet Center Inlet
6
5 4 3 2
1 1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
0 0:00
Time
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time Fig. 11. Variation of air velocity in cavity (January 16, 2008, east-facing). Fig. 14. Variation of air velocity in cavity (January 17, 2008, west-facing).
7
1 Outlet Center Inlet
0.8
5
Flow Rate [kg/s]
Velocity [m/s]
6
0.9
4 3 2
East West
0.7 0.6 0.5 0.4 0.3 0.2
1
0.1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time Fig. 12. Variation of air velocity in cavity (January 16, 2008, west-facing).
and outlets was 8.58 C on the eastern side, and 9.87 C C on the western side. The temperature differences between the inlets and outlets generate buoyancy effects that cause natural ventilation in the cavity. The variations in air velocity are shown in Figs. 11–14. The air velocity in the middle of the cavity was very low since the cross-sectional area was much greater than that of the air inlet. The amount of air induced into the cavity was used as a source of natural ventilation that functioned as a thermal buffer zone in the cavity. This influences the heating load variation and energy
0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
21:00
Time Fig. 15. Mass flow rates of air in cavity (January 16. 2008).
savings. Figs. 15 and 16 show the mass flow rates converted from the velocity described in Figs. 11–14. 3.2. Prediction for cavity temperature variation The temperature variation in the cavity formed a thermal buffer zone between the outdoor and indoor spaces. Since the temperature influenced the heating load in the building, it was necessary to predict the temperature variation with changes in outdoor weather conditions. In this study, the temperature of the cavity was
Y.-M. Kim et al. / Building and Environment 44 (2009) 2236–2244
east- and west-facing walls under clear, partly cloudy, and overcast conditions. They were tested using ANOVA to determine if the models were acceptable. The results are shown in Tables 2–4. Overall, four independent variables considered in the model were effective in predicting cavity temperature under the three sky conditions. The ANOVA test indicated that all prediction models were acceptable at a very low significance level. For the sky conditions considered in this study, the coefficient of determination was greater than 0.93 for the east- and west-facing facades. The cavity temperature was strongly correlated with the four independent variables. In particular, the outdoor air temperature was the most significant contributor to the variation in cavity temperature under all sky conditions which implies that the cavity temperature was strongly influenced by the outdoor temperature. Under clear conditions, the coefficients of determination in the models were 0.949 and 0.971 for the east- and west-facing conditions, respectively. This means that the variation in cavity temperature for the east- and west-facing facades was reduced by 94.9% and 97.1% when the four independent variables were changed. The coefficients of the slope of the models were examined under a 5% significance level. All of the slopes for the east- and west-facing facades were acceptable within the tested significance level. Under partly cloudy conditions, the linear correlation between the cavity temperature and the four variables was slightly stronger
1 0.9
East West
Flow Rate [kg/s]
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0:00
3:00
6:00
9:00
12:00
15:00
18:00
2241
21:00
Time Fig. 16. Mass flow rates of air in cavity (January 17, 2008).
predicted using a multiple linear regression method that minimized the Error Sum of Squares. The solar altitude and azimuth, irradiance, and outdoor temperature were used as independent variables in the prediction models. The data monitored between December and February were used for the prediction. Regression models were developed for the
Table 2 Prediction models for clear sky. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Altitude Azimuth Outdoor Air Irradiance ANOVA
t
Sig.
Unstandardized coefficients
Std. error
B
0.654 0.038 0.076 0.001 0.012 0.000 1.002 0.004 0.032 0.000 R2 ¼ 0.949, F(4,11910) ¼ 55521.3, Sig. ¼ 0.00
17.04 59.35 30.09 281.39 138.10
0.00 0.00 0.00 0.00 0.00
t
Sig.
26.18 70.68 50.14 382.13 171.25
0.00 0.00 0.00 0.00 0.00
t
Sig.
88.43 14.96 67.39 195.74 173.52
0.00 0.00 0.00 0.00 0.00
Std. error
4.682 0.053 0.034 0.002 0.051 0.001 1.075 0.005 0.020 0.000 R2 ¼ 0.971, F(4,11910) ¼ 99354.1, Sig. ¼ 0.00
Table 3 Prediction models for partly cloudy sky. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Altitude Azimuth Outdoor Air Irradiance ANOVA
Std. error
Unstandardized coefficients B
0.753 0.029 0.070 0.001 0.014 0.000 0.978 0.003 0.032 0.000 2 R ¼ 0.953, F(4,17184) ¼ 87213.1, Sig. ¼ 0.00
t
Sig.
98.94 15.12 85.34 249.75 210.82
0.00 0.00 0.00 0.00 0.00
Std. error
4.392 0.044 0.027 0.002 0.047 0.001 1.057 0.004 0.020 0.000 2 R ¼ 0.981, F(4,17184) ¼ 110928.1, Sig. ¼ 0.00
Table 4 Prediction models for overcast sky. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Altitude Azimuth Outdoor Air Irradiance ANOVA
t
Sig.
Std. error
0.407 0.025 0.083 0.001 0.017 0.000 0.895 0.002 0.033 0.000 R2 ¼ 0.931, F(4,24002) ¼ 81217.4, Sig. ¼ 0.00
Unstandardized coefficients B
16.36 86.04 67.40 382.75 178.36
0.00 0.00 0.00 0.00 0.00
t
Sig.
Std. error
3.607 0.034 107.58 0.019 0.001 13.99 0.037 0.000 91.94 1.024 0.003 313.59 0.023 0.000 294.32 R2 ¼ 0.959, F(4,24002) ¼ 139.376.8, Sig. ¼ 0.00
0.00 0.00 0.00 0.00 0.00
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than under a clear sky. The coefficients of determination were increased by 0.004 and 0.01 for the east- and west-facing facades, respectively. This means that the variation in cavity temperature was reduced by an additional 0.4% and 1% compared to the variation in cavity temperature under clear sky. Under overcast sky conditions, the correlation was weaker than under the other two sky conditions, but the coefficients of determination were still greater than 0.93. This means that the variation in cavity temperature can be reduced by at least 93% when the sun was completely covered by clouds causing insufficient solar irradiance. In summary, the outdoor air temperature was the strongest factor influencing the variation in cavity temperature. The influence of the outdoor air on cavity temperature became less as the sky conditions changed to partly cloudy and overcast. Solar altitude and irradiance were the next important contributors. Although the outdoor air was an important factor, it was related to the variation in irradiance from the sun. Hence, the irradiance should not be considered to be a minor factor in the model. In addition, the correlation between solar irradiance and outdoor air temperature needs to be examined further. 3.3. Validation for measured and simulated cavity temperatures The influence of natural ventilation occurring in the cavity of a double skin envelope on ventilation rates in the indoor space was analyzed. Since the data obtained from the field measurements were not enough to analyze the effect of natural ventilation, computer simulations were used to predict the influence of natural ventilation. Simulation software has its own computational algorithms and limitations, so the simulation results were not always similar to those from the measurements. To examine the range of variation between them, the measured data were validated using TRNSYS which has been widely applied to thermal analysis for buildings. For the east- and west-facing facades, the cavity temperature monitored from January 16 to 17 was compared with that generated by TRNSYS which used a portion of the monitored data as
Predicted Temperature [C]
30
West-facing y = 0.9299x - 0.9216 R2 = 0.9759
East-facing West-facing 20
10
0 -20
-10
0
10
20
30
East-facing y = 0.8037x - 1.1549 R2 = 0.9598
-10
-20
Measured Temerature [C] Fig. 17. Correlation between measured and predicted cavity temperatures.
input for the prediction. These two results were compared using linear regression analysis and its results are shown in Fig. 17. Overall, the correlation between them was strong and the model was acceptable at a low significance level as shown in the ANOVA test results in Table 5. The coefficients of determination for the eastand west-facing conditions were 0.9597 and 0.9758, respectively. This implies that the variation in the predicted cavity temperature was reduced by 95.97% and 97.58% when the monitored cavity temperature changed. In particular, the east-facing condition was exposed to more solar radiation since the building was rotated by 26 as shown in Fig. 2. The edge of the double skin on the eastfacing facade was adjacent to the south-facing wall. The heat that accumulated on the wall was transferred to the cavity space, and influenced the simulation results. In summary, the analysis’ results provide a strong foundation for additional simulations to predict the influence of natural ventilation in the cavity on ventilation rates and reduction of heating loads in spaces covered by double skin envelopes. 3.4. Prediction of ventilation rate and heating load reduction The variation of ventilation rates through the double skin envelope was predicted according to the conditions of air temperature in cavity. TRNSYS was used as a primary simulation tool for the prediction. Fig. 18 shows the variation in ventilation rates predicted for January. The air that filled the cavity could be supplied to the indoor space when the ratio of diffused solar radiation to global solar radiation was less than 0.69. This condition was generally considered to be a clear and partly cloudy day [15], and occurred only 17 times in January. On the 17 days when air could be supplied to the indoor space from the cavity, the total amount of air supplied by cavity was 67,866 m3. The maximum amount of air supplied to the indoor space was 8125.9 m3 per day, which replaced the indoor air 19.35 times. The indoor air could be replaced at least 0.81 times per day using the double skin envelope. It appears that the heating load was effectively reduced on the 17 days when the air from the cavity could be used. However, it does not appear that the double skin envelope was effective on the other days in January. This result implies that a clear and partly cloudy sky would be preferable when using a double skin envelope to reduce the heating load in winter. The double skin envelope could be used as a thermal buffer zone to mitigate the influence of outdoor air on the indoor space. The double skin envelope would contribute to a reduction in energy consumption by supplying air to the indoor space from the cavity at a desirable temperature, and would also function as insulation. The heating load of double and single skin envelopes was compared to each other using a TRNSYS simulation. It was assumed in the simulations that the internal skin of the double skin envelope was the single skin envelope, making the volume of the indoor space formed by the double skin envelope and that formed by the single skin envelop equal. The same building material properties were assumed for both cases.
Table 5 ANOVA test for correlation between predicted and measured cavity temperature. Factors
East-facing
West-facing
Unstandardized coefficients B (Constant) Measured ANOVA
t
Sig.
Std. error
1.155 0.016 0.804 0.003 R2 ¼ 0.959, F(1,2880) ¼ 68708.9, Sig. ¼ 0.00
Unstandardized coefficients B
71.83 262.12
0.00 0.00
t
Sig.
41.62 341.32
0.00 0.00
Std. error
0.922 0.019 0.930 0.003 R2 ¼ 0.976, F(1,2880) ¼ 116497.2, Sig. ¼ 0.00
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Ventilation Rate [ X 1,000m3/day]
9 8
0.8
7 6
0.6
5 4
0.4
3 0.2 0.0
2
Diffused/Global Irradiance Ventilation rate
1
6
11
16
1 21
26
31
0
30 25
Temperature [C]
Diffused Irradiance / Global Irradiance
10 1.0
Cavity Outdoor
20 15 10 5 0 -5 9:00
For the double skin envelope, two scenarios were assumed. One was that natural ventilation supplied from the cavity to indoor space was available only when the cavity temperature was greater than 22 C. It was assumed that the air in the cavity was supplied to the indoor space without being sent to an HVAC system. The other had no ventilation from cavity even if the cavity temperature was greater than 22 C. In this case, the cavity space functioned as a thermal buffer zone between indoor and outdoor. For the single skin envelope, the outdoor air was directly supplied to an HVAC system, otherwise the air supply procedure was the same as that for the double skin envelope. The daily variations in heating load for those three cases in January are shown in Fig. 19. When ventilation was available from cavity to indoor space, the variation of cavity temperature for a selected day in January is shown in Fig. 20 Overall, heating load was reduced by the contribution of preheated air from the cavity and the thermal buffer zone of the double skin. When the air temperature in the cavity was greater than 22 C, it was supplied to the indoor space. The amount of air supplied from the cavity was 67,866 m3 as discussed above, and this contributed to the reduction in the HVAC system load. Overall, the heating load for single skin was the greatest among three cases. The load for double skin without ventilation from cavity showed the lowest value. The difference in heating load caused by double skin with ventilation and without ventilation was 1%. It means that the heating load can be effectively reduced when the cavity in double skin functioned as a thermal buffer zone.
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Time [day] Fig. 18. Variation of solar irradiance and natural ventilation in cavity.
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Fig. 20. Temperature variation in cavity under natural ventilation condition (January 26).
However, this case does not provide any natural ventilation through the cavity. When the single skin envelope was used the heating load was greater than that of double skin envelope by 14.71% in January. Single skin does not appear to contribute to reducing heating load in buildings. Since the double skin with ventilation reduces heating load providing natural ventilation from the cavity, it is considered to be an environment-friendly envelope system that can be applied to building designs. When the double skin was applied to the building envelope, the heating load was reduced by the thermal buffer zone which resulted in energy savings. The insulation formed by the cavity contributed to a 14.71 % reduction in heating energy consumption when compared to the single skin envelope. This occurred because of the reinforcement of thermal resistance by the building envelope. The preheated air layer between the internal and external skins acted as additional insulation that reduced heat transfer through the envelope. The thermal buffer zone that formed in the cavity had a temperature which was greater than the outdoor temperature, and reduced the heat transfer through the envelope and cavity of the double skin.
4. Conclusions Field measurements and computer simulations were performed on an actual building with a double skin envelope to examine the influence of the double skin on heating load reduction. A summary of general findings of this study is as follows.
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Time [day] Fig. 19. Heating load variation for double and single skin envelope.
1. The variations in cavity temperature correspond to the patterns of vertical irradiance on the external skin. The air temperature in the cavity was influenced by irradiance when the sky was clear and partly cloudy. This resulted in ventilation of air to the indoor space with a high enough temperature for heating. However, the double skin envelope did not function effectively under overcast skies since there was not enough radiation to heat up the air in the cavity. 2. The double skin envelope on the east-facing facade did not receive enough solar radiation in winter, and failed to achieve any effective ventilation rate. This implies that the east-facing double skin envelope would not function beneficially in practice. However, the west-facing skin received enough solar radiation and succeeded in generating ventilation for the indoor space. It appears that the double skin envelope would effectively reduce the heating load in winter.
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3. Multiple linear prediction models indicated that the cavity temperatures of east- and west-facing walls were most significantly influenced by the variations in outdoor air temperature. This appeared to occur because the outdoor temperature was primarily influenced by the variations in solar altitude and irradiance under clear and overcast skies. This study suggests that the variation in outdoor temperature is a useful factor for application to control strategies when the windows on the internal skin are controlled based on the variation in cavity temperature. 4. Computer simulations indicate that the air in the cavity was heated to the required temperature without consuming additional energy when the ratio of diffused irradiance to global irradiance was smaller than 0.69. This occurred for 17 days during January. On those days, the total amount of indoor air could be replaced a minimum of 0.81 times, and a maximum of 19.35 times per day. The double skin envelope considered in this study effectively increased thermal resistance and reduced heat transfer. The cavity worked as a thermal buffer zone and contributed to reducing heating energy consumption by 14.71% in January.
5. Limitations and future work This study was performed in an actual building with a double skin envelope during the winter season. The results discussed in this study are only applicable for winter conditions. Buildings are exposed to all seasons for the entire year, and the various weather conditions influence their energy consumption. The cavity space of the double skin was effective in winter since the air in the cavity works as a thermal buffer zone. However, this would work against the cooling load in the summer and result in significant energy consumption when the double skin envelope becomes overheated due to an accumulation of solar irradiance in the cavity space. This study lacks an analysis for the summer period when a cooling load exists. Further study during the summer would be useful, and the analysis is being studied by the authors. The building considered in this study had double skin envelopes that faced east and west. In general, the double skin is considered to be effective when it is installed on the south facade of a building, since the influence of solar irradiance on the cavity space would be beneficial to energy savings. Further analysis under various building orientations would be useful. More energy can be saved if
the indoor air is re-circulated through the cavity, but recirculation was not considered in this study. The double skin envelope appears to save more energy than the single skin, however, a greater initial cost is necessary. The energy saved by a double skin envelope pays back its investment cost for its entire lifetime. An economical analysis would be beneficial. Computer simulations that have particular computational algorithms were used in this study to predict the variation in ventilation rates and energy savings for the double skin. Additional simulations using different software with various computational algorithms would be useful in providing more reliable results. Acknowledgement This work was supported by the Sustainable Building Research Center of Hanyang University which was supported by the SRC/ERC program of MEST (R11-2005-056-02002-1). References [1] ASHRAE Standards 62-1999. Ventilation for acceptable indoor air quality. ASHRAE; 1999. [2] ANSI/ASHRAE/IESNA Standards 90-1-2001. Energy standard for buildings except low-rise residential buildings. ASHRAE; 2001. [3] National building code for Korea. 2005. [4] Compagno A. Intelligent glass façade, material practice design. Boston Berlin, Germany: Birkhauser Publisher for Architecture; 1999. [5] Kim S, Song K. Determining photosensor conditions of a daylight dimming control system using different double-skin envelope configuration. Indoor Built Environ 2008;16:411–25. [6] Li-Cor, Inc.. LI-COR sensor instruction manual. Li-Cor, Inc.; 1991. [7] Kim S, Kim J. The impact of daylight fluctuation on a daylight dimming control system in a small office. Energy Build 2007;39:935–44. [8] Eicker U, Huber M, Seeberger P, Vorschulze C. Limits and potentials of office building climatisation with ambient air. Energy Build 2006;38:574–81. [9] Ballestini G, Carli M, Masiero N, Tombola G. Possibilities and limitations of natural ventilation in restored industrial archaeology buildings with a doubleskin façade in Mediterranean climates. Build Environ 2005;40:983–95. [10] Collet F, Serres L, Miriel J, Bart M. Study of thermal behavior of clay wall facing south. Build Environ 2006;41:307–15. [11] Saelens D, Roels S, Hens H. Strategies to improve the energy performance of multiple-skin façades. Building and Environment 2008;43:638–50. [12] Chel A, Hayak J, Kaushik G. Energy conservation in honey storage building using Trombe wall. Energy Build 2008;40:1643–50. [13] Chen T. A method for the direct generation of comprehensive numerical solar building transfer functions. Sol Energy 2003;74:123–32. [14] Catalina T, Virgone J, Blanco E. Development and validation of regression models to predict monthly heating demand for residential building. Energy Build 2008;40:1825–32. [15] Rea M. IESNA handbook. 9th ed. The Illuminating Engineering Society of North America; 2002.