797 Double skin façade glazed office buildings; A parametric study for optimized energy and thermal comfort performance H. Poirazis Lund University, Sweden ABSTRACT Highly glazed buildings are often considered to be airy, light and transparent with more access to daylight. Their performance, however, regarding to energy use and thermal comfort issues is often questioned. Passive solar systems such as Double Skin Façades are likely to improve the overall building performance if integrated properly. This article deals with energy and indoor climate simulations of double skin office buildings in Sweden using a dynamic energy simulation tool. Different highly glazed Double Skin Façade alternatives were studied in order to investigate the influence of (a) the geometry, (b) the ventilation strategy, (c) the construction (panes and shading devices) and (d) the control strategy on the building energy performance. Interesting results were obtained when comparing alternatives with different panes, control strategies and coupling of the Double Skin Façade with the HVAC system. The results are discussed and general suggestions for optimized performance are given. 1. INTRODUCTION Since the nineties there is a strong interest in office buildings with highly glazed facades. Many have been built worldwide and even more are being built. Architects often like the idea of a clean façade while owners relate the large glazing areas to a transparent image and users (occupants) to a more pleasant environment. However, the energy efficiency and the quality of indoor thermal environment that highly glazed buildings can provide are often questioned. In order to achieve improved indoor climate and reduced energy use, systems such as Double Skin facades can be used. Detailed knowledge of the possibilities and limitations of this system can lead to improved performance and successful building integration. A pilot study was carried out (on a component level) in order to select the alternatives to be further examined. Later on, these alternatives were simulated on a building level using a dynamic building energy simulation tool. The main aim of the pilot study was to understand the possibilities and limitations of the Double Skin Façade (DSF) cavity, while the main focus of the main study was to optimize their performance when integrated in the building. 2. BACKGROUND 2.1 The Glazed Office Buildings project This study is a part of the Glazed Office Buildings project, a project that aims at investigating possibilities and limitations as to energy use and indoor climate in highly glazed Single and Double Skin Façade office buildings in Nordic climates. 2.2 The Double Skin Facade (DSF) system The Double Skin Façade is a system consisting of two glass skins placed in such a way that air can flow in the intermediate cavity. The distance between the skins usually varies from 20 cm up to 2 m. For protection and heat extraction reasons the solar shading devices (usually venetian blinds) are placed inside the cavity. The ventilation of the cavity can be natural, fan supported or mechanical; the origin and destination of the air can also vary depending on the location, the use and the HVAC strategy of the building (Poirazis, 2004). 3. METHODS 3.1 General A pilot study of Double Skin Façade cavities was carried out aiming on gaining knowledge regarding the possibilities and limitations of the system. This pilot study was limited to a component level and the simulations were carried out for constant boundary conditions. The outputs focus on the cavity air and pane temperatures. A main study followed aiming on optimizing the performance of the DSF system and its building integration. The simulations were carried out on a single zone (office) level for a building located in Göteborg, Sweden. The outputs focus on energy use and thermal comfort comparisons. The methods and results of both studies are presented and discussed. 3.2 Calculation methods For the parametric studies a two dimensional simulation tool (WIS 3) was used; the tool was built on algorithms PALENC 2007 - Vol 2.indd 797 7/9/2007 1:24:41 µµ
798 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and based on international (CEN, ISO) standards, such as the ISO Standard 15099. For the main study a dynamic energy building simulation tool (IDA ICE 3.0) was used. 3.3 Façade mode The two modes studied are briefly described below: Typical double façade mode: The cavity is closed during the heating season for increased thermal insulation and opened during the cooling season for heat extraction purposes. Airflow window mode: all year round the air enters the cavity from inside the office space (exhaust air). The aim of this mode is to provide better boundary conditions all year round. During the heating season the air through the cavity ends up in the AHU for heat recovery purposes while during the cooling season the air is extracted to the outside. 3.4 Cavity geometry A multi storey (10, 20 and 30 m high) and a box window (3.5 m high) construction were considered. For all the cases during the pilot study the cavity width assumed was 3.5 m and the depth varied from 0.2m to 1.6m. Table 1: Glazing alternatives for the standard double skin façade mode; the ventilated cavity is between the outer and intermediate pane Case Outer pane Intermediate pane Inner pane DSF A Clear Clear 4mm Clear DSF B Solar control Clear Clear DSF C Solar control Clear Low E Coated DSF E Clear Solar control + lowe Clear Table 2: Glazing alternatives for the airflow window mode; the ventilated cavity is between the inner and intermediate pane Case Outer pane Intermediate pane AW A Clear Clear Clear AW B Clear Low E Coated Clear AW C Clear Solar control Clear AW D Solar control Clear Clear AW E Solar control Low E Coated Clear AW F Solar control Clear Clear Inner pane AW G Solar control Clear Low E hard coated 3.5 Glazing type For the typical double façade mode the thermal barrier (double glazing unit) is required as the inner layer while in the airflow window cases, it is required as the outer one. The glazing unit alternatives used for the simulations during the pilot study are shown in Tables 1 and 2. 3.6 Position of shading devices The position of shading devices inside the cavity was also changed during the pilot study in order to investigate its influence on the inner pane temperatures. 3.7 Environment During the pilot study, the DSF cavities were studied for a typical summer day and night, a typical winter day and night and an extreme summer day. For the main study all year round simulations were carried out for a building located in Göteborg, Sweden. 3.8 Other parameters In order to simulate the zone alternatives (used for the main study) close to reality, parameters such as occupancy, internal loads, HVAC strategy, infiltration rates, etc had to be inserted in the software. The tuning of these parameters was done extensively for a typical office building located in Sweden (Poirazis, 2005). 4. RESULTS 4.1 General The results from the pilot study are described briefly focusing mostly on the cavity air temperatures and the inner pane temperatures. The results of the main study concern mainly the energy use issues. 4.2 Pilot study A parametric study of naturally ventilated (typical double) multi storey height façades was carried out. The main focus of the study was to investigate the impact of geometry on the airflow and air temperatures along the cavity. Findings of this study are briefly described at the conclusions chapter. Typical double façade mode During winter (when the cavity is closed) the inner pane surface temperatures vary from 20 C to 24 C when shading is not applied (Figure 1); when shading is applied the temperature variation becomes somewhat narrower. Regardless shading DSF D has the highest inner surface and air temperatures. In order to study the effect of the shading device position inside the cavity for winter conditions a parametric study was carried out. During summer a 20 m multi storey high façade was selected for calculating the air temperature at the vertical and horizontal centre of the inner sub cavity (between the venetian blinds and the intermediate pane; Figure 2) and the inner pane temperatures (Figure 3). The shading devices are placed in different cavity positions (the distance shown in Figure 2 corresponds to the distance between the outer pane and the shading devices). PALENC 2007 - Vol 2.indd 798 7/9/2007 1:24:42 µµ
799 Although the air temperature increase is highly dependant on the depth of the sub cavity for all the cases (mostly for the DSF A and D), the inner surface temperature is not much influenced by it. However, the glazing type used is crucial for the inner layer temperatures. The cases with low E coating applied at the intermediate and inner pane (cases DSF C and D correspondently) are the coolest ones (around 28 C). Airflow window mode The inner surface temperatures for a typical winter and an extreme summer day were studied. For the AW A the inner surface temperature is almost 22 C for winter, 37 C for summer without shading and 48 C with shading (Figure 4). Figure 1: Temperature at the vertical and horizontal centre of a box window facade (typical winter day, closed 800 mm deep cavity, 3.5 m high, no shading devices) Figure 4: Inner surface temperatures for an airflow box window (no shading devices) Figure 2: Inner sub cavity air temperatures for box window facades (extreme summer day) Figure 3: Inner pane temperatures for box window facades (extreme summer day) When the intermediate clear pane is replaced by a low E coated one (AW B) the surface temperatures slightly increase. An intermediate solar control one (AW C) results in slightly higher inner pane temperatures during a typical winter day; for an extreme summer day, however, the highly absorbing intermediate pane results in increased cavity air temperatures, and consequentially (due to the relatively high thermal transmittance of the inner clear pane) high inner surface temperatures. When shading is applied the inner surface temperature increase is lower than the cases A and B. Since most of the radiation is absorbed at the intermediate pane, the shading device does not influence as much (as in the cases A and B) the air temperature of the cavity. When a body tinted solar control pane is placed as an outer one (cases D and E) the inner pane temperatures drop (compared with AW A, B and C) since the absorption takes place at the outer pane. Larger is the impact of the outer solar control pane when shading devices are applied since less absorption takes place in the blinds. In the last two cases an advance solar coated (low E + solar control) pane is applied in the outer pane resulting in lower temperatures during summer and slightly higher during winter. The low E inner pane of the AW G results PALENC 2007 - Vol 2.indd 799 7/9/2007 1:24:42 µµ
800 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and in slightly lower temperatures during winter. 4.3 Main Study Typical double façade mode For the naturally ventilated cavities nine different control set points were considered for the dampers (opening and closing according to different cavity air temperatures). The design parameters studied are the façade orientation and the type of glazing and shading devices. Each of these cases was simulated for the nine different set points and the alternatives with minimum total energy use were selected as optimum. The glazing type is crucial on the energy savings. As shown in Figure 5, regardless the façade orientation the DSF F (solar control+low E coating intermediate pane) performs best. The DSF A and D have increased heating demand (due to the high U value). The lower g value of DSF D (solar control outer pane) results in lower cooling and higher heating demand. In total there is a slightly higher total energy demand for the north but lower for the south orientated façade. The DSF E case has lower heating demand due to the low E inner coating; the cooling demand, however is slightly higher for the north and lower for the south orientated façade (compared with DSF D). Figure 6: % decrease of total energy demand for the north and south oriented typical double façade mode Larger appear to be the energy savings on the DSF A case with three clear panes due to the decreased cooling demand. When comparing the DSF cases D and E (both with solar control external pane) it appears that although the low E inner pane of the DSF E results in quite lower demand, the impact of opening is similar. Finally, for the DSF F case the influence of the opening is smaller. Figure 5: Energy use for the north and south oriented typical double façade mode In order to study the energy savings of the ventilated façade, a comparative study was carried out, regarding the DSF cases with optimum control set points (for the dampers) and the ones when the cavity opening was considered always closed. The latter can be considered as three pane single skin façade cases with intermediate blinds. The decrease of total (heating and cooling) demand is presented in Figure 6 while the decrease of cooling demand is presented in Figure 7. Figure 7: % decrease of cooling demand for the north and south oriented typical double façade mode From the above it becomes obvious that although the total energy savings are not large, the cooling demand drops dramatically. Airflow window mode In Figure 8 the energy use of the airflow window alternatives is presented. Generally, the north facing alternatives tend to perform better mainly due to the lower cooling demand. As expected, the AW A has the highest total (heating and cooling) demand. The outer solar control pane (AW D) results in lower cooling and slightly higher heating PALENC 2007 - Vol 2.indd 800 7/9/2007 1:24:42 µµ
801 demand. For the south orientation the decrease in total demand reaches the 11%. When a low E intermediate pane replaces the clear one (AW E) the heating demand drops drastically since the air temperature drop inside the cavity is smaller due to the better insulation of the outer skin (approximately 33% regardless orientation, compared with AW D).The cooling demand, however, increases since during the cooling period the low E intermediate pane does not allow heat transmittance to the outside. When the advanced low E + solar control pane is placed as outer one (AW F) the cooling demand drops while the heating demand increases (compared with AW E); the decrease in the total demand is more noticeable in the south orientation due to the larger cooling demand. Finally, when the clear inner pane is replaced by a low E one the total demand drops further. Figure 8: Energy use for the airflow window mode 5. CONCLUSIONS 5.1 Pilot study The relationship between height and depth of naturally ventilated cavities is crucial for the airflow rates. Minimum cavity depths should be assuring large air flows for efficient heat extraction and thus proper inner pane temperatures. The opening size is a key parameter for airflows large enough for sufficient heat extraction, since deeper cavities with the same openings don t assure larger airflows. Thus, the dampers should occupy as little space as possible for larger airflows and minimum cavity depth. Regarding the selection of panes, solar control outer panes have negative impact during winter (and positive during summer), since they result in lower inner pane temperatures and transmit less energy into the building. Generally, outer solar control panes reduce the effect of shading. The intermediate low E+solar control performs especially well during winter since the absorption which takes place in the intermediate pane results in less heat transmission losses through the outer one. For extreme summer conditions and when shading is applied, low E inner panes reduce the influence of high inner pane temperatures caused by increased air temperatures (inside the cavity) while more heat is extracted from the cavity by ventilation. The position of shading devices influences the air temperature of the inner sub cavity; the pane types, however, have much larger impact. Regarding the airflow window mode it was concluded that a highly absorbing outer pane or a low E inner one reduce the risk of overheating the inner pane during summer when shading is applied. Low U and g values for the outer skin of an airflow window are essential to its performance. 5.2 Main study For the typical double façade mode the alternatives performing best in terms of energy use are the DSF F (intermediate solar control + low E pane) and the DSF E (inner low E and outer solar control i.e. low g- and U-values. When allowing natural ventilation inside the cavity the energy use for cooling decreases dramatically, while the total demand is somewhat lower. Regarding the airflow window mode the cases performing best are the ones with solar control + low E outer panes. An inner low E pane further improves the façade s performance. ACKNOWLEDGEMENS This project was funded by the Swedish Energy Agency. REFERENCES ISO Standard 15099, 2003 Thermal Performance of Windows, Doors and Shading Devices - Detailed Calculations. International Organization for Standardization. Geneva. Poirazis, H., 2004. Double skin facades for office buildings; a literature review. Division of Energy and Building Design, Department of Architecture and Built Environment, Lund Institute of Technology, Lund University, Sweden. ISBN 91-85147 02-8 Poirazis, H., 2005. Single Skin Glazed Office buildings; energy use and indoor climate simulations. Division of Energy and Building Design, Department of Architecture and Built Environment, Lund Institute of Technology, Lund University, Sweden. ISBN 91-85147 11-7 WIS 3, TNO - Building and Construction Research, Delft, The Netherlands. IDA ICE 3.0- EQUA, www.equa.se, Stockholm, Sweden PALENC 2007 - Vol 2.indd 801 7/9/2007 1:24:43 µµ