Assessment of the double-skin façade
passive thermal buffer effect
Oliver Kinnane, PhD, MArch, BE
Tom Prendergast, MAI
Department of Civil, Structural and Environmental Engineering,
Trinity College Dublin.
[email protected]
ABSTRACT
Double Skin Façades (DSFs) are becoming increasingly popular architecture for commercial
office buildings. Although DSFs are widely accepted to have the capacity to offer significant passive
benefits and enable low energy building performance, there remains a paucity of knowledge with regard
to their operation. Identification of the most determinant architectural parameters of DSFs is the focus
of ongoing research. This paper presents an experimental and simulation study of a DSF installed on a
commercial building in Dublin, Ireland. The DSF is south facing and acts to buffer the building from
winter heat losses, but risks enhancing over-heating on sunny days. The façade is extensively monitored
during winter months. Computational Fluid Dynamic (CFD) models are used to simulate the convective
operation of the DSF. This research concludes DSFs as suited for passive, low energy architecture in
temperature climates such as Ireland but identifies issues requiring attention in DSF design.
INTRODUCTION
A Double Skin Façade (DSF) or Multi Skin Façade (MSF) is generally composed of a glazed
curtain offset from the line of the building envelope (Figure 1). DSFs have continued to increase in
popularity, particularly in commercial architecture, yet still today there remains a paucity of
comprehensive studies proving the benefit of DSFs in different climate regions, and at different seasons.
Although there are many published case studies e.g.(Hashemi, Fayaz, & Sarshar, 2010) (Pasquay, 2004)
a lack of reliable experimental data and validated simulation studies is oft commented in the literature
(Gertis, K., 1999) (H. Manz, Schaelin, & Simmler, 2004).
Ever more studies of DSF systems are required as their characteristics and operation are directly
related to the climate in which the building is located, with solar radiation, wind and ambient air
temperature all having an impact. This paper presents an experimental study focused on the temperature
profile in a DSF in the maritime Irish climate during winter. This experimental study will form the basis
for an extensive Computational Fluid Dynamics (CFD) study of different configurations of MSFs in the
maritime climate. This study will in turn enable an evaluation of the appropriateness of MSFs in the
context of the Irish climate and their ability for energy savings and comfort enhancement in Irish
buildings.
DSFs are generally designed for different operation in summer and winter conditions. During Irish
summer months DSFs generally operate in the ‘open’ mode. This implies that vents are opened at the
bottom and top of the façade cavity. The air in the cavity removes excess heat by means of convective
Oliver Kinnane is an Assistant Professor of Façade Engineering in the Department of Civil, Structural and Environmental Engineering,
Trinity College Dublin, Ireland. Tom Prendergast is a graduate of TCD and a Project Engineer at KEO International Consultants.
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
1
flow induced by the stack effect. This action prevents excessive heat accumulation in the cavity. If this
occurs unwanted heat can transmit into the internal spaces. This can have a significant impact on the
thermal comfort conditions within the building and create a greater necessity for the use of auxiliary
cooling systems, hence resulting in an increase in energy consumption.
When the air is cleared from within the cavity, the temperature of the building envelope skin is
lowered and heat transfer from the internal skin to the occupied space is reduced. Accordingly less heat
is transferred from the outside to the inside, and less energy is required to cool the space.
In winter common DSF operation utilizes a sealed cavity, with no air circulation. For the winter
scenario, the DSF cavity is warmer than the exterior temperature. As the air in the cavity is heated by the
sun the temperature of the envelope skin increases and the temperature difference across the envelope
skin reduces. Accordingly less heat is transferred from inside the building to the outside given a reduced
temperature differential between interior conditioned space and the adjacent thermal zone. Significantly
less energy is required to heat the space.
A greater proportion of research focuses on the evaluation and modeling of the summer operation
of DSFs (H. Manz & Frank, 2005) with a paucity of investigation of the winter thermal buffer effect.
Similarly the majority of studies investigate the airflow in DSFs with less emphasis on investigation of
the temperature profiles in the cavity. In an 11 story building in Iranian winter conditions Hashemi et al
th
th
(Hashemi et al., 2010) document a difference in temperature of 5–12 ◦C on the 7 floor and on 11 floor
7.5–10.5 ◦C more than the outside temperature. Vertical thermal stratification in the cavity is common in
DSFs. The heated air in the cavity rises due to natural buoyancy, and a drop in air velocities at the top of
the cavity leads to stratification. Thermal stratification is identified in monitored data (Hashemi et al.,
2010) and simulation studies of mechanically ventilated facades (Pfuhler, Sikorski, & Kuhn, 2012).
Hamza and Abohela (Hamza & Abohela, 2013) present an exploratory study of cavity stratification in
non-uniform DSFs. Thermal stratification in the cavity has been shown to be influenced by a number of
design and climatic parameters including solar radiation levels, shading device use and their colour,
depth of the cavity of the double-skin, glazing types on both façade layers and design of inlets and
outlets in relation to prevailing wind direction and speed amongst others.
This paper presents an experimental monitoring study of the temperature profile in the DSF of a
commercial building in Dublin, Ireland that will form the basis of an extensive and validated modeling
study of the appropriateness of DSF for this and similar climates. The impact of solar radiation levels,
surface and cavity temperatures on DSF operation are presented.
EXPERIMENTAL METHODOLOGY
Experimental monitoring of a case study DSF is presented with focus on temperature
characterization. Temperatures in the DSF were extensively monitored, with 9 temperature sensors
installed on the three floors of the cavity. Interior temperature and external temperature are also
monitored. Surface temperature readings were taken at intervals. Solar irradiance data was also attained
for the location.
The DSF under consideration in this study is installed on the upper stories of an office building in
Dublin, Ireland. It is a three-story façade (width x depth = 12m x 0.7m), is south facing and composed of
external double-glazing and a single interior sheet of glass. The air cavity includes timber sun-shading
louvers that drop approx. 750mm from the top of each level, are 250mm wide and horizontal. Automated
venetian blinds shade the building envelope skin, and adjust their angle throughout the day. A metal grill
divides each level of the DSF, which enable accessibility to each level and air movement between levels.
Although the DSF is analyzed in the ‘closed’ state there are 10mm gaps between the 100mm
louvers, which allow ingress and exhaust of air. Such small openings have been shown to generate
significant air flows (Gratia & De Herde, 2004).
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
2
a)
b)
c)
Figure 1 a) Double skin façade on upper stories of commercial building, b) corridor of double skin
façade, and c) vents in closed state, with air gaps evident between louvers.
Dublin is located on the eastern coast of Ireland, on the northwestern periphery of Europe (latitude:
53°20′N and longitude: 6°15′W). It has a temperate maritime climate, of mild winter and summer
seasons.
RESULTS
Winter temperatures were measured from January to April 2014. The following figures document
the typical observed temperature profile for the DSF.
Cavity temperatures. The temperature in the 3 Levels of the cavity over a typical 4-day period in
February is shown in Figure 2.
Figure 2. Temperature profile over 4 days of a typical sunny winter week, with outdoor
temperatures in an 8-15°C circadian swing. Solar radiation is shown in brown and
scaled to (W/m2)/10.
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
3
The results show that Tcav during the day and night exceeds To on all levels except Level 1 of the
cavity. The air temperature on Level 1 deviates little from the outdoor temperature but is often below
even when exposed to high solar radiation. This phenomena is further shown in Figure 3 and 4. The
outside temperature fluctuates in a 6°C range, whereas the temperature fluctuations in Level 2 and 3 are
in the range 20-28°C on different days. This is in contrast to other studys in winter conduitions that show
the fluctuation in Tcav to be less than that of the outside temperature (Hashemi et al., 2010). The
temperature difference between the cavity temperature on Level 2 (Tcav2) and outside is 18°C at midday
and 2-3°C at night. Similarly for Level 3, Tcav2 - To is approx. 20°C at maximum and 2-3°C at nighttime.
Maximum Tcav3 reaches 35-39°C, when the outdoor temperature is 15°C. These high cavity temperature
on the upper levels reduce the temperature difference across the building envelope, thereby impacting
the heat loss across this boundary.
Temperature and solar incidence. Figure 3 and Figure 4 show the solar irradiance and the
temperature on each level of the DSF for typical sunny and overcast days, charaterised by high and low
solar radiation. To differs by 5°C between the days shown. However, Tcav3 is almost 18°C higher on the
sunnier and warmer day, showing the significant impact of solar radiation on a closed cavity in winter in
sunny conditions.
Vertical thermal stratification is evident between the different façade levels with a large jump from
the inlet level (Tcav1) to the middle of the façade (Tcav2) of up to 12-15°C.
Figure 3. Temperature profiles for each level of the façade on a typical sunny day. Solar radiation
(brown) is scaled to (W/m2)/10.
In contrast on a typical overcast day the temperature in Tcav1 is 2-4 greater than To. It is difficult to
explain this given that all Levels of the cavity are exposed ot high solar radiation with no overshadowing. The surface temperatures of the walkway grill and glass at Level 1 are signicant lower than
temperatures at other levels and these possibly act to reduce the air temperatures in
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
4
Figure 4. Temperature profiles for each level of the façade cavity on a typical overcast day. Solar
radiation (green) is scaled to (W/m2)/10.
Surface and cavity temperatures. Outdoor, indoor and cavity air, and boundary layer surface
temperatures are shown in Figure 5. On this day the temperature in the interior space is controlled by the
BMS from rising above 24°C. The surface temperature of the internal face of the building envelope
boundary gains heat from the auxiliary internal space heating. Although Figure 5 shows temperatures for
a discrete day of average solar radiation, the temperature profiles display a common trend on the
different levels. The temperature gradient across the building envelope glazing surface drops in the
upper floors; Tenv_ext_3s - Tenv_int_3s = 3.5°C and Tenv_ext_3s - Tenv_int_3s = 1.5°C. The gradient in surface
temperature in Level 1 in contrast, increases Tenv_ext_1s - Tenv_int_1s = +5.8°C.
As expected, and similar to results reported by studies in hot arid climates Hamza et al (Hamza,
Gomaa and Underwood, 2007), the surface temperature on the in-cavity surface of the exterior glazing
of the DSF (Tdsf) is lower than the surface temperature of the in-cavity surface of the building envelope
glazing (Tenv_ext).
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
5
Figure 5. Temperatures outside, inside and on glass surfaces of DSF on a given day of average
solar radiation (18/03/14). The points close to the DSF boundary and building
envelope marked (s) represent surface temperature readings.
In Figure 5, Levels 2 and 3 cavity temperatures (Tcav2, Tcav3) are approximately equal to the indoor
air temperatures (TLev2a, TLev3a). On Level 1 the cavity air temperature is significantly lower than on Level
2 and 3 (approx. 5°C lower) and on Level 1 the air temperature in the cavity is lower than that in the
conditioned space in the building interior
SIMULATION STUDIES
Data presented in this paper is being used as the basis for a comprehensive simulation study of (i)
zonal energy analysis using EnergyPlus and (ii) CFD modeling study using ANSYS. Further monitoring
is planned during coming summer and winter seasons to enable validation of CFD and energy models.
This will enable assessment of the specific conditions a DSF can benefit building performance and the
optimum configuration and operation of the façade to enable passive heating and cooling and hence
energy savings for the building. Airflow patterns in the closed and open states are investigated for
turbulent and laminar patterns given different boundary conditions.
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
6
Figure 6. Energy model and CFD simulation of same model developed using EnergyPlus
Figure 7. CFD model developed to assess temperature and laminar flow patterns in the DSF
CONCLUSION
Due to these higher air temperatures in the cavity relative to the outdoor temperature the external
walls lose heat more slowly. This is beneficial to preheating of the inside spaces and heating energy
conservation. However, in the DSF close to the building envelope the air temperature is often
significantly higher than the heating set point implying a reversal of the standard winter temperature
gradient seen across single skin building envelopes. Hence, the DSF can act to increase the internal air
temperature even causing overheating, when the building is in free running mode. On days of high solar
radiation levels it is proposed that the cavity be ventilated. Again this is not the case in Level 1, where
the cavity temperature is regularly up to 10°C lower. Hence, the thermal buffer benefit of the DSF at the
lower level is not discernible.
In contrast to many studies in the literature this study documents a consistently lower inlet
temperature than outdoor air temperature. Saelens et al (Saelens, Roels, & Hens, 2004) demonstrated
that the difference between the inlet temperature and the outdoor air temperature depends on the solar
intensity and airflow rate. With solar radiation they showed the inlet temperature to be higher than the
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
7
outdoor temperature, as did other authors (Heinrich Manz, 2004) (Fuliotto, 2010). Based on a review of
these studies He et al (He, Shu, & Zhang, 2011) use a constant difference of +4°C difference in the
summer case and +2°C difference in the winter case, between the inlet temperature and outdoor air
temperature.
Based on standard heat loss assessment through the building envelope the DSF can be beneficial to
ACKNOWLEDGMENTS
The authors would like to thank Arup for enabling this research and for the case study building.
NOMENCLATURE
TcavX
=
air temperature in cavity dsf_Xs
T
=
surface temperature on external skin of dsf
Tenv_est_Xs
=
surface temperature on external skin of building envelope
=
surface temperature on internal skin of building envelope
Tenv_int_Xs
REFERENCES
Fuliotto, R. (2010). Experimental and numerical analysis of heat transfer and airflow on an
interactive building facade. doi:10.1016/j.enbuild.2009.07.006
Gertis, K. (1999). Sind neue Fassadenentwicklungen bauphysikalisch sinnvoll? Teil 1:
Transparente Wärmedämmung. Bauphysik, 21(Nr.1), 1–9.
Gratia, E., & De Herde, A. (2004). Optimal operation of a south double-skin facade. Energy and
Buildings, 36(1), 41–60. doi:10.1016/j.enbuild.2003.06.001
Hashemi, N., Fayaz, R., & Sarshar, M. (2010). Thermal behaviour of a ventilated double skin
facade in hot arid climate. Energy and Buildings, 42(10), 1823–1832. doi:10.1016/j.enbuild.2010.05.019
Hamza, N. & Abohela, I. (2012). Daylighting and thermal analysis of an obstructed double skin
façade in hot arid areas. In Proceedings of PLEA Conference, Munich, Germany.
Hamza, N. Gomaa, A. & Underwood, C. (2007). Daylighting and thermal analysis of an
unobstructed double skin façade in hot arid climates. In Proceedings of Clima 2007 Well Being Indoors.
Helsinki, Finland.
He, G., Shu, L., & Zhang, S. (2011). Double skin facades in the hot summer and cold winter zone
in China: Cavity open or closed? Building Simulation, 4(4), 283–291. doi:10.1007/s12273-011-0050-7
Huifen, Z., Yingchao, F., Fuhua, Y., Hao, T., Ying, Z., & Sheng, Y. (2013). Mathematical
Modeling of Double-Skin Facade in Northern Area of China. Mathematical Problems in Engineering,
2013. doi:10.1155/2013/712878
Manz, H. (2004). Total solar energy transmittance of glass double façades with free convection.
Energy and Buildings, 36(2), 127–136. doi:10.1016/j.enbuild.2003.10.003
Manz, H., & Frank, T. (2005). Thermal simulation of buildings with double-skin façades. Energy
and Buildings, 37(11), 1114–1121. doi:10.1016/j.enbuild.2005.06.014
Manz, H., Schaelin, A., & Simmler, H. (2004). Airflow patterns and thermal behavior of
mechanically ventilated glass double façades. Building and Environment, 39(9), 1023–1033.
doi:10.1016/j.buildenv.2004.01.003
Pasquay, T. (2004). Natural ventilation in high-rise buildings with double facades, saving or waste
of energy. Energy and Buildings, (4), 381–389. doi:10.1016/j.enbuild.2004.01.018
Pfuhler, J., Sikorski, E., & Kuhn, M. (2012). Modeling the Air Temperature Profiles in the Cavity
of a Double-skin Façade. International High Performance Buildings Conference. Retrieved from
http://docs.lib.purdue.edu/ihpbc/70
Saelens, D., Roels, S., & Hens, H. (2004). The inlet temperature as a boundary condition for
multiple-skin
facade
modelling.
Energy
and
Buildings,
36(8),
825–835.
doi:10.1016/j.enbuild.2004.01.005
30th INTERNATIONAL PLEA CONFERENCE
16-18 December 2014, CEPT University, Ahmedabad
8