Assessment of the double-skin façade passive thermal ... · a) b) c) Figure 1 a) Double skin...

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.

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

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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 (Hashemi et al., 2010) document a difference in temperature of 5–12 ◦C on the 7th floor and on 11th 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).


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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.


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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 over-shadowing. 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


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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).


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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.


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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


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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 Tdsf_Xs = surface temperature on external skin of dsf Tenv_est_Xs = surface temperature on external skin of building envelope Tenv_int_Xs = surface temperature on internal skin of building envelope REFERENCES Fuliotto, R. (2010). Experimental and numerical analysis of heat transfer and airflow on an

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Transparente Wärmedämmung. Bauphysik, 21(Nr.1), 1–9. Gratia, E., & De Herde, A. (2004). Optimal operation of a south double-skin facade. Energy and

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Assessment of the double-skin façade passive thermal ... · a) b) c) Figure 1 a) Double skin...

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