Energy Performance Assessment of Multiple Skin Facdes

发布于:2021-10-14 11:04:29

Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

Energy Performance Assessment of Multiple Skin Facades
Dirk Saelens, Ph.D.
ABSTRACT In this paper, the energy performance of an office respectively equipped with a conventional insulated glazing unit with exterior shading and with three multiple skin facade typologies (an airflow window, a supply air window and a naturally ventilated window) is modelled and discussed under typical Belgian weather conditions. It is found that multiple skin facades may improve the energy efficiency. However, the analysis shows that variants performing well in winter are not necessarily beneficial in summer. Combining typologies or changing the systems settings according to the particular situation will be necessary to obtain an overall all year round improvement. The results further indicate that evaluating the energy efficiency of multiple skin facades can not be performed by analysing the transmission losses and gains solely. It is imperative to take into account the enthalpy change of the cavity air and to perform a whole building energy analysis.

Jan Carmeliet, Ph.D.

Hugo Hens, Ph.D.
Member ASHRAE

1. INTRODUCTION The sensibility for environmental friendly and energy conscious building design urged the need to develop new facade technologies. In the search towards energy efficient and visually attractive facades, multiple skin facades (MSFs) are regularly presented as being valuable solutions to follow the desires of modern architecture. MSFs (also known as active envelopes, second skin facades, twin-facades, etc.) consist of two panes with in between a cavity through which air flows. The driving force for the airflow is natural or mechanical ventilation. In the cavity, usually a shading device is provided. Generally, distinction is made between naturally and mechanically ventilated MSFs. Extensive literature on MSF-typologies can, for example be found in Compagno [1995], Gertis [1999], Ziller [1999], Baker et al [2000], Oesterle et al [2001] and Arons and Glicksman [2001]. In literature, numerous papers describe how MSFs should work to improve the building’s energy efficiency. As many variants exist, the principles to reduce the energy demand strongly depend on the chosen typology. Some authors sum up the working principles and ideas to improve the energy efficiency without providing calculation results or experiments [Lieb, 2001; den Boer and Ham, 2001; Arons and Glicksman, 2001]. Gertis [1999] correctly points out that only few simulations have been made and that only few measurements are available to support the claimed benefits. Other researchers provide models to simulate specific MSF typologies. They, however, do not link the envelope level results to the building energy performance or do not couple the model to a building energy simulation program [Holmes, 1994; Park et al, 1989, Tanimoto and Kimura, 1997; Helbig, 1999]. Only few combinations of MSF-modelling and building energy simulation are available. Most of these papers are restricted to only one MSF-typology. Müller and Balowski [1983] analyse airflow windows, Oesterle et al [2001] give a comprehensive survey of double skin facades and Haddad and Elmahdy [1998] discuss the behaviour of supply air windows. In this paper we focus on the energy saving objectives of three MSF typologies used in a single office. To simulate the energy demand of the office, a cell centred control volume model, describing the MSF, is coupled to a dynamic energy simulation program. The results of the energy simulations are compared and confronted with the objectives found in literature. We focus on one storey high solutions: (1) a conventional facade with an insulated glazing unit (IGU), (2) a naturally ventilated double skin facade (DSF), (3) a mechanically ventilated airflow window (AFW) and (4) a mechanically ventilated supply air window (SUP) (Figure 1). The traditional solution consists of an insulating glazing unit with a U-factor of 1.23 W/(m?K) and a Solar Heat Gain Coefficient (SHGC) of 0.59. The window is equipped with an exterior roller blind. The SHGC of the traditional solution with lowered roller blind equals 0.14. If we add a clear glass pane in front of the sunshading and allow exterior air to enter the cavity, a naturally ventilated double skin facade (DSF) is created. If the cavity is mechanically ventilated and provides ventilation air to the office, a supply air window (SUP) is designed. In the case of an airflow window (AFW), the insulating glazing unit is placed at the outside and the single glass at the inside. The cavity is mechanically ventilated with interior air. All systems are equipped with a roller blind, which is lowered as soon as the incident solar radiation exceeds 100 W/m?. It should be noted that roller blinds do not assist with daylighting and cannot be varied as Venetian blinds can.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

Table of Nomenclature
Latin Symbols
AFW airflow window BESP building energy simulation program C cavity specific heat capacity ca (J/(kgK)) DSF double skin facade g gravitational acceleration (m/s) volumetric airflow rate ga (m?/s) total airflow rate through Ga MSF-cavity (m?/h) Gr recirculation airflow rate (m?/h) hygienic ventilation Gv airflow rate (m?/h) Gw waste airflow rate (m?/h) H cavity height (m) I incident solar radiation (W/m?) IGU MSF Q S SHGC SUP T U insulating glazing unit multiple skin facade heat transfer rate (W) surface Solar Heat Gain Coefficient (-) supply air window absolute temperature (K) U-factor, overall heat transfer coefficient (W/(m?K))

Subscripts
a b cav cond conv cplg d e g i inf r r12 s t v w air direct solar radiation cavity conduction convection airflow due to coupling with other zone diffuse solar radiation exterior enthalpy flow interior infiltration airflow recirculation net radiation to surface 1 from surface 2 surface solar radiation on a tilted surface ventilation waste

Greek Symbols
α ?p η ρ θ ρ τ absortion coefficient (-) pressure difference (Pa) thermal efficiency (-) air density (kg/m?) temperature (°C) reflection coefficient (-) transmission coefficient (-)

2. ENERGY EFFICIENCY OBJECTIVES

Energy efficiency is probably the main argument to choose MSFs as a facade concept. MSF-systems are presented as being superior to single skin facades both during the heating and the cooling season. The energy efficiency objectives obviously depend on the MSF-typology. Nevertheless, two main principles can be distinguished: (1) multiple skin facades may reduce the transmission losses in winter and the transmission gains in summer and (2) MSFs can either reuse the return air in order to use the collected solar energy or recover some of the transmission losses or expel the return air to avoid overheating and to remove the absorbed solar radiation.
LOAD
LOAD

Ga

Gv
transmission
transmission

Gv

Gv

Ga

Gv

1. conventional facade (IGU)
LOAD

2. naturally ventilated double skin facade (DSF)
LOAD

Ga
transmission

Ga Ga
transmission

Ga

Ga

Ga
4. mechanically ventilated supply window (SUP)

3. mechanically ventilated airflow window (AFW)

Figure 1: Diagram of different multiple skin facades. Ga is the airflow rate through the MSF cavity, Gv is the hygienic ventilation airflow rate.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

2.1 Transmission

Almost all studies dealing with energy efficiency of MSFs mention the reduction of the transmission losses as a major advantage over traditional cladding solutions. In winter, it seems almost logical that the addition of an extra layer results in an increase of the thermal resistance of the system. Furthermore, the incident solar energy can be better captured, which further decreases the transmission losses and even may provide gains. This is often referred to as the buffer or conservatory effect and is most often mentioned in the context of double skin facades [Oesterle et al, 2001; Lieb, 2001 and McKlintock, 2001]. In the case of mechanically ventilated AFWs, the air exhausted from the interior through the cavity, raises the cavity temperature and reduces the temperature difference with the interior, hence reducing the transmission losses. In this respect, numerous reports and studies consider the decrease of the equivalent or dynamical U-factors with increasing airflow rate [Müller and Balowski, 1983; Holmes, 1994 and Helbig, 1999]. In summer, the cavity temperature may increase considerably by the absorbed solar radiation. Consequently, the unwanted indirect solar heat gain must be controlled by adequate cavity ventilation. Most authors mention the position of the sunshading in the cavity as a major advantage as it is protected from weathering and soiling and it will not have to be pulled up to protect it from damage [Arons and Glicksman, 2001; Lieb, 2001; McKlintock, 2001; Oesterle et al, 2001 and den Boer and Ham, 2001].
2.2 Cavity air enthalpy change

An important characteristic of MSFs is the airflow through the cavity. Some authors suggest to use the return air as an energy source. During heating demand, part of the transmission losses may be recovered and some absorbed solar energy can be used. This principle and the possibility to naturally ventilate high rise buildings explain the popularity of double skin facades in for example Germany [Gertis 1999, Arons and Glicksman, 2001]. Extensive analysis of room ventilation with double skin facades can be found in Ziller [1996 and 1999] and Oesterle et al [2001]. Supply air windows in turn aim at preheating the ventilation air. Haddad and Elmahdy [1998] compared conventional windows to supply air windows and concluded that the latter design results in higher net monthly gains. In summer, there is no need for extra gains. The exhaust air from an AFW unit can be easily expelled to the exterior if the temperature becomes to high. However, as the air flowing through the cavity of a supply air window and through some double skin facade solutions is used as ventilation air, it cannot be expelled to discard the excessive solar heat. Haddad and Elmahdy [1998] state that this would only result in a small penalty on the cooling load.
3. SIMULATION 3.1 Calculation object

An annual energy simulation on an hourly basis under Belgian climatic conditions is performed for a one person office. The office measures 4.0 by 6.0 by 3.0 m (width by depth by height). The glass to wall ratio is approximately 56 %, the transparent surface measures 3.00 by 2.25 m, the opaque part of the facade consists of an insulated cladding system with an overall U-factor of 0.33 W/(m?·K). The setpoint temperature for heating is 21 °C with a night setback to 16 °C. The setpoint temperature for cooling is 26 °C. Internal gains due to occupancy, lighting and office appliances have been considered according to ASHRAE guidelines [ASHRAE, 1997]. All systems are equipped with a roller blind, which is lowered as soon as the incident solar radiation exceeds 75 W/m?. The solar properties of the glass and roller blind are summarised in Table 1. In the analysis, the load or energy demand of the office is defined as the energy needed to keep the office temperature between the setpoints. The load includes the ventilation energy but excludes the energy needed to power the fans nor does it take into account distribution losses or inefficiencies due to the control mechanism.
Table 1: Properties of the glass and the roller blind. The solar properties are valid for normal incidence. (Manufacturer’s data). α (?) 0.099 0.115 0.370 ρ (?) 0.075 0.302 0.480 τ (?) 0.826 0.583 0.150

clear glass low E glass roller blind

emissivity (-) 0.837 0.090 0.850

First, we analyse the results of the “basic variants”. For mechanically ventilated MSFs (AFW and SUP) only the hygienic ventilation rate flows through the cavity (Ga = Gv = 0.5 ACH or 36 m?/h [ANSI/ASHRAE, 1989]),

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

The airflow rate through the cavity (Ga) is kept constant throughout the entire calculation. The airflow rate (Ga) through the naturally ventilated variant (DSF) instead depends on the weather conditions and varies with time. The office is not naturally ventilated but is mechanically ventilated with the hygienic ventilation rate (Gv). The results are compared and confronted with the results of the conventional facade (IGU). Secondly, we explore to what extend the energy efficiency may be improved by altering the airflow rate or reusing the return air. Also MSFs without cavity ventilation, which acts as triple glazed units with a betweenglass shading device (DSF/SUP 0 and AFW 0) will be discussed under “parameter analysis”. The abbreviations referring to the mechanically ventilated variants are labelled with the airflow rate through the cavity (Ga) which is expressed as the air change of the office. The abbreviations for the naturally ventilated facades express the number of grids opened at the inlet and outlet of the cavity. For instance DSF 01 indicates the double skin facade with one inlet and outlet grid open (Table 2).
Table 2: Nomenclature and cavity airflow rate (Ga) for the different variants.

Ga

double skin facade DSF (m?/h) basic variants DSF 01 variable DSF 0 0 parameter analysis DSF 02 variable

Gv 0.5 ACH = 36 m?/h Ga is the airflow rate through the cavity (m?/h) Gv is the ventilation airflow rate (m?/h)
3.2 Multiple skin facade model

airflow window AFW (m?/h) AFW 0.5 36 AFW 0 0 AFW 1.0 72 AFW 2.0 144 0.5 ACH = 36 m?/h

supply air window SUP (m?/h) SUP 0.5 36 SUP 0 0 SUP 1.0 72 SUP 2.0 144 0.5 ACH = 36 m?/h

In this study, the thermal behaviour is calculated with a MSF-model which is based on a cell centred finite volume method [Saelens, 2002]. The MSF is divided into four or five vertical layers (depending on the position of the roller blind: raised or lowered) which are in turn divided into 32 parts along the height (Figure 2). For each volume, the heat balance is written. The thermal system constructed that way is then solved. At the cavity surfaces (S1, S2 and S3), three heat transfer modes are taken into account: conduction (Qcond), convection (Qconv) and radiation (Qr). The convective heat transfer (Qconv) in the presented model depends on the airflow rate and the temperature difference between the surface and the air. Radiation heat transfer (Qr) for each surface is calculated with the net-radiation method [Siegel and Howell, 1992]. In the cavities (C1 and C2), the heat transfer is governed by convection and enthalpy transport due to the airflow. Heat transfer between the two panes of the double glazing (Qglass) is a combination of conduction, convection and radiation and is calculated using manufacturers data. The absorbed solar energy (Qs) is calculated for each separate layer with an embedded technique described by Edwards [1977]. It is function of the angle of incidence and also accounts for partial shading of the panes. The heat transfer with the surroundings (Qe and Qi) is described with a combined heat transfer coefficient. Long wave radiation is taken into account by means of an equivalent outdoor temperature. Finally, it is assumed that the roller blind perfectly obstructs air exchange between both cavities, an assumption which was confirmed by tracer gas measurements [Saelens, 2002]. With the MSF-model, both mechanically and naturally ventilated MSFs can be evaluated. The airflow rate in the mechanical flow variant is a known variable and hence the thermal system can be solved. As the airflow rate and the temperature profiles in the naturally ventilated envelope are mutually dependent, an iterative solution of the thermal system is necessary. The pressure difference (?p) due to thermal buoyancy has been determined with the following expression [Liddament, 1996]: ? Tcav ? ?p = ρ 0 ? g ? H ? ? [1] ? T ? 1? ? (Pa) ? e ?
with ρ0 the air density (kg/m?), g the gravitational acceleration (m/s?), H the cavity height (m), Tcav the absolute average cavity temperature and Te the absolute exterior temperature (K). This pressure difference is balanced by the measured airflow resistance of the cavity: 1 opened inlet and outlet grid [2] G a = 56.9 ? ?p 0.51 (m?/h)

2 opened inlet and outlet grids [3] G a = 168.9 ? ?p 0.44 (m?/h) where ?p is the pressure difference (Pa) over the inlet or outlet grids. We restrict this study to one storey high MSFs, as the accuracy of the MSF-model has only been checked for these variants [Saelens et al, 2001b].

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

S0 Qs0 Qcond0 QS1 Qglass Qcond0
S0 S1 C1

S1 Qcond1 Qconv1 Qcond1 Qr12

C1 Qg1,out Qs2
Qconv2,e

S2 Qcond2 Qconv2,i Qcond2 Qr23
S2 C2 S3

C2 Qg2,out Qs3 Qconv3 Qg2,in Qr32

S3 Qcond3 Qi Qcond3

Ex

Qe

In

Qg1,in Qr21

outer pane of outer glazing inner pane of outer glazing exterior cavity

roller blind inner cavity interior glazing

Figure 2: Diagram of the numerical model for the airflow window with lowered shading device. 3.3 Coupling with energy simulation program

The MSF-model is coupled to TRNSYS, a commercially available dynamic building energy simulation program (BESP). The MSF-model passes the inner pane average surface temperature (Ts,MSF), the cavity exhaust air temperature and the total transmitted solar energy (IMSF) to the BESP. The simulation program in turn provides the MSF-model with the incident direct (Ib,t) and diffuse (Id,t) solar radiation, the angle of solar incidence (θ), the zone air (Ta,3) and average surface temperature (part of Tstar), the exterior air (Ta,e) and sky temperature (Tsky) and the cavity air inlet temperature (Ta,1 = Tinlet,1) (Figure 3). As the results of both programs are mutually dependent, an iteration between the MSF-model and the BESP is carried out until convergence is achieved. Measurements [Saelens et al, 2001b and Saelens, 2002] showed that the cavity air inlet temperature hardly ever equals the interior air temperature for AFWs or the exterior temperature for double skin facades and supply air windows. Figure 4 shows the measured inlet temperature of a naturally and mechanically ventilated experimental set-up as a function of the exterior and interior temperature respectively. Both plots show two distinct regions: 1. In the first region, the inlet temperatures are positioned parallel to the line where the exterior or interior temperature is equal to the inlet temperature. This region represents the period without solar radiation. The inlet temperature is determined by the transmission losses and gains through the bounding surfaces. For the naturally ventilated facade, most inlet temperatures are slightly higher than the exterior air indicating that the air heats up while passing through the inlet zone. The temperatures above the line are caused by flow reversal. For the mechanically ventilated airflow window, the cooling of the inlet temperature is caused by transmission losses through the exterior surface. The inlet temperature depends on the airflow rate. When the airflow rate is lower, the inlet temperature cools down more. 2. In the second region, the inlet temperatures are notably higher than the exterior or interior temperature. This region represents the day situation. Then, absorption of solar radiation causes heating, which is more pronounced for the naturally ventilated facade. Heating in the airflow window again depends on the airflow rate. Comparison of the + and o signs in Figure 4 shows that when the airflow rate is higher, the inlet temperature remains cooler. Moreover, sensitivity studies [Saelens et al, 2001a and Saelens, 2002] indicated that the results of the MSFmodel are particularly sensitive to the inlet temperature. To account for these phenomena, the inlet zone is modelled as a separate thermal zone in the BESP (zone 1 in Figure 3). In this zone the transmission losses or gains, the solar gains and dynamic effects are accounted for to calculate the cavity inlet temperature.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

Toutlet,MSF Ib,t Id,t Ta,e Tsky θ Ts,MSF
zone 1

Qinf,3+Qv,3+Qcplg,3 Ta,2
zone 2

IMSF Tstar,3

Ta,3 Rstar,3

Qgain,3

Ta,1 Tinlet,i
BESP climate MSF model
from MSF model to TYPE 56 average interior surface temperature MSF (K) outlet temperature (K) total solar energy transmitted through MSF (W) Ta,i Ts,i Tinlet,i

zone 3

BESP
from TYPE 56 to MSF model = = = zone air temperature (K) area averaged temperature of opaque surfaces (K) inlet temperature (K)

climatic conditions from TRNSYS to MSF model and TYPE 56 Ta,e Tsky Ib,t Id,t θ

= ambient exterior air = Ts,MSF temperature (K) = sky temperature (K) Toutlet,MSF = = direct solar radiation incident IMSF = on MSF (W/m?) = diffuse solar radiation incident on MSF (W/m?) = angle of incidence (deg)

Figure 3: Diagram of the coupling between the MSF-model and the building energy simulation program (BESP) for the airflow window.
35 exterior temperature (°C) 30 25 20 15 10 10 20 30 40 50 inlet temperature (°C) interior temperature (°C) 23
Ga = 128 m?/h Ga = 37 m?/h

22

21

20 20 21 22 23 24 25 26 inlet temperature (°C)

a. Naturally ventilated variant

b. Mechanically ventilated variant

Figure 4: Scatter plot of the inlet temperature of a naturally (a) and mechanically (b) ventilated MSF versus the exterior (a) and interior (b) temperature during summer conditions as measured in the Vliet test building experimental set-up [Saelens, 2002]..

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

4. SIMULATION RESULTS 4.1 Basic variants a. Heating season

Figure 5 gives the annual heating demand per unit of floor surface for the basic envelope solutions for four orientations. The IGUs are not outperformed by the MSFs. The most energy consuming solution (DSF 01) requires on average 22 % more energy than the traditional IGU-facade. The AFW-variant (AFW 0.5) uses on average 18 % less energy and the SUP-window (SUP 0.5) has an energy demand that is only half that of the IGU. The differences in energy use due to the orientation have the same order of magnitude for all solutions. Due to the lower availability of solar radiation the north oriented facade requires the most energy. In order to explain the differences it is necessary to break up the energy demand in its different components. Figure 6 shows the heating load (Figure 6.a), the transmission through the inner pane (losses are positive) (Figure 6.b) and the direct solar gains (Figure 6.c) during two January days for the basic variants facing south. The IGU has the highest transmission losses (Figure 6.b), approximately 30% higher than the double skin facade (DSF 01). However, the latter requires more heating energy (Figure 6.a). This can be attributed to the difference in direct solar gains. The extra pane added to MSFs, lowers the direct solar gains by approximately 20 % compared to IGUs. The absolute values of the solar gain differences are higher than the difference in transmission losses, which favoured the DSFs. For DSFs, the indirect solar gains are hindered by the position of the window with the highest thermal resistance at the office side of the cavity. Consequently, the IGU requires less heating energy compared to the DSF. The airflow window (AFW 0.5) has the lowest transmission losses (Figure 6.b) and the second lowest heating demand. Again the direct solar gains are about 20 % lower compared to the IGU, but now the indirect gains play a much more important role. The position of the glass with the highest thermal resistance at the outside of the cavity makes the AFW the only variant with transmission gains during the first day on Figure 6.b. The low transmission losses together with the high indirect solar gains cause the AFW to have a lower heating demand than the IGU. The supply air window (SUP 0.5) is the only variant where the ventilation air is supplied through the cavity. Compared to the double skin facade (DSF 01), which has approximately the same transmission losses and gains (Figure 6.b), the south oriented SUP-window uses 65% less heating energy on an annual basis (Figure 5.a). The preheating of the ventilation air is caused by solar gains and recovery of part of the transmission losses. Comparison of the load (Figure 6.a) and the transmission losses (Figure 6.b) shows that the order of energy efficiency is only partially reflected in the transmission graph. Consequently, dynamic or equivalent U-factors are not suited to be used as energy performance indicators.
20.0 annual heating demand (kWh/(m?.a)) annual cooling demand (kWh/(m?.a)) 0.0

15.0 north east south west

-5.0 north east south west

-10.0

10.0

-15.0

5.0

-20.0

0.0 IGU DSF 01 AFW 0.5 SUP 0.5

-25.0 IGU DSF 01 AFW 0.5 SUP 0.5

a. Heating demand

b. Cooling demand

Figure 5: Annual heating and cooling demand per unit of floor area for the basic variants. b. Cooling season

Figure 5.b gives the annual cooling demand per unit of floor surface for the basic envelope solutions for four orientations. The difference between the minimum and maximum cooling load is much more pronounced than it was for the heating demand. Also the order of efficiency for cooling differs from the order for heating. Again, the insulated glazing unit is not outperformed. On the contrary: it requires 32 % less cooling energy than the naturally ventilated DSFs. The AFWs have still higher cooling demands and the SUP-window surpasses them all. Figure 7 shows the cooling load (Figure 7.a), the transmission through the inner pane (gains are negative) (Figure 7.b) and the direct solar gains (Figure 7.c) during two August days for the variants facing south.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

The cooling demand differences are governed by the differences in direct and indirect solar gains. Because of the extra pane, MSFs have lower direct solar gains (Figure 7.c). Moreover, the direct solar gains of the IGU may even be higher if the weather conditions do not allow the exterior shading device to be lowered. This, however, was not included in the simulation. Regarding the indirect solar gains, reflected in the transmission gains, the IGU with exterior shading device is the best option (Figure 7.b). Its exterior shading device cools down very efficiently towards the exterior. The shading device of MSFs is situated in the cavity, which makes it more difficult to remove the absorbed solar heat. Consequently, the cavity warms up and inevitably increases the indirect solar gains. The indirect gains depend largely on the thermal resistance of the MSF’s inner pane. Figure 7.b indicates that the AFW again represents the highest gains as the single pane between the heated cavity and the interior has the lowest thermal resistance. As the IGU also has the lowest cooling load (Figure 7.a), it may be concluded that the annual cooling demand of this office set-up is mainly determined by the indirect solar gains. This is further clarified by the correspondence between the cooling load and the transmission gains through the inner pane (Figures 7.a and 7.b). The relation is much more pronounced than for the heating demand, but again the transmission through the inner pane does not entirely reflect the cooling load. The SUP-window has transmission gains that are comparable to that of the DSF. However, due to the ventilation air warming up, which was an advantage during heating, the cooling demand increases considerably, unlike the findings of Haddad and Elmahdy [1998]. This result indicates that natural ventilation of offices through DSFs might not be an ideal option in summer.
900 800 700 600 500 400 300 200 100 0 192

0
DSF 01 IGU AFW 0.5 SUP 0.5

-100 load [W] -200 -300 -400 -500 -600 IGU DSF 01 AFW 0.5 SUP 0.5 48 transmission through inner pane [W] 72 96

load [W]

216

240

a. load
transmission through inner pane [W] 300 250 200 150 100 50 0 -50 -100 -150 192 216

a. load
200 100 0 -100 -200 -300 -400 -500 -600 -700 48

IGU DSF 01 SUP 0.5 AFW 0.5 240

IGU DSF 01 SUP 0.5 AFW 0.5 72 96

b. transmission through inner pane
800 direct solar gain [W] direct solar gain [W] 700 600 500 400 300 200 100 0 192 216 240 IGU MSF 800 700 600 500 400 300 200 100 0

b. transmission through inner pane
shading device down IGU MSF

48

72

96

c. direct solar gain Figure 6: Load, transmission and solar gains during two January days. Excerpt from the annual energy calculations of the south facing office.

c. direct solar gain Figure 7: Load, transmission and solar gains during two August days. Excerpt from the annual energy calculations of the south facing office.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

4.2 Parameter analysis a. Double skin facades

The airflow rate in the naturally ventilated double skin facades is governed by thermal buoyancy and pressure differences due to wind. Thermal buoyancy causes an upward flow in the cavity, wind pressure differences may both cause upward and downward flow, depending on the particular situation. An experimental study [Saelens and Hens, 2001] on a similar DSF, situated at ground level, showed that the main direction of the flow was upwards. It was found that the main driving force in summer was thermal buoyancy. In winter, both driving forces governed the airflow. In the present simulations, the effect of wind on the airflow rate is not incorporated as it would require the knowledge of the pressure distribution over the building envelope for various wind directions. As a result, the results mainly characterise the summer conditions. A detailed analysis [Saelens, 2002] showed that the influence of taking into account wind-induced airflow on the energy demand on yearly basis is smaller than 3 %. In winter, the heating demand is expected to be underestimated as wind pressure effects are not accounted for. In summer, wind induced airflow may be desirable as long as it increases the ventilation rate and hence reduces the transmission gains. Table 3 shows that both the annual heating and cooling load are directly related to the number of open ventilation grids and consequently to the airflow rate. In winter, extra ventilation should be avoided as it lowers the cavity temperature and increases the transmission losses. Closing the grids of the naturally ventilated (DSF/SUP 0) facade results in a higher cavity temperature, which explains the 8 % lower annual heating demand compared to the ventilated DSF 01. However, for this particular south orientation the heating demand is still 20 % higher than that of the IGU-facade. In summer, it is useful to ventilate the cavity as much as possible as it effectively lowers the transmission gains and hence the cooling load. The cooling load is the highest if there is no airflow which can remove the absorbed solar heat (DSF/SUP 0). As a result, it is advisable to foresee adjustable grids which may be opened and closed according to the climatic conditions.
b. Airflow windows

The analysis of the basic airflow window variant (AFW 0.5) demonstrated that it was possible to lower the transmission losses and hence, the energy demand for heating by ventilating the cavity with interior air. In summer, it was possible to remove some of the absorbed solar energy and consequently lower the indirect solar gains. If the cavity is not ventilated (AFW 0), the annual heating demand raises (Table 3) as the resulting cavity temperature is lower than for the ventilated case. Obviously, the cooling demand increases because the absorbed solar energy is no longer removed from the cavity. The question raises if it is useful to increase the airflow rate through the cavity or to reuse the return air in order to further improve the energy efficiency? Increasing the airflow rate, lowers the temperature difference between the cavity and the office. Both during heating and cooling season a higher airflow rate therefore lowers the transmission losses or gains respectively. However, the translation to the overall building energy performance is not straightforward. To increase the airflow rate in the AFW (Ga), we have to increase the airflow rate through the office in order to conserve the mass balance of the office. There are three main possibilities to conserve the mass balance (Figure 8): (a) Provide additional exterior air. (b) The additional exterior air may be replaced by waste air from other zones (Ga = Gv + Gw). (c) Recirculation of air returning from the AFW-cavity (Ga = Gv + Gr). Table 3 shows that the resulting annual heating and cooling load are strongly influenced by the chosen option.
LOAD

LOAD

LOAD

Ga

Ga

Gv
transmission

transmission

transmission waste air

Gr
recirculation air

Ga

Ga
exterior air

Ga

Ga

Gw Gv
exterior air

Ga

Ga

Gv
exterior air

a. all exterior air b. waste air c. recirculation Figure 8: Diagram of different airflow windows set-ups. Ga is the airflow rate through the airflow window cavity, Gv is the office hygienic ventilation rate, Gw is the waste air airflow rate (Ga = Gw + Gv) with the restriction that Ga should be smaller than the building’s total hygienic ventilation rate and Gr is the recirculation airflow rate (Gr = Ga – Gv).

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

(1) In winter, it is not beneficial to increase the airflow rate above the office’s ventilation rate if the mass balance is to be satisfied with exterior air. The decrease in transmission losses is too small (Figure 9.a) compared to the surplus energy needed to heat up the exterior air. In summer, the higher airflow rate through the cavity reduces the transmission gains (Figure 9.b). However, the considerable decrease in annual cooling load is more a consequence of free cooling due to the increased airflow rate rather than it may be attributed to the decrease in indirect gains. (2) If the extra air to adhere to the office mass balance is waste air from another (for instance interior) zone with the same interior temperature, the decrease in transmission losses is genuine as there is no need to heat up exterior air. Enthalpy losses in the cavity do not have to be considered in this case. It should be stressed that the airflow rate through the cavity should not exceed the total building’s hygienic ventilation rate (Ga = Gv + Gw ≤ Gv,tot). Otherwise, the decrease in transmission losses is no longer genuine as the extra exterior air has to be taken into account. The AFW may be considered as an exhaust duct for the waste ventilation air and acts as a heat-exchanger. The efficiency of lowering the transmission losses compared to the insulated glazing unit (IGU) may be defined as: Q transmission,IGU ? Q transmission,AFW η= [-] [4] ρ a ? c a ? g a ? (θ i ? θ e )
where: Qtransmission,IGU and Qtransmission,AFW are the transmission losses of the IGU and AFW respectively, ρa is the air density, ca is the thermal capacity of air, ga is the cavity airflow rate and θi and θe are the interior and exterior air temperature respectively. The nominator represents the ventilation losses. The average efficiency is rather disappointing: only 26.0 % (AFW 0.5), 20.1 % (AFW 1.0) or 13.8 % (AFW 2.0) of the lost ventilation energy is used to lower the transmission losses. These numbers are much lower than the efficiency which can be obtained with a heat-exchanger unit on the exhaust ducts. In summer and even during the intermediate season, the cavity temperature may increase considerably causing undesired heating. It was demonstrated that the indirect solar gains represent a substantial part of the cooling load of AFWs. In order to decrease the transmission gains, increasing the airflow rate through the cavity proves to be efficient (Figure 9.b). Table 3 indicates that increasing the airflow rate is useful to lower the heating and cooling demand provided that the cavity is ventilated with waste air. The resulting energy reduction for cooling is more pronounced than the reduction for heating. (3) Some authors suggest to reuse the return air to use the collected solar energy and to recover some of the transmission losses. We have to bear in mind that not all returning air can be reused. Some of the air has to be replaced by fresh air (Gv), inevitably losing some of the gained energy (Figure 8.c). Table 3 shows however that on annual basis it is not useful to reuse the return air to decrease the heating nor the cooling load. Figure 10 shows the net energy gain of the AFW for different airflow rates during two January days for a south facing office. It is defined as the reduction of the transmission losses compared to the unventilated AFW variant (?Qtransmission) minus the enthalpy loss of the air flowing through the cavity: net energy gain = ?Q transmission ? ρ a c a g a (θ inlet ? θ outlet ) (W) [5] At the bottom of Figure 10, the resulting energy demand of the basic variant (AFW 0.5) is plotted. During daytime, when there is a heating demand, the gain in transmission losses is offset by the enthalpy loss of the cavity air (negative values of the net energy gain). This offset is even more pronounced if the airflow rate through the cavity is increased. Hence it is not beneficial to increase the airflow rate to lower the transmission losses if the cavity air will be reused. If the incident solar radiation is high enough, the AFW gains energy and acts as a solar collector. This is evidenced by the peak on the left part of Figure 10. The solar gain increases with increasing airflow rate. Nevertheless, the annual heating demand increases for higher airflow rates as the solar gains are less important than the net energy loss (Table 3). In summer, the cavity usually becomes quite hot. Increasing the ventilation rate lowers the indirect gains but it is obvious that reusing the air is useless as we want to expel all extra energy. Consequently, the cooling demand increases with increasing airflow rate (Table 3). The analysis showed that it is possible to further decrease the heating load by ventilating the cavity with waste air. This option was also advantageous to lower the cooling loads, but allowing free cooling with exterior air was the easiest way to control these loads. The reuse of air flowing through the cavity not only imposes the need for complex ductwork, mixing chambers and valves, it also proved to be not beneficial. Furthermore, it has to be noted that the present results have to be seen as best practice. A study of a real life building equipped with mechanically ventilated MSFs [Saelens and Hens, 1998], indicated for example that air infiltration in the cavity and badly insulated ducts may strongly affect the performance of an AFW.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

300 transmission through inner pane [W] 200 100 0 -100 -200

transmission through inner pane [W]

losses

200 100 0 -100 -200 -300 -400 -500 -600 -700 -800

losses

AFW 0 AFW 0.5 AFW 1.0 AFW 2.0 gains

gains
48 72

AFW 2.0 AFW 1.0 AFW 0.5 AFW 0
96 120

-300 168 192 216 240

a. winter b. summer Figure 9: Transmission losses and gains for airflow windows as a function of the cavity airflow rate (Ga).
40 30 20 net energy gain [W] 10 0 -10 -20 -30 -40 -50 -60 192 216 0 240 1000 500 1500

AFW 0.5 AFW 1.0 AFW 2.0

3000 2500 2000
load AFW 0.5 [W]

Figure 10: Above: The net energy gain (transmission losses reduction – enthalpy losses) of the airflow window for different airflow rates during a winter situation. Below: The resulting energy demand of the basic variant (AFW 0.5). c. Supply air windows

The basic variant of the supply air window (SUP 0.5) reduces the heat demand by preheating the ventilation air. Unfortunately, this causes a penalty on the cooling load. Changing the airflow rate affects the transmission losses and changes the enthalpy gains. If the airflow rate is increased, the cavity temperature tends more to the exterior temperature. This results in higher transmission losses during the heating season. During the cooling season, the transmission gains decrease. Consequently, increasing the airflow rate to change the transmission losses or gains is only advantageous during cooling demand. During the heating season, the transmission losses increase with increasing airflow rate. Furthermore, it is only useful to allow a higher airflow rate than the ventilation rate if the outlet temperature of the cavity air is higher than the interior air temperature. In winter, this is usually not the case (Figure 11.a), resulting in an important increase of the annual heating demand when the airflow rate is increased (Table 3). During cooling season, high airflow rates are in favour regarding transmission gains. Increasing the airflow rate lowers the raise in cavity temperature and hence the indirect solar gains. If the cavity outlet temperature surpasses the interior temperature, the enthalpy gain increases with increasing airflow rate because a bigger amount of only slightly cooler air enters the office. However, this does not result in a higher annual cooling load because of free cooling during night-time (Figure 11.b). Table 3 shows that the resulting annual cooling load decreases with increasing airflow rate. The analysis of the supply air windows shows that the advantage of increasing the airflow rate depends on the heating or cooling season. It is not useful to increase the airflow rate above the ventilation rate in winter as the cavity air usually is lower than the interior temperature. In summer, the increase of the airflow rate is useful as the free cooling effect results in a noticeable decrease of the cooling load. However, it should be stressed that night-time ventilation is not a MSF-specific contribution. This contradiction indicates that the ability of changing the airflow rate throughout the year, would offer a substantial energy saving potential. Such a system needs a sophisticated building energy management system to choose the optimal ventilation settings. Furthermore, the additional fan power will have to be considered. Also the combination of a SUP-window in winter and a DSF with sufficient cavity ventilation in summer could both lower the heating and cooling load. But again an advanced building energy management system is needed to choose the optimal settings. Moreover, in summer it may be necessary to cut off the office ventilation through the cavity, which may require extra ducting to provide the necessary hygienic ventilation.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

Table 3: Overview of annual heating and cooling demand per unit of floor area (kWh/(m?.a)) for a south oriented facade. Heating demand (kWh/(m?.a)) IGU 12.2 DSF/SUP 0 DSF 01 DSF 02 13.9 15.1 15.9 AFW AFW 0.5 AFW 1.0 AFW 2.0 0 10.1 9.4 41.0 112.6 / / 9.2 9.1 / / 9.5 10.3 Cooling demand (kWh/(m?.a)) IGU -5.1 DSF/SUP 0 DSF 01 DSF 02 -8.2 -6.7 -5.7 AFW AFW AFW AFW 0 0.5 1.0 2.0 -9.7 -8.8 -0.1 0.0 / / -8.4 -7.1 / / -8.8 -19.3

Insulated glazing unit Double skin facades Airflow windows 1. all exterior air 2. waste air 3. recirculation Supply air windows

SUP 0.5 5.2
600 400 200 load (W) 0 -200 -400 -600 -800 192 216 outlet temperature load

SUP 1.0 27.0

SUP 2.0 80.8
0

SUP 0.5 -21.1

SUP 1.0 -11.8

SUP 2.0 -4.4
80

70 50 temperature (°C) load (W) 30

-50 -100 -150

load

70 60 temperature (°C) 50

interior temperature -200 -250 -300 SUP 0.5 SUP 1.0 SUP 2.0 4872

40 30 20 10 0 -10 4896

interior temperature SUP 0.5 SUP 1.0 SUP 2.0

10 -10 -30 240

-350 -400 4848

outlet temperature

a. winter b. summer Figure 11: Load, interior air temperature and cavity outlet temperature of the supply air window for different airflow rates during two winter (a) and two summer days (b).
4. CONCLUSIONS

In this paper, the energy efficiency of different kinds of one-storey multiple skin facades was analysed, compared to objectives found in literature and compared to the performance of a traditional cladding system with exterior shading device. The reduction of the transmission losses, the possibility of recovering the transmission losses by the airflow, the position of the shading device sheltered from climatic conditions and the ability to remove the absorbed solar heat are the most commonly mentioned energy advantages. It is shown that it is possible to improve the building’s energy efficiency in some way by using multiple skin facades. Unfortunately, most typologies are incapable of lowering both the annual heating and cooling demand. Only by combining typologies or changing the system settings according to the particular situation, a substantial overall improvement over the traditional insulated glazing unit with exterior shading is possible. This implies that sophisticated control mechanisms are inevitable to make multiple skin facades work efficiently throughout the year. In order to correctly evaluate the energy efficiency an annual energy simulation focussing on both heating and cooling load is necessary. Furthermore, the analysis shows that the energy performance strongly depends on the way the cavity air is used. In order to correctly evaluate the energy efficiency of multiple skin facades, it is imperative not only to study the transmission gains and losses but also to take into account the enthalpy change of the cavity air and to perform a whole building energy analysis.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

5. ACKNOWLEDGEMENTS

This research is funded by a research grant of the Flemish Institute for the Promotion of Industrial Scientific and Technological Research (IWT: Vlaams Instituut voor de van het Wetenschappelijk-Technologisch Onderzoek in de Industrie). Their financial contribution is gratefully acknowledged.
6. REFERENCES

ANSI/ASHRAE 62-1989, Ventilation for Acceptable Indoor Air Quality. Arons D.M.M. and Glicksman L.R., 2001, Double Skin, Airflow Facades: will the Popular European Model work in the USA?, Proceedings of ICBEST 2001, International Conference on Building Envelope Systems and Technologies, Ottawa, Canada, Vol. 1:203-207. ASHRAE, 1997, 1997 ASHRAE Handbook - Fundamentals, Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Baker P.; D. Saelens; M. Grace; and T. Inoue 2000, Advanced Envelopes - Methodology, Evaluation and Design Tools, Final report IEA Annex 32 (IBEPA), Leuven: Acco. Compagno, A., 1995, Intelligent Glass Fa?ades.- Material, Practice, Design, Basel: Verlag für Architektur. den Boer T.L.J. and Ham M., 2001, Facade Orientation as a Major Design Consideration, Proceedings of ICBEST 2001, International Conference on Building Envelope Systems and Technologies, Ottawa, Canada, Vol. 1, pp. 45-49. Edwards, D.K. 1977, Solar Absorption by Each Element in an Absorber-Coverglass Array, Solar Energy, Vol. 19, pp. 401-402. Gertis, K. 1999, Sind neuere Fassadenentwickelungen bauphysikalisch sinnvoll? Teil 2: Glas-Doppelfassaden (GDF) (in German), Bauphysik, Vol. 21, pp. 54-66. Haddad K.H. and Elmahdy A.H., 1998, Comparison of the Monthly Thermal Performance of a Conventional Window and a Supply-Air Window, ASHRAE Transactions, Vol. 104, Part 1B, pp. 1261-1270. Helbig, S. , 1999, Energetische Bewertung von Zuluftfenstern (in German), Proceedings of the 10th International Symposium for Building Physics, Dresden, pp. 539-548. Holmes, M. J., 1994, Optimisation of the Thermal Performance of Mechanically and Naturally Ventilated Glazed Facades, Renewable Energy, Vol. 5, pp. 1091-1098. Liddament, M.W. 1996, A guide to Energy Efficient Ventilation, Warwick: Air Infiltration and Ventilation Centre. Lieb, R.D., 2001, Building Physical Values of Double Skin Facades, Proceedings of ICBEST 2001, International Conference on Building Envelope Systems and Technologies, Ottawa, Canada, Vol. 1, pp. 51-54. McKlintock, M., 2001, Case Study Analysis and Comparison of Naturally Vented, Double Skin Facade Performance, Proceedings of ICBEST 2001, International Conference on Building Envelope Systems and Technologies, Ottawa, Canada, Vol. 2, pp. 187-193. Müller H.; and Balowski M., 1983, Waste Air Ventilated Windows for Offices, Heizung Luftung Haustechnik, Vol. 34, n° 10, p. 412-417. Oesterle E.; Lieb R.-D., Lutz M., Heusler W. , 2001, Double-Skin Facades. – Integrated Planning, Munich: Prestel. Park, S.D.; H.S. Suh,; and S.H. Cho 1989, The Analysis of Thermal Performance in an Airflow Window System Model, Proceedings of the ASHRAE/DOE/BTECC/CIBSE Conference, Thermal Performance of the Exterior Envelopes of Buildings IV, pp. 361-375. Saelens, D.; and H. Hens, 1998, Active Envelopes - Essential in Urban Areas?, Proceedings of the 19th AIVC Annual Conference, Ventilation Technologies in Urban Areas, Oslo 28-30 September, pp. 467-476. Saelens, D.; and Hens, H., 2001, Experimental evaluation of naturally ventilated active envelopes, International Journal of Thermal Envelopes and Building Science, vol. 25, nr. 2, pp. 101-127. Saelens, D.; J. Carmeliet; and H. Hens, 2001, Modeling of Air and Heat Transport in Active Envelopes, Proceedings of ICBEST 2001, International Conference on Building Envelope Systems and Technologies, Ottawa, Canada, June 2001, pp. 243-247. Saelens, D.; J. Carmeliet; and H. Hens 2001, Evaluating the Thermal Performance of Active envelopes, Proceedings of Performance of Exterior Envelopes of Whole Buildings VIII: Integration of Building Envelopes, Clearwater Beach, Florida, December 2001, pp. 243-247. Saelens, D., 2002, Energy Performance Assessment of Multiple-Skin Facades, PhD dissertation, Leuven: KU Leuven, Laboratory for Building Physics. Siegel, R.; and Howell, J.R. 1992, Thermal Radiation Heat Transfer, London: Taylor and Francis. Tanimoto, J. and Kimura, K. , 1997, Simulation Study on an Airflow Window System with an Integrated Roll Screen., Energy and Buildings, nr. 26, pp. 317 - 325.

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Saelens D., Carmeliet J., and Hens H., 2003, Energy Performance Assessment of Multiple Skin Facades, International Journal of HVAC&R Research., vol. 9, nr. 2, pp.167-186.

Ziller, C.; Sedlacek, G.; Ruscheweyh, H; Oesterle, E. and Lieb R.-D, Naturliche Belüftung eines Hochhauses mit Doppelfassade (in German), KI, nr. 8, 1996, pp. 343-346. Ziller, C. 1999, Modellversuche und Berechnungen zur Optimierung der natürlichen Lüftung durch Doppelfassaden (in German), PhD dissertation, Aachen: Shaker Verlag.

Dirk Saelens, dr.ir. is member of the scientific staff at the Laboratory of Building Physics and the author to whom correspondence should be addressed. Jan Carmeliet, prof. dr. ir. is associate professor at the Laboratory of Building Physics. Hugo Hens, prof. dr. ir. is full professor and head of the Laboratory of Building Physics and ASHRAE member.

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