Strategies to improve the energy performance of multiple-skin facades

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Building and Environment 43 (2008) 638–650 www.elsevier.com/locate/buildenv

Strategies to improve the energy performance of multiple-skin facades Dirk Saelensa,, Staf Roelsb, Hugo Hensb a

VK Engineering, Building Services, Clemenceaulaan 87, B-1070 Brussel, Belgium Laboratory for Building Physics, Department of Civil Engineering, K.U. Leuven, Kasteelpark Arenberg 40, B-3001 Leuven, Belgium

b

Received 30 November 2005; received in revised form 6 April 2006; accepted 15 June 2006

Abstract This paper describes how to optimize the energy performance of single-story multiple-skin facades (MSF) by changing the settings of the fac- ades and HVAC-system according to the net energy demand of the building. The annual energy performance is analyzed for typical Belgian climatic conditions using a whole building simulation tool. Three MSF are scrutinised: a mechanically ventilated airflow window, a naturally ventilated double-skin fac- ade and a mechanically ventilated supply window. Their performance is compared against the performance of two traditional facades: a traditional window with exterior and interior shading device. It is shown that both the heating and cooling demand may significantly be improved by implementing control strategies such as controlling the airflow rate and the recovery of air returning from the multiple-skin facades. r 2006 Elsevier Ltd. All rights reserved.

1. Introduction The awareness for environmental friendly and energy conscious building design urges 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, twinfacades, etc.) consist of two panes separated by 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 [1–6]. 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 (e.g. [7,8]). However, calculation results or experimental results are seldom Corresponding author. Tel.: +32 2 414 07 77; fax: +32 475 98 07 36.

E-mail address: [email protected] (D. Saelens). 0360-1323/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2006.06.024

given. Other researchers present sophisticated models to simulate specific MSF typologies [9–15]. They, however, lack to link the envelope level results to the building energy performance or do not couple the model to a building energy simulation program. The coupled simulation of the building and the building components is however imperative to come to a correct assessment of the energy performance. It is the only way to model and assess the complex interaction between air flow in the fac- ade and the HVAC system and the building energy management system. Especially when the settings of the different systems change to optimize different performances, whole building simulations are essential. Only few combinations of MSF-modelling and building energy simulation are available. Most of this research is restricted to only one MSF-typology. Mu¨ller and Balowski [16] analyze airflow windows, Oesterle et al. [5] give a comprehensive survey of double skin facades and Haddad and Elmahdy [17] discuss the behavior of supply air windows. Recent studies have recognized that an optimal control strategy combined with an overall building analysis is necessary to increase the energy efficiency of MSFs [18–21]. In this paper, the energy demand of an office equipped with three MSF-typologies is discussed and optimized. The optimization strategies in this paper are restricted to the settings of the HVAC-system. Further optimizations which

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are not discussed here include the analysis of the system parameters such as the geometry of the fac- ade and the properties of the glazing and shading device. First, the building geometry and different models are presented. Then, different optimization strategies for all three MSFtypologies are derived from the energy performance of the non-optimized base case MSFs. Many of the optimization strategies need specific measuring equipment and have an impact on the construction of the HVAC-system. For all suggested strategies these implications are highlighted and commented on. Finally, the results are compared with the energy performance of optimized traditional solutions.

2. Case description Three multiple-skin fac- ades and two traditional cladding systems are analysed. The first MSF is a mechanically ventilated airflow window (AFW) (Fig. 1(a)). The outer glass is an argon filled low emission glass. The shading device is a roller blind, positioned in the cavity. The inner glass is a normal clear float glass. The cavity is mechanically ventilated with interior air. The second MSF is a naturally ventilated double-skin fac- ade (DSF) (Fig. 1(b)). The outer glass is a normal clear float glass. The shading device is the same roller blind and is also positioned in the cavity. The inner glass is a similar argon filled low emission glass. The cavity is naturally ventilated with exterior air. The third MSF is a supply window (SUP) (Fig. 1(c)). The glass configuration is similar as for the DSF, but the cavity is now mechanically ventilated with exterior air. The two traditional cladding systems have an

a

c

b

d

e

639

Table 1 Glass and shading device properties Glass

Single Double

Glass properties U-value (W/(m2 K))

g-value ()

5.67 1.23

0.85 0.43

Shading device properties at normal incidence Device a() r() Roller blind 0.83 0.09

t() 0.08

argon filled low emission glass. The shading device is an exterior (IGUe) (Fig. 1(d)) or interior (IGUe) (Fig. 1(e)) roller blind. Table 1 summarizes the properties of the glass and shading device. These five fac- ade systems are used in a narrow office building. The main facades look north-east and south-west. Fig. 2 shows a section of a typical floor equipped with an AFW. The fac- ade consists of an opaque part that covers the false ceiling zone (height ¼ 0.75 m) and a transparent part that runs from the floor covering to the false ceiling (height ¼ 3.25 m). The false ceiling contains the HVAC supply and return ducts. The floor is subdivided in three zones: a north-east external zone, an internal zone and a south-west external zone. Each zone has a depth of 4.50 m. In the analysis, the load or energy demand is presented as an average per unit of office floor and is subdivided per zone. It is defined as all the sensible energy needed to keep the office temperature between the setpoints. The energy demand includes the energy to make up the ventilation air, control inefficiencies and distribution losses or gains of the air ducts but excludes the energy needed to power the fans nor does it take into account the efficiency of the HVAC-plant. The setpoint temperature for heating is 21 1C with a night set-back to 16 1C. The setpoint temperature for cooling is 26 1C. There is no cooling during night-time. The office hours run from 7 a.m. till 7 p.m. Also during the weekend the set-back settings are applied. To maintain the temperatures between the setpoints a PID (proportional, integrating and differentiating) controller that senses the operative temperatures of each zone is implemented. The operative temperature is defined as a weighted average of the air temperature and mean surface temperature. Weighting factors are 0.55 and 0.45, respectively. Typical internal gains due to occupancy (7 W/m2), lighting (10 W/ m2) and office appliances (20 W/m2) are used. On top of the air conditioning, the offices are equipped with chilled ceilings, which is a common feature in offices with MSFs. All simulations use the climatic conditions of the Test Reference Year of Uccle, Belgium. 3. Modelling

Fig. 1. Schematic representation of the multiple skin facades and the traditional solutions.

To analyze the energy demand of offices equipped with MSFs, a modelling environment has been developed. The

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Fig. 2. Schematic representation of the simulated office floor for a typical AFW configuration.

environment consists of a model of the facades, a model of the office zones, a model of the heating and cooling system and a model of the building energy management system (Fig. 3). All models are combined into a commercially available dynamic building energy simulation program (BESP) TRNSYS 15.3 [22]. The thermal behavior of the MSF and traditional facades is calculated with a cell centered finite volume method [18] (Figs. 3(b)–(e)). The MSFs are 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. 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 (Qc) and radiation (Qr). The convective heat transfer (Qconv) depends on the flow regime, the airflow rate and the temperature difference between the surface and the air. An elaborate overview of the used equations can be found in [18]. Radiation heat transfer (Qr) for each surface is calculated with the netradiation method [23]. In the cavities (C1 and C2), the heat transfer is governed by convection and enthalpy transport due to the airflow. Heat transfer between both 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 [24]. 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, applied in this set-up, perfectly obstructs air exchange between both cavities. This assumption was confirmed by measurements [18] and CFD-simulations [25]. A comparison of the presented model for the mechanically ventilated airflow window and the naturally

ventilated double-skin facades against measurements is presented in [18]. The same study also analyzes the sensitivity of the energy demand to different modelling assumptions. A similar model is used to calculate the thermal behavior of the traditional solutions (IGUs). When the shading device is lowered, a naturally ventilated cavity is formed between the glazing and the roller blind. The cavity of the variant with exterior shading device is ventilated with exterior air (Fig. 3(d)). That of the variant with interior shading device is ventilated with interior air (Fig. 3(e)). The airflow rate in the mechanical flow variant is a known variable and hence the thermal system can be solved easily. As the airflow rate and the temperature profiles in the naturally ventilated cavity are mutually dependent, an iterative solution of the thermal system is used. Fig. 3(a) further shows the interconnection between the fac- ade model and the office zone for the mechanically ventilated airflow window. The MSF-model passes the inner pane average surface temperature (Ts,MSF), the cavity exhaust air temperature (Tout,MSF) and the total transmitted solar energy (IMSF) to the BESP. The latter is distributed over the floor area (qs in Fig. 3(a)). 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 (a), the zone air (Ta,3) and average surface temperature (part of Tstar,3), the exterior air (Ta,e) and sky temperature (Tsky). Saelens et al. [26] stress the importance of the inlet temperature on the energy performance of multiple-skin facades. Similarly, in cases where the return air of the multiple-skin fac- ade is to be reused, a correct modelling of the outlet temperature is equally important. Therefore the inlet (zone 1 in Fig. 3(a)) and outlet (zone 2 in Fig. 3(a)) zone have been modelled separately. The office zone is subdivided in a workspace zone (zone 3 in Fig. 3(a)) and a false ceiling zone (zone 4 in Fig. 3(a)) in which the HVAC return and supply ducts are placed. Several studies show the importance of taking into

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a

b

c

d

e

Fig. 3. Schematic representation of the modeling environment and the numerical models. Diagrams show the control volumes in case of lowered shading devices. (Legend: Qe: heat exchange with exterior surroundings; Qcond: conductive heat transfer in the layers; Qglas: heat transfer between the panes of the double glazing; Qs: absorbed solar radiation; Qc: convective heat transfer; Qrxy: net longwave radiation from surrounding surfaces y to surface x; Qi: heat exchange with interior surroundings; and Qg: enthalpy flow).

account the gains and losses from the supply and return duct in the false ceiling [27,26]. These heat fluxes are accounted for by coupling the losses and gains from the supply and return ducts to the false ceiling zone. As the results of the MSF-model and the BESP-models are mutually dependent, during each time step an iteration is carried out until convergence is achieved. In case of the traditional solutions, the inlet and outlet zone are not ventilated. The implementation of the surface temperature and the incoming solar radiation remains the same. When the shading is lowered, the interior or exterior temperature act as inlet temperature for the cavity. In case of the traditional solution with interior shading device the extra heat source from the air flowing out of the cavity is

added to the convective gain of the workspace zone (qgain,3 in Fig. 3(a)). The internal gains are split into a convective and radiative part according to the ASHRAE-guidelines [28]. The convective internal gains are added to the air temperature as a part of qgain,3 (Fig. 3(a)), the radiative internal gains are distributed over the internal surfaces proportional to the surface area (qg,r in Fig. 3(a)). The air handling unit provides heating and cooling. 4. Optimization strategies Energy efficiency is an important argument to choose MSFs as facade concept. MSF-systems are presented as

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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 expel the return air to avoid overheating or reuse the return air in order to use the absorbed solar energy and recover some of the transmission losses. Fig. 4 shows the energy demand per unit of floor area for the different fac- ade solutions without optimization strategies. For each fac- ade solution the results are split up per zone. In the non-optimized cases, the cavity ventilation of the mechanically ventilated MSFs (AFW and SUP) equals the hygienic ventilation rate (Ga ¼ Gv ¼ 20 m3/(h.m)). The airflow rate (Ga) through the naturally ventilated cavity (DSF) depends on the weather conditions and varies with time. As for the traditional cladding systems (IGUe and IGUi) the office zones are mechanically ventilated with the hygienic ventilation rate (Gv). During night-time and weekends the airflow rate is set back to a minimum airflow rate which equals half the required ventilation rate.

heating demand (kWh/(m2.a))

a

60 QH_I 50 40

QH_SW 38.0

QH_NE 36.2

35.3 31.6

30 17.9

20 10 0 IGUe

IGUi

AFW

DSF

SUP

Heating demand

cooling demand (kWh/(m2.a))

b

0 -10 -20

-17.8

-19.8

-30 -40 -50

QC_I

-39.2

-36.9

QC_SW

-45.3

QC_NE -60 IGUe

IGUi

AFW

DSF

SUP

Cooling demand

Fig. 4. Energy demand of non-optimized facades. (Legend: IGUe: exterior shading device; IGUi: interior shading device; AFW: airflow window; DSF: double-skin fac- ade; SUP: supply window; QH: heating demand; QC cooling demand; I: internal zone; SW: southwest zone; NE: northeast zone).

Analyzing the heating demand results (Fig. 4(a)), the IGUe shows to be the most energy consuming solution. Nevertheless, the traditional solutions—which have the highest transmission losses—are not really outperformed by the AFW and DSF. Much can be attributed to the differences in solar gains. Compared to the IGUe, the IGUi has an 8% lower heating demand due to the position of the roller blind at the inside. This produces higher indirect solar gains. The direct solar gains of the MSFs are considerably lower than that of the IGUs because of the extra pane. For the DSF this compensates for the lower transmission losses. The inner pane of the AFW has a lower thermal resistance than that of the DSF and moreover the AFW is ventilated with interior air, which results in even lower transmission losses. As a consequence the heating demand is about 20% lower compared to the IGUe. Because of the preheating of the ventilation air, the SUP outperforms all other variants and requires roughly half the heating energy needed by the other facade solutions. The cooling demand (QC in Table 1) strongly depends on the indirect solar gains. Despite the higher direct solar gains, the IGU with exterior shading device (IGUe on Fig. 4(b)) has the lowest cooling demand. The best option to reduce to indirect gains is to prevent the solar radiation from entering the building. The IGU with interior shading device (IGUi) not only has a high direct solar gain but also suffers from high indirect solar gains. As a result it consumes 55% more cooling energy than the IGUe. The AFW does not perform much better. It has lower direct solar gains compared to the IGUs and it also has lower indirect gains than the IGUi. However, the higher overall insulation level prevents the building to cool down during night-time. As a result, the AFW only performs somewhat better than the IGUi. The DSF is able to approach the performance of the IGUe as it has an inner pane with qhigh thermal resistance. This prevents the absorbed solar radiation from entering the office. The SUP has the poorest cooling performance and requires 2.5 times more energy than the IGUe. The advantage of preheating the ventilation now becomes a major disadvantage. Unfortunately, most MSF-typologies are incapable of lowering the heating and cooling demand simultaneously. Only by combining typologies or by changing system settings according to the particular situation, a substantial overall improvement over the traditional solutions is possible. This implies that control mechanisms are inevitable to make MSFs work efficiently throughout the entire year. Two important optimization strategies for MSFs are identified and will be explored and implemented. Where possible both strategies will be combined to achieve an optimal solution. 4.1. Airflow rate control The airflow rate in the office zones as well as in the MSFcavities may be changed to improve the energy efficiency.

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According to the fac- ade typology, these changes may not be independent. For the AFW and the SUP there is a relation between the overall building airflow rate and the airflow rate through the MSF-cavity. For the DSF, the cavity airflow rate is independent of the airflow rate through the office zones. Changing the airflow rate will also be used to allow free cooling of the building. 4.2. Recuperation of the return air The air passing through an MSF-cavity is heated by transmission losses through the inner pane and by impinging solar radiation. During heating conditions it may be interesting to use the return air from the MSFcavity as an energy source to heat the building. Some facade typologies use the MSF-cavity as a heat-exchanger to preheat the ventilation air. During cooling conditions there is no need to recover energy. Instead, the hot air returning from the MSFcavities is expelled. 5. Air flow window In this analysis we will discuss four strategies to improve the energy efficiency of AFWs: (1) (2) (3) (4)

a fixed change of the AFW cavity airflow rate; a controlled change of the AFW cavity airflow rate; recuperation of the air returning from the cavity; A combination of airflow rate and recuperation control.

643

5.1. Fixed airflow rate Let us first analyze the influence of the airflow rate assuming that the airflow rate is fixed throughout the entire simulation. In principle the three office zones are treated independently. As a consequence the airflow rate in a zone equals the cavity airflow rate in that zone. In this analysis we also consider what happens if the return air from the internal zone (Zone I in Fig. 2) is distributed towards the external zones. This distribution increases the airflow rate in the external zones and the AFW-cavity by 50%. There is no recuperation in the HVAC-system: the recuperation airflow rate (Gr in Fig. 2) equals zero. An increase of the airflow rate increases the cavity temperature and reduces the transmission losses. As long as the airflow rate does not exceed the total ventilation rate of the building, an increase of the cavity airflow rate is favorable to lower the heating demand. This is illustrated by the reduction of the heating demand for the cases with distribution (Table 2, comparison between case 1A and 1B). If the total AFW airflow rate exceeds the total buildings ventilation rate, extra exterior air has to be inserted into the building. The heating of this extra air has a higher energy cost than the reduction of the transmission losses. An optimal situation is achieved when the airflow rate through the facades equals the total ventilation airflow of the building. In such a way, AFWs are used as a heat exchanger for the exhaust air. In summer, an increase of the airflow rate helps to lower the cooling demand. Two effects are important: (1) the reduction of the indirect solar gains due to the increased airflow in the AFW; and (2) the free cooling effect by

Table 2 Energy demand of AFWs 1. AFW fixed airflow rate 1A. No distribution from internal zone Ga (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

20 31.6 36.9

40 67.1 24.7

80 121.4 19.6

120 155.2 18.6

1B. Distribution from internal zone Ga (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

20 30.1 33.7

40 63.3 23.1

80 112.8 18.7

120 142.3 17.5

2. AFW controlled airflow rate Ga,max (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

20 31.6 36.9

40 30.6 29.7

80 30.4 26.0

120 31.0 25.4

3. AFW recuperation Gr,,max (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

0 31.6 36.9

20 29.4 34.8

60 27.3 34.8

100 25.4 34.9

4. AFW controlled airflow rate and recuperation 20 Ga,,max (m3/(h m)) Gr,,max (m3/(h m)) 0 QH (kWh/(m2 a)) 31.6 QC (kWh/(m2 a)) 36.9

40 20 29.7 31.1

80 60 27.6 27.1

120 100 25.8 26.5

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increasing the exterior ventilation. Comparison of the results with and without distribution of the internal zone return air indicates that the latter is far more important. 5.2. Controlled airflow rate The airflow rate based on the ventilation rate is the most favorable option to reduce the heating demand. In summer, it is favorable to increase the airflow rate to lower the cooling demand. As a consequence, it is useful to implement a controller which chooses an optimal airflow rate according to the energy needs. By using a speed controllable fan or adjustable diaphragms in the airducts, the controller is able to change the airflow rate of each zone separately. The decision of the airflow rate controller is based on the energy mode of a particular zone. If a zone is in heating mode, the ventilation airflow is set. If a zone is in cooling mode, the maximum airflow rate is set as long as the exterior temperature is lower than the setpoint for cooling. The definitions of heating and cooling mode should be carefully chosen. Monitoring of the energy demand in the terminal units of a particular zone may result in an instable and non-optimal behavior. An example illustrates this. If the temperature of a particular zone surpasses the setpoint for cooling, the HVAC-system will provide active cooling. Now the HVAC-system is in cooling and the airflow rate will be increased. As soon as the temperature drops below the cooling setpoint, the cooling will stop and the airflow rate will return to normal. It is however desirable that the free cooling of the office continues to avoid the need for active cooling. Therefore the energy regime is determined from the interior temperature: as long as the interior temperature is lower than the average between the heating and cooling setpoint, the system is in heating mode. Otherwise, the system is in cooling. Table 2 (case 2) shows the heating (QH) and cooling demand (QC) as a function of the maximum allowed airflow rate. Note that during night-time and weekends the airflow rate is set back to a minimum airflow rate which equals half the required ventilation rate. Again, there is no recuperation in the HVAC-system: the recuperation airflow rate (Gr in Fig. 2) equals zero. During heating demand, the controller chooses the minimal ventilation rate and the heating demand hardly changes compared to the base case AFW (Ga,max equals 20 m3/(h m)). During cooling conditions the airflow rate is increased whenever useful. Without changing the heating demand the cooling demand may be reduced by up to 45%. As for the previous case, it should be noted that the cooling effect is mainly caused by free cooling. 5.3. Recuperation As a further step to save heating energy, recuperation of the return air is implemented. Recuperation tries to lower the energy demand by reusing that part of the return air

which is not needed for ventilation. Only when the total airflow rate exceeds the required ventilation airflow rate, recuperation of the AFW return air is possible. On top of the speed adjustable fan and the diaphragms, a mixing chamber allowing recuperation should be added to the HVAC-system. As the northeast and southwest facades have a different thermal behaviour, the system should be able to control both orientations separately. We analyze three methods of recuperation: (1) Continuous recuperation during heating demand. The Building Energy Management System (BEMS) always recuperates whenever a zone is in heating mode. The recuperation airflow rate equals maximum airflow rate minus the hygienic ventilation airflow rate. The definition of heating and cooling mode is again based on the interior zone temperature. During cooling conditions, recuperation is not useful and therefore disabled. (2) Recuperation based upon the outlet temperature of the cavity air. In the outlet temperature based strategy, the BEMS increases the airflow rate of a zone to the maximum recuperation rate whenever the outlet temperature is warmer than the setpoint temperature for heating and the system is in heating mode. The definition of heating and cooling mode is again based on the interior zone temperature. As in the previous case, recuperation is disabled during cooling conditions. (3) Recuperation based upon the sensible energy content of the supply air. The energy content algorithm minimizes the sensible energy content of the supply air to the HVAC-system. To do so, the BEMS has to be able to compare the sensible energy content of different settings. This is only possible if the BEMS can predict the outlet temperature of the different AFWs. By using the airflow window model, a correlation between the incoming solar radiation, the exterior and interior temperature, the airflow rate and the outlet temperature was established and implemented in the BEMS. At each timestep, the BEMS predicts the sensible energy content of the supply air based on the conditions during the previous timestep. Based on this information the BEMS chooses the airflow rate settings, which minimize the sensible energy content of the supply air to the HVAC-system. Fig. 5 shows for the three strategies the energy demand as a function of the maximum allowed recuperation airflow rate. The reduction of the heating demand strongly depends on the chosen strategy and the maximum allowed recuperation airflow rate. Implementing a system which always recuperates part of the return air during heating demand (strategy 1) proofs to be inefficient to lower the heating demand. The overall heating demand slightly increases while increasing the recuperation airflow rate (Fig. 5, cases AFW Rxx a). Closer

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analysis of the results shows that the heating demand for the internal zone decreases while the energy demand for the external zones increases. As the airflow rate towards the internal zone does not change, the lower heating demand for the internal zone is caused by the higher supply temperature for the HVAC-system. Recuperation has two effects on the external zones: (1) the transmission losses decrease because the airflow rate through the fac- ade increases; and (2) the energy content of the supply air for the HVAC-system changes. The first effect causes the heating demand of the external zones to be lower. This effect is however rather small. The second effect may both cause an increase or decrease of the heating demand. Let us consider the sensible energy content of

QH_I

50

QH_SW

40 28.2

27.3

26.8

25.4

AFW R100 e

30

QH_NE

34.7

AFW R100 t

30.9 29.4 32.9

31.8

AFW R60 e

31.6

AFW R60 t

heating demand (kWh/(m2.a))

60

20 10

AFW R100 a

AFW R60 a

AFW R20 e

AFW R20 t

AFW R20 a

AFW BC

0

Fig. 5. Heating demand of AFWs as a function of maximum allowed recuperation airflow rate and recuperation strategy. (Legend: BC: base case; Rxx: maximum allowed recuperation airflow rate; a: always recuperation, strategy 1; t: recuperation based on return temperature, strategy 2; e: recuperation based on supply air energy content, strategy 3, QH: heating demand; I: internal zone; SW: southwest zone; NE: north-east zone).

i

where ca is the specific heat of the air, Ga,i is the total airflow rate to zone i and ya,i is the air temperature of zone i. More negative energy contents indicate that the air needs more heating. The sensible energy content of the supply air with strategy 1 (always recuperation) during a cloudy day is more negative than the sensible energy content of outside ventilation air (compare Event and Ea in Fig. 6(a)). As a consequence, the heating demand increases by using strategy 1 during the cloudy day. During a sunny day, the sensible energy content of outside ventilation air is more negative during the afternoon when the sun shines on the southwest fac- ade than the sensible energy content of the recuperation strategies (compare Event and Ea,t and e in Fig. 6(b)). In this case, recuperation is useful to lower the heating demand. An optimal recuperation strategy only recuperates whenever it is useful. The decision whether recuperation is useful may be based on the outlet temperature of the airflow windows (strategy 2) or on the sensible energy content of the supply air in the HVAC-system (strategy 3). The temperature-based algorithm avoids to recuperate when on cloudy days. Nevertheless, it misses some opportunities during sunny days. Hence it performs better than continuous recuperation but is not yet optimal. Using the sensible energy content strategy (strategy 3), the advantages of full recuperation are exploited and the situation where the temperature of the return air from the AFWs is too low to improve the sensible energy content is avoided. A reduction of the heating demand up to 20% is now possible. Given the fact that the implementation is not straightforward and that the fan consumption again raises, recuperation may not necessarily be an economical

b

0 E vent E a (strategy 1) E t (strategy 2) E e (strategy 3)

0 E vent E a (strategy1) E t (strategy 2)

-500

other strategies

-1000

the supply air in point r in Fig. 2: X E supply ¼ ca  G a;i  ðysupply  ya;i Þ,

energy content supply air (W)

energy content supply air (W)

a

645

E e (strategy3) -500 E e (strategy 3)

-1000

E t (strategy 2)

E a (strategy 1)

E a (strategy 1) 6:00 h

12:00 h

18:00 h

6:00 h

12:00 h

18:00h

1062

1068

1074

-1500

-1500 768

774

780

786

792

1056

time (hours of the year)

time (hours of the year)

cloudy winter day

sunny winter day

1080

Fig. 6. Energy content of the supply air for different settings of the recuperation algorithm. The maximum recuperation rate (Gr) equals 60 m3/(h m). (Legend: vent: energy content in case of hygienic ventilation rate; a: energy content in case of always recuperation, strategy 1; t: energy content in case of recuperation based on return temperature, strategy 2; e: energy content in case of recuperation based on supply air energy content, strategy 3).

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solution. It may be noted that as a side effect also the cooling demand decreases with approximately 5%. 5.4. Controlled airflow rate and recuperation

demand decreases dramatically by supplying the ventilation air for the HVAC-system through the supply window (Ga,sup in Fig. 7(a) and Fig. 2). This allows to capture solar radiation and to reuse part of the transmission losses. Only one fan (F1 in Fig. 7(a)) is operated in this situation. If necessary, part of the air can be recirculated (Gr in Fig. 7). In summer, the additional energy from the SUPs is not needed. However it may be desirable to ventilate the SUPs to prevent overheating. To achieve this, a second fan (F2 in Fig. 7(b)) has to be operated and the mixing chamber has to be positioned in such a way that exterior air is supplied to the HVAC-plant instead of air coming from the SUPs (Fig. 7(b)). Three operation modes are analyzed:

A logical step to optimize both the heating and cooling demand is combining the airflow rate control and recuperation strategy. Table 4 indicates that both the heating and cooling load decrease with increasing airflow rate. The maximum heating load (QH) reduction equals 22%, the maximum cooling load (QC) reduction is 39%. These reductions are somewhat smaller than what is achieved when both strategies are used separately. To draw an overall conclusion it should again be noted that an increase of the airflow rate raises the fan energy consumption substantially. Hence, the reductions have to be weighed against this extra energy.

(1) always recuperation; (2) controlled recuperation; (3) controlled recuperation and free cooling.

6. Supply window

6.1. Always recuperation

To optimize the SUP results, important modifications to the HVAC-plant are necessary. Fig. 7 gives a schematic representation of the HVAC-plant. In winter the heating

This case is a variant on the base case. The supply air for the HVAC-plant is supplied through the SUP at all times. Also the airflow for the internal zones is supplied through

a

b

Fig. 7. Schematic representation of the simulated office floor for typical SUP configurations.

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the SUPs. As a consequence the airflow rate in the SUPs is 50% higher than the airflow rate in the zones (Ga,sup ¼ 1.5  Ga). The airflow rate and HVAC-settings are fixed throughout the year. Recirculation is not possible (Gr ¼ 0 in Fig. 5). The optimal airflow rate through the SUPs equals the ventilation airflow rate (Table 3). Both the heating and cooling load increase when the airflow rate through the SUPs increases. During heating conditions, the energy content drops with increasing airflow rate and concurrently the cavity is colder which increases the transmission losses. During cooling conditions, the energy content increases with increasing airflow rate. This negative effect is not countered by the lower cavity temperature which lowers transmission gains. 6.2. Controlled recuperation In this case the BEMS can extract the supply air for the HVAC-system from the exterior environment or from the SUPs. As long as the system is in heating mode, the supply air is provided through the SUPs and equals the hygienic ventilation rate (including the ventilation air needed for the internal zones) (Fig. 7(a)). The definition of the heating and cooling mode is the same as for the AFWs. Furthermore, the system may increase the supply airflow rate to Ga,sup,max to take advantage of high outlet temperatures in the SUPs. Note that this maximum airflow rate equals 1.5  Ga,max because the internal zone has to be ventilated as well. As for the AFWs, this control is based on the energy content of the return air rather than on the temperature of the return air. If the system is in cooling mode, the BEMS closes the connection between the SUPs and the HVACsystem (Fig. 7(b)). The airflow rate through the SUPs of the zones with cooling equals the maximal airflow rate (Ga,sup ¼ Ga,sup,max). The supply air to the HVAC-system equals the buildings ventilation airflow rate. Note that the SUPs are now ventilated with the second fan. Implementing the controlled recuperation strategy proofs to be a successful approach (Table 3). The heating load decreases because of the recuperation algorithm. As

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for the airflow windows, the heating demand decreases if a higher recuperation rate is allowed. The gains should be weighed against the increase of the fan consumption. The cooling demand decreases considerably because the warm return air from the SUPs is no longer recovered. As the cooling demand hardly depends on the airflow rate through the supply window, these results also show that the influence of the airflow rate on the transmission gains during summer conditions is relatively small. This was to be expected as the thermal resistance of the inner pane window is high. 6.3. Controlled recuperation and free cooling Essentially, this case uses the same strategy as the controlled recuperation case but it also allows free cooling. If the system is in cooling mode, the supply airflow rate to the HVAC-system (Gv) is now increased above the ventilation rate to Gv,max if the average zone temperature is warmer than the exterior temperature. The combination of the recuperation strategy and free cooling hardly changes the heating demand but further lowers the cooling demand. 7. Double-skin facade The optimization strategies of the naturally ventilated fac- ade focus on the position of the inlet grids. The idea is straightforward. During heating mode the grids are closed to create an extra insulating barrier. During cooling mode the grids are opened to ventilate the cavity and prevent overheating. In order to automatically control the airflow in the cavity, motorized dampers, powered adjustable grids or automated operable windows have to be installed. In this analysis, three situations are possible: (a) closed apertures (0 in Table 4); (b) normal inlet and outlet apertures (1 in Table 4); and (c) large inlet and outlet apertures (2 in Table 4). The pressure characteristics of the cavities with opened aperture are based on experimental

Table 3 Energy demand of SUPs SUP always recuperation Ga (m3/(h m)) Ga,sup (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

20 30 17.9 45.3

40 60 47.4 49.7

80 120 101.2 58.0

120 180 135.3 72.5

SUP controlled recuperation Ga,sup,max (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2.a))

30 17.9 45.3

60 17.2 17.9

120 16.4 17.1

180 15.4 16.7

SUP controlled recuperation and free cooling Ga,sup,max (m3/(h m)) Gv, max (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

30 20 17.9 45.3

60 40 17.3 15.4

120 80 16.4 14.2

180 120 15.5 13.5

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Table 4 Energy demand of DSFs DSF fixed grids Aperture QH (kWh/(m2 a)) QC (kWh/(m2 a))

0 34.5 22.2

1 36.2 19.8

2 36.9 18.5

DSF controlled grids Aperture QH (kWh/(m2 a)) QC (kWh/(m2 a))

0–1 34.4 20.0

0–2 34.3 18.7

0–1–2 34.3 18.7

0–2–F 34.0 11.8

SUP without recuperation Ga, sup (m3/(h m)) QH (kWh/(m2 a)) QC (kWh/(m2 a))

30 36.1 18.2

60 36.4 17.7

120 36.7 16.8

180 36.9 16.5

heating demand (kWh/(m2.a))

a

60 QH_I QH_SW QH_NE

50 40

38.0

36.2

35.3

31.6

30 20.8

19.9

20

34.0

28.0 17.9

18.2

10

SUP OPT

SUP BC

DSF OPT

DSF BC

AFW OPT

AFW BC

IGUi OPT

Ui BCIG

IGUe BC

IGUe OPT

0

Heating demand

b cooling demand (kWh/(m2.a))

0 -7.0

-10 -20

-11.8

-14.5 -17.8

-40 -50

-13.5

-19.8 -26.5

-30 -36.9

-39.2

QC_I

-45.3

QC_SW QC_NE

SUP best

SUP bc

DSF best

DSF bc

AFW best

AFW bc

Ui bestIG

IGUi bc

IGUe best

-60 IGUe bc

data available in [18]. The situation with normal apertures represents the base case. First we analyze the energy demand when the apertures are fixed at one position throughout the year. Closing the cavity decreases the heating demand, while increasing the cooling load compared with the results of the base case. Maximizing the ventilation decreases the cooling load but increases the heating load. This conflicting situation is resolved by implementing the above mentioned control mechanism. Now, both the heating and cooling demand are optimized. Increasing the maximum airflow rate in summer proves to be successful to control the cooling loads. Allowing the system to toggle between open, normal and large opening (0–1–2 in Table 4) does not produce better results. Implementing a simple open/close algorithm is hence sufficient to achieve an optimal result. Finally, also free cooling of the office is implemented (0–2–F in Table 4) and combined with the control mechanism. This technique significantly lowers the cooling demand, but is not a merit of the DSF. As an illustration, Table 4 compares the results of the SUP window where no recuperation is allowed with the results of the DSF with fixed apertures. Expectedly, the heating demand of both MSF-types is comparable. The cooling demand of the DSF is somewhat higher but needs no fan. As a consequence the overall energy efficiency of the DSF will probably outperform that of the SUP.

Cooling demand

8. Traditional facades A comparison of the base case traditional facades against the optimized MSFs shows that the optimization provides a considerable improvement of the MSF heating demand (Fig. 8). Only the heating demand of the DSF is only marginally improved. Except for the AFW, also the cooling demand of the optimized MSFs outperforms the cooling demand of the IGUe (Fig. 8). From an energy point of view, the optimized multiple-skin fac- ades may compete with non-optimized traditional fac- ade systems. It can be argued that this is no longer a fair comparison if the

Fig. 8. Energy demand of optimized facades compared against nonoptimized variants. (Legend: IGUe: exterior shading device; IGUi: interior shading device; AFW: airflow window; DSF: double-skin fac- ade; SUP: supply window; QH: heating demand; QC cooling demand; I: internal zone; SW: southwest zone; NE: northeast zone).

energy performance of the IGUs is not optimized as well. We consider three optimization strategies for the traditional facades: (1) a heat exchanger (HE) to decrease the ventilation energy; (2) night-time ventilation (NV); and (3) free cooling (FC) to temper the cooling demand.

ARTICLE IN PRESS D. Saelens et al. / Building and Environment 43 (2008) 638–650

8.1. Heat exchanger In order to recover part of the ventilation energy, a crossflow heat exchanger system is positioned between the supply and exhaust ducts. As a result the heating demand decreases significantly (Table 5, HE). Due to the heating of the supply air when the exterior temperature is higher than the exhaust temperature, the cooling demand somewhat raises. For a Belgian climate, the effect is rather limited (Table 5, HE). 8.2. Night ventilation The algorithm for night ventilation increases the ventilation during night-time whenever the maximum zone temperature of the previous day surpassed the average between the setpoint for heating and cooling and the instantaneous exterior temperature is lower than the instantaneous interior temperature. To avoid the need to turn on the heating during the morning, the nightventilation is turned off as soon as the zone temperature drops below the setpoint for heating. The algorithm is able to control each zone separately. This allows to ensure a maximal cooling of each zone and minimizes the fan energy. The cooling demand decreases substantially. Despite the temperature cut-off for ventilation there is a small raise of the heating demand. 8.3. Free cooling Whenever the building is in cooling mode and the exterior temperature is lower than the interior temperature the building is intensively ventilated. Implementation of free cooling decreases the cooling load and hardly changes the heating demand (Table 5, FC). 8.4. Combinations of heat exchanger, night ventilation and free cooling Optimizing both the heating and cooling demand is achieved by combining the heat-exchanger and the nightTable 5 Energy demand of IGUs IGUE Case QH (kWh/(m2 a))

BC 38.0

HE 26.0

NV 38.4

Case QH kWh/(m2 a)) QC (kWh/(m2 a))

FC 38.2 11.6

HE+NV 27.0 10.1

H+N+FC 26.6 7.0

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time ventilation (Table 5, HE+NV). Implementation of free cooling further decreases the cooling load without changing the heating demand (Table 5, H+N+FC). 9. Conclusions In this paper, different strategies to optimize the energy efficiency of multiple-skin facades are studied and compared with the results of traditional cladding systems under typical Belgian climate conditions. By implementing control strategies the energy efficiency of all facade systems was significantly improved. Further optimization is possible though by optimizing the system properties such as the geometry and the properties of the glazing and solar shading device. Using an algorithm based on the energy content of the return air, recuperation of the air returning from the airflow and supply window showed to be the most useful strategies to lower the heating demand. For the naturally ventilated double-skin facade controlled apertures with a simple open–close strategy were found beneficial. Controlling the airflow rate showed to be a successful approach to lower the cooling demand for all facade typologies. Though it must be noted that in most cases the cooling is mainly caused by free cooling rather than being a merit of the multiple-skin facades. Optimizing the naturally ventilated double-skin facade resulted in an efficient control of the cooling demand but could hardly improve the heating demand. For the airflow window, the optimization significantly lowered the heating demand but the optimized airflow window still suffers from high cooling demands. The supply window has the highest potential to benefit from the optimization techniques. It is able to considerably reduce the heating demand while providing an acceptable cooling demand. The traditional facade with exterior shading device, however, was found to provide the best solar protection. Acknowledgments This research is funded by a research grant of the Flemish Institute for the Promotion of Industrial Scientific and Technological Research (IWT: Vlaams Instituut ter Bevordering en Promotie van het WetenschappelijkTechnologisch Onderzoek in de Industrie). Their financial contribution is gratefully acknowledged. References

IGUI Case QH (kWh/(m2 a)) QC (kWh/(m2 a))

BC 35.3 39.2

HE 23.0 43.9

NV 35.7 22.0

Case QH kWh/(m2 a)) QC (kWh/(m2 a))

FC 35.5 25.1

HE+NV 23.5 22.7

H+N+FC 23.6 14.5

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