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Ventilated Façade with double chamber and flow control device
Accepted Manuscript Title: VENTILATED FAC ¸ ADE WITH DOUBLE CHAMBER AND FLOW CONTROL DEVICE Authors: Jaime Santa Cruz Astorqui, C´esar Porras-Amores PII: DOI: Reference:
S0378-7788(16)31461-X http://dx.doi.org/doi:10.1016/j.enbuild.2017.04.063 ENB 7559
To appear in:
ENB
Received date: Revised date: Accepted date:
14-11-2016 16-2-2017 21-4-2017
Please cite this article as: Jaime Santa Cruz Astorqui, C´esar Porras-Amores, VENTILATED FAC¸ADE WITH DOUBLE CHAMBER AND FLOW CONTROL DEVICE, Energy and Buildingshttp://dx.doi.org/10.1016/j.enbuild.2017.04.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
VENTILATED FAÇADE WITH DOUBLE CHAMBER AND FLOW CONTROL DEVICE
Jaime Santa Cruz Astorqui * and César Porras‐Amores Technical University of Madrid, School of Building Engineering, TEMA Research Group, Avda. Juan de Herrera, 6, 28040 Madrid, Spain; E‐Mail: [email protected] and [email protected] * Corresponding author: Jaime Santa Cruz Astorqui. E‐Mail: [email protected] Tel.: +34‐913367637
ABSTRACT Currently, ventilated facades are composed by an inner sheet, thermal insulation, ventilation chamber and exterior finish, a system that allows the heat absorbed by solar radiation to dissipate through natural ventilation of the air from the chamber. This article discusses the feasibility of adding a second air chamber parallel to the existing one, both interconnected by the bottom of the facade, and with a device at the top to regulate the air flow in the chambers, depending on the gradient of the existing temperature between inside and outside the building. The main objective is to evaluate the potential of this proposed system in the improvement of the energy efficiency of the building, using a steady model of Computational Fluid Dynamics (CFD). To this end, a comparative study of energy performance was carried out, as well as the thermal and fluid dynamic behavior, between the proposed two‐chamber system and the conventional ventilated facade system with closed joint, at different times of the year. The results show that the proposed system allows an increase of 38% efficiency in summer period, and 333% in winter period, compared to a conventional ventilated façade with closed joint. Keywords: ventilated façade; passive design; CFD simulation; energy efficiency
INTRODUCTION Currently, there are various systems of ventilated façade, all of which are characterized by incorporating a ventilated air chamber between the outer skin and the insulation (which is continuous along the entire façade). The main purpose of this type of façade is to dissipate the heat the outer skin absorbs when direct solar radiation falls on it, and thereby reduce the temperature of the inner sheet and therefore, the interior space of the building. The operation is based on natural air convection by effect of an increment of its temperature. Thus, an upward flow of air is created through the chamber, which dissipates heat from the outer skin to finally be expelled out through the top of the façade. In short, both the reduction of energy consumption of the cooling system in warm periods and the protection against solar radiation through the top layer of the façade, are the main benefits attributed to the ventilated façade [1]. In this sense, there are two large groups of ventilated façades: a) open joint and b) closed joint. Ventilated façades with open joints allow free circulation of air between the chamber and the outside through the existing joints between the pieces that make up the outer skin. In this type of façades, usually the air enters from the outside into the chamber through the joints of the lower half of the façade, and out gradually to the outside through the upper joints [2]. The performance of this type of façade has been previously studied by developing numerical models of computational fluid dynamics [3] and [4] as well as[5] experimentally by nonintrusive measuring airflow techniques [6] and [7]. In ventilated façades of closed joint, there are no open joints between the pieces that make up the outer skin, so the air enters the chamber through an opening or grid located at the lower part of the façade, and comes out through the other opening or vent in the crowning of the façade. The behavior and the thermal performance of the ventilated façades of closed joint has been studied through simulations of computational fluid dynamics [1], energy simulation [8] and scale prototypes [9]. In any case, ventilated façades are particularly suitable in climates with hot summer periods and orientations exposed to direct sunlight. Under these conditions, solar radiation can increase the surface temperature of a traditional façade by 60º C, so this heat dissipation through the ventilated chamber represents a significant energy savings in air conditioning systems. However, ventilated façades on the market have two weaknesses related to their degree of energy efficiency: First, in cold periods (the outdoor temperature is lower than the indoor comfort temperature), this type of façade is a drawback, since the ventilation of the chamber dissipates the heat from solar radiation falling on the façade, thus causing major losses of heating energy than a conventional façade with identical thermal resistance. In this sense, there are closure control systems of the chamber [10] which allow to occluding the air in the chamber and prevent its ventilation, being configured as a closed chamber capturing heat (greenhouse effect), which solves the problem. Second, in warm periods, the ventilation air circulating in the chamber gradually increases its temperature as it rises through the chamber so that in the lower region of the façade, the air in the chamber is about the outdoor temperature and therefore the system's efficiency is maximal. In the upper zone, the air of the chamber reaches a high temperature, so that the system loses its effectiveness. This gradual warming effect is maximized in ventilated façades of closed joint, not being possible that fresh air enters the intermediate zones of the same.
In this sense, to avoid the negative effects described, incorporating a second vent chamber is studied in combination with a device that regulates air flow between the chambers and the outside. In short, the proposed system achieves two effects: on the one hand, it is possible to drastically reduce the air temperature in the chamber adjacent to the insulation of the enclosure (in warm periods), and secondly, it is possible to confine the air inside the chamber, turning the enclosure in a solar collector (in cold periods). PROPOSED VENTILATED FAÇADE WITH DOUBLE CHAMBER The components, materials and mechanisms that make the proposed system are detailed below (Figures 1, 2 and 3). The membrane between the two chambers (1) is formed of a flexible sheet of polyethylene foam (or insulating material with similar characteristics), which is mounted in rolls as wide as the distance between uprights (13) of the ventilated façade. The union between the bands of membrane and the uprights is performed by a PVC bead of extruded sectioned in "H" (3) which provides sealing between the two chambers. To maintain fixed the membrane and ensure constant thickness of the chambers, it is fixed to the façade insulation (14) by polyethylene spacers (2) that self‐screw to the isolation. The outer chamber or ventilation chamber (6) is located between the membrane (1) and the outer skin (12) of the ventilated façade (which must be of closed joint). The inner chamber or inlet chamber (4) is located between the membrane (1) and the façade insulation (14). Both chambers are connected together at the lower part of the façade, and they open to the outside through the locking device located on the top of the cover (11). The closing device (Figure 2) aims the opening or closing of the chambers to the outside. This device consists of a cover plate (11) for protection against the rain, an intake grill (5) connected to the inlet chamber (4) a ventilation or exhaust grill (7) connected to the ventilation chamber (6), and a butterfly valve (8) actuated by a handle (9). The assembly is mounted on a steel tube frame (10) whose transverse dimension is equal to the total thickness of the façade on the top of its cover, and its longitudinal dimension coincides with the distance between the fixing uprights of the ventilated façade. By operating the handle (9) the position of the butterfly valve is switched (8) between closed and open, allowing the control of air flow between the outside and the chambers. The devices are fixed to the façade by mechanical fasteners (16), and to each other through the rack by screws. In the corners of the façade, it is necessary to place special modules (figure 3) which lack of a closing valve, and therefore are blind in their bottom and sides. The Table 1 includes the references in the figures 1, 2 and 3. The proposed system is currently in the process of patent application in the Spanish Patent and Trademark Office (OEPM, for its Spanish acronym). OPERATION OF VENTILATED FAÇADE WITH DOUBLE CHAMBER
In the figure 4 operation schemes of the proposed ventilated façade with double chamber are shown, compared to the operation of a conventional ventilated façade of closed joint. The proposed system is easily adaptable to any existing ventilated façade system of closed joint on the market (Figure 4 left) and it consists of the incorporation of a continuous membrane inside the existing chamber to get a double chamber, and a device located on the top of the façade, with a control mechanism for opening and closing of the two chambers. The system uses two modes of operation depending on the temperature gradient between the exterior and interior of the building: opened valve for hot periods and closed valve for cold periods. In hot periods, when the outdoor temperature is higher than the indoor comfort temperature (figure 4 center), the outside air penetrates the chamber adjacent to insulation (inlet chamber) through the coronation of the façade, and flows downstream, maintaining an average temperature very similar to the outdoor ambient temperature, since it is hardly affected by the heating of the outer skin. Upon reaching the bottom of the inlet chamber, the air passes into the outer chamber (ventilation chamber) and rises by natural convection when heated by direct contact with the outer skin (chimney effect), thus dissipating the heat absorbed by solar radiation. Through this system, the flow of heat into the building is reduced, which means greater energy efficiency. The movement of air through the chambers is ensured by the temperature gradient between the air entering the intake grill and the air ejected from the exhaust grill. This gradient will be greater the more direct solar radiation falling on the façade. The effectiveness of the system is based on the average air temperature in contact with the insulation (inlet chamber), it is significantly lower than the average air temperature in the chamber of a conventional ventilated façade, thus reducing heat gains from solar radiation by up to 38% in summer and 333% heat loss in winter (data obtained by CFD simulation of west façade according to the data indicated in table 4). In cold periods, when the outdoor temperature is lower than the indoor comfort temperature (figure 4 right), the butterfly valve of the input/output device is closed, confining the air into the chambers. When closing the valve, communication between the chambers is left open at the top, allowing air to flow in a circular direction between the two chambers. Solar radiation heats the outer skin of the façade, which conduct heat to the air of the outer ventilation chamber, which circulates by natural convection upwardly, going to the internal chamber through the top of the façade. From there, preheated air circulates downwardly, maintaining a low temperature gradient between the hot and cold sides of the insulation, thereby reducing heat losses by transmission through the facade. The continuous circulation of air between the two chambers can increase the amount of heat absorbed (in relation to a closed conventional chamber), thereby increasing the efficiency of the system. The adjustment of the device for controlling air flow is made to coincide with the on/off dates of the heating system, manipulating from the roof the butterfly valve which opens and closes the ventilation system. Optionally, the valve can be motorized for remote operation. ANALYSIS OF THE PROPOSAL
In order to evaluate the thermal behavior and dynamic fluid of the proposed double chamber system, models of computational fluid dynamics (CFD) have been developed. Specifically, there have been several simulations of ventilated façade, one corresponding to the conventional system and another to the proposed double chamber system at different times of year. The 2D computational domain has an air volume adjacent to the outer side of the ventilated façade. The computational domain is subjected to the action of gravity (g = 9.81 m/s2). In both models, the height of the facade corresponds to a 4‐storey building (10 meters high) and the adjacent air volume has dimensions of 10.6 m x 4.4 m. The conventional ventilated façade system is composed (from outside to inside) of the following ‐‐outer layers (15 mm), air chamber (50 mm), thermal insulation (75 mm), brick (120mm) and plaster (10 mm) ‐‐. On the other hand, the proposed double chamber system consists (from outside to inside) of the following ‐‐ outer layer (15 mm), ventilation chamber (50 mm), membrane of separation (15 mm), inlet chamber (45 mm), thermal insulation (60 mm), brick (120 mm) and plaster (10 mm) ‐‐. In this case, the total thickness of insulating material is the same as in the conventional ventilated façade (75 mm), as the membrane also considered an insulating element. The thermal properties of the materials used are detailed in Table 2. The equations of the numerical model were solved with a commercial CFD software, STAR‐CCM+. The simulations were performed at steady state by the Reynolds‐Averaged Navier‐Stokes (RANS) equations which have been widely used for fluid‐thermal and natural ventilation analysis [13] and [14]. The effects of the turbulence have been simulated by using the standard k‐epsilon turbulence model which has been used widely for practical engineering flow calculation [14], [15], [16] and [17]. Solar radiation has been simulated with the model discrete ordinates (DO). The unstructured mesh is made up of about 70,000 cells with a special refinement in regions where it is expected to have more complex flows such as inside the air chambers. The criterion of convergence to complete the simulation was set at 0.01% error for all the field variable of this problem. The numerical model has been validated from other works of façades previously published [2] and [18].
Boundary conditions
The boundary conditions defined have been selected to the specific characteristics of each studied ventilated façade systems. The Mach number (<0.01) and the variation in density due to the pressure gradient is small; therefore, the density variation is considered constant and the air flow incompressible. Although the flow is incompressible, in natural convection cases it is necessary to introduce the hypothesis Boussinesq for density not to be completely constant. This enables that the variations in density can be considered negligible, except where the effect of gravity appears. Solar radiation incident on the cladding has been simulated as an internal heat source on the outer surface of the ceramic tile. The added value is therefore the final radiation absorbed by the plate. The boundary conditions on the inner side of the façade (inside the building) have been fixed by a heat transfer coefficient of 8 W/m2K, which is the value typically used in building codes for interior flows, according to ASHRAE [19]. The walls of the outdoor air volume (except the top side) have been set to the value of the outdoor temperature in each of the cases. The top of the outdoor air volume has a constant atmospheric pressure so that it allows the input and output of flow through it. The figure 5 schematically represents the boundary conditions imposed on the simulation model.
The boundary conditions used in the simulation of both conventional façade and the proposed one, for the hot and cold period, are summarized in table 3, obtained by the Andalusian Energy Agency in Lebrija‐2 station, Sevilla (37.8oN‐5.96oO) to July 15 (hot period) and January 15 (cold period). The façade studied in both cases has a west orientation. Since the simulations are performed in stationary conditions, it has been adopted as the value of the irradiance (w/m2) the values of irradiation (wh/m2) of table 3. THERMAL AND FLUID‐DYNAMIC ANALISYS This section includes the most relevant results obtained from the simulations and the analysis thereof. The results include the thermal and fluid‐dynamic behavior of the conventional ventilated façade system and the ventilated façade system with double chamber with flow control device, in a time of heat (TindoorToutdoor). Warm season (TindoorToutdoor) The figure 8 shows the thermal behavior of ventilated façade systems (conventional and proposed) in the cold months.
Under conditions of outdoor temperature lower than comfort indoor temperature, conventional ventilated façades (left) do not take advantage of the heat direct sunlight involves (and to a lesser extent diffuse solar radiation). By installing the proposed air flow control device, it is possible to confine air within the chamber, which heats progressively in contact with the outer skin exposed to solar radiation, and therefore improves the thermal efficiency of the building. Specifically, in the simulation carried out, the heat gain of the building per unit area can be increased up to 333% (table 4) during the winter months. Therefore, the use of the proposed façade system is not limited to the period of heat (gains) but is also suitable for the cold months (losses) due to manual device for opening and closing grills which enables or disables ventilation in the chambers, depending on weather conditions. In figure 9 the behavior of air flow in the façade systems studied (conventional and double chamber) is shown. Lower air velocity in the double chamber is evident, because the air is confined, but also a recirculation between the two chambers which favors the transmission of heat from the outer plate to the insulation occurs. The figure 10 shows the temperature profiles at the outer face of the thermal insulation obtained by simulations. In the proposed solution, the temperature is reduced during the summer season (left), while in the winter period (right) it is increased, confirming the better performance of the constructive solution in times of increased energy expenditure. In addition to the above, in the cold season (right), the temperature in the lower part of the façade could be increase up to 20 oC, further reducing energy losses in this area of the façade. Similarly, in the summer (left), the temperature in the upper part of the façade could be reduced up to 20 oC, thus minimizing energy gains on the upper floors. These results demonstrate that the usual differences of comfort and energy efficiency between houses in the same building, are less affected with the proposed system, since the vertical thermal gradient in the air chambers is considerably reduced. Furthermore, the analysis of heat transfer coefficient on the inside of the façade is best to quantify the efficiency of the system because it represents the heat flow (gain or loss) per unit area (w/m2), as only the study of temperatures is not sufficient. The coefficients of heat transfer and temperature in the inside of the façade are shown in Table 4 below. The double chamber system allows an increase of 38% efficiency (summer period) and 333% (winter period), compared to a conventional ventilated façade with closed joint. These values could be increased by an optimized design of the ventilation chambers (inlet chamber and vent) size. The results show that changing the architectural design in construction elements of conventional systems of envelope can lead to significant improvements in the energy performance of the building. The Indoor temperature of the building can decrease (summer season) or increase (winter season) up to 10% with the outside double chamber system over the conventional system. Based on the above it can be confirmed that the proposed system overcomes the main weaknesses of the conventional ventilated façade system and can be used in any climate, orientation and a taller façade. CONCLUSIONS The work includes the thermal and fluid dynamic comparative analysis of a solution of conventional ventilated façade with closed joint and a ventilated façade system optimized with a double chamber and a mechanical
device for controlling the air flow. In addition, the schemes of the design of the proposed system and the description of numerical models of simulations used are included. The main conclusions are drawn below:
The change in the architectural design and comfort construction elements of conventional systems of envelope can lead to significant improvements in the energy and comfort performance of the building. With the proposed double chamber system, energy losses due to the building envelope can be reduced up to 38% (summer) while energy gains can increase up to 333% (winter season) compared to the conventional system of closed joint. The Indoor temperature of the building can decrease (summer season) or increase (winter season) up to 10% over the conventional system. The proposed solution has a similar cost to other conventional ventilated façades and is recommended for its application in both rehabilitation works and new buildings due to its simplicity of implementation. The proposed construction system overcomes the main weaknesses of the conventional ventilated façade with closed joint. First, the design of the system helps to reduce the vertical temperature gradient along the envelope, homogenizing the air temperature in the chambers. In the warmer months, the vertical temperature gradient can be reduced by up to 65% reduction. By minimizing the vertical thermal gradients, global consumption due to energy gains‐losses through the façade depends less on the height of the building, preventing the upper houses to present higher or lower indoor comfort and degree of energy efficiency than the lower houses. Furthermore, the proposed solution is not only limited to summer climatic conditions (TindoorToutdoor) and different façade orientations.
Given the complexity of the mechanisms of heat transfer in the case of ventilated façades it is recommended to incorporate tools of computer simulation in the design phase of façades as a decision tool for professionals, allowing to establish dynamic strategies of ventilation according to the climatic conditions of the region where the façade is installed. The research carried out shows the potential for improvement in energy efficiency of the building by choosing an optimized construction system. ACKNOWLEDGMENTS This study has been carried out as part of the BIA2013‐43061_R research project, funded by the Spanish Ministry of Economy, Industry and Competitiveness. REFERENCES 1. 2. 3.
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Gagliano, A., F. Nocera, and S. Aneli, Thermodynamic analysis of ventilated façades under different wind conditions in summer period. Energy and Buildings, 2016. 122: p. 131‐139. San Juan Guaita, C., Análisis del comportamiento térmico y fluido‐dinámico de las fachadas ventiladas de junta abierta. 2012. Giancola, E., et al., Experimental assessment and modelling of the performance of an open joint ventilated façade during actual operating conditions in Mediterranean climate. Energy and Buildings, 2012. 54: p. 363‐375. Sanjuan, C., et al., Energy performance of an open‐joint ventilated façade compared with a conventional sealed cavity façade. Solar Energy, 2011. 85(9): p. 1851‐1863. Suárez, M.J., et al., Energy evaluation of an horizontal open joint ventilated façade. Applied Thermal Engineering, 2012. 37: p. 302‐313. Sanjuan, C., et al., Experimental PIV Techniques Applied to the Analysis of Natural Convection in Open Joint Ventilated Facades. Energy Procedia, 2012. 30: p. 1216‐1225. Sanjuan, C., et al., Experimental analysis of natural convection in open joint ventilated façades with 2D PIV. Building and Environment, 2011. 46(11): p. 2314‐2325.
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Peci López, F. and M. Ruiz de Adana Santiago, Sensitivity study of an opaque ventilated façade in the winter season in different climate zones in Spain. Renewable Energy, 2015. 75: p. 524‐533. Iribar‐Solaberrieta, E., et al., Energy Performance of the Opaque Ventilated Facade. Energy Procedia, 2015. 78: p. 55‐60. Server, P.J., Device for the ventilation of double‐leaf façades with an inner air chamber. 2015, Google Patents. CD‐Adapco, STAR‐CCM+ 8.06.011 user´s guide. 2014. Instituto de Ciencias de la Construcción Eduardo Torroja and Instituto de la Construcción de Castilla y León. CTE WEB. Código técnico de la edificación web. 2007; Available from: http://cte‐web.iccl.es. Abanto, J., et al., Airflow modelling in a computer room. Building and Environment, 2004. 39(12): p. 1393‐1402. Porras‐Amores, C., et al., Assessing the potential use of strategies independent from the architectural design to achieve efficient ventilation: A Spanish case study. Building Services Engineering Research and Technology, 2014. Buratti, C., R. Mariani, and E. Moretti, Mean age of air in a naturally ventilated office: Experimental data and simulations. Energy and Buildings, 2011. 43(8): p. 2021‐2027. Kwon, K.S., et al., Analysing ventilation efficiency in a test chamber using age‐of‐air concept and CFD technology. Biosystems Engineering, 2011. 110(4): p. 421‐433. Mohammed, R.H., A simplified method for modeling of round and square ceiling diffusers. Energy and Buildings, 2013. 64(0): p. 473‐482. Sanjuan, C., et al., Development and experimental validation of a simulation model for open joint ventilated façades. Energy and Buildings, 2011. 43(12): p. 3446‐3456. Handbook, A. and S. Fundamentals, Edition, American Society of Heating. Refrigeration and Air‐ Conditioning Engineers, Inc., Atlanta, GA, 1985.
Figure 1. Plant and perspective of the integration of the membrane in a conventional ventilated façade
Figure 2. Opening/closing of the chambers
Figure 3. Special module blind on corner of the façade
Figure 4. Operating diagram of a conventional ventilated façade and ventilated façade with double chamber
Figure 5. Scheme of the boundary conditions imposed on simulation models
Figure 6. Thermal behavior in both façade systems in warm months 1: Interior Wall; 2: Thermal insulation; 3: Ventilated chamber; 3a: Inlet chamber; 3b: Ventilated chamber; 3c: separation membrane; 4: Exterior plate; 5: Air inlet to the chamber
Figure 7. Air flow behavior in both façade systems in warm months
Figure 8. Thermal behavior in both façade systems in cold months 1: Interior Wall; 2: Thermal insulation; 3: Ventilated chamber; 3a: Inlet chamber; 3b: Ventilated chamber; 3c: separation membrane; 4: Exterior plate; 5: Air inlet to the chamber
Figure 9. Air flow behavior in both façade systems in cold months
Figure 10. Temperatures on the outer face of the insulation (Tindoor= 22oC)
1. Separation membrane of the two chambers 2. Self‐drilling separator 3. PVC rod between membrane joints 4. Inlet chamber (air inlet) 5. Intake grill 6. Ventilation chamber (air outlet) 7. Ventilation or exhaust grill 8. Butterfly valve to open/close the chamber 9. Handle for actuating the valve
10. Tubular frame for device assembly 11. Cover 12. Exterior layer of the ventilated façade with closed joints 13. Ventilated façade uprights 14. Façade insulation 15. Brick Wall (interior wall) 16. Steel anchors to brick wall 17. Spring brake handle 18. Blind cover on bottom and side of corner module
Table 1: References in figures 1, 2 and 3 Structural element
Material
Air Chambers Ceramic tile Thermal insulation Separation membrane Perforated brick Interior plastering
Air Ceramics Extruded polystyrene Extruded polystyrene Ceramics Gypsum
Density (kg/m3] 1,18 2000 25 25 780 1000
Thermal conductivity [w/m.K] 0,026 1 0,025 0,025 0,35 0,57
Specific heat [J/Kg.K] 1004 800 1450 1450 1000 1000
Table 2. Thermal properties of the materials used [11] and [12] Date: Solar time: Indoor temperature Outdoor temperature Façade orientation: Direct irradiation Diffuse irradiation Global irradiation
Hot period 15‐July 14:00 22 38 Vertical‐west 377,3 117,0 494,3
Cold period 15‐Jan 14:00 22 11,5 Vertical‐west 186,6 114,2 300,8
H o C o C Wh/m² Wh/m² Wh/m²
Table 3. Summary of the boundary conditions applied in simulations Conventional VF
Cold
Warm
Proposed VF Cold Warm
Cold
o
C W/m2
Temperature
22.64
29.65
24.78 26.75
9%
‐10%
Conduction Heat Flux Radiation Heat Flux
4.80 0.34
56.98 4.23
20.79 35.48 1.46 2.55
333% 335%
‐38% ‐40%
W/m2
Heat Flux
5.14
61.20
22.25 38.03
333%
‐38%
W/m2
Table 4. Average values obtained on the inside of the facade.
Warm
Improvement (%)
S0378-7788(16)31461-X http://dx.doi.org/doi:10.1016/j.enbuild.2017.04.063 ENB 7559
To appear in:
ENB
Received date: Revised date: Accepted date:
14-11-2016 16-2-2017 21-4-2017
Please cite this article as: Jaime Santa Cruz Astorqui, C´esar Porras-Amores, VENTILATED FAC¸ADE WITH DOUBLE CHAMBER AND FLOW CONTROL DEVICE, Energy and Buildingshttp://dx.doi.org/10.1016/j.enbuild.2017.04.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
VENTILATED FAÇADE WITH DOUBLE CHAMBER AND FLOW CONTROL DEVICE
Jaime Santa Cruz Astorqui * and César Porras‐Amores Technical University of Madrid, School of Building Engineering, TEMA Research Group, Avda. Juan de Herrera, 6, 28040 Madrid, Spain; E‐Mail: [email protected] and [email protected] * Corresponding author: Jaime Santa Cruz Astorqui. E‐Mail: [email protected] Tel.: +34‐913367637
ABSTRACT Currently, ventilated facades are composed by an inner sheet, thermal insulation, ventilation chamber and exterior finish, a system that allows the heat absorbed by solar radiation to dissipate through natural ventilation of the air from the chamber. This article discusses the feasibility of adding a second air chamber parallel to the existing one, both interconnected by the bottom of the facade, and with a device at the top to regulate the air flow in the chambers, depending on the gradient of the existing temperature between inside and outside the building. The main objective is to evaluate the potential of this proposed system in the improvement of the energy efficiency of the building, using a steady model of Computational Fluid Dynamics (CFD). To this end, a comparative study of energy performance was carried out, as well as the thermal and fluid dynamic behavior, between the proposed two‐chamber system and the conventional ventilated facade system with closed joint, at different times of the year. The results show that the proposed system allows an increase of 38% efficiency in summer period, and 333% in winter period, compared to a conventional ventilated façade with closed joint. Keywords: ventilated façade; passive design; CFD simulation; energy efficiency
INTRODUCTION Currently, there are various systems of ventilated façade, all of which are characterized by incorporating a ventilated air chamber between the outer skin and the insulation (which is continuous along the entire façade). The main purpose of this type of façade is to dissipate the heat the outer skin absorbs when direct solar radiation falls on it, and thereby reduce the temperature of the inner sheet and therefore, the interior space of the building. The operation is based on natural air convection by effect of an increment of its temperature. Thus, an upward flow of air is created through the chamber, which dissipates heat from the outer skin to finally be expelled out through the top of the façade. In short, both the reduction of energy consumption of the cooling system in warm periods and the protection against solar radiation through the top layer of the façade, are the main benefits attributed to the ventilated façade [1]. In this sense, there are two large groups of ventilated façades: a) open joint and b) closed joint. Ventilated façades with open joints allow free circulation of air between the chamber and the outside through the existing joints between the pieces that make up the outer skin. In this type of façades, usually the air enters from the outside into the chamber through the joints of the lower half of the façade, and out gradually to the outside through the upper joints [2]. The performance of this type of façade has been previously studied by developing numerical models of computational fluid dynamics [3] and [4] as well as[5] experimentally by nonintrusive measuring airflow techniques [6] and [7]. In ventilated façades of closed joint, there are no open joints between the pieces that make up the outer skin, so the air enters the chamber through an opening or grid located at the lower part of the façade, and comes out through the other opening or vent in the crowning of the façade. The behavior and the thermal performance of the ventilated façades of closed joint has been studied through simulations of computational fluid dynamics [1], energy simulation [8] and scale prototypes [9]. In any case, ventilated façades are particularly suitable in climates with hot summer periods and orientations exposed to direct sunlight. Under these conditions, solar radiation can increase the surface temperature of a traditional façade by 60º C, so this heat dissipation through the ventilated chamber represents a significant energy savings in air conditioning systems. However, ventilated façades on the market have two weaknesses related to their degree of energy efficiency: First, in cold periods (the outdoor temperature is lower than the indoor comfort temperature), this type of façade is a drawback, since the ventilation of the chamber dissipates the heat from solar radiation falling on the façade, thus causing major losses of heating energy than a conventional façade with identical thermal resistance. In this sense, there are closure control systems of the chamber [10] which allow to occluding the air in the chamber and prevent its ventilation, being configured as a closed chamber capturing heat (greenhouse effect), which solves the problem. Second, in warm periods, the ventilation air circulating in the chamber gradually increases its temperature as it rises through the chamber so that in the lower region of the façade, the air in the chamber is about the outdoor temperature and therefore the system's efficiency is maximal. In the upper zone, the air of the chamber reaches a high temperature, so that the system loses its effectiveness. This gradual warming effect is maximized in ventilated façades of closed joint, not being possible that fresh air enters the intermediate zones of the same.
In this sense, to avoid the negative effects described, incorporating a second vent chamber is studied in combination with a device that regulates air flow between the chambers and the outside. In short, the proposed system achieves two effects: on the one hand, it is possible to drastically reduce the air temperature in the chamber adjacent to the insulation of the enclosure (in warm periods), and secondly, it is possible to confine the air inside the chamber, turning the enclosure in a solar collector (in cold periods). PROPOSED VENTILATED FAÇADE WITH DOUBLE CHAMBER The components, materials and mechanisms that make the proposed system are detailed below (Figures 1, 2 and 3). The membrane between the two chambers (1) is formed of a flexible sheet of polyethylene foam (or insulating material with similar characteristics), which is mounted in rolls as wide as the distance between uprights (13) of the ventilated façade. The union between the bands of membrane and the uprights is performed by a PVC bead of extruded sectioned in "H" (3) which provides sealing between the two chambers. To maintain fixed the membrane and ensure constant thickness of the chambers, it is fixed to the façade insulation (14) by polyethylene spacers (2) that self‐screw to the isolation. The outer chamber or ventilation chamber (6) is located between the membrane (1) and the outer skin (12) of the ventilated façade (which must be of closed joint). The inner chamber or inlet chamber (4) is located between the membrane (1) and the façade insulation (14). Both chambers are connected together at the lower part of the façade, and they open to the outside through the locking device located on the top of the cover (11). The closing device (Figure 2) aims the opening or closing of the chambers to the outside. This device consists of a cover plate (11) for protection against the rain, an intake grill (5) connected to the inlet chamber (4) a ventilation or exhaust grill (7) connected to the ventilation chamber (6), and a butterfly valve (8) actuated by a handle (9). The assembly is mounted on a steel tube frame (10) whose transverse dimension is equal to the total thickness of the façade on the top of its cover, and its longitudinal dimension coincides with the distance between the fixing uprights of the ventilated façade. By operating the handle (9) the position of the butterfly valve is switched (8) between closed and open, allowing the control of air flow between the outside and the chambers. The devices are fixed to the façade by mechanical fasteners (16), and to each other through the rack by screws. In the corners of the façade, it is necessary to place special modules (figure 3) which lack of a closing valve, and therefore are blind in their bottom and sides. The Table 1 includes the references in the figures 1, 2 and 3. The proposed system is currently in the process of patent application in the Spanish Patent and Trademark Office (OEPM, for its Spanish acronym). OPERATION OF VENTILATED FAÇADE WITH DOUBLE CHAMBER
In the figure 4 operation schemes of the proposed ventilated façade with double chamber are shown, compared to the operation of a conventional ventilated façade of closed joint. The proposed system is easily adaptable to any existing ventilated façade system of closed joint on the market (Figure 4 left) and it consists of the incorporation of a continuous membrane inside the existing chamber to get a double chamber, and a device located on the top of the façade, with a control mechanism for opening and closing of the two chambers. The system uses two modes of operation depending on the temperature gradient between the exterior and interior of the building: opened valve for hot periods and closed valve for cold periods. In hot periods, when the outdoor temperature is higher than the indoor comfort temperature (figure 4 center), the outside air penetrates the chamber adjacent to insulation (inlet chamber) through the coronation of the façade, and flows downstream, maintaining an average temperature very similar to the outdoor ambient temperature, since it is hardly affected by the heating of the outer skin. Upon reaching the bottom of the inlet chamber, the air passes into the outer chamber (ventilation chamber) and rises by natural convection when heated by direct contact with the outer skin (chimney effect), thus dissipating the heat absorbed by solar radiation. Through this system, the flow of heat into the building is reduced, which means greater energy efficiency. The movement of air through the chambers is ensured by the temperature gradient between the air entering the intake grill and the air ejected from the exhaust grill. This gradient will be greater the more direct solar radiation falling on the façade. The effectiveness of the system is based on the average air temperature in contact with the insulation (inlet chamber), it is significantly lower than the average air temperature in the chamber of a conventional ventilated façade, thus reducing heat gains from solar radiation by up to 38% in summer and 333% heat loss in winter (data obtained by CFD simulation of west façade according to the data indicated in table 4). In cold periods, when the outdoor temperature is lower than the indoor comfort temperature (figure 4 right), the butterfly valve of the input/output device is closed, confining the air into the chambers. When closing the valve, communication between the chambers is left open at the top, allowing air to flow in a circular direction between the two chambers. Solar radiation heats the outer skin of the façade, which conduct heat to the air of the outer ventilation chamber, which circulates by natural convection upwardly, going to the internal chamber through the top of the façade. From there, preheated air circulates downwardly, maintaining a low temperature gradient between the hot and cold sides of the insulation, thereby reducing heat losses by transmission through the facade. The continuous circulation of air between the two chambers can increase the amount of heat absorbed (in relation to a closed conventional chamber), thereby increasing the efficiency of the system. The adjustment of the device for controlling air flow is made to coincide with the on/off dates of the heating system, manipulating from the roof the butterfly valve which opens and closes the ventilation system. Optionally, the valve can be motorized for remote operation. ANALYSIS OF THE PROPOSAL
In order to evaluate the thermal behavior and dynamic fluid of the proposed double chamber system, models of computational fluid dynamics (CFD) have been developed. Specifically, there have been several simulations of ventilated façade, one corresponding to the conventional system and another to the proposed double chamber system at different times of year. The 2D computational domain has an air volume adjacent to the outer side of the ventilated façade. The computational domain is subjected to the action of gravity (g = 9.81 m/s2). In both models, the height of the facade corresponds to a 4‐storey building (10 meters high) and the adjacent air volume has dimensions of 10.6 m x 4.4 m. The conventional ventilated façade system is composed (from outside to inside) of the following ‐‐outer layers (15 mm), air chamber (50 mm), thermal insulation (75 mm), brick (120mm) and plaster (10 mm) ‐‐. On the other hand, the proposed double chamber system consists (from outside to inside) of the following ‐‐ outer layer (15 mm), ventilation chamber (50 mm), membrane of separation (15 mm), inlet chamber (45 mm), thermal insulation (60 mm), brick (120 mm) and plaster (10 mm) ‐‐. In this case, the total thickness of insulating material is the same as in the conventional ventilated façade (75 mm), as the membrane also considered an insulating element. The thermal properties of the materials used are detailed in Table 2. The equations of the numerical model were solved with a commercial CFD software, STAR‐CCM+. The simulations were performed at steady state by the Reynolds‐Averaged Navier‐Stokes (RANS) equations which have been widely used for fluid‐thermal and natural ventilation analysis [13] and [14]. The effects of the turbulence have been simulated by using the standard k‐epsilon turbulence model which has been used widely for practical engineering flow calculation [14], [15], [16] and [17]. Solar radiation has been simulated with the model discrete ordinates (DO). The unstructured mesh is made up of about 70,000 cells with a special refinement in regions where it is expected to have more complex flows such as inside the air chambers. The criterion of convergence to complete the simulation was set at 0.01% error for all the field variable of this problem. The numerical model has been validated from other works of façades previously published [2] and [18].
Boundary conditions
The boundary conditions defined have been selected to the specific characteristics of each studied ventilated façade systems. The Mach number (<0.01) and the variation in density due to the pressure gradient is small; therefore, the density variation is considered constant and the air flow incompressible. Although the flow is incompressible, in natural convection cases it is necessary to introduce the hypothesis Boussinesq for density not to be completely constant. This enables that the variations in density can be considered negligible, except where the effect of gravity appears. Solar radiation incident on the cladding has been simulated as an internal heat source on the outer surface of the ceramic tile. The added value is therefore the final radiation absorbed by the plate. The boundary conditions on the inner side of the façade (inside the building) have been fixed by a heat transfer coefficient of 8 W/m2K, which is the value typically used in building codes for interior flows, according to ASHRAE [19]. The walls of the outdoor air volume (except the top side) have been set to the value of the outdoor temperature in each of the cases. The top of the outdoor air volume has a constant atmospheric pressure so that it allows the input and output of flow through it. The figure 5 schematically represents the boundary conditions imposed on the simulation model.
The boundary conditions used in the simulation of both conventional façade and the proposed one, for the hot and cold period, are summarized in table 3, obtained by the Andalusian Energy Agency in Lebrija‐2 station, Sevilla (37.8oN‐5.96oO) to July 15 (hot period) and January 15 (cold period). The façade studied in both cases has a west orientation. Since the simulations are performed in stationary conditions, it has been adopted as the value of the irradiance (w/m2) the values of irradiation (wh/m2) of table 3. THERMAL AND FLUID‐DYNAMIC ANALISYS This section includes the most relevant results obtained from the simulations and the analysis thereof. The results include the thermal and fluid‐dynamic behavior of the conventional ventilated façade system and the ventilated façade system with double chamber with flow control device, in a time of heat (Tindoor
Under conditions of outdoor temperature lower than comfort indoor temperature, conventional ventilated façades (left) do not take advantage of the heat direct sunlight involves (and to a lesser extent diffuse solar radiation). By installing the proposed air flow control device, it is possible to confine air within the chamber, which heats progressively in contact with the outer skin exposed to solar radiation, and therefore improves the thermal efficiency of the building. Specifically, in the simulation carried out, the heat gain of the building per unit area can be increased up to 333% (table 4) during the winter months. Therefore, the use of the proposed façade system is not limited to the period of heat (gains) but is also suitable for the cold months (losses) due to manual device for opening and closing grills which enables or disables ventilation in the chambers, depending on weather conditions. In figure 9 the behavior of air flow in the façade systems studied (conventional and double chamber) is shown. Lower air velocity in the double chamber is evident, because the air is confined, but also a recirculation between the two chambers which favors the transmission of heat from the outer plate to the insulation occurs. The figure 10 shows the temperature profiles at the outer face of the thermal insulation obtained by simulations. In the proposed solution, the temperature is reduced during the summer season (left), while in the winter period (right) it is increased, confirming the better performance of the constructive solution in times of increased energy expenditure. In addition to the above, in the cold season (right), the temperature in the lower part of the façade could be increase up to 20 oC, further reducing energy losses in this area of the façade. Similarly, in the summer (left), the temperature in the upper part of the façade could be reduced up to 20 oC, thus minimizing energy gains on the upper floors. These results demonstrate that the usual differences of comfort and energy efficiency between houses in the same building, are less affected with the proposed system, since the vertical thermal gradient in the air chambers is considerably reduced. Furthermore, the analysis of heat transfer coefficient on the inside of the façade is best to quantify the efficiency of the system because it represents the heat flow (gain or loss) per unit area (w/m2), as only the study of temperatures is not sufficient. The coefficients of heat transfer and temperature in the inside of the façade are shown in Table 4 below. The double chamber system allows an increase of 38% efficiency (summer period) and 333% (winter period), compared to a conventional ventilated façade with closed joint. These values could be increased by an optimized design of the ventilation chambers (inlet chamber and vent) size. The results show that changing the architectural design in construction elements of conventional systems of envelope can lead to significant improvements in the energy performance of the building. The Indoor temperature of the building can decrease (summer season) or increase (winter season) up to 10% with the outside double chamber system over the conventional system. Based on the above it can be confirmed that the proposed system overcomes the main weaknesses of the conventional ventilated façade system and can be used in any climate, orientation and a taller façade. CONCLUSIONS The work includes the thermal and fluid dynamic comparative analysis of a solution of conventional ventilated façade with closed joint and a ventilated façade system optimized with a double chamber and a mechanical
device for controlling the air flow. In addition, the schemes of the design of the proposed system and the description of numerical models of simulations used are included. The main conclusions are drawn below:
The change in the architectural design and comfort construction elements of conventional systems of envelope can lead to significant improvements in the energy and comfort performance of the building. With the proposed double chamber system, energy losses due to the building envelope can be reduced up to 38% (summer) while energy gains can increase up to 333% (winter season) compared to the conventional system of closed joint. The Indoor temperature of the building can decrease (summer season) or increase (winter season) up to 10% over the conventional system. The proposed solution has a similar cost to other conventional ventilated façades and is recommended for its application in both rehabilitation works and new buildings due to its simplicity of implementation. The proposed construction system overcomes the main weaknesses of the conventional ventilated façade with closed joint. First, the design of the system helps to reduce the vertical temperature gradient along the envelope, homogenizing the air temperature in the chambers. In the warmer months, the vertical temperature gradient can be reduced by up to 65% reduction. By minimizing the vertical thermal gradients, global consumption due to energy gains‐losses through the façade depends less on the height of the building, preventing the upper houses to present higher or lower indoor comfort and degree of energy efficiency than the lower houses. Furthermore, the proposed solution is not only limited to summer climatic conditions (Tindoor
Given the complexity of the mechanisms of heat transfer in the case of ventilated façades it is recommended to incorporate tools of computer simulation in the design phase of façades as a decision tool for professionals, allowing to establish dynamic strategies of ventilation according to the climatic conditions of the region where the façade is installed. The research carried out shows the potential for improvement in energy efficiency of the building by choosing an optimized construction system. ACKNOWLEDGMENTS This study has been carried out as part of the BIA2013‐43061_R research project, funded by the Spanish Ministry of Economy, Industry and Competitiveness. REFERENCES 1. 2. 3.
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Figure 1. Plant and perspective of the integration of the membrane in a conventional ventilated façade
Figure 2. Opening/closing of the chambers
Figure 3. Special module blind on corner of the façade
Figure 4. Operating diagram of a conventional ventilated façade and ventilated façade with double chamber
Figure 5. Scheme of the boundary conditions imposed on simulation models
Figure 6. Thermal behavior in both façade systems in warm months 1: Interior Wall; 2: Thermal insulation; 3: Ventilated chamber; 3a: Inlet chamber; 3b: Ventilated chamber; 3c: separation membrane; 4: Exterior plate; 5: Air inlet to the chamber
Figure 7. Air flow behavior in both façade systems in warm months
Figure 8. Thermal behavior in both façade systems in cold months 1: Interior Wall; 2: Thermal insulation; 3: Ventilated chamber; 3a: Inlet chamber; 3b: Ventilated chamber; 3c: separation membrane; 4: Exterior plate; 5: Air inlet to the chamber
Figure 9. Air flow behavior in both façade systems in cold months
Figure 10. Temperatures on the outer face of the insulation (Tindoor= 22oC)
1. Separation membrane of the two chambers 2. Self‐drilling separator 3. PVC rod between membrane joints 4. Inlet chamber (air inlet) 5. Intake grill 6. Ventilation chamber (air outlet) 7. Ventilation or exhaust grill 8. Butterfly valve to open/close the chamber 9. Handle for actuating the valve
10. Tubular frame for device assembly 11. Cover 12. Exterior layer of the ventilated façade with closed joints 13. Ventilated façade uprights 14. Façade insulation 15. Brick Wall (interior wall) 16. Steel anchors to brick wall 17. Spring brake handle 18. Blind cover on bottom and side of corner module
Table 1: References in figures 1, 2 and 3 Structural element
Material
Air Chambers Ceramic tile Thermal insulation Separation membrane Perforated brick Interior plastering
Air Ceramics Extruded polystyrene Extruded polystyrene Ceramics Gypsum
Density (kg/m3] 1,18 2000 25 25 780 1000
Thermal conductivity [w/m.K] 0,026 1 0,025 0,025 0,35 0,57
Specific heat [J/Kg.K] 1004 800 1450 1450 1000 1000
Table 2. Thermal properties of the materials used [11] and [12] Date: Solar time: Indoor temperature Outdoor temperature Façade orientation: Direct irradiation Diffuse irradiation Global irradiation
Hot period 15‐July 14:00 22 38 Vertical‐west 377,3 117,0 494,3
Cold period 15‐Jan 14:00 22 11,5 Vertical‐west 186,6 114,2 300,8
H o C o C Wh/m² Wh/m² Wh/m²
Table 3. Summary of the boundary conditions applied in simulations Conventional VF
Cold
Warm
Proposed VF Cold Warm
Cold
o
C W/m2
Temperature
22.64
29.65
24.78 26.75
9%
‐10%
Conduction Heat Flux Radiation Heat Flux
4.80 0.34
56.98 4.23
20.79 35.48 1.46 2.55
333% 335%
‐38% ‐40%
W/m2
Heat Flux
5.14
61.20
22.25 38.03
333%
‐38%
W/m2
Table 4. Average values obtained on the inside of the facade.
Warm
Improvement (%)