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Energy assessment and optimization of perforated metal sheet double skin façades through Design Builder; A case study in Spain
Energy and Buildings 111 (2016) 326–336
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Energy assessment and optimization of perforated metal sheet double skin fac¸ades through Design Builder; A case study in Spain Jesús M. Blanco a,∗ , Aiert Buruaga a , Eduardo Rojí b , Jesús Cuadrado b , Belinda Pelaz b a b
Fluid Mechanics Department, School of Engineering, University of the Basque Country, UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain Mechanical Engineering Department, School of Engineering, University of the Basque Country, UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 27 July 2015 Received in revised form 23 September 2015 Accepted 20 November 2015 Available online 2 December 2015 Keywords: Double-skin perforated fac¸ades Thermal behavior Model validation Energy assessment Optimization
a b s t r a c t Double-skin perforated sheet fac¸ades, are enclosures composed of a perforated metallic sheet, air chamber and glass, is showing an increasing tendency in modern building design. In a previous research, their thermal behavior was addressed, taking into account several physical parameters such as, perforation rates, colors and materials, as well as the influence of wind penetration through a Matlab® model, validated through a fully experimental test campaign, monitoring metallic sheets during 1 year, for different configurations, within a range of 0–35% (perforation rates), black-white (colors) and galvanized steelaluminum (materials). Here, following this research, first of all, the behavior of such configurations is also fully addressed, through a complete Energyplus® model (design builder), which was validated through the abovementioned Matlab® model and experimental outputs. The relevant contribution shows a new parametric energy assessment taking into account additional variables such as the air-gap and location (according to the different climate zones defined for Spain). Finally, the influence of different enclosures on the cooling, heating and lighting loads (energy consumptions) of the building as a whole was obtained, demonstrating the suitability of the previously optimized configurations in terms of relative energy savings. This leads to set up a new methodology aiming to the optimization of design sustainability based on minimum energy consumption. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Many architects, engineers and scientists in general are nowadays devoted to the optimization of new building components. One of the big challenges experienced in the last decades is the relevance of the energy savings factor which is increasing substantially, due mainly to resources constraint [1] influencing the prescriptions given by the main European regulations [2–4] in terms of emissions and consumptions limitations. The use of double skin facades during the last century has been an important step in energy saving control for buildings (by using different combinations of glasses, venetian blinds, roller blinds [5] and even green walls [6]). Esthetically, the trend of hiding windows in modern buildings seriously affects the human perception, giving to the whole building a more abstract dimension [7], making it more interesting visually. Regarding to the double skin enclosure built with metal perforated sheet panels, this issue results even more important and the possibility of controlling the light
∗ Corresponding author. E-mail address: [email protected] (J.M. Blanco). http://dx.doi.org/10.1016/j.enbuild.2015.11.053 0378-7788/© 2015 Elsevier B.V. All rights reserved.
(and in consequence the solar energy gains) changing opening areas (perforations) depending on both the location and orientation and combining with new insulation materials in the facade [8] turns it into a more versatile solution (Fig. 1). This figure is only representative, the building and the perforated sheet are not scaled. Works devoted to double skin fac¸ades are mostly focused to horizontal louvers [9] (mostly venetian blinds), analyzing the energy savings of the building by these elements, even explaining different shading strategies by automatic moving louvers depending on the natural light intensity [10] or intelligent double skins [11] exposed to extreme climate conditions, from desert environments [12], cold weather [13] or tropical climates [14]. Many factors are involved, such as sun irradiation [15,16] and urban environment [17–19] among others. It has also been reported the implementation of renewable energies on this kind of fac¸ades [20–23] or renovation of existing buildings [24,25]. Other works improve the description of the thermal response of a ventilated fac¸ade and takes into account the influence that wind direction and location have on the velocity of the air inside the ventilated chamber [26], or analyzing temperature distribution along this cavity [27,28]. Most of these researches have been mostly based on EnergyPlus® [29].
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Nomenclature cp D DB DG DOE hcv hc H K Ir M Mt ¯ M MRT p r S SG t T TSC Xt X¯ Z
specific heat at constant pressure (J kg−1 K−1 ) screen material diameter (m) Design Builder® model double Glass Department Of Energy convective heat transfer coefficient from glass or shading layer to gap air (W m−2 K−1 ) surface-to-surface heat transfer coefficient for nonvented (closed) cavities (W m−2 K−1 ) height of shading layer (m) thickness of material (m) solar irradiance (W m−2 ) Matlab® model measured value of a variable at instant t. measured mean value of a variable at instant t. mean radiant temperature (K) sheet perforation rate (0/1) reflectivity coefficient = (1-˛-) (0/1) screen material spacing (m) single-glass time (h) temperature (K) screen transmittance predicted value of a variable at instant t. predicted mean value of a variable at instant t. pressure drop factor
Greek symbols absorptivity coefficient (0/1) ␣ thermal conductivity (W m−1 K−1 ) emissivity coefficient (0/1) ε k conductance (W m−2 K−1 ). Boltzmann constant (5.68 10−8 W m−2 K−4 ) IR diffuse transmittance ε diffuse emissivity Ir diffuse reflectance of the screen material SC 5 temperature of the surface of the shading layer that faces the gap (K) temperature of shading layer surface facing the zone 6 air (K) solar altitude angle in polar coordinates (radians) ∅ solar azimuth angle in polar coordinates (radians) density (kg m−3 ) ␥ screen material aspect ratio (dimensionless) Subscripts 0 relative to reference temperature (T0 = 283 K) relative to Bernouilli’s law B cv relative to convection relative to equivalent eq ext relative to the exterior conditions relative to the gap gap gl relative to the glass gnd relative to the ground in relative to inlet opening area relative to the shading device sh sky relative to the sky T relative to total Superscripts relative to diffuse dif eff relative to effective relative to long-wave lw
Fig. 1. General cross section of the office building model used in design builder.
But to the time, there are not detailed studies regarding perforated metal skin shadings. One of the few studies that approach to this subject, through monitoring the behavior of real aluminum sliding perforated panes is [30] but there is no evidence of any further mathematical or computer simulated assay reported about double-skin perforated sheet fac¸ades, apart from the previously mentioned [26]. One of the most complicated issues in the analysis of these types of skins is the geometry design as the plate is composed by a homogeneous distribution of small holes, making difficult the prediction of its general behavior in the real scenario, with gaps directly opened to the external environment [31]. Another relevant matter is the calculation and monitoring of sun irradiation. This has been developed in depth by [32], concluding that design builder software is well calibrated to estimate the incidence of sun irradiation over the material, both the diffuse and direct solar radiation. A great concern is the validation of numerical models through monitoring real scenarios being this the motivation of many researches [33–35]. This paper is the second part of a previous study [36] which analyzed the thermal behavior of real double-skin perforated sheet fac¸ades. The aim was to study the thermal behavior of this type of fac¸ades taking into account several physical parameters such as, perforation rates, colors and materials. In this model, the penetration of air through the metal sheet perforations has been considered. First of all, a theoretical model was built in Matlab® , based on a full energy balance. The experimental process for validation purposes consisted of monitoring metallic sheets mounted on a scaffold within a range of 0–35% (perforation rate), black-white (color) and galvanized steel-aluminum (material) in a large-scale test campaign. Excellent agreement was achieved between the outputs from the above-mentioned numerical model and the experimental outputs, so the model was considered validated. This work is aimed to analyze the impact of such types of perforated metal sheet skin facades on the internal illumination, heating and cooling energy gains and will be applied to a case study, comparing the behavior for different climatic zones in Spain through a new Energyplus® model, fully validated through that theoretical model and experimental data reported in the previous study. The outputs obtained will be used to optimize the configuration of such enclosures for each climatic zone, calculating the total energy savings of the building in different situations. Finally a comparative test for different enclosure configurations, such as single and double glazed and the corresponding doubleskin perforated sheet fac¸ades, reported relevant savings in energy consumptions with regard to equivalent traditional not shading enclosures. 2. Aims and methodology The main goals of this research are to calculate the energy savings of the building due to the use of perforated sheet panels in different climate zones defined in Spain. To achieve this, different software, such as Design Builder® and Matlab® was implemented. The first step was to build and validate a Design Builder® (powered by EnergyPlus® ) model using the Matlab® model that was
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previously validated through experimental testing. Due to the complexity of calculations and iterations to fully address the interior energy gains of the building, the validation has been based in the temperature of the external metal sheet. The next step was to evaluate the energy savings of the building by means of Design Builder® for different perforation rates, materials, colors, chamber widths and climatic areas as defined in Spain. This evaluation was done splitting the lighting, cooling and heating gains, with the objective of identifying their relative contribution in the total amount of energy transferred. Finally, the optimization of such enclosures (perforation rate based) on the different climatic areas defined in Spain was addressed as a case study, together with a comparative test for different glass type enclosure configurations regarding energy demands over a south oriented fac¸ade considered as the most unfavorable alternative.
addressing the thermal behavior of a double-skin fac¸ade and its influence on the interior of the building. Many parameters can be used to characterize glass and exterior/interior shading elements, considering convection in the air chamber but also the irradiative exchange between the two skins and the exterior [38]. The shading module including the main dimensions of the domain and the so called “screen” mode is depicted in Fig. 2. The virtual reference domain consists of a 10 meters wide, 3.5 m high and 8 m depth space unit, with 3.5 m × 10 m glass fac¸ade facing to the south. The airflow is permitted inside the shell (Fig. 2a). Four options of shading devices are available, such as: “shades”, “blinds”, “screens” and “switchable glazing” [39,40]. For this research, the “screen” type has been selected. It is composed of intersecting orthogonally crossed cylinders, Fig. 2b assuming their surface to be diffusely reflecting. The schematic configuration of the cross section defining our enclosure is depicted in (Fig. 3) [41].
3. EnergyPlus® model 3.1. Description The wide range of situations in which double-skin perforated sheet fac¸ades can be found makes it necessary to draw up a numerical model. The EnergyPlus® model presented below shows great potential for obtaining the optimum parameters of such enclosure, assessing energy savings [37] with regard to other alternatives,
Fig. 2. Parametric definition of the model: (a) domain extent (units in metres), (b) detail of the “screen” mode rendering with intersecting orthogonal crossed cylinders.
Fig. 3. Schematic configuration defining the enclosure (a) glazing system with two glass layers and an interior shading layer showing variables used in heat balance equations, (b) variation of gap air temperature with distance from the inlet for upward flow.
J.M. Blanco et al. / Energy and Buildings 111 (2016) 326–336 Table 1 Thermal properties of perforated sheets. Thermal properties for the sheets
[W/mK]
cp [J/kgK]
[kg/m3 ]
Aluminum anodized Clean metallic galvanized sheet
160 50
880 450
2.800 7.800
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this case completely open), height and width respectively. The temperature of the plate width is also considered to remain constant throughout, while conduction heat transfer is disregarded, because of the high conductivity and the low thickness of the sheets. 3.2. Screen beam transmittance
Table 2 Main irradiative properties of sheets and glass used for the model. Radiative properties adopted
˛ [0 /1 ]
ε [0 /1 ]
[0 /1 ]
r [0 /1 ]
Aluminum anodized Clean metallic galvanized sheet Matte black painting aluminum Glossy white painting aluminum
0.50 0.65 0.95 0.25
0.84 0.13 0.89 0.89
0.17 0.19 0.20 0.15
0.33 0.16 0.25 0.60
Table 3 Comparative thermal coefficients for single and double glassed enclosures.
U-value [W m−2 K−1 ] Total solar transmission [0 /1 ] Direct solar transmission [0 /1 ] Light transmission [0 /1 ]
DG 6-13-6 mm
SG 6 mm
2.708 0.697 0.604 0.781
6.121 0.810 0.775 0.881
Climate-related data, such as irradiance (Ir) and ambient temperature (Text ), including characteristics of the double-skin fac¸ade under analysis, such as perforation rate (p), solar absorptivity (˛) and emissivity (ε) of the perforated sheet-metal, transmittance () of the glass as well as other material related properties such as, thermal conductivity (), density (), and specific heat (cp ) of plate and glass are the main inputs. General thermal characteristics are those used in the model as per Table 1, whereas Table 2 is focused on irradiative characteristics for the different materials tested. Table 3 shows a summary of the thermal coefficients associated to single and double glassed enclosures respectively. A distinction must be drawn between radiant parameters, airspace parameters, and others, as shown in Table 4, according to the perforation rate. The model for the study of double-skin (glass/sheet-metal) perforated fac¸ades applies a system of non-linear equations to estimate the temperatures of glass and sheet-metal allowing the calculation of the heat balance for the whole building, representing the amount of heat to be provided via the air-conditioning system to offset energy gains and losses through the double-skin sheet. As simplifying hypotheses, the convection and temperature coefficients of the air space depend on top and bottom openings (in
The incident direct radiation affects the optical properties related to the direct component of transmitted radiation. Two components are included in the screen transmittance algorithm: One of them is the screen beam transmittance. This parameter is related to the blocking of the sun’s rays by the screen material, which takes in account the beam solar radiation that goes through the screen openings. The other one, the scattering transmittance, is the cause for the additional flux of transmitted beam solar radiation by diffuse reflectance (scattering) from the screen material [41]. The screen material aspect ratio between the diameter of the openings and their spacing in the screen model is the next one: =
D S
(1)
The optical ray trace modeling accounts for the reflected (scattered) transmittance of incident beam radiation derived by curve fitting results. The ray traces depend on the screen aspect ratios, diffuse screen reflectance, and relative solar azimuth and altitude angles. It is considered that the surface of the screen cylinders is diffusely reflecting with the properties of a Lambertian surface. For the estimation of the amount of transmitted flux due to the reflection, a hemispherical detector is used whereas for the calculation of the direct beam transmittance, the model takes the direction of the diffuse reflectance, the relative solar altitude angle ˛ and the relative solar azimuth angle ϕ for a given diffuse reflectance sc and screen aspect ratio . 3.3. Shading device EnergyPlus® provides the possibility to put shading devices on the exterior or interior of the window or between glass layers. For the perforated sheet fac¸ade model, the exterior shading device is chosen. Three types of thermal interactions are considered in the window shading device; the interaction between the shading layer and the adjacent glass (if there is a shade, a screen or a blind), between shading layer and the room in the case of interior shading and
Table 4 Definition of the parametric values. Aluminum anodized (p = 55%)
Glossy white aluminum (p = 55%)
Matte black aluminum (p = 55%)
0.7 0.11 0.039 0.7
0.55 0.225 0.378 0.55
0.55 0.315 0.405 0.55
0.55 0.022 0.429 0.55
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.02 50
0.02 50
0.02 160
0.02 160
0.02 160
Parametric values
Definition
(a) Radiant parameters: Solar transmittance Solar reflectance Thermal hemispherical emissivity Thermal transmittance
[0 /1 ] 1--˛ (1 − p) [0 /1 ] ε (1 − ) [0 /1 ]
[0 /1 ]
0.1 0.333 0.117 0.1
0.25 0.2775 0.0975 0.25
0.4 0.222 0.078 0.4
0.55 0.1665 0.0585 0.55
(b) Air space parameters: Shade to glass distance
e [m]
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.02 50
0.02 50
0.02 50
Top opening multiplier factor Bottom opening multiplier factor 1Left-side opening multiplier factor Right-side opening multiplier factor (c) Other parameters: Sheet thickness Sheet conductivity
t0 [0 /1 ] b0 [0 /1 ] l0 [0 /1 ] r0 [0 /1 ] K [m] [W m K−1 ]
Galvanized steel (p = 10%)
Galvanized steel (p = 25%)
Galvanized steel (p = 40%)
Galvanized steel (p = 55%)
Galvanized steel (p = 70%)
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Fig. 5. Wind effects over the central period: (a) wind speed distribution (m/s), (b) experimental temperatures for sheets and external (◦ C).
Fig. 4. External climate conditions for model validation in a 24 h period: (a) solar irradiance (W/m2 ), (b) external temperature (◦ C).
between the shading layer and the outside surrounding in case of exterior shading. The natural convection of the airflow between the shading element and the glass is also considered. This flow interacts with the temperature of the glazing and shading elements and is determined on the model described in the ISO Standard 15099 [42]. The exterior shading device considers: • The long-wave radiation (Ir) that is absorbed by the shading device from the surrounding, or transmitted by the shading element and absorbed by the adjacent glass. This surrounding are the sky and ground plus the shadowing device surface and the exterior buildings “seen” from the opening. • Shading device and adjacent glass inter-reflection of irradiation. • Direct and diffuse solar radiation absorbed by the shading device. • “Shading element- air chamber and exterior shading- outside air” convection effects. • Buoyancy effect induces natural convection airflow in the space between shading device and glass and this flow affects on the convection coefficients of shading-to-gap and glass-to-gap interactions. The heat transfer coefficient is calculated in Design Builder® as it follows: hcv = 2hc + 4v
(2)
4. Validation The EnergyPlus® model will be validated through the theoretical model previously reported [36], together with the experimental outputs, since a correct evaluation of the energy consumption for the whole building depends on the accuracy of these predictions. Fig. 4 shows the environment conditions considered for both models. Fig. 4a shows the hourly evolution of the solar irradiance (W/m2 ) whereas Fig. 4b shows the external temperature measured. Both graphs are based on a 24 h period, chosen for the day where the biggest solar irradiance measurements were recorded. Fig. 5 shows the wind effect over a particular period of time. In Fig. 5a, peak values of wind speed were detected, influencing the behavior of different configurations of sheets as can be seen in detail on Fig. 5b for the same central period. A constant perforation rate (p = 0.55) was considered in this figure, taking into account different colors and materials. Fig. 6 shows the comparative test carried out to validate the Design Builder® model (DB) through our previous Matlab® model (M) and experimental temperatures (EXP) of the sheets. Fig. 6a shows perforated sheet temperature evolution for galvanized steel, according to perforation rate p = 25% to the left and p = 55% to the right), Fig. 6b depicts perforated sheet temperature evolution for a perforation rate (p = 0.55), according to material (left galvanized steel and right anodized aluminum) and finally Fig. 6c exposes perforated sheet temperature evolution for a perforation rate (p = 0.55), according to color (left matte black aluminum and right glossy white aluminum).
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Fig. 6. Perforated sheet temperature evolution comparing Matlab and Design Building results for (a) different perforation rate using galvanized steel: left p = 0.25, right p = 0.55 (b) different material using perforation rate (p = 0.55),: left galvanized steel, right anodized aluminum, (c) different colors with perforation rate (p = 0.55), left matte black aluminum, right glossy white aluminum.
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A relevant matching is appreciated in terms of temperature prediction for both models and experimental values in all the cases studied, having detected maximum deviations of about 3 ◦ C, so with these considerations, the model of the double-skin fac¸ade in EnergyPlus® can be considered validated. To quantify the variations between the measured data, Design Builder data and Matlab for the metal sheet temperature, the square of the Pearson product moment correlation coefficient (R2 ) [26] is calculated using Eq. (2).
⎛
R2 = ⎝
t=1,n
t=1,n
Xt − X
Xt − X
Mt − M
2 t=1,n
⎞2
Mt − M
2
⎠
(3)
The variation of different calculations carried out comparing different materials, colors and perforation rates has been similar. To estimate the error between different parameters, the perforation rate (p = 0.55) has been chosen as reference value. Fig. 7 plots over the horizontal axe the temperature differences obtained between Matlab and the experimental test, whereas the temperature differences obtained between Design Builder and the experimental test are depicted over the vertical axe. The values obtained for R2 as can be visualized graphically means that the error is minimum so we can conclude that the values obtained are fully admissible.
Fig. 7. Error estimation through the square of the Pearson product moment correlation coefficient.
5. Results and discussion A standard case was chosen for study, which was simulated in a wide range of situations in order to determine the optimum design,
Fig. 8. Climate zones in Spain with averaged solar radiation values.
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Table 5 Monthly averaged climate data: (a) Zone I (Bilbao). (b) Zone V (Seville). Month
1
2
3
4
5
6
7
8
9
10
11
12
(a) Solar global irradiation (kWh/m2 day) Cloudless days/month Rainfall (mm) Average Humidity (%) Tmin (◦ C) Tmax (◦ C) Tavg (◦ C)
1.56 2.6 120 72 5.1 13.4 9.3
2.23 2.7 86 69 5.1 14.3 9.7
3.43 2.6 90 68 6.4 16.5 11.5
4.30 1.8 107 69 7.6 17.6 12.6
5.17 2.1 78 69 10.6 20.8 15.7
5.55 3.0 60 70 13.4 23.4 18.4
5.49 3.9 50 71 15.4 25.4 20.4
4.87 3.4 76 72 15.7 26.0 20.9
4.08 3.8 73 71 13.8 24.6 19.2
2.72 2.7 111 71 11.4 21.4 16.4
1.70 2.6 147 73 8.1 16.6 12.4
1.38 2.8 122 72 5.9 13.9 9.9
(b) Solar global irradiation (kWh/m2 day) Cloudless days/month Rainfall (mm) Average Humidity (%) Tmin (◦ C) Tmax (◦ C) Tavg (◦ C)
2.72 11.2 66 71 5.7 16.0 10.9
3.66 7.9 50 67 7.0 18.1 12.5
5.03 8.6 36 59 9.2 21.9 15.6
6.14 6.0 54 57 11.1 23.4 17.3
6.99 6.9 30 53 14.2 27.2 20.7
7.88 12.9 10 48 18.0 32.2 25.1
8.1 21.1 2 44 20.3 36.0 28.2
7.2 18.7 5 48 20.4 35.5 27.9
5.78 10.3 27 54 18.2 31.7 25
4.02 7.8 68 62 14.4 26.0 20.2
2.92 8.0 91 70 10.0 20.2 15.1
2.33 8.4 99 74 7.3 16.6 11.9
Fig. 9. Total energy savings (illumination, cooling and heating) with the use of perforated sheets of different materials, colors, perforation rates and air chamber gaps (5 and 30 cm) for zones I and V.
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taking into account for the whole enclosure, additional variables such as: the air-space width (e) and location [43] considering for the later variable the different climate zones defined in Spain, as shown in Fig. 8, including the averaged irradiance values for each of them. Table 5 [44,45] shows the monthly averaged climate data for two of the extreme zones such as zone I, characterized by cold and wet atmospheric conditions (Table 5a) being “Bilbao” one of the representative capitals in the north, and zone V characterized by hot and dry conditions (Table 5b) being “Seville” one of the representative capitals in the south. Weather conditions for the rest of zones reach intermediate values. For the assessment of the energy savings when installing double-skin perforated sheet fac¸ades, which is the first goal of this paper, a parametric study has been carried out over a south oriented fac¸ade. The annual consumption figures for the building with each type of enclosure were obtained, leading to draw the optimum layout of the enclosure in terms of energy efficiency, addressing the second objective. Different characteristics [46] of double-skin fac¸ades will be studied, considering the whole set of variables previously defined as will be fully described next. Fig. 9 displays the total energy savings (cooling, heating and illumination), for a year period in percentage, when a perforated sheet is incorporated to the primitive enclosure, for different colors, materials, perforation rates and air-space widths (e), which are compared for the two extreme climate zones above mentioned. Thus, Fig. 9a and b corresponds to an air-space width of e = 0.3 m (large) whereas Fig. 9c and d is for e = 0.05 m (very thin). It can be concluded that both color and material of perforated sheets have very little influence on the energy consumption of the building except for the galvanized steel. Analyzing the impact on total savings of energy due to different materials used in the perforated sheet, it can be concluded that there is not much difference between them in both zones (less than 5% between the most and less efficient material). In zone V, the black aluminum represents the less efficient material (as the sun radiation level is higher and the absorption coefficient associated to the black color is greater), but results for other materials are random. In zone I, the highest total energy savings are clearly reached with galvanized steel (5% more than using aluminum) this may be due to the higher density of the galvanized steel compared to aluminum, together with a lower transmittance helping to keep the heat inside the building in winter, increasing this way efficiency. The distinctive factor is the perforation rate, being directly responsible for the amount of direct solar radiation that enters the building (affecting the energy required for the climate control system to return to the comfort set points inside the building). According to this, for zone I (Bilbao) the optimum is reached around p = 0.35–0.45 but in the case of zone V (Seville), the figure shows the maximum around a slightly lower values of p = 0.10–0.30. Regarding perforation rate, as it is predictable in both zones, the general behavior is that the bigger the perforation, the lower is the impact of material changing, due to the reduction of the mass in the sheet. The air-space width (e) is another key parameter. It can be concluded that the lower the “e”, the higher the internal temperature as the heat of the metal sheet becomes more difficult to be dissipated. This can be easily explained for zone I, due to the cold winter temperatures, leading to higher energy savings through galvanized steel. Nevertheless, the analysis carried out for zone V reflects that the effect of the air chamber is not as relevant as initially was expected, due to the high temperature of the inlet air, leading to a decreasing of the convective effect. In this case, is the white aluminum the best material, closely followed by galvanized steel. Fig. 10 shows the total energy consumption (kWh per year) for a particular material such as galvanized steel shadowing with different perforation rates. Cooling, heating and illumination
Fig. 10. Cooling, heating and illumination energy consumption per year with galvanized steel perforated sheet shadowing for different perforation rates: (a) zone I, (b) zone V.
loads were considered separately in order to address their own contribution to the total load. Fig. 10a shows values for zone I whereas Fig. 10b is focused on zone V. It must be outlined that differences are quite remarkable with regard to the climate control system, requesting (for all the range of perforation rates) a much bigger amount of cooling rates for zone V and consequently much less amount of heating, as expected, due to the external climate conditions for both zones. Obviously, illumination loads are slightly increasing as the perforation rate decreases for both zones. The optimum configuration, regarding energy consumption can be obtained for around p = 0.4 for zone I and p = 0.25 for zone V, being consequent with the outputs previously obtained.
J.M. Blanco et al. / Energy and Buildings 111 (2016) 326–336 Table 6 Optimum range of perforation rates according to the climate zones in Spain for galvanized steel. Climate zone I II III IV V
Optimum range of p [0 /1 ] 0.35–0.45 0.30–0.40 0.25–0.40 0.20–0.35 0.10–0.30
335
differentiating lighting and climate control systems (heating and cooling respectively) provide an accurate picture for each scenario. On the one hand, the need for more heating (and much less cooling) in zone I, according to Fig. 11a, means that providing plates with a perforation rate p = 0.40, savings of about 22% with single glass and 16% with double glass were obtained when comparing with the same enclosure without shading. It is remarkable that even for zone I, in south fac¸ades without perforated metal sheet protection, the amount of energy needed for heating during the winter is lower than the required for cooling in summertime. On the other hand, for zone V, where major savings will be obtained with lower perforation rates due to the remarkable reduction in cooling load compared with curtain-wall fac¸ades, for an optimum solution with a perforation rate p = 0.25, as shown in Fig. 11b, the savings can reach up to 53% when compared with single glass and 49% in the case of double glass fac¸ades, which are not indeed negligible figures. In particular, results evident the greater proportion between cooling-heating achieved for zone V when compared for zone I.
6. Conclusions
Fig. 11. Cooling, heating and illumination energy consumption per year for single and double glass without perforated steel and with galvanized steel perforated sheet, considering optimum perforation rate for each zone: (a) zone I (p = 0.40), (b) zone V (p = 0.25).
This study has been carried out for the rest of zones, so Table 6 summarizes the optimum range of perforation rates for all the climatic zones in Spain for a particular material, such as galvanized steel. More adjusted values will depend upon the weather conditions of the particular location to be considered. Finally, Fig. 11 shows the energy consumption just for these optimum configurations of double-skin perforated sheet-metal fac¸ades compared to single gazed ones, but also to curtain walls without any sun protection, single (SG) and double (DG) glassed respectively, for both extreme locations. Consumptions
The implementation of double-skin perforated sheet fac¸ades is showing a marked tendency in modern building design, mostly based on esthetic aspects, whereas the basic energy saving principles are not taken into account. The real concern is that a wrong design of such enclosures unfortunately leads to a general discomfort conditions inside the building, together with a noticeable increase of the total energy consumption experienced by the building as a whole. In this paper, a theoretical model for predicting the thermal behavior of double skin perforated sheet fac¸ades has successfully been built through Energyplus® (design builder). Simulations have been performed for different configurations over a 24-h type-period in summer time and detailed information on the temperatures of these sheets in accordance with the different parameters (color, material and perforation rate) has been recorded. It was finally validated through a Matlab® model and experimental outcomes reported in a previous stage of this research. As in the simulation of Design Builder the detailed shape of the perforations in the metal sheet is not possible to be considered, the impact of the shape and size of the holes has not been analyzed in this paper. Regarding perforation rates, optimum configurations vary significantly depending on the climatic zone considered. For this case study p = 0.3 was obtained for a particular location inside zone I (cold climate conditions), whereas p = 0.2 resulted the optimum for another particular location inside zone V (hot climate conditions). Regarding material and color, galvanized steel, closely followed by white aluminum were considered the most appropriate combinations. A further energy study based on the heat flow exchanged across the enclosure, between sheet, glass and the interior of the building has also been carried out taking into account additional variables such as: the air-space width (e) and location according to different climate zones defined in Spain, focused over a south type oriented fac¸ade quantifying their influence on the total building load under which the climate control system is operating. Figures were obtained for annual energy savings. By way of example, the method was presented in detail for two different climate zones in Spain, reading savings as much as around 20% in energy consumption for the coldest areas, till around 45% for the hottest ones, not being therein negligible values. This method was extended to the rest of climatic zones, highlighting the optimized configurations of perforated sheets, based on a suggested range of the perforation rate (p) yielding from 0.1
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till 0.45 for all of the zones, so a specific study with this model should be addressed for any particular location considered. Results derived from this study show those different perforation rates of perforated sheet-metal are suitable for optimum use in climate zones where high cooling load is required by shading the intense solar radiation, but also in other less adverse climates, according to the procedure established here. In a forthcoming publication we are preparing now, based in the role of shape and size of the holes, these parameters are going to be carefully studied. Further work will be also focused on the effect of the different orientations, as here only south oriented fac¸ades were considered. Besides, the implementation of a Computational Fluid dynamics (CFD) tool to fully address the behavior of the air inside the chamber, according to different external weather conditions will be also studied, taking the advantage of the previous results reported here, regarding the thermal behavior of double skin perforated sheet fac¸ades. Acknowledgements The authors are deeply grateful to the Basque Government, which funded this research through project IT781-13, and to all those involved in the different stages for their guidance and invaluable help at all times in the arduous process of model validation. References [1] P. Nejat, F. Jomehzadeh, M.M. Taheri, M. Gohari, Abd. Zaimi, M. Majid, A global review of energy consumption, CO2 emissions and policy in the residential sector (with an overview of the top ten CO2 emitting countries), Renew. Sustain. Energy Rev. 43 (2015) 843–862. [2] R. Álvarez, S. Zubelzu, G. Díaz, A. López, Analysis of low carbon super credit policy efficiency in European Union greenhouse gas emissions, Energy 82 (2015) 996–1010. [3] I. Ballarini, E.P. Corgnati, V. Corrado, Use of reference buildings to assess the energy saving potentials of the residential building stock: the experience of TABULA project, Energy Policy 68 (2014) 273–284. [4] R. Galvin, M. Sunikka-Blank, Economic viability in thermal retrofit policies: learning from ten years of experience in Germany, Energy Policy 54 (2013) 343–351. [5] C. Oleskowicz-Popiel, M. Sobczak, Effect of the roller blinds on heat losses through a double-glazing window during heating season in Central Europe, Energy Build. 73 (2014) 48–58. [6] L. Malys, M. Musy, C. Inard, A hydrothermal model to assess the impact of green walls on urban microclimate and building energy consumption, Build. Environ. 73 (2014) 187–197. [7] L.G. Bakker, E.C.M. Hoes-van Oeffelen, R.C.G.M. Loonen, J.L.M. Hensen, User satisfaction and interaction with automated dynamic facades: a pilot study, Build. Environ. 78 (2014) 44–52. [8] J.P. Jelle, S. Kalnæs, T. Gao, Low-emissivity materials for building applications: a state-of-the-art review and future research perspectives, Energy Build. 96 (2015) 326–356. [9] D. Saelens, W. Parys, J. Roofthooft, A. Tablada de la Torre, Reprint of “Assessment of approaches for modeling louver shading devices in building energy simulation programs”, Energy Build. 68 (2014) 799–810. [10] S. Grynning, B. Time, B. Matusiak, Solar shading control strategies in cold climates: heating, cooling demand and daylight availability in office spaces, Solar Energy 107 (2014) 182–194. [11] J.W. Moon, S.H. Yoon, S. Kim, Development of an artificial neural network model based thermal control logic for double skin envelopes in winter, Build. Environ. 61 (2013) 149–159. [12] H. Sabry, A. Sherif, M. Gadelhak, M. Aly, Balancing the daylighting and energy performance of solar screens in residential desert buildings: examination of screen axial rotation and opening aspect ratio, Solar Energy 103 (2014) 364–377. [13] S. Paiho, I. Pinto Seppä, C. Jimenez, An energetic analysis of a multifunctional fac¸ade system for energy efficient retrofitting of residential buildings in cold climates of Finland and Russia, Sustain. Cities Soc. 15 (2015) 75–85. [14] A.N. Sadeghifam, S.M. Zahraee, M.M. Meynagh, I. Kiani, Combined use of design of experiment and dynamic building simulation in assessment of energy efficiency in tropical residential buildings, Energy Build. 86 (2015) 525–533. ˜ [15] B. Arranz, E. Rodríguez-Ubinas, C. Bedoya-Frutos, S. Vega-Sánchez, Evaluation of three solar and daylighting control systems based on Calumen II, Ecotect and radiance simulation programmes to obtain an energy efficient and healthy interior in the experimental building Prototype SD10, Energy Build. 83 (2014) 225–236.
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Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Energy assessment and optimization of perforated metal sheet double skin fac¸ades through Design Builder; A case study in Spain Jesús M. Blanco a,∗ , Aiert Buruaga a , Eduardo Rojí b , Jesús Cuadrado b , Belinda Pelaz b a b
Fluid Mechanics Department, School of Engineering, University of the Basque Country, UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain Mechanical Engineering Department, School of Engineering, University of the Basque Country, UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 27 July 2015 Received in revised form 23 September 2015 Accepted 20 November 2015 Available online 2 December 2015 Keywords: Double-skin perforated fac¸ades Thermal behavior Model validation Energy assessment Optimization
a b s t r a c t Double-skin perforated sheet fac¸ades, are enclosures composed of a perforated metallic sheet, air chamber and glass, is showing an increasing tendency in modern building design. In a previous research, their thermal behavior was addressed, taking into account several physical parameters such as, perforation rates, colors and materials, as well as the influence of wind penetration through a Matlab® model, validated through a fully experimental test campaign, monitoring metallic sheets during 1 year, for different configurations, within a range of 0–35% (perforation rates), black-white (colors) and galvanized steelaluminum (materials). Here, following this research, first of all, the behavior of such configurations is also fully addressed, through a complete Energyplus® model (design builder), which was validated through the abovementioned Matlab® model and experimental outputs. The relevant contribution shows a new parametric energy assessment taking into account additional variables such as the air-gap and location (according to the different climate zones defined for Spain). Finally, the influence of different enclosures on the cooling, heating and lighting loads (energy consumptions) of the building as a whole was obtained, demonstrating the suitability of the previously optimized configurations in terms of relative energy savings. This leads to set up a new methodology aiming to the optimization of design sustainability based on minimum energy consumption. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Many architects, engineers and scientists in general are nowadays devoted to the optimization of new building components. One of the big challenges experienced in the last decades is the relevance of the energy savings factor which is increasing substantially, due mainly to resources constraint [1] influencing the prescriptions given by the main European regulations [2–4] in terms of emissions and consumptions limitations. The use of double skin facades during the last century has been an important step in energy saving control for buildings (by using different combinations of glasses, venetian blinds, roller blinds [5] and even green walls [6]). Esthetically, the trend of hiding windows in modern buildings seriously affects the human perception, giving to the whole building a more abstract dimension [7], making it more interesting visually. Regarding to the double skin enclosure built with metal perforated sheet panels, this issue results even more important and the possibility of controlling the light
∗ Corresponding author. E-mail address: [email protected] (J.M. Blanco). http://dx.doi.org/10.1016/j.enbuild.2015.11.053 0378-7788/© 2015 Elsevier B.V. All rights reserved.
(and in consequence the solar energy gains) changing opening areas (perforations) depending on both the location and orientation and combining with new insulation materials in the facade [8] turns it into a more versatile solution (Fig. 1). This figure is only representative, the building and the perforated sheet are not scaled. Works devoted to double skin fac¸ades are mostly focused to horizontal louvers [9] (mostly venetian blinds), analyzing the energy savings of the building by these elements, even explaining different shading strategies by automatic moving louvers depending on the natural light intensity [10] or intelligent double skins [11] exposed to extreme climate conditions, from desert environments [12], cold weather [13] or tropical climates [14]. Many factors are involved, such as sun irradiation [15,16] and urban environment [17–19] among others. It has also been reported the implementation of renewable energies on this kind of fac¸ades [20–23] or renovation of existing buildings [24,25]. Other works improve the description of the thermal response of a ventilated fac¸ade and takes into account the influence that wind direction and location have on the velocity of the air inside the ventilated chamber [26], or analyzing temperature distribution along this cavity [27,28]. Most of these researches have been mostly based on EnergyPlus® [29].
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Nomenclature cp D DB DG DOE hcv hc H K Ir M Mt ¯ M MRT p r S SG t T TSC Xt X¯ Z
specific heat at constant pressure (J kg−1 K−1 ) screen material diameter (m) Design Builder® model double Glass Department Of Energy convective heat transfer coefficient from glass or shading layer to gap air (W m−2 K−1 ) surface-to-surface heat transfer coefficient for nonvented (closed) cavities (W m−2 K−1 ) height of shading layer (m) thickness of material (m) solar irradiance (W m−2 ) Matlab® model measured value of a variable at instant t. measured mean value of a variable at instant t. mean radiant temperature (K) sheet perforation rate (0/1) reflectivity coefficient = (1-˛-) (0/1) screen material spacing (m) single-glass time (h) temperature (K) screen transmittance predicted value of a variable at instant t. predicted mean value of a variable at instant t. pressure drop factor
Greek symbols absorptivity coefficient (0/1) ␣ thermal conductivity (W m−1 K−1 ) emissivity coefficient (0/1) ε k conductance (W m−2 K−1 ). Boltzmann constant (5.68 10−8 W m−2 K−4 ) IR diffuse transmittance ε diffuse emissivity Ir diffuse reflectance of the screen material SC 5 temperature of the surface of the shading layer that faces the gap (K) temperature of shading layer surface facing the zone 6 air (K) solar altitude angle in polar coordinates (radians) ∅ solar azimuth angle in polar coordinates (radians) density (kg m−3 ) ␥ screen material aspect ratio (dimensionless) Subscripts 0 relative to reference temperature (T0 = 283 K) relative to Bernouilli’s law B cv relative to convection relative to equivalent eq ext relative to the exterior conditions relative to the gap gap gl relative to the glass gnd relative to the ground in relative to inlet opening area relative to the shading device sh sky relative to the sky T relative to total Superscripts relative to diffuse dif eff relative to effective relative to long-wave lw
Fig. 1. General cross section of the office building model used in design builder.
But to the time, there are not detailed studies regarding perforated metal skin shadings. One of the few studies that approach to this subject, through monitoring the behavior of real aluminum sliding perforated panes is [30] but there is no evidence of any further mathematical or computer simulated assay reported about double-skin perforated sheet fac¸ades, apart from the previously mentioned [26]. One of the most complicated issues in the analysis of these types of skins is the geometry design as the plate is composed by a homogeneous distribution of small holes, making difficult the prediction of its general behavior in the real scenario, with gaps directly opened to the external environment [31]. Another relevant matter is the calculation and monitoring of sun irradiation. This has been developed in depth by [32], concluding that design builder software is well calibrated to estimate the incidence of sun irradiation over the material, both the diffuse and direct solar radiation. A great concern is the validation of numerical models through monitoring real scenarios being this the motivation of many researches [33–35]. This paper is the second part of a previous study [36] which analyzed the thermal behavior of real double-skin perforated sheet fac¸ades. The aim was to study the thermal behavior of this type of fac¸ades taking into account several physical parameters such as, perforation rates, colors and materials. In this model, the penetration of air through the metal sheet perforations has been considered. First of all, a theoretical model was built in Matlab® , based on a full energy balance. The experimental process for validation purposes consisted of monitoring metallic sheets mounted on a scaffold within a range of 0–35% (perforation rate), black-white (color) and galvanized steel-aluminum (material) in a large-scale test campaign. Excellent agreement was achieved between the outputs from the above-mentioned numerical model and the experimental outputs, so the model was considered validated. This work is aimed to analyze the impact of such types of perforated metal sheet skin facades on the internal illumination, heating and cooling energy gains and will be applied to a case study, comparing the behavior for different climatic zones in Spain through a new Energyplus® model, fully validated through that theoretical model and experimental data reported in the previous study. The outputs obtained will be used to optimize the configuration of such enclosures for each climatic zone, calculating the total energy savings of the building in different situations. Finally a comparative test for different enclosure configurations, such as single and double glazed and the corresponding doubleskin perforated sheet fac¸ades, reported relevant savings in energy consumptions with regard to equivalent traditional not shading enclosures. 2. Aims and methodology The main goals of this research are to calculate the energy savings of the building due to the use of perforated sheet panels in different climate zones defined in Spain. To achieve this, different software, such as Design Builder® and Matlab® was implemented. The first step was to build and validate a Design Builder® (powered by EnergyPlus® ) model using the Matlab® model that was
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previously validated through experimental testing. Due to the complexity of calculations and iterations to fully address the interior energy gains of the building, the validation has been based in the temperature of the external metal sheet. The next step was to evaluate the energy savings of the building by means of Design Builder® for different perforation rates, materials, colors, chamber widths and climatic areas as defined in Spain. This evaluation was done splitting the lighting, cooling and heating gains, with the objective of identifying their relative contribution in the total amount of energy transferred. Finally, the optimization of such enclosures (perforation rate based) on the different climatic areas defined in Spain was addressed as a case study, together with a comparative test for different glass type enclosure configurations regarding energy demands over a south oriented fac¸ade considered as the most unfavorable alternative.
addressing the thermal behavior of a double-skin fac¸ade and its influence on the interior of the building. Many parameters can be used to characterize glass and exterior/interior shading elements, considering convection in the air chamber but also the irradiative exchange between the two skins and the exterior [38]. The shading module including the main dimensions of the domain and the so called “screen” mode is depicted in Fig. 2. The virtual reference domain consists of a 10 meters wide, 3.5 m high and 8 m depth space unit, with 3.5 m × 10 m glass fac¸ade facing to the south. The airflow is permitted inside the shell (Fig. 2a). Four options of shading devices are available, such as: “shades”, “blinds”, “screens” and “switchable glazing” [39,40]. For this research, the “screen” type has been selected. It is composed of intersecting orthogonally crossed cylinders, Fig. 2b assuming their surface to be diffusely reflecting. The schematic configuration of the cross section defining our enclosure is depicted in (Fig. 3) [41].
3. EnergyPlus® model 3.1. Description The wide range of situations in which double-skin perforated sheet fac¸ades can be found makes it necessary to draw up a numerical model. The EnergyPlus® model presented below shows great potential for obtaining the optimum parameters of such enclosure, assessing energy savings [37] with regard to other alternatives,
Fig. 2. Parametric definition of the model: (a) domain extent (units in metres), (b) detail of the “screen” mode rendering with intersecting orthogonal crossed cylinders.
Fig. 3. Schematic configuration defining the enclosure (a) glazing system with two glass layers and an interior shading layer showing variables used in heat balance equations, (b) variation of gap air temperature with distance from the inlet for upward flow.
J.M. Blanco et al. / Energy and Buildings 111 (2016) 326–336 Table 1 Thermal properties of perforated sheets. Thermal properties for the sheets
[W/mK]
cp [J/kgK]
[kg/m3 ]
Aluminum anodized Clean metallic galvanized sheet
160 50
880 450
2.800 7.800
329
this case completely open), height and width respectively. The temperature of the plate width is also considered to remain constant throughout, while conduction heat transfer is disregarded, because of the high conductivity and the low thickness of the sheets. 3.2. Screen beam transmittance
Table 2 Main irradiative properties of sheets and glass used for the model. Radiative properties adopted
˛ [0 /1 ]
ε [0 /1 ]
[0 /1 ]
r [0 /1 ]
Aluminum anodized Clean metallic galvanized sheet Matte black painting aluminum Glossy white painting aluminum
0.50 0.65 0.95 0.25
0.84 0.13 0.89 0.89
0.17 0.19 0.20 0.15
0.33 0.16 0.25 0.60
Table 3 Comparative thermal coefficients for single and double glassed enclosures.
U-value [W m−2 K−1 ] Total solar transmission [0 /1 ] Direct solar transmission [0 /1 ] Light transmission [0 /1 ]
DG 6-13-6 mm
SG 6 mm
2.708 0.697 0.604 0.781
6.121 0.810 0.775 0.881
Climate-related data, such as irradiance (Ir) and ambient temperature (Text ), including characteristics of the double-skin fac¸ade under analysis, such as perforation rate (p), solar absorptivity (˛) and emissivity (ε) of the perforated sheet-metal, transmittance () of the glass as well as other material related properties such as, thermal conductivity (), density (), and specific heat (cp ) of plate and glass are the main inputs. General thermal characteristics are those used in the model as per Table 1, whereas Table 2 is focused on irradiative characteristics for the different materials tested. Table 3 shows a summary of the thermal coefficients associated to single and double glassed enclosures respectively. A distinction must be drawn between radiant parameters, airspace parameters, and others, as shown in Table 4, according to the perforation rate. The model for the study of double-skin (glass/sheet-metal) perforated fac¸ades applies a system of non-linear equations to estimate the temperatures of glass and sheet-metal allowing the calculation of the heat balance for the whole building, representing the amount of heat to be provided via the air-conditioning system to offset energy gains and losses through the double-skin sheet. As simplifying hypotheses, the convection and temperature coefficients of the air space depend on top and bottom openings (in
The incident direct radiation affects the optical properties related to the direct component of transmitted radiation. Two components are included in the screen transmittance algorithm: One of them is the screen beam transmittance. This parameter is related to the blocking of the sun’s rays by the screen material, which takes in account the beam solar radiation that goes through the screen openings. The other one, the scattering transmittance, is the cause for the additional flux of transmitted beam solar radiation by diffuse reflectance (scattering) from the screen material [41]. The screen material aspect ratio between the diameter of the openings and their spacing in the screen model is the next one: =
D S
(1)
The optical ray trace modeling accounts for the reflected (scattered) transmittance of incident beam radiation derived by curve fitting results. The ray traces depend on the screen aspect ratios, diffuse screen reflectance, and relative solar azimuth and altitude angles. It is considered that the surface of the screen cylinders is diffusely reflecting with the properties of a Lambertian surface. For the estimation of the amount of transmitted flux due to the reflection, a hemispherical detector is used whereas for the calculation of the direct beam transmittance, the model takes the direction of the diffuse reflectance, the relative solar altitude angle ˛ and the relative solar azimuth angle ϕ for a given diffuse reflectance sc and screen aspect ratio . 3.3. Shading device EnergyPlus® provides the possibility to put shading devices on the exterior or interior of the window or between glass layers. For the perforated sheet fac¸ade model, the exterior shading device is chosen. Three types of thermal interactions are considered in the window shading device; the interaction between the shading layer and the adjacent glass (if there is a shade, a screen or a blind), between shading layer and the room in the case of interior shading and
Table 4 Definition of the parametric values. Aluminum anodized (p = 55%)
Glossy white aluminum (p = 55%)
Matte black aluminum (p = 55%)
0.7 0.11 0.039 0.7
0.55 0.225 0.378 0.55
0.55 0.315 0.405 0.55
0.55 0.022 0.429 0.55
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.02 50
0.02 50
0.02 160
0.02 160
0.02 160
Parametric values
Definition
(a) Radiant parameters: Solar transmittance Solar reflectance Thermal hemispherical emissivity Thermal transmittance
[0 /1 ] 1--˛ (1 − p) [0 /1 ] ε (1 − ) [0 /1 ]
[0 /1 ]
0.1 0.333 0.117 0.1
0.25 0.2775 0.0975 0.25
0.4 0.222 0.078 0.4
0.55 0.1665 0.0585 0.55
(b) Air space parameters: Shade to glass distance
e [m]
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.30 0.05 1 1 1 1
0.02 50
0.02 50
0.02 50
Top opening multiplier factor Bottom opening multiplier factor 1Left-side opening multiplier factor Right-side opening multiplier factor (c) Other parameters: Sheet thickness Sheet conductivity
t0 [0 /1 ] b0 [0 /1 ] l0 [0 /1 ] r0 [0 /1 ] K [m] [W m K−1 ]
Galvanized steel (p = 10%)
Galvanized steel (p = 25%)
Galvanized steel (p = 40%)
Galvanized steel (p = 55%)
Galvanized steel (p = 70%)
330
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Fig. 5. Wind effects over the central period: (a) wind speed distribution (m/s), (b) experimental temperatures for sheets and external (◦ C).
Fig. 4. External climate conditions for model validation in a 24 h period: (a) solar irradiance (W/m2 ), (b) external temperature (◦ C).
between the shading layer and the outside surrounding in case of exterior shading. The natural convection of the airflow between the shading element and the glass is also considered. This flow interacts with the temperature of the glazing and shading elements and is determined on the model described in the ISO Standard 15099 [42]. The exterior shading device considers: • The long-wave radiation (Ir) that is absorbed by the shading device from the surrounding, or transmitted by the shading element and absorbed by the adjacent glass. This surrounding are the sky and ground plus the shadowing device surface and the exterior buildings “seen” from the opening. • Shading device and adjacent glass inter-reflection of irradiation. • Direct and diffuse solar radiation absorbed by the shading device. • “Shading element- air chamber and exterior shading- outside air” convection effects. • Buoyancy effect induces natural convection airflow in the space between shading device and glass and this flow affects on the convection coefficients of shading-to-gap and glass-to-gap interactions. The heat transfer coefficient is calculated in Design Builder® as it follows: hcv = 2hc + 4v
(2)
4. Validation The EnergyPlus® model will be validated through the theoretical model previously reported [36], together with the experimental outputs, since a correct evaluation of the energy consumption for the whole building depends on the accuracy of these predictions. Fig. 4 shows the environment conditions considered for both models. Fig. 4a shows the hourly evolution of the solar irradiance (W/m2 ) whereas Fig. 4b shows the external temperature measured. Both graphs are based on a 24 h period, chosen for the day where the biggest solar irradiance measurements were recorded. Fig. 5 shows the wind effect over a particular period of time. In Fig. 5a, peak values of wind speed were detected, influencing the behavior of different configurations of sheets as can be seen in detail on Fig. 5b for the same central period. A constant perforation rate (p = 0.55) was considered in this figure, taking into account different colors and materials. Fig. 6 shows the comparative test carried out to validate the Design Builder® model (DB) through our previous Matlab® model (M) and experimental temperatures (EXP) of the sheets. Fig. 6a shows perforated sheet temperature evolution for galvanized steel, according to perforation rate p = 25% to the left and p = 55% to the right), Fig. 6b depicts perforated sheet temperature evolution for a perforation rate (p = 0.55), according to material (left galvanized steel and right anodized aluminum) and finally Fig. 6c exposes perforated sheet temperature evolution for a perforation rate (p = 0.55), according to color (left matte black aluminum and right glossy white aluminum).
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Fig. 6. Perforated sheet temperature evolution comparing Matlab and Design Building results for (a) different perforation rate using galvanized steel: left p = 0.25, right p = 0.55 (b) different material using perforation rate (p = 0.55),: left galvanized steel, right anodized aluminum, (c) different colors with perforation rate (p = 0.55), left matte black aluminum, right glossy white aluminum.
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A relevant matching is appreciated in terms of temperature prediction for both models and experimental values in all the cases studied, having detected maximum deviations of about 3 ◦ C, so with these considerations, the model of the double-skin fac¸ade in EnergyPlus® can be considered validated. To quantify the variations between the measured data, Design Builder data and Matlab for the metal sheet temperature, the square of the Pearson product moment correlation coefficient (R2 ) [26] is calculated using Eq. (2).
⎛
R2 = ⎝
t=1,n
t=1,n
Xt − X
Xt − X
Mt − M
2 t=1,n
⎞2
Mt − M
2
⎠
(3)
The variation of different calculations carried out comparing different materials, colors and perforation rates has been similar. To estimate the error between different parameters, the perforation rate (p = 0.55) has been chosen as reference value. Fig. 7 plots over the horizontal axe the temperature differences obtained between Matlab and the experimental test, whereas the temperature differences obtained between Design Builder and the experimental test are depicted over the vertical axe. The values obtained for R2 as can be visualized graphically means that the error is minimum so we can conclude that the values obtained are fully admissible.
Fig. 7. Error estimation through the square of the Pearson product moment correlation coefficient.
5. Results and discussion A standard case was chosen for study, which was simulated in a wide range of situations in order to determine the optimum design,
Fig. 8. Climate zones in Spain with averaged solar radiation values.
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Table 5 Monthly averaged climate data: (a) Zone I (Bilbao). (b) Zone V (Seville). Month
1
2
3
4
5
6
7
8
9
10
11
12
(a) Solar global irradiation (kWh/m2 day) Cloudless days/month Rainfall (mm) Average Humidity (%) Tmin (◦ C) Tmax (◦ C) Tavg (◦ C)
1.56 2.6 120 72 5.1 13.4 9.3
2.23 2.7 86 69 5.1 14.3 9.7
3.43 2.6 90 68 6.4 16.5 11.5
4.30 1.8 107 69 7.6 17.6 12.6
5.17 2.1 78 69 10.6 20.8 15.7
5.55 3.0 60 70 13.4 23.4 18.4
5.49 3.9 50 71 15.4 25.4 20.4
4.87 3.4 76 72 15.7 26.0 20.9
4.08 3.8 73 71 13.8 24.6 19.2
2.72 2.7 111 71 11.4 21.4 16.4
1.70 2.6 147 73 8.1 16.6 12.4
1.38 2.8 122 72 5.9 13.9 9.9
(b) Solar global irradiation (kWh/m2 day) Cloudless days/month Rainfall (mm) Average Humidity (%) Tmin (◦ C) Tmax (◦ C) Tavg (◦ C)
2.72 11.2 66 71 5.7 16.0 10.9
3.66 7.9 50 67 7.0 18.1 12.5
5.03 8.6 36 59 9.2 21.9 15.6
6.14 6.0 54 57 11.1 23.4 17.3
6.99 6.9 30 53 14.2 27.2 20.7
7.88 12.9 10 48 18.0 32.2 25.1
8.1 21.1 2 44 20.3 36.0 28.2
7.2 18.7 5 48 20.4 35.5 27.9
5.78 10.3 27 54 18.2 31.7 25
4.02 7.8 68 62 14.4 26.0 20.2
2.92 8.0 91 70 10.0 20.2 15.1
2.33 8.4 99 74 7.3 16.6 11.9
Fig. 9. Total energy savings (illumination, cooling and heating) with the use of perforated sheets of different materials, colors, perforation rates and air chamber gaps (5 and 30 cm) for zones I and V.
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taking into account for the whole enclosure, additional variables such as: the air-space width (e) and location [43] considering for the later variable the different climate zones defined in Spain, as shown in Fig. 8, including the averaged irradiance values for each of them. Table 5 [44,45] shows the monthly averaged climate data for two of the extreme zones such as zone I, characterized by cold and wet atmospheric conditions (Table 5a) being “Bilbao” one of the representative capitals in the north, and zone V characterized by hot and dry conditions (Table 5b) being “Seville” one of the representative capitals in the south. Weather conditions for the rest of zones reach intermediate values. For the assessment of the energy savings when installing double-skin perforated sheet fac¸ades, which is the first goal of this paper, a parametric study has been carried out over a south oriented fac¸ade. The annual consumption figures for the building with each type of enclosure were obtained, leading to draw the optimum layout of the enclosure in terms of energy efficiency, addressing the second objective. Different characteristics [46] of double-skin fac¸ades will be studied, considering the whole set of variables previously defined as will be fully described next. Fig. 9 displays the total energy savings (cooling, heating and illumination), for a year period in percentage, when a perforated sheet is incorporated to the primitive enclosure, for different colors, materials, perforation rates and air-space widths (e), which are compared for the two extreme climate zones above mentioned. Thus, Fig. 9a and b corresponds to an air-space width of e = 0.3 m (large) whereas Fig. 9c and d is for e = 0.05 m (very thin). It can be concluded that both color and material of perforated sheets have very little influence on the energy consumption of the building except for the galvanized steel. Analyzing the impact on total savings of energy due to different materials used in the perforated sheet, it can be concluded that there is not much difference between them in both zones (less than 5% between the most and less efficient material). In zone V, the black aluminum represents the less efficient material (as the sun radiation level is higher and the absorption coefficient associated to the black color is greater), but results for other materials are random. In zone I, the highest total energy savings are clearly reached with galvanized steel (5% more than using aluminum) this may be due to the higher density of the galvanized steel compared to aluminum, together with a lower transmittance helping to keep the heat inside the building in winter, increasing this way efficiency. The distinctive factor is the perforation rate, being directly responsible for the amount of direct solar radiation that enters the building (affecting the energy required for the climate control system to return to the comfort set points inside the building). According to this, for zone I (Bilbao) the optimum is reached around p = 0.35–0.45 but in the case of zone V (Seville), the figure shows the maximum around a slightly lower values of p = 0.10–0.30. Regarding perforation rate, as it is predictable in both zones, the general behavior is that the bigger the perforation, the lower is the impact of material changing, due to the reduction of the mass in the sheet. The air-space width (e) is another key parameter. It can be concluded that the lower the “e”, the higher the internal temperature as the heat of the metal sheet becomes more difficult to be dissipated. This can be easily explained for zone I, due to the cold winter temperatures, leading to higher energy savings through galvanized steel. Nevertheless, the analysis carried out for zone V reflects that the effect of the air chamber is not as relevant as initially was expected, due to the high temperature of the inlet air, leading to a decreasing of the convective effect. In this case, is the white aluminum the best material, closely followed by galvanized steel. Fig. 10 shows the total energy consumption (kWh per year) for a particular material such as galvanized steel shadowing with different perforation rates. Cooling, heating and illumination
Fig. 10. Cooling, heating and illumination energy consumption per year with galvanized steel perforated sheet shadowing for different perforation rates: (a) zone I, (b) zone V.
loads were considered separately in order to address their own contribution to the total load. Fig. 10a shows values for zone I whereas Fig. 10b is focused on zone V. It must be outlined that differences are quite remarkable with regard to the climate control system, requesting (for all the range of perforation rates) a much bigger amount of cooling rates for zone V and consequently much less amount of heating, as expected, due to the external climate conditions for both zones. Obviously, illumination loads are slightly increasing as the perforation rate decreases for both zones. The optimum configuration, regarding energy consumption can be obtained for around p = 0.4 for zone I and p = 0.25 for zone V, being consequent with the outputs previously obtained.
J.M. Blanco et al. / Energy and Buildings 111 (2016) 326–336 Table 6 Optimum range of perforation rates according to the climate zones in Spain for galvanized steel. Climate zone I II III IV V
Optimum range of p [0 /1 ] 0.35–0.45 0.30–0.40 0.25–0.40 0.20–0.35 0.10–0.30
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differentiating lighting and climate control systems (heating and cooling respectively) provide an accurate picture for each scenario. On the one hand, the need for more heating (and much less cooling) in zone I, according to Fig. 11a, means that providing plates with a perforation rate p = 0.40, savings of about 22% with single glass and 16% with double glass were obtained when comparing with the same enclosure without shading. It is remarkable that even for zone I, in south fac¸ades without perforated metal sheet protection, the amount of energy needed for heating during the winter is lower than the required for cooling in summertime. On the other hand, for zone V, where major savings will be obtained with lower perforation rates due to the remarkable reduction in cooling load compared with curtain-wall fac¸ades, for an optimum solution with a perforation rate p = 0.25, as shown in Fig. 11b, the savings can reach up to 53% when compared with single glass and 49% in the case of double glass fac¸ades, which are not indeed negligible figures. In particular, results evident the greater proportion between cooling-heating achieved for zone V when compared for zone I.
6. Conclusions
Fig. 11. Cooling, heating and illumination energy consumption per year for single and double glass without perforated steel and with galvanized steel perforated sheet, considering optimum perforation rate for each zone: (a) zone I (p = 0.40), (b) zone V (p = 0.25).
This study has been carried out for the rest of zones, so Table 6 summarizes the optimum range of perforation rates for all the climatic zones in Spain for a particular material, such as galvanized steel. More adjusted values will depend upon the weather conditions of the particular location to be considered. Finally, Fig. 11 shows the energy consumption just for these optimum configurations of double-skin perforated sheet-metal fac¸ades compared to single gazed ones, but also to curtain walls without any sun protection, single (SG) and double (DG) glassed respectively, for both extreme locations. Consumptions
The implementation of double-skin perforated sheet fac¸ades is showing a marked tendency in modern building design, mostly based on esthetic aspects, whereas the basic energy saving principles are not taken into account. The real concern is that a wrong design of such enclosures unfortunately leads to a general discomfort conditions inside the building, together with a noticeable increase of the total energy consumption experienced by the building as a whole. In this paper, a theoretical model for predicting the thermal behavior of double skin perforated sheet fac¸ades has successfully been built through Energyplus® (design builder). Simulations have been performed for different configurations over a 24-h type-period in summer time and detailed information on the temperatures of these sheets in accordance with the different parameters (color, material and perforation rate) has been recorded. It was finally validated through a Matlab® model and experimental outcomes reported in a previous stage of this research. As in the simulation of Design Builder the detailed shape of the perforations in the metal sheet is not possible to be considered, the impact of the shape and size of the holes has not been analyzed in this paper. Regarding perforation rates, optimum configurations vary significantly depending on the climatic zone considered. For this case study p = 0.3 was obtained for a particular location inside zone I (cold climate conditions), whereas p = 0.2 resulted the optimum for another particular location inside zone V (hot climate conditions). Regarding material and color, galvanized steel, closely followed by white aluminum were considered the most appropriate combinations. A further energy study based on the heat flow exchanged across the enclosure, between sheet, glass and the interior of the building has also been carried out taking into account additional variables such as: the air-space width (e) and location according to different climate zones defined in Spain, focused over a south type oriented fac¸ade quantifying their influence on the total building load under which the climate control system is operating. Figures were obtained for annual energy savings. By way of example, the method was presented in detail for two different climate zones in Spain, reading savings as much as around 20% in energy consumption for the coldest areas, till around 45% for the hottest ones, not being therein negligible values. This method was extended to the rest of climatic zones, highlighting the optimized configurations of perforated sheets, based on a suggested range of the perforation rate (p) yielding from 0.1
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till 0.45 for all of the zones, so a specific study with this model should be addressed for any particular location considered. Results derived from this study show those different perforation rates of perforated sheet-metal are suitable for optimum use in climate zones where high cooling load is required by shading the intense solar radiation, but also in other less adverse climates, according to the procedure established here. In a forthcoming publication we are preparing now, based in the role of shape and size of the holes, these parameters are going to be carefully studied. Further work will be also focused on the effect of the different orientations, as here only south oriented fac¸ades were considered. Besides, the implementation of a Computational Fluid dynamics (CFD) tool to fully address the behavior of the air inside the chamber, according to different external weather conditions will be also studied, taking the advantage of the previous results reported here, regarding the thermal behavior of double skin perforated sheet fac¸ades. Acknowledgements The authors are deeply grateful to the Basque Government, which funded this research through project IT781-13, and to all those involved in the different stages for their guidance and invaluable help at all times in the arduous process of model validation. References [1] P. Nejat, F. Jomehzadeh, M.M. Taheri, M. Gohari, Abd. Zaimi, M. Majid, A global review of energy consumption, CO2 emissions and policy in the residential sector (with an overview of the top ten CO2 emitting countries), Renew. Sustain. Energy Rev. 43 (2015) 843–862. [2] R. Álvarez, S. Zubelzu, G. Díaz, A. López, Analysis of low carbon super credit policy efficiency in European Union greenhouse gas emissions, Energy 82 (2015) 996–1010. [3] I. Ballarini, E.P. Corgnati, V. Corrado, Use of reference buildings to assess the energy saving potentials of the residential building stock: the experience of TABULA project, Energy Policy 68 (2014) 273–284. [4] R. Galvin, M. Sunikka-Blank, Economic viability in thermal retrofit policies: learning from ten years of experience in Germany, Energy Policy 54 (2013) 343–351. [5] C. Oleskowicz-Popiel, M. Sobczak, Effect of the roller blinds on heat losses through a double-glazing window during heating season in Central Europe, Energy Build. 73 (2014) 48–58. [6] L. Malys, M. Musy, C. Inard, A hydrothermal model to assess the impact of green walls on urban microclimate and building energy consumption, Build. Environ. 73 (2014) 187–197. [7] L.G. Bakker, E.C.M. Hoes-van Oeffelen, R.C.G.M. Loonen, J.L.M. Hensen, User satisfaction and interaction with automated dynamic facades: a pilot study, Build. Environ. 78 (2014) 44–52. [8] J.P. Jelle, S. Kalnæs, T. Gao, Low-emissivity materials for building applications: a state-of-the-art review and future research perspectives, Energy Build. 96 (2015) 326–356. [9] D. Saelens, W. Parys, J. Roofthooft, A. Tablada de la Torre, Reprint of “Assessment of approaches for modeling louver shading devices in building energy simulation programs”, Energy Build. 68 (2014) 799–810. [10] S. Grynning, B. Time, B. Matusiak, Solar shading control strategies in cold climates: heating, cooling demand and daylight availability in office spaces, Solar Energy 107 (2014) 182–194. [11] J.W. Moon, S.H. Yoon, S. Kim, Development of an artificial neural network model based thermal control logic for double skin envelopes in winter, Build. Environ. 61 (2013) 149–159. [12] H. Sabry, A. Sherif, M. Gadelhak, M. Aly, Balancing the daylighting and energy performance of solar screens in residential desert buildings: examination of screen axial rotation and opening aspect ratio, Solar Energy 103 (2014) 364–377. [13] S. Paiho, I. Pinto Seppä, C. Jimenez, An energetic analysis of a multifunctional fac¸ade system for energy efficient retrofitting of residential buildings in cold climates of Finland and Russia, Sustain. Cities Soc. 15 (2015) 75–85. [14] A.N. Sadeghifam, S.M. Zahraee, M.M. Meynagh, I. Kiani, Combined use of design of experiment and dynamic building simulation in assessment of energy efficiency in tropical residential buildings, Energy Build. 86 (2015) 525–533. ˜ [15] B. Arranz, E. Rodríguez-Ubinas, C. Bedoya-Frutos, S. Vega-Sánchez, Evaluation of three solar and daylighting control systems based on Calumen II, Ecotect and radiance simulation programmes to obtain an energy efficient and healthy interior in the experimental building Prototype SD10, Energy Build. 83 (2014) 225–236.
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