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Effects of solar shading devices on energy requirements of standalone office buildings for Italian climates
Applied Thermal Engineering 54 (2013) 190e201
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Effects of solar shading devices on energy requirements of standalone office buildings for Italian climates Laura Bellia, Francesco De Falco, Francesco Minichiello* DII, University of Naples Federico II, Naples, Italy
h i g h l i g h t s < Solar shading devices on a building reduce annual energy requests of the systems. < The energy saving has been evaluated for an office building in Italian climates. < These savings have been evaluated considering heating, cooling and lighting systems. < In warm summer climates (Palermo), the highest saving has been obtained (about 20%). < Building and shading device characteristics influence the energy savings.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 October 2012 Accepted 19 January 2013 Available online 9 February 2013
In Europe, the building energy demand is about 40% of the total energy requirement. In order to obtain significant energy saving in this sector, the European Energy Performance Building Directive (EPBD) 2002/91/CE and the EPBD Recast (Directive 2010/31/UE) promote the use of passive strategies for buildings, which improve indoor thermal conditions above all in summer and so allow the reduction of size and energy requirements of air conditioning systems. This paper analyzes the influence of external solar shading devices on the energy requirements of a typical air-conditioned office building for Italian climates. A type of office building widespread in Europe has been considered. The energy saving related to the solar shading refers only to summer air conditioning, but the evaluation has been carried out for the entire year, by using a building energy simulation code. The energy demand of the main technical systems (heating, cooling and lighting) and the energy saving related to the use of solar shading devices have been evaluated, as a function of the most significant parameters, such as the climate, the geometrical characteristics of the shadings and the building, the thermal transmittance of the building envelope and the building orientation. The solar shading devices have shown the highest energy efficiency for warm summer climates: for example, the global annual energy saving related to the use of suitable shading devices has been evaluated between 8% for Milan (the coldest climate) and 20% (for Palermo, the warmest one). Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Building Office Solar shading Louver Overhang Energy saving Dynamic simulation Italian climates
1. Introduction With reference to the total final energy requirement, 29% depends on the household sector [1,2]. In particular, as regards Europe, the building energy demand is about 40% of the global energy requirement. In order to obtain significant energy saving in this sector, several strategies have been proposed related to air conditioning systems [3,4]. On the other hand, the European Energy
* Corresponding author. DII, University of Naples Federico II, Napoli, P. le Tecchio 80, 80125 Italy. Tel.: þ39 081 2538665; fax: þ39 081 2390364. E-mail addresses: [email protected], [email protected] (F. Minichiello). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.01.039
Performance Building Directive (EPBD) 2002/91/CE [5] and the EPBD Recast (Directive 2010/31/UE) [6] promote the use of passive strategies for buildings, which improve indoor thermal conditions above all in summer and so allow the reduction of size and energy requirements of air conditioning systems [7,8]. This allows also a useful reduction of peaks for summer electric energy demand [9]. Among the various passive strategies proposed for opaque [9,10] and transparent [11,12] building envelope, the use of suitable shading devices for building transparent components is noteworthy. Nowadays, this solution is still more useful, as the “shift toward better insulated building envelopes, reduced air-infiltration rates . is leading to indoor spaces that are more sensitive to solar gain” [13].
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The main function of a shading system is the protection of the building transparent envelope from solar radiation in summer conditions, so preventing overheating by blocking the access of unwanted energy flow into the building. In fact, although several factors contribute to summer heat gain (e.g., electrical equipment, occupants, lights, etc.), in hot climates a significant fraction derives from solar heat gain entering through the fenestration areas. Among the several shading solutions for limiting this thermal load, intercepting the solar radiation before it reaches the glazed area, through the use of external shading devices, is the most effective one [14,15]. It is noteworthy that “fenestration products fully shaded from the outside reduce solar heat gain by as much as 80%” [16]. Palmero-Marrero and Oliveira [17] investigated the effects of louver shading devices applied to a building for different climates (Mexico, Cairo, Lisbon, Madrid, London), assessing their impact on indoor thermal conditions and energy demand compared to a building without shading devices. Tzempelikos and Athienitis [18] evaluated the effects of shading device characteristics, shading control and glazing area on cooling and lighting energy needs for a building located in Montreal; an exterior roller blade has been considered as shading device. Datta [19] analyzed the influence of fixed horizontal louver shading devices on thermal performance of a building for Italian climates, considering a simple 2-zone building with high external wall U-value (1.691 W m2 K1); the study was focused on a louver shading device applied to the south-facing window. Overhang application on electrochromic windows and the related energy saving for commercial buildings in USA are analyzed by Lee and Tavil [20], while in Ref. [21] thermotropic glass with active dimming control for solar shading is investigated. Florides et al. [22] show that the cooling load reduction related to the use of window shading is about 8e9% for modern houses in Cipro. David et al. [23] analyze the thermal effects and the visual efficiency of solar shades, proposing simple indices to consider both these aspects. In Ref. [15], an experimental configuration of external shading devices applied to apartment houses in South Korea is presented; daylight aspects and energy savings for heating and cooling are evaluated, by using also an energy simulation program. Abu-Zour et al. [24] propose a new design of solar collector integrated into solar louvers. Anyway, few research investigations have been carried out to evaluate the energy performances of solar shading devices applied to buildings for Italian climates [19]; moreover, in the energy analysis on building shading devices, the energy request of the lighting system has been rarely considered [18], as well as a complete building instead of a single-zone building. Hence, in this paper, an extended analysis on energy saving related to external solar shading devices for Italian climates is presented. The investigation has been carried out by using EnergyPlus, an accurate building energy simulation program [25]; EnergyPlus is an energy and thermal load simulation program which can help the user to size appropriate HVAC equipment, improve energy performance of buildings and related systems, develop life cycling cost analyses, etc. It is noteworthy that shading models used by EnergyPlus have been validated by means of experimental investigation [13,26]. The approach based on building performance simulation has been conveniently used in several research works related to the building energy efficiency [27,28]. Three different Italian climates (cities of Palermo, Rome and Milan) and a typical air-conditioned office building have been analyzed for both winter and summer. The above mentioned localities have been chosen as representative of typical Italian
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climates: Palermo is the hottest among them, Milan the coldest, while Rome is characterized by an intermediate climate. The energy requirements of the main technical systems (heating, cooling and lighting) have been evaluated. The influence of various parameters on the energy saving related to the use of suitable solar shadings (overhangs on the south facade and louvers on the eastewest facades) has been analyzed. The most important examined parameters are: geometrical characteristics of shadings and building, thermal transmittance of the building envelope, type of lighting control system and building orientation. The aim of this paper is to provide simplified criteria for engineers and architects in order to: a) establish if the use of external solar shading devices for standalone office buildings is suitable or not, in terms of energy requirements, for various Italian climates (but this evaluation is applicable also to other similar climates); b) in the case of energetic suitability, quantify obtainable energy saving and optimize the geometry and positioning of the shading devices, as a function of various parameters. The analysis has been carried out considering above all the end user electric energy (neither the primary energy, nor the CO2 equivalent emissions); only a simplified evaluation of the full life cycle energy consumption related to the production of shading devices has been also reported. 2. Case study In Fig. 1, the modeled office building is shown, while the main characteristics of the complex building-systems and boundary design conditions are reported in Table 1. The air conditioning
Fig. 1. The simulated office building: volumetric scheme and plan of the typical floor.
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system of the building is characterized by fan-coils connected to an electric water chiller/heat pump; the values of global heating COP and cooling EER are reported in Table 1. The choice of the optimal shading device for a given window depends above all on the orientation of the facade and on the apparent path of the sun. Therefore, in Fig. 2 the modeled solar shadings applied to the examined building are shown, i.e. overhangs on the south facade and louvers on the eastewest facades, coherently with the practice and the indications from scientific literature [16,30]. The main characteristics of the selected shading devices are reported in Table 2. As regards the louver configuration and, in particular, the slat angle of 25 (Fig. 2 and Table 2), they have been chosen on the basis of a carried-out energy optimization, as reported in Section 3.8. It can be also noted that this paper refers to Mediterranean climates, so the optimal energy solutions are different from those of Central-Northern Europe, whose cold climates usually require high solar gains in winter and, thus, not fixed shading devices for buildings (or no shading). In Fig. 3, typical applications of external solar shading devices in Italy are shown. 3. Results Several parameters have been varied for the case study, in order to better analyze the effects of solar shading devices on the building energy requirements, as regards heating, cooling and lighting systems; unless otherwise specified, the values of the most important parameters are those reported in Tables 1 and 2. It can be noted that because the short sides of the examined building are eastewest oriented (except for the Section 3.8), their fenestration area is little compared to the long sides, therefore the influence of the louvers on the energy requirements is much minor compared to the overhangs (on long south side). Moreover, it can be noted that the use of the solar shadings leads to energy saving only for the cooling system, while it leads to an increase in energy requirements for both heating and lighting systems (because less solar radiation, useful for daylighting and Fig. 2. The considered solar shading devices: louvers and overhangs. Table 1 Main characteristics of the analyzed building, systems and climates. Width (NeS direction) Height Surface to volume ratio Window to wall ratio (WWR) for each facade Window area TSUMMER-SET-POINT UGLAZING UFRAME UROOF Occupancy level Illuminance level on work plane Metabolic rate Cooling SEER (fan-coils þ water chiller)
12.8 m 10.5 m 0.28 m1 30% (unless otherwise specified) South exposure: 100 m2 North exposure: 100 m2 26 C 2.7 W m2 K1 3.6 W m2 K1 1.0 W m2 K1 0.111 persons/m2
Length Plan area and volume Infiltration airflow rate Window height East exposure: 23 m2
32.3 m 413.4m2e4341.1 m3 0.5 ACH 1.5 m West exposure: 16 m2
0.70 20 C 0.58 W m2 K1 0.90 W m2 K1 From 9:00 a.m. to 18 p.m., 5 days/week Office: 400 lx atrium and corridors: Thermal load related to the office 20 W/m2 100 lx toilets: 200 lx electrical equipment 0.9 met/person Clothing thermal resistance 1 clo for winter, 0.5 clo for summer 2.8 (Milan) Heating SCOP (fan-coils þ 2.8 (Milan) 2.7 (Rome) heat pump) 3.0 (Rome) 2.6 (Palermo) 3.2 (Palermo)
Note: HVAC systems only operate with building occupancy Weather data from Italian Climatic data collection “Gianni De Giorgio” [29] Location Milan (Linate) Latitude 45.43 , Longitude 9.28 Cooling degree-days (base 10 C) 1771 2454 Heating degree-days (base 18 C) Maximum monthly global horizontal radiation (average daily total) 5539 Wh m2 (July)
Glass g-value TWINTER-SET-POINT Uwall UBASEMENT FLOOR Occupancy scheduling
Rome (Fiumicino) Latitude 41.80 , Palermo (Punta Raisi) Longitude 12.23 Latitude 38.18 , Longitude 13.10 2174 3082 1514 744 6121 Wh m2 (July) 6714 Wh m2 (July)
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Table 2 Characterization of selected shading devices. Overhang Orientation
Louver South
Projection 1.0 m (unless otherwise specified) Vertical offset from top of window 0.0 m Horizontal window overlap 0.0 m Note: the geometrical parameters reported in this table are also shown in Fig. 2.
Orientation
East, west
Blades of the louver Vertical offset from top of window Horizontal window overlap Blade depth Angle Vertical spacing Distance from window
4 (unless otherwise specified) 0.0 m 0.0 m 0.7 m 25 0.3 m 0.3 m
Material of the selected shading devices (for both overhangs and louvers): aluminium. Thermal conductivity ¼ 221.0 W/mK; specific heat ¼ 896 J/kg K; density ¼ 2740 kg/m3; solar reflectance ¼ 0.61; infrared emittance: 0.25.
winter heating, enters the building when solar shadings are adopted). Therefore, the best energy solution for the building minimizes the overall annual energy demand. A preliminary analysis has been carried out showing that in winter, for Italian climates, the thermal energy picking-up is suitable for the analyzed office building. For instance, with reference to the hottest locality (Palermo), we have verified that in the period from 1 December to 31 March (identified as conventional winter period by the Italian rules for the climatic zone of Palermo) cooling is required only in 117 h, i.e. about 4% of the total hours (2904 h). 3.1. Influence of the overhang depth Energy requirements of the examined building were calculated, adopting the overhang typologies reported in Table 2, for Palermo, Rome and Milan climates. Moreover, the energy demands for each type of overhang and climate were compared to those related to the reference building with no shading devices. The overhang depth has been fixed to 0.5 m, 1.0 m and 1.5 m, a variation similar to that reported in Ref. [30]. The results are reported in Fig. 4.
For Palermo climate (Fig. 4A), the best energy performances have been obtained for overhangs with depth of 1.0 m e savings of about 20% in terms of total electric energy for heating, cooling and lighting (as regards heating and cooling, the electric energy is that required by all the equipment of the HVAC system, i.e. fan-coils, electric chiller/heat pump, pumps, etc.). The saving drops to 16% when adopting a 0.5 m or 1.5 m overhang. Similar results have been obtained (Fig. 3B and C) for Rome (savings between 9% and 15%) and Milan (1e8%). For all the climates investigated, 1.0 m overhangs represent the best solution in terms of energy demand reduction for the examined building, but it should be noted that the optimal overhang depth depends on the window height (1.5 m in the reference case). The influence of the geometrical characteristics of the shadings (louvers) on the eastewest facades is examined in a successive section. Even though this paper refers to the end user electric energy, also a simplified analysis has been carried out on the full life cycle energy consumption in the production of shading devices. The “cradle-to-gate” approach (instead of the “cradle-to-site” one) has
Fig. 3. Typical applications of external solar shading devices.
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Fig. 4. Influence of the overhang depth on the annual energy requests for Palermo (A), Rome (B) and Milan (C).
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Fig. 5. Influence of the climatic conditions on the annual energy requests for shaded building and no-shaded one.
been used, so neglecting the energy related to transport from factory gate to construction site [31]. With reference to the case of 1.0 m overhangs, the global mass of the overhangs and louvers has been evaluated (about 470 kg, considering extruded aluminium as
material), and the embodied energy value of 200 MJ/kg has been used for the aluminium production. The results show that the primary energy saved by using the solar shading devices equalizes the embodied energy by 3 years for the most irradiated locality
Fig. 6. Influence of the building height on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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(Palermo) and by 5 years for the less irradiated one (Milan). The actual value (0.46) of the thermal to electrical conversion efficiency for Italy has been considered. Obviously, if extruded recycled aluminium is used, much minor embodied energy value has to be considered (about 17 MJ/kg), and therefore the above cited time period is significantly minor. Finally, it is noteworthy taking into account that aluminium is a recyclable material. In the following analyses, only the end user electric energy saving has been considered.
warmest climate in summer, 15% reduction for Rome (intermediate climate), and 8% for Milan (the coldest of the considered climates). It can be noted that only for Milan, the chiller (cooling) energy requirements are similar or lower than the heating ones. It is also noticeable that, for the considered climates, the percentage energy savings related only to the cooling system are between 26% and 29%.
3.2. Influence of the climate
In Fig. 6, results relative to the same building, but with 11 instead of 3 floors, are shown, in order to verify if the percentage energy saving connected to the solar shadings depends also on the height of the building. This possible dependence is negligible. In fact, comparing the results for Palermo (Fig. 6A), the energy saving values are 17%, 22% and 17% (for 0.5 m, 1.0 and 1.5 m overhangs), very similar to 16%, 20% and 16% obtained for the building of 3 floors (Fig. 4A). Also for Milan (7%, 8% and 1% versus 6%, 8% and 1%, i.e. Fig. 6B versus Fig. 4B), the dependence of the percentage energy savings on the number of building floors is negligible.
The influence of the different Italian climates has been evaluated considering three typical localities of Southern, Central and Northern Italy, i.e. Palermo, Rome and Milan. As expected, the energy saving potential related to the shading system is greater in climates characterized by warmer summer, because the cooling energy requirements are higher compared to the other considered climates. With reference to Fig. 5, 1.0 m overhangs allow 20% reduction of total energy demand for Palermo, which has the
3.3. Influence of the building height
Fig. 7. Influence of the building external wall thermal insulation on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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3.4. Influence of the building thermal insulation The examined building is now considered for two different conditions of thermal insulation: no thermal insulation (U-values: 1.5 W m2 K1 for flat roof and ground floor, 2.1 W m2 K1 for external wall) and high thermal insulation (U-values: 0.15 W m2 K1 for flat roof and ground floor, 0.25 W m2 K1 for external wall). For each insulation level, the effects of the shading systems on building energy demand are investigated, for both Palermo and Milan climates (Fig. 7). For Palermo climate (Fig. 7A), the considered shading system allows a 14% energy saving for the uninsulated building and a 24% saving for the well insulated building. It can be also noted that the insulated building envelope leads to a relevant increase in cooling energy demand compared to the uninsulated building (69% with no shading system, 59% with shading system). The cooling energy demand represents the highest energy requirement for this warm climate, leading to a relevant increase in global energy demand (39% and 29%).
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For colder climate of Milan, the global energy savings are 3% for the uninsulated building and 16% for the well insulated building (Fig. 7B). It can be noted that for Milan, in the case of uninsulated building, the energy saving related to the use of solar shadings is negligible, because the cold winter climate leads to heating energy requirements significantly higher than the summer cooling ones. This result is coherent with those of other previous research papers [19]. So, the level of thermal insulation of the building envelope plays a significant role on the total energy requirements, both in warm and cold climates. In the case of cold climate and very low level of building insulation, the solar shadings are not useful. 3.5. Influence of the external wall thermal mass The results reported in Fig. 8 for Palermo and Milan show that the influence of the external wall thermal mass on the energy saving related to the use of solar shadings is negligible (external wall internal heat capacity: 134.8 kJ m2 K1 for high thermal mass,
Fig. 8. Influence of the building thermal mass on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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Fig. 9. Influence of the building window to wall ratio (WWR) on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
12.8 kJ m2 K1 for low thermal mass). In fact, for Palermo (Milan) this energy saving is about 20% (8%) independently of the level of the external wall thermal mass (see Fig. 4 versus Fig. 8). In case of high thermal mass, a future investigation considering also suitable night ventilation (not present in this case study) could be useful.
3.6. Influence of the window to wall ratio In the previous sections, a typical window to wall ratio (WWR ¼ 30%) has been considered for the examined building [18,32] (WWR represents, for each facade, the ratio between the
Fig. 10. Schemes of the lighting control system: two steps (A), linear with off (B).
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window area and the total area of the wall). In this section, the influence of WWR of the building on the energy saving related to the use of solar shadings has been investigated for Palermo and Milan (Fig. 9). The following main considerations can be drawn: In case of WWR ¼ 60%, higher percentage energy savings can be obtained by using solar shadings compared to the case of WWR ¼ 30% (for Palermo these saving rises from 20% to 28%, for Milan from 8% to 15%); In case of WWR ¼ 60%, energy saving slightly increases when using 1.5 m overhangs instead of 1.0 m overhangs, because the height of the windows of the new building (WWR ¼ 60%) is major compared to the reference building (WWR ¼ 30%) and, thus, overhangs with higher depth work slightly better; As expected, passing from WWR ¼ 30% to WWR ¼ 60%, the global energy requirements increase, above all for Palermo (25% adopting shading devices, 38% without shading devices); Both for Palermo and Milan, the global energy requirements of the building with WWR ¼ 30% without shadings are about equal to those of the building with WWR ¼ 60% and the shadings. This result not only confirms that highly glazed building require more energy, but also shows that the use of suitable shading devices can eliminate or significantly reduce
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this increase; this conclusion is coherent with that obtained by other researchers [32]. 3.7. Influence of the lighting control system In this section, the influence of the lighting control system on the energy saving related to the use of solar shadings has been investigated for Palermo and Milan. The following types of lighting control system have been considered: one step, two steps, linear with off (the schemes of the second and the third type are shown in Fig. 10). The results reported in Fig. 11 show that the energy saving related to the use of solar shadings is almost independent of the type of lighting control system: in fact, these savings are about 20% for Palermo and 8% for Milan, independently of the selected lighting control system. Besides, in Fig. 11 it can be seen that the linear control system leads to significant energy savings for lighting compared to the case of one step control system (37e42%), even if the savings are minor when the overall energy demand is considered (8e9%). It is also noticeable that, passing from a building with one step lighting control system and no shadings (the worst case) to the same building with linear lighting control system and 1.0 m overhangs (the best solution), significant global energy savings are obtained (26% and 15% for Palermo and Milan).
Fig. 11. Influence of the lighting control system on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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Fig. 12. Influence of the building orientation on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo and Milan.
3.8. Influence of the orientation of the building The same building has been analyzed, but with the long sides oriented toward east and west (instead of south and north). In this case, instead of overhangs, the shading system more suitable for the long sides is represented by a louver unit with 10 blades, height equal to 0.15 m and inclination of 25 (this configuration has been chosen on the basis of a carried-out energy optimization). On the contrary, the short side oriented toward south is better shaded by means of overhangs instead of louvers; no shading device has been considered for north side. In Fig. 12 the results relative to this building are reported for Palermo and Milan. The following main considerations can be drawn: - The use of shadings for Milan is even negative e the global energy requirements increase of 9%, due above all to the significant rising in the lighting energy demand (133%); - For Palermo only little global energy savings (5%) are obtained e in fact, the reduction of the energy requirement for cooling is significant (31%), but also the increase in lighting energy demand is relevant (189%); - As expected, the energy behavior of this building is worse compared to the reference typical building with long sides oriented toward south and north (for example, as regards the case of the building without shading, a 10% increase in global energy requirements has been obtained for Palermo, 6% for Milan). 4. Conclusions This paper analyses the influence of solar shading devices on the energy requirements of a typical standalone office building, for three Italian climates. Energy performances achievable by applying external solar shading to an air-conditioned building have been evaluated for both winter and summer, by means of a suitable building energy simulation code, EnergyPlus. The energy requirements for heating, cooling and lighting have been analyzed, as a function of the main boundary conditions. The aim of this paper is to provide simplified criteria for engineers and architects in order to: a) establish if external solar
shading devices for standalone office buildings are suitable or not, in terms of energy requirements, for a given Italian climate (but this evaluation is applicable also to other similar climates); b) in the case of energetic suitability, quantify obtainable energy saving and optimize the geometry and positioning of the shading devices, as a function of the main parameters. The analysis has been carried out considering above all the end user electric energy. The following principal conclusions can be highlighted (with reference to the use of suitable overhangs on south-facing facades and louvers on the east and west facing facades, unless otherwise specified): - The energy savings are higher when choosing overhangs with depth of 1.0 m e but this depth is strictly correlated to the height of the windows; - In the optimal case (overhangs with depth of 1.0 m), the global energy saving is equal to about 20% for Palermo (the hottest climate), 15% for Rome (intermediate climate) and 8% for Milan (the coldest one), and this confirms that shadings are preferable in warm climates; - For Palermo, the considered shading system allows a 14% energy saving for an uninsulated building and a 24% saving for a highly insulated building. For colder climate of Milan, these saving values are 3% and 16%. This shows that for cold winter climates, in the case of uninsulated building, the energy saving related to the use of solar shadings is negligible, because the heating energy requirements are significantly higher than the summer cooling ones (on which solar shadings have a positive influence); - The window to wall ratio WWR of the building influences significantly the energy saving related to the use of solar shadings (for example, in the case of WWR ¼ 60%, compared to the case of WWR ¼ 30%, the global energy requirements increase, above all for Palermo, and higher percentage energy savings can be obtained by using solar shading e for Palermo this saving rises from 20% to 28%, for Milan from 8% to 15%); moreover, the global energy requirements of the building with WWR ¼ 30% without shadings are about equal to those of the building with WWR ¼ 60% and the shadings. This result shows that the use of suitable shading devices can eliminate or significantly reduce the increase of energy demand typical of
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highly glazed building; this conclusion is coherent with that obtained by other researchers [32]; - The global energy saving connected with the use of shadings is almost independent of the type of lighting control system; - Negligible influence on the global energy saving connected with the use of shadings is related to the height of the examined building and the external wall thermal mass; - When the long sides of the building are oriented toward east and west instead of north and south, the energy behavior of the building worsens (for example, as regards the case of the building without shading, a 10% increase in global energy requirements has been obtained for Palermo, 6% for Milan), and the use of shadings for Milan is even negative (the global energy requirements increase of 9%), while for Palermo only little global energy savings (5%) are obtained. Finally, these further general conclusions can be derived: - The long sides of the building have to be oriented toward north and south, if possible; - The depth of the overhangs (on the south side) depends also on the window height (for example, the depth of 1 m is suitable for window height of 1.5 m, but the optimal depth should be increased for higher windows). A useful further research will be focused on the evaluation of a suitable relationship between climatic characteristics and energy convenience in using solar shading devices on buildings. Nomenclature T U
temperature, C unitary thermal transmittance, W m2 K1
Acronyms MI referred to Milan, e PA referred to Palermo, e RM referred to Rome, e SCOP mean winter seasonal energy efficiency ratio, e SEER mean summer seasonal energy efficiency ratio, e WWR window to wall ratio, e References [1] International Energy Agency, Worldwide Trends in Energy Use and Efficiency, Authoring Institution: International Energy Agency (IEA), Head of Communication and Information Office, Paris, France, 2008 (last access: December 2012), http://www.iea.org/publications/freepublications/publication/Indicators_20081.pdf. [2] M.J. Suarez, C. Sanjuan, A.J. Gutierrez, J. Pistono, E. Blanco, Energy evaluation of an horizontal open joint ventilated facade, Appl. Therm. Eng. 37 (2012) 302e313. [3] A. Capozzoli, P. Mazzei, F. Minichiello, D. Palma, Hybrid HVAC systems with chemical dehumidification for supermarket applications, Appl. Therm. Eng. 26 (2006) 795e805. [4] F. Ascione, L. Bellia, A. Capozzoli, F. Minichiello, Energy saving strategies in airconditioning for museums, Appl. Therm. Eng. 29 (2009) 676e686.
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[5] European Parliament, EPBD e Energy Performance of Buildings Directive. Directive 2002/91/EC of the European Parliament and of the Council of the European Union, European Parliament, 2002. [6] European Parliament, EPBD Recast e Energy Performance of Buildings Directive. Directive 2010/31/EU of the European Parliament and of the Council of the European Union, European Parliament, 2010. [7] G.H. dos Santos, N. Mendes, Numerical analysis of passive cooling using a porous sandy roof, Appl. Therm. Eng. 51 (2013) 25e31. [8] M. Kayfeci, A. Kecebas, E. Gedik, Determination of optimum insulation thickness of external walls with two different methods in cooling applications, Appl. Therm. Eng. 50 (2013) 217e224. [9] A. Synnefa, M. Santamouris, I. Livada, A study of the thermal performance of reflective coatings for the urban environment, Sol. Energy 80 (2006) 968e981. [10] F. Ascione, L. Bellia, P. Mazzei, F. Minichiello, Solar gain and building envelope: the surface factor, Build. Res. Inform. 38 (2) (2010) 187e205. [11] T. Inoue, Solar shading and daylighting by means of autonomous responsive dimming glass: practical application, Energy Build. 35 (2003) 463e471. [12] P.A.B. James, A.S. Bahaj, Smart glazing solutions to glare and solar gain: a ‘sick building’ case study, Energy Build. 37 (2005) 1058e1067. [13] B.A. Lomanowski, J.L. Wright, Modeling fenestration with shading devices in building energy simulation: a practical approach, in: Proceedings of Building Simulation 2009, Glasgow, Scotland, July 27e30, 2009, pp. 976e983. [14] E. Gratia, A. De Herde, The most efficient position of shading devices in a double-skin façade, Energy Build. 39 (2007) 364e373. [15] G. Kim, H.S. Lima, T.S. Limb, L. Schaeferc, J.T. Kim, Comparative advantage of an exterior shading device in thermal performance for residential buildings, Energy Build. 46 (2012) 105e111. [16] ASHRAE, 2009 ASHRAE Handbook e Fundamentals, Chapter 15 (Fenestration), ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers), Atlanta, 2009. [17] A.I. Palmero-Marrero, A.C. Oliveira, Effect of louver shading devices on building energy requirements, Appl. Energy 87 (2010) 2040e2049. [18] A. Tzempelikos, A.K. Athienitis, The impact of shading design and control on building cooling and lighting demand, Sol. Energy 81 (2007) 369e382. [19] G. Datta, Effect of fixed horizontal louver shading devices on thermal performance of building by TRNSYS simulation, Renew. Energy 23 (2001) 497e507. [20] E.S. Lee, A. Tavil, Energy and visual comfort performance of electrochromic windows with overhangs, Build. Environ. 42 (2007) 2439e2449. [21] T. Inoue, M. Ichinose, N. Ichikawa, Thermotropic glass with active dimming control for solar shading and daylighting, Energy Build. 40 (2008) 385e393. [22] G.A. Florides, S.A. Kalogirou, S.A. Tassau, L.C. Wrobel, Modeling of the modern houses of Cyprus and energy consumption analysis, Energy 25 (2000) 915e937. [23] M. David, M. Donn, F. Garde, A. Lenoir, Assessment of the thermal and visual efficiency of solar shades, Build. Environ. 46 (2011) 1489e1496. [24] A.M. Abu-Zour, S.B. Riffat, M. Gillott, New design of solar collector integrated into solar louvres for efficient heat transfer, Appl. Therm. Eng. 26 (2006) 1876e1882. [25] D.B. Crawley, L.K. Lawrie, C.O. Pedersen, R.J. Liesen, D.E. Fisher, R.K. Strand, R.D. Taylor, R.C. Winkelmann, W.F. Buhl, Y.J. Huang, A.E. Erdem, ENERGYPLUS, a new-generation building energy simulation program, in: Proceedings of Building Simulation ’99, vol. 1, 1999, pp. 81e88. [26] P.G. Loutzenhiser, H. Manz, S. Carl, H. Simmler, G.M. Maxwell, Empirical validation of solar gain models for a glazing unit with exterior and interior blind assemblies, Energy Build. 40 (2008) 330e340. [27] F.R. Mazarrón, J. Cid-Falceto, I. Cañas, An assessment of using ground thermal inertia as passive thermal technique in the wine industry around the world, Appl. Therm. Eng. 33e34 (2012) 54e61. [28] M.R. Hall, K.B. Najim, C.J. Hopfe, Transient thermal behaviour of crumb rubber-modified concrete and implications for thermal response and energy efficiency in buildings, Appl. Therm. Eng. 33e34 (2012) 77e85. [29] Last access: December 2012. Authoring institution: USA Department of energy (DOE), apps1.eere.energy.gov/buildings/energyplus/weatherdata_sources.cfm. [30] A.I. Palmero-Marrero, A.C. Oliveira, Evaluation of a solar thermal system using building louvre shading devices, Sol. Energy 80 (5) (2006) 545e554. [31] G.P. Hammon, C.I. Jones, Embodied energy and carbon in construction materials, Proc. Inst. Civ. Eng. Energy 161 (2) (2008) 87e98. [32] H. Poirazis, A. Blomsterberg, M. Wall, Energy simulation for glazed office buildings in Sweden, Energy Build. 40 (2008) 1161e1170.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Effects of solar shading devices on energy requirements of standalone office buildings for Italian climates Laura Bellia, Francesco De Falco, Francesco Minichiello* DII, University of Naples Federico II, Naples, Italy
h i g h l i g h t s < Solar shading devices on a building reduce annual energy requests of the systems. < The energy saving has been evaluated for an office building in Italian climates. < These savings have been evaluated considering heating, cooling and lighting systems. < In warm summer climates (Palermo), the highest saving has been obtained (about 20%). < Building and shading device characteristics influence the energy savings.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 October 2012 Accepted 19 January 2013 Available online 9 February 2013
In Europe, the building energy demand is about 40% of the total energy requirement. In order to obtain significant energy saving in this sector, the European Energy Performance Building Directive (EPBD) 2002/91/CE and the EPBD Recast (Directive 2010/31/UE) promote the use of passive strategies for buildings, which improve indoor thermal conditions above all in summer and so allow the reduction of size and energy requirements of air conditioning systems. This paper analyzes the influence of external solar shading devices on the energy requirements of a typical air-conditioned office building for Italian climates. A type of office building widespread in Europe has been considered. The energy saving related to the solar shading refers only to summer air conditioning, but the evaluation has been carried out for the entire year, by using a building energy simulation code. The energy demand of the main technical systems (heating, cooling and lighting) and the energy saving related to the use of solar shading devices have been evaluated, as a function of the most significant parameters, such as the climate, the geometrical characteristics of the shadings and the building, the thermal transmittance of the building envelope and the building orientation. The solar shading devices have shown the highest energy efficiency for warm summer climates: for example, the global annual energy saving related to the use of suitable shading devices has been evaluated between 8% for Milan (the coldest climate) and 20% (for Palermo, the warmest one). Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Building Office Solar shading Louver Overhang Energy saving Dynamic simulation Italian climates
1. Introduction With reference to the total final energy requirement, 29% depends on the household sector [1,2]. In particular, as regards Europe, the building energy demand is about 40% of the global energy requirement. In order to obtain significant energy saving in this sector, several strategies have been proposed related to air conditioning systems [3,4]. On the other hand, the European Energy
* Corresponding author. DII, University of Naples Federico II, Napoli, P. le Tecchio 80, 80125 Italy. Tel.: þ39 081 2538665; fax: þ39 081 2390364. E-mail addresses: [email protected], [email protected] (F. Minichiello). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.01.039
Performance Building Directive (EPBD) 2002/91/CE [5] and the EPBD Recast (Directive 2010/31/UE) [6] promote the use of passive strategies for buildings, which improve indoor thermal conditions above all in summer and so allow the reduction of size and energy requirements of air conditioning systems [7,8]. This allows also a useful reduction of peaks for summer electric energy demand [9]. Among the various passive strategies proposed for opaque [9,10] and transparent [11,12] building envelope, the use of suitable shading devices for building transparent components is noteworthy. Nowadays, this solution is still more useful, as the “shift toward better insulated building envelopes, reduced air-infiltration rates . is leading to indoor spaces that are more sensitive to solar gain” [13].
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The main function of a shading system is the protection of the building transparent envelope from solar radiation in summer conditions, so preventing overheating by blocking the access of unwanted energy flow into the building. In fact, although several factors contribute to summer heat gain (e.g., electrical equipment, occupants, lights, etc.), in hot climates a significant fraction derives from solar heat gain entering through the fenestration areas. Among the several shading solutions for limiting this thermal load, intercepting the solar radiation before it reaches the glazed area, through the use of external shading devices, is the most effective one [14,15]. It is noteworthy that “fenestration products fully shaded from the outside reduce solar heat gain by as much as 80%” [16]. Palmero-Marrero and Oliveira [17] investigated the effects of louver shading devices applied to a building for different climates (Mexico, Cairo, Lisbon, Madrid, London), assessing their impact on indoor thermal conditions and energy demand compared to a building without shading devices. Tzempelikos and Athienitis [18] evaluated the effects of shading device characteristics, shading control and glazing area on cooling and lighting energy needs for a building located in Montreal; an exterior roller blade has been considered as shading device. Datta [19] analyzed the influence of fixed horizontal louver shading devices on thermal performance of a building for Italian climates, considering a simple 2-zone building with high external wall U-value (1.691 W m2 K1); the study was focused on a louver shading device applied to the south-facing window. Overhang application on electrochromic windows and the related energy saving for commercial buildings in USA are analyzed by Lee and Tavil [20], while in Ref. [21] thermotropic glass with active dimming control for solar shading is investigated. Florides et al. [22] show that the cooling load reduction related to the use of window shading is about 8e9% for modern houses in Cipro. David et al. [23] analyze the thermal effects and the visual efficiency of solar shades, proposing simple indices to consider both these aspects. In Ref. [15], an experimental configuration of external shading devices applied to apartment houses in South Korea is presented; daylight aspects and energy savings for heating and cooling are evaluated, by using also an energy simulation program. Abu-Zour et al. [24] propose a new design of solar collector integrated into solar louvers. Anyway, few research investigations have been carried out to evaluate the energy performances of solar shading devices applied to buildings for Italian climates [19]; moreover, in the energy analysis on building shading devices, the energy request of the lighting system has been rarely considered [18], as well as a complete building instead of a single-zone building. Hence, in this paper, an extended analysis on energy saving related to external solar shading devices for Italian climates is presented. The investigation has been carried out by using EnergyPlus, an accurate building energy simulation program [25]; EnergyPlus is an energy and thermal load simulation program which can help the user to size appropriate HVAC equipment, improve energy performance of buildings and related systems, develop life cycling cost analyses, etc. It is noteworthy that shading models used by EnergyPlus have been validated by means of experimental investigation [13,26]. The approach based on building performance simulation has been conveniently used in several research works related to the building energy efficiency [27,28]. Three different Italian climates (cities of Palermo, Rome and Milan) and a typical air-conditioned office building have been analyzed for both winter and summer. The above mentioned localities have been chosen as representative of typical Italian
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climates: Palermo is the hottest among them, Milan the coldest, while Rome is characterized by an intermediate climate. The energy requirements of the main technical systems (heating, cooling and lighting) have been evaluated. The influence of various parameters on the energy saving related to the use of suitable solar shadings (overhangs on the south facade and louvers on the eastewest facades) has been analyzed. The most important examined parameters are: geometrical characteristics of shadings and building, thermal transmittance of the building envelope, type of lighting control system and building orientation. The aim of this paper is to provide simplified criteria for engineers and architects in order to: a) establish if the use of external solar shading devices for standalone office buildings is suitable or not, in terms of energy requirements, for various Italian climates (but this evaluation is applicable also to other similar climates); b) in the case of energetic suitability, quantify obtainable energy saving and optimize the geometry and positioning of the shading devices, as a function of various parameters. The analysis has been carried out considering above all the end user electric energy (neither the primary energy, nor the CO2 equivalent emissions); only a simplified evaluation of the full life cycle energy consumption related to the production of shading devices has been also reported. 2. Case study In Fig. 1, the modeled office building is shown, while the main characteristics of the complex building-systems and boundary design conditions are reported in Table 1. The air conditioning
Fig. 1. The simulated office building: volumetric scheme and plan of the typical floor.
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system of the building is characterized by fan-coils connected to an electric water chiller/heat pump; the values of global heating COP and cooling EER are reported in Table 1. The choice of the optimal shading device for a given window depends above all on the orientation of the facade and on the apparent path of the sun. Therefore, in Fig. 2 the modeled solar shadings applied to the examined building are shown, i.e. overhangs on the south facade and louvers on the eastewest facades, coherently with the practice and the indications from scientific literature [16,30]. The main characteristics of the selected shading devices are reported in Table 2. As regards the louver configuration and, in particular, the slat angle of 25 (Fig. 2 and Table 2), they have been chosen on the basis of a carried-out energy optimization, as reported in Section 3.8. It can be also noted that this paper refers to Mediterranean climates, so the optimal energy solutions are different from those of Central-Northern Europe, whose cold climates usually require high solar gains in winter and, thus, not fixed shading devices for buildings (or no shading). In Fig. 3, typical applications of external solar shading devices in Italy are shown. 3. Results Several parameters have been varied for the case study, in order to better analyze the effects of solar shading devices on the building energy requirements, as regards heating, cooling and lighting systems; unless otherwise specified, the values of the most important parameters are those reported in Tables 1 and 2. It can be noted that because the short sides of the examined building are eastewest oriented (except for the Section 3.8), their fenestration area is little compared to the long sides, therefore the influence of the louvers on the energy requirements is much minor compared to the overhangs (on long south side). Moreover, it can be noted that the use of the solar shadings leads to energy saving only for the cooling system, while it leads to an increase in energy requirements for both heating and lighting systems (because less solar radiation, useful for daylighting and Fig. 2. The considered solar shading devices: louvers and overhangs. Table 1 Main characteristics of the analyzed building, systems and climates. Width (NeS direction) Height Surface to volume ratio Window to wall ratio (WWR) for each facade Window area TSUMMER-SET-POINT UGLAZING UFRAME UROOF Occupancy level Illuminance level on work plane Metabolic rate Cooling SEER (fan-coils þ water chiller)
12.8 m 10.5 m 0.28 m1 30% (unless otherwise specified) South exposure: 100 m2 North exposure: 100 m2 26 C 2.7 W m2 K1 3.6 W m2 K1 1.0 W m2 K1 0.111 persons/m2
Length Plan area and volume Infiltration airflow rate Window height East exposure: 23 m2
32.3 m 413.4m2e4341.1 m3 0.5 ACH 1.5 m West exposure: 16 m2
0.70 20 C 0.58 W m2 K1 0.90 W m2 K1 From 9:00 a.m. to 18 p.m., 5 days/week Office: 400 lx atrium and corridors: Thermal load related to the office 20 W/m2 100 lx toilets: 200 lx electrical equipment 0.9 met/person Clothing thermal resistance 1 clo for winter, 0.5 clo for summer 2.8 (Milan) Heating SCOP (fan-coils þ 2.8 (Milan) 2.7 (Rome) heat pump) 3.0 (Rome) 2.6 (Palermo) 3.2 (Palermo)
Note: HVAC systems only operate with building occupancy Weather data from Italian Climatic data collection “Gianni De Giorgio” [29] Location Milan (Linate) Latitude 45.43 , Longitude 9.28 Cooling degree-days (base 10 C) 1771 2454 Heating degree-days (base 18 C) Maximum monthly global horizontal radiation (average daily total) 5539 Wh m2 (July)
Glass g-value TWINTER-SET-POINT Uwall UBASEMENT FLOOR Occupancy scheduling
Rome (Fiumicino) Latitude 41.80 , Palermo (Punta Raisi) Longitude 12.23 Latitude 38.18 , Longitude 13.10 2174 3082 1514 744 6121 Wh m2 (July) 6714 Wh m2 (July)
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Table 2 Characterization of selected shading devices. Overhang Orientation
Louver South
Projection 1.0 m (unless otherwise specified) Vertical offset from top of window 0.0 m Horizontal window overlap 0.0 m Note: the geometrical parameters reported in this table are also shown in Fig. 2.
Orientation
East, west
Blades of the louver Vertical offset from top of window Horizontal window overlap Blade depth Angle Vertical spacing Distance from window
4 (unless otherwise specified) 0.0 m 0.0 m 0.7 m 25 0.3 m 0.3 m
Material of the selected shading devices (for both overhangs and louvers): aluminium. Thermal conductivity ¼ 221.0 W/mK; specific heat ¼ 896 J/kg K; density ¼ 2740 kg/m3; solar reflectance ¼ 0.61; infrared emittance: 0.25.
winter heating, enters the building when solar shadings are adopted). Therefore, the best energy solution for the building minimizes the overall annual energy demand. A preliminary analysis has been carried out showing that in winter, for Italian climates, the thermal energy picking-up is suitable for the analyzed office building. For instance, with reference to the hottest locality (Palermo), we have verified that in the period from 1 December to 31 March (identified as conventional winter period by the Italian rules for the climatic zone of Palermo) cooling is required only in 117 h, i.e. about 4% of the total hours (2904 h). 3.1. Influence of the overhang depth Energy requirements of the examined building were calculated, adopting the overhang typologies reported in Table 2, for Palermo, Rome and Milan climates. Moreover, the energy demands for each type of overhang and climate were compared to those related to the reference building with no shading devices. The overhang depth has been fixed to 0.5 m, 1.0 m and 1.5 m, a variation similar to that reported in Ref. [30]. The results are reported in Fig. 4.
For Palermo climate (Fig. 4A), the best energy performances have been obtained for overhangs with depth of 1.0 m e savings of about 20% in terms of total electric energy for heating, cooling and lighting (as regards heating and cooling, the electric energy is that required by all the equipment of the HVAC system, i.e. fan-coils, electric chiller/heat pump, pumps, etc.). The saving drops to 16% when adopting a 0.5 m or 1.5 m overhang. Similar results have been obtained (Fig. 3B and C) for Rome (savings between 9% and 15%) and Milan (1e8%). For all the climates investigated, 1.0 m overhangs represent the best solution in terms of energy demand reduction for the examined building, but it should be noted that the optimal overhang depth depends on the window height (1.5 m in the reference case). The influence of the geometrical characteristics of the shadings (louvers) on the eastewest facades is examined in a successive section. Even though this paper refers to the end user electric energy, also a simplified analysis has been carried out on the full life cycle energy consumption in the production of shading devices. The “cradle-to-gate” approach (instead of the “cradle-to-site” one) has
Fig. 3. Typical applications of external solar shading devices.
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Fig. 4. Influence of the overhang depth on the annual energy requests for Palermo (A), Rome (B) and Milan (C).
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Fig. 5. Influence of the climatic conditions on the annual energy requests for shaded building and no-shaded one.
been used, so neglecting the energy related to transport from factory gate to construction site [31]. With reference to the case of 1.0 m overhangs, the global mass of the overhangs and louvers has been evaluated (about 470 kg, considering extruded aluminium as
material), and the embodied energy value of 200 MJ/kg has been used for the aluminium production. The results show that the primary energy saved by using the solar shading devices equalizes the embodied energy by 3 years for the most irradiated locality
Fig. 6. Influence of the building height on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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(Palermo) and by 5 years for the less irradiated one (Milan). The actual value (0.46) of the thermal to electrical conversion efficiency for Italy has been considered. Obviously, if extruded recycled aluminium is used, much minor embodied energy value has to be considered (about 17 MJ/kg), and therefore the above cited time period is significantly minor. Finally, it is noteworthy taking into account that aluminium is a recyclable material. In the following analyses, only the end user electric energy saving has been considered.
warmest climate in summer, 15% reduction for Rome (intermediate climate), and 8% for Milan (the coldest of the considered climates). It can be noted that only for Milan, the chiller (cooling) energy requirements are similar or lower than the heating ones. It is also noticeable that, for the considered climates, the percentage energy savings related only to the cooling system are between 26% and 29%.
3.2. Influence of the climate
In Fig. 6, results relative to the same building, but with 11 instead of 3 floors, are shown, in order to verify if the percentage energy saving connected to the solar shadings depends also on the height of the building. This possible dependence is negligible. In fact, comparing the results for Palermo (Fig. 6A), the energy saving values are 17%, 22% and 17% (for 0.5 m, 1.0 and 1.5 m overhangs), very similar to 16%, 20% and 16% obtained for the building of 3 floors (Fig. 4A). Also for Milan (7%, 8% and 1% versus 6%, 8% and 1%, i.e. Fig. 6B versus Fig. 4B), the dependence of the percentage energy savings on the number of building floors is negligible.
The influence of the different Italian climates has been evaluated considering three typical localities of Southern, Central and Northern Italy, i.e. Palermo, Rome and Milan. As expected, the energy saving potential related to the shading system is greater in climates characterized by warmer summer, because the cooling energy requirements are higher compared to the other considered climates. With reference to Fig. 5, 1.0 m overhangs allow 20% reduction of total energy demand for Palermo, which has the
3.3. Influence of the building height
Fig. 7. Influence of the building external wall thermal insulation on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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3.4. Influence of the building thermal insulation The examined building is now considered for two different conditions of thermal insulation: no thermal insulation (U-values: 1.5 W m2 K1 for flat roof and ground floor, 2.1 W m2 K1 for external wall) and high thermal insulation (U-values: 0.15 W m2 K1 for flat roof and ground floor, 0.25 W m2 K1 for external wall). For each insulation level, the effects of the shading systems on building energy demand are investigated, for both Palermo and Milan climates (Fig. 7). For Palermo climate (Fig. 7A), the considered shading system allows a 14% energy saving for the uninsulated building and a 24% saving for the well insulated building. It can be also noted that the insulated building envelope leads to a relevant increase in cooling energy demand compared to the uninsulated building (69% with no shading system, 59% with shading system). The cooling energy demand represents the highest energy requirement for this warm climate, leading to a relevant increase in global energy demand (39% and 29%).
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For colder climate of Milan, the global energy savings are 3% for the uninsulated building and 16% for the well insulated building (Fig. 7B). It can be noted that for Milan, in the case of uninsulated building, the energy saving related to the use of solar shadings is negligible, because the cold winter climate leads to heating energy requirements significantly higher than the summer cooling ones. This result is coherent with those of other previous research papers [19]. So, the level of thermal insulation of the building envelope plays a significant role on the total energy requirements, both in warm and cold climates. In the case of cold climate and very low level of building insulation, the solar shadings are not useful. 3.5. Influence of the external wall thermal mass The results reported in Fig. 8 for Palermo and Milan show that the influence of the external wall thermal mass on the energy saving related to the use of solar shadings is negligible (external wall internal heat capacity: 134.8 kJ m2 K1 for high thermal mass,
Fig. 8. Influence of the building thermal mass on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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Fig. 9. Influence of the building window to wall ratio (WWR) on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
12.8 kJ m2 K1 for low thermal mass). In fact, for Palermo (Milan) this energy saving is about 20% (8%) independently of the level of the external wall thermal mass (see Fig. 4 versus Fig. 8). In case of high thermal mass, a future investigation considering also suitable night ventilation (not present in this case study) could be useful.
3.6. Influence of the window to wall ratio In the previous sections, a typical window to wall ratio (WWR ¼ 30%) has been considered for the examined building [18,32] (WWR represents, for each facade, the ratio between the
Fig. 10. Schemes of the lighting control system: two steps (A), linear with off (B).
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window area and the total area of the wall). In this section, the influence of WWR of the building on the energy saving related to the use of solar shadings has been investigated for Palermo and Milan (Fig. 9). The following main considerations can be drawn: In case of WWR ¼ 60%, higher percentage energy savings can be obtained by using solar shadings compared to the case of WWR ¼ 30% (for Palermo these saving rises from 20% to 28%, for Milan from 8% to 15%); In case of WWR ¼ 60%, energy saving slightly increases when using 1.5 m overhangs instead of 1.0 m overhangs, because the height of the windows of the new building (WWR ¼ 60%) is major compared to the reference building (WWR ¼ 30%) and, thus, overhangs with higher depth work slightly better; As expected, passing from WWR ¼ 30% to WWR ¼ 60%, the global energy requirements increase, above all for Palermo (25% adopting shading devices, 38% without shading devices); Both for Palermo and Milan, the global energy requirements of the building with WWR ¼ 30% without shadings are about equal to those of the building with WWR ¼ 60% and the shadings. This result not only confirms that highly glazed building require more energy, but also shows that the use of suitable shading devices can eliminate or significantly reduce
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this increase; this conclusion is coherent with that obtained by other researchers [32]. 3.7. Influence of the lighting control system In this section, the influence of the lighting control system on the energy saving related to the use of solar shadings has been investigated for Palermo and Milan. The following types of lighting control system have been considered: one step, two steps, linear with off (the schemes of the second and the third type are shown in Fig. 10). The results reported in Fig. 11 show that the energy saving related to the use of solar shadings is almost independent of the type of lighting control system: in fact, these savings are about 20% for Palermo and 8% for Milan, independently of the selected lighting control system. Besides, in Fig. 11 it can be seen that the linear control system leads to significant energy savings for lighting compared to the case of one step control system (37e42%), even if the savings are minor when the overall energy demand is considered (8e9%). It is also noticeable that, passing from a building with one step lighting control system and no shadings (the worst case) to the same building with linear lighting control system and 1.0 m overhangs (the best solution), significant global energy savings are obtained (26% and 15% for Palermo and Milan).
Fig. 11. Influence of the lighting control system on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo (A) and Milan (B).
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Fig. 12. Influence of the building orientation on the annual energy requests in the cases of shaded building and no-shaded one, for Palermo and Milan.
3.8. Influence of the orientation of the building The same building has been analyzed, but with the long sides oriented toward east and west (instead of south and north). In this case, instead of overhangs, the shading system more suitable for the long sides is represented by a louver unit with 10 blades, height equal to 0.15 m and inclination of 25 (this configuration has been chosen on the basis of a carried-out energy optimization). On the contrary, the short side oriented toward south is better shaded by means of overhangs instead of louvers; no shading device has been considered for north side. In Fig. 12 the results relative to this building are reported for Palermo and Milan. The following main considerations can be drawn: - The use of shadings for Milan is even negative e the global energy requirements increase of 9%, due above all to the significant rising in the lighting energy demand (133%); - For Palermo only little global energy savings (5%) are obtained e in fact, the reduction of the energy requirement for cooling is significant (31%), but also the increase in lighting energy demand is relevant (189%); - As expected, the energy behavior of this building is worse compared to the reference typical building with long sides oriented toward south and north (for example, as regards the case of the building without shading, a 10% increase in global energy requirements has been obtained for Palermo, 6% for Milan). 4. Conclusions This paper analyses the influence of solar shading devices on the energy requirements of a typical standalone office building, for three Italian climates. Energy performances achievable by applying external solar shading to an air-conditioned building have been evaluated for both winter and summer, by means of a suitable building energy simulation code, EnergyPlus. The energy requirements for heating, cooling and lighting have been analyzed, as a function of the main boundary conditions. The aim of this paper is to provide simplified criteria for engineers and architects in order to: a) establish if external solar
shading devices for standalone office buildings are suitable or not, in terms of energy requirements, for a given Italian climate (but this evaluation is applicable also to other similar climates); b) in the case of energetic suitability, quantify obtainable energy saving and optimize the geometry and positioning of the shading devices, as a function of the main parameters. The analysis has been carried out considering above all the end user electric energy. The following principal conclusions can be highlighted (with reference to the use of suitable overhangs on south-facing facades and louvers on the east and west facing facades, unless otherwise specified): - The energy savings are higher when choosing overhangs with depth of 1.0 m e but this depth is strictly correlated to the height of the windows; - In the optimal case (overhangs with depth of 1.0 m), the global energy saving is equal to about 20% for Palermo (the hottest climate), 15% for Rome (intermediate climate) and 8% for Milan (the coldest one), and this confirms that shadings are preferable in warm climates; - For Palermo, the considered shading system allows a 14% energy saving for an uninsulated building and a 24% saving for a highly insulated building. For colder climate of Milan, these saving values are 3% and 16%. This shows that for cold winter climates, in the case of uninsulated building, the energy saving related to the use of solar shadings is negligible, because the heating energy requirements are significantly higher than the summer cooling ones (on which solar shadings have a positive influence); - The window to wall ratio WWR of the building influences significantly the energy saving related to the use of solar shadings (for example, in the case of WWR ¼ 60%, compared to the case of WWR ¼ 30%, the global energy requirements increase, above all for Palermo, and higher percentage energy savings can be obtained by using solar shading e for Palermo this saving rises from 20% to 28%, for Milan from 8% to 15%); moreover, the global energy requirements of the building with WWR ¼ 30% without shadings are about equal to those of the building with WWR ¼ 60% and the shadings. This result shows that the use of suitable shading devices can eliminate or significantly reduce the increase of energy demand typical of
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highly glazed building; this conclusion is coherent with that obtained by other researchers [32]; - The global energy saving connected with the use of shadings is almost independent of the type of lighting control system; - Negligible influence on the global energy saving connected with the use of shadings is related to the height of the examined building and the external wall thermal mass; - When the long sides of the building are oriented toward east and west instead of north and south, the energy behavior of the building worsens (for example, as regards the case of the building without shading, a 10% increase in global energy requirements has been obtained for Palermo, 6% for Milan), and the use of shadings for Milan is even negative (the global energy requirements increase of 9%), while for Palermo only little global energy savings (5%) are obtained. Finally, these further general conclusions can be derived: - The long sides of the building have to be oriented toward north and south, if possible; - The depth of the overhangs (on the south side) depends also on the window height (for example, the depth of 1 m is suitable for window height of 1.5 m, but the optimal depth should be increased for higher windows). A useful further research will be focused on the evaluation of a suitable relationship between climatic characteristics and energy convenience in using solar shading devices on buildings. Nomenclature T U
temperature, C unitary thermal transmittance, W m2 K1
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