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Solar gains in the glazing systems with sun-shading
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729 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece Solar gains in the glazing systems with sun-shading G. Oliveti, N. Arcuri, R. Bruno, M. De Simone University of Calabria, Italy distribution. There are fundamentally two methods to determine the value of g. Carrying out an experimental calorimetrical measurement: a portion of unitary surfa- ce is irradiated and the energy transmitted is determined calorimetrically; by calculation methods, these are ba- sed on the knowledge of the optical properties of each layer making up the window system and the shading system (transmission coefficient, absorption and reflec- tion of the beam and diffused solar radiation) (Tilmann E. Kuhn et Al., 2000). The resolution of the optical pro- blem in the system composed of window and shading leads to the determination of the transmission coeffi- cient of the solar radiation τ and of the energy absor- bed from each layer. The resolution of the thermal field successively allows the determination of the energy lost internally owing to convection and infrared irradiation, and therefore definitively of the factor g of the system. In this paper are determined the values that the total solar energy transmittance g assumes in a shading sy- stem made up of parallel slats (Venetian blinds) coupled with a double clear glazing, or a low-emissive glazing. The Venetian blind with differently inclined slats can be positioned outside the glazing, in the interpane of the glazing, or inside the room. Moreover, with reference to an example case the effect of the shading system on the energy demand required to maintain the room at a tem- perature of 20°C in the winter and 26°C in the summer cooling period, was determined. The optical performances of the shading-window sy- stem and the thermal analysis of the room considered were determined using the ParaSol dynamic simulation program developed at the Lund Institute of Technology, Sweden (Parasol v3.0, 2007). The code implemented the calculation procedure of Standard ISO 15099 (ISO/DIS 15099, 1999) to determine the mean monthly values of the solar transmission coefficient τ and of the total solar energy transmittance g. 2. THE CASE EXAMPLE A 4x4x3 m room in a building used as an office was con- sidered. The module presents a single dispersive external wall with a 1.80x1.20 m glazed surface. The thermal tran- smittance of the opaque surface is 0.40 W/m 2 K, whereas for the glazed surface, two types of glazing were considered whose thermal and optical properties are shown in table 1. ABSTRACT This paper presents the optical and thermal performan- ces of different glazed surfaces coupled with a shading system struck by solar radiation. The mean monthly values of the total solar energy transmittance g are de- termined and of the transmission coefficient τ of the window area, the shading system, the compound win- dow shading system, with the shading placed external- ly, inside and in the glazing interpane. The effect of the shading system on the solar contributions to heating and cooling is determined with reference to a case study. 1. INTRODUCTION It is known that buildings constitute one of the sectors with the highest energy consumption. For this reason in archi- tectonic design attempts are being made to utilize the pos- sibilities offered by passive solar systems, which if well- sized and regulated assure important solar contributions in the winter and reduced ones in the summer, with con- sequent reductions of energy demand for air-conditioning. Among the passive systems, glazed surfaces allow a di- rect solar gain since radiation enters directly into the rooms and is absorbed by the walls which act as absor- bers and accumulators. The control of incoming radiation usually occurs by means of shading systems. Often their effect on buildings energy performances is not evaluated accurately, owing to the unavailability of data relative to the effective optical behaviour, when they are coupled to different types of window systems on the market. Similarly to glazed surfaces, the optical behaviour of the shading system, and of the compound system made up of the window and the shading system, is characte- rized by the corresponding total solar energy transmit- tance g (global solar transmittance), which indicates the incident solar energy fraction transmitted through the considered system. Factor g is made up of two par- ts: that transmitted directly, characterized by the solar transmission coefficient τ, and that which is absorbed and successively lost to the indoor air q i . In the shaded windows, the value of g depends on the type of shading system and on its position (external, interpane, inside the room), on the type of glass, the wind velocity, the ventilation in the gap between shading and glass, the di- rection of the incident solar radiation and on its spectral PALENC 2007 - Vol 2.indd 729 7/9/2007 1:24:25 μμ
Transcript of Solar gains in the glazing systems with sun-shading
7292nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece
Solar gains in the glazing systems with sun-shading
G. Oliveti, N. Arcuri, R. Bruno, M. De SimoneUniversity of Calabria, Italy
distribution. There are fundamentally two methods to determine the value of g. Carrying out an experimental calorimetrical measurement: a portion of unitary surfa-ce is irradiated and the energy transmitted is determined calorimetrically; by calculation methods, these are ba-sed on the knowledge of the optical properties of each layer making up the window system and the shading system (transmission coefficient, absorption and reflec-tion of the beam and diffused solar radiation) (Tilmann E. Kuhn et Al., 2000). The resolution of the optical pro-blem in the system composed of window and shading leads to the determination of the transmission coeffi-cient of the solar radiation τ and of the energy absor-bed from each layer. The resolution of the thermal field successively allows the determination of the energy lost internally owing to convection and infrared irradiation, and therefore definitively of the factor g of the system. In this paper are determined the values that the total solar energy transmittance g assumes in a shading sy-stem made up of parallel slats (Venetian blinds) coupled with a double clear glazing, or a low-emissive glazing. The Venetian blind with differently inclined slats can be positioned outside the glazing, in the interpane of the glazing, or inside the room. Moreover, with reference to an example case the effect of the shading system on the energy demand required to maintain the room at a tem-perature of 20°C in the winter and 26°C in the summer cooling period, was determined.The optical performances of the shading-window sy-stem and the thermal analysis of the room considered were determined using the ParaSol dynamic simulation program developed at the Lund Institute of Technology, Sweden (Parasol v3.0, 2007). The code implemented the calculation procedure of Standard ISO 15099 (ISO/DIS 15099, 1999) to determine the mean monthly values of the solar transmission coefficient τ and of the total solar energy transmittance g.
2. THE CASE EXAMPLE
A 4x4x3 m room in a building used as an office was con-sidered. The module presents a single dispersive external wall with a 1.80x1.20 m glazed surface. The thermal tran-smittance of the opaque surface is 0.40 W/m2K, whereas for the glazed surface, two types of glazing were considered whose thermal and optical properties are shown in table 1.
ABSTRACT
This paper presents the optical and thermal performan-ces of different glazed surfaces coupled with a shading system struck by solar radiation. The mean monthly values of the total solar energy transmittance g are de-termined and of the transmission coefficient τ of the window area, the shading system, the compound win-dow shading system, with the shading placed external-ly, inside and in the glazing interpane. The effect of the shading system on the solar contributions to heating and cooling is determined with reference to a case study.
1. INTRODUCTION
It is known that buildings constitute one of the sectors with the highest energy consumption. For this reason in archi-tectonic design attempts are being made to utilize the pos-sibilities offered by passive solar systems, which if well-sized and regulated assure important solar contributions in the winter and reduced ones in the summer, with con-sequent reductions of energy demand for air-conditioning.Among the passive systems, glazed surfaces allow a di-rect solar gain since radiation enters directly into the rooms and is absorbed by the walls which act as absor-bers and accumulators. The control of incoming radiation usually occurs by means of shading systems. Often their effect on buildings energy performances is not evaluated accurately, owing to the unavailability of data relative to the effective optical behaviour, when they are coupled to different types of window systems on the market. Similarly to glazed surfaces, the optical behaviour of the shading system, and of the compound system made up of the window and the shading system, is characte-rized by the corresponding total solar energy transmit-tance g (global solar transmittance), which indicates the incident solar energy fraction transmitted through the considered system. Factor g is made up of two par-ts: that transmitted directly, characterized by the solar transmission coefficient τ, and that which is absorbed and successively lost to the indoor air qi . In the shaded windows, the value of g depends on the type of shading system and on its position (external, interpane, inside the room), on the type of glass, the wind velocity, the ventilation in the gap between shading and glass, the di-rection of the incident solar radiation and on its spectral
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Table 1: Thermal and optical properties of glazing.U
W/m2K g τClear glazing 2.88 0.77 0.69Low-emissive glazing 1.62 0.60 0.49
A blue Venetian blind was chosen for the shading sy-stem with 22 mm-wide slats and 28 mm interpane, po-sitioned inside the room or interpane. In this case the air gap of the glazing is 30 mm wide. The 50 mm-wide Venetian blind collocated externally is grey with a 42 mm interpane. The slats were inclined at an angle of θ = 0° (horizontal position), 30°, 60° and 90° (vertical position). The climatic data of the city of Rome were considered with reference to the mean monthly day of solar irradiation and of outdoor air temperature (UNI 10349, 1994). The dispersive surface of the modulo was orientated to the South, East and West. Regarding the use of the room it was supposed that is occupied from 8 o’clock in the morning until 5 o’clock in the afternoon, from Monday to Friday. Moreover, the following sup-positions were made in the energy simulations:- the solar radiation only affects the glazed surface and the shading system is active when the irradiation excee-ds the value of 150 W/m2;- the plant intervenes when the indoor air temperature is less than 20°C in winter heating, and over 26°C in summer cooling;- the ventilation flow is at 2 air changes per hour;- the internal loads are those corresponding to the pre-sence of a person, a PC and a printer. The load owing to the lighting plant is 12.5 W/m2.All the evaluations leave out of consideration the as-pects connected with the use of natural light.
3. THE OPTICAL AND THERMAL PERFORMANCES
The simulations carried out on an hourly basis were used to determine the mean month daily values of the transmission coefficient of the solar radiation τ and of the total solar energy transmittance g of three configura-tions: glazing alone, shading alone, and the compound shading-glazing system. Figure 1 shows the monthly trends of the total solar energy transmittance g for the clear glazing and low-emissive glazing at different exposures. The variability field of g ranges from 60% to 70% for the former and from 47% to 55% for the latter. For window exposed to the South, g assumes the highest values in the winter and has greater monthly variability compared to the East and West orientations, which can be retained equivalent. The transmission coefficient τ proves on average less than g by 8% for clear glazing and less than g by 19% for low-emissive glazing.
Figure 1: Monthly values of the total solar energy transmittance for clear glazing (C) and for low-emissive glazing (L) at varying of orientation.
Figure 2 shows the trend of the total solar energy tran-smittance g and of the transmission coefficient τ for ex-ternal Venetian blinds exposed to the South with diffe-rently inclined slats. The two transmission coefficients have qualitatively the same trend. The highest values are found when the slats are placed horizontally: radia-tion transmission is highest in the winter and is mainly beam radiation, whereas in the summer it is prevalently transmitted by reflection (P. Pfrommer et Al., 1996).
Figure 2: Trends of the transmission coefficient τ and of the total solar energy transmittance g at variation of the angle of incli-nation θ of the external Venetian blind slats. South orientation, clear glazing.
The lower shading capacity is obtained using horizontal slats. The highest, for θ = 90° with τ = 0 and g = qi = 7%. It should be stressed that the convection-radiation contribution qi increases with the angle θ. Fig. 3 shows similar trends of τ and g in the case where the shading system is placed inside the room. The trends highlight the importance of the convection and radiation exchanges consequent on the heating of the shading, for θ = 90° τ =0 and g = 0.8.
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Figure 3: Trends of the transmission coefficient τ and of the total solar energy transmittance g at variation of the angle of inclination θ of the internal Venetian blind slats. South orientation, clear glazing.
If the system formed by the clear glazing and the Vene-tian blind placed externally, in the interpane, or inside the room is considered, the values of the solar gain g for orientation to the South at variation of angle θ, are shown in figure 4.
Figure 4: Trends of the total solar energy transmittance g at varia-tion of the position of the shading and of the angle of inclination θ. South orientation, clear glazing.
The internal Venetian blind gives rise to values of the total solar energy transmittance comparable to those obtained with the non-shaded window area. An effective capacity shading, as is known, is obtained with the external Ve-netian blind. The reduction in the case of θ = 90° is ma-ximum, with values of g that reduce on average by 0.53 to 0.04. Also for θ = 0° the reduction is significant, with g which from 0.62 becomes 0.20. Inserting interpane Venetian blind leads to values of the total solar energy transmittance between 0.28 and 0.46 with a very contai-ned monthly variability. The inclination of the slats has little influence on the transmission phenomenon in the winter months in the case of external Venetian blinds.For exposure to the East the monthly variability of the total solar energy transmittance g is less evident, as fi-gure 5 shows. Also in this case g assumes maximum values when the Venetian blind is placed internally and reduces on average by 40% when it is positioned in the interpane and by 80% when it is external.
Figure 5: Trends of the total solar energy transmittance g at varia-tion of the position of the shading and of the angle of inclination θ. East orientation, clear glazing.
Figure 6 shows, for exposure to the South, the mean monthly values of g obtained using low-emissive gla-zing, at variation of the position of the shading system and of the angle of inclination of the slats. The trends obtained for exposure to the East are shown in figure 7.The use of low-emissive glazing in general leads to a re-duction of the total solar energy transmittance g that de-creases on average by 17% for internal Venetian blinds, by 50% for those placed in the interpane and by 26% for ex-ternal ones. Moreover, it can be noted that the inclination of the slats has a reduced influence on system shading per-formances if the Venetian blind is placed in the interpane.
Figure 6: Trends of the total solar energy transmittance g at varia-tion of the position of the shading and of the angle of inclination θ. South orientation, low-emissive glazing.
Figure 7: Trends of the total solar energy transmittance g at varia-tion of the position of the shading and of the angle of inclination θ. East orientation, low-emissive glazing.
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4. EFFECT OF THE SHADING SYSTEMS ON SO-LAR GAIN
The effects of the shading system on the energy demand of the air-conditioned space in winter heating and sum-mer cooling were evaluated, considering the monthly energy balances. The results obtained are relative to the room considered, having multi-layered walls and an average value of the internal thermal capacity. Under the action of the two external forcing agents, air tempe-rature and solar irradiation, temperature control of the indoor air is obtained by means of the energy contribu-tions of the plants. Two series of simulations were conducted: in the pre-sence of shading systems and in their absence, and the energy supplied to the control space by means of the plants, determined. The code allows the losses to the outdoor air to be maintained. In this way the contribu-tions of the solar radiation are computed indirectly as the difference between the energy supplied to the con-
trol space by means of the plants in the presence and in the absence of shading systems. Table 2 shows, for the clear glazing, the yearly variations of the solar contribu-tions due to the presence of the shading systems in the heating and cooling of the room, at variation of orien-tation, shading position and inclination of the slats. The table points out that the reduction of the solar contribu-tions in cooling is more important with respect to hea-ting, and that gives rise overall to an energy benefit on a yearly basis. These effects appear for all orientations, with a greater benefit for exposure to the East and West and Venetian blinds placed externally. Lesser benefits are obtained by placing the shading in the interpane. In the case of internal positioning they become of little im-portance if compared to the previous collocations. The results obtained with the low-emissive glazing are shown in table 3. The effects produced by the shaded system are attenuated because of the lower solar radia-tion transmission coefficient through the glazing.
Table 2: Yearly variations of the solar contributions owing to the presence of shading systems in heating and cooling at Variation of exposure, placing of the shading system and inclination θ of the slats. Clear glazing.
External Inperpane InternalHeating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ)
Sout
h
0° 529,70 -932,09 238,01 -495,32 62,43 -55,4630° 784,60 -973,91 379,30 -559,29 117,80 -112,5660° 793,23 -1005,34 409,05 -624,85 144,84 -160,4890° 825,02 -1060,25 422,18 -673,30 167,67 -202,87
Eas
t
0° 112,76 -947,01 54,02 -492,53 13,86 -80,6730° 174,47 -1158,16 86,54 -664,65 27,26 -153,3960° 182,25 -1189,75 93,87 -723,82 34,13 -208,4690° 190,61 -1221,82 95,41 -758,21 39,16 -254,36
Wes
t
0° 120,17 -930,79 60,92 -472,43 17,29 -40,3830° 181,14 -1146,17 94,09 -637,24 32,30 -98,0960° 187,57 -1175,54 102,69 -700,07 39,37 -150,1790° 196,78 -1206,01 105,90 -741,06 44,78 -194,30
Table 3: Yearly variations of the solar contributions owing to the presence of shading systems in heating and cooling at variation of exposure, placing of the shading system and inclination θ of the slats. Low-emissive glazing.
External Inperpane InternalHeating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ)
Sout
h
0° 410,70 -749,38 294,14 -648,93 33,65 -8,4130° 609,39 -784,39 436,06 -665,45 63,78 -39,2660° 617,31 -812,39 416,37 -664,66 76,31 -59,3890° 643,12 -859,68 398,27 -657,48 86,67 -77,81
Eas
t
0° 87,17 -757,05 65,48 -636,58 8,07 -29,0230° 132,95 -928,84 96,31 -777,62 15,43 -66,9360° 139,15 -955,95 95,07 -760,15 18,49 -92,9290° 145,84 -983,78 90,50 -735,01 20,80 -114,93
Wes
t
0° 92,37 -749,83 72,54 -639,08 8,79 -3,5430° 137,31 -924,15 102,02 -776,81 17,43 -30,8360° 142,52 -950,56 100,98 -757,22 20,70 -54,2290° 149,81 -978,02 97,56 -728,94 23,11 -74,56
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5. CONCLUSIONS
The optical and thermal properties of window systems composed of clear glazing or of low-emissive glazing coupled to a shading system made up of a Venetian blind, have been quantified. The position of the shading system modifies the value of the total solar energy tran-smittance in an important way when the Venetian blind is placed externally, in a more contained way when pla-ced in the interpane, whereas it proves ineffective when placed internally. The total solar energy transmittance, for clear glazing, undergoes a mean yearly reduction of 85% in the case of an external shading system, and of 46% in the case when the shading is placed in the inter-pane. These percentages become respectively 85% and 65% in the case of low-emissive glazing. The effect of inclination of the slats is important for an-gles ranging from 0° to 30° in the case of Venetian blinds placed externally or in the interpane, and more contained for greater angles. The monthly variability of the total solar energy transmittance depends on the orientation and is more contained for exposure to the East and West.For the purpose of room air-conditioning, the placing of the system shading externally proves more effecti-ve in cooling and less so in heating. These effects are attenuated in the case of shading systems placed in the interpane. Finally, the shading system of the surfaces exposed to the East and West give rise to greater energy benefits both in heating and in cooling.
REFERENCES
ISO/DIS 15099 (1999), Thermal Performance of Windows, Doors and Shading Devices – Detailed Calculation.Parasol v3.0 (2007). User’s Manual, Division of Energy and building Design, Department of Architecture and Build Environ-ment, Lund Institute of technology, Sweden.Pfrommer P., Lomas K. J., Kupke Chr. (1996). Solar radiation tran-sport through slat-type blinds: a new model and its application for thermal simulation of buildings. Solar Energy Vol. 57, n° 2, pp. 77-91.Tilmann E. Kuhn, Buhler C., Werner J. Platzer (2000). Evalua-tion of overheating protection with sun-shading system. Solar Energy Vol. 69, Nos 1-6, pp. 59-74.UNI 10349 (1994). Heating and cooling of buildings. Climatic data.
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