Solar gains in the glazing systems with sun-shading

Transcription

1 729 Solar gains in the glazing systems with sun-shading G. Oliveti, N. Arcuri, R. Bruno, M. De Simone University of Calabria, Italy ABSTRACT This paper presents the optical and thermal performances 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 determined and of the transmission coefficient τ of the window area, the shading system, the compound window shading system, with the shading placed externally, 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 architectonic design attempts are being made to utilize the possibilities offered by passive solar systems, which if wellsized and regulated assure important solar contributions in the winter and reduced ones in the summer, with consequent reductions of energy demand for air-conditioning. Among the passive systems, glazed surfaces allow a direct solar gain since radiation enters directly into the rooms and is absorbed by the walls which act as absorbers 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 characterized by the corresponding total solar energy transmittance g (global solar transmittance), which indicates the incident solar energy fraction transmitted through the considered system. Factor g is made up of two parts: 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 direction of the incident solar radiation and on its spectral distribution. There are fundamentally two methods to determine the value of g. Carrying out an experimental calorimetrical measurement: a portion of unitary surface is irradiated and the energy transmitted is determined calorimetrically; by calculation methods, these are based on the knowledge of the optical properties of each layer making up the window system and the shading system (transmission coefficient, absorption and reflection of the beam and diffused solar radiation) (Tilmann E. Kuhn et Al., 2000). The resolution of the optical problem in the system composed of window and shading leads to the determination of the transmission coefficient of the solar radiation τ and of the energy absorbed 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 system 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 temperature of 20 C in the winter and 26 C in the summer cooling period, was determined. The optical performances of the shading-window system 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 (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 considered. The module presents a single dispersive external wall with a 1.80x1.20 m glazed surface. The thermal transmittance 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. PALENC Vol 2.indd 729 7/9/2007 1:24:25 µµ

2 730 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Table 1: Thermal and optical properties of glazing. U g τ W/m 2 K Clear glazing Low-emissive glazing A blue Venetian blind was chosen for the shading system with 22 mm-wide slats and 28 mm interpane, positioned 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 suppositions were made in the energy simulations: - the solar radiation only affects the glazed surface and the shading system is active when the irradiation exceeds the value of 150 W/m 2 ; - 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 presence of a person, a PC and a printer. The load owing to the lighting plant is 12.5 W/m 2. All the evaluations leave out of consideration the aspects 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 configurations: 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 transmittance g and of the transmission coefficient τ for external Venetian blinds exposed to the South with differently inclined slats. The two transmission coefficients have qualitatively the same trend. The highest values are found when the slats are placed horizontally: radiation 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 inclination 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 q i 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. PALENC Vol 2.indd 730 7/9/2007 1:24:26 µµ

3 731 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 Venetian 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 5: Trends of the total solar energy transmittance g at variation θ. East orientation, clear glazing. Figure 6 shows, for exposure to the South, the mean monthly values of g obtained using low-emissive glazing, 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 reduction of the total solar energy transmittance g that decreases on average by 17% for internal Venetian blinds, by 50% for those placed in the interpane and by 26% for external ones. Moreover, it can be noted that the inclination of the slats has a reduced influence on system shading performances if the Venetian blind is placed in the interpane. Figure 4: Trends of the total solar energy transmittance g at variation 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 Venetian blind. The reduction in the case of θ = 90 is maximum, with values of g that reduce on average by 0.53 to Also for θ = 0 the reduction is significant, with g which from 0.62 becomes Inserting interpane Venetian blind leads to values of the total solar energy transmittance between 0.28 and 0.46 with a very contained 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 figure 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 6: Trends of the total solar energy transmittance g at variation θ. South orientation, low-emissive glazing. Figure 7: Trends of the total solar energy transmittance g at variation θ. East orientation, low-emissive glazing. PALENC Vol 2.indd 731 7/9/2007 1:24:26 µµ

4 732 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 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 summer 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 temperature and solar irradiation, temperature control of the indoor air is obtained by means of the energy contributions of the plants. Two series of simulations were conducted: in the presence 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 contributions of the solar radiation are computed indirectly as the difference between the energy supplied to the control 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 contributions due to the presence of the shading systems in the heating and cooling of the room, at variation of orientation, shading position and inclination of the slats. The table points out that the reduction of the solar contributions in cooling is more important with respect to heating, 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 importance 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 radiation 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 Internal Heating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ) 0 529,70-932,09 238,01-495,32 62,43-55, ,60-973,91 379,30-559,29 117,80-112, , ,34 409,05-624,85 144,84-160, , ,25 422,18-673,30 167,67-202, ,76-947,01 54,02-492,53 13,86-80, , ,16 86,54-664,65 27,26-153, , ,75 93,87-723,82 34,13-208, , ,82 95,41-758,21 39,16-254, ,17-930,79 60,92-472,43 17,29-40, , ,17 94,09-637,24 32,30-98, , ,54 102,69-700,07 39,37-150, , ,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. South East West South East West External Inperpane Internal Heating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ) Heating (MJ) Cooling (MJ) 0 410,70-749,38 294,14-648,93 33,65-8, ,39-784,39 436,06-665,45 63,78-39, ,31-812,39 416,37-664,66 76,31-59, ,12-859,68 398,27-657,48 86,67-77, ,17-757,05 65,48-636,58 8,07-29, ,95-928,84 96,31-777,62 15,43-66, ,15-955,95 95,07-760,15 18,49-92, ,84-983,78 90,50-735,01 20,80-114, ,37-749,83 72,54-639,08 8,79-3, ,31-924,15 102,02-776,81 17,43-30, ,52-950,56 100,98-757,22 20,70-54, ,81-978,02 97,56-728,94 23,11-74,56 PALENC Vol 2.indd 732 7/9/2007 1:24:26 µµ

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 transmittance in an important way when the Venetian blind is placed externally, in a more contained way when placed 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 interpane. These percentages become respectively 85% and 65% in the case of low-emissive glazing. The effect of inclination of the slats is important for angles 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 effective 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 (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 Environment, Lund Institute of technology, Sweden. Pfrommer P., Lomas K. J., Kupke Chr. (1996). Solar radiation transport through slat-type blinds: a new model and its application for thermal simulation of buildings. Solar Energy Vol. 57, n 2, pp Tilmann E. Kuhn, Buhler C., Werner J. Platzer (2000). Evaluation of overheating protection with sun-shading system. Solar Energy Vol. 69, Nos 1-6, pp UNI (1994). Heating and cooling of buildings. Climatic data. PALENC Vol 2.indd 733 7/9/2007 1:24:26 µµ