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A new value of average beam solar heat gain coefficient for innovative daylighting systems
Energy and Buildings 33 (2001) 519±524
A new value of average beam solar heat gain coef®cient for innovative daylighting systems A. Tsangrassoulisa,*, C. Pavloua, M. Santamourisa, Wilfried Pohlb, Christof Scheiringb a
Group of Building Environmental Studies, University of Athens, University Campus, Building Physics 5, Athens 15784, Greece b Bartenach LichtLabor, Rinnerstrasse 14, A-6071, Aldrans, Innsbruck, Austria Received 13 March 2000; accepted 20 July 2000
Abstract The majority of the fenestration systems are available in the market today are of the type whose solar gain properties can be determined through the use of a computational procedure. However there is a class of fenestration systems (or attachments) that cannot be handled with the conventional procedures like solar path diagrams and shading masks. This class includes the daylight systems. These systems are an innovative technology for sunlight redirection or sunlight exclusion. A new formula for the solar heat gain coef®cient (SHGC) is introduced, which can help in the discussion process concerning the optimization of solar gains through the above mentioned systems. This formula is solar positioned and radiation weighted value and can provide an accurate way to estimate the energy ¯ux through the window and can be used as a performance rating methodology for the above mentioned systems. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Heat gain; Daylighting systems
1. Introduction The solar heat gain coef®cient (SHGC) is the fraction of solar radiant ¯ux incident on a fenestration system that enters a building as heat gain. It includes directly transmitted as well as absorbed and re-emitted components (ISO/DIS 9050) [1]. The incident radiation in this case is assumed to be parallel and in perpendicular direction to the surface of the probe. SHGC of conventional glazing systems containing only parallel layers with specular, nondiffusing optical properties, is essentially constant from 0 to 408 of incident angles. Since during the year period the angle of incidence could exceed 408 (depending on the orientation of the fenestration system) it is evident that angle depended properties of the SHGC should be taken into account for energy performance calculations. In order to resolve the above mentioned problem the concept of shading coef®cient (SC) was inserted. This coef®cient is the ratio of the SHGC for a given glazing to that for a single pane clear glass and it is not applicable to the framing elements of the fenestration system. The advantage of the SC is that for single pane clear and tinted glass, the SC *
Corresponding author. E-mail address: [email protected] (A. Tsangrassoulis).
is very nearly constant with angle of incidence over a wide range of incidence angle. The National Fenestration Rating Council [2] has selected a single number SHGC value for normal incidence to indicate solar gain performance of complete fenestration system. Furthermore the ASHRAE Handbook of Fundamentals [3] has described the SC method of determining solar heat gain for building heating and cooling load calculations. A SHGC value for normal incidence cannot properly characterize the performances of the glazing over a year's worth of their performance, unless this value can be coupled with the information about SHGC behavior at other angles of incidence to determine solar gains over all irradiation conditions encountered in the course of a year. The measurement of beam SHGC at Bartenbach LichtLabor have been performed according to ISO/DIS 9050, but the measurement procedure is not restricted on perpendicular incidence. This procedure allows the characterization of complex glazings with strongly angle dependent behavior. 2. Experimental procedure The device used to obtain beam SHGC-values is a direct calorimetric measurement equipment. The test method is based on the measurement of the energy collected by a black
0378-7788/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 8 8 ( 0 0 ) 0 0 0 7 5 - X
520
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
3. Discussion
Fig. 1. Schematic representation of the measuring device.
absorber behind the sample in a steady state condition. The absorber allows the measurement of the transmitted solar radiation. The heat extracted from the absorber is removed by a water loop while the determination of the extracted heat is performed by measuring inlet, outlet water temperature and water ¯ow rate. The whole system is housed in a climatic chamber, in which prede®ned conditions (surrounding air temperature, wind speed) can be used. As a source of radiation a solar simulator lamp is used (HMI 2.5 kW). In addition natural sunlight can be used through a heliostat system. The device is presented in (Fig. 1). For steady-state conditions the measured cooling power of the water loop is given by q gfg
K TAbs
TUmg
where q is the measured cooling power, fg the global radiation, TUmg the surrounding air temperature and TAbs the absorber temperature. To minimize the second term on the right-hand side of the above equation, the cooling power is controlled to keep the temperature difference small having as a target to reduce the in¯uence of K. A summary of the technical data concerning the measuring device is presented in Table 1. There was an attempt to obtain as many SHGC data as possible for various angles of incidence since the large amount of data produced appears to be important for obtaining an accurate characterization of the optically complex fenestration systems. Table 1 Technical data of the measuring device Measuring principle
Calorimetric
Maximum size of probe Solar simulator
1m 1m HMI 2.5 kW (AM 1.5), 600 W/m2, beam divergence 1.78 f 1m Aluminium painted black (Nextel Velvet) movable 0±308C controlled 0±15 m/s
Heliostat Absorber Climatic chamber Wind speed
As mentioned above, for Fresnel type glazings, SHGC values are essentially constant for incident angles below 408. When more angularly selective glazings are introduced the simple relationship between shading factor and solar heat gain is no longer valid and the shading factor becomes a variable rather than a constant [4]. Vertical windows, sloped glazings and roof skylight windows in general receive signi®cant quantities of beam radiation at angles greater than this over the course of a year especially during summer months for vertical glazings and for nearly every day for horizontal ones. Any calculations of solar gain above 408, therefore, must account for the reduced SHGC values at these angles. Although the majority of the fenestration systems sold today are of the type whose solar gain properties can be determined through the use of a computational procedure [5±9] there is a class of systems for sunlight redirection that cannot be handled with the existing methodologies. In the present paper three of these systems have been measured 1. System no. 1: high specular movable lamella embedded in insulation glass from Rosenheimer Glastechnik GmbH. 2. System no. 2: high specular movable lamella behind insulation glass from WAREMA Renkhoff GmbH. 3. System no. 3: movable prism behind insulation glass from Huppe Form GmbH. The data collected from the measurement procedure were used to investigate the in¯uence of SHGC values of the examined systems to thermal behavior of buildings and to develop a single SHGC value to characterize the performance of these systems on monthly basis. 4. The impact of angle of incidence depended beam SHGC to thermal behavior of buildings In order to investigate this impact, simulation activities took place to estimate the thermal and cooling loads, on monthly basis, in a test-room (Fig. 2) equipped with the systems under examination. The data that have been used as input to the simulation procedure (TRNSYS software was used [10]) are the following. Measured beam SHGC values for various angles of incidence. Climatic data for Athens (longitudnal 238430 E, latitude 378580 N). Ground temperature, January to April and October to December 168C, May to September 208C. Cooling and heating systems with infinite power working all day. Set point in summer period (May to September) 248C.
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
521
heating loads is increased as the cooling period is approached. The above mentioned effect can be veri®ed by the fact that the relative differences of heating load for measured and constant SHGC values are positive while the opposite happens during the cooling season. Simulation results suggest that the systems under examination, block a large part of the solar radiation during summer months, which in turn causes lower values of cooling loads in comparison with the cooling loads that have been calculated using single SHGC value. Examining the simulation data it is evident that the systems play the role of a shading device especially during summer months. Fig. 2. Schematic representation of the test-room.
5. Simplified procedure
Set point in winter period (January to April and October to December) 208C. Infiltration 0.1 ACH. No use of natural or mechanical ventilation techniques. The convective percentage of the solar energy that enters the room is considered equal to 45, 40 and 14% for systems 1, 2 and 3, respectively. The simulation results are presented in Tables 2 and 3. Average monthly values of heating and cooling loads have been estimated using both measured and constant single values (supplied by manufacturers) of the SHGC. These latter values are 0.2, 0.25,0.27 for systems 1, 2 and 3, respectively. It is evident that the relative differences of heating load for measured and single SHGC values are smaller from that of cooling. The reason is thought to be that the systems under examination have lower values of SHGC for high values of sun's elevation angles in comparison to the constant SHGC value. As a consequence the relative difference in monthly
As mentioned in the introduction a single SHGC value cannot properly characterize the performances of the examined systems over a year's worth of their performance, unless the single SHGC value can be coupled with information about SHGC behavior at other angles of incidence to determine solar gains over all irradiation conditions encountered in the course of a year. The optical properties of the windows are almost always quoted for near normal incidence. In many climates, radiation values has their maximum for large angles of incidence so using the near normal value of SHGC is not correct. Since the value of SHGC is depended on the time (i.e. position of the sun), a single solar positioned and radiation weighted value of SHGC is introduced for the systems under examination. For the estimation of the above mentioned SHGC value the sky vault is divided in a number of patches (158 in elevation and 108 in azimuth) which can be increased or
Table 2 Simulation results of cooling loads for measured and constant beam SHGC values Cooling load (KWh/m2) Set point 248C System 1
System 2
System 3
Month
Variable
Constant
Var/Con%
Month
Variable
Constant
Var/Con%
Month
Variable
Constant
1 2 3 4 5 6 7 8 9 10 11 12
± ± ± ± 0.00 0.03 0.38 1.62 1.31 ± ± ±
± ± ± ± 2.10 4.08 5.78 8.46 5.50 ± ± ±
± ± ± ± ± ± 1425 422 320 ± ± ±
1 2 3 4 5 6 7 8 9 10 11 12
± ± ± ± 0.02 0.10 0.81 3.32 3.96 ± ± ±
± ± ± ± 3.46 5.54 7.52 10.73 7.71 ± ± ±
± ± ± ± 14323 5370 834 223 95 ± ± ±
1 2 3 4 5 6 7 8 9 10 11 12
± ± ± ± 1.76 1.23 2.70 7.23 6.32 ± ± ±
± ± ± ± 3.32 5.51 7.61 11.01 7.79 ± ± ±
Var/Con% ± ± ± 89 347 182 52 23 ± ± ±
522
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
Table 3 Simulation results of heating loads for measured and constant beam SHGC values Heating load (KWh/m2) Set point 208C System 1
System 2
Month
Variable
Constant
1 2 3 4 5 6 7 8 9 10 11 12
10.5 11.5 12.1 6.0 ± ± ± ± ± 4.0 5.1 8.4
11.07 11.26 10.14 2.86 ± ± ± ± ± 3.60 5.44 9.48
Var/Con% 5 2 17 53 ± ± ± ± ± 10 6 12
System 3
Month
Variable
Constant
1 2 3 4 5 6 7 8 9 10 11 12
9.00 10.0 10.77 4.67 ± ± ± ± ± 2.93 3.63 6.86
10.14 10.30 9.14 2.07 ± ± ± ± ± 2.91 4.53 8.52
Var/Con% 13 3 15 56 ± ± ± ± ± 1 25 24
Month
Variable
Constant
Var/Con%
1 2 3 4 5 6 7 8 9 10 11 12
10.5 10.68 10.09 2.92
9.83 9.98 8.81 1.81
6 7 13 38
3.23 4.77 8.82
2.66 4.23 8.21
18 11 7
decreased according to the number of measurements that have been performed. For each sky patch Ð which is visible to the fenestration system Ð the number of hours that the sun is in its area is estimated. Radiation should be taken into account, since it is a common situation a system to have high value of beam SHGC while at the same time radiation values are low (i.e. early morning or late evening). Finally the proposed weighted SHGC value can be calculated for each month (SHGCmw) for any tilt or orientation of the daylighting system as follows: PNtp SHGC tp Ravg SHGCmw i1PNtp i1 tp Ravg
manufacturers should provide data of the above proposed SHGC value for various sites. In order to simplify the calculation, a simple model [11] for the estimation of normal radiation can be introduced. The transmittance of the atmosphere is calculated as follows K T A0 A1 exp cos sun zenith angle
The parameter time tp in each patch is referred to the time that the sun path is in the area of a specific patch of sky while the parameter radiation value Ravg is the average normal radiation that is assigned to a sky patch. Ntp denotes the total number of patches. It is evident that the calculated SHGC value is radiation dependent and thus site dependent. Consequently, the
where H is the height of the site in kilometers.
where A0 0:4237
0:00821 6
H2
A1 0:5055 0:00595 6:5
H2
K 0:2711 0:01858 2:5
H2
Table 4 Monthly radiation-time averaged beam SHGC (as percentage) Month
System 1
System 2
System 3
1 2 3 4 5 6 7 8 9 10 11 12
23.2 19.3 12.8 6.6 2.4 0.96 1.2 4.8 9.3 15.8 21.5 24.9
32.3 27.3 19.8 11 4.2 1.6 2.1 8.1 15.1 22.9 30.4 34
23.6 23.6 22.9 18.3 9.8 5.4 6.2 16.3 20.3 23.5 23.7 23.5
Fig. 3. Energy transmitted through system 1 as calculated using (a) a constant value of SHGC (provided by manufacturer), (b) the proposed value of SHGC and (c) real measurements.
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
523
area) that enters a south oriented window have been performed by using (a) measured values of SHGC (using Athens radiation data), (b) single constant value supported by the manufacturer (using Athens radiation data) and (c) proposed new value of SHGC (using model radiation data) (Figs. 3±5). From the ®gures presented above, it is evident that the energy transmitted by the fenestration system using measurements of SHGC is in quite signi®cant agreement with the energy values as calculated using the radiation and solar positioned formula for the SHGC. The use of this formula reproduces the monthly trend quite well. 6. Conclusions
Fig. 4. Energy transmitted through system 2 as calculated using (a) a constant value of SHGC (provided by manufacturer), (b) the proposed value of SHGC and (c) real measurements.
The Table 4 presents the values of the monthly radiationtime averaged SHGC (as percentage). Thus, for the heating and cooling period the above values are: System 1 Cooling period: 3.7% Heating period: 17.7% System 2 Cooling period: 6.2% Heating period: 25.3% System 3 Cooling period: 11.5% Heating period: 22.7% In order to examine the validity of the above mentioned values a comparison of radiation monthly values (in Athens
In this paper, a new formula for the calculation of beam SHGC has been proposed utilizing calorimetric measurements of three innovative daylighting systems. After the investigation on the impact of the three innovative daylighting systems in the energy behavior of a testroom, a simple formula for the estimation of SHGC value (solar position and radiation weighted) was introduced. This formula can properly characterize the performance of the examined systems over a year's worth of their performance since it can be coupled with information about SHGC behavior at other angles of incidence to determine solar gains over all irradiation conditions encountered in the course of a year. The formula can be used to compare the above mentioned systems without complicated simulation tools. The new value is radiation and thus site dependent; it can be used during the decision process; it is a viable way of determining the performance of the above mentioned systems, producing data that agrees when real measurements of SHGC are used. The results from the evaluation as presented in the paper clearly show that the methodology provides an accurate way to estimate the energy ¯ux through the window. References
Fig. 5. Energy transmitted through system 3 as calculated using (a) a constant value of SHGC (provided by manufacturer), (b) the proposed value of SHGC and (c) real measurements.
[1] ISO 900, Glass in building Ð determination of light transmittance, solar direct transmittance, total solar energy transmittance, UV transmittance and related glazing factors, 1990. [2] NFRC 300-94, Procedures for determining solar optical properties of simple fenestration products, NFRC, 1994. [3] ASHRAE, ASHRAE Handbook-Fundamentals, Fenestration, 1993 (Chapter 27). [4] R. McCluney, Sensitivity of fenestration solar gain to source spectrum and angle of incidence, ASHRAE Transactions 102 (1996) 2. [5] E.U. Finlayson, et al., Window 4: documentation of calculation procedures, Publ. LBL-33943/UC-350, Lawrence Berkeley Laboratory, Energy & Environment Division, Berkeley, CA 94720, 1993.
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A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
[6] WIS reference manual, Dick van Dijk & John Goulding, TNO Building and Construction, October 1996. [7] E. Shaviv, On the optimum design of shading devices for windows, Energy and Buildings for Temperate Climates, PLEA 88, Pergamon Press, Elmsford, New York, 1988, pp. 279±284. [8] A. Beck, W. Korner, O. Gross, J. Fricke, Making better use of natural light with a light-redirecting double glazing system, Solar Energy 66 (3) (1999) 215.
[9] Y. Blanchet, M. Girard, Thermal and visual effectiveness of shading devices, in: Proceedings of the Third European Conference on Architecture, Florence, 17±21 May 1993. [10] TRNSYS, A Transient System Simulation Programme, University of Winsconsin-Madison, Madison, WI, 1983. [11] H.C. Hottel, A simple model for estimating the transmittance of direct solar radiation through clear atmospheres, Solar Energy 18 (1976) 129.
A new value of average beam solar heat gain coef®cient for innovative daylighting systems A. Tsangrassoulisa,*, C. Pavloua, M. Santamourisa, Wilfried Pohlb, Christof Scheiringb a
Group of Building Environmental Studies, University of Athens, University Campus, Building Physics 5, Athens 15784, Greece b Bartenach LichtLabor, Rinnerstrasse 14, A-6071, Aldrans, Innsbruck, Austria Received 13 March 2000; accepted 20 July 2000
Abstract The majority of the fenestration systems are available in the market today are of the type whose solar gain properties can be determined through the use of a computational procedure. However there is a class of fenestration systems (or attachments) that cannot be handled with the conventional procedures like solar path diagrams and shading masks. This class includes the daylight systems. These systems are an innovative technology for sunlight redirection or sunlight exclusion. A new formula for the solar heat gain coef®cient (SHGC) is introduced, which can help in the discussion process concerning the optimization of solar gains through the above mentioned systems. This formula is solar positioned and radiation weighted value and can provide an accurate way to estimate the energy ¯ux through the window and can be used as a performance rating methodology for the above mentioned systems. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Heat gain; Daylighting systems
1. Introduction The solar heat gain coef®cient (SHGC) is the fraction of solar radiant ¯ux incident on a fenestration system that enters a building as heat gain. It includes directly transmitted as well as absorbed and re-emitted components (ISO/DIS 9050) [1]. The incident radiation in this case is assumed to be parallel and in perpendicular direction to the surface of the probe. SHGC of conventional glazing systems containing only parallel layers with specular, nondiffusing optical properties, is essentially constant from 0 to 408 of incident angles. Since during the year period the angle of incidence could exceed 408 (depending on the orientation of the fenestration system) it is evident that angle depended properties of the SHGC should be taken into account for energy performance calculations. In order to resolve the above mentioned problem the concept of shading coef®cient (SC) was inserted. This coef®cient is the ratio of the SHGC for a given glazing to that for a single pane clear glass and it is not applicable to the framing elements of the fenestration system. The advantage of the SC is that for single pane clear and tinted glass, the SC *
Corresponding author. E-mail address: [email protected] (A. Tsangrassoulis).
is very nearly constant with angle of incidence over a wide range of incidence angle. The National Fenestration Rating Council [2] has selected a single number SHGC value for normal incidence to indicate solar gain performance of complete fenestration system. Furthermore the ASHRAE Handbook of Fundamentals [3] has described the SC method of determining solar heat gain for building heating and cooling load calculations. A SHGC value for normal incidence cannot properly characterize the performances of the glazing over a year's worth of their performance, unless this value can be coupled with the information about SHGC behavior at other angles of incidence to determine solar gains over all irradiation conditions encountered in the course of a year. The measurement of beam SHGC at Bartenbach LichtLabor have been performed according to ISO/DIS 9050, but the measurement procedure is not restricted on perpendicular incidence. This procedure allows the characterization of complex glazings with strongly angle dependent behavior. 2. Experimental procedure The device used to obtain beam SHGC-values is a direct calorimetric measurement equipment. The test method is based on the measurement of the energy collected by a black
0378-7788/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 8 8 ( 0 0 ) 0 0 0 7 5 - X
520
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
3. Discussion
Fig. 1. Schematic representation of the measuring device.
absorber behind the sample in a steady state condition. The absorber allows the measurement of the transmitted solar radiation. The heat extracted from the absorber is removed by a water loop while the determination of the extracted heat is performed by measuring inlet, outlet water temperature and water ¯ow rate. The whole system is housed in a climatic chamber, in which prede®ned conditions (surrounding air temperature, wind speed) can be used. As a source of radiation a solar simulator lamp is used (HMI 2.5 kW). In addition natural sunlight can be used through a heliostat system. The device is presented in (Fig. 1). For steady-state conditions the measured cooling power of the water loop is given by q gfg
K TAbs
TUmg
where q is the measured cooling power, fg the global radiation, TUmg the surrounding air temperature and TAbs the absorber temperature. To minimize the second term on the right-hand side of the above equation, the cooling power is controlled to keep the temperature difference small having as a target to reduce the in¯uence of K. A summary of the technical data concerning the measuring device is presented in Table 1. There was an attempt to obtain as many SHGC data as possible for various angles of incidence since the large amount of data produced appears to be important for obtaining an accurate characterization of the optically complex fenestration systems. Table 1 Technical data of the measuring device Measuring principle
Calorimetric
Maximum size of probe Solar simulator
1m 1m HMI 2.5 kW (AM 1.5), 600 W/m2, beam divergence 1.78 f 1m Aluminium painted black (Nextel Velvet) movable 0±308C controlled 0±15 m/s
Heliostat Absorber Climatic chamber Wind speed
As mentioned above, for Fresnel type glazings, SHGC values are essentially constant for incident angles below 408. When more angularly selective glazings are introduced the simple relationship between shading factor and solar heat gain is no longer valid and the shading factor becomes a variable rather than a constant [4]. Vertical windows, sloped glazings and roof skylight windows in general receive signi®cant quantities of beam radiation at angles greater than this over the course of a year especially during summer months for vertical glazings and for nearly every day for horizontal ones. Any calculations of solar gain above 408, therefore, must account for the reduced SHGC values at these angles. Although the majority of the fenestration systems sold today are of the type whose solar gain properties can be determined through the use of a computational procedure [5±9] there is a class of systems for sunlight redirection that cannot be handled with the existing methodologies. In the present paper three of these systems have been measured 1. System no. 1: high specular movable lamella embedded in insulation glass from Rosenheimer Glastechnik GmbH. 2. System no. 2: high specular movable lamella behind insulation glass from WAREMA Renkhoff GmbH. 3. System no. 3: movable prism behind insulation glass from Huppe Form GmbH. The data collected from the measurement procedure were used to investigate the in¯uence of SHGC values of the examined systems to thermal behavior of buildings and to develop a single SHGC value to characterize the performance of these systems on monthly basis. 4. The impact of angle of incidence depended beam SHGC to thermal behavior of buildings In order to investigate this impact, simulation activities took place to estimate the thermal and cooling loads, on monthly basis, in a test-room (Fig. 2) equipped with the systems under examination. The data that have been used as input to the simulation procedure (TRNSYS software was used [10]) are the following. Measured beam SHGC values for various angles of incidence. Climatic data for Athens (longitudnal 238430 E, latitude 378580 N). Ground temperature, January to April and October to December 168C, May to September 208C. Cooling and heating systems with infinite power working all day. Set point in summer period (May to September) 248C.
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
521
heating loads is increased as the cooling period is approached. The above mentioned effect can be veri®ed by the fact that the relative differences of heating load for measured and constant SHGC values are positive while the opposite happens during the cooling season. Simulation results suggest that the systems under examination, block a large part of the solar radiation during summer months, which in turn causes lower values of cooling loads in comparison with the cooling loads that have been calculated using single SHGC value. Examining the simulation data it is evident that the systems play the role of a shading device especially during summer months. Fig. 2. Schematic representation of the test-room.
5. Simplified procedure
Set point in winter period (January to April and October to December) 208C. Infiltration 0.1 ACH. No use of natural or mechanical ventilation techniques. The convective percentage of the solar energy that enters the room is considered equal to 45, 40 and 14% for systems 1, 2 and 3, respectively. The simulation results are presented in Tables 2 and 3. Average monthly values of heating and cooling loads have been estimated using both measured and constant single values (supplied by manufacturers) of the SHGC. These latter values are 0.2, 0.25,0.27 for systems 1, 2 and 3, respectively. It is evident that the relative differences of heating load for measured and single SHGC values are smaller from that of cooling. The reason is thought to be that the systems under examination have lower values of SHGC for high values of sun's elevation angles in comparison to the constant SHGC value. As a consequence the relative difference in monthly
As mentioned in the introduction a single SHGC value cannot properly characterize the performances of the examined systems over a year's worth of their performance, unless the single SHGC value can be coupled with information about SHGC behavior at other angles of incidence to determine solar gains over all irradiation conditions encountered in the course of a year. The optical properties of the windows are almost always quoted for near normal incidence. In many climates, radiation values has their maximum for large angles of incidence so using the near normal value of SHGC is not correct. Since the value of SHGC is depended on the time (i.e. position of the sun), a single solar positioned and radiation weighted value of SHGC is introduced for the systems under examination. For the estimation of the above mentioned SHGC value the sky vault is divided in a number of patches (158 in elevation and 108 in azimuth) which can be increased or
Table 2 Simulation results of cooling loads for measured and constant beam SHGC values Cooling load (KWh/m2) Set point 248C System 1
System 2
System 3
Month
Variable
Constant
Var/Con%
Month
Variable
Constant
Var/Con%
Month
Variable
Constant
1 2 3 4 5 6 7 8 9 10 11 12
± ± ± ± 0.00 0.03 0.38 1.62 1.31 ± ± ±
± ± ± ± 2.10 4.08 5.78 8.46 5.50 ± ± ±
± ± ± ± ± ± 1425 422 320 ± ± ±
1 2 3 4 5 6 7 8 9 10 11 12
± ± ± ± 0.02 0.10 0.81 3.32 3.96 ± ± ±
± ± ± ± 3.46 5.54 7.52 10.73 7.71 ± ± ±
± ± ± ± 14323 5370 834 223 95 ± ± ±
1 2 3 4 5 6 7 8 9 10 11 12
± ± ± ± 1.76 1.23 2.70 7.23 6.32 ± ± ±
± ± ± ± 3.32 5.51 7.61 11.01 7.79 ± ± ±
Var/Con% ± ± ± 89 347 182 52 23 ± ± ±
522
A. Tsangrassoulis et al. / Energy and Buildings 33 (2001) 519±524
Table 3 Simulation results of heating loads for measured and constant beam SHGC values Heating load (KWh/m2) Set point 208C System 1
System 2
Month
Variable
Constant
1 2 3 4 5 6 7 8 9 10 11 12
10.5 11.5 12.1 6.0 ± ± ± ± ± 4.0 5.1 8.4
11.07 11.26 10.14 2.86 ± ± ± ± ± 3.60 5.44 9.48
Var/Con% 5 2 17 53 ± ± ± ± ± 10 6 12
System 3
Month
Variable
Constant
1 2 3 4 5 6 7 8 9 10 11 12
9.00 10.0 10.77 4.67 ± ± ± ± ± 2.93 3.63 6.86
10.14 10.30 9.14 2.07 ± ± ± ± ± 2.91 4.53 8.52
Var/Con% 13 3 15 56 ± ± ± ± ± 1 25 24
Month
Variable
Constant
Var/Con%
1 2 3 4 5 6 7 8 9 10 11 12
10.5 10.68 10.09 2.92
9.83 9.98 8.81 1.81
6 7 13 38
3.23 4.77 8.82
2.66 4.23 8.21
18 11 7
decreased according to the number of measurements that have been performed. For each sky patch Ð which is visible to the fenestration system Ð the number of hours that the sun is in its area is estimated. Radiation should be taken into account, since it is a common situation a system to have high value of beam SHGC while at the same time radiation values are low (i.e. early morning or late evening). Finally the proposed weighted SHGC value can be calculated for each month (SHGCmw) for any tilt or orientation of the daylighting system as follows: PNtp SHGC tp Ravg SHGCmw i1PNtp i1 tp Ravg
manufacturers should provide data of the above proposed SHGC value for various sites. In order to simplify the calculation, a simple model [11] for the estimation of normal radiation can be introduced. The transmittance of the atmosphere is calculated as follows K T A0 A1 exp cos sun zenith angle
The parameter time tp in each patch is referred to the time that the sun path is in the area of a specific patch of sky while the parameter radiation value Ravg is the average normal radiation that is assigned to a sky patch. Ntp denotes the total number of patches. It is evident that the calculated SHGC value is radiation dependent and thus site dependent. Consequently, the
where H is the height of the site in kilometers.
where A0 0:4237
0:00821 6
H2
A1 0:5055 0:00595 6:5
H2
K 0:2711 0:01858 2:5
H2
Table 4 Monthly radiation-time averaged beam SHGC (as percentage) Month
System 1
System 2
System 3
1 2 3 4 5 6 7 8 9 10 11 12
23.2 19.3 12.8 6.6 2.4 0.96 1.2 4.8 9.3 15.8 21.5 24.9
32.3 27.3 19.8 11 4.2 1.6 2.1 8.1 15.1 22.9 30.4 34
23.6 23.6 22.9 18.3 9.8 5.4 6.2 16.3 20.3 23.5 23.7 23.5
Fig. 3. Energy transmitted through system 1 as calculated using (a) a constant value of SHGC (provided by manufacturer), (b) the proposed value of SHGC and (c) real measurements.
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area) that enters a south oriented window have been performed by using (a) measured values of SHGC (using Athens radiation data), (b) single constant value supported by the manufacturer (using Athens radiation data) and (c) proposed new value of SHGC (using model radiation data) (Figs. 3±5). From the ®gures presented above, it is evident that the energy transmitted by the fenestration system using measurements of SHGC is in quite signi®cant agreement with the energy values as calculated using the radiation and solar positioned formula for the SHGC. The use of this formula reproduces the monthly trend quite well. 6. Conclusions
Fig. 4. Energy transmitted through system 2 as calculated using (a) a constant value of SHGC (provided by manufacturer), (b) the proposed value of SHGC and (c) real measurements.
The Table 4 presents the values of the monthly radiationtime averaged SHGC (as percentage). Thus, for the heating and cooling period the above values are: System 1 Cooling period: 3.7% Heating period: 17.7% System 2 Cooling period: 6.2% Heating period: 25.3% System 3 Cooling period: 11.5% Heating period: 22.7% In order to examine the validity of the above mentioned values a comparison of radiation monthly values (in Athens
In this paper, a new formula for the calculation of beam SHGC has been proposed utilizing calorimetric measurements of three innovative daylighting systems. After the investigation on the impact of the three innovative daylighting systems in the energy behavior of a testroom, a simple formula for the estimation of SHGC value (solar position and radiation weighted) was introduced. This formula can properly characterize the performance of the examined systems over a year's worth of their performance since it can be coupled with information about SHGC behavior at other angles of incidence to determine solar gains over all irradiation conditions encountered in the course of a year. The formula can be used to compare the above mentioned systems without complicated simulation tools. The new value is radiation and thus site dependent; it can be used during the decision process; it is a viable way of determining the performance of the above mentioned systems, producing data that agrees when real measurements of SHGC are used. The results from the evaluation as presented in the paper clearly show that the methodology provides an accurate way to estimate the energy ¯ux through the window. References
Fig. 5. Energy transmitted through system 3 as calculated using (a) a constant value of SHGC (provided by manufacturer), (b) the proposed value of SHGC and (c) real measurements.
[1] ISO 900, Glass in building Ð determination of light transmittance, solar direct transmittance, total solar energy transmittance, UV transmittance and related glazing factors, 1990. [2] NFRC 300-94, Procedures for determining solar optical properties of simple fenestration products, NFRC, 1994. [3] ASHRAE, ASHRAE Handbook-Fundamentals, Fenestration, 1993 (Chapter 27). [4] R. McCluney, Sensitivity of fenestration solar gain to source spectrum and angle of incidence, ASHRAE Transactions 102 (1996) 2. [5] E.U. Finlayson, et al., Window 4: documentation of calculation procedures, Publ. LBL-33943/UC-350, Lawrence Berkeley Laboratory, Energy & Environment Division, Berkeley, CA 94720, 1993.
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[6] WIS reference manual, Dick van Dijk & John Goulding, TNO Building and Construction, October 1996. [7] E. Shaviv, On the optimum design of shading devices for windows, Energy and Buildings for Temperate Climates, PLEA 88, Pergamon Press, Elmsford, New York, 1988, pp. 279±284. [8] A. Beck, W. Korner, O. Gross, J. Fricke, Making better use of natural light with a light-redirecting double glazing system, Solar Energy 66 (3) (1999) 215.
[9] Y. Blanchet, M. Girard, Thermal and visual effectiveness of shading devices, in: Proceedings of the Third European Conference on Architecture, Florence, 17±21 May 1993. [10] TRNSYS, A Transient System Simulation Programme, University of Winsconsin-Madison, Madison, WI, 1983. [11] H.C. Hottel, A simple model for estimating the transmittance of direct solar radiation through clear atmospheres, Solar Energy 18 (1976) 129.