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Evaluation of the CIE overcast sky model against Japanese data
ELSEVIER
Energy
and Buildings
27 ( 1998)
175-l 77
Evaluation of the CIE: overcast sky model against Japanese data T. Muneer * Received
1 May
1997; received
in revised
form 9 May
1997
Abstract The present study wasundertakento evaluatetheCIE overcastskymodelfor Japanusingthedatanowemergingfrom the CIE International Daylight Measurement Programme.KyushuUniversity in Japanis the nationalandinternationalco-ordinatingcentrefor this programme. Usingdatafrom only thosedays whichexperienced a completeovercastit hasbeenshownhereinthattheabovemodelalwaysunderestimates the averagevertical illuminance.The analysisindicatesthat a uniform sky is a betterrepresentation of the overcastluminancedistribution. 0 1998ElsevierScienceS.A. Keywrds:
CIE overcast
sky model; Radiance:
Luminance
1. Introduction The solarenergy incident on any vertical o: slopingsurface consistsof three components-i.e., beam, sky-diffuse and ground-reflected energy. Most meteorological networks record horizontal total and sky-diffuse irradiance. While calculation of beamenergy is a routine task once solar geometry is known, the sky-diffuse and ground-reflected components are, in general, more difficult to obtain owing to their anisotropic nature. Under the prevailing overcast and semiovercast climate the sky-diffuse energy component is the dominant oneandthus understandingits physicsis important. Study and characterisationof the overcastsky radianceand luminance distribution has been a subject of great interest. The CIE overcast sky model is basedon tt-,ework of Moon and Spencer [ I] which postulatesa value of b = 2 in Eq. ( 1) to describethe luminancedistribution. However, Steven and Unsworth [ 21 have shownthat for a generalone-dimensional anisotropic model, radiance or luminance distribution may be representedas: L,IL,=(l+b*sin~)!(l+b)
(1)
where L, and LZ are respectively the luminance from a patch of sky at an angle of @and the zenith. b is l:heluminance (or radiance) distribution index. Integration of this distribution, acrossthe sky-vault provides the ratio of energy received by any sloping surface to the horizontal diffuse energy, TF * Tel.: +44 napier.ac.uk
131 4552531;
0378.7788/98/$19,00 P1180378-7788(
bx:
+44
0 1998 Elsevier 97)00078-5
131 4552204:
e-mail:
t.muneer@
Science S.P.. All rights reserved
defined here asthe ‘tilt factor’. TF is analogousto the radiation configuration factor and may be expressedin terms of the surfaceslope, /3 and 6: TF=cos’(P/2)
+ [26/n-(3+2b)]
X (sir@-p*co@-
min2( p/2))
(2)
For a vertical surface underan isotropic sky (b = 0) TF = 0.5. TF respectively having a value of lessthan or greater than 0.5 indicates an anisotropic distribution skewedtowards the zenith and the horizon. For the CIE overcast sky model (b = 2) TF becomesequal to 0.4. In this note measuredirradianceand illuminance data from Fukuoka ( SouthernJapan) have beenusedto study the nature of the overcast luminance distribution.
2. Evaluation of the model Overcast sky data were classified for the present study usingthe parameterK,. the ratio of horizontal global to extraterrestrial irradiance, asthe measureof the amount of overcast, e.g.. a heavy overcast would yield low values of K,. It hasbeen shownby the author [ 31 that for valuesof K, 5 0.35 the horizontal diffuse andglobal irradianceare of equalmagnitude thus indicating the presenceof an overcastcloud layer. In this respectattention is drawn towards Fig. 5 of author’s study of the solar climate of Fukuoka, Japan [ 31. The mentioned plot showsthe variation of the diffuse to global irradiance ratio against K, and displays a horizontal trend upto the limiting value of K, = 0.35.
176
T. Muneer
/ Energ?:
-*
l . .
and Buildings
x
175-I
77
.
C learness Fig. I. Evaluation
2 7 ( 1998)
of the CIE overcast
An examination of Fig. 1 shows that there is a signifcant scatter for overcast sky condition but the average ratio of vertical to horizontal illuminance is much more than the expected value of 0.4. Table 1 sheds further light on this evaluation. It appears that with a thickening of the cloud the luminance distribution tends to be rnc3re uniform. Fig. 1 as well as Table 1 also show that the variation of the ratio of vertical to horizontal illuminance is of differing magnitude during the summer and winter months. This phenomenon may be explained in the light of the work carried out by Perez et al. [4]. They have shown that sky clarity as well as solar altitude and the atmospheric water vapour content significantly influence the luminance distribution. Of course the latter two of the above mentioned parameters assume differing values during summer and winter seasons. In particular, Southern Japan experiences warm, dry summers while rain is frequently expected during winters,
x
. north
Index,
n easl
I south
x WSSI
I
Kt
sky model against Fukuoka
data.
It has been pointed out in the literature that one of the deficiencies of the CIE overcast sky model is that it does not distinguish between thin and heavy overcast. It is well known that the Moon and Spencer model [ 11, with a value of b = 2 in Eqs. ( 1) and (2) above is applicable only when the complete sky canopy is covered with uniform, dark clouds [461. Figure 7 of Ref. [4] clearly demonstratesthe changing luminancedistribution with varying amountof overcast (dark overcast to bright overcast). Also, Mahdavi et al. [ 61 undertook a study on evaluation of daylight factors using measured data. The list of possiblesourcesof disagreementbetween simulated and measureddaylight factors were highlighted. One of these was pointed out as deviation of the prevailing sky condition during the measurementfrom the CIE standard overcast sky model. Similarly, a study undertaken by Nong et al. [ 71 using measureddata from Hokkaido in Japanhas shown that around 39% of the time the ratio of vertical to
T. Muneer/ Table 1 Averaged
ratios of vertical/horizontal
illuminance
(TF)
Energy
and Buildings
27 (1998)
175-l 77
177
for banded clearness index
K, Lower
Upper
Summer
Winter
Limit
Limit
North
East
South
West
North
East
South
West
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0.62 0.58 0.55 0.5 1 0.50 0.48 0.46
0.62 0.59 0.53 0.2 1 0.5 1 0.:12 0.52
0.62 0.56 0.51 0.52 0.52 0.5 1 0.48
0.61 0.55 0.50 0.52 0.5 1 0.48 0.41
0.52 0.50 0.49 0.48 0.50 0.48 0.45
0.52 0.50 0.47 0.46 0.44 0.44 0.50
0.56 0.48 0.48 0.46 0.45 0.47 0.52
0.58 0.53 0.5 1 0.49 0.52 0.53 0.48
horizontal illuminance exceeded the predictions of the Moon and Spencer’s model. Their comments are in agreement with those of the present author. In this respect attention is also drawn towards a recent article by Enarun and Littlefair [ 81 and the accompanying discussion by the present author wherein a plot using Edinburgh data has been included to drive this point home. The above findings have potentially important consequences in terms of daylight use and its penetration within buildings. Presently, the Moon and Spencer model [ 1] is the defacto standard for obtaining the incident illuminance and the daylight factor tables are also based on this model. Present and other research [7] has indicated that the above model will lead to a significant underestimation of the available daylight within building interiors. This inadvertently leads to either higher air-conditioned loads (due to an overdesign on the part of the daylight designer) or the lack of motivation to use appropriate glass facade and thus a larger dependency on electric lighting. Thus, there is a need to examine closely the
short comings of the Moon and Spencer model, in particular to locations in the tropical and temperate belt. References [ 1] P. Moon, D. Spencer, Trans. Illum. Eng. Sac. 37 ( 1942) 707-726 London. [2] M.D. Steven, M.H. Unsworth,Q.J.R. Met. Sac. 106 (1980) 57-61. [3] T. Muneer, LR&T 27 (4) (1995) 223-230. [4] R. Perez, R. Seals, J. Michalsky, .I. Illum. Eng. Sac. 22 (1) ( 1993) lO17. ]5] N. Ruck, Discussion on modelling skylight angular luminance distribution from routine irradiance measurements. in: R. Perez, R. Seals. J. Michalsky (Eds.), J. Illum. Eng. Sac.. 22 (1) (1993) 17. [6] A. Mahdavi, L. Berberidou-Kallivoka, P. Matthew, K. Jen Tu, 3. Illum. Eng. Sac. 22 ( 1) ( 1993) 40-44. [7] L. Nong, H. Nakamura, Y. Koga, N. Igawa, Classification of daylight and solar radiation data into three categories of sky condition: overcast sky, Proc. of the Second Lux Pacifica Conf., CIO-Cl5 (November IO13,1993). [ 81 D. Enarun. P.J. Littlefair, LR&T 27 ( 1995) 53-58.
Energy
and Buildings
27 ( 1998)
175-l 77
Evaluation of the CIE: overcast sky model against Japanese data T. Muneer * Received
1 May
1997; received
in revised
form 9 May
1997
Abstract The present study wasundertakento evaluatetheCIE overcastskymodelfor Japanusingthedatanowemergingfrom the CIE International Daylight Measurement Programme.KyushuUniversity in Japanis the nationalandinternationalco-ordinatingcentrefor this programme. Usingdatafrom only thosedays whichexperienced a completeovercastit hasbeenshownhereinthattheabovemodelalwaysunderestimates the averagevertical illuminance.The analysisindicatesthat a uniform sky is a betterrepresentation of the overcastluminancedistribution. 0 1998ElsevierScienceS.A. Keywrds:
CIE overcast
sky model; Radiance:
Luminance
1. Introduction The solarenergy incident on any vertical o: slopingsurface consistsof three components-i.e., beam, sky-diffuse and ground-reflected energy. Most meteorological networks record horizontal total and sky-diffuse irradiance. While calculation of beamenergy is a routine task once solar geometry is known, the sky-diffuse and ground-reflected components are, in general, more difficult to obtain owing to their anisotropic nature. Under the prevailing overcast and semiovercast climate the sky-diffuse energy component is the dominant oneandthus understandingits physicsis important. Study and characterisationof the overcastsky radianceand luminance distribution has been a subject of great interest. The CIE overcast sky model is basedon tt-,ework of Moon and Spencer [ I] which postulatesa value of b = 2 in Eq. ( 1) to describethe luminancedistribution. However, Steven and Unsworth [ 21 have shownthat for a generalone-dimensional anisotropic model, radiance or luminance distribution may be representedas: L,IL,=(l+b*sin~)!(l+b)
(1)
where L, and LZ are respectively the luminance from a patch of sky at an angle of @and the zenith. b is l:heluminance (or radiance) distribution index. Integration of this distribution, acrossthe sky-vault provides the ratio of energy received by any sloping surface to the horizontal diffuse energy, TF * Tel.: +44 napier.ac.uk
131 4552531;
0378.7788/98/$19,00 P1180378-7788(
bx:
+44
0 1998 Elsevier 97)00078-5
131 4552204:
e-mail:
t.muneer@
Science S.P.. All rights reserved
defined here asthe ‘tilt factor’. TF is analogousto the radiation configuration factor and may be expressedin terms of the surfaceslope, /3 and 6: TF=cos’(P/2)
+ [26/n-(3+2b)]
X (sir@-p*co@-
min2( p/2))
(2)
For a vertical surface underan isotropic sky (b = 0) TF = 0.5. TF respectively having a value of lessthan or greater than 0.5 indicates an anisotropic distribution skewedtowards the zenith and the horizon. For the CIE overcast sky model (b = 2) TF becomesequal to 0.4. In this note measuredirradianceand illuminance data from Fukuoka ( SouthernJapan) have beenusedto study the nature of the overcast luminance distribution.
2. Evaluation of the model Overcast sky data were classified for the present study usingthe parameterK,. the ratio of horizontal global to extraterrestrial irradiance, asthe measureof the amount of overcast, e.g.. a heavy overcast would yield low values of K,. It hasbeen shownby the author [ 31 that for valuesof K, 5 0.35 the horizontal diffuse andglobal irradianceare of equalmagnitude thus indicating the presenceof an overcastcloud layer. In this respectattention is drawn towards Fig. 5 of author’s study of the solar climate of Fukuoka, Japan [ 31. The mentioned plot showsthe variation of the diffuse to global irradiance ratio against K, and displays a horizontal trend upto the limiting value of K, = 0.35.
176
T. Muneer
/ Energ?:
-*
l . .
and Buildings
x
175-I
77
.
C learness Fig. I. Evaluation
2 7 ( 1998)
of the CIE overcast
An examination of Fig. 1 shows that there is a signifcant scatter for overcast sky condition but the average ratio of vertical to horizontal illuminance is much more than the expected value of 0.4. Table 1 sheds further light on this evaluation. It appears that with a thickening of the cloud the luminance distribution tends to be rnc3re uniform. Fig. 1 as well as Table 1 also show that the variation of the ratio of vertical to horizontal illuminance is of differing magnitude during the summer and winter months. This phenomenon may be explained in the light of the work carried out by Perez et al. [4]. They have shown that sky clarity as well as solar altitude and the atmospheric water vapour content significantly influence the luminance distribution. Of course the latter two of the above mentioned parameters assume differing values during summer and winter seasons. In particular, Southern Japan experiences warm, dry summers while rain is frequently expected during winters,
x
. north
Index,
n easl
I south
x WSSI
I
Kt
sky model against Fukuoka
data.
It has been pointed out in the literature that one of the deficiencies of the CIE overcast sky model is that it does not distinguish between thin and heavy overcast. It is well known that the Moon and Spencer model [ 11, with a value of b = 2 in Eqs. ( 1) and (2) above is applicable only when the complete sky canopy is covered with uniform, dark clouds [461. Figure 7 of Ref. [4] clearly demonstratesthe changing luminancedistribution with varying amountof overcast (dark overcast to bright overcast). Also, Mahdavi et al. [ 61 undertook a study on evaluation of daylight factors using measured data. The list of possiblesourcesof disagreementbetween simulated and measureddaylight factors were highlighted. One of these was pointed out as deviation of the prevailing sky condition during the measurementfrom the CIE standard overcast sky model. Similarly, a study undertaken by Nong et al. [ 71 using measureddata from Hokkaido in Japanhas shown that around 39% of the time the ratio of vertical to
T. Muneer/ Table 1 Averaged
ratios of vertical/horizontal
illuminance
(TF)
Energy
and Buildings
27 (1998)
175-l 77
177
for banded clearness index
K, Lower
Upper
Summer
Winter
Limit
Limit
North
East
South
West
North
East
South
West
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.05 0.10 0.15 0.20 0.25 0.30 0.35
0.62 0.58 0.55 0.5 1 0.50 0.48 0.46
0.62 0.59 0.53 0.2 1 0.5 1 0.:12 0.52
0.62 0.56 0.51 0.52 0.52 0.5 1 0.48
0.61 0.55 0.50 0.52 0.5 1 0.48 0.41
0.52 0.50 0.49 0.48 0.50 0.48 0.45
0.52 0.50 0.47 0.46 0.44 0.44 0.50
0.56 0.48 0.48 0.46 0.45 0.47 0.52
0.58 0.53 0.5 1 0.49 0.52 0.53 0.48
horizontal illuminance exceeded the predictions of the Moon and Spencer’s model. Their comments are in agreement with those of the present author. In this respect attention is also drawn towards a recent article by Enarun and Littlefair [ 81 and the accompanying discussion by the present author wherein a plot using Edinburgh data has been included to drive this point home. The above findings have potentially important consequences in terms of daylight use and its penetration within buildings. Presently, the Moon and Spencer model [ 1] is the defacto standard for obtaining the incident illuminance and the daylight factor tables are also based on this model. Present and other research [7] has indicated that the above model will lead to a significant underestimation of the available daylight within building interiors. This inadvertently leads to either higher air-conditioned loads (due to an overdesign on the part of the daylight designer) or the lack of motivation to use appropriate glass facade and thus a larger dependency on electric lighting. Thus, there is a need to examine closely the
short comings of the Moon and Spencer model, in particular to locations in the tropical and temperate belt. References [ 1] P. Moon, D. Spencer, Trans. Illum. Eng. Sac. 37 ( 1942) 707-726 London. [2] M.D. Steven, M.H. Unsworth,Q.J.R. Met. Sac. 106 (1980) 57-61. [3] T. Muneer, LR&T 27 (4) (1995) 223-230. [4] R. Perez, R. Seals, J. Michalsky, .I. Illum. Eng. Sac. 22 (1) ( 1993) lO17. ]5] N. Ruck, Discussion on modelling skylight angular luminance distribution from routine irradiance measurements. in: R. Perez, R. Seals. J. Michalsky (Eds.), J. Illum. Eng. Sac.. 22 (1) (1993) 17. [6] A. Mahdavi, L. Berberidou-Kallivoka, P. Matthew, K. Jen Tu, 3. Illum. Eng. Sac. 22 ( 1) ( 1993) 40-44. [7] L. Nong, H. Nakamura, Y. Koga, N. Igawa, Classification of daylight and solar radiation data into three categories of sky condition: overcast sky, Proc. of the Second Lux Pacifica Conf., CIO-Cl5 (November IO13,1993). [ 81 D. Enarun. P.J. Littlefair, LR&T 27 ( 1995) 53-58.