- Email: [email protected]
Daylighting for energy conservation in the tropics: The Lumen method and the OTTV
~
Energy Vol. 21, No. 6, pp. 505-510, 1996
Pergamon
0360-5442(95)00108-5
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-5442/96 $15.00+0.00
DAYLIGHTING FOR ENERGY CONSERVATION IN THE TROPICS: THE LUMEN METHOD AND THE OTTV S. CHIRARATTANANON,t J. NOORITANON and R. BALAKA Division of Energy Technology, Asian Institute of Technology, Bangkok, Thailand
(Received 12 November 1993; received for publication 30 October 1995)
Abstract--We describe a potential use of the Lumen Method for daylighting applications in commercial buildings in a tropical climate. Calculations of the overall thermal-transfer value show that daylighting and judicious choice of the composition of the building envelope optimize lighting and minimize air-conditioning loads in buildings. Copyright © 1996 Elsevier Science Limited.
INTRODUCTION
The Energy Conservation Promotion (ECP) Act is being implemented in Thailand, t a Southeast-Asian country with a tropical climate. The ECP Act sets standards for energy performance of the building envelope, lighting, and air-conditioning. An overall thermal-transfer value (OTTV) is used, in a manner similar to ASHRAE 90A and Singapore standards, as an index which represents the average heat transfer into a building through the building envelope. Daylighting has been proposed as an energy-efficient practice for commercial buildings. 2 There is a need to adopt a suitable daylight calculation tool, as well as consolidating information on daylight characteristics. The new Lumen Method for daylight calculation of the Illumination Engineering Society (IES) of the U.S.A. 3 is here used as a method with good potential. Most building professionals recognize the benefits of daylighting but concerns are often expressed about the effect of radiant energy on the cooling load for the air-conditioning system. We describe the interplay between reductions in the requirements for electrical lighting and increasing cooling loads for a reference building. DAYLIGHTING AND THE LUMEN METHOD
Daylighting in a tropical climate In a tropical climate, the sky is cloudy for most hours of the year, with a monthly average cloudcover index (on a digital scale) from 5.9 to 9.0. 4 Clear-sky and overcast-sky conditions are rare. In Thailand, the efficacy of daylight is generally high (105-115 lm-W -~ ). The diffuse horizontal illuminance is distributed above 10,000 lux for over 93% of the time (Fig. 1). In a previous study, it was shown that daylighting in existing buildings generally saves up to 50% of the electric-lighting energy. 2 Nevertheless, the use of shading devices is widespread. The Lumen Method and daylight assessment The Lumen Method for side lighting (from windows) in daylight calculation relates the illuminance at the workplane to the skylight and the reflected ground light in the form E = Te(ExvkCUk+ ExvgCUg), 3 where Te = net transmittance of the window which accounts for the light-loss factor, glazing transmittance, etc., Exvk= exterior vertical illuminance from the half sky, CUk = coefficient of utilization from sky luminance, E~v~= exterior vertical illuminance from the ground, and CU s = coefficient of utilization from the ground. Let E~hkbe the exterior horizontal illuminance from the half sky and ExH the exterior horizontal illuminance from both sun and sky. The ratio Exvk/Exhk, as well as the window and room dimensions, form a set of parameters used in determining values for the coefficients of utilization, while ExH is used to determine Exvs. Values for Exvkand Exvg must include the view factor and additional illuminance effects due to shading devices and large exterior surfaces of adjacent buildings. The IES tTo whom all correspondence should be addressed. EGYZ~-6-F
505
S. Chirarattananon et al
506
0.18
>'
0.16 0.14 0.12
--
0.1
0
"O
0.08 0.06 0.04
O
z
0.02 0
5
10 15 20 25 30 35 40 45 50 55 60 85 Illuminance
Level (klux)
Fig. 1. Diffuse illuminance in Thailand.
publication suggests a so-called "lune method" to correct for such effects) Studies on the Lumen Method include field measurements in Bangkok, Kathmandu, and Shanghai. For fenestration without a shading device, the Lumen Method is adequate. For cases with exterior shading devices, application of the lune method has not proved completely satisfactory.
A case study on daylight assessment in Bangkok A room with window setback is shown in Fig. 2. Let Exv be the exterior vertical, half-sky illuminance at the window and Exh the corresponding horizontal illuminance. Measurements under cloudy sky conditions yield the ratios shown on the upper line in Table 1. Corresponding values calculated using the lune method on the second line of Table 1 show that the lune method overestimates the E J E x h ratio but underestimates E~vg. Such discrepancies appear to be persistent. On the other hand, using a set of measured Exv and Exvg values and calculated value for the illuminance at interior points, agreement is
Ex,4 , ExH ~::~U-ExfllIC~UI~ "4~O. / ~ '~ 0l" Interior Exterior
Fig. 2. The configuration of the study room and the measurementpoints.
Table 1. Illuminanceratios. Assessment Method Measurement Lune method
E~ / F.~,
Rems E~/F_~
2.0
2.2
0.058
3.6
0.023
F_~/F_~
Daylighting for energy conservation
I
"R : 450 400 ~350 300 250 2O0 150 100
a CalculatedResult
507
• MeasuredResult
I
"!
~ , ,
10
~~__.
90
30 50 70 Percent of Room Depth
Fig. 3. A comparison of calculated and measured daylight illuminance.
obtained with measurement results (see Fig. 3). Apparently, the issue is assessments of Exv, Exh and Exvg from Exvk, Exhkand ExH. DAYLIGHTING AND COOLING ENERGY
To illustrate the quantitative benefits of daylighting a reference building is described in the following. Daylight illuminance and supplementary-electric lighting are to be assessed via the Lumen Method, and the corresponding cooling requirement estimated using the OTTV of different envelope compositions.
A description of the reference building Figure 4 shows the plan of a floor of a building to be used as reference for the illustration. Each facade of the building faces one of the cardinal directions. The floor space is divided into a core zone and the periphery. The floor is divided into 8 areas, comprising 4 comer rooms and 4 normal rooms. The windows are identical and evenly distributed with a fixed height of 1.5 m. The width of each window assumes a value corresponding to each of the assumed value of window-area to wall-area ratio (WWR). The floor space is daylighted with supplementary electric lighting to achieve a uniform illuminance of 500 lux. The periphery space is cooled by an air-conditioning system with an assumed value for the coefficient of performance (COP). The electric luminaries are uniformly spaced on the ceiling. The reflectance values for the ceiling, wall, and floor are 0.7, 0.5, and 0.3, respectively. The light-loss factor is 0.7. The lamp efficacy is 65 lm-W -~. The average electric power requirement of this airconditioned, daylighted floor has been assessed for three types of glazing, two types of wall and two
i I I
7.5 m.
10m.
Core Area
; r
I<
25 m. Fig. 4. Plan view of a floor of the reference building.
7.5 m.
S. Chirarattananon et al
508
Table 2. Composition and thermal properties of envelope components.
Envelope Component Glazing
Pertinent Property Thickness mm.
1. Clear 2. Reflective glass,
gray 3. Low-E double glazing Opaque Wall 1. Brick 2. Brick and gypsum board
Shading Coefficient 0.92 0.59
6-12-6
0.45
Material Cement-plastered brick Plastered brick 100mm., air-gap 50ram, gypsum board 10mm.
Visible Transmission 0.88 0.39
U-valuffUf), W.m-2_ K-l 5.79 5.98 1.42
0.36
U-value (Uw), W -m"2-K"l 3.07
1.07
values of COP. Table 2 shows property details. The opaque walls are considered heavy and the exterior wall surface is assumed to have a solar absorptivity of 0.3. Values of the COP are 2 and 3.
The Lumen Method for assessment of electric lighting Starting with a uniform sky with a horizontal illuminance of 5000 lux and given a WWR of 0.05, the daylight illuminance for points on a workplane in a comer room with clear glazing are obtained by superposing the daylight illuminance from windows from both facades. Results are given in Table 3(a). Most of the points at 10% distance from the facade require no supplementary-electric lighting. Using the Lumen Method for calculation of the electric power required for supplementary lighting to achieve a uniform illuminance of 500 Lux, the required electric power per cell (of area 1.5 x 1.5 m 2) is shown for each sample cell in Table 3(b). The total supplementary power for the room is 564.2 W for the given values of the horizontal illuminance and the WWR. Such calculation is also performed for each of the exterior illuminance level displayed in Fig. 1. Based on such results, the annual-average electric power required for supplementary lighting in such a room is obtained as PL (WWR, clear glazing) = Y-~;EL;, where EL; = electric power required for the room corresponding to a given exterior horizontal illuminance level i, andf~ = normalized frequency of occurrence of the horizontal illuminance at the level i. Similar calculations may be made for the normal room. The annual-average electric power requirement for lighting the floor is the sum of the requirements for all rooms of the floor. Calculations for two other glazing types and other values of WWR produce the results shown in Table 4.
Table 3. (a).
Daylight illuminance (lux) in a comer room. (b).
(a)
(b)
Percentage of the total distance from the facade 90 % 50 % 493.3 510.7 75.2 92.6 57.7 75.2
Electric power required for supplementary lighting (Wm-:).
Comer 10% 925.6 510.7 493.3
10 % 50 % 90 %
Percentage of the total distance from the facade 90e 50 % 10 % 0.3
0
17.4 18.1
16.7 17.4
0 0 0.3
Percentage distance from facade
Comer
10 % 50 % 90 %
Percentage distance from facade
Daylighting for energy conservation
509
Table 4. Annual-average electric power for supplementary lighting for the floor.
Avon ;e supplementary iigl ~ing power, W . Clear glazing RFT Grey glazing Low-E double glazinBs 10,398 lO,398 10,398 2,325 3,761 4,604 1,075 2,255 3,327 661 1,627 2,637 479 1,310 2,237 793 229 1,563
WWR 0.0 0.05 0.10 0.15 0.20 0.46
The 0173/and external heat gain The O'lq'V formulation aims to capture the annual average of the heat flux transferred into the room during daylight hours through the building envelope. The formulation adopted for Thailand and used in this study was presented in 1989 and its accuracy was confirmed by simulation via the DOE-2 c o d e . 4 For an envelope of the reference building comprising a brick wall and clear glazing on the windows, the o T r v is OTTV = (Uw)(TD~q)(1 - WWR) + (Uf)(DT)(WWR) + (SF)(SC)(WWR) = 33.79 + 142.4 WWR (W-m-2), where TDeq is the equivalent temperature difference across the wall = 1 I°C for our heavy wall with light color, DT = temperature difference across the glazing = 5°C, SF = solar factor = 160 W-m -2. The other thermal property values are given in Table 2. The result shows that the heat gain increases with an increase in WWR, while Table 4 indicates a decreasing trend for the power requirement for supplementary lighting.
Electric power for lighting and cooling We assume that the electric power required for supplementary lighting becomes part of the cooling load for the air-conditioning system. The annual-average cooling load due to external heat gain and the electric lighting per unit area of envelope for one floor of the building is CL/A = OTTV + PL/A, where PL/A is the electric-lighting power per unit area of the envelope (W-m -2) derived from PL. For an air-conditioned space, the annual-average electric power required for lighting and cooling per unit area of envelope for onefloor is PLC/A = (CLIA)/COP + PL/A (W-m-2). Figure 5 shows a set of graphs of PLC/A, each calculated for an opaque wall comprising a brick and gypsum board with COP = 3. Each of the 3 lower lines corresponds to one of the given types of glazing material. The graphs also show the corresponding power requirement if no daylighting is implemented and lighting is totally electric. The graphs clearly confirm the energy benefits of daylighting. The curve for the line corresponding to the case for clear glazing shows a sharp optimum point at a low WWR value. The line corresponding to the case of a low-E, double-glazed window shows a less sensitive response to increasing WWR. Figure 6 shows the PLC/A graphs for windows glazed with reflective grey glass, each of the combi-
• Clear glazing, no dayligMing • Gray glazing, no daylighting
• Low-E glazing, with daylighting x Gray glazing, with dayllghting = Low-____EE glazing, no dayllghtin____._._ggg• Clear glazing, with daylighting 80
_~ 70 -*-C ~~ 60
i
~_.._--~~:j
30 20
l
lO 0
0
0.05
0.1
0.15
0.2
Window-to
~
~
0.25
0.3
!
!
0.35
0.4
Wall Ratio
Fig. 5. Daylighting and electric power for various glazings.
0.45
510
S. Chirarattananon et al
nations of the two types of opaque walls and the two COP values. Clearly, the better performing wall and air-conditioning systems yield benefits but these do not seem to affect the ranges of WWR at the optimum points. CONCLUSION The Lumen Method for daylight calculations appears to be sufficiently mature for use in a national energy-conservation program. We have shown how it can be used with OTTV calculations to assess lighting and cooling-energy performance of a building envelope. By choosing appropriate envelope parameters such as the WWR, optimum performance can be achieved. • Brick wall, COP2 • Brick-air gap-gypsum, COP2
x Brick wall, COP3 I • Brick-air gap-gypsum, COP3
I
70 D....
so J
~
QN
30 i1~11
lO
0 0.05
i
i
!
i
i
~
i
0.1
0.15
0.2
0.25
0,3
0,35
0.4
0.45
Window-to-Wall Ratio Fig. 6. Daylightingand electric power, wall type and COP effects.
REFERENCES 1. S. Chirarattananon and B. Limmechokchai, Energy--The International Journal 9, 269 (1994). 2. S. Chirarattananon, J. Kaewkiew and S. Sushil, RERIC Int. Energy J. 14, 33 (June 1992). 3. IES Committees on Calculation Procedure, "IES Recommended Practice for the Lumen Method of Daylight Calculations", RP-23-1989, IESNA, New York, U.S.A. (1989). 4. S. Chirarattananon, P. Rakwanmsuk and J. Kaewkiew, "A Proposed Building Performance Standard for Thailand: an Introduction and a Preliminary Assessment", Proc. of the ASHRAE Far East Conf. on Air-Conditioning in Humid Climate, Kuala Lumpur, Malaysia (Nov. 1989).
Energy Vol. 21, No. 6, pp. 505-510, 1996
Pergamon
0360-5442(95)00108-5
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-5442/96 $15.00+0.00
DAYLIGHTING FOR ENERGY CONSERVATION IN THE TROPICS: THE LUMEN METHOD AND THE OTTV S. CHIRARATTANANON,t J. NOORITANON and R. BALAKA Division of Energy Technology, Asian Institute of Technology, Bangkok, Thailand
(Received 12 November 1993; received for publication 30 October 1995)
Abstract--We describe a potential use of the Lumen Method for daylighting applications in commercial buildings in a tropical climate. Calculations of the overall thermal-transfer value show that daylighting and judicious choice of the composition of the building envelope optimize lighting and minimize air-conditioning loads in buildings. Copyright © 1996 Elsevier Science Limited.
INTRODUCTION
The Energy Conservation Promotion (ECP) Act is being implemented in Thailand, t a Southeast-Asian country with a tropical climate. The ECP Act sets standards for energy performance of the building envelope, lighting, and air-conditioning. An overall thermal-transfer value (OTTV) is used, in a manner similar to ASHRAE 90A and Singapore standards, as an index which represents the average heat transfer into a building through the building envelope. Daylighting has been proposed as an energy-efficient practice for commercial buildings. 2 There is a need to adopt a suitable daylight calculation tool, as well as consolidating information on daylight characteristics. The new Lumen Method for daylight calculation of the Illumination Engineering Society (IES) of the U.S.A. 3 is here used as a method with good potential. Most building professionals recognize the benefits of daylighting but concerns are often expressed about the effect of radiant energy on the cooling load for the air-conditioning system. We describe the interplay between reductions in the requirements for electrical lighting and increasing cooling loads for a reference building. DAYLIGHTING AND THE LUMEN METHOD
Daylighting in a tropical climate In a tropical climate, the sky is cloudy for most hours of the year, with a monthly average cloudcover index (on a digital scale) from 5.9 to 9.0. 4 Clear-sky and overcast-sky conditions are rare. In Thailand, the efficacy of daylight is generally high (105-115 lm-W -~ ). The diffuse horizontal illuminance is distributed above 10,000 lux for over 93% of the time (Fig. 1). In a previous study, it was shown that daylighting in existing buildings generally saves up to 50% of the electric-lighting energy. 2 Nevertheless, the use of shading devices is widespread. The Lumen Method and daylight assessment The Lumen Method for side lighting (from windows) in daylight calculation relates the illuminance at the workplane to the skylight and the reflected ground light in the form E = Te(ExvkCUk+ ExvgCUg), 3 where Te = net transmittance of the window which accounts for the light-loss factor, glazing transmittance, etc., Exvk= exterior vertical illuminance from the half sky, CUk = coefficient of utilization from sky luminance, E~v~= exterior vertical illuminance from the ground, and CU s = coefficient of utilization from the ground. Let E~hkbe the exterior horizontal illuminance from the half sky and ExH the exterior horizontal illuminance from both sun and sky. The ratio Exvk/Exhk, as well as the window and room dimensions, form a set of parameters used in determining values for the coefficients of utilization, while ExH is used to determine Exvs. Values for Exvkand Exvg must include the view factor and additional illuminance effects due to shading devices and large exterior surfaces of adjacent buildings. The IES tTo whom all correspondence should be addressed. EGYZ~-6-F
505
S. Chirarattananon et al
506
0.18
>'
0.16 0.14 0.12
--
0.1
0
"O
0.08 0.06 0.04
O
z
0.02 0
5
10 15 20 25 30 35 40 45 50 55 60 85 Illuminance
Level (klux)
Fig. 1. Diffuse illuminance in Thailand.
publication suggests a so-called "lune method" to correct for such effects) Studies on the Lumen Method include field measurements in Bangkok, Kathmandu, and Shanghai. For fenestration without a shading device, the Lumen Method is adequate. For cases with exterior shading devices, application of the lune method has not proved completely satisfactory.
A case study on daylight assessment in Bangkok A room with window setback is shown in Fig. 2. Let Exv be the exterior vertical, half-sky illuminance at the window and Exh the corresponding horizontal illuminance. Measurements under cloudy sky conditions yield the ratios shown on the upper line in Table 1. Corresponding values calculated using the lune method on the second line of Table 1 show that the lune method overestimates the E J E x h ratio but underestimates E~vg. Such discrepancies appear to be persistent. On the other hand, using a set of measured Exv and Exvg values and calculated value for the illuminance at interior points, agreement is
Ex,4 , ExH ~::~U-ExfllIC~UI~ "4~O. / ~ '~ 0l" Interior Exterior
Fig. 2. The configuration of the study room and the measurementpoints.
Table 1. Illuminanceratios. Assessment Method Measurement Lune method
E~ / F.~,
Rems E~/F_~
2.0
2.2
0.058
3.6
0.023
F_~/F_~
Daylighting for energy conservation
I
"R : 450 400 ~350 300 250 2O0 150 100
a CalculatedResult
507
• MeasuredResult
I
"!
~ , ,
10
~~__.
90
30 50 70 Percent of Room Depth
Fig. 3. A comparison of calculated and measured daylight illuminance.
obtained with measurement results (see Fig. 3). Apparently, the issue is assessments of Exv, Exh and Exvg from Exvk, Exhkand ExH. DAYLIGHTING AND COOLING ENERGY
To illustrate the quantitative benefits of daylighting a reference building is described in the following. Daylight illuminance and supplementary-electric lighting are to be assessed via the Lumen Method, and the corresponding cooling requirement estimated using the OTTV of different envelope compositions.
A description of the reference building Figure 4 shows the plan of a floor of a building to be used as reference for the illustration. Each facade of the building faces one of the cardinal directions. The floor space is divided into a core zone and the periphery. The floor is divided into 8 areas, comprising 4 comer rooms and 4 normal rooms. The windows are identical and evenly distributed with a fixed height of 1.5 m. The width of each window assumes a value corresponding to each of the assumed value of window-area to wall-area ratio (WWR). The floor space is daylighted with supplementary electric lighting to achieve a uniform illuminance of 500 lux. The periphery space is cooled by an air-conditioning system with an assumed value for the coefficient of performance (COP). The electric luminaries are uniformly spaced on the ceiling. The reflectance values for the ceiling, wall, and floor are 0.7, 0.5, and 0.3, respectively. The light-loss factor is 0.7. The lamp efficacy is 65 lm-W -~. The average electric power requirement of this airconditioned, daylighted floor has been assessed for three types of glazing, two types of wall and two
i I I
7.5 m.
10m.
Core Area
; r
I<
25 m. Fig. 4. Plan view of a floor of the reference building.
7.5 m.
S. Chirarattananon et al
508
Table 2. Composition and thermal properties of envelope components.
Envelope Component Glazing
Pertinent Property Thickness mm.
1. Clear 2. Reflective glass,
gray 3. Low-E double glazing Opaque Wall 1. Brick 2. Brick and gypsum board
Shading Coefficient 0.92 0.59
6-12-6
0.45
Material Cement-plastered brick Plastered brick 100mm., air-gap 50ram, gypsum board 10mm.
Visible Transmission 0.88 0.39
U-valuffUf), W.m-2_ K-l 5.79 5.98 1.42
0.36
U-value (Uw), W -m"2-K"l 3.07
1.07
values of COP. Table 2 shows property details. The opaque walls are considered heavy and the exterior wall surface is assumed to have a solar absorptivity of 0.3. Values of the COP are 2 and 3.
The Lumen Method for assessment of electric lighting Starting with a uniform sky with a horizontal illuminance of 5000 lux and given a WWR of 0.05, the daylight illuminance for points on a workplane in a comer room with clear glazing are obtained by superposing the daylight illuminance from windows from both facades. Results are given in Table 3(a). Most of the points at 10% distance from the facade require no supplementary-electric lighting. Using the Lumen Method for calculation of the electric power required for supplementary lighting to achieve a uniform illuminance of 500 Lux, the required electric power per cell (of area 1.5 x 1.5 m 2) is shown for each sample cell in Table 3(b). The total supplementary power for the room is 564.2 W for the given values of the horizontal illuminance and the WWR. Such calculation is also performed for each of the exterior illuminance level displayed in Fig. 1. Based on such results, the annual-average electric power required for supplementary lighting in such a room is obtained as PL (WWR, clear glazing) = Y-~;EL;, where EL; = electric power required for the room corresponding to a given exterior horizontal illuminance level i, andf~ = normalized frequency of occurrence of the horizontal illuminance at the level i. Similar calculations may be made for the normal room. The annual-average electric power requirement for lighting the floor is the sum of the requirements for all rooms of the floor. Calculations for two other glazing types and other values of WWR produce the results shown in Table 4.
Table 3. (a).
Daylight illuminance (lux) in a comer room. (b).
(a)
(b)
Percentage of the total distance from the facade 90 % 50 % 493.3 510.7 75.2 92.6 57.7 75.2
Electric power required for supplementary lighting (Wm-:).
Comer 10% 925.6 510.7 493.3
10 % 50 % 90 %
Percentage of the total distance from the facade 90e 50 % 10 % 0.3
0
17.4 18.1
16.7 17.4
0 0 0.3
Percentage distance from facade
Comer
10 % 50 % 90 %
Percentage distance from facade
Daylighting for energy conservation
509
Table 4. Annual-average electric power for supplementary lighting for the floor.
Avon ;e supplementary iigl ~ing power, W . Clear glazing RFT Grey glazing Low-E double glazinBs 10,398 lO,398 10,398 2,325 3,761 4,604 1,075 2,255 3,327 661 1,627 2,637 479 1,310 2,237 793 229 1,563
WWR 0.0 0.05 0.10 0.15 0.20 0.46
The 0173/and external heat gain The O'lq'V formulation aims to capture the annual average of the heat flux transferred into the room during daylight hours through the building envelope. The formulation adopted for Thailand and used in this study was presented in 1989 and its accuracy was confirmed by simulation via the DOE-2 c o d e . 4 For an envelope of the reference building comprising a brick wall and clear glazing on the windows, the o T r v is OTTV = (Uw)(TD~q)(1 - WWR) + (Uf)(DT)(WWR) + (SF)(SC)(WWR) = 33.79 + 142.4 WWR (W-m-2), where TDeq is the equivalent temperature difference across the wall = 1 I°C for our heavy wall with light color, DT = temperature difference across the glazing = 5°C, SF = solar factor = 160 W-m -2. The other thermal property values are given in Table 2. The result shows that the heat gain increases with an increase in WWR, while Table 4 indicates a decreasing trend for the power requirement for supplementary lighting.
Electric power for lighting and cooling We assume that the electric power required for supplementary lighting becomes part of the cooling load for the air-conditioning system. The annual-average cooling load due to external heat gain and the electric lighting per unit area of envelope for one floor of the building is CL/A = OTTV + PL/A, where PL/A is the electric-lighting power per unit area of the envelope (W-m -2) derived from PL. For an air-conditioned space, the annual-average electric power required for lighting and cooling per unit area of envelope for onefloor is PLC/A = (CLIA)/COP + PL/A (W-m-2). Figure 5 shows a set of graphs of PLC/A, each calculated for an opaque wall comprising a brick and gypsum board with COP = 3. Each of the 3 lower lines corresponds to one of the given types of glazing material. The graphs also show the corresponding power requirement if no daylighting is implemented and lighting is totally electric. The graphs clearly confirm the energy benefits of daylighting. The curve for the line corresponding to the case for clear glazing shows a sharp optimum point at a low WWR value. The line corresponding to the case of a low-E, double-glazed window shows a less sensitive response to increasing WWR. Figure 6 shows the PLC/A graphs for windows glazed with reflective grey glass, each of the combi-
• Clear glazing, no dayligMing • Gray glazing, no daylighting
• Low-E glazing, with daylighting x Gray glazing, with dayllghting = Low-____EE glazing, no dayllghtin____._._ggg• Clear glazing, with daylighting 80
_~ 70 -*-C ~~ 60
i
~_.._--~~:j
30 20
l
lO 0
0
0.05
0.1
0.15
0.2
Window-to
~
~
0.25
0.3
!
!
0.35
0.4
Wall Ratio
Fig. 5. Daylighting and electric power for various glazings.
0.45
510
S. Chirarattananon et al
nations of the two types of opaque walls and the two COP values. Clearly, the better performing wall and air-conditioning systems yield benefits but these do not seem to affect the ranges of WWR at the optimum points. CONCLUSION The Lumen Method for daylight calculations appears to be sufficiently mature for use in a national energy-conservation program. We have shown how it can be used with OTTV calculations to assess lighting and cooling-energy performance of a building envelope. By choosing appropriate envelope parameters such as the WWR, optimum performance can be achieved. • Brick wall, COP2 • Brick-air gap-gypsum, COP2
x Brick wall, COP3 I • Brick-air gap-gypsum, COP3
I
70 D....
so J
~
QN
30 i1~11
lO
0 0.05
i
i
!
i
i
~
i
0.1
0.15
0.2
0.25
0,3
0,35
0.4
0.45
Window-to-Wall Ratio Fig. 6. Daylightingand electric power, wall type and COP effects.
REFERENCES 1. S. Chirarattananon and B. Limmechokchai, Energy--The International Journal 9, 269 (1994). 2. S. Chirarattananon, J. Kaewkiew and S. Sushil, RERIC Int. Energy J. 14, 33 (June 1992). 3. IES Committees on Calculation Procedure, "IES Recommended Practice for the Lumen Method of Daylight Calculations", RP-23-1989, IESNA, New York, U.S.A. (1989). 4. S. Chirarattananon, P. Rakwanmsuk and J. Kaewkiew, "A Proposed Building Performance Standard for Thailand: an Introduction and a Preliminary Assessment", Proc. of the ASHRAE Far East Conf. on Air-Conditioning in Humid Climate, Kuala Lumpur, Malaysia (Nov. 1989).