11632nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

Estimation of outdoor illuminance for passive solar architecture in Malaysia

A. Zain-Ahmed, H. Omar, M.Y. Alwi, M. Omar, S. AhmedUniversiti Teknologi MARA, Malaysia

sive solar systems. The uncertainties in the best evalu-ation models reflect the uncertainties in measured data (Myers, 2005 and Younes et al, 2005). Daylight data are required in daylighting and energy efficient designs in buildings as daylighting can reduce artificial light-ing consumption from 50 to 80% (Bodart and de Herde, 2001). It is therefore imperative to have good basic measured data for more accurate simulations such as vertical illuminance which can be estimated from sky luminance data (Li et al., 2005).In the last decade, studies on local daylighting and illu-minance have been carried out in the Asia Pacific region (Li et al, 1996 and Chirarattananon et al, 2002). Li et al (2002) continued with their work and commented that ac-curate cooling load calculations depend greatly on long-term routine measurements which are most effective and accurate method of establishing a reliable database.Illuminance from the sky dome is not measured at any meteorological station in Malaysia. Illuminance and daylight availability are important parameters in day-lighting design. Global solar irradiation, however is measured at major meteorological stations around the country. It is useful, therefore to have global luminous efficacy in order to estimate the global illuminance from global solar irradiation. Several attempts have been made to determine the mean and hourly global lu-minous efficacy values for the sky at Shah Alam, Klang Valley, Malaysia, which is about 15 km away from Ma-laysia’s capital city, Kuala Lumpur. The latest value of global luminous efficacy is reported here and compari-sons are made with previous work.

2. OBJECTIVES

This work is aimed at producing site specific mean global luminous efficacy. The Klang Valley uses meteorologi-cal data from the Subang station which is about 10 km away from the city of Shah Alam and 15 km from Kuala Lumpur. Global solar irradiation data is available from this station and global illuminance may be derived from solar irradiation data using global luminous efficacy, KG. The results of the present study are then compared with the results of previous work for the same location.

ABSTRACT

Illuminance and daylight availability are important pa-rameters in daylighting design for passive, low energy architecture. In Malaysia, the only climatic parameter that can be used for determining daylight availability is global solar irradiation, which is measured at major me-teorological stations around the country. However, it is possible to estimate the global illuminance from global solar irradiation data if the global luminous efficacy is known. Several attempts have been made to determine the mean and hourly global luminous efficacy values for the sky at Shah Alam in the Klang Valley, Malaysia, which is about 15 km away from Malaysia’s capital city, Kuala Lumpur. Global solar irradiation data is available from this station and global illuminance may be derived from solar irradiation data using global luminous effica-cy, KG. Past studies in the determination of KG involved the measurement of global illuminance using simple outdoor luxmeters and simultaneous data collection of global solar irradiance using a pyranometer. The present study used a sky scanner to measure both parameters simultaneously from the sky dome. The results of the present study were then compared with the results of previous work for the same location. The mean global luminous efficacy was calculated for each set of data using the General Illuminance Model. The monthly average illuminance was then estimated using the ob-tained KG while the irradiance data used were from the Model Year Climate (MYC) data. The illuminance was also calculated using the Du Mortier-Perraudeau-Page (DPP) Model recommended for intermediate or aver-age sky types. The results of KG obtained from the 2 periods of measurements were then compared with that produced by the DPP Model. This paper shows that the sky scanner results are closer to the data produced from the DPP model than those obtained from the separate luxmeter-pyranometer experimental set-up.Keywords : Global luminous efficacy, daylighting, il-luminance, irradiance, intermediate sky

1. INTRODUCTION

Solar and daylight data are very important and useful data for the design, development and evaluation of pas-

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1164 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

3. METHODOLOGY

This study used a 4th generation sky scanner of EKO make to measure the sky luminance and radiance simul-taneously at Shah Alam. Two sets of data were used, one set measured in 2005 and the second set measured in 2006. The 2005 set was measured over 31 days in August and the second set of data were obtained from 7 days of measurement in April 2006. The experimen-tal set up was installed on the 21st floor of the tallest building in the university campus which has 20 floors to ensure no obstruction from nearby buildings and trees. A desktop computer (housed in a weatherproof cabin) was linked to the sky scanner to download the data au-tomatically. The equipment were monitored every day to ensure security and uninterrupted data acquisition.The Malaysian sky has been established as an intermedi-ate sky (Zain-Ahmed et al, 1998 and Zain-Ahmed, 2000) and therefore equations that are suitable for this type of sky were selected to calculate the global luminous efficacy. The mean global luminous efficacy was calculated for each set of data using the General Illuminance Model given asKG = Luminance (lm/W) (1) RadianceandKG = Illuminance (lm/W) (2) IrradianceUsing Equation 2, the illuminance was estimated while the irradiance data used were from the Model Year Cli-mate (MYC) data (Zain-Ahmed, 1998). The illuminance was also calculated using the Du Mortier-Perraudeau-Page Model (1994) recommended for intermediate or average sky types.The Du Mortier-Perraudeau-Page (DPP) Model is given as KG = (KBIb + KDId)/Ig (3)WhereKB : beam luminous efficacyKD : diffuse luminous efficacyIb : hourly beam solar irradiationIg : hourly diffuse solar irradiationId : hourly global solar irradiationand Ib = (Ig - Id)The diffuse luminous efficacy is given as KD = 144 -29CC (4)where CC is the cloud cover in oktas.Muneer (1997) recommends the average value of 104 lm/W for KB which was calculated based on the models developed by Navvab et al. (1988) and Aydinli and Ko-chmann (1983). The results of KG obtained from the 2 periods of meas-urements were then compared with that produced by the DPP Model. The results of this study were also com-

pared with the global luminous efficacy value produced in 2000 during which a luxmeter (to measure global illuminance) and pyranometer (to measure irradiance) were used simultaneously. A sky scanner was not avail-able at that time. This set of data were obtained during measurements done over a period of 8 days in February, 2000 in Shah Alam.

4. RESULTS AND DISCUSSION

The mean values of hourly global illuminance and hourly global irradiance data are shown in Figures1and 2. The correlations are very good (R2 = 0.99) for both sets of measurements. The measurements made in August 2005 gave a global luminous efficacy of 106 lm/W while the April 2006 measurements produced a Kg of 112 lm/W.

Figure 1.Thirty-one (31)-day data (August 2005), KG = 106 lm/m2

Figure 2. Seven (7)-day data (April 2006), KG = 112 lm/m2

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11652nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

Figure 3. Mean hourly illuminance for 31-day period in August 2005

Figure 4. Mean Hourly irradiance for 31-day period in August 2005

Figure 5. Mean hourly illuminance during 7-day period in April 2006

Figure 6. Mean hourly irradiance during 7-day period in April 2006

The distribution curves for the 31-day period and the 7-day period shows similarity. However, the peak values of illuminance and irradiance (40,000 lx and 370 W/m2 respectively) are higher for the 31-day period of meas-urements as compared to the peak values (35,000 lx and 300 W/m2 respectively) for the 7-day period.The comparison results for the global luminous effica-cies are shown in Table 1. Global luminous efficacy val-ues are well-reported for overcast skies and very few for average (or intermediate) skies. Values of KG for over-cast skies vary from 86 to 117 lm/W. Table 1 : Comparison of Global Luminous Efficacies from Various Sources

Sky Type Source KG (lm/W)

Overcast Moon and Spencer (1942)

117

Overcast Muneer & Angus (1993)

110

Overcast Hopkinson (1966) 86Overcast Tregenza (1980) 120Overcast Perez et al. (1990) 110Average Blackwell (1954) 116Average (Shah Alam Sky)

Zain-Ahmed (2000)(using Du Mortier-Perraudeau-Page Model)

112

Average (Shah Alam Sky)

Zain-Ahmed (2000)(Measured over 8 days)

114

Average (Shah Alam Sky)

This study(Measured over 31 days)

106

Average (Shah Alam Sky)

This study(Measured over 7 days)

112

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1166 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

Blackwell first reported in 1954 that for average skies, the KG value is 116 lm/W. We initially applied the DPP Model to calculated the Global Luminous Efficacy for the Malaysian sky which turned out to be 112 lm/W (Zain-Ahmed, et al, 2000). The input parameters were from the Model Year Climate Data produced by Zain-Ahmed et al, (1998) which were derived from more than 10 years of Malaysian climate data. At the same time, outdoor measurements were conduct-ed using outdoor luxmeters and pyranometers. The KG produced a value of 114 lm/W. Recently more measure-ments were made using a sky scanner while the more recent measurements produced 2 values, 106 lm/W and 112 lm/W respectively. Although the earlier value was produced from 31 days of measurements and the lat-ter was done during a period of 7 days only, the 7-day measurement is deemed to be more representative of the normal sky conditions in Malaysia. This may also be due to haze conditions which occurred during the month of August 2005 when the measurements were made.

5. CONCLUSION

This study has shown that the global luminous efficacy is best determined using field measurements of illumi-nance and irradiance simultaneously. However, it is not always possible to capture illuminance data routinely as illuminance data are not commonly used by many. Sim-ilarly, direct and diffuse irradiation data are not standard measurements in Malaysia. Although it is possible to use the DPP Model to estimate illuminance in Malaysia, the unavailability of direct and diffuse irradiation data makes this exercise difficult.This study therefore recommends that the global effi-cacy value can now be used to estimate the global il-luminance from global irradiance data that are available from most major meteorological stations in Malaysia. Architects require such data for daylighting considera-tions in their designs.From Equations (1) and (2) the luminance and global illuminance data may be estimated byL = 112 x R (5)And ILg =112 x Ig (6)WhereL : luminanceR : radianceILg: global illuminance.This paper has also shown that the sky scanner results are closer to the data produced from the DPP model than those obtained from the separate luxmeter-pyranometer experimental set-up. This also implies that further field measurements need to be done in order to improve the

quality of subsequent any estimated data and evaluations. In the mean time, estimations of outdoor illuminance may be adequately done using Equation (6) when irradi-ance data are available and no other inputs are required.

ACKNOWLEDGEMENTS

The authors are grateful to Universiti Teknologi MARA for providing the equipment and the Ministry of higher Education for partial funding of the project.

REFERENCES

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