Daylight is the one light source that most closely matches human visual response (Ahmaed, 2002). Daylighting is dynamic in nature, composed of diffuse skylight, reflected light and intense, directional sunlight, all changing in intensity, direction and spectrum as the time and weather change (Leslie, 2003). Beyond our natural affinity for daylight, studies have proven that daylighting has a profound effect on human mood. In the other word, lighting can be used to strengthen the human circadian rhythm, which determines various physiological functions such as sleep, digestion, alertness, depression and probably general health (Hashmi, 2008). Daylight is much more effective than electric lighting at entraining the circadian system; this is because the circadian system responds only to high levels of blue light, such as those found in daylight. Studies have revealed that daylight is three to four times more effective on circadian rhythm than fluorescent lamps and twenty times more effective than incandescent lamps (Hashmi, 2008). Building occupants value the view that windows provide. View gives the occupant visual information about weather, time of day and surrounding activities. Views also can be aesthetically pleasing and give a respite from closely focused study (Leslie, 2003).
Simulation analyses as well as field-monitoring studies have reported that daylighting controls can result in significant lighting energy savings ranging from 30 to 77% (Ihm et al., 2009). Electric energy savings also result in fewer power plant emissions that contribute to acid rain, air pollution and global warming (Leslie, 2003).
At the present time, up to 20% of generated power is consumed for illumination in the country which has main contribution in fuel consumption and air pollution. for instance in 2005, gross power of 178071x106 kwh was generated in the country, by consumption of, 32832x106 m3 natural gas, 2612x106 L diesel and 6329x106 L fuel oil. Consumption of these fuels has resulted in air pollutants as nitrogen oxides, hydrocarbons, sulfur dioxide, carbon dioxide, sulfur tri oxide and carbon mono oxide in large quantities of 147661x103, 5302x103, 140220x103, 95899792x103, 2139x106 and 198x106 kg, respectively (Tavanir Organisation, 2009).
Daylight availability defined in terms of the external skylight illuminance available on an unobstructed horizontal plane for a certain percentage of daytime working hours or for specified periods (Rahim and Mulyadi, 2000). Such data can be obtained either by measurements or by calculations from other meteorological quantities (Alshaibani, 2001).
Since, the launch of International Daylight Measurement Program (IDMP) by the International Commission on Illumination (CIE, Commission Internationale de lEclairage) in 1991, measurement of daylight illuminance has been undertaken and reported from various parts of the world (Chirarattananon et al., 2007). Since then, however, a few Asian stations have been established in Japan, Hong Kong, Thailand and Singapore, measurement of luminance and illuminance has not yet been undertaken in Iran.
The first step in evaluating the visual performance and energy efficiency provided by daylight requires an accurate estimation of the daylight entering a building (Li et al., 2004). This can be performed by experimental equations or computer simulation programs. Daylighting simulations have achieved a high standard of accuracy and realism in the last decade (Lehara and Glicksmanb, 2007) which mostly predict indoor daylight illuminance based on measured weather data.
In respect to no reliable data in Iran upon which to assess the impact of daylight on workplaces adopting a proper method to predict daylight availability could be the viable alternative.
Tehran province is surrounded by 11 industrial parks with maximum distance of 125 km from Tehran. This study was undertaken to estimate outdoor illuminances on both horizontal and vertical surfaces, as well as potentiality of energy saving by daylighting in industrial parks of Tehran Province.
MATERIALS AND METHODS
This study contains two main stages, (1) developing two adequate models for predicting of outdoor horizontal and vertical illuminances in industrial parks of Tehran and (2) estimation of indoor illuminance in an assumed workplace hence predicting of electrical energy conservation by daylighting in this workplace.
Developing Outdoor Illuminance Models
For the purpose of developing adequate models, measurement of global illuminances
was conducted on both horizontal and south oriented vertical surfaces by means
of special measuring sets in three established stations [Hamadan, Eshtehard
(near of Tehran) and Kerman]. Daylight values were monitored over 15 days between
12 July and 1 August 2007 at 1 h intervals from 9 am to 3 pm. Synchronically
global horizontal and south facing vertical illuminances were calculated using
equations proposed by Illuminating Engineering Society of North America IESNA.
In respect to more occurrences of clear skies in measuring period (88%) just
data related in clear skies were taken in to account. All of collected data
were entered in statistical sheet of SPSS software and adequate regression models
were developed between calculated and measured values of horizontal and vertical
|| Properties of industrial parks of Tehran Province
Details on measuring stations, monitoring periods and adopted equations, could
be observed in authors earlier studies (Shekari et al., 2008; Golmohammadi
et al., 2009). Based on IESNA equations horizontal and vertical illuminances
of a working year (294 days) were calculated for 11 industrial parks of Tehran
Province from of 8 to 16 h and subsequently were modified with fitted models.
Indeed 2646 data for each industrial park and hence 29106 data on horizontal
or vertical surface were obtained for all industrial parks. Table
1 shows geographical coordinate and other characteristics of industrial
parks. Vertical external illuminance can provide more accurate information than
the horizontal one to determine the average indoor illuminance (Li and Lam,
2000). Therefore, in this study, vertical illuminances were applied to predict
potentiality of energy conservation and statistical analyses were made on horizontal
Prediction of Indoor Illuminances and Energy Conservation
The outdoor required vertical illuminance (Exv) for maintaining
internal horizontal illuminance of 500 Lx, was estimated using the following
equations derived from Lumen method (Rea Marks, 2000).
where, Eitotal is total interior horizontal illuminance on a reference point from sidelighting in Lx, Edes is desired indoor illuminance (500 Lx), Aw and As are, the area of the window wall in m2 and the area of the window in m2, respectively.
where, Eig is interior horizontal illuminance on a reference point from the ground in Lx, Exg is exterior vertical illuminance from the ground on the window in Lx, CUg and τ are, respectively coefficient of utilization from the ground and net transmittance of the window wall.
where, Ei is, interior horizontal illuminance on a reference point from sidelighting in Lx.
where, cusky and Exv are, respectively coefficient of utilization from the sky and exterior required vertical illuminance on the window to maintain interior desired illuminance in Lx.
Cumulative frequency distribution of outdoor illuminance can indicate the percentage of working year in which a given illuminance is exceeded. Based on this distribution annual electrical energy saving for an assumed workplace was estimated by following equation (Li and Lam, 2000).
where, Ea is, annual energy savings in electric lighting (kwh), LPD is, installed lighting power density (w m-1), A and Ha are, respectively the floor area in m2 and the annual operating hours of the electric lighting system and F is, the fraction of the working year when daylighting can maintain the required indoor design illuminance.
Following regression models were developed to predict horizontal and vertical illuminances in the workplaces:
where, Ehm and Ehc are, respectively predicted horizontal illuminance and calculated horizontal illuminance in KLx.
where, Evsm and Evsc are, respectively predicted South facing vertical illuminance and calculated South facing vertical illuminance in KLx.
The minimum, maximum, mean and standard deviation of total annual data on horizontal plane found to be 12.45, 108.12, 66.48 and 24.71 KLx, respectively. Table 2 shows these values for all industrial parks.
Minimum, maximum and mean hourly values of annual data for all workplaces from
8 to 16 found to be (16.93, 64.92 and 42.28 KLx), (31.76, 82.62 and 59.97 KLx),
(44.27, 96.33 and 73.85 KLx), (52.11, 104.95 and 82.53 KLx), (54.01, 108.12
and 85.32 KLx), (49.76, 105.96 and 82 KLx), (40.2, 98.4 and 72.83 KLx), (26.52,
85.65 and 58.6 KLx) and (12.45, 68.61 and 40.96 KLx), respectively. Annual values
of four industrial parks at different standard times are shown in Table
3. The maximum and minimum values of mean hourly illuminances were associated
with Charmshahr (85.65 KLx) and Eshtehard (39.49 KLx), respectively. Among all
indusrial parks as it is shown in Table 4, the maximum frequencies
of horizontal illuminances (12%), were related in range of 56.73-64.06 KLx and
the minimum frequencies (3%) were in range of 20.0-27.34 KLx.
|| Quantities of horizontal illuminances (KLx)
|| Annual horizontal illuminance values (KLx) for each standard
|*Upper, medium and lower rows are related in minimum, maximum
and mean values, respectively
|| Frequency percent of various illuminance ranges (KLx) in
|1: Eshtehard, 2: Parand, 3: Pishva, 4: Charmshahr, 5: Kharazmi,
6: Shams-Abad, 7: Abbas-Abad, 8: ALi-Abad, 9: Firooz Kooh, 10: Nasir-Abad,
11: Nazar-Abad and 12: Total
Also, 19% of all frequencies were related in values more than 93 KLx. Mean
hourly values of horizontal illuminances in June (with mean monthly illuminance
of 91.61 KLx) were considerably higher and in December (with mean monthly illuminance
of 38.33 KLx) were less than other months, respectively (Fig.
1). Exterior vertical illuminances of 46.5 Klx was obtained to maintain
average internal illuminance of 500 Lx at reference point of 0.5 depth of an
assumed workplace in Shams Abad with following properties:
Width of 30 m, depth of 12 m from window wall to the rear wall, height of 4 m, window width of 6 m, window height of 3 m, net transmittance of the window of 0.9, exterior vertical illuminance from the ground on the window of 1 KLx, coefficient of utilization from the sky also from the ground of 0.078.
|| Mean hourly/monthly horizontal illuminances in Tehran industrial
||Cumulative percent of working hours with external south facing
vertical illuminance exceeded a given value in industrial parks of Tehran
Regarding to cumulative percents of outdoor vertical illuminaces (Fig. 2), the exterior vertical illuminace of 46.5 KLx will be occurred for more than 55% of working hours in industrial parks of Tehran. It means that for more than 55% of a working year, indoor illuminance of 500 Lx could be maintained by daylight solely.
Based on Eq. 5, for this workplace with installed lighting power density of 9 w m-2 and daily operating time of 10 h, the annual energy saving in electric lighting will be 5240 kwh. This amount of energy saving in just one workplace among 2995 active units of Tehran would cause to decrease in annual consumption of 964.16 m3 natural gas, 73.36 L diesel and 183.4 L fuel oil as well as would result in decrease in annual emission of air pollutants as nitrogen oxides (4.3 kg), hydrocarbons (0.1 kg), sulfur dioxide (4.08 kg), carbon dioxide (2827.18 kg), sulfur tri oxide (52.4 kg) and carbon mono Oxide (5.24). Taking in to account the current cost of 2000 Rials ($0.22) per kwh of generated power, the annual conservation of 10480000 Rials ($1152.8) could be saved due to 5240 kwh annual energy saving.
In this study, daylight availability on horizontal and vertical planes as well as energy saving by daylighting were estimated for 11 industrial parks of Tehran Province. The maximum calculated value among 29106 data exceeded 108 KLx which is as high as further expectation for a subtropical region as Iran.
Results of this study exhibited a great variation of illuminances during a working year so that maximum hourly values of annual data were 2 times more than the minimum values in all places also the maximum value of mean hourly illuminances (85.32 KLx) found to be two times more than the minimum corresponding value (40.96 KLx) at all the time. On the other hand horizontal illuminances more than 100 KLx occured more frequently than other places (9.7%) suggesting high daylight values in Tehran industrial parks.
In this study, calculation of vertical illuminances were based on new equations proposed by IESNA (2000), resulted in fitted model different from researchers prior model (Shekari et al., 2008).
Revision of Excel data sheets of illuminances revealed that there are different outdoor horizontal illuminances while occurrences a given outdoor vertical illuminance in different times which verifies the idea of Li and Lam (2000) that vertical external illuminance can provide more accurate information than the horizontal one to determine the average indoor illuminance.
While reearchers agree on the positive impact of daylighting, there is a disagreement in corresponding quantifying energy saving potential. In this study, an annual electrical energy conservation of 55% was estimated for an assumed workplace in Tehran industrial parks whereas daylighting case studies exhibit energy savings of 33 to 60% (Leslie, 2003; Chirarattananon et al., 2007; Pattanasethanon et al., 2008; Roisin, 2008; Ihm et al., 2009). In these studies, differences in calculation methods, building locations and related characteristics have resulted in different findings on energy saving by daylighting.
In conclusion findings exhibited high daylight availability and good potentiality of energy saving in industrial parks of Tehran by daylight so that implementation of daylighting in just one active unit of each industrial park of Tehran, would result in decrease up to 31.826 t in emission of air pollutants and 10608.584 m3 in fuel consumption which are equal to conservation of 115280000 Rials ($12680.8). For more accurate data long term measurement of illuminances must be made in Iran.
This study is based on the first author M.Sc. thesis which was conducted in Hamadan University of Medical Sciences, Iran in 2008.