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Analysis and prediction of daylighting and energy performance in atrium spaces using daylight-linked lighting controls
Applied Energy 112 (2013) 1016–1024
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Analysis and prediction of daylighting and energy performance in atrium spaces using daylight-linked lighting controls Stanley K.H. Chow ⇑, Danny H.W. Li, Eric W.M. Lee, Joseph C. Lam Building Energy Research Group, Department of Civil and Architectural Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region
h i g h l i g h t s " Daylight-linked lighting control and energy performance for atrium is studied. " Field measurement of automatic dimming control shows 93% energy saving. " Field measurement of manual on–off control shows 95% energy saving. " Atrium illuminance is correlated with daylight factor for energy saving prediction.
a r t i c l e
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Article history: Received 13 October 2012 Received in revised form 10 December 2012 Accepted 14 December 2012 Available online 12 January 2013 Keywords: Atrium Daylight Greenhouse gases Daylight-linked lighting controls Energy savings
a b s t r a c t In subtropical Hong Kong, a certain amount of electricity is used to create visually comfortable interior spaces through electric lighting, which is the second major electricity-consuming item in commercial buildings, accounting for 20–30% of total electricity use. The burning of fossil fuels for electricity generation has many adverse effects on the environment. Daylighting is an important and useful strategy for enhancing visual comfort and reducing the need for the electricity consumed by light fittings. The rational use of daylight through tools such as photoelectric lighting controls can effectively reduce buildings’ electricity consumption and the related pollutants and greenhouse gas emissions. Daylighting design techniques are often best demonstrated via field measurements that provide reliable operational and energy performance data for establishing design guidelines. An atrium provides an environmentally controlled indoor public space that introduces daylight into the hearts of large buildings. In circulation areas such as corridors, people expect the way ahead to be sufficiently lit and daylight-linked lighting controls can deliver excellent energy savings. This paper presents the daylighting and energy performance of an atrium space using daylight-linked lighting controls. The cost, energy and environmental issues related to various daylight illuminances are estimated and design implications are discussed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Renewable technologies are substantially safer than their alternatives, offering a solution that meets the present increasing demand for electrical power and addresses many of the environmental and social problems associated with fossil and nuclear fuels. People are paying increasing attention to high-quality, renewable solar energy [1]. Solar energy is freely available and can be easily harnessed to reduce our reliance on hydrocarbonbased energy in both passive and active designs [2]. Daylighting is an effective and sustainable development strategy for enhancing visual comfort, energy-efficiency and green building developments [3]. Daylight is considered the best source of light for good color ⇑ Corresponding author. Tel.: +852 3762 2122; fax: +852 2305 5093. E-mail address: [email protected] (S.K.H. Chow). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.12.033
rendering because it is the one light source that most closely matches the human visual response [4]. People desire high-quality natural lighting in their work environments [5] although it has been noted that discomfort due to visual effects (glare, headaches, deregulation of the circadian rhythm leading to depression) is frequently reported [6]. Electric lighting is the largest component of the internal load in air-conditioned office buildings [7]. Rational uses of daylight, such as photoelectric lighting controls, can effectively reduce buildings’ energy consumption and the related pollutants and greenhouse gas emissions. In a side-lit space, vertical windows can provide a large amount of daylight near the window facades, but the daylight levels dwindle rapidly as the distance from the window increases. Core daylighting, that is, daylight provision in areas situated at considerable distances from façades and windows, is currently one of the main challenges in sustainable building design [8]. An atrium allows daylight to penetrate into
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the core of a building, contributing not only cultural and architectural content, but also the potential to resolve many environmental issues [9]. Daylighting is one of the basic components of passive solar building design and its estimation is essential [10]. Studies have been conducted on the effect of atrium characteristics on daylight levels on the atrium floor and along working planes in adjoining spaces [11]. Daylight may vary substantially with every floor. With shallow atria or the spaces on the top few floors, more incidental light is received from the sky while the daylight levels in deep atria or the lower atria spaces are chiefly influenced by reflected light [12]. In circulation areas such as corridors, people expect the way ahead to be lit sufficiently. With day-lit corridors, daylight-linked lighting controls can deliver remarkable energy savings [13]. Daylighting design techniques are often best elaborated through field measurements that provide reliable operational and energy performance data to establish design guidelines [14]. Empirical data obtained via systematic measurements, however, would help confirm the usefulness, suitability and accuracy of theoretical models and provide much needed information to building professionals [15]. On-site recorded results for computing the energy savings, greenhouse gas reductions, cost implications and visual performances can convince building owners and occupants to implement appropriate operation and management strategies while ensuring that officials implement apposite policies [16]. This study examined the daylighting and energy performance of an atrium building incorporating daylight-linked lighting controls. Economic and environmental benefits are assessed. Daylight factors and illuminance are predicted. Data measurements and analyses are described and reported. The implications for daylighting designs and building energy strategies are also discussed. This can provide the actual savings and benefits to building professionals for designing appropriate daylight-linked lighting controls in atrium and convince the building owners and occupants to implement such daylighting schemes.
Fig. 1. Vertical section of the building.
2. Background information The institutional building featured here is a purpose-built, 13-storey block located on the periphery of a Hong Kong industrial zone situated along the southern coast of China within the subtropical region, at a latitude of 22.3N and a longitude of 114.2E. The institution is surrounded by buildings in three cardinal orientations (i.e. north, south and west), with a highway to the east. 2.1. Atrium geometry The building was designed with a skylight and an enclosed, stepped atrium to harvest daylight and provide natural ventilation through environmental conservation measures. Fig. 1 shows the vertical section of the building. The stepped atrium increases daylight and improves sky views by splaying the well walls away from the vertical, but the sky component, and hence the daylight factor, of the lower floors are significantly reduced [17]. The atrium dimensions are 11.5 m 9.5 m 40.7 m (height). The lower part of the atrium from 2/F to 9/F consists of typical classrooms, with four open corridors surrounding the atrium. The upper part, between 10/F and 13/F, is a cubic atrium (i.e. 15 m 15 m 15 m) and contains mainly mechanical and office rooms. There is a glazed skylight with a building integrated photovoltaic (BIPV) system installed on the atrium roof aperture. The skylight constitutes around 40% of the plan area obstruction (PAO), which is defined as the percentage of roof blockage when viewed from a plan perspective with the focal point at infinity. Fig. 2 displays the skylight and the PAO. The reduction of light caused by the glazing ranges from 20% (10% for the structure and 10% for the glazing) [18] to as high
Fig. 2. Skylight and the PAO.
as 80% [19]. It has been suggested that the addition of a glazed roof over an atrium well can reduce daylight levels in spaces adjacent to the well by 45% [20].
2.2. Lighting system The daylighting performance and energy use based on daylightlinked lighting controls were evaluated in the 9/F corridors. The atrium corridors are used for circulation among different classrooms,
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Fig. 3. Photoelectric senor at 9/F.
with dimensions of 2.65 m (width), 14.65 m (along the atrium) and 2.22 m (height). The lighting system contains 67 ceiling-mounted, energy-efficient T5 fluorescent tubes, with rated power ranging from 14 W to 35 W. Each T5 fluorescent tube is regulated by one dimmable high frequency electronic regulating ballast that can dim the lamp output smoothly and uniformly. The lighting circuitry for each corridor is separated. The lamp fittings can be dimmed from 1% to 100% via the analog control input. The maximum lighting load is 2261 W plus electronic ballast load. The lighting power density, defined as the electrical power consumed by lighting installations per unit floor area in the corridor, is 14.6 W/m2.
3. Daylight-linked lighting controls 3.1. Illuminance and energy savings The illuminance level of the electric lighting was measured at night (i.e. around 8 pm) when all of the lights were on and no occupants were walking through the corridors. The illuminance levels
along the midway of the four corridors were recorded. The mean illuminance was found to be around 200 lx. The mean luminous efficacy for 14 W and 35 W fluorescent tubes is 95 and 101 lm/W, respectively, which is higher than the minimum allowable efficacy at 87 and 94 lm/W [21]. To provide the minimum pre-set lighting level along the corridors, one adjustable photoelectric sensor with a measuring range from 4 to 1600 lx was mounted on the ceiling of each of the four corridors and used to regulate and record the light intensity. The vertical distance between the top of the atrium and the light sensor was 15.4 m. Four sensors were installed in the corners of the corridors to take illuminance measurements [22], as shown in Fig. 3. They detected both daylight and reflected light from the corridor surfaces to provide a ‘closed loop’ control dimming system. Because it is a narrow plan walkway, a single-zone control was considered adequate [23]. The lighting level received was transmitted to a differential dimming controller through C-bus protocol to vary the light output of the fluorescent lamps using the dimmable electronic ballasts. The illuminance was recorded at 5-min intervals, daily, from 9 am to 6 pm between January and June 2012. Setting aside any missing data and including instrumentation malfunction and power failure, 77,000
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Fig. 4. Hourly average of illuminance.
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Fig. 6. Average illuminance of the 4 zones.
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" # Ev El 3
ð1Þ
r
where Ev is the illuminance (lx), El is the mean illuminance (lx), r is the standard deviation (lx) and E is the expectation operator (dimensionless). The skewness value for Zones 1–4 are +1.39, +0.81, +2.26 and +0.41, respectively. A positive skew value indicates that the frequency on the right side is lower than on the left side, such that the bulk of the values lie to the left of the mean. This skewness can be explained by the overall lower illuminance level harvested. For daylighting design and calculation, a cumulative frequency distribution of the indoor daylight illuminance can indicate the percentage of the working year in which a given illuminance is exceeded. This is valuable for estimating the necessary use of artificial lighting and the probable energy savings delivered by the daylight-linked lighting controls. Fig. 8 presents the cumulative frequency distribution of illuminance levels in the 4 zones. Although Zone 4 scored the highest illuminance level, Zone 1 scored
14000 9/F Zone 1
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0
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75%
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0%
200 0 Jan
Feb
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Fig. 5. Monthly average of illuminance.
0 100 150 200 250 300 350 400 450 500 600 700 800 900 1000 1500 2000 2500 3000 3500 More
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c¼E
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measurements were recorded. The illuminance received was also transmitted to a light intensity level logging system for recording. The measurement of electric lighting consumption was conducted using a single-phase AC power management system that included four power meters. Each power meter displays the readings in kW h with built-in software to capture the measured data from the power meter and transmit them to a microcomputer for storage and subsequent analysis. The data were recorded at 5-min intervals, daily, from 9 am to 6 pm between January and June 2012 when the illuminance set-point was set to 100 lx. This is in accordance with the Chartered Institute of Building Services Engineers (CIBSEs) Code for Interior Lighting [24], which states that circulation areas such as corridors should have a design illuminance level of 100 lx. Fig. 4, which displays the respective hourly illuminance averages reveals that the daylight illuminance begins to increase in early morning, reaches its peak value at around noon and gradually decreases in the afternoon. The minimum illuminance was 98 lx in Zone 3 at 6 pm while the maximum was 1280 lx in Zone 4 at 12 noon. The average illuminance ranged from 128 lx at 6 pm to 726 lx at 12 noon. Fig. 5 shows the monthly illuminance averages, which indicate that the daylight illuminance increases from January to May and decreases in June. This drop in June can be explained by the increased frequency of rain in June compared to May. Similar to the hourly average, the illuminance in Zone 4 is maintained at a higher level than in the other zones. The minimum monthly average illuminance was 222 lx in Zone 3 in January while the maximum was 887 lx in Zone 4 in May. The overall average illuminance ranged from 306 lx in January to 640 lx in May. Fig. 6 displays the average illuminance of the four zones. The average illuminance ranged from 267 lx in Zone 3 to 658 lx in Zone 4 while the overall illuminance was 452 lx. The average illuminance in Zone 3 (west) was far less than in the other zones, probably because there is a lift machine room located in the eastern portion of the roof area that screens a certain amount of daylight in this zone. Fig. 7 presents the frequency distribution for the illuminance levels of the four zones, respectively. The asymmetry of the frequency distribution is evident in the left skewness of the illuminance levels in Zones 1 and 2, representing a higher proportion of the illuminance level at the lower band. The left skewness is further enhanced in Zone 3, where 7300, 11,000 and 13,200 out of 40,000 records were at 100, 150 and 200 lx, respectively. The skewness c is statistically and quantitatively expressed by:
Illuminance Level (lx) Fig. 8. Cumulative frequency distribution of illuminance levels of the 4 zones.
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Energy (kWh)
1020
22.0
95%
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21.0 92% 20.5
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90% Jan
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Daily Average Energy Saving (kWh)
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Overall
Energy Saving Percentage
Fig. 9. Monthly daily average energy saving.
highest frequency at a set-point of 100 lx. This means that the energy savings at a set-point of 100 lx in Zone 1 are greater than in other zones. However, it should be noted that if the set-point is at a higher level, say 200 lx, then Zones 2 and 4 will score higher frequencies than Zone 1.
MPBP was computed to be 3.42 years. For an energy saving strategy in practice, one set of equipment is sufficient and the capital cost is HK$6620 (US$849). Accordingly, the MPBP was computed to be 0.86 year. Pollutants created by electricity generation are considered one of the main causes of the local air pollution. Based on the electricity and gas emissions produced by one of the local power companies, as of 31 December 2011 [25], the reductions in various emissions were projected. Using the dimming controls for 9/F, the projected annual energy saving is 7585 kW h and the annual average CO2, SO2, NOx and particulate emissions could be reduced by 5869, 78, 7.5 and 0.37 kg, respectively. Given that in Hong Kong most electricity is consumed by building stocks [26] and the atrium is a key feature in modern architecture, the environmental benefits of widely applying daylight-linked lighting controls in Hong Kong would be considerable.
4. Prediction and potential savings 4.1. Lighting control by on–off status
Cost, energy and environmental issues are often used to assess the performance of solar energy systems. In Hong Kong, electricity is mainly produced by two local power companies using fossil fuels. The pollutants created by electricity generation are considered one of the main causes of the local air pollution. The economic benefits in terms of the monetary payback period (MPBP) of the lighting system of the institutional building with atrium design under study can be evaluated by comparing the electricity tariff saved to the capital cost of the system (i.e. daylight linked lighting controls). Fig. 9 displays the monthly daily average energy savings. The daily energy savings ranged from a low of 20.5 kW h in January to a high of 21.3 kW h in June and their respective energy savings percentages were 91% and 94%. The overall energy saved was 93% or 21 kW h in the four corridors on 9/F with daylight-linked lighting control systems when the illuminance was set at 100 lx. Fig. 10 displays the energy savings by zone. The energy savings from Zones 1–4 were 957, 744, 823 and 908 kW h, respectively. Zone 1 scored greater energy savings than Zone 4, even though the average illuminance of Zone 4 was higher than that of Zone 1. This result echoes the result in Fig. 8 when Zone 1 scored a higher cumulative frequency at a set-point of 100 lx. In addition, the energy savings for Zone 3 were also higher than those for Zone 2, even though the average illuminance of Zone 3 was the lowest overall. The total energy saved during the measured period was 3429 kW h. Using the current commercial general electric tariff of HK$1.02 (US$0.131) per kW h for Hong Kong, the savings in electricity charges for the monitored period of 165 days was computed to be HK$3498 (US$448). The annual savings in electricity charges was projected to be HK$7738 (US$992). The current system involves four sets of equipment to measure the illuminance of the four zones. The total capital cost of light level sensors, dimmer controls and cable and conduits was HK$26480 (US$3395). The
An examination of the lighting control by on–off status was conducted in the 10/F corridor of the building. An on–off control is designed to switch electrical lighting on and off automatically as the lighting levels rise and fall within a set of predetermined levels [27]. A special problem with the on–off lighting controls is rapid switching, particularly during unstable weather conditions when the daylight varies around the switching illuminance level. This can annoy occupants and reduce lamp life [10]. However, on–off lighting controls are more suited to well-lit environments with low set-point illuminance levels. A variant of the on–off control is the ‘differential switching’ or ‘dead-band’ photoelectric control that has two switching illuminances: one at which the lights are switched on, and another, higher illuminance at which the lights are switched off. The main advantage of this is that it can reduce rapid switching when the illuminance swings around the desired level [28]. As Fig. 11 shows, the lighting system contains 31 wall-mounted, energy-efficient compact fluorescent tubes with a rated power of 32 W each. The maximum lighting load is 992 W plus the electronic ballast load. The compact fluorescent tubes were wired in three separate circuitries. The on–off switching was manually operated according to visual judgments of the illuminance levels. The illuminance in the corridor was measured hourly from 9 am to 6 pm for seven consequent days per month
Energy (kWh)
3.2. Economic and environmental benefits
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Fig. 10. Energy saving by zoning.
Zone 4 (North) Energy Saving Percentage
Fig. 11. Compact fluorescent tubes at 10/F.
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Daylight Illuminance Level (lx) Fig. 12. Cumulative frequency distribution.
Fig. 13. Photoelectric light sensors at 6/F to 8/F.
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4.2. Illuminance and atrium geometry
0
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Fig. 14. Relationship between daily average illuminance and vertical distance from atrium roof aperture.
1000
Illuminance (lx)
To study the relationship between the daylight illuminance and the depth of the atrium, three ceiling-mounted photoelectric light level sensors were installed near the corner of two corridors in Zones 3 and 4 from 6/F to 8/F along the same plummet line, to measure the illuminance. Fig. 13 shows the locations of the photo sensors. The vertical distances of the sensors, measured from the skylight to 8/F, 7/F and 6/F were 18.7 m, 22.0 m and 25.3 m, respectively. The illuminance was recorded at 5-min intervals, daily, from 9 am to 6 pm between January and June 2012. The data were transmitted to an illuminance level logging system for recording. Fig. 14 shows the daily mean illuminance for the three floors. The daily illuminance average dropped from 437 lx at 8/F, to 332 lx at 7/F and to 249 lx at 6/F. An inverse relationship between the recorded illuminance levels and the vertical distance was observed. Fig. 15 displays the hourly average illuminance values for the three floors. Generally, the daylight illuminance began to increase in the early morning, reached the peak value at 12 noon and gradually decreased in the afternoon. The higher floor (i.e. 8/F) reached a higher level of illuminance than the lower floors. The illuminance scored a minimum value of 108 lx at 6 pm and a maximum value of 438 lx at 12 noon on 6/F; a minimum of 128 lx at 6 pm and a maximum of 641 lx at 12 noon on 7/F; and a minimum of 169 lx at 6 pm and a maximum of 788 lx at 12 noon on 8/F. The average illuminance was 249, 332 and 437 lx on 6/F, 7/F and 8/F, respectively. Fig. 16 displays the monthly averages of illuminance values for the three floors. The daylight illuminance increases from January to
26
437
25.3
Vertical Distance (m)
500
Daily Average Illuminance (lx)
from April to June 2012 by a hand-held light-level meter (Testo 540, manufactured in Germany) which is capable of measuring from 0 to 99,999 lx. The energy for a full lighting load during the measuring period was estimated to be 208.3 kW h. Totally 210 measurements were recorded. The average illuminance was computed to be 687 lx. The result showed that 199 of the measurements met the CIBSE recommended illuminance level of 100 lx for circulation areas, whereas 11 measurements were below 100 lx when much of the daylight was obstructed by clouds during overcast conditions and in the evening hours. It has been reported that there is no simple analytical expression for lighting energy savings under such an on–off control [28]. For daylighting designs and calculations, a cumulative frequency distribution of daylight availability can be used to determine the necessary use of electrical lighting and the probable energy savings from on–off controls [29]. Fig. 12 displays the cumulative frequency distribution of daylight availability and reveals that around 95% of the time, the artificial light fittings could be switched off at a set-point of 100 lx. The potential energy saving over the monitored 21 days and a whole year could be 198 and 3440 kW h, respectively. Financially, the annual electricity tariff saving was projected to be HK$3509. In terms of environmental issues, the annual CO2 emissions could be reduced by 2580 kg.
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Hour of Day Fig. 15. Hourly average of the illuminance.
May and decreases in June. The drop in June can be explained by more frequent rain in June than in May. Similar to the hourly average, the higher floor (i.e. 8/F) reached a higher level of illuminance than the lower floors. The maximum monthly average illuminance was 600 lx on 8/F in May while the minimum was 168 lx on 6/F in January. The overall average illuminance ranged from 249 lx on 6/F to 437 lx on 8/F. Fig. 17 plots the cumulative frequency distribution for the daylight illuminance levels on the three floors. The overall illuminance level on 8/F scored a higher cumulative frequency at various set-points, thus distinguishing it from the cumulative
S.K.H. Chow et al. / Applied Energy 112 (2013) 1016–1024
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Fig. 16. Monthly average of the illuminance.
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Daylight Factor (%)
Fig. 19. Prediction of daylight factor and illuminance.
Cumulative Frequency of Time for Saving (%)
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Illuminance Set Point (lx) Fig. 17. Cumulative frequency distribution at various illuminance set-points.
frequency distribution for the daylight illuminance levels on 9/F. As the recommended illuminance level for a corridor is 100 lx, the cumulative percentages around this level are highlighted in Fig. 18. The cumulative percentages are around 88%, 84% and 78% for 8/F, 7/F and 6/F, respectively. The projected annual energy savings, if the same lighting system implemented in the 9/F corridors were installed, would be 7263, 6933 and 6432 kW h for 8/F, 7/ F and 6/F, respectively.
daylight factors from 6/F to 10/F is 0.965, indicating that 96.5% of the variation in illuminance levels can be explained by the changes in daylight factors. The illuminance on the atrium sides from 11/F to 13/F were extrapolated by correlating the DF ratios between the floors. The results show that the high illuminance allows daylight to provide sufficient illuminance on the respective floors so that artificial lighting can be reduced. Because 11/F is a mechanical floor, there is no glazed area facing the atrium. As Figs. 20 and 21 illustrate, 12/F and 13/F are office floors with three sides facing the atrium and having glazed areas. The total glazed area is about 25%. There are 20 lighting panels with a total power of 2400 W on each on 12/F and 13/F at a center distance of 800 mm from the glazing facing the atrium. The total estimated energy consumption is 14,400 kW h per year. As the office cubicles are shallow in dimension, single-zone evaluation is adopted in this study of the daylight performance [33]. The average interior daylight Ein in individual office cubicles on 12/F and 13/F can be predicted by combining the Littlefair Eq. (2) [34].
Ein ¼ 4.3. Prediction of daylight factor and illuminance Predicting daylight levels within an atrium building can sometimes be difficult or uncertain [10]. The British Standard Code of Practice for daylighting [30] uses the average daylight factor (DFavs) [31,32] as a way to judge a daylit space. The daylight factor is the instantaneous ratio of interior illuminance at the measurement points, to the exterior illuminance on a horizontal plane due to an unobstructed sky. It can be predicted using scale model measurements or computer simulations. When studying an actual building, analytical formulae could be used as another way to estimate the daylight in an atrium. In this study, the DF from 6/F to 13/F were predicted using the Littlefair formula [29] Appendix A, which uses a sky component and an internally reflected component. Fig. 19 displays the relationships among the floor levels (i.e. vertical distance from atrium aperture), daylight factors and the daily average illuminance levels. The coefficient of determination between the 5-min interval measured illuminance levels and the
Cumulative Frequency of Time for Saving (%)
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80% 6/F, 78%
70% 0
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Illuminance Set Point (lx) Fig. 18. Cumulative frequency distribution of illuminance at set-point of 100 lx.
Ev tW Að1 RÞ
ð2Þ
where Ev is the predicted daylight in the atrium; t and W are the visual transmittance and area of the window, respectively; A is the total interior surface area for the office; and R is the overall mean reflectance of the office. According to the CIBSE Code for Interior Lighting [24], working places hosting general tasks such as office work should have a design illuminance level of 500 lx. Based on the physical parameters of the two floors, the ratio of Ev to Ein is 5.37. Hence, a daylight illuminance level of 2685 lx is required to fulfil the Code. The predicted daylight illuminance is 1786 and 2275 lx for 12/F and 13/F, respectively. The predicted average interior daylight in individual offices on 12/F and 13/F are 333 and 424 lx, respectively, which suggest that about 68% and 87% of the energy used for artificial lighting can be saved on 12/F and 13/F, respectively. However, the high frequency dimming control in the present study does not have the ideal characteristic of light output being perfectly proportional to the power consumed. The light output can be roughly assumed to be proportional to the power consumed, but the lamps cannot be dimmed to total extinction [28]. In normal operation, their residual light output and power consumption occur throughout working hours, even if the illuminance level far exceeds the design value. However, such operations may be less noticeable and less disturbing to occupants [28]. There is a linear correlation between the light output and the input power for ideal operation of the high frequency dimming controls. Whenever the total illuminance on the working plane (daylight plus electric light for a closed loop control) exceeds the desired illuminance, the lamps will be dimmed to the minimum light output fraction with the corresponding power fraction. For other conditions, the lighting power consumption can be computed with the input values of the minimum light output fraction, the power fraction, the target illuminance, the working plane illuminance and the full light output
1023
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Fig. 20. 12/F office layout.
for energy saving from artificial lighting through daylight-linked lighting control or manual on–off status control. Fig. 22 displays the total energy savings per year. The energy saving for 10/F is much lower than for other floors because the daylight harvest was incorporated into the lighting system in the building design stage. The lighting power density of the 10/F corridor is 6.4 W/m2, which is 44% that of the 9/F corridor. The energy savings for 12/F and 13/F were not higher than those for the lower floors because 12/F and 13/F are used for offices, which require higher illuminance levels than the circulation areas on the lower floors. The total potential energy savings per year represented by the total area of the histogram is 43,003 kW h. The annual saving in electricity charges was projected to be HK$43863 (US$5623). If light level sensors and dimmer controls were installed on all of the floors studied, then the capital cost was estimated to be HK$46340 (US$5941). Accordingly, the MPBP was computed to be 1.06 years.
4.4. Prediction of overall potential benefits
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The measured and projected illuminance on the floors studied (i.e. from 6/F to 13/F) of the institutional building with atrium design revealed a daylight harvest because the atrium design allows
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Fig. 22. Total energy saving per annum.
13/F
Energy Saving Percentage
illuminance based on the linear correlation [28,35]. Given this power fraction, 59% and 75% of the energy used for artificial lighting can be saved on 12/F and 13/F, respectively. This illustrates that the power fraction of the high frequency dimming control delivers a 16% deduction of overall energy savings.
Energy Saving (kWh)
Fig. 21. Fenestration of 12/F and 13/F.
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The annual average CO2, SO2, NOx and particulate emissions could be reduced by 32,252, 440, 42.4 and 2.1 kg, respectively. 5. Conclusions Atriums in buildings have been widely used as environmental conservation measures. Effective control of the electric lighting in an atrium is essential if the full benefits of daylight are to be realized. The daylighting and energy performance of an atrium using daylight-linked lighting controls was evaluated and around 93% of the energy spent could be saved in the four corridors on 9/F with a daylight-linked lighting control system when the illuminance was set at 100 lx. Annually, a lighting energy of 7585 kW h could be saved and the average CO2, SO2, NOx and particulate emissions could be reduced by 5869, 78, 7.5 and 0.37 kg, respectively. The monetary payback period was determined to be 0.86 year. Another study on the lighting control by on–off status was conducted in the building’s 10/F corridor. When the illuminance set-point was 100 lx, around 95% of the artificial lighting time could be switched off. The projected annual savings was estimated to be 3440 kW h. A further study was conducted using three ceiling-mounted photoelectric light level sensors installed near the corner of two corridors in Zones 3 and 4 from 6/F to 8/F along the same plummet line to measure the light intensity level. The recorded illuminance in these areas began increasing in the early morning, reaching the peak value at around noon and gradually decreasing in the afternoon. The cumulative percentages were around 88%, 84% and 78%, respectively, for 8/F, 7/F and 6/F with an illuminance level of 100 lx for a corridor. The projected annual energy savings are 7263, 6933 and 6432 kW h, respectively. The measured illuminance levels between 6/F and 10/F closely match those of the daylight factors predicted by formulae. The illuminance on the atrium sides from 11/F to 13/F were extrapolated by correlating the daylight factor ratios between the floors. The predicted average interior daylight levels in individual offices on 12/F and 13/F were 333 and 424 lx, respectively, which suggests that 68% and 87% of the lighting for 12/F and 13/F could be switched off to save energy if a design illuminance level of 500 lx for general office work was adopted. However, if the power fraction of the high frequency dimming control is considered, then the energy saved from not using artificial lighting is reduced to 59% and 75% on 12/F and 13/F, respectively. The measured and projected illuminance on the floors studied (i.e. 6/F to 13/F) revealed a daylight harvest because the atrium design allows for energy savings from artificial lighting by daylightlinked lighting control or manual on–off status control. The total potential energy saving per year is 43,003 kW h and the annual average CO2, SO2, NOx and particulate emissions could be reduced by 32,252, 440, 42.4 and 2.1 kg, respectively. The results support the assertion that installing daylight-linked lighting controls in daylit corridors could produce remarkable energy savings. These findings can provide much needed information for building professionals, building owners and operators striving to develop their energy conservation strategies and management schemes. Acknowledgements The work described in this paper was fully supported by a grant from the Central Policy Unit of the Government of Hong Kong Special Administrative Region and the Research Grants Council of the Special Administrative Region, China (Project No. CityU1011-PPR-10). References [1] Tiwari GN, Sharma R. Technical performance evaluation of stand-alone photovoltaic array for outdoor field conditions of New Delhi. Appl Energy 2012;92:644–52.
[2] Muneer T, Younes S. The all-sky meteorological radiation model: proposed improvements. Appl Energy 2006;83:436–50. [3] Li DHW. A review of daylight illuminance determinations and energy implications. Appl Energy 2010;87:2109–18. [4] Li DHW, Tsang EKW. An analysis of daylighting performance for office buildings in Hong Kong. Build Environ 2008;43(9):1446–58. [5] Roche L, Dewey E, Littlefair PJ. Occupant reactions to daylight in offices. Light Res Technol 2000;32(3):119–26. [6] Ochoa CE, Aries MBC, Loenen EJV, Hensen JLM. Considerations on design optimization criteria for windows providing low energy consumption and high visual comfort. Appl Energy 2012;95:238–45. [7] Li DHW, Wong SL. Daylighting and energy implications due to shading effects from nearby buildings. Appl Energy 2007;84:1199–209. [8] Linharta F. Performance of anidolic daylighting systems in tropical climates – parametric studies for identification of main influencing factors. Sol Energy 2010;84(7):1085–94. [9] Hung WY, Chow WK. A review on architectural aspects of atrium buildings. Archit Sci Rev 2001;44(3):285–96. [10] Tiwari GN. A modified model for estimation of daylight factor for skylight integrated with dome roof structure of mud-house in New Delhi India. Appl Energy 2010;87:3037–50. [11] Wright J, Letherman K. Illuminance in atria: review of prediction methods. Light Res Technol 1998;30(1):1–10. [12] Aizlewood M. The daylighting of atria: a critical review. ASHRAE Trans 1995;101:841–57. [13] Li DHW, Lam JC. An analysis of lighting energy savings and switching frequency for a daylit corridor under various indoor design illuminance levels. Appl Energy 2003;76(4):363–78. [14] Doulos L, Tsangrassoulis A, Topalis FV. The role of spectral response of photosensors in daylight responsive systems. Energy Build 2008;40(4): 588–99. [15] Knight IP. Measured energy savings due to photocell control of individual luminaries. Light Res Technol 1999;31(1):19–22. [16] Atif MR, Love JA, Littlefair P. Daylighting monitoring protocols and procedures for buildings. A Report of Task 21/Annex 29: Daylight in Building; 1997. [17] Samant S. A critical review of articles published on atrium geometry and surface reflectances on daylighting in an atrium and its adjoining spaces. Archit Sci Rev 2010;53(2):145–56. [18] Beker N, Fanchiotti A, Steemers K. Daylighting in architecture: a European reference book. London: James and James; 1993. [19] Song K, Boyer L. Instrumentation system for evaluating daylighting performance in sunlit atria with design-stage scale models. ASHRAE Trans 1994;100:696–702. [20] Calcagni B, Parongini M. Daylight factor prediction in atria building designs. Sol Energy 2004;76(6):669–82. [21] Electrical and Mechanical Services Department. Code of practice for energy efficiency of lighting installations, 2007 ed. Electrical and Mechanical Services Department, Hong Kong Special Administrative Region, 2007. [22] Kim K, Boyer LL. Development of daylight prediction methods for atrium design. In: Proceedings from the international daylighting conference 2, Long Beach, CA; 1986. [23] Li DHW, Tsang EKW. An analysis of measured and simulated daylight illuminance and lighting savings in a daylit corridor. Build Environ 2005;40(7): 973–82. [24] Chartered Institute of Building Services Engineers. CIBSE Code for Interior Lighting. London: CIBSE; 1994. [25] China Light and Power Co. Ltd. website:. [26] Electrical and Mechanical Services Department. Hong Kong Energy end-use data. Hong Kong Special Administrative Region: Electrical and Mechanical Services Department; 2009. [27] Li DHW, Cheung KL, Wong SL, Lam TNT. An analysis of energy-efficient light fittings and lighting controls. Appl Energy 2010;87:558–67. [28] Littlefair PJ. Predicting lighting energy use under daylight linked lighting controls. Build Res Inform 1998;26(4):208–22. [29] Littlefair PJ. Daylight prediction in atrium buildings. Sol Energy 2002;73(2):105–9. [30] British Standards Institution. Code of practice for daylighting. British Standard BS8206, Part 2. BSI, London; 1992. [31] Littlefair PJ. Average daylight factor: a simple basis for daylight design. Build Res Inform Paper 15/88. Construction Research Communications, Garston, UK; 1998. [32] Lynes JA. A sequence for daylighting design. Light Res Technol 1979;11(2):102–6. [33] Lynes JA, Littlefair PJ. Lighting energy saving from daylight: estimation at the sketch design stage. Light Res Technol 1990;22(3):129–37. [34] Littlefair PJ. The illuminance distribution of an average sky. Light Res Technol 1981;13(4):192–8. [35] Ihm P, Nemri A, Krarti M. Estimation of lighting energy savings from daylighting. Build Environ 2009;44(3):509–14.
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Analysis and prediction of daylighting and energy performance in atrium spaces using daylight-linked lighting controls Stanley K.H. Chow ⇑, Danny H.W. Li, Eric W.M. Lee, Joseph C. Lam Building Energy Research Group, Department of Civil and Architectural Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region
h i g h l i g h t s " Daylight-linked lighting control and energy performance for atrium is studied. " Field measurement of automatic dimming control shows 93% energy saving. " Field measurement of manual on–off control shows 95% energy saving. " Atrium illuminance is correlated with daylight factor for energy saving prediction.
a r t i c l e
i n f o
Article history: Received 13 October 2012 Received in revised form 10 December 2012 Accepted 14 December 2012 Available online 12 January 2013 Keywords: Atrium Daylight Greenhouse gases Daylight-linked lighting controls Energy savings
a b s t r a c t In subtropical Hong Kong, a certain amount of electricity is used to create visually comfortable interior spaces through electric lighting, which is the second major electricity-consuming item in commercial buildings, accounting for 20–30% of total electricity use. The burning of fossil fuels for electricity generation has many adverse effects on the environment. Daylighting is an important and useful strategy for enhancing visual comfort and reducing the need for the electricity consumed by light fittings. The rational use of daylight through tools such as photoelectric lighting controls can effectively reduce buildings’ electricity consumption and the related pollutants and greenhouse gas emissions. Daylighting design techniques are often best demonstrated via field measurements that provide reliable operational and energy performance data for establishing design guidelines. An atrium provides an environmentally controlled indoor public space that introduces daylight into the hearts of large buildings. In circulation areas such as corridors, people expect the way ahead to be sufficiently lit and daylight-linked lighting controls can deliver excellent energy savings. This paper presents the daylighting and energy performance of an atrium space using daylight-linked lighting controls. The cost, energy and environmental issues related to various daylight illuminances are estimated and design implications are discussed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Renewable technologies are substantially safer than their alternatives, offering a solution that meets the present increasing demand for electrical power and addresses many of the environmental and social problems associated with fossil and nuclear fuels. People are paying increasing attention to high-quality, renewable solar energy [1]. Solar energy is freely available and can be easily harnessed to reduce our reliance on hydrocarbonbased energy in both passive and active designs [2]. Daylighting is an effective and sustainable development strategy for enhancing visual comfort, energy-efficiency and green building developments [3]. Daylight is considered the best source of light for good color ⇑ Corresponding author. Tel.: +852 3762 2122; fax: +852 2305 5093. E-mail address: [email protected] (S.K.H. Chow). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.12.033
rendering because it is the one light source that most closely matches the human visual response [4]. People desire high-quality natural lighting in their work environments [5] although it has been noted that discomfort due to visual effects (glare, headaches, deregulation of the circadian rhythm leading to depression) is frequently reported [6]. Electric lighting is the largest component of the internal load in air-conditioned office buildings [7]. Rational uses of daylight, such as photoelectric lighting controls, can effectively reduce buildings’ energy consumption and the related pollutants and greenhouse gas emissions. In a side-lit space, vertical windows can provide a large amount of daylight near the window facades, but the daylight levels dwindle rapidly as the distance from the window increases. Core daylighting, that is, daylight provision in areas situated at considerable distances from façades and windows, is currently one of the main challenges in sustainable building design [8]. An atrium allows daylight to penetrate into
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the core of a building, contributing not only cultural and architectural content, but also the potential to resolve many environmental issues [9]. Daylighting is one of the basic components of passive solar building design and its estimation is essential [10]. Studies have been conducted on the effect of atrium characteristics on daylight levels on the atrium floor and along working planes in adjoining spaces [11]. Daylight may vary substantially with every floor. With shallow atria or the spaces on the top few floors, more incidental light is received from the sky while the daylight levels in deep atria or the lower atria spaces are chiefly influenced by reflected light [12]. In circulation areas such as corridors, people expect the way ahead to be lit sufficiently. With day-lit corridors, daylight-linked lighting controls can deliver remarkable energy savings [13]. Daylighting design techniques are often best elaborated through field measurements that provide reliable operational and energy performance data to establish design guidelines [14]. Empirical data obtained via systematic measurements, however, would help confirm the usefulness, suitability and accuracy of theoretical models and provide much needed information to building professionals [15]. On-site recorded results for computing the energy savings, greenhouse gas reductions, cost implications and visual performances can convince building owners and occupants to implement appropriate operation and management strategies while ensuring that officials implement apposite policies [16]. This study examined the daylighting and energy performance of an atrium building incorporating daylight-linked lighting controls. Economic and environmental benefits are assessed. Daylight factors and illuminance are predicted. Data measurements and analyses are described and reported. The implications for daylighting designs and building energy strategies are also discussed. This can provide the actual savings and benefits to building professionals for designing appropriate daylight-linked lighting controls in atrium and convince the building owners and occupants to implement such daylighting schemes.
Fig. 1. Vertical section of the building.
2. Background information The institutional building featured here is a purpose-built, 13-storey block located on the periphery of a Hong Kong industrial zone situated along the southern coast of China within the subtropical region, at a latitude of 22.3N and a longitude of 114.2E. The institution is surrounded by buildings in three cardinal orientations (i.e. north, south and west), with a highway to the east. 2.1. Atrium geometry The building was designed with a skylight and an enclosed, stepped atrium to harvest daylight and provide natural ventilation through environmental conservation measures. Fig. 1 shows the vertical section of the building. The stepped atrium increases daylight and improves sky views by splaying the well walls away from the vertical, but the sky component, and hence the daylight factor, of the lower floors are significantly reduced [17]. The atrium dimensions are 11.5 m 9.5 m 40.7 m (height). The lower part of the atrium from 2/F to 9/F consists of typical classrooms, with four open corridors surrounding the atrium. The upper part, between 10/F and 13/F, is a cubic atrium (i.e. 15 m 15 m 15 m) and contains mainly mechanical and office rooms. There is a glazed skylight with a building integrated photovoltaic (BIPV) system installed on the atrium roof aperture. The skylight constitutes around 40% of the plan area obstruction (PAO), which is defined as the percentage of roof blockage when viewed from a plan perspective with the focal point at infinity. Fig. 2 displays the skylight and the PAO. The reduction of light caused by the glazing ranges from 20% (10% for the structure and 10% for the glazing) [18] to as high
Fig. 2. Skylight and the PAO.
as 80% [19]. It has been suggested that the addition of a glazed roof over an atrium well can reduce daylight levels in spaces adjacent to the well by 45% [20].
2.2. Lighting system The daylighting performance and energy use based on daylightlinked lighting controls were evaluated in the 9/F corridors. The atrium corridors are used for circulation among different classrooms,
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Fig. 3. Photoelectric senor at 9/F.
with dimensions of 2.65 m (width), 14.65 m (along the atrium) and 2.22 m (height). The lighting system contains 67 ceiling-mounted, energy-efficient T5 fluorescent tubes, with rated power ranging from 14 W to 35 W. Each T5 fluorescent tube is regulated by one dimmable high frequency electronic regulating ballast that can dim the lamp output smoothly and uniformly. The lighting circuitry for each corridor is separated. The lamp fittings can be dimmed from 1% to 100% via the analog control input. The maximum lighting load is 2261 W plus electronic ballast load. The lighting power density, defined as the electrical power consumed by lighting installations per unit floor area in the corridor, is 14.6 W/m2.
3. Daylight-linked lighting controls 3.1. Illuminance and energy savings The illuminance level of the electric lighting was measured at night (i.e. around 8 pm) when all of the lights were on and no occupants were walking through the corridors. The illuminance levels
along the midway of the four corridors were recorded. The mean illuminance was found to be around 200 lx. The mean luminous efficacy for 14 W and 35 W fluorescent tubes is 95 and 101 lm/W, respectively, which is higher than the minimum allowable efficacy at 87 and 94 lm/W [21]. To provide the minimum pre-set lighting level along the corridors, one adjustable photoelectric sensor with a measuring range from 4 to 1600 lx was mounted on the ceiling of each of the four corridors and used to regulate and record the light intensity. The vertical distance between the top of the atrium and the light sensor was 15.4 m. Four sensors were installed in the corners of the corridors to take illuminance measurements [22], as shown in Fig. 3. They detected both daylight and reflected light from the corridor surfaces to provide a ‘closed loop’ control dimming system. Because it is a narrow plan walkway, a single-zone control was considered adequate [23]. The lighting level received was transmitted to a differential dimming controller through C-bus protocol to vary the light output of the fluorescent lamps using the dimmable electronic ballasts. The illuminance was recorded at 5-min intervals, daily, from 9 am to 6 pm between January and June 2012. Setting aside any missing data and including instrumentation malfunction and power failure, 77,000
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Fig. 6. Average illuminance of the 4 zones.
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" # Ev El 3
ð1Þ
r
where Ev is the illuminance (lx), El is the mean illuminance (lx), r is the standard deviation (lx) and E is the expectation operator (dimensionless). The skewness value for Zones 1–4 are +1.39, +0.81, +2.26 and +0.41, respectively. A positive skew value indicates that the frequency on the right side is lower than on the left side, such that the bulk of the values lie to the left of the mean. This skewness can be explained by the overall lower illuminance level harvested. For daylighting design and calculation, a cumulative frequency distribution of the indoor daylight illuminance can indicate the percentage of the working year in which a given illuminance is exceeded. This is valuable for estimating the necessary use of artificial lighting and the probable energy savings delivered by the daylight-linked lighting controls. Fig. 8 presents the cumulative frequency distribution of illuminance levels in the 4 zones. Although Zone 4 scored the highest illuminance level, Zone 1 scored
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measurements were recorded. The illuminance received was also transmitted to a light intensity level logging system for recording. The measurement of electric lighting consumption was conducted using a single-phase AC power management system that included four power meters. Each power meter displays the readings in kW h with built-in software to capture the measured data from the power meter and transmit them to a microcomputer for storage and subsequent analysis. The data were recorded at 5-min intervals, daily, from 9 am to 6 pm between January and June 2012 when the illuminance set-point was set to 100 lx. This is in accordance with the Chartered Institute of Building Services Engineers (CIBSEs) Code for Interior Lighting [24], which states that circulation areas such as corridors should have a design illuminance level of 100 lx. Fig. 4, which displays the respective hourly illuminance averages reveals that the daylight illuminance begins to increase in early morning, reaches its peak value at around noon and gradually decreases in the afternoon. The minimum illuminance was 98 lx in Zone 3 at 6 pm while the maximum was 1280 lx in Zone 4 at 12 noon. The average illuminance ranged from 128 lx at 6 pm to 726 lx at 12 noon. Fig. 5 shows the monthly illuminance averages, which indicate that the daylight illuminance increases from January to May and decreases in June. This drop in June can be explained by the increased frequency of rain in June compared to May. Similar to the hourly average, the illuminance in Zone 4 is maintained at a higher level than in the other zones. The minimum monthly average illuminance was 222 lx in Zone 3 in January while the maximum was 887 lx in Zone 4 in May. The overall average illuminance ranged from 306 lx in January to 640 lx in May. Fig. 6 displays the average illuminance of the four zones. The average illuminance ranged from 267 lx in Zone 3 to 658 lx in Zone 4 while the overall illuminance was 452 lx. The average illuminance in Zone 3 (west) was far less than in the other zones, probably because there is a lift machine room located in the eastern portion of the roof area that screens a certain amount of daylight in this zone. Fig. 7 presents the frequency distribution for the illuminance levels of the four zones, respectively. The asymmetry of the frequency distribution is evident in the left skewness of the illuminance levels in Zones 1 and 2, representing a higher proportion of the illuminance level at the lower band. The left skewness is further enhanced in Zone 3, where 7300, 11,000 and 13,200 out of 40,000 records were at 100, 150 and 200 lx, respectively. The skewness c is statistically and quantitatively expressed by:
Illuminance Level (lx) Fig. 8. Cumulative frequency distribution of illuminance levels of the 4 zones.
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Energy (kWh)
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Fig. 9. Monthly daily average energy saving.
highest frequency at a set-point of 100 lx. This means that the energy savings at a set-point of 100 lx in Zone 1 are greater than in other zones. However, it should be noted that if the set-point is at a higher level, say 200 lx, then Zones 2 and 4 will score higher frequencies than Zone 1.
MPBP was computed to be 3.42 years. For an energy saving strategy in practice, one set of equipment is sufficient and the capital cost is HK$6620 (US$849). Accordingly, the MPBP was computed to be 0.86 year. Pollutants created by electricity generation are considered one of the main causes of the local air pollution. Based on the electricity and gas emissions produced by one of the local power companies, as of 31 December 2011 [25], the reductions in various emissions were projected. Using the dimming controls for 9/F, the projected annual energy saving is 7585 kW h and the annual average CO2, SO2, NOx and particulate emissions could be reduced by 5869, 78, 7.5 and 0.37 kg, respectively. Given that in Hong Kong most electricity is consumed by building stocks [26] and the atrium is a key feature in modern architecture, the environmental benefits of widely applying daylight-linked lighting controls in Hong Kong would be considerable.
4. Prediction and potential savings 4.1. Lighting control by on–off status
Cost, energy and environmental issues are often used to assess the performance of solar energy systems. In Hong Kong, electricity is mainly produced by two local power companies using fossil fuels. The pollutants created by electricity generation are considered one of the main causes of the local air pollution. The economic benefits in terms of the monetary payback period (MPBP) of the lighting system of the institutional building with atrium design under study can be evaluated by comparing the electricity tariff saved to the capital cost of the system (i.e. daylight linked lighting controls). Fig. 9 displays the monthly daily average energy savings. The daily energy savings ranged from a low of 20.5 kW h in January to a high of 21.3 kW h in June and their respective energy savings percentages were 91% and 94%. The overall energy saved was 93% or 21 kW h in the four corridors on 9/F with daylight-linked lighting control systems when the illuminance was set at 100 lx. Fig. 10 displays the energy savings by zone. The energy savings from Zones 1–4 were 957, 744, 823 and 908 kW h, respectively. Zone 1 scored greater energy savings than Zone 4, even though the average illuminance of Zone 4 was higher than that of Zone 1. This result echoes the result in Fig. 8 when Zone 1 scored a higher cumulative frequency at a set-point of 100 lx. In addition, the energy savings for Zone 3 were also higher than those for Zone 2, even though the average illuminance of Zone 3 was the lowest overall. The total energy saved during the measured period was 3429 kW h. Using the current commercial general electric tariff of HK$1.02 (US$0.131) per kW h for Hong Kong, the savings in electricity charges for the monitored period of 165 days was computed to be HK$3498 (US$448). The annual savings in electricity charges was projected to be HK$7738 (US$992). The current system involves four sets of equipment to measure the illuminance of the four zones. The total capital cost of light level sensors, dimmer controls and cable and conduits was HK$26480 (US$3395). The
An examination of the lighting control by on–off status was conducted in the 10/F corridor of the building. An on–off control is designed to switch electrical lighting on and off automatically as the lighting levels rise and fall within a set of predetermined levels [27]. A special problem with the on–off lighting controls is rapid switching, particularly during unstable weather conditions when the daylight varies around the switching illuminance level. This can annoy occupants and reduce lamp life [10]. However, on–off lighting controls are more suited to well-lit environments with low set-point illuminance levels. A variant of the on–off control is the ‘differential switching’ or ‘dead-band’ photoelectric control that has two switching illuminances: one at which the lights are switched on, and another, higher illuminance at which the lights are switched off. The main advantage of this is that it can reduce rapid switching when the illuminance swings around the desired level [28]. As Fig. 11 shows, the lighting system contains 31 wall-mounted, energy-efficient compact fluorescent tubes with a rated power of 32 W each. The maximum lighting load is 992 W plus the electronic ballast load. The compact fluorescent tubes were wired in three separate circuitries. The on–off switching was manually operated according to visual judgments of the illuminance levels. The illuminance in the corridor was measured hourly from 9 am to 6 pm for seven consequent days per month
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Fig. 11. Compact fluorescent tubes at 10/F.
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To study the relationship between the daylight illuminance and the depth of the atrium, three ceiling-mounted photoelectric light level sensors were installed near the corner of two corridors in Zones 3 and 4 from 6/F to 8/F along the same plummet line, to measure the illuminance. Fig. 13 shows the locations of the photo sensors. The vertical distances of the sensors, measured from the skylight to 8/F, 7/F and 6/F were 18.7 m, 22.0 m and 25.3 m, respectively. The illuminance was recorded at 5-min intervals, daily, from 9 am to 6 pm between January and June 2012. The data were transmitted to an illuminance level logging system for recording. Fig. 14 shows the daily mean illuminance for the three floors. The daily illuminance average dropped from 437 lx at 8/F, to 332 lx at 7/F and to 249 lx at 6/F. An inverse relationship between the recorded illuminance levels and the vertical distance was observed. Fig. 15 displays the hourly average illuminance values for the three floors. Generally, the daylight illuminance began to increase in the early morning, reached the peak value at 12 noon and gradually decreased in the afternoon. The higher floor (i.e. 8/F) reached a higher level of illuminance than the lower floors. The illuminance scored a minimum value of 108 lx at 6 pm and a maximum value of 438 lx at 12 noon on 6/F; a minimum of 128 lx at 6 pm and a maximum of 641 lx at 12 noon on 7/F; and a minimum of 169 lx at 6 pm and a maximum of 788 lx at 12 noon on 8/F. The average illuminance was 249, 332 and 437 lx on 6/F, 7/F and 8/F, respectively. Fig. 16 displays the monthly averages of illuminance values for the three floors. The daylight illuminance increases from January to
26
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from April to June 2012 by a hand-held light-level meter (Testo 540, manufactured in Germany) which is capable of measuring from 0 to 99,999 lx. The energy for a full lighting load during the measuring period was estimated to be 208.3 kW h. Totally 210 measurements were recorded. The average illuminance was computed to be 687 lx. The result showed that 199 of the measurements met the CIBSE recommended illuminance level of 100 lx for circulation areas, whereas 11 measurements were below 100 lx when much of the daylight was obstructed by clouds during overcast conditions and in the evening hours. It has been reported that there is no simple analytical expression for lighting energy savings under such an on–off control [28]. For daylighting designs and calculations, a cumulative frequency distribution of daylight availability can be used to determine the necessary use of electrical lighting and the probable energy savings from on–off controls [29]. Fig. 12 displays the cumulative frequency distribution of daylight availability and reveals that around 95% of the time, the artificial light fittings could be switched off at a set-point of 100 lx. The potential energy saving over the monitored 21 days and a whole year could be 198 and 3440 kW h, respectively. Financially, the annual electricity tariff saving was projected to be HK$3509. In terms of environmental issues, the annual CO2 emissions could be reduced by 2580 kg.
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Hour of Day Fig. 15. Hourly average of the illuminance.
May and decreases in June. The drop in June can be explained by more frequent rain in June than in May. Similar to the hourly average, the higher floor (i.e. 8/F) reached a higher level of illuminance than the lower floors. The maximum monthly average illuminance was 600 lx on 8/F in May while the minimum was 168 lx on 6/F in January. The overall average illuminance ranged from 249 lx on 6/F to 437 lx on 8/F. Fig. 17 plots the cumulative frequency distribution for the daylight illuminance levels on the three floors. The overall illuminance level on 8/F scored a higher cumulative frequency at various set-points, thus distinguishing it from the cumulative
S.K.H. Chow et al. / Applied Energy 112 (2013) 1016–1024
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Fig. 16. Monthly average of the illuminance.
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Fig. 19. Prediction of daylight factor and illuminance.
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120% 100%
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Illuminance Set Point (lx) Fig. 17. Cumulative frequency distribution at various illuminance set-points.
frequency distribution for the daylight illuminance levels on 9/F. As the recommended illuminance level for a corridor is 100 lx, the cumulative percentages around this level are highlighted in Fig. 18. The cumulative percentages are around 88%, 84% and 78% for 8/F, 7/F and 6/F, respectively. The projected annual energy savings, if the same lighting system implemented in the 9/F corridors were installed, would be 7263, 6933 and 6432 kW h for 8/F, 7/ F and 6/F, respectively.
daylight factors from 6/F to 10/F is 0.965, indicating that 96.5% of the variation in illuminance levels can be explained by the changes in daylight factors. The illuminance on the atrium sides from 11/F to 13/F were extrapolated by correlating the DF ratios between the floors. The results show that the high illuminance allows daylight to provide sufficient illuminance on the respective floors so that artificial lighting can be reduced. Because 11/F is a mechanical floor, there is no glazed area facing the atrium. As Figs. 20 and 21 illustrate, 12/F and 13/F are office floors with three sides facing the atrium and having glazed areas. The total glazed area is about 25%. There are 20 lighting panels with a total power of 2400 W on each on 12/F and 13/F at a center distance of 800 mm from the glazing facing the atrium. The total estimated energy consumption is 14,400 kW h per year. As the office cubicles are shallow in dimension, single-zone evaluation is adopted in this study of the daylight performance [33]. The average interior daylight Ein in individual office cubicles on 12/F and 13/F can be predicted by combining the Littlefair Eq. (2) [34].
Ein ¼ 4.3. Prediction of daylight factor and illuminance Predicting daylight levels within an atrium building can sometimes be difficult or uncertain [10]. The British Standard Code of Practice for daylighting [30] uses the average daylight factor (DFavs) [31,32] as a way to judge a daylit space. The daylight factor is the instantaneous ratio of interior illuminance at the measurement points, to the exterior illuminance on a horizontal plane due to an unobstructed sky. It can be predicted using scale model measurements or computer simulations. When studying an actual building, analytical formulae could be used as another way to estimate the daylight in an atrium. In this study, the DF from 6/F to 13/F were predicted using the Littlefair formula [29] Appendix A, which uses a sky component and an internally reflected component. Fig. 19 displays the relationships among the floor levels (i.e. vertical distance from atrium aperture), daylight factors and the daily average illuminance levels. The coefficient of determination between the 5-min interval measured illuminance levels and the
Cumulative Frequency of Time for Saving (%)
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Illuminance Set Point (lx) Fig. 18. Cumulative frequency distribution of illuminance at set-point of 100 lx.
Ev tW Að1 RÞ
ð2Þ
where Ev is the predicted daylight in the atrium; t and W are the visual transmittance and area of the window, respectively; A is the total interior surface area for the office; and R is the overall mean reflectance of the office. According to the CIBSE Code for Interior Lighting [24], working places hosting general tasks such as office work should have a design illuminance level of 500 lx. Based on the physical parameters of the two floors, the ratio of Ev to Ein is 5.37. Hence, a daylight illuminance level of 2685 lx is required to fulfil the Code. The predicted daylight illuminance is 1786 and 2275 lx for 12/F and 13/F, respectively. The predicted average interior daylight in individual offices on 12/F and 13/F are 333 and 424 lx, respectively, which suggest that about 68% and 87% of the energy used for artificial lighting can be saved on 12/F and 13/F, respectively. However, the high frequency dimming control in the present study does not have the ideal characteristic of light output being perfectly proportional to the power consumed. The light output can be roughly assumed to be proportional to the power consumed, but the lamps cannot be dimmed to total extinction [28]. In normal operation, their residual light output and power consumption occur throughout working hours, even if the illuminance level far exceeds the design value. However, such operations may be less noticeable and less disturbing to occupants [28]. There is a linear correlation between the light output and the input power for ideal operation of the high frequency dimming controls. Whenever the total illuminance on the working plane (daylight plus electric light for a closed loop control) exceeds the desired illuminance, the lamps will be dimmed to the minimum light output fraction with the corresponding power fraction. For other conditions, the lighting power consumption can be computed with the input values of the minimum light output fraction, the power fraction, the target illuminance, the working plane illuminance and the full light output
1023
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Fig. 20. 12/F office layout.
for energy saving from artificial lighting through daylight-linked lighting control or manual on–off status control. Fig. 22 displays the total energy savings per year. The energy saving for 10/F is much lower than for other floors because the daylight harvest was incorporated into the lighting system in the building design stage. The lighting power density of the 10/F corridor is 6.4 W/m2, which is 44% that of the 9/F corridor. The energy savings for 12/F and 13/F were not higher than those for the lower floors because 12/F and 13/F are used for offices, which require higher illuminance levels than the circulation areas on the lower floors. The total potential energy savings per year represented by the total area of the histogram is 43,003 kW h. The annual saving in electricity charges was projected to be HK$43863 (US$5623). If light level sensors and dimmer controls were installed on all of the floors studied, then the capital cost was estimated to be HK$46340 (US$5941). Accordingly, the MPBP was computed to be 1.06 years.
4.4. Prediction of overall potential benefits
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The measured and projected illuminance on the floors studied (i.e. from 6/F to 13/F) of the institutional building with atrium design revealed a daylight harvest because the atrium design allows
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Fig. 22. Total energy saving per annum.
13/F
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illuminance based on the linear correlation [28,35]. Given this power fraction, 59% and 75% of the energy used for artificial lighting can be saved on 12/F and 13/F, respectively. This illustrates that the power fraction of the high frequency dimming control delivers a 16% deduction of overall energy savings.
Energy Saving (kWh)
Fig. 21. Fenestration of 12/F and 13/F.
1024
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The annual average CO2, SO2, NOx and particulate emissions could be reduced by 32,252, 440, 42.4 and 2.1 kg, respectively. 5. Conclusions Atriums in buildings have been widely used as environmental conservation measures. Effective control of the electric lighting in an atrium is essential if the full benefits of daylight are to be realized. The daylighting and energy performance of an atrium using daylight-linked lighting controls was evaluated and around 93% of the energy spent could be saved in the four corridors on 9/F with a daylight-linked lighting control system when the illuminance was set at 100 lx. Annually, a lighting energy of 7585 kW h could be saved and the average CO2, SO2, NOx and particulate emissions could be reduced by 5869, 78, 7.5 and 0.37 kg, respectively. The monetary payback period was determined to be 0.86 year. Another study on the lighting control by on–off status was conducted in the building’s 10/F corridor. When the illuminance set-point was 100 lx, around 95% of the artificial lighting time could be switched off. The projected annual savings was estimated to be 3440 kW h. A further study was conducted using three ceiling-mounted photoelectric light level sensors installed near the corner of two corridors in Zones 3 and 4 from 6/F to 8/F along the same plummet line to measure the light intensity level. The recorded illuminance in these areas began increasing in the early morning, reaching the peak value at around noon and gradually decreasing in the afternoon. The cumulative percentages were around 88%, 84% and 78%, respectively, for 8/F, 7/F and 6/F with an illuminance level of 100 lx for a corridor. The projected annual energy savings are 7263, 6933 and 6432 kW h, respectively. The measured illuminance levels between 6/F and 10/F closely match those of the daylight factors predicted by formulae. The illuminance on the atrium sides from 11/F to 13/F were extrapolated by correlating the daylight factor ratios between the floors. The predicted average interior daylight levels in individual offices on 12/F and 13/F were 333 and 424 lx, respectively, which suggests that 68% and 87% of the lighting for 12/F and 13/F could be switched off to save energy if a design illuminance level of 500 lx for general office work was adopted. However, if the power fraction of the high frequency dimming control is considered, then the energy saved from not using artificial lighting is reduced to 59% and 75% on 12/F and 13/F, respectively. The measured and projected illuminance on the floors studied (i.e. 6/F to 13/F) revealed a daylight harvest because the atrium design allows for energy savings from artificial lighting by daylightlinked lighting control or manual on–off status control. The total potential energy saving per year is 43,003 kW h and the annual average CO2, SO2, NOx and particulate emissions could be reduced by 32,252, 440, 42.4 and 2.1 kg, respectively. The results support the assertion that installing daylight-linked lighting controls in daylit corridors could produce remarkable energy savings. These findings can provide much needed information for building professionals, building owners and operators striving to develop their energy conservation strategies and management schemes. Acknowledgements The work described in this paper was fully supported by a grant from the Central Policy Unit of the Government of Hong Kong Special Administrative Region and the Research Grants Council of the Special Administrative Region, China (Project No. CityU1011-PPR-10). References [1] Tiwari GN, Sharma R. Technical performance evaluation of stand-alone photovoltaic array for outdoor field conditions of New Delhi. Appl Energy 2012;92:644–52.
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