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Harvesting daylight with LED or T5 fluorescent lamps? The role of dimming
Accepted Manuscript Title: Harvesting daylight with LED or T5 fluorescent lamps? The role of dimming Authors: L.T. Doulos, A. Tsangrassoulis, P.A. Kontaxis, A. Kontadakis, F.V. Topalis PII: DOI: Reference:
S0378-7788(16)31177-X http://dx.doi.org/doi:10.1016/j.enbuild.2017.02.013 ENB 7374
To appear in:
ENB
Received date: Revised date: Accepted date:
13-10-2016 20-1-2017 6-2-2017
Please cite this article as: L.T.Doulos, A.Tsangrassoulis, P.A.Kontaxis, A.Kontadakis, F.V.Topalis, Harvesting daylight with LED or T5 fluorescent lamps? The role of dimming, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2017.02.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Harvesting daylight with LED or T5 fluorescent lamps? The role of dimming L.T. Doulos* 1,2, A. Tsangrassoulis3, P.A.Kontaxis2,4, A. Kontadakis3, and F.V.Topalis2 1,*
School of Applied Arts, Lighting Design, Hellenic Open University, Patra, Greece, [email protected]. *corresponding author 2 Lighting Laboratory, National Technical University of Athens, Athens, Greece 3 University of Thessaly, Dept. of Architecture, Volos, Greece 4 Technological Educational Institute of Athens, Greece
Abstract While the efficiency of the Lighting Emitting Diodes (LED) is increasing and new installations are being implemented, their control due to the adoption of daylight harvesting systems, hasn’t been sufficiently examined yet, although this can reduce further energy consumption. A crucial parameter for achieving improved performance is the proper matching of a dimming system with the luminaires used, since this can significantly affect energy savings not only among various luminaires with T5 tubular fluorescent lamps but also among various LED luminaires. Furthermore, dimming can affect the power quality of the lighting installation depending on the technology used, making it an important factor during the selection phase. The aim of the present paper is to quantify energy savings among different LED and T5 fluorescent luminaires that are commonly used with daylight harvesting systems in offices and investigate their impact on power quality. Measurements using four commercial LED and four T5 luminaires were performed estimating the mathematical function which correlates their light output with their associate power consumption. Using the measured data, a set of hourly annual simulations were performed for a photosensor installed in a typical office room, quantifying the relative differences in energy savings among various cases examined together with the power factor values. Keywords Daylight harvesting, Dimmable LED, Dimmable T5 fluorescent lamps, Energy savings, Lumen maintenance, Photosensor, Power Factor. Highlights
Best-fit functions between consumed power and light output (LO, %) are introduced not only for luminaires with T5 lamps and Electronic Dimmable Ballasts (EDBs) but also for LED luminaires with dimmable drivers.
A new methodology for the determination of energy savings differences between different EDBs and LED dimmable drivers is proposed.
The scope of the methodology is the optimum selection of luminaires with regard to the amount of energy savings during daylight harvesting or lumen maintenance control strategy.
The role of dimming concerning the fluctuation of Power Factor (PF) during a year using daylight harvesting strategy is investigated.
1. Introduction Lighting comprising a large percentage of the buildings energy’ balance [1, 2]. While trying to reduce the energy consumption, a large number of new or retrofitting projects are realized, based mainly on the concept of improving the luminaire’s luminous efficacy. Although the retrofitting approach for energy savings can be influenced by a number of parameters that are irrelevant to the lighting technology, such as the annual financial turnover [3], it seems that daylight controls are frequently excluded. This is because design teams want to simplify the retrofit project, lowering its cost, even though daylight harvesting can offer significant opportunities for energy savings. The adoption of daylight controls is negatively affected by a number of reasons such as the complex design process, the unreliability of the calculation methodologies and the users’ acceptance [4]. Although there is a large number of studies in literature examining techniques which reduce lighting energy consumption [5-16], the number of studies examining possible energy savings through daylight harvesting and LED sources is quite limited [17,18]. Pandharipande and Caicedo [17] proposed an energy efficient illumination control of a LED system using a number of photosensors and examining various occupancy scenarios. Wang and Tan [18] also used LED fixtures in order to exploit the dimming procedure and to develop an overall illumination control based on a neural network mapping model. Daylight responsive dimming systems controlled by photosensors adjust the electric lighting level according to the amount of daylighting impinging on the photosensor. Thus, the
daylight availability and the potential energy saving estimation tools are crucial for the final decisions. Yu and Su [19] have categorized the estimation tools into four groups: software simulations, field measurements, empirical formulae and estimation algorithms. However, sometimes the obtained energy savings aren’t as high as expected because of the users’ interaction [20]. Thus, tuning up the lighting system, according to the occupants needs can result in higher energy savings [21]. Dimming can also have an impact on power quality. Chiradeja et al [22] examined the effects of harmonics for both electromagnetic and electronic ballasts for T8 lamps and for electronic ballasts for T5 lamps, estimating energy savings from the corresponding replacement. Albu et al. [23] performed light and power quality measurements and showed that LED luminaires have greater luminous efficacy than luminaires with tubular florescent lamps but In terms of power quality, the current harmonic distortion level was attenuated if the light sources were dimmed. However, on what concerns the color temperature between T5 fluorescent lamps and LED fixtures no significant differences were found and they were rated equally pleasant for the same illuminance level [24]. While there is not a standard method to evaluate and characterize LED dimming performance [25] there are also other important features that must be considered such as dimming range, drop out, flicker, color shift, and noise [26]. The power factor of LED is high when they function without dimming [25] while it generally reduced as LEDs are dimmed [27]. The present paper examines and compares the relative differences in energy savings and power factor of a number of dimmable LED luminaires and T5 fluorescent lamps when these are dimmed with photosensors. 2. Measurements Two groups of luminaires were tested, one group (I) of LED luminaires and one group (II) of T5 tubular fluorescent lamps:
Group I: LED downlight luminaires from four different manufactures. o
1 X 17 Watt Chip on Board (COB) LED
o
1 X 20 Watt Chip on Board (COB) LED
o
1 X 20 Watt Chip on Board (COB) LED
o
1 X 22 Watt Chip on Board (COB) LED
Group II: T5 tubular fluorescent lamps from four different manufactures [6]. o
1 X 28 Watt T5 tubular lamp
o
2 X 28 Watt T5 tubular lamps
o
1 X 49 Watt T5 tubular lamp
o
1 X 54 Watt T5 tubular lamp
The measurements took place in the Photometry Laboratory of the National Technical University of Athens. The test space was a dark room (20m x 7m x 4m) with black mat painted surfaces (Figure 1). The ambient temperature was maintained constant at 26oC and input voltage at 230Vac, 50 Hz through the voltage stabilizer for all tested conditions [6]. All luminaires were aged and the appropriate preheat time was applied before the first measurement. Suggestions for avoiding lighting sources emitting cold white light were taken into account for the experiments of the present paper [28], thus all selected lighting sources had CCT of 4000K.
Figure 1. LED luminaire (left photo) on goniophotometer A and T5 luminaire (right photo) attached on goniophotometer B.
Photometric and electric measurements were taken for various dimming levels. Every luminaire was manually dimmed using the DALI protocol (either through a PC interface (group I) or 0V to 10V digital converter (group II)), from a maximum to a minimum control signal in tandem procedure [6, 14]. The measurement of the luminous intensity was performed according to the CIE standards No. 121 (1996) and No. 70 (1987) [29, 30]. Two types of goniophotometers (A and B) were used. Every luminaire of the group I was attached to the goniophotometer A at a distance of 2.18 m from the head of the photometer (Figure 1, left) while for group II luminaires were attached to the goniophotometer B at a distance of 9.90 m (Figure 1, right). The light output was measured as an illuminance value at the same distance from the luminaires for each goniophotometer. In both cases, the photometer sensors were placed inside a mat black painted cylinder with baffles, protected from stray light. The field of view of the photometer sensor covered the entire emitting surface of each tested luminaire. The measurements of the light output were monitored every 5 minutes
and were taken only if the light output was stabilized, by testing if the standard deviation between the last two measurements was less than 2%. This procedure varied from 15 min to 20 min for all cases [6, 31]. A single-phase power analyzer was used for the measurement of active, reactive and apparent power as well as of the power factor for various dimming levels of the tested LED drivers and EDBs. The dimming range was calculated for each individual luminaire from an upper limit of 100% (maximum dimming level) to a minimum level near of 0%. The values were the same when the lamps were measured from maximum to minimum light output (decreasing control signal from 100% to 0%) or from minimum to maximum light output (increasing control signal from 100% to 0%). In all cases, no LED or T5 lamp was extinguished before reaching the reported minimum light output. 2.1 Light output and relative consumed power versus control signal Figures 2 to 5 present each luminaire relative light output and consumed power against the control signal. The test results show significant deviations between the luminaires of group II. Most of the tested drivers and electronic dimming ballasts (EDBs) showed a non-linear relationship. Linear relationships can be assumed only for specific LED drivers and for a specific range of control signals.
Figure 2. Light output versus control signal for group I luminaires (LED luminaires).
Figure 3. Light output versus control signal for group II luminaires (T5 luminaires).
Figure 4. Relative consumed power versus control signal for group I luminaires (LED luminaires).
Figure 5. Relative consumed power versus control signal for group II luminaires (T5 luminaires). 2.2 Relative consumed power versus light output Figures 6 and 7 present the relative consumed power against light output percentage for all luminaires. It is evident that the same light output corresponds to different consumed power even for luminaires of the same test group. For example in group I, when 40% relative light output is needed, luminaire D LED has 47.5% relative consumed power while luminaire A LED gives 38.5% respectively. (Figure 6). Using the same aforementioned light output (40%), luminaire A T5 (group II) has 49.5% relative consumed power, while luminaire B T5 has 41.6% (Figure 7). It seems that when both group luminaires are dimmed near 0%, with LED drivers, they consume less energy when compared to EDBs. Table 1 shows the polynomial interpolation results between normalized power consumption (y) and normalized light output (x). Both parameters varied between 0 to 1. In both groups, the coefficient of determination (R2) is higher than 0.999 indicating that data are quite close to the fitted polynomial function. These best-fit functions can be used to predict energy savings when a daylight harvesting system or a system with lumen maintenance capabilities is adopted.
Figure 6. Relative consumed power versus light output for group I luminaires (LED luminaires).
Figure 7. Relative consumed power versus light output for group II luminaires (T5 luminaires). Table 1. Derived functions from polynomial interpolation between light output (%) (x, 0 to 1) and consumed power (y, 0 to 1), for both groups I and II. Group I I I I II II II
Luminaire A LED B LED C LED D LED A T5 B T5 C T5
II
D T5
Derived functions y=-0.257x +0.658x -0.357x2+0.914x+0.041 y=-0.335x4+0.836x3-0.542x2+0.968x+0.072 y=0.733x4-1.065x3+0.519x2+0.742x+0.068 y=-0.949x4+2.467x3-2.205x2+1.650x+0.034 y=2.004x5 - 5.135x4 + 4.858x3 - 2.053x2 + 1.164x + 0.158 y=8.257x6-21.977x5+21.863x4-9.299x3+1.211x2+0.815x+ 0.123 y=0.941x4 -1.623x3+0.843x2+0.623x+0.196 y=-11.655x6+36.079x5-40.844x4+20.876x3-4.795x2+1.218x+ 0.122 4
3
R2 0.9999 0.9999 0.9996 0.9997 0.9999 0.9999 0.9999 0.9999
2.3 Power factor versus control signal In addition to the photometric and electric measurements for various dimming levels, power factors were measured as well (Figures 8 and 9), within the whole dimming range for all
tested luminaires. Power factor values ranged from 0.96 at maximum light output to 0.35 at minimum light output for group I luminaires, while, for group II the values ranged from 0.99 to 0.63 respectively. All LED luminaires in group I performed poorly with luminaire A LED being the best among them (ranging from 0.94 to 0.37) and luminaire D LED being the worst of all (ranging from 0.91 to 0.35). The differences between the two groups are not only due to the different operational principle between the LED drivers and EDBs but also due to the different technology between LEDs and T5 lamps. Table 2 presents the power factor (PF ranging from 0 to 1) as a function of a normalized control signal (nCS ranging from 0 to 100%) as this was derived from a polynomial interpolation. Coefficients of determination values (R2) are quite high, ranging from 0.9918 to 0.9997 for group I luminaires and from 0.9928 to 0.9973 for group II luminaires.
Figure 8. Power factor values versus dimming percentage for group I luminaires (LED luminaires).
Figure 9. Power factor values versus dimming percentage for group II luminaires (T5 luminaires).
Table 2. Power factor (y, 0 to 1) as function of control signal (x, 0 to 1) for both groups I and II. Group
Luminaire
I
A LED
I
B LED
I
C LED
I
D LED
II
A T5
II
B T5
II
C T5
II
D T5
Derived functions 6 5 y = -5,343x + 17,354x - 22,587x4 + 15,864x3 - 7,249x2 + 2,533x + 0,370 y = -8,654x6 + 29,995x5 - 40,636x4 + 27,687x3 - 10,776x2 + 2,982x + 0,363 y = -10,922x6 + 36,079x5 - 46,652x4 + 30,634x3 - 11,745x2 + 3,209x + 0,360 y = 38,262x6 - 118,950x5 + 138,040x4 - 71,764x3 + 14,135x2 + 0,910x + 0,276 y = -4,540x6 + 8,062x5 + 1,789x4 - 11,039x3 + 6,685x2 - 0,610x + 0,635 y = 9,620x6 - 31,847x5 + 40,273x4 - 23,681x3 + 5,844x2 - 0,044x + 0,823 y = 4,905x6 - 15,991x5 + 18,447x4 - 8,951x3 + 1,872x2 - 0,072x + 0,756 y = 8,425x6 - 24,810x5 + 25,831x4 - 11,396x3 + 2,382x2 - 0,172x + 0,722
R2 0.9997 0.9992 0.9997 0.9918 0.9973 0.9928 0.9966 0.9949
3. Simulations The simulations were performed using DAYSIM software [32] which has been used in previous photosensor studies to calculate daylight levels on the working surface and the ceiling sensor [6, 33, 34]. The room that was used for the simulations is a typical space in an office building with dimensions of 3.5 x 5.4 x 2.7 m with one external façade [35, 36] and openings on the south exposed façade (Figure 10). Two types of electric lighting systems were examined using six surface mounted luminaires in a uniform layout. The first one uses group I LED luminaires (Figure 11) while the second one uses group II luminaires with parabolic louvers.
Figure 10. Vertical cross section of the office room and location of the photosensor (left) and layout of south façade of the office room (right).
The installed power was lower than 16 W/m2 (8-9W/m2 for LEDs and 11-12W/m2 for T5 lamps) according to the Greek regulation for the energy performance of buildings and EN 15193-2007 [37, 38] for the two systems while the average maintenance lighting levels on the working surface (0.80 m height) were above 500 lux with a predefined design illuminance of 500lux and uniformity (minimum to average value) larger than 0.6 according to EN 12464 – 1 [39] for both systems. Since the lighting system is inside the perimeter zone as this is defined by EN 15193-2007 [38], it can be controlled with one sensor. A photosensor with an ideal cosine spatial sensitivity distribution was placed in the geometrical center of the room mounted on the ceiling. The point beneath the photosensor was used as the work plane illuminance reference at the standard desk height. Wall, ceiling and floor reflectance are 0.65, 0.85 and 0.20 accordingly while glazing transmittance is 0.55.
Figure 11. Layout of the office space with six surface mounted LED luminaires in a uniform layout and a photosensor placed in the geometrical center of the room. Simulations have been performed on an hourly basis for a typical year using Athens Greece TMY. Three control strategies were tested:
Daylight harvesting (closed loop control algorithm)
Daylight harvesting (integral reset control algorithm)
Lumen maintenance
The operational equations of Rubistein et al [40] were used in order for the control algorithms for daylight harvesting to be satisfied (Table 3). The nighttime condition was added in order to determine the photosensor signal SEm with absence of daylight and the task illuminance IEm, when only artificial light is received from the sensors’ field of view. The simulations gave the daylight illuminance ID(t) on the work plane and the daylight component of the signal that was produced by the photosensor SD(t) on an hourly basis for a typical year,
using as operation schedule, all days from 8:00 a.m. to 18:00 p.m. The same conditional expression for the closed loop control algorithm was used for all the tested luminaires.
Table 3. Operational equations for different control algorithms [40] Control
Transfer Function
Conditional Task Illuminance
Algorithm ST (t ) SEm
Integral Reset
Closed
S (t ) I T (t ) I D (t ) I Em 1 D S Em
S (t ) 1 D S Em
MST (t ) SEm 1
Loop
or
1 MS D (t ) S Em I T (t ) I D (t ) I Em 1 - MSEm
1 MS D (t ) S Em 1 - MSEm
M
I D (t cal ) I D (t cal )S Em I Em S D (t cal )
Expression I D (t ) I Em S D (t ) S Em
I D (t ) I D (t cal ) S D (t ) S D (t cal )
ST(t): Signal produced by photosensor (time dependent), SD(t): Daylight component of ST(t), SD(tcal): Daylight component of ST(t) at calibration time, SE(t): Electric light component of ST(t), δ: Fractional output of electric lights (δmin ≤ δ ≤1). Full light output δ=1, minimum light output δ= δmin., IEm : Task illuminance level for δ=1 without daylight, SEm : Signal produced by photosensor for δ=1 without daylight, IT(t): Total light at task (time dependent), ID(t): Daylight at task (time dependent), ID(tcal): Daylight at task at calibration time, IE(t): Electric light at task (time dependent)
Commissioning and calibration procedures of the photosensor were adapted from previous studies [6-8, 10, 11, 34, 41-43] in order for a reliable performance of the daylight responsive dimming system to be achieved. Given the necessary inputs for the operational equations of each control algorithm (Table 3), the appropriate dimming percentage δ was determined using the photosensor’s output value and control algorithm. Consumed power is a function of this dimming percentage according to the equations presented in Table 1. The energy savings due to lumen maintenance were calculated by varying δ from 80% to 100% for the same time interval 40000h according to Royer [44] and the US Department of Energy [45] results for high performance fluorescent lamps and LEDs with low junction temperatures. Moreover, since the simulations were performed on hourly basis, power factor values were estimated as a function of δ using the equations from Table 2.
4. Results 4.1 Dimming and Daylight harvesting In Tables 4 and 5 the monthly and annual average energy savings are presented for the closed loop and integral reset algorithms for all tested luminaires. For the closed loop algorithm (Table 4) the absolute maximum value of the monthly average energy savings was 87.00% for the LED A luminaire in June and July while for group II it was 78.99% for T5 B luminaire for the same months. The maximum annual average energy saving value was 79.28% for the LED A in group I and 72.63% for the T5 B in group II. The minimum average monthly values were 61.03% for the LED A luminaire and 55.88% for T5 C both in December while the minimum annual average value of energy savings was 74.09% for the LED D and 67.37% for T5 A. Using integral reset algorithm (Table 5) the maximum value of the monthly and annual energy savings was 87.05% (from April to August) and 83.32% for LED A respectively. When examining group II, these values were 79.04% (from May to August) and 75.92%for T5 B. The minimum value of the monthly and annual energy savings was 69.72% (December) and 78.34% for the LED D respectively. For group II these values were 62.94% (December) and 70.35%for T5 C.
Table 4. Average monthly and annual energy savings for all luminaires for the closed loop algorithm Energy savings (%) for Closed Loop algorithm Luminaire
Group I (LED luminaires)
Group II (luminaires with T5 lamps)
LED A
LED B
LED C
LED D
T5 A
T5 B
T5 C
T5 D
January
69.60
66.64
68.66
64.50
59.05
64.64
58.92
63.28
February
76.63
73.41
75.49
71.29
65.00
70.77
64.69
69.57
March
81.80
78.41
80.34
76.32
69.43
74.87
68.87
73.98
April
83.65
80.20
82.09
78.16
71.05
76.27
70.41
75.61
May
85.82
82.34
84.18
80.50
73.02
77.97
72.33
77.57
June
87.00
83.52
85.34
81.96
74.13
78.99
73.41
78.64
July
87.00
83.52
85.34
81.95
74.13
78.99
73.40
78.63
August
85.97
82.48
84.32
80.67
73.16
78.09
72.46
77.71
September
83.08
79.68
81.58
77.78
70.58
75.93
69.99
75.10
October
76.32
73.18
75.14
71.43
64.96
70.23
64.62
69.25
November
68.32
65.42
67.35
63.37
57.94
63.30
57.83
62.03
December
66.01
63.18
65.13
61.03
55.94
61.43
55.88
59.98
Annual
79.28
76.01
77.92
74.09
67.37
72.63
66.91
71.79
Table 5. Average monthly and annual energy savings for all luminaires for the integral reset algorithm Energy savings (%) for Integral Reset algorithm Luminaire
Group I (LED luminaires)
Group II (luminaires with T5 lamps)
LED A
LED B
LED C
LED D
T5 A
T5 B
T5 C
T5 D
January
77.17
74.00
75.94
72.19
65.65
70.95
65.26
69.97
February
83.94
80.53
82.45
78.81
71.42
76.61
70.84
76.01
March
86.54
83.06
84.89
81.43
73.70
78.62
72.99
78.21
April
87.05
83.56
85.38
82.02
74.17
79.03
73.45
78.67
May
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
June
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
July
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
August
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
September
86.87
83.38
85.21
81.82
74.01
78.89
73.29
78.52
October
80.28
77.01
79.01
75.30
68.34
73.68
67.92
72.88
November
75.54
72.46
74.24
70.83
64.28
69.14
63.89
68.35
December
74.45
71.40
73.17
69.72
63.29
68.16
62.94
67.35
Annual
83.32
79.96
81.81
78.34
70.95
75.92
70.35
75.38
For group I (LED luminaires) the maximum difference in energy savings between the luminaires is 5.19% (between luminaire LED D and A) for the closed loop algorithm and 4.99% (also between luminaire D and A) for the integral reset algorithm. For group II (luminaires with T5 lamps) the above mentioned differences are equal to 5.72% (between luminaire T5 C and B) and 5.57% (also between luminaire D and A) respectively (Table 6). The differences in energy savings between luminaires with the same technology, concerning just one factor EDBs or LED drivers, resulted in significant values for both cases. Table 6. Annual energy savings from minimum to maximum and their differences between the luminaires for closed loop algorithm and integral reset algorithm. Group I (LED luminaires) Group II (luminaires with T5 lamps) Annual energy Energy saving Annual energy Energy saving Luminaire Luminaire savings (%) difference (%) savings (%) difference (%) Closed loop algorithm LED D 74.09 T5 C 66.91 LED B 76.01 1.92 T5 A 67.37 0.47 LED C 77.92 3.83 T5 D 71.79 4.88 LED A 79.28 5.19 T5 B 72.63 5.72 Integral reset algorithm LED D 78.34 T5 C 70.35 LED B 79.96 1.62 T5 A 70.95 0.60 LED C 81.81 3.47 T5 D 75.38 5.02 LED A 83.32 4.99 T5 B 75.92 5.57
Without taking into account the percentage of hours on an annual basis with lighting levels (from daylight and artificial lighting) exceeding the designed value (i.e 500 lux), integral reset seems to have better results for energy savings. However, illuminance levels on the working plane must meet the target illuminance value during daylight and thus the proper choice of the control algorithm should not be based only on the energy savings achieved. Daylight harvesting closed loop control algorithm can perform much better than integral reset, providing sufficient electrical light to meet target illuminance for more hours during the day [6]. 4.2 Dimming and lumen maintenance While closed loop algorithm is more suitable for daylight applications, integral reset can be used in lumen maintenance control strategies. During the design phase of a lighting system, the decline in the lumen output is typically accounted for by applying a maintenance factor (MF). Since MF value is less than 1, the initial light levels are above the design illuminance target. During lumen maintenance control, luminaires are initially dimmed to a percentage equal to the MF. In an effort to present the differences in energy savings when lumen maintenance control is used, we selected a predefined value of the maintenance factor, 80% for all tested luminaires [44, 45]. Figures 12 and 13 show the relative consumed power as a function of the dimming level in the range between 80% and 100% for both tested groups. The results show deviations in energy consumption for the same dimming level. For dimming level 80%, luminaire C LED gives 75% relative consumed power, while luminaire D LED gives 82% relative consumed power. For group II luminaires, the minimum and maximum values of consumed power ranged from 72% for luminaire B T5 to 82% for luminaire A T5.
Figure 12. Relative consumed power for dimming range 80% to 100% for group I (LED luminaires) and lumen maintenance control strategy.
Figure 13. Relative consumed power for dimming range 80% to 100% for group II (luminaires with T5 lamps) and lumen maintenance control strategy. Consumption
Figures 14 and 15 present the differences in energy savings for both tested luminaire groups within the selected dimming range for lumen maintenance strategy of 80% to 100%. The lower and upper lines of the box plots represent the energy consumptions when the dimming level is 80% and 100% respectively.
Figure 14. Differences in energy savings between the LED luminaires in group I for the lumen maintenance control strategy for dimming level 80% to 100%.
Figure 15. Differences in energy savings between the LED luminaires in group II for the lumen maintenance control strategy for dimming level 80% to 100%.
The maximum difference between the luminaires of group I, was estimated at 4.25% (between luminaire LED D and C) and 6.02% (between luminaire T5 A and B) for group II (luminaires with T5 lamps) (Table 7). The differences in energy savings between luminaires with the same technology resulted in significant differences for both cases (due to EDB and driver only) and thus the optimum choice of both EDB and LED drivers is required not only for daylight harvesting but also for lumen maintenance.
Table 7. Energy savings for the tested luminaires using lumen maintenance strategy (Using the same time interval between sequentially cleaning and re-lamping of the luminaires for high performance fluorescent lamps and LEDs with low junction temperatures [44, 45]). Group I (LED luminaires) Energy savings Energy saving Luminaire from lumen difference maintenance (%) (%) D LED 9.06 B LED 10.51 1.46 A LED 11.31 2.25 C LED 13.31 4.25
Group II (luminaires with T5 lamps) Energy savings Energy saving Luminaire from lumen difference maintenance (%) (%) T5 A 9.84 T5 C 10.03 0.20 T5 D 12.79 2.96 T5 B 15.86 6.02
4.3 Dimming and Power Factor Figures 16 to 19 present the frequency distribution of power factor values during the annual operation using closed loop and integral reset algorithms.
Figure 16. Annual frequency distribution of power factor values during the annual operation. Closed loop algorithm with Group I luminaires.
Figure 17. Annual frequency distribution of power factor values during the annual operation. Closed loop algorithm with Group II luminaires.
Figure 18. Annual frequency distribution of power factor values during the annual operation. Integral reset algorithm with Group I luminaires.
Figure 19. Annual frequency distribution of power factor values during the annual operation. Integral reset algorithm with Group II luminaires The lowest PF values are these of the LED luminaires for both closed loop and integral reset algorithm. In this group, PF values between 0.3 and 0.5 occur for more than 80% of the operational hours. In group II, PF values are significantly increased with 80% of operational hours in the range of 0.7-0.9. Between the two control algorithms, integral reset has shown lower PF values for the same operation schedule. The low power factor values could have undesirable technological and economic consequences. The reduced power factor could result to grid frequency declination, current increase and thus safety-related issues and many other harmful effects in the electrical grid [46] that could result in resonance in distribution networks, increased transmission and distribution losses. For the economic consequences there might be potentially higher electricity costs resulting from a power factor charge utility. Thus the information of fluctuation of PF during dimming procedure could be used in optimization strategies in order to avoid low PF values and bad behavior of LEDs. Finally, figures 20 and 21 present the monthly frequency percentages of PF value fluctuation of a T5 B luminaire and a LED A luminaire, as an example. For luminaire T5 B, during June and July PF values vary from 0.8 to 0.9 on the total of the operational schedule time, while for January, during 65.5% of the operational schedule time. As a result, someone can expect the fluctuation of PF of lower values during the summer.
Figure 20. Monthly frequency of power factor values during annual operation using photosensors with closed loop algorithm for T5 B luminaire.
Figure 21. Monthly frequency of power factor values during annual operation using photosensors with closed loop algorithm for LED A luminaire. 5. Conclusions The selection of the proper luminaire is crucial during the design phase not only by satisfying the requirements imposed by regulations as far as it concerns average illuminance and uniformity but also for their energy performance. It is evident that it is also crucial to identify and estimate all parameters involved with their dimming performance. While LED technology is evolving, becoming more and more energy efficient, there is still a lack of accurate energy calculations when dimming is included in cases with daylight harvesting or lumen maintenance. In contrast to other type of lamps, power factor values for the LED lamps [27] / luminaires generally became worse when dimmed. While LED power quality performance seems to be independent on the choice of dimmer [27], further
research is needed not only to provide high power factor equipment for on/off function [47 – 49] but also for dimming. Thus, the dimming procedure of LED technology needs more investigation in order to be integrated in daylight responsive systems. Four dimmable LED luminaires (Group I) and four luminaires with T5 florescent lamps were tested. Light output versus control signal, consumed power versus control signal, consumed power versus light output and power factor versus control signal were measured. The bestfit functions a) between consumed power and light output and b) between power factor and control signal were estimated in order to be used in the simulations. A typical office room was selected [6] and a number of lighting systems were installed together with a daylight harvesting system. A reference signal of photosensor using daylight harvesting and lumen maintenance strategies was created using DAYSIM. The reference signal was used to all the tested luminaires in order to check the influence of different EDBs and LED drivers on a specific control signal in order to quantify the annual energy savings for every case. The analysis of the results shows significant differences in energy savings among the tested luminaires. The annual maximum difference in energy savings between the tested LED luminaires was 5.19% for closed loop, 4.99% for integral reset and 4.25% for lumen maintenance while between the tested T5 luminaires it was 5.72%, 4.57% and 6.02% respectively. Thus the optimum choice of the dimmable luminaire is required in early stage of lighting design, either uses LED or T5 fluorescent lamps. In general:
LED luminaires have performed better in overall annual energy savings than the luminaires with T5 lamps. The reason is mostly due to the LED driver, because at low dimming percentages, it consumes less energy than EDBs.
PF values can be a critical parameter during the design phase of a lighting system especially if a daylight harvesting system is to be adopted.
Integral reset algorithm results in lower PF values in comparison to those of the closed loop algorithm.
The frequency distribution of power factor values during the annual operation using photosensors can be used as a new criterion [12] for the commissioning procedure of a daylight responsive system.
During design procedure of a daylight harvesting system the choice of the appropriate technology between LED and T5 fluorescent lamps can be affected by the low PF values of LEDs. As a recommendation, In order to avoid a bad choice, data bases with control functions for control voltage and power factor for a large number of EDBs and LED drivers
and luminaires must be created. This will make lighting designers tol be able to select the most suitable technology for their project. Moreover the function between consumed power and Light Output (LO, %) of luminaires should be known during the design phase in an effort to properly match the photosensor and luminaires, exploiting the full potential of energy savings. It should be pointed out, that the aforementioned functions can increase the accuracy of the simulations making, in turn, more accurate the calculation of the return of investment in retrofit projects. The proposed methodology will force also EDB and LED driver manufacturers to provide not only more energy efficient equipment but also to improve their electrical characteristics. References [1] Li D.H.W, Mak A.H.L., Chan W.W.H., Cheng C.C.K. Predicting energy saving and life-cycle cost analysis for lighting and daylighting schemes. International Journal of Green Energy 2009, 6, 4, 359-370. [2] Li D. H.W., K.L. Cheung, S.L. Wong, Tony N.T. Lam. An analysis of energy-efficient light fittings and lighting controls. Applied Energy 2010, 87 2, 558–567. [3] Zografakis N., K. Karyotakis, K. P. Tsagarakis, Implementation conditions for energy saving technologies and practices in office buildings: Part 1. Lighting, Renewable and Sustainable Energy Reviews 16 (2012) 4165– 4174 [4] Bellia L, Fragliasso F, Stefanizzi E, Why are daylight-linked controls (DLCs) not so spread? A literature review, Building and Environment (2016), doi: 10.1016/j.buildenv.2016.06.040. [5] Doulos, L., Tsangrassoulis, A., Topalis, F.V. The role of spectral response of photosensors in daylight responsive systems. Energy and Buildings 2008, 40, 588–599. [6] Doulos, L., Tsangrassoulis, A., Topalis, F.V. Quantifying energy savings in daylight responsive systems: The role of dimming electronic ballasts. Energy and Buildings 2008, 40, 36–50. [7] Mistrick R., Sarkar A. A study of daylight-responsive photosensor control in five daylighted classrooms, Leukos 2005, 3, 1, 51-74. [8] Mistrick R., Chen C., Bierman A., Felts D. A comparison of photosensor-controlled electronic dimming systems in a small office, Journal of the Illuminating Engineering Society 2000, 29, 1, 66-80. [9] Li D.H.W., Lam JC. An investigation of daylight performance and energy saving in a daylit corridor. Energy Build 2003, 35, 365–73. [10]Choi A., Mistrick RG. On the prediction of energy savings for a daylight dimming system. J Illum Eng Soc 1997, 26, 2, 77–90. [11] Mistrick R., Thongtipaya J. Analysis of daylight photocell placement and view in a small office. Journal of the Illuminating Engineering Society 1997, 26, 2, 150-160. [12] Doulos L., Tsangrassoulis A., Topalis F.V., Multi-criteria decision analysis to select the optimum position and proper field of view of a photosensor, Energy Conversion and Management 86 (2014) 1069–1077. [13] Littlefair PJ, Motin A. Lighting controls in areas with innovative daylight systems: a study of sensor type. Light. Res. Technol. 2001, 33 (1) 59–73. [14] Choi A, Song K, Kim Y. The characteristics of photosensors and electronic dimming ballasts in daylight responsive dimming systems. Building and Environment 2005, 40, 39–50.
[15] D.H.W. Li , Angela C.K. Cheung, Stanley K.H. Chow, Eric W.M. Lee, Study of daylight data and lighting energy savings for atrium corridors with lighting dimming controls Energy and Buildings 72 (2014) 457–464 [16]Topalis F.V. and L.T.Doulos, Ambient Light Sensor Integration, Handbook of Advanced Lighting Technology, 2016, Springer International Publishing, Cham, 1—28, ISBN 978-3319-00295-8, DOI 10.1007/978-3-319-00295-8_33-1, http://dx.doi.org/10.1007/978-3319-00295-8_33-1 [17]Pandharipande A., D. Caicedo, Daylight integrated illumination control of LED system based on enhanced presence sensing, Energy and Buildings 43 (4) (2011) 944–950. [18]Z. Wang, Y.K. Tan, Illumination control of LED systems based on neural network model and energy optimization algorithm, Energy and Buildings 62 (2013) 514–521. [19] Yu X., Y. Su, Daylight availability assessment and its potential energy saving estimation – A literature review, Renewable and Sustainable Energy Reviews 52 (2015) 494–503. [20] Aghemo C., L. Blaso, A. Pellegrino. Building automation and control systems: A case study to evaluate the energy and environmental performances of a lighting control system in offices. Automation in Construction 43 (2014) 10–22. [21] Wen Y.J., A. M. Agogino, Personalized dynamic design of networked lighting for energyefficiency in open-plan offices, Energy and Buildings 43 (2011) 1919–1924 [22]Chiradejaa P., A. Ngaopitakkulb, C. Jettanasen, Energy savings analysis and harmonics reduction for the electronicballast of T5 fluorescent lamp in a building’s lighting system, Energy and Buildings 97 (2015) 107–117. [23] Albu HC, L. Halonen, E. Tetri, F. Pop and D. Beu, Luminous and power quality analysis of office building light sources Lighting Res. Technol. 2013; 45: 740–751 [24] Viitanen J., J. Lehtovaara, E. Tetri and L. Halonen (2013) User Preferences in Office Lighting: A Case Study Comparing LED and T5 Lighting, LEUKOS, 9:4, 261-290 [25] Wei M. and K. W. Houser, Status of Solid-State Lighting Based on Entries to the 2010 US DOE Next Generation Luminaire Competition LEUKOS, 8, 4, 2012, 237 – 259 [26] Dyble M, Narendran N, Bierman A, Klein T. 2005. Impact of dimming white LEDs: Chromaticity shifts due to different dimming methods. Fifth International Conference on Solid State Lighting. San Diego (CA). SPIE Proceedings Vol. 59411G [27] US Department of Energy, Pacific Northwest National Laboratory, CALiPER Retail Lamps Study 3.1: Dimming, Flicker and Power Quality Characteristics of LED A Lamps, December 2014. [28] Behar-Cohen F., C. Martinsons, F. Viénot, G. Zissis, A. Barlier-Salsi, J.P. Cesarini, O. Enouf, M. Garcia, S. Picaud, D. Attia, Light-emitting diodes (LED) for domestic lighting: Any risks for the eye?, Progress in Retinal and Eye Research 30 (2011) 239-257. [29]CIE, The photometry and goniophotometry of luminaires, No.121, 1996. [30]CIE, The measurement of absolute luminous intensity distributions, No.70, 1987. [31]NLPIP, Specifier Report: Dimming electronic ballasts, 7 (3) 1999. [32]DAYSIM, Daylighting Analysis software to predict the annual daylight availability and electric lighting use in a building. http://irc.nrc-cnrc.gc.ca/ie/light/daysim.html [33] L. Doulos, A. Tsangrassoulis, F.V. Topalis, A critical review of simulation techniques for daylight responsive systems, in: Proceedings of the European Conference on Dynamic Analysis, Simulation and Testing applied to the Energy and Environmental performance of Buildings DYNASTEE, Athens, Greece, 2005. [34] F. Reinhart, C. Jones, Electric lighting energy savings for an on/off photocell control – a comparative simulation study using DOE2.1 and DAYSIM, in: Proceedings of eSim 2004, Vancouver, Canada, (2004), pp. 183–189. [35] IEA Task 27, Perfomance of solar fac¸ade components, Subtask A report, D. van Dijk, 2001. [36] SWIFT Project, ENK 6-CT1999-00012, Final report, 2003.
[37]Greek Regulation of Energy Performance of Buildings, D6-5825_KENAK_FEK_407-B2010. [38] European Norm 15193: Energy performance of buildings — Energy requirements for lighting, 2007. [39] European Norm 12464 -1: Light and lighting — lighting of work places Part 1: Indoor work places, 2011. [40]F. Rubinstein, G. Ward, R. Verderber, Improving the performance of photo-electrically controlled lighting systems, Journal of the Illuminating Engineering Society 18 (1) (1989) 70-94. [41] NLPIP, Specifier Report: Photosensors, 6 (1) (1998) [42]F. Rubinstein, D. Avery, J. Jennings, On the calibration and commissioning of lighting controls, in: Proceedings of Right Light 4, Copenhagen, Denmark, (1997), pp. 225–230 [43]S. Ranasinghe, R. Mistrick, A study of photosensor configuration and performance in a daylighted classroom space, Journal of the Illuminating Engineering Society 32 (2) (2003) 3–20. [44] M. Royer, Lumen Maintenance and Light Loss Factors: Consequences of Current Design Practices for LEDs, LEUKOS, 10, 2, (2014) 77-86. [45] US Department Of Energy, Lumen Maintenance and Light Loss Factors: Consequences of Current Design Practices for LEDs, September 2013. [46] Power Electronics Basics, Operating principles, Design, Formulas and Applications, Yuriy Rozanov, Sergey Ryvkin, Evgeny Chaplygin, Pavel Voronin, CRC Press, Taylor & Francis Group,6000Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742, ISBN-13: 978-1-4822-9880-2, 2016 [47] Sin-Woo Lee; Hyun-Lark Do, Boost Integrated Two-Switch Forward AC-DC LED Driver with High Power Factor and Ripple-Free Output Inductor Current, IEEE Transactions on Industrial Electronics, Volume: PP, Issue: 99, 16 January 2017, DOI 10.1109/TIE.2017.2652407 [48] Kai-Yu Hu; Yi-Yang Tsai; Sheng-Ming Wang; Chien-Hung Tsai, High power factor boost PFC controller with feed forward adaptive on-time control, Consumer Electronics, 2016 IEEE 5th Global Conference on Consumer Electronics, 11-14 Oct. 2016, DOI: 10.1109/GCCE.2016.7800535 [49] Kumar Modepalli; Leila Parsa, Single Stage LED Driver with Power Factor Correction for Illumination and Visible Light Communication, IEEE Industry Applications Magazine, Volume: PP, Issue: 99, 26 December 2016, DOI: 10.1109/MIAS.2016.2600731
S0378-7788(16)31177-X http://dx.doi.org/doi:10.1016/j.enbuild.2017.02.013 ENB 7374
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Received date: Revised date: Accepted date:
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Please cite this article as: L.T.Doulos, A.Tsangrassoulis, P.A.Kontaxis, A.Kontadakis, F.V.Topalis, Harvesting daylight with LED or T5 fluorescent lamps? The role of dimming, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2017.02.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Harvesting daylight with LED or T5 fluorescent lamps? The role of dimming L.T. Doulos* 1,2, A. Tsangrassoulis3, P.A.Kontaxis2,4, A. Kontadakis3, and F.V.Topalis2 1,*
School of Applied Arts, Lighting Design, Hellenic Open University, Patra, Greece, [email protected]. *corresponding author 2 Lighting Laboratory, National Technical University of Athens, Athens, Greece 3 University of Thessaly, Dept. of Architecture, Volos, Greece 4 Technological Educational Institute of Athens, Greece
Abstract While the efficiency of the Lighting Emitting Diodes (LED) is increasing and new installations are being implemented, their control due to the adoption of daylight harvesting systems, hasn’t been sufficiently examined yet, although this can reduce further energy consumption. A crucial parameter for achieving improved performance is the proper matching of a dimming system with the luminaires used, since this can significantly affect energy savings not only among various luminaires with T5 tubular fluorescent lamps but also among various LED luminaires. Furthermore, dimming can affect the power quality of the lighting installation depending on the technology used, making it an important factor during the selection phase. The aim of the present paper is to quantify energy savings among different LED and T5 fluorescent luminaires that are commonly used with daylight harvesting systems in offices and investigate their impact on power quality. Measurements using four commercial LED and four T5 luminaires were performed estimating the mathematical function which correlates their light output with their associate power consumption. Using the measured data, a set of hourly annual simulations were performed for a photosensor installed in a typical office room, quantifying the relative differences in energy savings among various cases examined together with the power factor values. Keywords Daylight harvesting, Dimmable LED, Dimmable T5 fluorescent lamps, Energy savings, Lumen maintenance, Photosensor, Power Factor. Highlights
Best-fit functions between consumed power and light output (LO, %) are introduced not only for luminaires with T5 lamps and Electronic Dimmable Ballasts (EDBs) but also for LED luminaires with dimmable drivers.
A new methodology for the determination of energy savings differences between different EDBs and LED dimmable drivers is proposed.
The scope of the methodology is the optimum selection of luminaires with regard to the amount of energy savings during daylight harvesting or lumen maintenance control strategy.
The role of dimming concerning the fluctuation of Power Factor (PF) during a year using daylight harvesting strategy is investigated.
1. Introduction Lighting comprising a large percentage of the buildings energy’ balance [1, 2]. While trying to reduce the energy consumption, a large number of new or retrofitting projects are realized, based mainly on the concept of improving the luminaire’s luminous efficacy. Although the retrofitting approach for energy savings can be influenced by a number of parameters that are irrelevant to the lighting technology, such as the annual financial turnover [3], it seems that daylight controls are frequently excluded. This is because design teams want to simplify the retrofit project, lowering its cost, even though daylight harvesting can offer significant opportunities for energy savings. The adoption of daylight controls is negatively affected by a number of reasons such as the complex design process, the unreliability of the calculation methodologies and the users’ acceptance [4]. Although there is a large number of studies in literature examining techniques which reduce lighting energy consumption [5-16], the number of studies examining possible energy savings through daylight harvesting and LED sources is quite limited [17,18]. Pandharipande and Caicedo [17] proposed an energy efficient illumination control of a LED system using a number of photosensors and examining various occupancy scenarios. Wang and Tan [18] also used LED fixtures in order to exploit the dimming procedure and to develop an overall illumination control based on a neural network mapping model. Daylight responsive dimming systems controlled by photosensors adjust the electric lighting level according to the amount of daylighting impinging on the photosensor. Thus, the
daylight availability and the potential energy saving estimation tools are crucial for the final decisions. Yu and Su [19] have categorized the estimation tools into four groups: software simulations, field measurements, empirical formulae and estimation algorithms. However, sometimes the obtained energy savings aren’t as high as expected because of the users’ interaction [20]. Thus, tuning up the lighting system, according to the occupants needs can result in higher energy savings [21]. Dimming can also have an impact on power quality. Chiradeja et al [22] examined the effects of harmonics for both electromagnetic and electronic ballasts for T8 lamps and for electronic ballasts for T5 lamps, estimating energy savings from the corresponding replacement. Albu et al. [23] performed light and power quality measurements and showed that LED luminaires have greater luminous efficacy than luminaires with tubular florescent lamps but In terms of power quality, the current harmonic distortion level was attenuated if the light sources were dimmed. However, on what concerns the color temperature between T5 fluorescent lamps and LED fixtures no significant differences were found and they were rated equally pleasant for the same illuminance level [24]. While there is not a standard method to evaluate and characterize LED dimming performance [25] there are also other important features that must be considered such as dimming range, drop out, flicker, color shift, and noise [26]. The power factor of LED is high when they function without dimming [25] while it generally reduced as LEDs are dimmed [27]. The present paper examines and compares the relative differences in energy savings and power factor of a number of dimmable LED luminaires and T5 fluorescent lamps when these are dimmed with photosensors. 2. Measurements Two groups of luminaires were tested, one group (I) of LED luminaires and one group (II) of T5 tubular fluorescent lamps:
Group I: LED downlight luminaires from four different manufactures. o
1 X 17 Watt Chip on Board (COB) LED
o
1 X 20 Watt Chip on Board (COB) LED
o
1 X 20 Watt Chip on Board (COB) LED
o
1 X 22 Watt Chip on Board (COB) LED
Group II: T5 tubular fluorescent lamps from four different manufactures [6]. o
1 X 28 Watt T5 tubular lamp
o
2 X 28 Watt T5 tubular lamps
o
1 X 49 Watt T5 tubular lamp
o
1 X 54 Watt T5 tubular lamp
The measurements took place in the Photometry Laboratory of the National Technical University of Athens. The test space was a dark room (20m x 7m x 4m) with black mat painted surfaces (Figure 1). The ambient temperature was maintained constant at 26oC and input voltage at 230Vac, 50 Hz through the voltage stabilizer for all tested conditions [6]. All luminaires were aged and the appropriate preheat time was applied before the first measurement. Suggestions for avoiding lighting sources emitting cold white light were taken into account for the experiments of the present paper [28], thus all selected lighting sources had CCT of 4000K.
Figure 1. LED luminaire (left photo) on goniophotometer A and T5 luminaire (right photo) attached on goniophotometer B.
Photometric and electric measurements were taken for various dimming levels. Every luminaire was manually dimmed using the DALI protocol (either through a PC interface (group I) or 0V to 10V digital converter (group II)), from a maximum to a minimum control signal in tandem procedure [6, 14]. The measurement of the luminous intensity was performed according to the CIE standards No. 121 (1996) and No. 70 (1987) [29, 30]. Two types of goniophotometers (A and B) were used. Every luminaire of the group I was attached to the goniophotometer A at a distance of 2.18 m from the head of the photometer (Figure 1, left) while for group II luminaires were attached to the goniophotometer B at a distance of 9.90 m (Figure 1, right). The light output was measured as an illuminance value at the same distance from the luminaires for each goniophotometer. In both cases, the photometer sensors were placed inside a mat black painted cylinder with baffles, protected from stray light. The field of view of the photometer sensor covered the entire emitting surface of each tested luminaire. The measurements of the light output were monitored every 5 minutes
and were taken only if the light output was stabilized, by testing if the standard deviation between the last two measurements was less than 2%. This procedure varied from 15 min to 20 min for all cases [6, 31]. A single-phase power analyzer was used for the measurement of active, reactive and apparent power as well as of the power factor for various dimming levels of the tested LED drivers and EDBs. The dimming range was calculated for each individual luminaire from an upper limit of 100% (maximum dimming level) to a minimum level near of 0%. The values were the same when the lamps were measured from maximum to minimum light output (decreasing control signal from 100% to 0%) or from minimum to maximum light output (increasing control signal from 100% to 0%). In all cases, no LED or T5 lamp was extinguished before reaching the reported minimum light output. 2.1 Light output and relative consumed power versus control signal Figures 2 to 5 present each luminaire relative light output and consumed power against the control signal. The test results show significant deviations between the luminaires of group II. Most of the tested drivers and electronic dimming ballasts (EDBs) showed a non-linear relationship. Linear relationships can be assumed only for specific LED drivers and for a specific range of control signals.
Figure 2. Light output versus control signal for group I luminaires (LED luminaires).
Figure 3. Light output versus control signal for group II luminaires (T5 luminaires).
Figure 4. Relative consumed power versus control signal for group I luminaires (LED luminaires).
Figure 5. Relative consumed power versus control signal for group II luminaires (T5 luminaires). 2.2 Relative consumed power versus light output Figures 6 and 7 present the relative consumed power against light output percentage for all luminaires. It is evident that the same light output corresponds to different consumed power even for luminaires of the same test group. For example in group I, when 40% relative light output is needed, luminaire D LED has 47.5% relative consumed power while luminaire A LED gives 38.5% respectively. (Figure 6). Using the same aforementioned light output (40%), luminaire A T5 (group II) has 49.5% relative consumed power, while luminaire B T5 has 41.6% (Figure 7). It seems that when both group luminaires are dimmed near 0%, with LED drivers, they consume less energy when compared to EDBs. Table 1 shows the polynomial interpolation results between normalized power consumption (y) and normalized light output (x). Both parameters varied between 0 to 1. In both groups, the coefficient of determination (R2) is higher than 0.999 indicating that data are quite close to the fitted polynomial function. These best-fit functions can be used to predict energy savings when a daylight harvesting system or a system with lumen maintenance capabilities is adopted.
Figure 6. Relative consumed power versus light output for group I luminaires (LED luminaires).
Figure 7. Relative consumed power versus light output for group II luminaires (T5 luminaires). Table 1. Derived functions from polynomial interpolation between light output (%) (x, 0 to 1) and consumed power (y, 0 to 1), for both groups I and II. Group I I I I II II II
Luminaire A LED B LED C LED D LED A T5 B T5 C T5
II
D T5
Derived functions y=-0.257x +0.658x -0.357x2+0.914x+0.041 y=-0.335x4+0.836x3-0.542x2+0.968x+0.072 y=0.733x4-1.065x3+0.519x2+0.742x+0.068 y=-0.949x4+2.467x3-2.205x2+1.650x+0.034 y=2.004x5 - 5.135x4 + 4.858x3 - 2.053x2 + 1.164x + 0.158 y=8.257x6-21.977x5+21.863x4-9.299x3+1.211x2+0.815x+ 0.123 y=0.941x4 -1.623x3+0.843x2+0.623x+0.196 y=-11.655x6+36.079x5-40.844x4+20.876x3-4.795x2+1.218x+ 0.122 4
3
R2 0.9999 0.9999 0.9996 0.9997 0.9999 0.9999 0.9999 0.9999
2.3 Power factor versus control signal In addition to the photometric and electric measurements for various dimming levels, power factors were measured as well (Figures 8 and 9), within the whole dimming range for all
tested luminaires. Power factor values ranged from 0.96 at maximum light output to 0.35 at minimum light output for group I luminaires, while, for group II the values ranged from 0.99 to 0.63 respectively. All LED luminaires in group I performed poorly with luminaire A LED being the best among them (ranging from 0.94 to 0.37) and luminaire D LED being the worst of all (ranging from 0.91 to 0.35). The differences between the two groups are not only due to the different operational principle between the LED drivers and EDBs but also due to the different technology between LEDs and T5 lamps. Table 2 presents the power factor (PF ranging from 0 to 1) as a function of a normalized control signal (nCS ranging from 0 to 100%) as this was derived from a polynomial interpolation. Coefficients of determination values (R2) are quite high, ranging from 0.9918 to 0.9997 for group I luminaires and from 0.9928 to 0.9973 for group II luminaires.
Figure 8. Power factor values versus dimming percentage for group I luminaires (LED luminaires).
Figure 9. Power factor values versus dimming percentage for group II luminaires (T5 luminaires).
Table 2. Power factor (y, 0 to 1) as function of control signal (x, 0 to 1) for both groups I and II. Group
Luminaire
I
A LED
I
B LED
I
C LED
I
D LED
II
A T5
II
B T5
II
C T5
II
D T5
Derived functions 6 5 y = -5,343x + 17,354x - 22,587x4 + 15,864x3 - 7,249x2 + 2,533x + 0,370 y = -8,654x6 + 29,995x5 - 40,636x4 + 27,687x3 - 10,776x2 + 2,982x + 0,363 y = -10,922x6 + 36,079x5 - 46,652x4 + 30,634x3 - 11,745x2 + 3,209x + 0,360 y = 38,262x6 - 118,950x5 + 138,040x4 - 71,764x3 + 14,135x2 + 0,910x + 0,276 y = -4,540x6 + 8,062x5 + 1,789x4 - 11,039x3 + 6,685x2 - 0,610x + 0,635 y = 9,620x6 - 31,847x5 + 40,273x4 - 23,681x3 + 5,844x2 - 0,044x + 0,823 y = 4,905x6 - 15,991x5 + 18,447x4 - 8,951x3 + 1,872x2 - 0,072x + 0,756 y = 8,425x6 - 24,810x5 + 25,831x4 - 11,396x3 + 2,382x2 - 0,172x + 0,722
R2 0.9997 0.9992 0.9997 0.9918 0.9973 0.9928 0.9966 0.9949
3. Simulations The simulations were performed using DAYSIM software [32] which has been used in previous photosensor studies to calculate daylight levels on the working surface and the ceiling sensor [6, 33, 34]. The room that was used for the simulations is a typical space in an office building with dimensions of 3.5 x 5.4 x 2.7 m with one external façade [35, 36] and openings on the south exposed façade (Figure 10). Two types of electric lighting systems were examined using six surface mounted luminaires in a uniform layout. The first one uses group I LED luminaires (Figure 11) while the second one uses group II luminaires with parabolic louvers.
Figure 10. Vertical cross section of the office room and location of the photosensor (left) and layout of south façade of the office room (right).
The installed power was lower than 16 W/m2 (8-9W/m2 for LEDs and 11-12W/m2 for T5 lamps) according to the Greek regulation for the energy performance of buildings and EN 15193-2007 [37, 38] for the two systems while the average maintenance lighting levels on the working surface (0.80 m height) were above 500 lux with a predefined design illuminance of 500lux and uniformity (minimum to average value) larger than 0.6 according to EN 12464 – 1 [39] for both systems. Since the lighting system is inside the perimeter zone as this is defined by EN 15193-2007 [38], it can be controlled with one sensor. A photosensor with an ideal cosine spatial sensitivity distribution was placed in the geometrical center of the room mounted on the ceiling. The point beneath the photosensor was used as the work plane illuminance reference at the standard desk height. Wall, ceiling and floor reflectance are 0.65, 0.85 and 0.20 accordingly while glazing transmittance is 0.55.
Figure 11. Layout of the office space with six surface mounted LED luminaires in a uniform layout and a photosensor placed in the geometrical center of the room. Simulations have been performed on an hourly basis for a typical year using Athens Greece TMY. Three control strategies were tested:
Daylight harvesting (closed loop control algorithm)
Daylight harvesting (integral reset control algorithm)
Lumen maintenance
The operational equations of Rubistein et al [40] were used in order for the control algorithms for daylight harvesting to be satisfied (Table 3). The nighttime condition was added in order to determine the photosensor signal SEm with absence of daylight and the task illuminance IEm, when only artificial light is received from the sensors’ field of view. The simulations gave the daylight illuminance ID(t) on the work plane and the daylight component of the signal that was produced by the photosensor SD(t) on an hourly basis for a typical year,
using as operation schedule, all days from 8:00 a.m. to 18:00 p.m. The same conditional expression for the closed loop control algorithm was used for all the tested luminaires.
Table 3. Operational equations for different control algorithms [40] Control
Transfer Function
Conditional Task Illuminance
Algorithm ST (t ) SEm
Integral Reset
Closed
S (t ) I T (t ) I D (t ) I Em 1 D S Em
S (t ) 1 D S Em
MST (t ) SEm 1
Loop
or
1 MS D (t ) S Em I T (t ) I D (t ) I Em 1 - MSEm
1 MS D (t ) S Em 1 - MSEm
M
I D (t cal ) I D (t cal )S Em I Em S D (t cal )
Expression I D (t ) I Em S D (t ) S Em
I D (t ) I D (t cal ) S D (t ) S D (t cal )
ST(t): Signal produced by photosensor (time dependent), SD(t): Daylight component of ST(t), SD(tcal): Daylight component of ST(t) at calibration time, SE(t): Electric light component of ST(t), δ: Fractional output of electric lights (δmin ≤ δ ≤1). Full light output δ=1, minimum light output δ= δmin., IEm : Task illuminance level for δ=1 without daylight, SEm : Signal produced by photosensor for δ=1 without daylight, IT(t): Total light at task (time dependent), ID(t): Daylight at task (time dependent), ID(tcal): Daylight at task at calibration time, IE(t): Electric light at task (time dependent)
Commissioning and calibration procedures of the photosensor were adapted from previous studies [6-8, 10, 11, 34, 41-43] in order for a reliable performance of the daylight responsive dimming system to be achieved. Given the necessary inputs for the operational equations of each control algorithm (Table 3), the appropriate dimming percentage δ was determined using the photosensor’s output value and control algorithm. Consumed power is a function of this dimming percentage according to the equations presented in Table 1. The energy savings due to lumen maintenance were calculated by varying δ from 80% to 100% for the same time interval 40000h according to Royer [44] and the US Department of Energy [45] results for high performance fluorescent lamps and LEDs with low junction temperatures. Moreover, since the simulations were performed on hourly basis, power factor values were estimated as a function of δ using the equations from Table 2.
4. Results 4.1 Dimming and Daylight harvesting In Tables 4 and 5 the monthly and annual average energy savings are presented for the closed loop and integral reset algorithms for all tested luminaires. For the closed loop algorithm (Table 4) the absolute maximum value of the monthly average energy savings was 87.00% for the LED A luminaire in June and July while for group II it was 78.99% for T5 B luminaire for the same months. The maximum annual average energy saving value was 79.28% for the LED A in group I and 72.63% for the T5 B in group II. The minimum average monthly values were 61.03% for the LED A luminaire and 55.88% for T5 C both in December while the minimum annual average value of energy savings was 74.09% for the LED D and 67.37% for T5 A. Using integral reset algorithm (Table 5) the maximum value of the monthly and annual energy savings was 87.05% (from April to August) and 83.32% for LED A respectively. When examining group II, these values were 79.04% (from May to August) and 75.92%for T5 B. The minimum value of the monthly and annual energy savings was 69.72% (December) and 78.34% for the LED D respectively. For group II these values were 62.94% (December) and 70.35%for T5 C.
Table 4. Average monthly and annual energy savings for all luminaires for the closed loop algorithm Energy savings (%) for Closed Loop algorithm Luminaire
Group I (LED luminaires)
Group II (luminaires with T5 lamps)
LED A
LED B
LED C
LED D
T5 A
T5 B
T5 C
T5 D
January
69.60
66.64
68.66
64.50
59.05
64.64
58.92
63.28
February
76.63
73.41
75.49
71.29
65.00
70.77
64.69
69.57
March
81.80
78.41
80.34
76.32
69.43
74.87
68.87
73.98
April
83.65
80.20
82.09
78.16
71.05
76.27
70.41
75.61
May
85.82
82.34
84.18
80.50
73.02
77.97
72.33
77.57
June
87.00
83.52
85.34
81.96
74.13
78.99
73.41
78.64
July
87.00
83.52
85.34
81.95
74.13
78.99
73.40
78.63
August
85.97
82.48
84.32
80.67
73.16
78.09
72.46
77.71
September
83.08
79.68
81.58
77.78
70.58
75.93
69.99
75.10
October
76.32
73.18
75.14
71.43
64.96
70.23
64.62
69.25
November
68.32
65.42
67.35
63.37
57.94
63.30
57.83
62.03
December
66.01
63.18
65.13
61.03
55.94
61.43
55.88
59.98
Annual
79.28
76.01
77.92
74.09
67.37
72.63
66.91
71.79
Table 5. Average monthly and annual energy savings for all luminaires for the integral reset algorithm Energy savings (%) for Integral Reset algorithm Luminaire
Group I (LED luminaires)
Group II (luminaires with T5 lamps)
LED A
LED B
LED C
LED D
T5 A
T5 B
T5 C
T5 D
January
77.17
74.00
75.94
72.19
65.65
70.95
65.26
69.97
February
83.94
80.53
82.45
78.81
71.42
76.61
70.84
76.01
March
86.54
83.06
84.89
81.43
73.70
78.62
72.99
78.21
April
87.05
83.56
85.38
82.02
74.17
79.03
73.45
78.67
May
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
June
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
July
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
August
87.05
83.57
85.39
82.02
74.18
79.04
73.45
78.68
September
86.87
83.38
85.21
81.82
74.01
78.89
73.29
78.52
October
80.28
77.01
79.01
75.30
68.34
73.68
67.92
72.88
November
75.54
72.46
74.24
70.83
64.28
69.14
63.89
68.35
December
74.45
71.40
73.17
69.72
63.29
68.16
62.94
67.35
Annual
83.32
79.96
81.81
78.34
70.95
75.92
70.35
75.38
For group I (LED luminaires) the maximum difference in energy savings between the luminaires is 5.19% (between luminaire LED D and A) for the closed loop algorithm and 4.99% (also between luminaire D and A) for the integral reset algorithm. For group II (luminaires with T5 lamps) the above mentioned differences are equal to 5.72% (between luminaire T5 C and B) and 5.57% (also between luminaire D and A) respectively (Table 6). The differences in energy savings between luminaires with the same technology, concerning just one factor EDBs or LED drivers, resulted in significant values for both cases. Table 6. Annual energy savings from minimum to maximum and their differences between the luminaires for closed loop algorithm and integral reset algorithm. Group I (LED luminaires) Group II (luminaires with T5 lamps) Annual energy Energy saving Annual energy Energy saving Luminaire Luminaire savings (%) difference (%) savings (%) difference (%) Closed loop algorithm LED D 74.09 T5 C 66.91 LED B 76.01 1.92 T5 A 67.37 0.47 LED C 77.92 3.83 T5 D 71.79 4.88 LED A 79.28 5.19 T5 B 72.63 5.72 Integral reset algorithm LED D 78.34 T5 C 70.35 LED B 79.96 1.62 T5 A 70.95 0.60 LED C 81.81 3.47 T5 D 75.38 5.02 LED A 83.32 4.99 T5 B 75.92 5.57
Without taking into account the percentage of hours on an annual basis with lighting levels (from daylight and artificial lighting) exceeding the designed value (i.e 500 lux), integral reset seems to have better results for energy savings. However, illuminance levels on the working plane must meet the target illuminance value during daylight and thus the proper choice of the control algorithm should not be based only on the energy savings achieved. Daylight harvesting closed loop control algorithm can perform much better than integral reset, providing sufficient electrical light to meet target illuminance for more hours during the day [6]. 4.2 Dimming and lumen maintenance While closed loop algorithm is more suitable for daylight applications, integral reset can be used in lumen maintenance control strategies. During the design phase of a lighting system, the decline in the lumen output is typically accounted for by applying a maintenance factor (MF). Since MF value is less than 1, the initial light levels are above the design illuminance target. During lumen maintenance control, luminaires are initially dimmed to a percentage equal to the MF. In an effort to present the differences in energy savings when lumen maintenance control is used, we selected a predefined value of the maintenance factor, 80% for all tested luminaires [44, 45]. Figures 12 and 13 show the relative consumed power as a function of the dimming level in the range between 80% and 100% for both tested groups. The results show deviations in energy consumption for the same dimming level. For dimming level 80%, luminaire C LED gives 75% relative consumed power, while luminaire D LED gives 82% relative consumed power. For group II luminaires, the minimum and maximum values of consumed power ranged from 72% for luminaire B T5 to 82% for luminaire A T5.
Figure 12. Relative consumed power for dimming range 80% to 100% for group I (LED luminaires) and lumen maintenance control strategy.
Figure 13. Relative consumed power for dimming range 80% to 100% for group II (luminaires with T5 lamps) and lumen maintenance control strategy. Consumption
Figures 14 and 15 present the differences in energy savings for both tested luminaire groups within the selected dimming range for lumen maintenance strategy of 80% to 100%. The lower and upper lines of the box plots represent the energy consumptions when the dimming level is 80% and 100% respectively.
Figure 14. Differences in energy savings between the LED luminaires in group I for the lumen maintenance control strategy for dimming level 80% to 100%.
Figure 15. Differences in energy savings between the LED luminaires in group II for the lumen maintenance control strategy for dimming level 80% to 100%.
The maximum difference between the luminaires of group I, was estimated at 4.25% (between luminaire LED D and C) and 6.02% (between luminaire T5 A and B) for group II (luminaires with T5 lamps) (Table 7). The differences in energy savings between luminaires with the same technology resulted in significant differences for both cases (due to EDB and driver only) and thus the optimum choice of both EDB and LED drivers is required not only for daylight harvesting but also for lumen maintenance.
Table 7. Energy savings for the tested luminaires using lumen maintenance strategy (Using the same time interval between sequentially cleaning and re-lamping of the luminaires for high performance fluorescent lamps and LEDs with low junction temperatures [44, 45]). Group I (LED luminaires) Energy savings Energy saving Luminaire from lumen difference maintenance (%) (%) D LED 9.06 B LED 10.51 1.46 A LED 11.31 2.25 C LED 13.31 4.25
Group II (luminaires with T5 lamps) Energy savings Energy saving Luminaire from lumen difference maintenance (%) (%) T5 A 9.84 T5 C 10.03 0.20 T5 D 12.79 2.96 T5 B 15.86 6.02
4.3 Dimming and Power Factor Figures 16 to 19 present the frequency distribution of power factor values during the annual operation using closed loop and integral reset algorithms.
Figure 16. Annual frequency distribution of power factor values during the annual operation. Closed loop algorithm with Group I luminaires.
Figure 17. Annual frequency distribution of power factor values during the annual operation. Closed loop algorithm with Group II luminaires.
Figure 18. Annual frequency distribution of power factor values during the annual operation. Integral reset algorithm with Group I luminaires.
Figure 19. Annual frequency distribution of power factor values during the annual operation. Integral reset algorithm with Group II luminaires The lowest PF values are these of the LED luminaires for both closed loop and integral reset algorithm. In this group, PF values between 0.3 and 0.5 occur for more than 80% of the operational hours. In group II, PF values are significantly increased with 80% of operational hours in the range of 0.7-0.9. Between the two control algorithms, integral reset has shown lower PF values for the same operation schedule. The low power factor values could have undesirable technological and economic consequences. The reduced power factor could result to grid frequency declination, current increase and thus safety-related issues and many other harmful effects in the electrical grid [46] that could result in resonance in distribution networks, increased transmission and distribution losses. For the economic consequences there might be potentially higher electricity costs resulting from a power factor charge utility. Thus the information of fluctuation of PF during dimming procedure could be used in optimization strategies in order to avoid low PF values and bad behavior of LEDs. Finally, figures 20 and 21 present the monthly frequency percentages of PF value fluctuation of a T5 B luminaire and a LED A luminaire, as an example. For luminaire T5 B, during June and July PF values vary from 0.8 to 0.9 on the total of the operational schedule time, while for January, during 65.5% of the operational schedule time. As a result, someone can expect the fluctuation of PF of lower values during the summer.
Figure 20. Monthly frequency of power factor values during annual operation using photosensors with closed loop algorithm for T5 B luminaire.
Figure 21. Monthly frequency of power factor values during annual operation using photosensors with closed loop algorithm for LED A luminaire. 5. Conclusions The selection of the proper luminaire is crucial during the design phase not only by satisfying the requirements imposed by regulations as far as it concerns average illuminance and uniformity but also for their energy performance. It is evident that it is also crucial to identify and estimate all parameters involved with their dimming performance. While LED technology is evolving, becoming more and more energy efficient, there is still a lack of accurate energy calculations when dimming is included in cases with daylight harvesting or lumen maintenance. In contrast to other type of lamps, power factor values for the LED lamps [27] / luminaires generally became worse when dimmed. While LED power quality performance seems to be independent on the choice of dimmer [27], further
research is needed not only to provide high power factor equipment for on/off function [47 – 49] but also for dimming. Thus, the dimming procedure of LED technology needs more investigation in order to be integrated in daylight responsive systems. Four dimmable LED luminaires (Group I) and four luminaires with T5 florescent lamps were tested. Light output versus control signal, consumed power versus control signal, consumed power versus light output and power factor versus control signal were measured. The bestfit functions a) between consumed power and light output and b) between power factor and control signal were estimated in order to be used in the simulations. A typical office room was selected [6] and a number of lighting systems were installed together with a daylight harvesting system. A reference signal of photosensor using daylight harvesting and lumen maintenance strategies was created using DAYSIM. The reference signal was used to all the tested luminaires in order to check the influence of different EDBs and LED drivers on a specific control signal in order to quantify the annual energy savings for every case. The analysis of the results shows significant differences in energy savings among the tested luminaires. The annual maximum difference in energy savings between the tested LED luminaires was 5.19% for closed loop, 4.99% for integral reset and 4.25% for lumen maintenance while between the tested T5 luminaires it was 5.72%, 4.57% and 6.02% respectively. Thus the optimum choice of the dimmable luminaire is required in early stage of lighting design, either uses LED or T5 fluorescent lamps. In general:
LED luminaires have performed better in overall annual energy savings than the luminaires with T5 lamps. The reason is mostly due to the LED driver, because at low dimming percentages, it consumes less energy than EDBs.
PF values can be a critical parameter during the design phase of a lighting system especially if a daylight harvesting system is to be adopted.
Integral reset algorithm results in lower PF values in comparison to those of the closed loop algorithm.
The frequency distribution of power factor values during the annual operation using photosensors can be used as a new criterion [12] for the commissioning procedure of a daylight responsive system.
During design procedure of a daylight harvesting system the choice of the appropriate technology between LED and T5 fluorescent lamps can be affected by the low PF values of LEDs. As a recommendation, In order to avoid a bad choice, data bases with control functions for control voltage and power factor for a large number of EDBs and LED drivers
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