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Daylight Impact on Energy Performance of Internal Lighting

Giuseppe Parise (Fellow IEEE) Luigi Martirano (S.M.IEEE) Electrical Engineering Department University of Roma "La Sapienza"

parise@ieee.org martirano@uniromal.it

Abstract Daylight in interior offers a considerable contribute of energy allowing to reduce the level of electric lighting. The

key to design an integrated lighting system is the electric lighting control strategy. An optimal lighting system has to consider the different presence of daylight in the zones to arrange the most favorable luminaires layout. An automatic control system could be employed to switch or dim selected groups of luminaires. The paper suggests a criterion to evaluate the yearly daylight impact on energy performance of internal lighting according to daylight availability, the lighting system layout and the control system arranged. The evaluation is useful for designers to estimate the operation costs in reference to the initial costs.

Keywords - lighting systems; daylight; energy saving, ecodesign, management

I. INTRODUCTION

In large buildings a significant component of the energy used is spent in illuminating the interior of the building. In recent years the European Union EU has actively promoted political campaigns toward energy efficiency. The design of a lighting system has to be comprehensive in regard to the basic design constitution well as its operation and management. The comprehensive design has to survey and define both aspects during the different design phases and during the overall operational life cycle of the system. A new methodology has been suggested to contribute in reducing the gap between the traditional system design studies and their counterpart studies associated with system management aspect both aimed in reducing energy spent by light systems. To identify the cases that a system engineer/designer has to consider, the following two design steps shall be considered. For the basic constitution of the internal lighting system, the design studies shall identify the best efficient solutions for guaranteeing the illuminance E in night time (fig. I). For the management in day time, the system designer needs the evaluation of all the daylight contributions of illuminance ED in the interior that request a lighting system modulation as a lighting duty.

The efficiency could be improved for the basic design adopting new equipment (lamps, control gear, etc.) with high performance and by arranging lighting design practices (localised task lighting systems) in order to guarantee the best illuminance level with the mImmum power. The effectiveness in energy saving could be improved: - in the design stage adopting Building Automation and Control Systems BACS to avoid energy waste for unoccupied and daylight hours; - in the management stage organizing a Technical Building Management System TBMS. TBMS by means of BACS: - provides complex and integrated energy saving functions based on the actual use of a building, depending on the user's real needs to avoid unnecessary energy use. - offers data and diagnostic information for a safer and well-organized operation and maintenance. Frequently encountered examples of excessive use of electric lighting are supplying full electric lighting during periods when a room is partially or completely unoccupied and when daylight is available to satisfy lighting requirements partially or totally. Lighting control strategies can minimize the excessive use of electric lighting in buildings.

Figure I. Case study: illuminance distribution in "night time Note: the black line is the 300 isolux.

The use of day lighting ED to illuminate a space should be integrated with the electric lighting system of the

illuminance i'1Eo=E-Eo and generally requires sophisticated costly lighting controls for successful implementation. In most cases, for effective daylighting the lighting control system must be capable of control in dimming or switching small sectors of interior spaces independently.

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a)

b) Figure 2. Case study: illuminance distribution in "night time "and in "day time" on March 21 - 10: 30 AM, with

artificial lighting switched off (a) and switched all on (b). Note: the black line is the 300 isolux.

The paper suggests an adaptive criterion to arrange the lighting system considering the presence of natural daylight and a practical methodology to evaluate the prospected mean daylight availability in a room and the yearly energy consumption of the combined electric lighting systems with daylighting.

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II. YEARLY DAYLIGHT IMPACT

The general criteria adopted in lighting design of interior consider generally only a condition of "night time". The lighting system in a room adopts generally a symmetrical geometry and luminaires with symmetrical optic. This arrangement provides the required horizontal illuminance E over the room area b *d , being b the width and d the depth. A good practice is to consider symmetrical rows of luminaires with an interdistance of the order of their mounting height that guarantees a certain degree of uniformity and avoiding glare discomfort (figure 1). A complete lighting design (luminaires arrangements and control group system) has to consider also a condition of "day time" with the presence of natural daylight to permit an adequate management of the lamps. This is because daylight, being no uniform and dynamic in nature, provides illumination Eo(x,y,t) that varies in intensity, both spatially (x,y in the room area) and temporally (daytime t) (figure 2a and 2b). In fact, the daylight illuminance Eo(x,y,t) is a function of the geometric position in the room and is variable in the time (fig.3). In each point (x,y) of a room, natural interior illuminance is equal to:

Eo(x,y,t) = DF(x,y) *Eext (t) [lux] (1)

Eext (t) is the external illuminance value according to the latitude considered and variable in the daytime.

DF(x,y) is a fundamental measure of the quantity of daylight in an indoor point as the ratio of external and internal illuminance. The DF(x, y) factors on each point x,y of the room depend on the room and window characteristics and they are independent about the external illuminance.

To exploit daylight as a source of illumination that

dS _______________ 8 _____ 9 ____ �JO distance (m)

Figure 3. Illuminance profiles E, Eo, E-Eo and E+Eo on the middle axis (bl2) in a room with a d depth. Area E*d and dark brown area (E-Eo)*d are proportional to electric power spent in night time and to be integrated in

day time respectively.

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provides an illuminance Eo(x,y,t) in the room zones, it is necessary: - in the design process, to coordinate the artificial uniform lighting system with the daylight availability with a no uniform penetration in the room; - in the management time, to establish an interactive link between daylighting mean values ED in the room zones and the electric lighting system that has to

provide the integration £lEo=E-Eo (figure 3). In particular, considering the daylight availability factor Eo/E, the control could operate by reducing the

electric power to provide only £lED: - if daylight availability is present but not sufficient

to satisfy illuminance level (Eo/E<l), dimming and switching single lamp or luminaire;

- if daylight is present and sufficient to satisfy illuminance level (Eo/E2':l), switching off the system.

The energetic impact of daylighting on the electric lighting could be expressed by a k factor :

k = £lED IE =1 - ED / E [p.u.] (2)

where ED is the daylight mean illuminance In the considered area.

Yearly hours in percent

Figure 4. Dresler diagrams show yearly percentage of hours in intervals for wich Eex! is available or exceeded at

assigned latitude

A practical methodology to evaluate the daylight impact on the electric lighting of a room is to consider

an external yearly average illuminance EextA and three characteristic DF values. The average annually ED could be evaluated by the proposed following methodology that considers as reference Daylight Factors DF and Dresler Diagrams DDs (figure 4).

DDs offer to evaluate the annually hours with a diffuse exterior daylight illuminance as a function of latitude and of different working intervals for a total yearly time t.

The Dresler diagrams furnish different i levels of yearly hours percentage 1/t with a given external illuminance value Eext,j (lux) according to the latitude considered (fig. 4) and in reference to the total

operation time t= I:jtj. It is possible to evaluate the external yearly average illuminance EextA by:

EextA = I:iEext,i tJt.[lux] (3)

The DF average value for a room or a zone furnishes the quantity of daylight present in the room according to exterior daylight. DF depends on:

Geometry of the room (b, d)

Fenestration (h)

Presence of external obstruction (10 factor).

Figure 5 Geometric data for daylight availability

Consequently the average value of the daylight illuminance in a room or a zone of a room is equal to:

Eo= DF A *EextA [lux] (4) To consider also the presence of external obstruction Standard EN15193 [6] suggests a method to evaluate the daylight availability by a qualitative factor Dc:

Dc = (4. 13+20h-1.36IoE) 101: f [p.u.] (5)

where:

1: is the direct hemispherical transmission of fenestration (typical 0.85 - 0.9)

h = w*hl2.5hu (figure 5) IDE is a factor equal to

2,5 for d>2.5hu d/hu, for d<2.5hu

w represents the window wide respect the wide of the room (figure 5)

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h is the height of the window [m] hu is the height of the window on the working plane

[m] 10 is an obstruction factor [p.u.] (figure 6) equal to:

cos(1,5*yo) for Yo <600

o for Yo >600 f is the factor accounting for frame of fenestration

system and dirt on glazing (typical 0.6 - O.S).

Yooe

Figure 6 Obstruction factor

Standard [6] specifies the following relations: Dc>3% strong daylight availability, 3%>Dc>2% medium daylight availability, 2%>Dc> 1 % weak daylight availability, Dc<l % none daylight availability,

Standard EN15193 allows to evaluate graphically the daylight area AD and the no daylight area AND of the room according to the windows and room dimensions. In particular standard EN 15193 says that the maximum possible depth aD of daylight area AD that receives daylight through windows results equal to:

aD = 2.5 x hu [m] (6) Assuming 2h := 2.5 hu the equation becomes:

aD := 2h [m] (7)

In another paper [10] authors suggest to evaluate the daylight availability and penetration in a room considering DF values, in order to subdivide the room in three zones (Figure 7): - Window Zone WZ with a depth from the window side awz that approximately could be assumed equal to the height of the window (h) above the floor. - Transition Zone TZ with a depth from the window side aTZ that approximately could be assumed equal to 2h. The sum of WZ and TZ could be assumed equal to AD of EN15193. - Interior Zone IZ with no daylight availability. The area corresponds with the area AND of EN15193. Average values of DF for each zone (WZ, TZ and IZ) result a function of Dc (Table I).

Table I Dc>3% 3%>Dc>2% 2%>Dc>l% Dc<l% Strong Medium Weak None

DFwz 10% 7% 4% 2% DFTz 2% 2% 1% 0% DF,z 0% 0% 0% 0%

Detailed calculation using more accurate modeling of geometric relations should be used to determine the daylight penetration.

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The proposed zones are valuable easily by using a lighting software as Dialux or Relux and analyzing the punctual values of DF in the room. Lighting system could be arranged in rows parallel to the window according to the dimension of the room and the three zones WZ, TZ, IZ. A good practice is to consider a matrix of luminaires with a step normal to the window equal to h. 1. WZ The luminaires could be generally simply

switched on/off. Eventual dimming or switching by step control could be useful only for weak daylight days.

2. TZ The luminaires could be continuously controlled by dimming or switching by step.

3. IZ The luminaires could be generally simply switched on/off.

� o

"U c

'3:

I

WZ !

- ;

awz=h

Figure 7. The room could be subdivided in three zones WZ, TZ and IZ. The sum of WZ and TZ areas is equal to the daylight area AD. The depth of WZ and TZ are function of the height of the window h.

III. ENERGETIC EVALUATION

Energy savings arising from an installation that takes account of natural lighting in the room can be estimated as long as you know the prospected use of the room and therefore the value of average illuminance on the working plane and the number of hours activities planned for the building. In order to evaluate the impact of control systems in lighting performance the authors suggest a criterion based on the daylight factor that allows to use three ED reference daylight values (fig.S):

ED.WZ= DFwz*EextA[luX] (S) E D,TZ= DFTZ*EextA[luX] (9) E D,IZ= 0 [lux] (10)

Assuming (fig.9):

Ll Ewz = E -DFwz*EextA[lux]

Ll ETZ = E - DFTZ*EextA [lux]

Ll ETZ = E [lux]

(11)

(12)

(13)

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3000

h 1500

�ED(X) 2000

l ;[ i 1500

En'� � ! '"' 1000

� 500 """""'-- Eo,rz-Q Em 0

0 1 2 3 4 5 6 7 8 9 10 distance (m)

Figure 8. Three reference values of DF and daylIght

illuminance values for the three different zones: WZ, TZ, IZ.

1400

1200

1000

800 � � l 600

400

200

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Eo,wz

E

L'.Ewz-O

----EO,TZ

L'.EIZ E

L'.ETZ I EO,IZ r-t h �3� h � � d-2h8

distance(m)

Figure 9. i1Ewz, i1ETz, i1E1Z profiles.

9 ?o

(negative values of �Ewz and �ETZ have to be considered obviously equal to zero), it is possible to estimate the k factor as:

k= �EwzlE*h/d+�ETZIE*h/d+ (1-2h/d) [p.u] (14)

By knowing k, the installed electric power P of the lighting system and the operation time t, it is possible to estimate the daylight impact on energy performance of internal lighting as:

Wcs= k P t [kWh] (15)

Reference values of operation time t are available in the standard EN 15193 [6].

IV. CASE STUDY

Let's consider a lOx 10 m room with a full fenestration with an height h=3 m, without external obstruction, in a location with a latitude equal to 45°, working hours 8 AM-4 PM, total operation time t = 2000 hours [6] and design illuminance E=300 lux. Considering geometric data shown in figure 10 it is possible to estimate:

h=3/(2.5*0.85)= 1.41 IDE=2.5

10=1 (no obstruction)

Figure 10. Case study geometric data

By adopting (5) it is possible to evaluate the daylight : Dc=(4.13+20*0.56-1.36*2.5)*1 *0.85*0.7=7 (strong) The Table 1 offers the values of the three DF factors:

DFwz=10% DFTZ = 2% DFJZ = 0%

Figure II. Case study: illuminance distribution in day time on March 21 - 10: 30 AM, with artificial lighting switched on in three different groups: WZ=O%, TZ=25%, IZ=lOO%. Note: the black line is the 300 isolux.

From Dresler diagrams it is possible to estimate the following values available or exceeded of the external illuminance Eext,i in the tilt percentages of hours:

i=l, tl/t =60% , (t1=1200h), Eext1=15000 lux i=2, t2/t = 10%, ( t2=200h), Eext2= 13000 lux i=3, t3/t = 15 % ,( t3=300h), Eext3= 1 0000 lux i=4, t4/t =10% , (t4=200h), Eext4=7500 lux, i=5, t5/t =5% (t5= 100h), Eext5=2700 lux.

By means of (3) the external average illuminance value is evaluated equal to 12685 lux. By adopting expressions equations (8) (9) and (10), the internal average daylight illuminance in the three different zones results equal to:

ED,wz=1O%*12685 := 1250 lux

ED,TZ=2%*12685:= 250 lux ED,IZ=O lux

and by (11), (12) and (13) the integrations �E result:

�Ewz=O lux

�ETZ=50 lux

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�EIZ=300 lux

By adopting equation (14), considering that hid is equal to 0.3 in this case study , k factor results:

k=50/300*0.3+(1-2*0.3)=0.45

A sample of arrangement of luminaires according to the three zones is a 3x3matrix of 4x 18 watt luminaires (22.5 watt with auxiliaries), organized in 3 different groups of control, one for each zone WZ, TZ and IZ. Figure 11 shows illuminance distribution in day time with artificial lighting switched on in 3 different groups (WZ=O%, TZ=25%, IZ= 100%). Assuming t=2000 h, it is possible to estimate:

W=9x4x22.5x2000 = 1620 kWh

and

Wcs=0.45*1620 = 730 kWh

v. CONCLUSIONS

Intelligent management of energy efficiency, optimizing of costs and quality, requires imagination that can reveal opportunities. In the energy management of buildings an effective program could be follow the two basic aspects of efficiency in the performance and effectiveness in the lighting duty of energy saving. To estimate the real energy spent by lighting systems is an important goal in the designing time since an accurate evaluation could help and guide designers towards the most appropriate choice among different lighting control solutions. The paper suggests a criterion to evaluate the daylight impact on energy performance of internal lighting according to daylight availability, the lighting system layout and the control system arranged. The evaluation is useful for designers to estimate the operation costs in reference to the initial costs and to plan an adequate management for an effective operation.

BIOGRAPHIES

Giuseppe Parise (M'82-SM'03-Fellow '10)) received his Master's degree in Electrical Engineering from the University of Rome in 1972. He has been at Department of Electrical Engineering of the University of Rome "La Sapienza" ever since 1973 and is currently a Full Professor of Electrical Power Systems. He has authored about 200 papers and two patents. Since 1983, he has been a member of Superior Council of Ministry of Public Works. He is active in IEEEIIAS (past Member at Large of Executive Board), Chair of Electrical Power Systems Sapienza University researchers, chair of Italy Section Chapter IA34, member of subcommittees, in CEI (Italian Electrical Commission), in ABIT (Electrical Italian Association), he is

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past President of ABIT Rome's Section and President of Rome's Electrical Professional Engineering Voluntary Committee. Since 1975 he has been a Registered Professional Engineer; he has been designer of Power Electrical Systems in Buildings Complexes like in Roma Sapienza University City and Engineering Faculty, Polyclinic Umberto I, Italian Parliament, Campus Biomedical Research Center.

Luigi Martirano (StM'98-M02-SMll) received the M.S.

and Ph.D. degrees in Electrical Engineering from the University of Rome, Italy, in 1998 and 2002, respectively. In 2000, he joined the Department of Electrical Engineering of the University of Rome "La Sapienza". He is currently an Assistant Professor of Building Automation and Energy Management at the Engineering Faculty and of Lighting Systems at the Architecture Faculty. He is the author or coauthor of more than 60 papers and a co inventor of one international patent. His research activities cover power systems design, planning, safety, lightings, home and building automation, energy management. He is a senior member of the IEEE Industry Applications Society, of the ABIT (Italian Association of Electrical and Electronics Engineers) and of the CEI (Italian Electrical Commission) Technical Committees CT205 and SC315. He has been Registered Professional Engineer.

References: [1] G. Parise, L. Martirano, Ecodesign of lighting systems. Industry Applications Magazine, IEEE, Volume: 17-2011. [2] G. Parise, L. Martirano, Impact of building automation, controls and building management on energy performance of lighting systems, 2009 IEEE-I&CPS, Calgary Canada, 3-7 May 2009. [3] L. Martirano, Lighting systems to save energy in educational classrooms, 2011 IEEE-EEEIC, Rome 8-11 May 2011. [4] EN 12464-1 "Light and lighting - Lighting of work places - Part 1: Indoor work places" Standard 2002 [5] Standard 189 for the Design of High Performance Green Buildings, ANSI-ASHRAB, American Society of Heating, Refrigerating and Air-Conditioning Engineers, January 2009. [6] EN15193_1 "Energy performance of buildings - Energy requirements for lighting - part 1: Lighting energy estimation", March 2005. [7] R. Verderber, o. Morse, J. Jewell, Building design: impact on the lighting control system for a daylighting

strategy, IEEE Transactions on Industry Applications, Vol. 25, march April 1989. [8] J. Love, The evolution of performance indicators for the evaluation of daylighting systems, lAS-IEEE Annual Meeting, 1992. [9] D. Fischer, Interior lightings, Electric Power Applications, lEE Proceedings B, Volume: 133 , Issue: 2, 1986. [10] G. Parise, L. Martirano Combined Electric Light and Daylight Systems Ecodesign 2011 IEEE-lAS, Annual meeting Orlando FL, USA 9-13 October 2011

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