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Energy and Buildings 43 (2011) 2572–2582

Contents lists available at ScienceDirect

Energy and Buildings

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nergy saving potential and strategies for electric lighting in future Northuropean, low energy office buildings: A literature review

arie-Claude Dubois ∗, Åke Blomsterbergnst. of Architecture and Built Environment, Div. of Energy and Building Design, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden

r t i c l e i n f o

rticle history:eceived 28 January 2011eceived in revised form 10 June 2011ccepted 3 July 2011

eywords:fficeightingaylight harvesting

a b s t r a c t

This article presents key energy use figures and explores the energy saving potential for electric lighting inoffice buildings based on a review of relevant literature, with special emphasis on a North European con-text. The review reveals that theoretical calculations, measurements in full-scale rooms and simulationswith validated lighting programs indicate that an energy intensity of around 10 kWh/m2 yr is a realistictarget for office electric lighting in future low energy office buildings. This target would yield a significantreduction in energy intensity of at least 50% compared to the actual average electricity use for lighting(21 kWh/m2 yr in Sweden). Strategies for reducing energy use for electric lighting are presented and dis-cussed, which include: improvements in lamp, ballast and luminaire technology, use of task/ambient

ccupancy controlsanual or automatic dimming

otential electricity savingslluminance

indowshading devices

lighting, improvement in maintenance and utilization factor, reduction of maintained illuminance levelsand total switch-on time, use of manual dimming and switch-off occupancy sensors. Strategies based ondaylight harvesting are also presented and the relevant design aspects such as effects of window char-acteristics, properties of shading devices, reflectance of inner surfaces, ceiling and partition height arediscussed.

© 2011 Elsevier B.V. All rights reserved.

eflectance

ontents

1. Introduction: energy use in office buildings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25722. Energy saving potential and strategies for office lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2573

2.1. Actual energy use for office lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25732.2. Energy saving potential for office lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25732.3. Strategies to reduce energy use for lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2574

2.3.1. Strategies related to electric lighting installations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25742.3.2. Strategies related to daylight harvesting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2577

3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2580Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2580References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2580

. Introduction: energy use in office buildings

Commercial buildings, and primarily office buildings, arelassified among the buildings presenting the highest energy

Abbreviations: CFL, compact fluorescent lamp; DLQ, designer’s lighting quality;EG, electroencephalography; HF, high frequency; LCD, liquid crystal display; LED,ight emitting diodes; LENI, lighting energy numeric indicator (kWh/m2 yr); LOR,ight output ratio; LPD, lighting power density (W/m2); MF, maintenance factor;PD, normalised power density (W/m2 100 lx); U, utilance; WWR, window-to-wall

atio.∗ Corresponding author. Tel.: +46 46 222 7629.

E-mail address: marie-claude.dubois@ebd.lth.se (M.-C. Dubois).

378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.enbuild.2011.07.001

consumption. The total annual energy use in office buildingsvaries in the range 100–1000 kWh/m2 yr, depending on thegeographic location, use and type of office equipment, opera-tional schedules, type of envelope, use of HVAC systems, typeof lighting, etc. [1]. In Northern Europe, office energy inten-sity lies in the range 269–350 kWh/m2 yr and for offices allover Europe, it is about 306 kWh/m2 yr, with mean electricindex 150 kWh/m2 yr and mean fuel index 158 kWh/m2 yr [2].Recently, an inventory of energy use in 123 Swedish office

buildings of different age revealed that office buildings have anenergy intensity of 210 kWh/m2 yr in average, with a high elec-tricity use by square meter (93 kWh/m2 yr excluding heating)[3,4].

M.-C. Dubois, Å. Blomsterberg / Energy an

Nomenclature

Etask illuminance on the task area (lx)Swindow window area (m)Sfloor floor area (m)

Greek letters˚lum luminous flux emitted by the luminaire (lm)˚ luminous flux emitted by the lamp (lm)

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lamp˚TA Luminous flux reaching the task area (lm)

The recent 2010 Energy Performance of Buildings DirectiveEPBD) places a high demand on building professionals to pro-uce (and eventually retrofit) office buildings to near-zero energyse levels. The good news is that according to previous research,odern office buildings have a high energy savings potential [5,6].

lectric lighting is one area where energy savings are possible ateasonable cost in new buildings as well as in retrofit projects.ne recent study [7] indicated that investments in energy-efficient

ighting is one of the most cost-effective ways to reduce CO2 emis-ions and many studies show that electricity use for lighting coulde reduced by 50% using existing technology [8,9].

This article explores the potential and strategies for energy sav-ngs in office lighting including control systems mainly in Northernurope with some specific information from Sweden. The articles based on a literature review carried out as part of the Swedishroject ‘Energy-efficient office buildings with low internal gains:imulations and design guidelines’.

. Energy saving potential and strategies for office lighting

.1. Actual energy use for office lighting

Globally, lighting is an important issue in minimizing over-ll energy consumption [10]. In Sweden, for example, lightingccounts for around 10% of total energy consumption in the coun-ry, and this area offers considerable potential for energy savings11].

In commercial buildings, lighting constitutes generally 20–45%f electricity demand [6] but it varies a lot from one building tonother and the consumption of electric lighting can sometimes bes much as 40% of the gross energy consumption in some build-ngs [8]. The most significant environmental impact (80–90%) ofighting is generated during the operation of the lighting system;he cost of an electric lighting installation typically represents only5% of total costs, while electricity use during operation repre-ents around 70% of total costs [12]. In Sweden, lighting normallyccounts for 25–30% of electricity use in non-residential premises4,12,13]. The recent inventory of 123 office buildings of vary-ng age by the Swedish Energy Agency [4] revealed an averagenergy intensity of 21 kWh/m2 yr for office lighting and an aver-ge installed lighting power density (LPD) of 10.5 W/m2, whicharies according room type: 13.1 W/m2 for individual office rooms,2.4 W/m2 for landscape offices, and 8.6 W/m2 for common roomsincluding corridors). Note that in 1990, office electric lighting was0 kWh/m2 yr in Sweden; a reduction of around 9 kWh/m2 yr hashus occurred in 20 years [3]. Average LPDs of 7–11 W/m2 is achiev-ble using efficient lamp circuits (based on T8 i.e. 26 mm fluorescentubes and standard electronic high frequency ballasts) for generalffice lighting of 300–500 lx [see 14].

Regarding the average LPD, Hanselear et al. [10] noted that thisidely used indicator does not take into account the requirement

or the mean illuminance. According to these authors, the nor-alized power density (NPD, expressed in W/m2 100 lx), which is

d Buildings 43 (2011) 2572–2582 2573

calculated as the power density divided by the mean maintainedilluminance on a reference plane is a more interesting metric sinceit allows a simple and straightforward comparison between dif-ferent spaces with different illuminance requirements. Dependingon room size, room surface reflectance, light source and the appli-cation, actual target NPD-values for efficient lighting installationswith fluorescent lamps and a high degree of installation mainte-nance should be 1.9–2.3 W/(m2 100 lx) [14].

Assuming, for example, a task illuminance (Etask) target of 500 lx,we obtain:

LPD = NPD × Etask = 1.9–2.3 W/(m2 100 lx)

× 500 lx = 9.5–11.5 W/m2 (1)

These values are in line with the values measured in the recentSwedish inventory reported earlier [4].

2.2. Energy saving potential for office lighting

According to Borg [15], an existing office (in Sweden) usesaround 23 kWh/m2 yr for electric lighting whereas a modernadvanced installation may only use 11 kWh/m2 yr. If occupancyand daylight sensors are integrated in the installation, the annualenergy consumption for lights may come down to as low as5 kWh/m2 yr.

Recently, Bülow-Hübe [9] investigated the daylight availabilityand electricity use for lights in offices located in Gothen-burg, Sweden, through simulations using the validated programsRayfront/RADIANCE and DAYSIM. The study included single-celland open-plan offices with three different facades (30, 60 and 100%window-to-wall ratios). Assuming a LPD of 12 W/m2, annual elec-tricity use for lighting was calculated to be 28 kWh/m2 yr if thelights were switched on 9 h/day, 5 days/week. Assuming manuallight switching, with a mix of active and passive users, the elec-tricity use obtained dropped to 20–23 kWh/m2 yr. With a perfectlycommissioned photosensor dimming system, and mixed users,lighting electricity use obtained was in the range 11–18 kWh/m2 yr.This study thus demonstrated by simulation that it is possible tocut down electricity use for office lighting by about 50% (from 23to 11 kWh/m2 yr) using existing technology, and in comparison toa case with manual on/off switch near the door.

In other countries, Santamouris et al. [16] reported the findingsof a large monitoring campaign in 186 office buildings in Greece,where the specific energy consumption of the buildings for heat-ing, cooling, and lighting purposes, as well as the consumptionfor office equipment were monitored. The data for electric light-ing showed average energy use ranging from 15 to 25 kWh/m2 yrdepending on type of building. Around 50% of the buildings pre-sented a lighting consumption inferior to 11 kWh/m2 yr while forthe majority of buildings (86%), the consumption was less than20 kWh/m2 yr.

The European standard EN-15193 [17] presents LENI (Light-ing Energy Numeric Indicators) prescribing installed LPD for smallindividual office rooms of 10 W/m2 with preferable target around8 W/m2 (for ‘normal’ illuminance levels for offices). Taking intoconsideration a reference annual time of use (2500 h) and vari-ous lighting control strategies, the calculated annual energy useranges from 20 to 7 kWh/m2 yr, which shows the large potential forenergy savings through control strategies (up to 65% reduction). Forlarge office rooms (>12 m2), this standard recommends an installedLPD under 12 W/m2 with preferable target under 10 W/m2, which

results in annual energy use in the range 30–17 kWh/m2 yr depend-ing on the selected lighting control strategy (see Table 1). Acombination of occupancy sensors and daylight dimming providesthe lowest energy intensity values.

2574 M.-C. Dubois, Å. Blomsterberg / Energy and Buildings 43 (2011) 2572–2582

Table 1Guidelines for installed LPD (W/m2), reduction factors and LENI (kWh/m2 yr).

Type of room LPD (W/m2) Reduction factor LENI (kWh/m2yr)

Manual control Absence/presencecontrol

Daylight control Manual control + Absence/presencecontrol

+ Daylight control

Individual office rooms (>10 m2)Obl. 10 0.8 0.75 0.56 20 15 8Pref. 8 0.8 0.75 0.56 16 12 7

Large office rooms (>12 m2)Obl. 12 1 0.90 0.77 30 27 21Pref. 10 1 0.90 0.77 25 23 17

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CorridorObl. 8 1 0.75 0.Pref. 6 1 0.75 0.

These various sources indicate that it is possible to achievenergy savings of the order of 45–65% depending on room type andontrol strategy, and this, with existing technology. Loe [14] alsoresented detailed calculations showing that 50% savings are pos-ible when an installation has a task and building lighting approachnd is controlled to provide illumination only when needed1; helso demonstrated with simple calculations that it is likely thatreater savings are achievable.

Also worth noting: besides direct electricity savings due toeduced use of lights, indirect energy savings can also be obtainedecause of the reduced heat production and cooling needs [10,18].owever, in cold climates, there might be an increase in energy use

or heating, which is likely to be smaller than the electricity savings.he next section examines the strategies to implement in order toeach such low energy intensities.

.3. Strategies to reduce energy use for lighting

Strategies to reduce energy use for electric lighting in officesnclude:

) Strategies directly related to the electric lighting installation:• Improvement in lamp technology;• Improvement in ballast technology;• Improvement in luminaire technology;• Use of task/ambient lighting;• Improvement in maintenance factor;• Improvement in utilance or utilization factor;• Reduction of maintained illuminance levels;• Reduction of switch-on time;• Occupancy sensors and/or manual/automatic dimming.) Strategies related to daylight harvesting:• Effect of latitude and orientation;• Effect of window characteristics;• Effect of shading devices;• Effect of reflectance of inner surfaces;• Effect of ceiling height;• Effect of partition height.

The next sections discuss each strategy in detail. An overview ofhe related energy savings is presented at the end (see Table 2).

.3.1. Strategies related to electric lighting installations

.3.1.1. Improvement in lamp technology. Although T5 fluorescentamps have existed for 15 years, recent statistics (for Sweden)4] indicate that many existing lighting installations still use T8r even, older T12 lamps, which have a much lower luminous

1 In his calculations, about half the savings were due to the task/ambient lightingpproach and about half to the controls applied to the task/ambient lighting system.

20 15 915 11 6

efficacy (lm/W). Replacing T12 with T8 lamps can save up to 10% ofthe energy consumption while giving 10% more light [19]. Newer T516 mm lamps have even higher efficacies (90–104 lm/W) achievinga 40% reduction in energy use (compared to T12 lamps of 60 lm/Wwith magnetic ballasts) but these lamps need different fittings[13,19]. Note that the replacement rate of lighting systems is low,e.g. about 3% per year in Sweden, which implies that it takes about33 years to replace old lighting installations with new, energy-efficient ones [13]. Since 1995, not even 40% of lighting installationshave been changed and it will take another 20 years before thepotential for energy savings is fully exploited [13].

Recent statistics for Sweden show that fluorescent lamps withconventional ballasts represent nearly half (46%) of the installedelectric lighting power density in offices [6]. Also, traditional incan-descent lamps represent 12% of total installed lighting powerdensity [4]. A theoretical calculation [3] has shown that chang-ing all fluorescent tubes to T5 tubes and all incandescent lightbulbs to compact fluorescent lamps (CFL) in Sweden would reducethe energy intensity for electric lighting in offices by 5.5 and1.4 kWh/m2 yr respectively. The research by Santamouris et al. [16]in Greece, which consisted of a large monitoring campaign in 186office buildings, showed that the replacement of the existing lampswith fluorescent lamps with an efficacy of 80 lm/W, would reducethe total energy requirements for lighting by up to 35%; the use ofvery high efficacy lamps (117 lm/W) provided a reduction of up to55%.

Incandescent lamps which are changed for a CFL are directly eco-nomical and provide up to 15 times increase in durability for thesetypes of lamps [4]. Changing a conventional fluorescent tube with aT5 tube can allow saving electricity use by up to 80% (including sav-ings from the HF ballast, better luminaire and occupancy + daylightdimming) and at the same time obtain flicker-free light. Also, newlamps, especially the T5 lamps, contain less mercury than olderlamps and have a longer lamp life, which means that fewer lampsneed to be disposed of in time [15].

According to Borg [15], today’s most energy-efficient practicescenarios use modern technologies available on the market, whichmeans extensive use of occupancy and daylight sensors, T5 or metalhalide light sources and efficient luminaires. Current predictionsreveal, however, that light emitting diodes (LEDs) should providethe majority of light sources by 2035 [20]. The light efficacy of LEDsis increasing very quickly; it has nearly doubled every other year. In2009, white LEDs with a light efficacy of 100 lm/W were available[21]. However, it is expected that more traditional light sources willhave a major role to play for some time yet, which means that it willbe developments in the design and control of lighting installationsthat are likely to provide substantial energy saving opportunities

in the immediate future [22].

2.3.1.2. Improvement in ballast technology. In the past, ballasts wererelatively simple wire-wound devices and consumed an apprecia-

M.-C. Dubois, Å. Blomsterberg / Energy and Buildings 43 (2011) 2572–2582 2575

Table 2Overview of energy saving strategies and relative energy saving potential.

Energy saving strategy Relative saving potential References

1 Improvement in lamp technology 10% (T12 to T8) 40%a (T12 to T5) [19][13,19]2 Improvement in ballast technology 4–8% [19]3 Improvement in luminaire technology 40%b [13]4 Use of task/ambient lighting 22–25% [14,25]5 Improvement in maintenance factor 5%c [12]6 Improvement in utilization factor Depends on application and context7 Reduction of maintained illuminance levels 20% (500 to 400 lx) [34]8 Reduction of total switch-on time 6%d [3]9 Use of manual dimming 7–25% [33,45,46]10 Use of switch-off occupancy sensors 20–35% [33,48,49]11 Use of daylight dimming 25–60%e [54,74]

a However, this number also includes improvements due to HF ballast and improvement in luminaire.b However, this number also includes dimming (occupancy and daylight) and improved (HF) ballasts.c About 5% of light output would be lost each year without a proper maintenance programme.

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d By reducing average existing total switch-on time to only 2600 h/year.e This number is highly dependent on the climate, the shading strategy and the b

avings appear to be much higher than a more realistic case with e.g. manual on/of

le amount of energy – typically 10–20% of the lamp wattage [14].odern ballasts however often use electronic circuits. High fre-

uency (HF) electronic control ballasts, which can be used withoth T8- and T5-tubes, use less than half the energy required by theonventional wire-wound types [19]. Inefficient ballasts are beingteadily phased out across the European Union following the adop-ion of the Ballast Directive 2000/55/EC [23]. Since November 21,005, only low loss magnetic ballasts with typical efficiencies of 85%depending on the lamp power) and high frequency electronic con-rol gear with efficiency values more than 92% are allowed [10,23].

Recent statistics [4] for Sweden indicate that fluorescent tubesith HF-ballasts, with T8 and T5-tubes, represent so far only 27%

f the total installed lighting power density in offices in spite of theact that HF ballasts have existed for 20 years. HF lighting has manydvantages: an improved lighting quality, flicker-free lighting, andeduction in power demand, compatibility with occupation sensingnd daylight control, better controllability and longer life [20].

.3.1.3. Improvement in luminaires. Lighting equipment essentiallyonsists of a lamp, controls and control gear if needed, and a lumi-aire, each contributing to the overall efficiency [19]. New lightingxtures reflect light in such a way that more light can be usedhere needed and less light gets lost in the light fixture itself [14].hile the introduction of T5 lamps in 1995 allowed a 40% reduc-

ion in energy use compared to T12 lamps, the combination of neweflector material in lighting fixtures with dimming (daylight andccupancy) allows achieving another 40% energy reduction. Thesemprovements combined mean that modern lighting installations

ay use only about one fifth (20%) of the energy used by oldernstallations [13].

The efficiency of the luminaire is defined by light output ratioLOR):

OR = ˚lum

˚lamp(2)

here ˚lum is the initial luminous flux released by the luminairend ˚lamp is the initial luminous flux released by the lamp. TheOR describes the efficiency of the luminaire in emitting lamp fluxn lumens into the interior space. Its value is determined by theptical layout, the quality of the optical materials (reflectors, dif-users, filters, etc.), the ambient temperature of the lamp and theequirements for preventing glare. The use of new materials, suchs coated reflectors and holographic diffusers, allows LOR values of

5% and higher to be obtained [10].

.3.1.4. Use of task/ambient lighting. Loe [14,22] suggested an alter-ative approach to lighting design which consists of separating the

ne for comparison (if compared to a case where lights are on 100% of the time, theh at the door).

elements of task lighting and building or amenity lighting and tocontrol them both independently, but in an integrated way. Accord-ing to him, this is not a new approach as it was used in the early partof the 20th century when lighting was extremely expensive, bothin terms of the electricity it consumed and the cost of equipment,particularly lamps. Rogers [24] also suggested using task/ambientelectric lighting system where daylight could provide an ambientlevel of light adequate for circulation and general tasks, and elec-tric task lighting could provide higher localized illumination. Thisapproach is already used in Denmark, where relatively low gen-eral illuminance levels (50–100–200 lx) required in the office areoften provided by a combination of electric light and daylight, andillumination on the task (500 lx) is achieved with individual tasklamps. Worth noting that the Danish system is based on previousresearch which has shown that more uniform (monotonous) light-ing normally demands higher illuminance levels and that users arenormally more satisfied with control over their own task lamp [25].

In addition to better integration with daylight, a task/ambientsystem often yields lower LPDs, resulting in a greater base levelof energy savings, because illuminance is only provided when andwhere needed [24]. In experiments achieved by Veitch and New-sham [26] in the nineties where nine light conditions includingthree levels of LPDs (9, 14, 25 W/m2) and three levels of designers’lighting quality (DLQ) were evaluated by temporary office work-ers, it was shown that lighting systems incorporating both taskand ambient lighting (9 W/m2, measured LPD including task light-ing) were rated as providing better quality lighting than systemswithout task lighting (14 W/m2). In recent experiments in Denmark[25], installations combining low level general daylighting/lightinglevels with task lighting achieved total LPDs of 5.4 W/m2 includ-ing the task lamp and respecting the Danish code DS700 (500 lxon task, 200 lx in immediate surroundings, 100 lx in remote sur-roundings and 50 lx for general lighting). This installation resultedin 25% reduction in electricity use compared to a standard energy-efficient installation. Loe [14] also presented calculations showingenergy savings of around 22% (compared to fixed general lightingsolution) by simply using a combination of general lighting level(200 lx) combined with task lighting.

However, some researchers [27] expressed reserves about theuse of task lighting: because of the increased risk of visual fatigue,desktop lamps should not be used for prolonged periods of time,and never as the sole light source. This was also an important resultof the Danish study reported earlier: users complained that the

general light level, and especially the amount of daylight, was toolow [25]. Govén et al. [28] showed that the level of backgroundluminance, and particularly the luminance of the walls, has aninfluence on visual, emotional and even biological aspects. They

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ecommended wall luminance of around 100 cd/m2 for future light-ng applications (office context with 500 lx on task).

According to Borg [15], no more than 20 W should be allowedor one task lamp to reach the recommended 500 lx in the task area,ncluding the electricity consumption of the ballast. Technologiessing 6 W and less have been demonstrated with integrated LEDs

n Swedish tests and are now being offered by several manufac-urers [15]. In the Danish study [25], the recommended energyntensity of task lamps was 1–2 W/m2 and for the general light-ng, it was 5–6 W/m2, which resulted in total annual energy use of.6 kWh/m2 yr for electric lighting using daylight-responsive dim-ing and assuming normal work hours i.e. from 8:00 to 17:00, five

ays a week.

.3.1.5. Improvement in maintenance factor. The maintenance fac-or (MF) is the ratio of the average illuminance on the workinglane after a certain period of use of a lighting installation to the

nitial average illuminance obtained under the same conditions forhe installation. The maintenance factor takes into account lampurnouts, lamp lumen maintenance, luminaire dirt depreciationnd room surface reflectance maintenance. The rate of reduction oflluminance is influenced by the equipment choice, routine cleaningf the lamp, luminaire, and room surfaces and the environmentalnd operating conditions. In offices, schools and shops, which doot normally become very dirty, the lighting output can be reducedy up to 5% per year [12]. This reduction in light output dependsn the fact that lighting fixtures, light sources, walls and ceiling,ecome dirtier and also some lamps get older or burn. A highaintenance factor (cleaning) together with an effective main-

enance programme promotes energy efficient design and limitshe installed lighting power requirements [10]. A high degree ofnstallation maintenance involves cleaning of the luminaires everyear, and of room surfaces every three years, as well as bulk lampeplacement every 10,000 h [14].

.3.1.6. Improvement in utilance and utilization factor. An importantrinciple of energy efficient lighting design is to make the mostf any light sources available by directing the light to where it iseeded [22]. Rea [29] introduced the term ‘application efficacy’ that

s, first, based upon the lamp and luminaire combination ratherhan, as usually considered, solely on lamp luminous efficacy. Forxample, in architectural lighting, some specifiers recognize thathe most effective lamp and luminaire combination for a givenpplication is often not the one with the highest lamp luminousfficacy. The widespread use of tungsten halogen sources in dis-lay and downlighting applications, despite the low lamp luminousfficacy of these technologies relative to others, suggests that lampuminous efficacy is only partially related to the effectiveness of aighting installation.

The application efficacy is related to the utilance factor U defineds (from [10,14]):(4)U = ˚TA

˚lumwhere ˚TA is the initial luminous flux

eaching the task area and ˚lum is the luminous flux released fromhe luminaire. The utilance U relates the luminous flux from theuminaires to the luminous flux on the target area. It depends on:

the arrangement of the luminaires in the room in relation to theposition of the task area;the luminous intensity distribution of the luminaires and thespacing to height ratio;the reflectance of the surroundings, which determines the indi-rect contribution.

The LOR and the utilance U are combined in what is called thetilization factor UF defined as UF = ˚TA/˚lamp. The value of thetilance is even more important than the LOR in reaching energy

d Buildings 43 (2011) 2572–2582

efficiency targets and target utilance values for efficient interiorlighting have been recently proposed in the literature (see [10]).

2.3.1.7. Reduction of maintained illuminance levels. Recommendedmaintained illuminance levels are prescribed over the task area onthe reference surface which may be horizontal, vertical or inclined.In the USA and Canada for example, an illuminance of 500 lx onthe work plane is recommended for office work [30,31]. In the UK,the working plane illuminances recommended for offices are in therange 300–500 lx, the lower limit being recommended for mainlycomputer-based work and the upper limit for mainly paper-basedwork [32]. In Sweden, an illuminance of 500 lx is also recommendedon the task area for individual office rooms while 300 lx are nor-mally accepted as general lighting level for landscape offices [12].

Many studies indicate that office workers generally preferilluminance levels which are lower than recommended by thestandards [33–38]. However, one extensive study under office con-ditions has shown that people prefer artificial lighting in addition tothe normal daylighting present in an office environment: average800 lx on top of the prevailing daylight contribution [39]. Also, sev-eral other studies [40–41] have indicated a preference for very highilluminance levels (including daylight) ranging from 0 to 3000 lx.Recently, Fotios and Cheal [42] demonstrated that the preferredilluminance is significantly influenced by the range of illuminancesavailable to the research participant (the stimulus range), and thatoccupants tend to select the middle point of the range available.They concluded that studies with different stimulus range willlead to different estimates of preferred illuminance, with studieswith large range resulting in higher preferred illuminance selected.These authors thus proposed to give occupants a restricted rangeof illuminances to choose from, this range being chosen so thatthe expected preferred illuminance will be less than the stan-dard 500 lx, meaning they will be satisfied with their environmentdespite an illuminance less than 500 lx, and energy consumptionwill be reduced.

Boyce et al. [34] also claimed that lighting practice that uses500 lx as the target for maintained illuminance is excessive. Byusing 400 lx as a design criterion, a 20% decrease in energy con-sumption could be gained together with a likely increase in thepercentage of office workers who are within 100 lx of their pre-ferred illuminance. Tregenza et al. [43] claimed that a universallypreferred illuminance does not exist since they found that in bothseasons the range of illuminance deemed acceptable is greater thanthe range considered as unacceptable. Boyce [44] noted a lack ofassociation between illuminances and their subjectively viewedsuitability when subjects were carrying out realistic tasks, i.e. tasksfor which visibility requirements were satisfied at relatively lowlevels of illuminance. Loe [22] suggested recommending a band ofadjustable task illuminance for particular situations rather than aminimum level, which would probably be more appropriate andyield higher energy savings as some individuals would probablyselect lower illuminance levels than the recommended levels.

2.3.1.8. Reduction of switch-on time. The total number of units ofelectricity consumed by the lighting installation will also obvi-ously be affected by the length of time the lighting is switched on.The European standard EN 15193 [17] recommends a total utiliza-tion time for electric lighting in offices of 2500 h (2250 daytimehours + 250 nighttime hours). In the Swedish context, for example,recent calculations have shown that reducing time of use of elec-tric lighting in offices to 2600 h/yr would reduce energy intensityfor electric lighting by 1.3 kWh/m2 yr thus going from the actual

21 kWh/m2 yr to 19.7 kWh/m2 yr [3].

An annual time budget of 2500 h corresponds to about 48 hper week and thus 9.6 h per day (5 days/week) of total switch-on time, which is feasible even taking into consideration flextime.

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imiting the range of total switch-on time implies in practice thatighting systems must be switched-off after work hours, a practice

hich is unfortunately still not implemented in many countries.lso, even with a maximum switch-on time of 2500 h and a LPD of0 W/m2, the resulting annual energy use is 25 kWh/m2 yr. There-ore, in order to reach a total energy intensity for lighting of around0 kWh/m2 yr, it is necessary to either reduce the LPD to aroundW/m2 or to switch-off lights at least 60% of the time, by usingontrol systems such as manual dimming, occupancy switch-offnd/or daylight dimming.

.3.1.9. Use of manual/automatic dimming and occupancy sensors.everal studies have generated promising results showing thatlectrical energy use can be substantially reduced by using lightingontrol systems such as manual dimming and occupancy sen-ors. For manual dimming, the electric lighting energy savingsbtained range between 7–25% [33,45,46]. Note that Moore et al.47] reported on a survey of user attitudes toward control sys-ems and the luminous conditions they produce in 14 similar UKffice buildings. They observed that controllable systems wereypically operated at 50% of maximum output but they did notpecify the exact corresponding electricity savings. For switch-ff occupancy sensors, lighting electricity savings range from0 to 35% [33,48,49].

According to Guo et al. [50], automated control systems savenergy compared to manual switching, but differences betweenbserved savings and industry estimated savings that result fromhe application of these systems are often observed [50,51]. Studiesomparing energy use after installation of occupancy sensors withanual switching on as the baseline, show energy savings of about

5% in private offices with a sensor time delay setting of 20 min,hich is the lower bound for manufacturer claims [50].

.3.2. Strategies related to daylight harvestingMost commercial spaces have enough daylight next to windows

o eliminate the need for electric lighting [52] (apart for build-ngs located in the far north of Scandinavia where there is hardlyny daylight in the winter). The exploitation of daylight, com-only referred to as ‘daylight utilization’ or ‘daylight harvesting’,

s recognized as an effective means to reduce the artificial lightingequirements of nondomestic buildings. Daylight utilization mayllow energy savings compared to electric lighting due to its higheruminous efficacy. For a given quantity of illumination, light fromlear blue skies delivers the least amount of heat gain [53].

Research has shown that daylight-linked lighting control sys-ems such as automatic on/off and continuous dimming have theotential to reduce the electrical energy consumption in officeuildings by as much as 30–60% [54]. Bodart and De Herde [55]xamined previous literature on the subject and concluded thatt is difficult to evaluate the energy savings coming from lightimming as a function of daylight availability. For office buildingsith classical windows (no specific daylight system), they foundrevious research indicating electricity savings for lights rangingrom 20% to 77%. They achieved a study by computer simulationsith ADELINE and TRNSYS for the Belgian climate and showed thataylight harvesting allows reductions of electric lighting consump-ion of 50–80%, which would yield primary energy savings for theuilding of up to 40%, considering a glazing type normally used

n offices. They found that the savings obtained depended on thelazing visual transmittance, facade configuration, orientation ofpening, room width as well as reflectance of interior walls. Inanada, Athienitis and Tzempelikos [18] developed a simulation

ethodology and carried out simulations for a typical office room

5 m × 5 m × 3 m) located in Montreal with a Vision Control windoweasuring 2 m × 4 m on the facade facing 10◦ east of south with no

bstructions. Comparing with an office space where lights are on

d Buildings 43 (2011) 2572–2582 2577

100% during all working hours, they obtained electricity savings forlights (with the window system and controllable highly reflectivevenetian blinds plus light dimming) reaching 76% on overcast daysand 92% on clear days.

In parallel to this, research has also shown that in spite of fewpromising laboratory test results and computer predictions, mostdaylight-linked systems do not provide the anticipated energy sav-ings when installed in real buildings [54]. Post-occupancy studiescarried out in real buildings have shown that the actual energy per-formance of daylit buildings is invariably markedly worse than thatpredicted at the design stage [53]. Significant among the variousreasons for this is the inability of the standard predictive methodsto account for realistic conditions [56].

Dimming electric lights based on available daylight is alsoexpensive with significant equipment (dimming ballasts) and com-missioning costs. Papamichael et al. [52] claimed that to date, onlya small fraction of side-lit dimming applications operate satisfac-torily. While useful in low daylight areas, dimming is not reallynecessary in areas with high levels of daylight, where dimming isonly useful during the early morning and late afternoon [52]. More-over, dimming ballasts are less efficient than non-dimming ballastsand consume 10–20% power even at the lowest possible light out-put [52]. While it is true that occupants might be more distractedby the dramatic changes in light levels caused by switching (asopposed to a smooth dimming function), switching will generallyonly occur twice a day, during early morning and late afternoon orearly evening hours. Moore et al. [47] also claimed that there is lit-tle merit in equipping locally dimmable systems with photoelectriccontrols, given the general dislike of photoelectric control reportedin the literature.

In general, the literature reveals a number of reports of switch-ing behavior being related to daylight availability [45,57–61].However, a number of studies have indicated no relation-ship between daylight availability and electric lighting use[35,47,62–65] and even higher levels of electric light with higherexternal illuminances [61]. Begemann et al. [61] proposed two pos-sible explanations to this phenomenon. First, occupants could beattempting to balance the brightness of window areas with thoseof the interior. Secondly, the phenomenon could be caused byoccupant adaptation to exterior conditions prior to entering thebuilding, and too short a period elapsing to allow full adaptationto interior conditions prior to switching. This shows that daylightharvesting cannot rely purely on occupant behavior, some form ofautomatic control must be provided either as automatic switch-off or photoelectric dimming, otherwise there is a risk that moreenergy will be used.

Despite all these arguments, we have to consider the fact thatdaylight utilization is just one of the many arguments for admittingdaylight in buildings. In recent years, research has identified thebenefits of daylight and sunlight in buildings for the health andwell-being of occupants, including its necessity for the regulationof circadian rhythms [66,67]. A number of studies have stressed theimportance of daylight and windows and have demonstrated theirnumerous positive effects on building occupants. Most importantlyfor office environments, daylight is generally preferred to electriclighting [68].

In addition, daylight remains a predominant factor in how aspace is revealed and perceived by its users. Daylight presence has asignificantly positive contribution to lighting quality and makes aninterior space look more attractive. Glare studies have shown thatglare is tolerated much more from a daylight source than from itsartificial equivalent [69]. Significantly less incidents of eyestrain are

reported by people whose workstations received large proportionsof natural light [70]. Also, Sutter et al. [71] showed that high lumi-nance contrasts were more tolerated when the window occupieda large portion of the visual field.

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However, daylight, because of its variability and intensity, posesdditional challenges and needs to be carefully considered toealize its potential to provide healthy and comfortable office envi-onments. Boyce [72] argued that daylight offers no guarantees ofbetter work experience. Tzempelikos and Athienitis [73] warned

hat large fenestration areas often result in excessive solar gains andighly varying heating and cooling loads. Daylighting could lead tonet increase in energy consumption if the additional cooling loadue to daylight (i.e. including the solar component) exceeds thenergy saved due to reduced electric lighting, or if the net heatains and losses through the fenestration do not compensate forhe lighting energy saved [53]. In fact, an all too common scenarion overglazed buildings is where the blinds are down to controllare and the lights are on [73]. A full consideration of the potentialor daylighting to save energy should, at some point, account alsoor the thermal effects of daylight [53]. Fortunately, many recenttudies indicate that daylight harvesting can provide electric light-ng savings with ‘reasonable’ fenestration areas (around 30–40%f facade area) including the use of shading devices when needed9,55,73]. The next sections examine the effect of some designarameters on daylight utilization potential.

.3.2.1. Effect of latitude and orientation. The potential for day-ighting during winter is limited in the Nordic countries, due tohe high latitudes and restricted daylight availability in the win-er. Reinhart [74] studied the influence of various design variablesn the daylight availability and electric lighting requirements inpen plan office spaces using DAYSIM. Five climatic centers whichepresent the ambient daylight conditions of 186 North Americanetropolitan Areas were identified. For these five climatic centers,

ver 1000 office settings were investigated which feature varyingxternal shading situations, glazing types, facade orientations, ceil-ng designs and partition arrangements. The daylight performancef the offices was expressed in terms of their daylight autonomyistributions and energy savings for an ideally dimmed lightingystem. His results indicated that energy savings were falling withising latitude and total annual solar radiation. An analysis of theonthly energy savings for the five sites studied in the United

tates and Canada showed that most differences appear in the win-er months due to shorter day lengths in the North. In particular,he Vancouver region was characterized by dark overcast winterkies.

Concerning orientation, Bodart and De Herde [55] found thatorth oriented room consumption was always higher than for otherrientations but they did not use solar shading devices in theiresearch so the savings obtained for the south, east and west ori-ntations may not be realistic given the fact that shading devicesould probably be used in reality for these orientations. They alsooted that the influence of orientation was minor and even non-xistent. They explained this by the fact that daylighting availabilityas so important that the diffuse and the external reflected lightingortions added to the internal reflected daylight were sufficient toeach the minimum lighting requirement.

A user assessment survey by Osterhaus [75] in real daylit officepaces achieved in 1992–1994 in nine office buildings in the USAnd Germany where 250 questionnaires were distributed to indi-idual office workers indicated that north-facing windows (allurvey sites were located in the northern hemisphere), have aower impact than those facing other directions. Respondents inffices with northern orientation reported presence of daylightlare somewhat less frequently. However, there was no evidencehat windows facing other directions created higher levels of glare.

hile it can reasonably be expected that north-facing windows cre-te fewer concerns for glare discomfort, the author was surprisedhat east and west-facing windows showed no higher levels thanouth-facing windows. The author mentioned, however, that the

d Buildings 43 (2011) 2572–2582

beautiful view over certain landscape elements may have mitigatedthe effect of daylight glare for some orientations.

2.3.2.2. Effect of window characteristics. The size, shape, position,orientation and number of windows influence the daylight indoors,as does the framing and transmittance of glazing [27]. Gratia and DeHerde [76] provided some recommendations regarding windows:

(1) Generally, the higher the window is, the better the lowest partof the room is lit and the deeper the naturally lit zone is.

(2) The area of the window plays a primordial role.

In his simulations, Reinhart [74] found that changing from ahigh (75%) to a low (35%) transmittance glazing reduced energysavings by about 20 percentage points for the peripheral office. Heoutlined, however, that care has to be taken that such energy sav-ings on the electric lighting side are not compromised by additionalcooling loads. Therefore, he suggested that an ‘adequate’ blind con-trol strategy had to be chosen. In Belgium, Bodart and De Herde[55] performed a simulation study where they observed that theelectricity consumption did not vary linearly with the glazing trans-mittance. They observed that when an illuminance level of 500 lxwas reached, any daylight availability had no more influence onthe artificial light consumption. They concluded that a high visi-ble transmittance is beneficial for the lighting energy consumptionbut that beyond a certain value, the benefits decrease. They showedthat in an office room, an increase in the ratio Swindow/Sfloor from 16to 32% reduced the electric lighting consumption by 12% for glaz-ing with 20% visible transmittance and by 36% for glazing with 81%visible transmittance. However, these numbers did not include theadditional energy use for heating and cooling caused by the largerwindow.

In general, highly glazed facades, often with poor shading, havebecome very common. This, together with the extra heat gainsfrom the electric lighting made necessary by deep floor plans,and the wider use of false ceilings, increase the risk of overheat-ing [76]. Poirazis et al. [5] showed that office buildings in Swedenwith fully glazed facades are likely to have a higher energy use forheating and cooling than buildings with conventional facades (e.g.30% window to external wall area). In their simulation study, thetotal energy use (heating, cooling, electricity for pumps, fans andoffice equipment) of the 30% glazed building ranged from 123 to136 kWh/m2 yr and that of the 100% glazed building ranged from143.0 to 176 kWh/m2 yr. In the ‘best’ case, the additional energyuse of the 100% glazed building was 15% higher compared to theenergy use of the 30% glazed building. The authors also outlinedthat one of the main arguments for using increased glazed areas inbuildings is the provision of better indoor environment due to day-light. However, increased window area does not necessarily lead toa reduction in energy use for lighting the building properly. Glareproblems that can be caused by the large amount of daylight enter-ing a highly glazed working space often reduce the quality of visualcomfort and shading devices are used more frequently in highlyglazed buildings often maintaining the same levels of daylight usedin a building with a conventional facade (see also [9]).

Tzempelikos and Athienitis [73] showed that for 30% window-to-wall ratio (WWR) and a south orientation, daylight providesperipheral offices in Montreal with 500 lx on the work plane 76% ofthe working time in a year. They showed that increasing the WWRabove 30% did not result in significant increase in useful daylightin the room (9% more for 80% WWR). Therefore, the 30% WWR wasidentified by the authors as the daylighting saturation region for

south-facing facades in Montreal.

2.3.2.3. Effect of shading devices. According to Küller [27], thereduction in thickness of the exterior walls and the increased use of

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lazing in the facades of modern architecture has made the designf good daylighting more difficult. He explained that various kindsf shading devices might be necessary not only to avoid overheatingut also to control the interior lighting and avoid glare from win-ows. For computer tasks, where the normal line of sight is moreorizontal than for reading or handwriting tasks, glare from win-ows is usually a considerable concern and needs to be carefullyontrolled [75].

Shading devices generally reduce the amount of daylight avail-ble in a space. Christoffersen et al. [77] noted that for venetianlinds, the best utilization of daylight is achieved with horizon-al slats, because this evens out the big differences in luminancesetween the window zone and the rear wall zone. They explainedhat if the slats are tilted downwards (+45◦), light comes mainlyrom the ground and thus reduces illuminance levels by 75–90% inhe case of white venetian blinds and by slightly less in the case ofenetian blinds with reflective slats. They explained that it is thusssential to raise the slats when not needed to provide natural lightn the space. They also showed that for light shelves, the only sit-ation in which the light shelf increases the illuminance level ishen it is specularly reflecting and hits direct sunlight from a rela-

ive azimuth angle so small that most of the reflected light hits theeiling. Under overcast sky, the light shelf reduces the illuminanceevel on the working plane because it directly cuts off part of theiew to the sky.

Galasiu et al. [54] achieved a field experiment in Ottawa (Canada,at 45.24◦N), where the performance of two commercial photocon-rolled lighting systems, continuous dimming and automatic on/off,as evaluated as a function of various configurations of manual

nd photocontrolled automatic venetian blinds. They showed thatnder clear sky and without blinds, both lighting control systemseduced the lighting energy consumption on average by 50–60%hen compared to lights fully on from 6 a.m. to 6 p.m. These sav-

ngs, however, dropped by 5–45% for the dimming system, and by–80% for the automatic on/off system with the introduction ofarious static window blind configurations. The savings in lightingnergy were more significant when the lighting control systemsere used with photocontrolled blinds. This was due to the capa-

ility of the blinds to adjust their position automatically in directesponse to the variable daylight levels.

According to Reinhart [74], one error source for overoptimisticnergy savings predictions in office buildings is the treatmentf blinds. It is often assumed that blinds are retracted all yearound (maximum daylight availability) while the lighting is alwaysctivated during office hours. In his open-plan office study, the sim-lation results revealed that the daylight availability in peripheralffices allowed for electric lighting energy savings between 25%nd 60% for an ideally commissioned, dimmed lighting system.e observed, however, that electric lighting energy savings for aimmed lighting system in an open plan office decisively dependedn the underlying blind control strategy.

A number of researchers have attempted to investigate whetherccupants of office buildings use the shading devices accordingo predictable patterns and if so, if these patterns are dependentn factors such as window orientation, time of day, sky condition,eason, latitude, and workstation position. Galasiu and Veitch [68]resented an exhaustive review of these researches, which allowso conclude the following:

) Some research indicates no relation between exterior environ-mental conditions and use of shading devices [78].

) Sunlight entering the space (especially more than 2 m from thefacade) triggers the use of shading devices [79–83].

) Solar radiation levels above 250–300 W/m2 normally induce asignificant percentage of blind utilization [81,84,85].

d Buildings 43 (2011) 2572–2582 2579

4) Below 50–60 W/m2, occupants most certainly do not use shad-ing devices [81,86].

5) Most people operate the blinds based on perceptions formedover long periods of time, rather than primarily in response tocurrent conditions [81,86].

6) Once closed, the blind will remain closed the entire day[78,80,81,84].

Other research has suggested that when sky luminance is main-tained under 2500 cd/m2, only a minority of occupants would wantto lower the window blinds [87,88]. In a more recent study aboutdaylighting of the New York Times Headquarters building [89], athreshold value of 2000 cd/m2 was used, based on the assump-tions that the primary task involved a LCD computer monitor withan average luminance of 200 cd/m2. The window was within theoccupant’s peripheral field of view so that a maximum luminanceratio of 10:1 between window and task was just acceptable, andthat the average background luminance was 50–100 cd/m2. It wasalso based on subjective survey results that found that there wasa 50% probability that blinds would be lowered when the averagewindow luminance was 2100 cd/m2 [88,89].

2.3.2.4. Effect of inner surface reflectances. The use of brightercolours for inner walls is necessary to maximize the reflection ofnatural light in the space and even the reflection of electric lightson walls. According to the European Standard EN 12464 [90], pro-posed ranges of useful reflectances for the major interior surfacesare:

• ceiling 0.6–0.9• walls 0.3–0.8• working planes 0.2–0.6• floor 0.1–0.5

Loe [22] noted that there have been examples when the correcthorizontal task illuminance has been provided, but the occupantswere dissatisfied because the room appeared gloomy. Often theproblem was caused by low reflectance wall finishes in combina-tion with luminaires which provided little light on vertical surfaces,and hence the room did not appear ‘light’ and was deemed to beunder-lit and therefore unsatisfactory.

In Reinhart’s open-plan office study [74], the simulationsshowed that reducing partition reflectance seriously reduced theamount of daylight at second row offices (for landscape officelayouts) and should be avoided if daylighting is desired. He alsopointed out that increasing the ceiling reflectivity has a positiveeffect on energy savings and leads to a more uniform distribu-tion of daylight throughout the space. Gratia and De Herde [76]presented general guidelines for the design of low energy officebuildings in Belgium, based on a series of parametric simulationsusing the programs TAS, OPTI and ADELINE 2,0 (Superlite). The sim-ulations were carried out for a rectangular, 3041 m2 office buildingwith five floors and 60% WWR. Results of their simulations indi-cated average horizontal task illuminance values of 7552, 7617 and7127 lx with ceiling reflectances of 70, 80 and 0% respectively, fora situation with a clear sky on March 15th. They recommendedhigh reflectance values (70–80%) for the ceiling, especially when alight shelf is used because the light shelf redirects daylight towardthe ceiling. For walls, they obtained average horizontal task illumi-nance values of 7552, 8149 and 7105 lx with wall reflectances of45, 80 and 0% respectively. Therefore, they recommended keeping

wall reflectances above 50% to maximize reflection of daylight inthe space. For the floors, they obtained average horizontal task illu-minance values of 7552, 9080 and 7314 lx with floor reflectances of15, 80 and 0% respectively. This surface and the surface of desks in

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he office play a major role in light distribution due to geometricalonsiderations (exposure to skylight) and therefore, they recom-ended selecting floor and desk reflectances above 50%. However,

he authors noted that the floors are often relatively dark in order toacilitate maintenance and a compromise has to be made in order toimultaneously meet the requirements of visual comfort and main-enance. Finally, they mention that light colours of desks also allowreduction of contrast between the paper and the desk surface,hich contributes to visual comfort.

For the furniture, a reflectance value between 25 and 45% haseen recommended [91]. Osterhaus [75] also advised to select lightonitor screen borders and lighter desktop surfaces to keep lumi-

ance ratios low. Too dark desk and/or furniture surfaces may giveise to high contrasts and unacceptable luminance ratios in theirect field of view. On the other hand, too bright surfaces can yieldisturbing reflections and glare.

.3.2.5. Effect of ceiling height. Few studies have been found abouthe effect of ceiling height. In his open-plan office study, Reinhart74] found that the ceiling was a crucial design element for day-ighting as the majority of daylight that penetrates into a buildingeyond the 1st work station is reflected from the ceiling at leastnce. He found that second row offices receive considerably lessaylight even though a reduced partition height and increased ceil-

ng reflectances can double electric lighting energy savings up to0%. He also noted that reducing the ceiling height from 9 ft (2.74 m)o 8 ft (2.44 m) cuts energy savings for electric lighting in half.

.3.2.6. Effect of partition height. In Reinhart’s open-plan officetudy [74], it was also shown that lowering partition heights from4 in. (162.6 cm) to 48 in. (121.9 cm) nearly doubled energy sav-

ngs for the automated and manually controlled blind scenario.nother benefit of reduced partition heights between peripheralnd second row offices was that the latter get a partial view outside.n the other hand, lower partitions reduce the acoustical separa-

ion between two work spaces. A smart design option might be toroup work places that require intense communication betweeno-workers in peripheral and second row offices and reserve innerpaces with higher partitions for more noise sensitive tasks, as pro-osed by the author. Another solution might be to use transparentr semi-transparent partitions.

. Conclusions

Key figures for energy consumption and energy saving poten-ial for office lighting were presented based on a review of relevantiterature, with a special emphasis on a North European context.he review reveals that the replacement of older lighting instal-ations (T12 fluorescent lamps) with modern energy-efficient T5amps with HF ballasts could provide up to 40% energy savings. Andditional 40% energy savings could be obtained by using a combi-ation of more energy-efficient luminaires, task/ambient lighting,ccupancy switch-off and daylight dimming, making it possible tochieve totally 80% energy savings compared to older T12 fixedighting installations.

The review reveals that measurements in full-scale rooms16,25], theoretical calculations [14,15] as well as simulations9,17] with validated lighting programs indicate that an energyntensity of around 10 kWh/m2 yr is a realistic target for electricighting in future low energy office buildings as well as in officeuilding retrofits. This target, which assumes typical illuminance

evels for office rooms, would yield a significant reduction in energy

ntensity of at least 50% compared to the actual average elec-ric lighting use (21 kWh/m2 yr in Sweden). Note, however, thathis figure may vary according to room type (i.e. individual officeooms versus landscape rooms and common rooms). The review

d Buildings 43 (2011) 2572–2582

also indicates that lower energy intensities are even achievable byaccepting e.g. lower installed illuminance levels (500 to 400 lx) andtask/ambient lighting using very energy-efficient task lamps (6 Wavailable).

Strategies for reducing energy use for electric lighting werealso presented and discussed, which include: improvements inlamp, ballast and luminaire technology, use of task/ambient light-ing, improvement in maintenance and utilization factor, reductionof maintained illuminance levels and total switch-on time, useof manual dimming and occupancy sensors. Strategies based ondaylight harvesting were also addressed and some design aspectssuch as latitude and orientation, window characteristics, shadingdevices, reflectance of inner surfaces, ceiling and partition heightwere discussed.

This review generally shows that cost-effective energy savingsmay be achieved by simply improving the electric lighting systemi.e. by replacing or planning the electric lighting installation withenergy-efficient T5 fluorescent lamps-luminaires and CFL (or evenLED lamps) for task lighting and a combination of task/ambientlighting design, manual or automatic dimming, automatic switch-off occupancy sensors. Daylight harvesting based on photoelectricdimming can provide additional energy savings but at a highercommissioning cost. For peripheral office spaces where plenty ofdaylight is available, a simple daylight-sensitive switch-off systemmay be more cost-effective than continuous daylight dimming. Thereview revealed also that a number of studies have indicated thatdaylight harvesting can be achieved in peripheral spaces with ‘rea-sonable’ window-to-wall ratios (WWR) of no more than 30–40%. Anincrease in WWR does not provide substantial additional lightingenergy savings and creates risks for overheating, glare and a con-sequent abusive use of shading devices, with associated reduceddaylight benefits.

Some barriers to the proposed energy saving strategies in a realcontext also need to be addressed in future research. These barriersmay be related to the difficulty to switch-off lights at night due toextended office hours and flextime, user-acceptance issues relatedto the proposed low light levels or to the occupancy switch-off anddaylight dimming control systems, etc. Moreover, the difficulty touse daylight in core areas in deep building plans and glare problemsrelated to the excessive exposure to daylight in peripheral officespaces are important issues which will demand attention.

Acknowledgements

The authors thank SBUF (the development fund of the SwedishBuilding trade), CERBOF (Centre for energy and resource efficientconstruction and management of buildings) and NCC ConstructionSweden for funding this research project.

References

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[2] A. Lagoudi, M. Loizidou, M. Santamouris, D. Asimakopoulos, Symptoms expe-rienced, environmental factors and energy consumption in office buildings,Energy and Buildings 24 (1996) 237–243.

[3] L. Stengård, Global Energifakta, Statens energimyndigheten http://195.67.76.248/Global/Energifakta/F%C3%B6rb%C3%A4ttrad%20energistatistik/Festis/STIL2-belysning-stengard.pdf, last accessed 11 January 2009.

[4] Statens energimyndighetenen, Energi i våra lokaler: Resultat från Energimyn-dighetens STIL2-projekt, Delrapport från Energimyndighetens projekt Förbät-trad energistatistik i samhället, 2010, www.energimyndigheten.se/stil2, lastaccessed 27 January 2011.

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[6] Statens energimyndigheten, Förbättrad energistatistik för lokaler – “StegvisSTIL” Rapport år 1: Inventeringar av kontor och förvaltningsbyggnader, 2006,ISSN 1403-1892.

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