Improving Energy Performance of Green Blds

8/8/2019 Improving Energy Performance of Green Blds

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Building and Environment 42 (2007) 32613276

Improving energy performance of school buildings while ensuringindoor air quality ventilation

Rachel Becker a, , Itamar Goldberger a , Monica Paciuk b

a Department of Structural Engineering and Construction Management, Faculty of Civil and Environmental Engineering,TechnionIsrael Institute of Technology, Haifa 32000, Israel

b National Building Research Institute, Technion, Haifa, 32000, Israel

Received 6 June 2006; received in revised form 20 July 2006; accepted 28 August 2006

Abstract

Energy conscious design of school buildings, as well as deemed-to-satisfy provisions in a Performance Based Energy Code, shouldaddress the problem known as the energy efciencythermal comfortindoor air quality dilemma (EE-TC-IAQ Dilemma). In warmand moderate climates, the large internal heat sources usually found in school buildings prevent achieving thermal comfort withoutactive cooling in summer, but are not sufcient to eliminate the need for heating in winter. Commonly used air-conditioners do notimprove air quality, while natural ventilation induces uncontrolled energy losses. In this study, a step by step process was used for thedevelopment of deemed-to-satisfy design solutions, which cope with the EE-TC-IAQ Dilemma, for a performance based code.A distinction is made between improving building design variables and improving ventilation schemes. Results indicate thatimplementation of improved ventilation schemes in an otherwise well designed energy-conscious building result in savings of 2830%and 1718% for northern and southern classroom orientations, respectively.r 2006 Elsevier Ltd. All rights reserved.

Keywords: School buildings; Indoor air quality; Energy performance; Thermal comfort; Ventilation; Thermal insulation; Shading

1. Introduction

The methodology and results presented in this paperwere developed within the framework of an extendedIsraeli research program, which aims at the establishmentof a modern Building Energy Code that addresses allbuilding occupancies.

The Code will enable two design options: a performance-based option that requires a comparative assessment of

computationally-estimated energy expenditure against acalculated energy budget, and a prescriptive option. Forthe latter the Code will provide a set of solutions that areconsidered deemed-to-satisfy, and are based on a systema-tic investigation and identication of preferred sets of solutions. The reference energy budget will be based onsuch a preferred solution set.

The identication of preferred solutions for schoolbuildings must address the inherent conict that stemsfrom the wish to save energy while having to provideadequate indoor air quality in addition to thermal comfort.

School buildings include nowadays various functionalspaces. However, classrooms are still the most commonfunctional space. They occupy the largest area of everyschool building, and host the largest part of daily activitiesand occupants. One of the most dominant features of a

classroom is its high occupancy density, which results invery large values of the internal heat sources (approxi-mately 5 kW), as well as of the internal emissions of bodyodors, water vapor and CO 2 , causing an increasing concernwith regard to the indoor air quality youngsters areexposed to along the major period of their growing upyears [1,2].

Extensive natural or controlled ventilation, intended toremove internally generated contaminants, without activeheating or cooling, is only seldom sufcient for theprovision of required thermal comfort conditions.

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In summer, whenever external temperatures are lowerthan the required indoor temperature, ventilation canremove the excessive heat load produced by both incidentsolar radiation and internal sources, thus allowing theachievement of comfortable indoor temperatures. How-ever, whenever external temperatures exceed the required

comfort level temperature, active mechanical cooling mustbe provided.Heating classrooms in winter is signicantly assisted by

the large internal heat sources, which can replace a largepart of the heating energy demand. However, whenexternal temperatures are smaller than the required indoortemperature, ventilation removes most of the internal heatload as well as the heat gains from solar radiation. Naturalnon-controlled ventilation leads then to excessive energylosses, as well as to chilling draughts and loss of thermalcomfort. Controlled ventilation in winter is thus essential.

Consequently, simultaneous catering for thermal com-fort, indoor air quality and energy conservation in schoolsis a design dilemma with apparently no obvious solution.

In the sequel, we denote this dilemma by: the EE-TC-IAQ Dilemma (energy efciencythermal comfortin-door air quality dilemma). Efcient energy design of theclassroom wing of school buildings should thus beprimarily concerned with providing optimal solutions ableto cope with the above dilemma.

Within the framework of a performance-based buildingcode, the aim is to propose a range of engineering validpreferred solutions that establish a basis for creative androutine design, but not to prescribe an optimal uniquesolution, which may be too restrictive and become an

unreasonable barrier to creative design.This paper presents the methodology adopted for

deriving such preferred solutions that cope with theEE-TC-IAQ Dilemma in school buildings, as well as someresults that may be of general interest though they werederived for a typical local Mediterranean climate (seeAppendix for some typical data). Locally signicantobservations or conclusions that are not relevant elsewherehave been intentionally omitted.

2. Energy performance, thermal comfort and IAQ in schools

Most of the literature concerned with energy perfor-mance of school buildings is devoted to savings via specicfeatures such as utilization of solar energy [36], construc-tion features, such as thermal insulation, thermal mass, andshading [5,79] , HVAC performance [1013] , and geother-mal pumps [1417] . However, basic assumptions regardingthermal comfort, indoor air quality, occupancy andacclimatization schedules, internal loads, and architecturalfeatures of the school building are not identical in thevarious publications, and are usually based on localpreferences. Consequently, even when similar climaticconditions prevail, conclusions cannot be regarded assufciently general. Moreover, the topic of energy perfor-mance of schools located in the Mediterranean regions

climatic conditions and culture has not been exploredat all.

The basic assumptions relevant to this papers metho-dology, analysis and results are presented in Sections2.22.4 below, while Section 2.1 briey summarizes theliterature concerned with the EE-TC-IAQ Dilemma.

2.1. Coping with the EE-TC-IAQ Dilemma in schools literature overview

Despite the obvious need to cope with the EE-TC-IAQDilemma in school buildings, and the wealth of literatureemphasizing the need to improve IAQ on one hand[1,2,18,19] , and on the other one addressing the impacton acclimatization energy imposed by direct IAQ ventila-tion [4,18,20,21] , only a few publications were found thatpresent integrated solutions accompanied by thermal andenergy analysis. Of these, only a few were concerned withthe main classroom wings, addressing the inherent largeinternal loads as well as the more stringent IAQ require-ments that exist there.

Studies since 1997 include Davanagere and colleagues[22], who studied the effect of the new ASHRAE-62 IAQrequirements for school buildings on life cycle costs whenusing different HVAC systems, but did not address thearchitectural or construction features of the building;Dorer and Weber [23], who pointed to the signicance of an integrated evaluation of the energy and IAQ response of a multi-story school building, but addressed only twospecic features: natural night ventilation enabled by twomodes of window opening, and shaft ventilation via a

double glazed fac - ade; Kavanaugh and Xie [16], whodemonstrated the signicance of addressing the fan energyrequired for ventilation, either IAQ ventilation or heatrecovery ventilation, in the total energy analysis; Erikssonand Whalstrom [24], who analyzed the performance of ahybrid ventilation system based on a solar chimneyimplemented in a Swedish school, utilizing multi-zone airtransfer to model effects of wind conditions as well as dooropening strategies; Becker and Paciuk [25], who showed theimproved effect on total energy loads of various IAQventilation and night ventilation schemes, enabled byutilizing the buffering effect of an existing central atrium,but did not address their impact on total electricityconsumption.

Additional articles are devoted to some specic solu-tions or features of the EE-TC-IAQ Dilemma in otherfunctional spaces of school buildings. Recent publicationsaddress atria [26], staff rooms and auditoria [19], and sporthalls [27].

The literature survey revealed that research in the areahas been mostly concerned with improving the implemen-tation and control of natural ventilation, or replacingsome specic features of the HVAC system. No publica-tions were found that address enhanced energy conserva-tion by utilizing some of the regular, though specic,architectural features of classroom wings (such as the

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layout, which always includes corridors, lobbies or atriathat are vacant during classroom hours), and the specialopportunity they provide for rational design and manage-ment of mechanically controlled day and night ventilationschemes that are concerned with solving the EE-TC-IAQDilemma.

2.2. Typical architectural features

Most Israeli schools consist of two to three story-highbuildings. Fig. 1 depicts some schematic congurations of the classroom wings in these buildings, as derived byanalyzing various plans of existing schools. In the absenceof restrictions on orientation, classroom windows may faceany direction.

It is noticed that classrooms are usually clustered to thesides of a corridor or around a hall, with similar clustersstacked repeatedly throughout the building height. In a onesided corridor conguration, the opposite corridor wallincludes very large openings, which provide sufcientdaylight. When operable windows are installed in theopenings, they will usually consist of sliding wings, whichare closed in winter and open in summer. A two sidedcorridor or an internal hall does not have sufcient accessto natural delighting, and these spaces are served byelectrical lighting. Despite the lack of natural ventilation insuch corridors and lobbies, it is not common to equip themwith mechanical ventilation.

The classrooms schematic layout is given in Fig. 2 .Dimensions are typical of Israeli classrooms, which aredesigned for an occupancy of up to 40 pupils, based onDesign Guidelines provided by the Ministry of Education.In most cases the entrance door is located on the wallopposite to the windows, with only one wall including

windows. The blackboard is located on the transverse wall,so that when pupils are facing the board windows are ontheir left, allowing left-sided day lighting on their desks.

2.3. Thermal comfort

The American ASHRAE Comfort Standard 55 [28], andthe International ISO Standard 7730 [29] are considered asstandard tools for the evaluation of thermal comfort levels bymeans of the thermal predicted mean vote (PMV) and thepredicted percentage of thermally satised (PPS) occupantsin the vicinity of neutral thermal sensation, for differentcombinations of hygro-thermal conditions and personalfactors. These Standards are essentially identical, as bothhave adopted Fangers formulation and accompanyingvalues of the equations parameters [30]. However, researchconducted in various countries has indicated that signicantdiscrepancies exist between the predicted values and actualresponses in occupied buildings. Statistically valid studies of thermal comfort in school buildings are scarce, and publisheddata includes only those conducted in Japan [31,32], Brazil[33], Hawaii [34], and Singapore [19]. The discrepanciesbetween the actual responses and the predicted values weredifferent in the various studies, with no uniform direction on

the comfort scale. This indicates that local aspects affect thedeviation from Fangers formula, and that assessment of thermal comfort conditions cannot fully rely on the resultsderived by means of the ASHRAE and ISO Standards.

The literature search indicated that data on thermalresponse and thermal comfort in local Israeli, or Medi-terranean schools is not available. No specic eld studywas conducted within the framework of the currentresearch program. However, studies performed previouslyby the authors in Israeli ofces and residential buildings[3537] indicated that preferred conditions are usuallysomewhat cooler, in summer as well as in winter, thanthose predicted by the standard tools.

Assuming that the preferences in schools are similar tothose found in other buildings, the set points for thepresent thermal analysis were established as follows:

Assuming a 50% RH and a classroom-adequate low airvelocity of 0.1m/s, PMV was established for all possiblecombinations of the variables given in Table 1 . The rangeof metabolic rates, M , addresses the need to satisfy thecomfort requirements of seated pupils as well as of thestanding teacher. The ranges of clothings thermal resis-tance, Icl, stem from the difference in pupils clothing oruniform and that of teachers.

A plot of air temperature, T a , versus PMV is shown inFig. 3 . Combinations leading to PMV o 0.5 and

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Outside, shaded horizontal ly Atr ium Corridor/Hall

Interior non conditioned space Interior conditioned space

Fig. 1. Schematic presentation of some classroom wing congurations inIsraeli school buildings.

6.8m

7.2m

Fig. 2. Schematic presentation of classroom layout.



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PPS o 90% are denoted by too cold. Combinationsleading to PMV 4 +0.5 and PPS o 90% are denoted bytoo hot. Combinations with PMV within the range 0.5to +0.5 and PPS X 90% are denoted by acceptable.Circular ticks denote combinations with regular clothingand xs denote those for which clothing restrictions are

applied (e.g., long sleeves only).It was assumed that in schools, similarly to preferencesrecorded in residential and ofce buildings, the cooler sidetemperatures of the acceptable range would be preferredand considered as adequate for providing thermal comfort.

Consequently, T a 20.5 1C, was established as the set-point for winter heating, and T a 24.5 1C for summercooling. Conrming the statistical validity of these valueswas beyond the scope of the current research program.

2.4. Indoor air quality

Requirements for indoor air quality are determined inNorthern America and some other countries by means of the American Standard ASHRAE 62 [38], whereas in mostEuropean, as well as in some other countries, the prevailingbackground document is the European pre-standard prEN13779 [39]. These two documents are not identical, neitherin concept nor in the established norms. The Americanstandard stipulates a minimum value of 8 l/s/person for therequired ux of fresh air in classrooms based on anindoor maximum excess CO 2 concentration of 700ppm.The European document denes three performance levels,IDA1, IDA2, and IDA3, based on CO 2 excess concentra-tions of 800, 1000, and 1500 ppm, respectively. Obviously,even the most stringent European level IDA1 is more

lenient than the unique American level. For classrooms,where there is no smoking, prEN 13779 enables aventilation rate of 4 l/s/person for the IDA3 level, whichis half the ventilation rate established by ASHRAE. A localIsraeli regulation is under preparation, which intends tocompromise between the two mentioned documents. Using

this foreseen regulatory provision, a fresh air require-ment of approximately 5 l/s/person ensues, leading to arequired ventilation rate of 720 m 3/h in the given 40-pupilclassroom (i.e., $ 5 air changes p/h).

3. Improving energy performance of schools while copingwith the EE-TC-IAQ Dilemma

3.1. Methodology

The following methodology is suggested for identifyingthe range of solutions for construction of schools in warm

climates that may be considered as energy efcient whileproviding thermal comfort and indoor air quality, and thusaccepted as deemed-to-satisfy solutions for school build-ings in the forthcoming Energy Code:

A distinction is made between the modication of building design variables (such as orientation, size of windows, thermal insulation, internal mass, color and hueof facades, etc.) to bring about energy savings whileproviding the required amount of fresh air by means of direct ventilation (presented in Section 3.2 below), and themodication of ventilation schemes with the intention of further diminishing energy losses (presented in Section 3.3).Taking into account the uncontrolled energy lossesassociated with natural ventilation, only actively controlledventilation schemes are considered.

The effects of building design variables are studied on abasic representative three-story module of the classroomwing, comprised of construction elements with the stan-dard thermal properties. This module is designated as abasic building. The analysis yields a so called preferredbuilding, which achieves improved energy performance.The modied ventilation schemes are then applied to themodule representing the preferred building, and enablethe selection of preferred ventilation schemes. Perturba-tions of some building features are then used to re-examine

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winter

19.5

20.0

20.5

21.0

21.5

22.0

22.5

-1.0 -0.5 0.0 0.5 1.0

PMV

T e m p e r a

t u r e

( C )

Too Hot

Too Cold

Acceptable

Summer

22.5

23.0

23.5

24.0

24.5

25.0

25.5

26.0

26.5

-1.0 -0.5 0.0 0.5 1.0PMV

T e m p e r a

t u r e

( C )

Too Hot

Too Cold

Acceptable

Fig. 3. T a vs. PMV in winter and summer, for the various combinations of M, Icl, and MRT.

Table 1Combinations of variables used for thermal comfort analysis

Item (1) Summer (2) Winter (3)

M (W/m 2) 5870 5870I cl (m

2K/W) 0.070.11 0.160.24T a (1C) 2326 2022MRT ( 1C) T a +1 T a 1

M , metabolic rate (W/m 2); I cl , clothing insulation index (m21K/W); T a , air

temperature ( 1C); MRT, mean radiant temperature ( 1C).



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previous conclusions regarding the selection of the pre-ferred building.

Due to different efciencies of heating, cooling, lightingand ventilation devices, energy demands cannot besummed up and must rst be converted to electrical energyor fuel equivalents. As local devices use only electrical

power, the target function for energy conservation waschosen as the total annual electrical energy demand per1 m 2 of classroom oor area.

Thermal and energy performance analysis was per-formed by means of the public-domain program Energy-Plus [40], which enables the prediction of detailed energydemand as well as of electricity required for the variouselectro-mechanical devices and systems.

Some common assumptions, which stem from non-energy-related functional requirements of schools, aremade throughout the investigation. In the present studytheir values are as follows:

Occupancy periods: all classrooms are fully occupiedduring classroom hours the entire school day, whichlasts from 8:00 to 17:00 Sundays through Thursdays andfrom 8:00 to 13:00 on Fridays (loads were adjusted forthe intermission periods between classroom hours),while no activity takes place on Saturdays. The schoolyear extends from September 1 until July 14, except forofcial holidays.Indoor climate control: heating and cooling in everyclassroom are provided by means of an electrically-operated split air-conditioner controlled by an air-temperature thermostat. Heating is turned on at 7:00,

while cooling only at 8:00. The heating period lasts fromOctober 1 to March 31, and the cooling period fromApril 1 to July 14.Ventilation provision: scheduled controlled ventilation(IAQ ventilation as well as summer night ventilation) isachieved by means of electrical fans providing thedesignated air ow rates in or out of a given space(between ve air changes/h for IAQ ventilation to avarying night ventilation of 030 air changes/h). Inaddition, a constant inltration background rate of 0.5 h

1 prevails at all times.Lighting control: electrical lighting, controlled bydimmers, provides backup lighting to ensure at least300Lux on all students desks during occupancy. Thetotal maximum heating load, using efcient lightingdevices, amounts to 750 W/class.Internal heating loads: internal loads exist duringoccupancy, and stem from lighting (maximum 750 W),people (4000 W), and computers (200W).School location: the school is located in the Mediterra-nean region, which is simulated by Jerusalems TypicalMeteorological Year. A stand-alone building is assumed,so that no shading is provided by other buildings.Construction features: internal oors are heavy-weight(pre-stressed concrete slabs with 14cm effectivethickness) with dark ooring ( a 0.7). Internal wall

and ceiling surfaces are light colored ( a 0.4). Theconstruction features of the external and internal wallshave been part of the investigated design factors, asexplained in Section 3.2.

3.2. Effects of building design factors

The prevailing assumption in this part of the analysis isthat air is drawn into every classroom directly from theoutside by means of a fan designed to provide thedesignated air ow rate of 720 m 3/h during occupancy.No active ventilation is provided beyond occupancy hours.Improving energy efciency of the building is the primaryconcern at this stage, leading to the selection of apreferred building that yields the most signicant energysavings. The module representing this building will thenserve as the base-reference for investigating the potential of energy improvements achieved by utilizing modiedventilations schemes.

3.2.1. Investigated factorsEnergy performance analysis of the module with direct

ventilation addresses the factors presented in column 1 of Table 2 . Column 3 presents the set of regularly used valuesof these factors. The effect of each factor was studiedseparately, with this factor varied in its entire feasiblerange, as presented in column 2 for locally prevalentvalues, while the other factors are kept at their regularvalues (unless indicated otherwise in the text).

3.2.2. Results and discussionDetailed results of a thermal and energy analysis always

depend on the climatic conditions and basic assumptions,and are thus of a local nature. This section presents anddiscusses only the main observations that may be of generalinterest for the Mediterranean climatic zone and othersimilarly warm and moderate climates. All energy con-servation achievements are compared to the performanceof the Basic Building module with construction featuresaccording to column 3 in Table 2 .

Results indicated that the following four factors had anegligible effect on total annual heating and coolingdemands: external wall structure, roof structure, internalpartition structure, and facade color and hue. It was thusconcluded that for these factors the regularly used valuescan be considered in all further analyses. Moreover, thisimplies that for the sake of a local Energy Code, norestrictions should be imposed on these factors in thedeemed-to-satisfy solutions.

Analysis of results for bottom oor structure revealedthat this factor has very little impact for classrooms witheastern and western window orientations. For northernand southern orientations insulating the oor is benecialin the heating season without excessively increasing thecooling demand, thus enabling a total decrease of up to 3%

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in the total annual electricity demand. The recom-mendation that follows is to opt for an insulated oorwhenever it is economically justied by the constructiontechnology.

The remaining ve factors (window orientation, windowarea, glazing type, external wall thermal resistance, androof thermal resistance) had much larger effects on annual

acclimatization energy and electricity demands, as de-scribed below.

Fig. 4 shows the effect of classroom window orientationon annual electrical energy demands for the internal masswall (InM) with three levels of wall insulation: 12 cmconcrete wall without any insulation (d I 0 cm), and samewall with external insulation of 3 cm expanded polystyrene(d I 3 cm), as well as with 12 cm (d I 12 cm). All wallsinclude a 4 cm external stone cladding.

Similar trends can be observed for the poorly insulated,as well as for the highly insulated cases. Due to solarradiation, heating demand is lowest for the southernorientation and largest for the northern one, while coolingdemand is lowest for the northern and southern orienta-tions and largest for the eastern and western ones.Electricity demand for lighting is smallest for the south western orientation and largest for the eastern one. Thedifference between the eastern and western orientation canprobably be attributed to the generally larger level of cloudiness in the morning hours. Consequently, values of total electrical energy demand expected in the easternand western orientations are more than 20% larger thanthose expected in the northern and southern orienta-tions. Furthermore, addressing the need to prevent directpenetration of sun radiation in summer, it is concluded thateastern and western orientations of classroom windows are

not desirable, and should be avoided (eastern and westernorientation have thus been excluded from the currentstudy).

It is also observed that heating demand is largest, asexpected, for the non-insulated walls, whereas coolingdemand is smallest for these walls. This trend is observedalso in Fig. 5 , which shows the effect of wall thermal

resistance (surface to surface), r , for the InM wall, onannual electrical energy demand for classrooms withnorthern and southern windows. As for every otherbuilding type, heating demand decreases with an increasein thermal insulation. However, thermal insulation pre-vents the withdrawal of heat generated the large internalheat loads, increasing the cooling demand as the wallsthermal resistance increases. Nonetheless, even for thesouthern orientation, where cooling is more dominant thanfor the northern orientation, the graph of total annualdemand retains a monotonous decreasing concave trendalong the entire feasible range of thermal resistancevalues. Similar effects and trends were obtained for roof insulation. For a specic building, economic considerationsmust thus be taken into consideration in order to establishthe optimal insulation thickness for different insula-tion materials and construction types. However, theminimal deemed-to-satisfy thermal resistance for wallsand roofs to be stipulated by the Code cannot depend onthe cost of different solutions, but should rather bedetermined according to an energy criterion. This criterioncan be chosen so that at the prescribed value, a majordecrease (say, 90%) of the feasible total electricityreduction range is achieved. For the given climate, thevalue of rX 0.85 m 2 K/W was derived for external walls andrX 2.0m 2 K/W for the roof.

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Table 2Building design factors investigated in order to detect the preferred building when direct IAQ ventilation prevails

Design factor (1) Range in investigation (2) Regular value (3) Recommended value (4)

Orientation 0360 1 0360 1 North (0 1), or South (180 1)External wall structure Internal mass (InM), equally

distributed mass (EDM), orexternal mass (ExM)

Internal mass (InM) No restrictions

Wall thermal-resistance(m 2K/W)

0.052.55 0.551.0 (0.85 for basicbuilding)

rX 0.85

Bottom oor structure On ground, or thermallyinsulated

On ground When economically justied thermally insulated, otherwise on ground

Roof structure Regular, or inverted Roof Regular roof No restrictionsRoof thermal-resistance(m 2K/W)

0.052.55 0.552.05 (1.65 for basicbuilding)

rX 2.00

Internal partition structure Lightweight, or heavyweight Heavyweight No restrictionsFac- ade color and hue Light, gray, or dark Light No restrictionsWindow area 819.4% of oor area 14.7% of oor area Southern: X 10% Northern:

X 13%Window glazing type 1. Single, SC 1 , T vis 0.96 2.

Double with: SC: 0.960.15 T vis :0.840.05

Single, SC 1 T vis 0.96 Double Glazing. Southern:0.55 4 SC 4 0.5 0.55 X T vis 4 0.5Northern: SC X 0.6 for tintedglazing (no restriction for Low_Eglazing) T vis X 0.6



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Fig. 6 shows the effect of window relative area (denedby the ratio of glazed window area to oor area) on annualelectrical energy demands in the northern and southernorientations. Increased window area improves energyperformance in both orientations. However, even withthe largest windows, demand in the northern orientationremains larger than the largest demand in the southernorientation, when windows are smallest. Increasing thewindow area is much more effective in the northernorientation, but the graphs reveal that the marginalimprovement achieved beyond a northern glazed ratio of $ 13% (window size at least 4.0 1.6 m) or beyond asouthern glazed ratio of $ 10% (window size at least5.0 1.0 m) is negligible. Deemed-to-satisfy window areasare thus X 13% and X 10% of oor area in the northernand southern orientations, respectively.

Figs. 7 and 8 show the effects of window glazing type (asdened by its light transmission coefcient, T vis , andshading coefcient, SC) on annual electrical energy

demands for northern and southern orientations.

Two groups are observed according to their glazing:LE&Cthat includes double glazed windows with low-Eand clear glass ( T vis 0.7 to 0.84, SC 0.51 to 0.96) aswell as the single glazed clear glass window ( T vis 0.96,SC 1.0), and T that includes windows with tinted glass(T vis 0.050.62, SC 0.150.72). Properties of readilyavailable commercial double-glazed windows have beenused in the investigation. Consequently, the U -value wasnot identical for all the double glazed windows, and rangedwithin 1.783.23 W m

2 K1 . Moreover, even within the

same group, SC values do not vary monotonously withT vis , thus, when the effect of one factor is considered, theother is not constant, neither changing exactly in the samedirection. In addition, for similar SC values, low- E glazinghas much larger T vis values than tinted glazing. Thesecharacteristics of glazing properties explain the distinctdifference in the overall energy-related behavior of the twoglazing groups and the observed jumpiness of thegraphs. For LE&C glazing, the regular double-glazed

window with clear glass ( T vis 0.84, SC 0.96) yields the

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-

10

20

30

40

- 0.5 1.0 1.5 2.0 2.5 3.0

r (m 2K/W)

A n n u a

l E l e c t r i c i

t y ( k W h / m

2 )

Total -South

Cooling -South

Heating -South

Total -North

Cooling -North

Heating -North

Fig. 5. Effect of wall thermal resistance, r, on annual electrical energy demand for classrooms with northern and southern windows.

-

10

20

30

40

- 45 90 135 180 225 270 315 360Window Orientation (Degrees Clockwise)

A n n u a

l E l e c t r

i c i t y ( k W h / m

2 )Total - dI=0 cm

Total - dI=3 cm

Total - dI=12 cm

Cooling - dI=0 cm

Cooling - dI=3 cm

Cooling - dI=12 cm

Heating - dI=0 cm

Heating - dI=3 cm

Heating - dI=12 cm

Lighting

Ventilation

Fig. 4. Effects of classroom window orientation (degrees clockwise from north) on annual electrical energy demands for three levels of thermal insulationthickness d I 0, 3, and 12 cm.



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lowest annual energy demand, but for the variety of investigated glazing types the ranges of T vis and SC hardlyaffect the expected total electrical energy demands.Consequently, from an energy viewpoint, all the glazingtypes in this group are equivalent, and no preference couldbe established. For T glazing, the expected total electricalenergy demand generally increases with decreasing T vis aswell as with decreasing SC, mainly due to the effects of these factors on electricity demand for lighting. For thesouthern orientation, as long as T vis 4 0.5 and SC 4 0.5,the increased demand is moderate in comparison to theexpected electrical energy demand with LE&C glazing,whereas for the northern orientation only T vis X 0.6 andSC X 0.6 yield a comparatively moderate increase.

In the pursuit of recommended deemed-to-satisfy solu-tions the effect of light transmission on glare and theeffect of penetrating solar radiation on summer thermalcomfort in classes with southern orientation windows

should be addressed as well. Two indicators have thus beendened:

Discomfort glare severity indicator (DGSI), indicatingthe annual cumulative severity of excessive discomfortglare, GI. It is given by

DGSI Xt G

GI 20, (1)

where t G denotes the hours for which GI 4 20.Summer thermal discomfort severity indicator (STDSI),

indicating the cumulative severity of excessive meanradiant temperature in summer. It is given by:

STDSI Xt MRT

MRT T a , (2)

where MRT is the mean radiant temperature obtainedby EnergyPlus at the room center, T a is the room air

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20

25

30

35

40

0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20

A n n u a

l E l e c t r i c i

t y ( k W h / m

2 )

Southern Orientation Northern Orientation

Aw/Af (-)

Fig. 6. Effect of window relative area, Aw /Af , on annual electrical energy demand for classrooms with northern and southern window orientations.

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1Tvis

A n n u a

l E l e c t r i c i

t y ( k W h / m

2 )

Total - South (LE&C)Cooling - South (LE&C)Heating - South (LE&C)Lighting - South (LE&C)Total - North (LE&C)Cooling - North (LE&C)Heating - North (LE&C)Lighting - North (LE&C)Total - South (T)Cooling - South (T)Heating - South (T)Lighting - South (T)Total - North (T)Cooling - North (T)Heating - North (T)Lighting - North (T)

Fig. 7. Effect of window glazing light transmission coefcient, T vis , on annual electrical energy demand for classrooms with northern and southernwindow orientations.



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temperature, and t MRT denotes the hours for whichMRT 4 T a during the cooling season.

The effects of T vis

and SC on DGSI and STDSI,respectively, are illustrated in Figs. 9 and 10 .

As expected, DGSI and STDSI generally decrease withdecreasing T vis and SC, but their values in the southernorientation are substantially larger than those obtained inthe northern orientation. Accepting the performance levelof northern classes with regular clear glass double-glazedwindows ( T vis 0.84, SC 0.96) as a threshold level,glazing with 0.55 X T vis and 0.55 4 SC should be preferredin the southern orientation.

Combining these recommendations with those derivedwhen observing Fig. 7 , the following deemed-to-satisfyranges are suggested: for northern classroom windows

T vis X 0.6 and, when tinted glazing is used, SC X 0.6;

for southern classroom windows0.55 X T vis 4 0.5 and0.55 4 SC 4 0.5.

The entire set of recommendations derived for thedeemed-to-satisfy Energy Code solutions are summarizedin column 4 of Table 2 . The implementation of theserecommendations yields the so called preferred building.The preferred building does not have windows inwestern or eastern orientations, which may cause energydemands by some 7% to 21% larger than those predictedfor the northern and southern orientations, as well asexcessive glare and direct solar radiation on pupils. Therepresentative modules of the preferred building aredenoted by IAQ_5_N and IAQ_5_S for the northern andsouthern orientations, respectively. Minor electrical energysavings of 2% are obtained in northern classrooms incomparison to the electrical energy demand of the

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Fig. 8. Effect of Window Glazing Shading Coefcient, SC, on annual electrical energy demand for classrooms with northern and southern windoworientations.

0

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Tvis

D G S I

South North

Fig. 9. Effect of window glazing light transmission coefcient, T vis , on the excessive glare index, DGSI, for classrooms with northern and southernwindow orientations.



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commonly constructed Basic Building, but a slightincrease of 2.5% is expected for the southern classroomsdue to the preferred use of glazing with lower T vis and SCin order to prevent glare and excessive mean radianttemperatures.

The preferred building modules will be used in thenext steps for the investigation of modied ventilationstrategies and for the evaluation of their effect onadditional energy savings. Results will be compared tothose obtained for IAQ_5_N and IAQ_5_S.

3.3. Effects of ventilation schemes

3.3.1. Night ventilationThe existence of ventilation fans for the provision of

IAQ ventilation enables the introduction of summer nightventilation without adding new equipment. The analysis inthis section is based on the assumption that during thecooling season the fan is operated automatically and

exterior air is introduced into the classrooms wheneverT a T o X 0.5 1C. This night ventilation scheme is denotedby the symbol DN.

Fig. 11 shows the effect of night ventilation rate, N , ontotal annual electrical energy demands.

As expected, fan electricity demand increases withincreasing N , while active cooling electricity demanddecreases. A very at region in the total annual electricitydemand is evident for air change rates greater than 5 h

1 ,with an optimum at N E 10 h

1 .The annual electricity saving enabled by the implemen-

tation of DN night ventilation is 11.7% and 13% innorthern and southern orientations, respectively.

Based on these results, it was deduced that deemed-to-satisfy solutions should recommend the provision of night ventilation during the cooling season at a rate of N 4 5 h

1 .In order to check whether previously recommended

values for the building factors, which were derived in

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Cooling_S

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Cooling_N

Ventilation_N

Fig. 11. Effect of night ventilation rate, N , on annual electrical energy demand for classrooms with northern and southern window orientations.

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S T D S I

South North

Fig. 10. Effect of window glazing shading coefcient, SC, on the excessive mean radiant temperature index, STDSI, for classrooms with northern andsouthern window orientations.



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Section 3.2, need to be altered after the addition of nightventilation, a sensitivity analysis was conducted at theN 10 h

1 level. Perturbations around the preferred valuewere investigated separately for every factor. None of theprevious recommendations had to be changed.

3.3.2. Effects of improved IAQ and night ventilation schemesThe basic assumption in the next step is that the specicgeometrical conguration of a school building and itsoccupation pattern provide an additional feature forimproving energy saving. The main contribution stemsfrom the large public spaces in front of classrooms(corridor, hall, or atrium), which are only partiallyoccupied during intermissions between lectures and arecompletely vacated during class hours. When open toexternal air, these spaces respond passively to thesurrounding conditions and tend to develop air tempera-tures beyond the comfort limits during many hours. Themain hypothesis is that with adequate ventilation schemes,these spaces can be utilized as a buffer zone, which iswarmer than the external air in winter and cooler insummer, thus enabling pre heating of incoming IAQventilation air in winter, and increasing heat losses throughthe door-sided wall of the classrooms in summer. Theimproved thermal comfort level in the buffer zones is initself a minor additional-value byproduct that is notdiscussed quantitatively in this paper.

IAQ ventilation schemes that were considered, and theirsymbols, include:

ID exterior air drawn directly intoclassrooms and withdrawn directly tothe outside.

IDC exterior air drawn directly intoclassrooms and withdrawn to the outsidethrough a window-closed corridor.

IVC exterior air drawn into classroomsthrough the window-closed corridor andwithdrawn directly to the outside.

ISDCWVC a combination of IDC in summer andIVC in winter.

Night ventilation schemes that were considered, and theirsymbols, include:

ND exterior air drawn directly into classrooms andwithdrawn directly to the outside.

NDC exterior air drawn directly into classrooms andwithdrawn to the outside through a window-closedcorridor.

NVC exterior air drawn into classrooms through thewindow-closed corridor and withdrawn directly tothe outside.

The ventilation combinations that have been analyzed inorder to study the effect of ventilation schemes are listed inTable 3 .

In order to portray the adverse effect of IAQ ventilationon energy demand the analysis includes the cases with noactive ventilation, IAQ_0_N and IAQ_0_S, which cannotensure an adequate indoor air quality, but are otherwise

identical to the preferred building modules IAQ_5_Nand IAQ_5_S.

Fig. 12 shows the resulting annual electrical energydemands for the investigated combinations.

It can be noticed that without the provision of IAQventilation, heating demand is almost nil for the southernas well as for the northern orientation due to the largeinternal heat sources. Provision of the required minimumIAQ ventilation rate of 5 h

1 increases heating demandsignicantly (by more than 700%), although somewhatreducing the cooling demand (by less than 11%). At thispoint, additional runs, which are not presented here, wereperformed in order to obtain the optimal summer daytimeventilation rate. It was found to be smaller than therequired 5 h

1 . Consequently, this optimum has nopractical signicance and the IAQ air change rate of 5 h

1 is kept throughout the year. Results obtained for theimproved ventilation schemes, as analyzed below, are thuscompared to those obtained for cases IAQ_5_N andIAQ_5_S.

Changing IAQ ventilation from ID to IVC (schemesDIR_NC0_N and DIR_NC0_S compared to IAQ_5_N

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Table 3Combinations of analyzed ventilation schemes

Option index Night ventilation IAQ ventilation

South (1) North (2) Scheme (4) Rate (h1) (5) Scheme (6) Rate (h

1) (7)

IAQ_0_S IAQ_0_N 0 0IAQ_5_S IAQ_5_N 0 ID 5IAQ_5_NC10_S IAQ_5_NC10_N ND 10 ID 5DIR1_NC0_S DIR1_NC0_N 0 IVC 5DIR2_NC0_S DIR2_NC0_N 0 IDC 5MIX1_NC0_S MIX1_NC0_N 0 ISDCWVC 5MIX2_NC10_S MIX2_NC10_N ND 10 ISDCWVC 5MIX3_NC10_S MIX3_NC10_N NVC 10 ISDCWVC 5MIX4_NC10_S MIX4_NC10_N NDC 10 ISDCWVC 5



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and IAQ_5_S) proves to be very efcient for reducing theheating energy demand in both orientations. Exterior airwarms up while passing through the closed corridor,which, although not actively heated is at an elevatedtemperature compared to the external ambient conditions.In both orientations the corridor warms up due to heatgains transferred through the separation wall with the

adjacent classroom. A south facing corridor gains addi-tional substantial heat from solar radiation and serves as asun space. Consequently, the pre-warmed fresh airenables a reduction of some 23% in heating electricity forthe southern classrooms and 68% for the northern ones.However, due to the above detailed mechanisms, thisventilation scheme, when applied in summer, may increasethe cooling electricity demand for the northern classroomby some 29%. For the southern classrooms the incrementis not that large (only 13%) since the north facingcorridor experiences much smaller solar gains duringsummer.

Changing IAQ ventilation from ID to IDC (schemesDIR2_NC0_N and DIR2_NC0_S) reduces the bufferspace temperatures in summer, and consequently summercooling electricity increments are now only 15% and 7.5%in northern and southern classroom window orientations,respectively.

The combination of IVC in winter and IDC in summer(ISDCWVC, as implemented in schemes MIX1_NC0_Nand MIX1_NC0_S) can reduce heating electricity demandby 68% and 23% for northern classrooms and southernclassrooms, respectively, while increasing cooling demandsby only 15% and 7.5%, respectively. Consequently, it mayreduce total electrical energy demands by 11% for northernclassrooms and only 1% for southern ones.

Adding night ventilation to the daytime ISDCWVC IAQventilation further reduces cooling energy. Results inFig. 12 indicate that for the northern classroom, ND andNDC night ventilation are much more effective than NVC,with NDC (scheme MIX4_NC10_N) yielding a totalannual reduction of 28% (in comparison to IAQ_5_N).For the southern classroom, the three night ventilation

schemes yield similar improvements, with a total annualreduction of nearly 17% (in comparison to IAQ_5_S).These total reductions are larger than the 11.7% and 13%reductions derived by night time ventilation alone (seeSection 3.3.1).

Combining the most efcient schemes for winter andsummer improvements yielded the PREF_VENT_N andPREF_VENT_S possible solutions for the northern andsouthern orientations, respectively (see Fig. 12 ). Theseinclude: In winterfor both orientations a closed corridorwith IAQ ventilation according to the IVC scheme(reducing heating electricity by 68% for the northernclassroom and 23% for the southern one). In summerforboth orientations an open corridor with IAQ as well asnight ventilation according to the IDC scheme (reducingcooling electricity by 23% for the northern classroom and28% for the southern one). The total electricity reductionsobtained by these schemes (in comparison to IAQ_5_Nand IAQ_5_S) are 30% for the northern classrooms and18% for the southern ones.

Consequently, it was deduced that improved ventilationschemes are largely effective for northern classrooms,whereas for southern classrooms they yield only minorenergy reductions.

Recommended deemed to satisfy solutions thus include:closing corridor windows in winter and opening them in

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_ S

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Lighting Ventilation Heating Cooling

Fig. 12. Effects of various ventilation schemes on annual electrical energy demand for classrooms with northern and southern window orientations.



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summer for both window orientations. For northernclassroomsIAQ ventilation according to the IVC schemein winter and the ID or IDC scheme in summer, andsummer night ventilation according to the ND or NDCscheme.

For southern classroomsany combination of IAQventilation and summer night ventilation.

It may also be noted that the recommended ventilationschemes reduce electricity demands below the levels of thenon-ventilated cases IAQ_0_N and IAQ_0_S, thus redu-

cing the severity of the EE-TC-IAQ Dilemma.

3.3.3. Integration of resultsThe preferred building design variables deduced in

Section 3.2 were used in Sections 3.3.1 and 3.3.2 in orderto derive the preferred ventilation schemes. In orderto check whether the two sets of decisions can besimply integrated, sensitivity analyses were conducted.In each analysis one building design variable wasvaried while the ventilation schemes and all other designvariables were kept at their preferred solutions and values.Fig. 13 shows the analysis results for total electricitydemand in southern and northern orientations (indicatedby S and N, respectively) versus the external wallsstructure (location of mass indicated by InM, ExM orEDM) and thermal resistance. Except for a shift in theoverall range of total electricity demand, the trends of the lines for the buildings with a direct IAQ ventilationscheme and no night ventilation (indicated by Basic)are similar to those obtained for the buildings withpreferred ventilation schemes (indicated by PrefVent).Similar results were obtained for other variables, indicatingthat the recommendations derived in Section 3.2 arerelevant to the buildings with preferred ventilation schemesas well.

4. Conclusions

The large ventilation rates required in crowded spaces inorder to provide adequate indoor air quality may causesignicant energy losses in school buildings. Seeking toestablish deemed-to-satisfy solutions as the base-line for aperformance-based Energy Code for schools, two taskshave been accomplished: (i) establishing the preferred set of building design variables for schools that are energyefcient while providing acceptable levels of indoor air

quality, as well as thermal and visual comfort, and (ii)establishing a set of preferred ventilation schemes whichenhance the energy efciency without reducing otherperformance levels. The rst task is a classical energyefciency problem, leading in the present study to theprovision of the preferred building which served as areference for the second task. It may enable a minorelectrical energy saving of 2% in northern classrooms incomparison to the electrical energy demand of thecommonly constructed basic building, but may cause aslight increase of 2.5% for southern classrooms due to thepreferred use of glazing with lower lighting and solartransmittance in order to prevent glare and excessive meanradiant temperatures.

The existence of large, usually non-occupied, publicspaces in school buildings enables their utilization as bufferzones for pre-warming the fresh ventilation air in winter,and for increasing removal of heat stored in classroomsoors and walls in summer. Analysis of various combina-tions of ventilation schemes with identical ventilation ratesduring class hours, revealed that additional electricitysavings can be achieved, allowing overcoming the de-creased energy performance induced by indoor air quality(IAQ) ventilation. The preferred ventilation mode for theanalyzed weather conditions (which represent the moderateMediterranean climate) includes improved schemes for

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0 0.5 1 1.5 2 2.5 3

Thermal Resistance (m 2K/W)

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EDM-PrefVent-S

InM-PrefVent-N

ExM-PrefVent-N

EDM-PrefVent-N

Basic-NBasic-S

PrefVent-S

PrefVent-S

Fig. 13. Sensitivity analysis of total electricity demand in southern and northern classrooms to external wall structure (InM, ExM and EDM) and thermalresistance for the basic building and the one with preferred ventilation schemes.



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year round IAQ ventilation combined with night ventila-tion during the summer cooling period. The main outcomesinclude:

(1) Corridor windows should be closed in winter and openin summer.

(2) The preferred IAQ ventilation schemes in northern aswell as in southern classrooms are composed of: Inwinterdrawing fresh outside air into the classroomsthrough closed corridors, halls or atrium. In summer drawing exterior air directly into the classrooms andexhausting it via open corridors, halls or atrium.

(3) The preferred summer night ventilation in northernclassrooms is composed of drawing exterior air directlyinto the classroom and exhausting it directly to theoutside or via open corridors, halls or atrium. For

southern classrooms all night ventilation schemes havevery similar effects.

(4) When compared to the energy performance of thepreferred building, the total electrical energy savingachieved by the application of the preferred ventilationschemes may be in the order of 28% to 30% for north

facing classrooms and 17% to 18% for south facing ones.(5) Sensitivity analyses showed that recommendations forthe building design variables need not to be alteredwhen the Preferred Ventilation Schemes are applied.

Acknowledgments

The paper is based on a research project supported bythe Israeli Ministry of National Infrastructures and theRachel Shalon Foundation.

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Fig. A.1. Ambient air temperature and dew-point temperature variations in January (winter) and June (summer) in Jerusalem.



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[33] Xavier AADP, Lamberts R. Indices of thermal comfort developedfrom eld survey in Brazil. ASHRAE Transactions 2000;106:4558.

[34] Kwok AG. Thermal comfort in tropical classrooms. ASHRAETransactions 1998;104(1B):103147.

[35] Paciuk M. The role of personal control of the environment in thermalcomfort and satisfaction at the workplace. In: Proceedings of the 21th

annual conference of the environmental design research association.Coming of Age, Champaign-Urbana, April 69, 1990. p. 30312.[36] Paciuk M. An expanded model of thermal comfort in the ofce

environment. In: Proceedings of the 6th international conference onindoor air quality and climate, indoor air vol. 93, Helsinki, July 48,1993. p. 4954.

[37] Becker R, Paciuk M. Thermal comfort in residential buildings failure to predict by standard model. Building and EnvironmentJournal. 2006, Submitted for publication.

[38] ANSI/ASHRAE 62-1999. Ventilation for acceptable indoor airquality. American Society of Heating, Refrigerating and Air-Conditioning Engineers.

[39] CEN, prEn13779:1999 E. Ventilation for buildingsperformance

requirements for ventilation and air-conditioning systems. CEN,European committee for standardization, Brussels.[40] US DOE. Energy efciency and renewable energy, building

technologies program and software tools. Website: / http://www.eere.energy.gov/buildings/energy_tools/energyplus/ S . USA Depart-ment of Energy.

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http://www.eere.energy.gov/buildings/energy_tools/energyplus/http://www.eere.energy.gov/buildings/energy_tools/energyplus/http://www.eere.energy.gov/buildings/energy_tools/energyplus/http://www.eere.energy.gov/buildings/energy_tools/energyplus/

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