An Approach for the Evaluation of Energy

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An approach for the evaluation of energy andcost efciency of glass fac ades

Ikbal Cetiner a ,*, Ertan Ozkan b

a Istanbul Technical University, Faculty of Architecture, Taskisla, 80191 Taksim/Istanbul, Turkeyb Beykent University, Istanbul, Turkey

Received 22 May 2004; received in revised form 18 September 2004; accepted 3 October 2004

Abstract

Glass fac ades, particularly in high-rise buildings,increase in energyconsumption forheating, cooling andventilation. This causes too highrunning cost of mechanical systems. Double skin glass fac ade is a system that decreases these disadvantages, by providing natural ventilation,preventing solar heat gain, controlling daylight, etc. This paper aims to investigate the appropriateness of double skin glass fac ades inmoderate climate, such as Istanbul, in terms of the energy and cost efciency when compared to single skin glass fac ades. For this purpose, anapproach is proposed to determine the efcient alternatives. It comprises to generate standard fac ade alternatives by considering theobjectives, constraints and performance criteria, and to evaluate their energy and cost efciency for both single and double skin glass fac ades.In conclusion, themost energyefcient double skin glass fac adeis about 22.84%more efcient than themost energyefcient single skin glassfac ade is. Additionally, the most cost efcient single skin glass fac ade is about 24.68% more efcient than the most cost efcient double skinglass fac ade is.# 2004 Elsevier B.V. All rights reserved.

Keywords: Energy efciency; Cost efciency; Double skin glass fac ade; Computer simulation; Life cycle cost analysis

1. Introduction

In high-rise buildings, glass fac ades are often preferreddue to the short application time, low maintenance, beingrain screen as well as being lightweight, aesthetic anddurable. They cannot be set up with openings as the windowsfor ventilation due to wind effect. Therefore, the systemincreases the loads of cooling and ventilation systems. Thisbrings about too high cost in use and an increase in energyconsumed for running mechanical systems. To decrease all

these disadvantages, intelligent fac ade systems have beendeveloped. These are considered as a multiple functionalelement to reconcile the conicting needs such as heating,cooling, ventilating and lighting. Double skin glass fac ade isone of these systems.

Double skin glass fac ades are composed of two glassskins and a large cavity in between. The intermediate cavityfunctions basically as a thermal buffer zone that reduces heat

losses and provides passive heat gains from solar energy,where the system has facility to open outside for ventilation.Additionally, the casements in the inner skin can be openedto the cavity in high-rise building that is exposed to severewinds. These facilities ensure a natural form of ventilationand night-time cooling of the building and thus reducesenergy consumption needed for running air conditioningsystem [1]. The further advantages of the double skin glassfac ades are the applicability of effective solar shading in thecavity and the easy ventilation of warm cavity air by stack

effect, which causes the air to rise and convective heat tolose. Thus, the passive ventilation and convection reducesthe temperature of the air in the cavity. This results indecreasing the surface temperatures of the skins, loweringrate of heat transfer between two surfaces of the inner skin.The meaning of this is that the space close to the inner skincan be better utilized as a result of increased thermal comfortconditions [2].

The winds acting on a high-rise building do not permitexternal shading devices to x on the surface. However, an

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* Corresponding author.

0378-7788/$ see front matter # 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.enbuild.2004.10.007


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adequately ventilated sun-shading system in the intermedi-ate cavity can have almost the same effect as an externalinstallation, and it will be much more ef cient than interiorshading devices in rooms [3]. In addition, the investmentmade for the fac ade is also expected to be economical fromthe viewpoint of the building owner. In central Europe, these

fac ades are about twice the cost of conventional curtainwalls. In the U.S.A., they are likely to be four to ve timesmore expensive. The extra costs are racked up by theexpense of engineering these systems, the amount of specialglass required, and an unfamiliarity with these systemsamong the trades, which leads to higher installation costs[4].

Here is to investigate the appropriateness of double skinglass fac ades in moderate climates, such as Istanbul, in termsof the energy and cost ef ciency. For this purpose, anapproach for determining the ef cient alternatives isproposed.

2. Proposed approach

The aim of the study is to generate standard fac adealternatives in the context of performance approach andevaluate their energy and cost ef ciency. It includes theformulation of the problem, building of a model helping theevaluation of the alternatives, the comparison of thealternatives and an application ( Fig. 1). Every stage includesselection elements aiding the decision. According to thisapproach, all selection elements constitute a system and theyare arranged as input, process and output. For the energy

load and cost analyses of fac ade alternatives, computersimulation techniques and life cycle cost analysis (LCCA)are used.

2.1. Formulation of the problem

As a rst stage of the approach objectives/constraints,performance criteria and alternative solutions are deter-mined and formulated. To provide ef ciency, the elementsare determined according to environmental factors and userrequirements as being done in the studies based uponperformance concept. The steps followed in this process areas follows.

2.1.1. Environmental factorsThe environmental factors that affect energy and cost

ef ciency of fac ade are considered as natural articialenvironmental factors and economic factors.

Natural articial environmental factors are external andinternal factors. External factors are external climaticvariables, which are composed of temperature, humidity,solar radiation and wind, and arti cial environmentalvariables, which are related to orientation of building, formof building, building intervals, position of building andthermal optical properties of building skin. Internal factors

are ambient temperature, humidity, mean radiant tempera-ture and air movement, which contribute to provide internalclimatic comfort. The economic factors affecting the costef ciency of the fac ade are admitted as initial andmaintenance costs that are composed of energy, cleaningand repairing costs.

2.1.2. User requirementsUser requirements are the conditions that users need to

perform their activities depending on the factors affectingthe fac ade in terms of the ef ciency aimed.

2.1.3. Objectives/constraintsObjectives are determined in relation to natural articial

environmental factors, economic factors and user require-ments to evaluate the energy and cost ef ciency of the glassfac ade. Achieving these is possible by realizing the sub-objectives.

Constraints are restriction conditions that directly impacton the energy and cost ef ciency of the glass fac ade. Theseconditions are determined by utilizing from rules, regula-tions, speci cations and standards.

2.1.4. Performance requirements/properties/criteriaDetermination of performance requirements is important

in order to describe properties and criteria expected from theglass fac ade. Properties, which can be accepted as acriterion, are the features that glass fac ade must have interms of intended ef ciency. These differ according to theperformance expected from the fac ade. For instance, in thelast studies done on glass fac ade, properties of the fac ade

have been improved to consume less energy as a result of gradually decreasing natural energy sources and increasingenvironmental pollution. Performance criteria are deter-mined to evaluate whether alternatives are appropriate forobjectives. For this reason, criteria that are used to evaluatethe intended ef ciency have to be arranged in accordancewith properties expected from alternatives.

2.1.5. AlternativesAll possible alternatives should be generated in the frame

of selection elements determined with respect to userrequirements and environmental factors in order to select themost ef cient alternative. The followed way to generatealternatives is as follows.

2.1.5.1. Fac ade components. All components forming adouble skin glass fac ade are rst determined.

2.1.5.2. Sub-variables. The sub-variables can be consid-ered as the component properties directly affecting energyand cost ef ciency. The possible cases for these propertiesare decided in the frame of selecting elements includingobjectives, constraints and performance criteria. Forexample, the position of single or double glazing unitaffects the energy ef ciency.

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2.1.5.3. Generating fac ade alternatives. As a result of thecombination between the fac ade components and the sub-variables, many alternatives can be generated. To prevent thecomplexity that will be able to become during evaluations, acode system is developed for these alternatives. This systemformed considering the component properties is seen in

Table 1 . The cases selected for every sub-variable determinethe number of the columns included in the code of analternative. Considering all possible cases, a great number of alternatives canbe generated. The important point is, here, toselect appropriate cases in terms of aimed ef ciency,considering performance criteria.

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Fig. 1. The proposed approach.


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2.2. Building the model

Intheprocessofmodelling,theevaluationoftheenergyandcost ef ciency of alternatives with an objective approach isplanned. This is mainly possible as a result of the energy loadandlifecycle costanalysesof alternatives.In bothanalysestheef ciencies of glass fac ade alternatives are investigated inconjunction with the sub-variables as dimension, form, posi-tion, texture, color, material and joints. Evaluating the effectsof thesub-variableson energy consumptionandtotalcost, it ispossible to determine the appropriate alternatives in terms of the intended ef ciencies.For instance, comparing the alterna-

tives formed with single or doubleglass thebest con gurationcan be determined in terms of energy and cost ef ciencies.

2.2.1. Energy load analyses of alternativesEnergy load analysis aims to determine the effects of

heating and cooling energy consumption on the fac adealternatives generated in conjunction with the objectives,constraints and performance criteria. It is completed in threestages as follows.

2.2.1.1. Determining thermal and optic properties. Ther-mal and optic properties of fac ade alternatives changedepending on external effects and the properties of glassfac ade components. Since external effects can be controlledselecting appropriate materials for components or takingsome measures during fac ade design, the computationvalues belonging to these effects have to be known. On theother hand, to what extent the properties of the fac adecomponents affect the thermal and optic properties of thealternatives should be investigated with sensitive analyses.The thermal and optic properties affecting the energy andcost ef ciency of the fac ade are:

- heat transmission;- solar energy transmission;

- air-tightness;- heat storage capacity.

The values related to these properties have to be determinedin conjunction with dimension, position, joint, color andmaterial of fac ade components.

2.2.1.2. Determining heat gain and losses. To determinethe heat gain and losses of the alternatives, in addition to thethermal and optic properties of the glass fac ade, the effectsof both the variables related to building and environment(geometric properties, building function, building position,

the distance to the neighbor buildings, surface exposure,etc.) and the properties related to mechanical system (systemtype, energy type, operation hours, etc.) should also beconsidered.

Considering all these, the heat gain and losses of thealternatives can be assembled into four groups:

- gain and losses resulting from heat transmission;- gain and losses from solar radiation;- gain and losses resulting from air in ltration;- gain and losses resulting from mass effect.

2.2.1.3. Determining total energy loads. To evaluate ene-rgy loads of alternatives, the annual sum of their heat gainand losses are determined as follows:E H E HT E HS E HA E HM (1)

E C E CT E CS E CA E CM (2)

E A E H E C (3)where E A is the annual energy load; E H , E C the annualheating and cooling loads; E HT , E CT the heating and coolingloads resulting from heat transmission; E HS , E CS the heatingand cooling loads resulting from solar radiation; E HA , E CAthe heating and cooling loads resulting from air in ltration;


Table 1Coding fac ade alternatives


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E HM , E CM the heating and cooling loads resulting from masseffect.

In the following, the energy consumption of thealternatives are then calculated in connection with theirannual energy loads and the ef ciency of heating andcooling systems:

E HC E Hh (4)

E CC E C

c(5)

E AC E HC E CC (6)where E AC is the annual energy consumption; E HC , E CC theannual heating and cooling energy consumption; h, c theef ciencies of heating and cooling systems (%).

2.2.2. Life cycle cost analyses of alternativesThe amount of energy needed for the heat gain/losses

through a double skin fac ade affects the heating and coolingcosts. To determine total life cycle cost of alternatives, usingextra component and different materials for reducing energygain and losses or the money needed for cleaning, repairing,etc. are also taken into consideration. The economicanalyses of alternatives is done as follows.

2.2.2.1. Determining initial costs. The initial costs of thealternatives are calculated according to the square meteranalyses of the fac ade components and their workmanship,vehicle and material costs per square meter. This calculationcan be viewed below:

C I Xt

m1 Am M m (7)

where C I is the initial cost; m the number of components; Amthe areas of components; M m the total of workmanship,vehicle and material costs of every component per squaremeter.

2.2.2.2. Determining maintenance costs. Maintenancecosts include both energy costs and cleaning/repairingexpenses made for fac ade components at certain periods.

Energy costs in the life cycle period : Energy costs areenergy consumption costs resulting from heat gain and

losses. These costs are calculated as follows:C H E HC C UH (8)C C E CC C UC (9)where C H is the annual heating energy cost; C C the annualcooling energy cost; C UH the heating energy unit cost; C UCthe cooling energy unit cost.

The annual energy consumption costs are the costs thatare regularlypaid every year in the assumed life period of thefac ade, so these costs which will be paid in the future mustbe converted to present values. For this conversion, theenergy costs are multiplied by present value factor (PVF)which takes into account discount rate and life cycle period

[5]. In this paper, supposing that energy costs do not changein the life cycle period, present value of total life cycleenergy costs is calculated as follows:

PVF 1

1 r n

where n is the life cycle period (number of years); r thediscount rate (%):

C PLH C H Xt

n1

11 r n

(10)

C PLC C C Xt

n1

11 r n

(11)

C PLE C PLH C PLC (12)where C PLH is the present value of life cycle heating cost;C PLC the present value of life cycle cooling cost; C PLE thepresent value of total life cycle energy cost.

Cleaning/repairing costs in life cycle period : These costs

should be determined according to the data arranged for theexisting glass fac ades and converted to present values as thestages followed for calculating energy costs. The conversioncalculations are seen below:

C PLA C A Xt

n1

11 r n

(13)

where C A is the annual cleaning cost; C PLA the present valueof life cycle cleaning cost:

C PLR C R Xt

n1

11 r n

(14)

where C R is the annual repairing cost; C PLR the present valueof life cycle repairing cost:

C PLM C PLE C PLA C PLR (15)where C PLM is the present value of life cycle maintenancecost.

2.2.2.3. Determining total life cycle costs. To calculatetotal life cycle costs of alternatives, considering both initialcosts and present values of life cycle maintenance costs, thefollowing formula is used:

TLCC C I C PLM (16)

where TLCC is the total life cycle cost; C I the initial cost;C PLM the present value of life cycle maintenance cost.

2.3. Evaluation of alternatives

The alternatives generated considering objectives, con-straints and performance criteria are evaluated in conjunc-tion with sub-variables. In other words, the effects of thesub-variables on energy or cost ef ciency of the alternativesare investigated. For the evaluation, the results achievedfrom the energy and cost analyses of the alternatives are rstconverted to the ef ciency values and then these alternatives



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are grouped according to the disparities of sub-variables.In the conversion, assuming that the alternatives withthe highest energy consumption and life cycle cost do nothave any bene t, their ef ciencies are considered aszero. And then how ef cient other alternatives are whencompared with the zero ef cient alternatives are calculated.

The results can be illustrated with graphics or tables toevaluate the effect of every sub-variable on energy and costef ciency.

2.4. The comparison of the alternatives

In the comparisons, the success provided by thealternative, being minimum energy consumption andtotal life cycle cost is taken into consideration. Thealternatives having the highest percentage in terms of energy consumption and total life cycle cost are acceptedas the most ef cient ones. Consequently, the following canbe determined:

the most useful alternative whose energy consumption isminimum in terms of country sources;

the most protable alternative whose life cycle cost isminimum in terms of building owners.

3. Application

The proposed approach is applied on an of ce buildingassumed to be in Istanbul. The cases that the building isenclosed on all fac ades by either single or double skin areconsidered. The energy and cost ef ciency of the alternative

solutions is evaluated depending on the different con g-urations and material types of the fac ade components. Thedata related to the of ce building selected for thisapplication are as follows:

Buildingsizes

36 m 36 m Numberof oors

30

Ceilingheight

3.30 m Total oorarea

38880 m 2

Total fac adearea

13896 m 2 Exteriorshading

No

Type of

heatingenergy

Natural gas Type of

coolingenergy

Electric

Ventilationrate

20 L/s m2 Inltrationrate

0.4 ach

3.1. Formulation of the problem

3.1.1. Environmental factors- The data related to the external climatic variables are

taken from the Meteorology Station of Goztepe inIstanbul.

- Articial environmental variables is admitted as follows:

The position of the building Istanbul, Go ztepeThe orientation of the building Latitude: 41 8 N; longitude: 28 8 EThe height from sea 39.00 mBuilding intervals It is assumed that surrounding

buildings are far

- The ambient temperature is assumed to be minimum 20 8 Cand maximum 26 8 C to achieve comfort conditions [6].

- The initial costs are achieved from the interviews with therms constructing curtain wall in Turkey.

- Maintenance costs, which are considered as the energycosts, are calculated depending on the energy loads of thefac ade alternatives.

3.1.2. User requirementsUser requirements in terms of the energy and cost

ef ciency are to provide the interior environmental comfortand reduce the total life cycle cost of the fac ade.

3.1.3. Objectives/constraintsThe objective is to determine the most ef cient

alternatives in terms of energy consumption and total lifecycle cost.

In our country, the heat transmission coef cient (U value,W/m 2 K) that external skin must have in terms of energyconservation is labeled in TS 825 depending on the climaticzones [7]. However, there are not any standards related to thesolar heat gain coef cient (SHGC value).

3.1.4. Performance criteria

The performance requirements expected from anenergy and cost ef cient fac ade can be determined asenergy conservation, air tightness, natural ventilation,economy and interior comfort. A fac ade to meet theserequirements should have high heat transmission resis-tance, control sunlight, provide natural ventilation,prevent air in ltration, has a low life cycle cost andensure comfort conditions. For this reason, the skin shouldperform the following criteria.

3.1.4.1. The criteria related to energy efciency.- According to TS 825 U values for external wall and double

glazed windows should be 0.60 and 2.80 W/m 2 K,respectively [7].

- Glass fac ade should have a low SHGC to prevent solargain.

- It is assumed that ventilation is provided for all fac adeseffectively.

- There is a certain amount of air in ltration towards insideon all fac ades.

- Total energy consumption resulting from heat gain andlosses of the fac ade is the main criterion in evaluatingenergy ef ciency. The alternative with the lowest energyconsumption is the most ef cient one in terms of energyusage.

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3.1.4.2. The criteria related to economic ef ciency.- The initial costs affecting the total life cycle cost are

considered in the evaluation of the cost ef ciency.- Maintenance cost which is the main criterion in terms of

cost ef ciency is admitted as the annual energy costsdetermined with respect to energy consumption. Cleaningand repairing costs are not considered in calculating themaintenance costs because there are no available accuratedata related to these costs in our country.

- The service life of the glass fac ade is supposed 30 years.

3.1.5. AlternativesIn linewith the proposed approach, the skin alternatives is

determined as follows.

3.1.5.1. Fac ade components. For this application, the

following are approved for the components:

- The glass types used in the application are clear, re ectiveand low-E glasses. The data related to these glasses aregiven in the Table 2 .

- In the single skin glass fac ade, double glazing unit is used.- In the double skin fac ade, the width of the interval cavity

and the position of the Venetian blind are decided in theresult of sensitive analyses donewith WIS program, whichis used for computing the thermal and optic properties of the window system [8]. The analyses made for thedifferent fac ade congurations indicate that the width of internal cavity results in the slightly variation of the U values whereas it does not affect the SHGC values. Inaddition, using the Venetian blind affects both the U andSHGC values. The best result is achieved when theVenetian blind is positioned near the external skin. Theresults for an example of glazing con guration can be seen

in the Table 3 . The similar results are also achieved with


Table 2The data related to the glasses

Thickness (mm) Clear glass Re ective glass Low-E glass

Solar energyDirect transmittance 6 0.770 0.208 0.574Reectance to outdoor 6 0.070 0.152 0.218Reectance to indoor 6 0.070 0.328 0.144

DaylightVisual transmittance 6 0.880 0.300 0.825Reectance to outdoor 6 0.080 0.121 0.041Reectance to indoor 6 0.080 0.265 0.055

EmissivityEmissivity to outdoor 6 0.840 0.840 0.100Emissivity to indoor 6 0.840 0.550 0.840

Table 3The effects of the width of the cavity and the position of solar control device on the U and SHGC values

Cavity width(mm)

Congurations Heat transmissioncoef cient, U value (W/m 2 K)

Solar heat gaincoef cient, SHGC

300 1.91 0.39600 1.92 0.39900 1.93 0.39

1200 1.93 0.391500 1.93 0.39

300 1.85 0.16600 1.85 0.16900 1.86 0.16

1200 1.86 0.161500 1.86 0.16

300 1.85 0.14600 1.85 0.14900 1.86 0.14

1200 1.86 0.141500 1.86 0.14


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aim, the data related to the weather variables for Istanbul(dry bulb temperatures, dew point temperatures, horizontalsolar radiation, wind speed), the position, dimension andmaterial properties of the fac ade components (external skin,cavity, solar control device and internal skin) are de nedwithin the program. U and SHGC values achieved in the

result of the simulation are seen in Table 6 , including thegroups that are arranged depending on the positions of thesingle or double-glazing.

3.2.1.2. The heat gain and losses. The heat gain and lossesresulting from heat transmission, solar radiation, airinltration and mass effect of the alternatives are computedwith ENER-WIN simulation program, which has been usedfor the analysis of energy load [9].

3.2.1.3. The total energy loads. The total energy loadswhichare the sumof theheating andcooling loads (accordingto the Eqs. (1)(3)) and the annual energy consumptionvalues calculated depending on the ef ciency of the energysystem(according to theEqs. (4)(6))areseenin Table6 .Theannual energy consumption values resulting from the fac adeare the values per square meter of total oor area.

3.2.2. The total life cycle cost analyses of the alternatives3.2.2.1. The initial costs. The unit costs per square meterfor all the components are rst determined, averaging thevalues taken from three rms constructing curtain wall. Andthen the initial costs are calculated according to Eq. (7). Theresults can be viewed in Table 6 .

3.2.2.2. The life cycle energy costs. The life cycle energycosts calculated according to the Eqs. (8)(12) are includedin Table 6 . The data needed for the calculations are selectedas follows:

Annual energyloads

The results determinedby the simulation

Fuel type Natural gas and electricThe unit costs

of natural gas32478.43 kWh/TL(for Istanbul in 2003)

The unit costof electric

120.755 kWh/TL(for Istanbul in 2003)

Life period 30 years

Discount rate 15%The ef ciency

of the coolingsystem

100%

The ef ciencyof the heatingsystem

70% (according to BREDigest 355)

3.2.2.3. The total life cycle costs of the alternatives. Thetotal life cycle costs are the sum of the initial costs and thelife cycle energy costs of the alternatives (See Table 6 ).

3.3. The evaluation of the alternatives

The fac ade alternatives are evaluated in terms of energyconsumption and total life cycle costs by considering theglass/glazing types, position of glasses and solar controldevices.

3.3.1. The energy consumption of the alternativesThe fac ade alternatives are selected and grouped

according to the glass/glazing types, positions and solarcontrol devices. Thereafter, the most ef cient alternative inevery group is compared with each other. The results aregiven in Figs. 24 in conjunction with both energy and costef ciency. In Fig. 2, there are the single and double glazingalternatives for three glass types. It shows the effect of glass/ glazing types on the energy and cost ef ciency of thealternatives. In Fig. 3, the best alternatives for every positionof glazing are compared with each other to determine theeffect of the position of glass/glazing types on the energyconsumption of the alternatives. The effect of using solarcontrol device for the most ef cient alternatives in everygroup is illustrated in Fig. 4. The alternatives are evaluatedin relation to the sub-variables below:

3.3.1.1. The effect of the glass/glazing types. In both singleand double skin con gurations, the best results for U andSHGC values are achieved in the solutions with low-Eglasses (Table 6 ).

The alternatives with the low-E glass are more energyef cient than the other alternatives for both single anddouble skin. The double skin alternative formed with the

low-E glass on both skins is the most ef cient one in terms of energy consumption. In the double skin con gurations,using the double-glazing on both skins is an ef cientsolution for all glasses ( Fig. 2).

3.3.1.2. The effect of the position of glass/glazing type-s. The congurations formed with double-glazing on bothskins are the most ef cient alternatives in terms of energyef ciency. In the con gurations formed with both single anddouble-glazing, using double-glazing on external skin ismore ef cient than using it on internal skin ( Fig. 3).

3.3.1.3. The effect of using solar control device. UsingVenetian blind causes to decrease the total energy load forboth single and double skin alternatives. This is due to thereduction in the U and SHGC values of the alternatives(Fig. 4).

3.3.2. The total life cycle costs of the alternativesIn this step the effects of the sub-variables on the total life

cycle costs are evaluated as well as in the assessment of theenergy consumption of the alternatives.

3.3.2.1. The effect of glass/glazing types. The alternativeswith the low-E glass are more cost ef cient than the



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Table 6WIS, ENER-WIN and LCCA results for all alternatives

No. Code U (W/m 2 K) SHGC ATEL (million kWh) ATEC (kWh/m 2 ) IC (million $) LCEC (million $) TLCC (million $)

I D1N 2.95 0.55 3.54 129.89 1.56 1.47 3.03D1Y 2.70 0.24 3.42 125.53 2.18 1.25 3.43D2N 2.64 0.16 3.43 126.19 1.57 1.20 2.77D2Y 2.53 0.11 3.37 123.89 2.28 1.14 3.43D3N 2.34 0.44 3.10 113.89 1.55 1.20 2.75D3Y 2.21 0.19 3.12 114.71 2.27 1.07 3.34

II SS11N 2.90 0.55 3.49 128.27 2.75 1.45 4.20SS11Y 2.55 0.11 3.40 124.93 3.47 1.23 4.70SS12N 2.60 0.16 3.34 122.89 2.81 1.27 4.07SS12Y 2.37 0.09 3.26 119.88 3.53 1.15 4.67SS21N 2.53 0.11 3.35 122.99 2.80 1.18 3.98SS21Y 2.70 0.24 3.40 124.98 3.52 1.15 4.67SS22N 2.64 0.31 3.32 121.95 2.83 1.13 3.96SS22Y 2.40 0.12 3.27 120.16 3.54 1.10 4.64

III SD11N 2.21 0.47 2.93 107.72 3.04 1.20 4.24SD11Y 2.06 0.13 2.99 110.03 3.76 1.04 4.80SD12N 2.06 0.23 2.91 107.03 3.16 1.05 4.21SD12Y 1.97 0.12 2.88 105.70 3.88 0.93 4.81SD13N 1.93 0.39 2.68 98.44 3.14 0.98 4.12SD13Y 1.86 0.14 2.79 102.59 3.86 0.89 4.75SD21N 2.09 0.17 2.99 109.68 3.09 1.01 4.10SD21Y 2.34 0.44 3.01 110.43 3.81 0.98 4.79SD22N 1.94 0.09 2.88 105.93 3.21 0.91 4.12SD22Y 1.93 0.07 2.90 106.47 3.93 0.91 4.83SD23N 1.79 0.11 2.73 100.43 3.19 0.85 4.04SD23Y 1.79 0.07 2.77 101.78 3.91 0.84 4.75

IV DS11N 2.20 0.47 2.91 107.05 3.00 1.20 4.19DS11Y 2.09 0.27 292 107.26 3.72 1.08 4.80DS12N 2.08 0.33 2.88 105.98 3.06 1.10 4.17DS12Y 1.99 0.23 2.79 102.64 3.78 0.97 4.75DS21N 2.04 0.08 3.00 110.22 3.11 1.02 4.13DS21Y 1.97 0.10 2.88 105.79 3.83 0.92 4.74

DS22N 1.96 0.11 2.78 102.03 3.17 0.92 4.09DS22Y 1.88 0.09 2.86 104.95 3.89 0.90 4.79DS31N 1.82 0.38 2.57 94.47 3.13 0.93 4.06DS31Y 2.02 0.25 2.88 105.73 3.85 1.00 4.86DS32N 1.70 0.30 2.50 91.81 3.13 0.86 3.99DS32Y 1.93 0.22 2.78 102.03 3.85 0.92 4.77

V DD11N 1.86 0.42 2.59 95.25 3.29 1.01 4.30DD11Y 1.79 0.22 2.64 97.10 4.01 0.91 4.92DD12N 1.77 0.26 2.62 96.37 3.41 0.93 4.34DD12Y 1.72 0.18 2.60 95.55 4.13 0.87 5.00DD13N 1.70 0.35 2.47 90.86 3.39 0.87 4.26DD13Y 1.66 0.18 2.53 92.86 4.11 0.80 4.92DD21N 1.77 0.11 272 99.79 3.40 0.87 4.26DD21Y 1.72 0.08 2.67 98.23 4.12 0.83 4.94DD22N 1.70 0.09 2.66 97.58 3.52 0.83 4.35DD22Y 1.65 0.07 2.60 95.66 4.24 0.79 5.03DD23N 1.61 0.10 2.57 94.29 3.50 0.78 4.28DD23Y 1.57 0.07 2.54 93.16 4.22 0.75 4.97DD31N 1.62 0.34 2.39 87.99 3.38 0.83 4.21DD31Y 1.76 0.21 2.63 96.63 4.10 0.86 4.96DD32N 1.57 0.25 2.40 88.13 3.50 0.78 4.29DD32Y 1.70 0.18 2.56 94.10 4.22 0.82 5.04DD33N 1.50 0.30 2.29 84.23 3.48 0.76 4.24DD33Y 1.61 0.17 2.49 91.54 4.20 0.79 4.98

Heat transmission coef cient (U ), solar heat gain coef cient (SHGC), annual total energy loads (ATEL), annual total energy consumption (ATEC), initial cost(IC), life cycle energy cost (LCEC), total life cycle cost (TLCC).


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other alternatives for both single and double skin.The reason of this is that using low-E glass reducesthe life cycle energy costs of the alternatives. In thedouble skin con gurations, for instance, using doublelow-E glass on both skins instead of double clearglass increases the total cost by 1.15% inasmuch as itdecreases the life cycle energy costs by 24.49% ( Fig. 2 andTable 6 ).

3.3.2.2. The effect of the position of glass/glazing type-s. Single skin con gurations are clearly more cost ef cientthan the double skin con gurations because of their lowerinitial costs. Similarly, in the double skin con gurations, thealternatives that single glazing is used on both skins are the

most ef cient con gurations in terms of the total life cyclecost (Fig. 3 and Table 6 ).

3.3.2.3. The effect of using solar control device. Whereasthe total life cycle energy costs of the alternativeswith blind are lower than those without due to their lowerenergy consumption, their total costs are higher than thosewithout because of the higher initial costs ( Fig. 4 andTable 6 ).

4. Concluding remarks

In line with the evaluations made in terms of energy andcost ef ciency, the following are inferred:

Double skin con gurations are more energy ef cientthan single skin con gurations. In the double skincon gurations, the most energy ef cient alternative isthe con guration, which is coded as DD33N, formedwith the using of double low-E glass on both skins. Thisalternative is 22.84% more ef cient than the mostenergy ef cient single skin con guration, which iscoded as D3N, formed with the using of double low-Eglass.

Single skin con gurations are more cost ef cient thandouble skin con gurations. The single skin con gura-

tion, which is coded as D3N, formed with the using of double low-E glass has the least total life cycle cost.This alternative is 24.68% more ef cient than the mostcost ef cient double skin con guration, which iscoded as DD32N, formed with the using of doublelow-E glass on external skin and re ective glass oninternal skin.

Among the double skin con gurations, DD33N, which isthe most energy ef cient alternative, is about 33.91%more ef cient than SS11N, which has the highest energyconsumption. On the other hand, SS22N, which is themost cost ef cient alternative, is about 7.7% moreef cient than DD22N, which has the highest life cyclecost.

Using solar control device reduces both the energy andcost ef ciency of the alternatives.

In conclusion, the proposed approach makes possibleboth to generate fac ade alternatives by considering the o-bjectives, constraints and performance criteria and evaluatetheir energy and cost ef ciency. In the future studies, det-ermining the limit values for energy consumption and totallife cycle cost and standardizing the ef cient componentproperties that will be able to provide these values are in-tended.


Fig. 3. The effect of the position of the glass/glazing types.

Fig. 4. The effect of using solar control device.

Fig. 2. The effect of the glass/glazing types.


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References

[1] M.Wigginton, BauenmitGlass, Detail,Deutschland,March1998, p.309.[2] A. Compagno, Intelligente Glasfassaden Material, Anwendung,

Gestaltung = Intelligent Glass Facades, Birkhauser, Basel, Switzerland,2002, p. 118.

[3] E. Oesterle, R. Lieb, M. Lutz, W. Heusler, Double-skin Facades, Prestel

Verlag, Munich, Germany, 2001, p. 53.[4] W. Lang, T. Herzog, Using Multiple Glass Skins to Clad Buildings,

Architectural Record, July 2000, McGraw-Hill, UK, 2000, p. 182.[5] H. Marshall, R.T. Ruegg, Energy Conservation in Buildings: An

Economics Guidebook for Investment Decisions, NBS Handbook

132, U.S. Department of Commerce, National Bureau of Standards,Washington, USA, 1980, p. 131.

[6] ANSI/ASHRAE 55, Thermal Environmental Conditions for HumanOccupancy, American Society of Heating, Refrigerating and Air-Con-ditioning Engineers, Inc., Atlanta, U.S.A., 1992.

[7] Turkish Standard, Thermal Insulation in Buildings, Turkish StandardInstitute, Ankara, Turkey, 1998, p. 17.

[8] D. Van Dijk, J. Goulding, WIS Reference Manual, TNO Building andConstruction Research, Delft, Netherlands, 1996.

[9] L.O. Degelman, ENER-WIN User s Manual, Texas A&M University,Texas, USA, 1999.


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An Approach for the Evaluation of Energy

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