Effects of ceiling fans on the thermal comfort of students in … · 2019. 9. 16. · Effects of ceiling fans on the thermal comfort of students in learning environments of Bayero

Effects of ceiling fans on the thermal comfort of students in learningenvironmentsofBayeroUniversity,Kano,Nigeria

SaniM.Ali1,BrettD.Martinson1,SuraAl-Maiyah2,andMarkGaterell1

1UnversityofPortsmouth, SchoolofCivil EngineeringandSurveying,Portsmouth, PortlandBuilding,PortlandStreet,Portsmouth,PO13AH,UK;2UniversityofSalford,SchooloftheBuiltEnvironment,Manchester,UK.

Abstract: It iswell known that thermal comfort is influencedbymajorphysical parameters; air and radianttemperatures,humidity,andairspeedincombinationwithpersonalattributes;clothinginsulationandactivitylevel. Although temperature is conventionally considered in adaptive thermal comfort model, as the mostimportant physical parameterwhere cooling is involved,moderate air speed can enhance thermal comfortduringhighertemperatures.Throughconvectiveandevaporativecooling,ceilingfanscoolpeoplebycausingsweatfromtheoccupant’sbodytoevaporate.ThenorthernpartofNigeria,beinginthetropics,isknownforhighertemperatureregimesformostpartoftheyear.Theuseofairconditioningtoachievethermalcomfortisnotsustainable,foreconomicreasonsandthelackofstableelectricalenergy.Therefore,amajorityofnaturallyventilatedspacescouldbekeptthermallycomfortablewiththecontrolofceilingfansandoperablewindows.AspartofaresearchworkonlearningenvironmentsinaNorthernNigerianuniversity,thisstudyreportsontheeffectsofceilingfansonthethermalcomfortperceptionofthestudentsintwolecturetheatres.Airspeed,airandradianttemperatures,relativehumidityweremeasured,concurrentlycomfortsurveyswereundertakeninthe spaces, from which activity levels and clothing insulations were obtained. Adaptive thermal comfortstandards,ASHRAE55andEN15251,statethatthermalcomfortcanbemaintainedasairtemperatureriseswiththeuseofceilingfansoperatingatmoderatespeed.Theresultsshowthatreductionsof31%and22%inoverheating from the two lecture theatreswere realised,asa resultof ceiling fansusage,measuredby thedegreehour’sexceedanceindicator.Theseresultswerefurthercorroboratedbythestudents’acceptanceofthermalconditionsofthelecturetheatresattemperaturesaboveTmax.

Keywords:Ceilingfans,thermalcomfort,overheating,Africa,tropics

1. IntroductionThereisnorainsandnocloudcoverinthedryseasoninNorthernNigeria,resultinginwarmweatherconditionsandmakingindoorenvironmentsthermallyuncomfortable.Outsideairtemperature especially in April can reach as high as 40 °C necessitating the use of airconditionerstokeepacoolenvironment.However,thisiscomplicatedbythelackofstableenergysuppliesinNigeria(Akande2010).Thismakestheuseofairmovementtofacilitateindoor comfort very attractive not only in Kano, a city inNorthernNigeria, but in all hotclimatesaroundtheworld(Nicol2004).Evenbeforetheadventoffossilsfuels,humanbeingslearnttheartofexcludingtheeffectsofextremeweatherfromtheirdwellingunits,inhighlatitudeareasandelsewhere,inthecoldseasonfireswerekindled,layersofclothingaddedtokeepwarm,andmassivewallsandroofconstructedtostoreandutilizesolarradiation.During the hot season however, lighter clothing was preferred, people changed theiractivities,otherssleptoutdoorsandinthedaytimetreeshadesweresoughtforrelaxationandhandheldfanswerewidelyused inordertokeepcool (Candido,deDearetal.2010,Inusa andAlibaba 2017, Li, Zhou et al. 2017). Gradually buildingsweremade to performenvironmentally with natural ventilation through openings; doors, windows and otherarchitectural openings (Candido, de Dear et al. 2010). With the invention of electrically

PROVISIONAL PROCEEDINGS WINDSOR CONFERENCE 12th-15th April 2018

poweredfans,ceilingandmovablepersonalfansbecomepopularinthehotanddryclimates,anditwasonlyinthefirsthalfoftheTwentiethCenturythatairconditioningwasinvented(De Decker 2014). Although air conditioning (AC) is widely used as a means of meetingthermalcomfortrequirementswhereavailabilityandaffordabilityofenergypermits,ceilingfansaretechnicallysimple,canbeoperatedbynon-technicaloccupants,areinexpensiveandwithrelatively lowelectricalenergyuse(Aynsley2005,Voss,Vossetal.2013). Zhaietal.(2013) found that theaverageenergyconsumedby the fans formaintainingcomfortwaslowerthan10Wperperson,makingairmovementanenergy-efficientwaytodelivercomfortinwarmenvironments. Fans are further different fromACs, because the latter provide auniformthermalenvironmentinaspace,whichmaynotbeagreeabletoalloccupants,whilefans,especiallypersonalones,allowthecreationofdifferentmicroclimates(Zhai,Zhangetal.2013).

Airvelocityisusedtoinfluencethermalcomfortofoccupantsbyencouragingheatlossfromtheirbodiesthroughconvectionandevaporation(McIntyre1978,SchiavonandMelikov2008).ItisalsounderstoodfromtheguidanceofTM52thatceilingfanswhenoperatedundermoderatelycontrolledairspeed,enhancesthermalperceptionsofindooroccupants(CIBSETM522013).Accordingly,theguidancespecifiesthatanairvelocityofbetween0.3m/sand0.8m/s, raises the upper comfort temperature boundary (Tmax). This is reiterated by theASHRAEstandard55-2013whichstatesthatacontrolledincreaseinairspeedfrom0.2m/sto1.2m/sinanoccupiedarearaisestheupperacceptableoperativetemperature(ASHRAE2013).

AresearchconductedbyAynsley(2005)furthersuggeststhatanairspeedofabout1m/siscapableofoffsettinga3°Cincreaseinindoortemperature,anda3m/seffectsabout7°C . SimilarlyNicol&Humphreys (1973) in an analysis on thermal comfort conducted inNorthernIndiaandIraq,foundthatairmovementcanresultinthereductionoftemperaturebyasmuchas4°C,thiswasfurtherconfirmedbySharma&Ali(1986)whendevelopingatropical summer index with Indian subjects. These studies and similar others led theinternationalthermalcomfortstandardstoputforwardarelationshipbetweenthecomforttemperatureandtheincreaseinairvelocityasdemonstratedinFigure1(CEN2007).

Figure1:Airspeedrequiredtooffsetincreasedtemperature(CEN2007)

Ceilingfansarecommonfeaturesofinteriorspacesintropicalandsub-tropicalregions(Nicol2004,Candido,deDearetal.2010).Althoughtheusefulnessofceilingfansisnotindoubt,unlikeinhotclimaticregions,theyarenotcommonlyusedinthetemperateandthehigher latituderegions.Thiscouldbepartlybecauseheating requirementsare fargreaterthanthecoolingneeds.However,somebelievee.g.(DeDecker2014),thatceilingfan’susageandpopularitywereaffectedbythelimitof0.2m/sindoorairmovementrecommendedbyASHRAEstandard55andISO7730,whichwasperhapsintroducedtoavoiddraftsindoors.


Thislimitisthesamethewholeyearroundforbothwinterandsummerseasons.Whileinthewinter, airmovement indoors couldbe counter-productive, it isdesirable in the summer.Fortunately,thetwolastASHRAErevisions,whichbroughtinASHRAE55-2013,tookcareofthethresholdbyvaryingtheairspeedfrom0.2m/supto1.2m/s,andforhigheractivitylevelsover1.3metthereisnolimit(Nicol,Humphreysetal.2012).

Figure2:Photographsoflearningenvironmentsfurnishedwithceilingfans

ThispaperthereforeseekstofurtherinvestigatewhetherceilingfanscouldkeepthethermalcomfortofanindoorenvironmentatareasonablelevelandtoevaluatethelevelsofcontributiontheymakeinenhancingthethermalqualitiesoflearningenvironmentsinBayeroUniversity,Kano.Thisistobeachievedbyevaluatinglevelsofoverheatingintwoselectedlecture theatres, through physical measurements and survey data. Figure 2 showsphotographsofsomelearningenvironmentsfurnishedwithceilingfansintheUniversitytofacilitateindoorcomfort.

2. FieldworkThestudywascarriedoutinBayeroUniversity,Kano(BUK).Kano,issituatedonlatitude12°Nandlongitude8.17°E,intheSavannahregionofWestAfrica.ItisthesecondlargestandmostpopulouscityinNigeriaafterLagos.Maximumoutdoortemperaturereaches40°CinAprilandMayandgoesdownto12°CinDecemberandJanuary(Mohammed,Abdulhamidetal.2015).Itreceivesanaverageof3,117hoursofsunlightannuallyanditissunny71%ofdaylighthours. Relative humidity hovers between 15% and 70% and Kano receives its highestprecipitationofabout900mminAugust(InusaandAlibaba2017).Beingsituatedwithinlowlatitudescombinedwithhighsolarradiationandlowhumidity,KanoregionisclassifiedashavingahotanddryclimateaccordingtoKoppen’sclassification.ThereforeinKanocooling,minimizingheatgain,diversionofdirectsunlightandhumidificationarerequiredforindoorcomfort.

ThefieldworkwasundertakenfromAugust2016toMay2017,andwasconductedonthreedifferentoccasions;duringtherainyseasonofAugust,2016(warmandwet),theninJanuary,2017(winterseason)whenitwascoolanddryandfinally inMay,2017(summerseason)whenitwashotanddry.Theselectedlecturetheatresforthestudywerechosenfromtwoofthethreecampusesoftheuniversity:NewcampusandAminuKanoTeaching


hospital (AKTH), and respectively from the Faculties of Earth and Environmental Sciences(FEES)andClinicalSciences.Thereforeforbrevity,thenewcampustheatrewillbereferredto as “FEES” and the one at the Teaching Hospital as “AKTH”. The characteristics of thetheatresareshowninTable1.

2.1PhysicalMeasurementsDuringthefieldworks,boththephysicalmeasurementsandsurveyswereconductedbasedonprocedures consistentwithASHRAE standard55-2013.Anumberof instrumentswereusedtomeasurethethermalcomfortparameters.Airtemperatureandvelocityandrelativehumiditywerespotmeasuredandonlyairtemperatureandrelativehumiditywerelogged.HoboMX1102wereusedtologairtemperatureandrelativehumidity,150mmmattfinishedglobesfittedwithHobopendantscapturedtheradianttemperatureandTesto435-2meterwasusedforairvelocity.Thespotmeasurementswereconductedinfivelocationsineachtheatreat1.1mabovethefloor.Inthefloorplansofboththeatres,asshowninthetopofFigure3,measurementlocationsareshownincolouredletters.Fivelocationsinboththeatrescontaining the “TLCS” were the points of the measurements. The measurements wereconductedintwosituations,duringoccupiedandunoccupiedconditions.PhotographsoftheinteriorsofthelecturetheatresandexternalviewsarealsoshowninFigure3.

A B

CharacteristicsoftheLecture

Theatres

Table1:DesignCharacteristicsoftheLearningEnvironmentsCapacity(seats)

Volume(m3)

Floorarea(m2)

AverageHeight(m)

Window-wallorientation

No. ofCeilingfans

FloorSituation

Window –WallRatio

FEES 120 1,368 263 5.2 East/North/West

12 Tiered 30%

AKTH 120 1,829 381 4.8 North/South 14 Tiered 54%


Figure3:Floorplans,internalandexternalviewsofthelecturetheatres:A=AKTHandB=FEES

2.2SubjectiveMeasurementsPaper-based questionnaires were prepared containing seven sections covering; thermal,acousticandvisualcomfort,indoorairquality,clothingensembles,sketchesforoccupantstoindicate their locations and demographic information. As part of an extended PhD workinvolving an assessment of the indoor environmental quality (IEQ) parameters of variouslearning facilities, this study is reporting the thermal comfort aspect, which is directlyinfluencedbytheairmovement.Atotalof459questionnaires(123and336fortheAKTHandFEESrespectively)weresubsequentlydistributed,filledandcollectedback,forallthethreeoccasions.SevenpointLikerttypeASHRAEthermalsensationscaleswereusedtoassessboththethermalconditionsandtheairmovementinthespacesasshowninTables2,3,4and5.

Table2:Thermalcomfortacceptabilityscale1 2 3 4 5 6 7

VeryComfortable

Comfortable Slightly

comfortable

Okay Slightlyuncomfortable

Uncomfortable Veryuncomfortable

Table3:Thermalsensationscale-3

Unacceptable

-2

Unacceptable

-1

Acceptable

0

Acceptable

1

Acceptable

2

Unacceptable

3

Unacceptable

Cold Cool Slightlycool Neither Slightlywarm Warm Hot

Table4:Thermalpreferencescale-3 -2 -1 0 1 2 3

Wantingcold

Wantingcool Wantingslightlycool

Wantingnochange

Wantingslightlywarm

Wantingwarm Wantinghot

Table5:Airmovementacceptabilityscale-3 -2 -1 0 1 2 3

Toodraughty

Draughty Slightlydraughty

Okay Slightlystill Still Toostill

Both the physicalmeasurement and the survey resultswere used in evaluating thethermalconditionsofthetwotheatresbyfollowingthegroupingmethodsystemadoptedbyAl-Maiyah,MartinsonandElkhadi(2015).The7-pointscalewasconvertedintothree-pointscalebymergingtheresponsesinthefirsttwocategoriesintoone’comfortable’categoryandmerging the last two categories into ‘uncomfortable’ while the three central categoriesformedthe‘moderatelycomfortable’.SimilarlytherecommendationsofASHRAEStandard55(2013)andCEN15251(2007)werefollowed.Furthertothis,degreehour’sexceedance,anindicatorofoverheating,wasusedtodeterminethedeviationofthermalconditionsinthetheatresfromtheCEN15251adaptivecomfortthreshold.Thepredictedmeanvote(PMV)


model and adaptive approach using operative temperature were also employed in theanalysis.SimilarlythechartinFigure1,relatingtheairmovementandcomforttemperature,wasusedtodeterminethelikelycontributionoftheairvelocitytocomfortinthespaces.Thevaluesof themeasuredandderived thermal comfortparameters found in the spacesaredisplayedinTable6.Itisworthnotinghoweverthat,theairtemperature,relativehumidityandairvelocitymeasurements inthespaceswereforoccupiedsituations,theunoccupiedvaluesarenotverycriticalforthisstudy,becauseceilingfanswereseldomusedduringthewinterinKano,asshownbythelowairvelocities.

Table6:MeasuredandDerivedThermalComfortIndices

Parameters/Theatres FEES AKTH

Aug/Sept(warm&wet)

Jan/Feb(cool&dry)

Apr/May(hot&dry)

Aug/Sept(warm&wet)

Jan/Feb(cool&dry)

Apr/May(hot&dry)

Airtemp(˚C)

Standarddeviation

26.80

0.82

29.20

0.31

34.40

0.25

27.20

0.73

25.30

0.44

35.60

0.26

Airvelocity(m/s)

Standarddeviation

0.61

0.05

0.04

0.03

0.65

0.06

0.58

0.07

0.06

0.04

0.63

0.05

Externalairtemp(˚C)

Standarddeviation

30.40

1.28

26.40

0.74

34.80

1.72

27.90

1.95

25.40

0.61

36.50

1.76

Relativehumidity(%)

Standarddeviation

Clothinginsulation

69.60

2.48

0.66

16.00

1.03

0.72

41.90

2.29

0.65

60.20

1.06

0.65

18.30

1.87

0.71

36.40

2.52

0.60

Operativetemp(˚C)

Standarddeviation

26.97

0.39

29.51

2.13

33.36

1.87

27.30

0.51

25.60

1.75

35.00

1.76

Operat.temp(nofan)(˚C)

Standarddeviation

27.10

0.30

29.45

2.25

32.55

1.86

27.45

0.42

25.60

1.77

34.55

1.72

Runningmeantemp(˚C) 27.40 26.80 34.40 27.40 25.40 36.50

Predictedmeanvotes 0.24 1.35 1.56 0.36 0.38 2.02

Actualmeanvotes(AMV)

Standarddeviation

0.57

1.54

-0.78

1.30

1.34

1.02

0.46

0.85

-1.03

0.69

1.49

0.96

Neutraltemp(˚C) 27.80 26.70 30.20 27.80 27.20 30.80

Comforttemprange(˚C) 24.8-30.8 23.7-29.7 27.2-33.2 24.8–30.8 24.2-30.2 27.8-33.8

3. MeasuredresultsTheair,meanradiantandexternaltemperatures,airvelocitiesandrelativehumidityarethemainparametersmeasuredandreportedinTable6above.Thetablealsocontainsvaluesthatwere derived, including operative temperature (Top), running mean temperature (Trm),predicted mean vote (PMV), the adaptive neutral temperature (Tcomft) and comforttemperaturerange,similarlyfanmodifiedneutralandcomforttemperaturerangeareshown.Otherderivedvaluesfromthequestionnaires include:actualmeanvote(AMV)andactualpercentagedissatisfied(APD),whichareprocessedfromtheresultsoftheanswersobtainedfromthesurveyquestionnaires.


The operative temperature (Top) is an important parameter in assessing the likelythermal comfortof theoccupantsof abuilding, knownasdry resultant temperature,butrenamedasoperativetemperaturetoalignwithASHRAEandISOstandards.Itisasimplifiedmeasureofhumanthermalperceptionoftemperaturederivedfrommeanairtemperature,meanradianttemperatureandairspeed(seeEquation1).Wheretheairspeedislessthan0.1m/s, the radiative and convective heat transfers may be similar, so Top becomes theaverage of the air and mean radiant temperatures (Nicol, Humphreys et al. 2012). ThecalculatedTopwithandwithouttheinfluenceoffansarealsoshowninTable6above.

𝑇𝑜𝑝 = 𝑇𝑚𝑟 + ()*×√-./)-0√-./)

Equation1

WhereTaristheairtemperature,TmristhemeanradianttemperatureandVaistheairspeed(m/s).

TheoperativetemperaturevalueswereobtainedbyprocessingthevaluesofairandmeanradianttemperaturesinEquation1aboveandwereusedtodeterminetheadaptivethermalcomforttemperaturerangesandneutraltemperature.PMVwascalculatedusingtheCentre for the Built Environment (CBE) thermal comfort tool for ASHRAE, the clothinginsulation(clo)valueswereobtainedfromthequestionnaireswhilethemetabolicrate(met)of1.2metforseatingandlisteningwasused(Tyler2013).AMVisthemeanofthethermalsensationvotesofallparticipantsofasurveyinarealworldsettingasopposedtoPMV,whichislaboratorybased.Asmentionedearlier,thisstudycombinedthethreecentralcategories(-1,0&+1)ofthethermalsensationscaleandassessedthemasacceptable,whiletheAPDwascalculatedfromtheshareofthetwoextremecategories(-3&-2)and(+2&+3) fromthethermalsensationvotes.

A-AirTemperature B-RelativeHumidity C-AirVelocities

Figure4:SeasonalAirTemperatures,RelativeHumidityandAirVelocitiesintheTheatres

Table6andFigure4revealthattheAprilairtemperaturevalueswerethehighestinboththeatres,aswasexpected, itwasthehottestperiodoftheyearinKano,withtheairtemperaturereachingashighas35.6˚CandwasrecordedinAKTH.Itisunderstoodfromthetable that the internal air temperatureswere following the external temperatures in thespacesduringmid-seasonandsummer,butthatwasnotthecaseforFEESduringthewinter.TheairvelocityvaluesrecordedwerebothhighestandlowestinFEES,andwereexpectedlyhigherinAprilandlowestinJanuary,whenfanswerenotoperated,theystoodat0.65m/s(SD=0.06)and0.04m/s(SD=0.03)respectively.Thedesigncapacitiesofthetwotheatresareequal:thatis120seats,buttheoccupancylevelsduringthesurveysweredifferent.AKTH


wasoccupiedbyaboutonethirdofitsdesigncapacityacrossthethreesurveys,whileFEESwasfulltoitsdesigncapacityonallthethreeoccasions.

4. SurveyresultsThesurveyswereundertakenacrossinthethreeseasons,femalesaccountedfor22%ofthestudents-dominated respondents and 75% of them were 26 years and above. From theclothingensemblessectionofthequestionnaire,clovalueswerefoundtodifferacrosstheseasons.Thehighestmeanvalueof0.72clo(SD=0.13)wasrecordedinJanuaryandtheleastof 0.60 clo (SD = 0.11) was recorded in April. Whereas metabolic rate for lecturing andlisteningwasfixedat1.2met.

ItisduringthesummerthattheeffectofhightemperatureismoreproblematicintheKanoregion,thereforetheanalysisofthepossibleoverheatingusingthesubjectivevoteswasrestrictedtothesummerresultsonly.Itisalsonoteworthythatduringthisseasonceilingfanswereoperatedpracticallyineverynaturallyventilatedbuildingintheregion,thereforethethermalacceptabilitylevelsinbothspaceswerecalculatedbasedonthisfact.ThelevelsofthermalacceptabilityshownbytherespondentsinAKTHandFEESwererespectively75%and81%.IndoorclimatesofthelearningenvironmentsduringthesurveywereonaveragefourdegreeswarmerthantheASHRAEcomfortstandardprescriptionsbutcausedlessthermaldiscomfortthanexpected.Howeverdespitethehighlevelsofacceptance,56%and37%oftherespondentsreportedthatthetheatreswererespectively“hot”.Onthequestionoftheirpreferences,30%inAKTHand53%inFEESpreferredcoolerenvironments,andsurprisinglyupto5%oftheminAKTHwantedtobewarmed.InAKTHupto96%oftherespondentswerehappywith theair speedof0.63m/swhile80%showed theiracceptanceof0.65m/sairspeed in FEES. Figure 5 shows the thermal acceptability, sensation, preference andacceptabilityofairmovementofthestudentsinboththeatresduringtheseason.

A-Thermalacceptability

B–Thermalsensation

4%

5%

20%

13%

29%

17%

2%

17%

2%

17%

18%

13%

13%

7%

11%

12%

AKTH

FEES

Comfortable Moderatelycomfortable Uncomfortable

0%

0%

0%

0%2%

2%

7%

8%

7%

8%

28%

46%

44%

19%

12%

18%

AKTH

FEES

Comfortable Moderatelycomfortable Uncomfortable


C–Thermalpreference

D–AirmovementFigure5:ThermalAcceptability,SensationandPreferenceandairmovementduringtheSummer

5. OverheatingAnalysisA space is said to be overheated during the occupied hours when the operativetemperature exceeds a threshold comfort temperature. Similarly the severity of theoverheatinginanygivendayisafunctionofitsdurationandariseintemperatureabovethethreshold(CIBSETM522013).TM52(2013)offersapassmarktoanyindoorspacethatmeetsanytwoofthefollowingthreecriteria:

• Thresholdtemperatureshouldnotbeexceededbymorethan3%ofoccupiedhoursperyear;

• Dailyweightedexceedanceshallbelessthanorequaltosixdegreehours;and• Operativetemperaturenotexceedingthethresholdupperlimit(Tupp).

A-AKTHtemperaturetimeseries

9%

36%

22%

17%

36%

27%

14%

7%

14%

7%

2%

3%

4%

3%

0%

0%

AKTH

FEES

Wantcooler Wantnochange Wantwarmer

7%

3%

20%

7%

27%

24%

16%

17%

16%

18%

11%

11%

0%

12%

4%

8%

AKTH

FEES

Acceptable Moderatelyacceptable Unacceptable


B-FEEStemperaturetimeseriesFigure6:Temperaturetimeseriesofthetheatres

ThechartsinFigure6showthetemperaturetimeseriesofAKTHandFEESrespectively,fortheentireperiodofthefieldworks,boundedbyupperandlowertemperatures(TmaxandTmin). Various other values of temperatures were displayed in the charts, external (Tex),internal (Tar-in),mean radiant (Tmr), runningmeanoutdoor (Trm) and fanassistedmodifiedupper(Tmax-fan).Theupperlimittemperature(Tmax)asdefinedbytheinternationalcomfortstandardswasfoundtoberaisedasaresultoftheactionoftheceilingfansinthespacesby2°C(Tmax-fan).Usingtherunningmeantemperature(Trm)asanindicator,itcanbeseenfromthechartsthat,forthemajorityoftheperiodtheTrmwaswithintheoriginalcomfortzone,inlinewiththeadaptivethermalcomfortapproach(ATC)for80%acceptability(seeequations2and3)(CEN2007).However,inbothspaces,theTrmcrossedtheTmaxinFEEStheatrefromMarch,26onwardsandfromApril02,inAKTH.However,duetothefans’actionthetheatresbecameacceptable,ascanbeseenfromthechartsthattheTrmdidnotcrossthenewlimit(Tmax-fan).

Tmin=0.33Trm+15.8……………………………………………………..(2)

Tmax=0.33Trm+21.8……………………………………………………(3)

WhereTminandTmaxarethelowerandupperrangesofallowabletemperaturesfor80%acceptabilitylimitsandTrmistheexponentiallyweightedrunningmeanoutdoortemperature(CEN2007).

Thecharts in figure7showsthepercentagesofexceedances (x-axis)andnumberofdegreedeviationawayfromtheneutraltemperature(y=0)inAKTHandFEESrespectivelyfortheoccupiedperiodofthesurveys.Thechartsindicatethepercentagesoftime,inthetheatreswhentheTmaxwascrossedfortheentireperiod.Thedottedandyellowlinesinthecharts(Tmax-fanandTupp)denotetheactionofceilingfansinthetheatresasaresultofwhichoverheatingwasreducedby31%and22%inAKTHandFEESandreducesdiscomfortto15%and10%ofthetimerespectively.ThechartsinFigure8however,showthepercentageofdegreeday’sexceedancestheinternaltemperaturesledtooverheating,butthefans’actionsreducedthediscomfort to lessthan5% inbothspaces.Thisconfirmsthat introducingtheceilingfanscanimprovethethermalqualitiesofnaturallyventilatedindoorspaceseveninsub-SaharanAfricaasopinedbyNicol(2004).


A-AKTHTheatre

B-FEESTheatre

Figure7:Percentageofexceedancesinthetheatres

A-AKTH

54%0% 85%

-8

-3

2

7

12

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Degree

deviatio

nfrom

neu

tral

Percentageofexceedances

Tmax Tmin Tmax-fan Tupp

8.8% 68.4% 90.4%100.0%

-8

-3

2

7

12

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Degree

deviatio

nfrom

neu

tral

Percentageofexceedances

Tmin Tmax Tmax-fan Tupp

0

5

10

15

20

25

30

35

40

45

60% 65% 70% 75% 80% 85% 90% 95% 100%

Degree

hou

rsexcee

dances

Percentageofdegreedaysexceedances


B-FEES

Figure8:Percentageofdegreeday’sexceedancesinthetheatres

6. DiscussionThe ASHRAE Standard 55-2013, which sums up the recommendations of the majorinternationalcomfortstandards,specifiesthevaluesofairvelocityrequiredtocompensateforelevatedtemperatures.Thevalues,rangingfrom0.2m/supto1.2m/s,aresaidtooffsetelevatedtemperaturesabovesummercomfortthresholdunderoccupantcontroluptoalimitof30°C.Theresultsandsubsequentanalysisfromthisstudyindicatethattheoverallthermalsensationinboththeatreswaswarmduringthesummer.TheseconditionswereindicatedbyPMVmodel, following theprovisionsof the thermal comfort standards suchas (ISO77302005).SimilarlytheoverheatinganalysisfromFigures6,7and8alsoconfirmedthatthetwospaceswereoverheatedduringtheseason,howeverthecoolingeffectbroughtaboutbytheaction of the ceiling fans made them acceptable to vast majority of the occupants. Theincrementof2°CincomforttemperatureasaresultoftheelevatedairspeedwasobtainedusingtheASHRAE55orISO7730orCEN15251chartsshowninFigure1.Itistobenotedthatthehighestairvelocitymeasuredduringthesurveysinthisstudywas0.65m/s,whichoffset2°C,itthereforemeansthatonlyabout1.3°Ccouldfurtherbeoffsetshouldtheairvelocityreachtheallowable1.2m/susingthesamechart.

The study founddifferences inmagnitude in the resultsofAMVwith thoseofPMVduringthesurveys.Thisisshownbycorrelatingthedifferencesinthermalmeanvotes(PMVminusAMV)againsttheairvelocity,theregressionlinedepictsastrongnegativerelationship,meaningthatwithanincreaseinairvelocitythedifferencebetweenthetwoindicesreduces(seefigure9).Thisisinagreementwithstudiesconductedinsimilarclimaticregionsoftheworld(Brager,Paliagaetal.2004,Nicol2004,Candido,deDearetal.2010,Zhai,Zhangetal.2013). The resultsof theAMVduring the surveysweredifferent to thoseofPMVmodel,though thedifferenceswerenot so large, it still shows that PMV/PPDmodel predicted awarmerperceptionthanwasfoundinactualityduringbothsummerandwinter,thisisalsoin agreement with especially the adaptive thermal comfort studies around the world(HumphreysandNicol2002,BurattiandRicciardi2009,Nicol,Humphreysetal.2012).

0

5

10

15

20

25

30

35

40

45

60% 65% 70% 75% 80% 85% 90% 95% 100%

Degree

hou

rsexcee

dances

Percentageofdegreedaysexceedances


Figure9:Airvelocityversusthermalmeanvotes(PMV–AMV)

ThesummerPMVmodel results for thespaces (+1.56and+2.02 forFEESandAKTHrespectively)clearlyshowthatthespaceswereuncomfortablywarm,whiletheAMVresultsshowtheywereslightlywarm.Ontheotherhand,therequirementsoftheadaptivethermalcomfort approach of CEN 15251 (2007) for buildings type II stated that the operativetemperatures(Top)inindoorspacesshouldliewithintheupperandlowerboundaries(TmaxandTmin)ofthecalculatedcomfortrangetemperatures.FromthesameTable6above,itcanbeseenthatthecalculatedTopinthespacesacrossalltheseasons,withexceptionofAKTHduring the summer, fell within the said boundaries. However, when the Tmax-fan wasintroducedasaresultoffanaction,thethermalconditionsinAKTHalsobecomeacceptable.Thelowestboundaryofthecomfortrangeduringthewinterwas23.7˚C,whiletheupperboundary during the summer was 33.8 ˚C. This adequately contained the highest pointreachedbytheTopandthereforesignifiesthatEN15251couldthereforebeusedinpredictingthermalconditionsinKanoregion.

Nevertheless, it seems that theprovisionsmadeby international comfort standardsweredonewithlessconsiderationofthesub-SaharanAfricainmind.Forexample,usingthesestandards’recommendations,thedatapresentedinTable6andFigures6,7and8indicatethat the spaces were overheated during the summer, and although the ceiling fans hadgreatlyenhancedtheirthermalqualitiesandbecameacceptabletomostoftheoccupants,thespacesstilldidnotsatisfyallthethreeoverheatingcriteriarecommendedbyCIBSETM52.This could be explained by the fact that this comfort standard considered only the UKsituations when compiling the thresholds. Similarly, one of the acceptability conditionsimposedbyASHRAE55onprevailingmeanoutdoortemperaturelimitisarangeofbetween10°Cand33.5°C,andinthisstudyallthemeansummertemperaturesrecordedwerefoundtobeabovethislimit.

7. ConclusionThe study investigated the possibility of overheating in two lecture theatres in BayeroUniversity,Kano,andhowceilingfansraisedthelevelsoftheirthermalacceptability.Variousphysical parametersweremeasuredwhich culminated in calculating comfort indices andconcurrently the occupantwere subjected to a survey to determine their actual comfortperceptions.ThephysicalmeasurementsandsurveyswereconductedfromAugust2016toMay 2017 and comparisons were made between the experimental and surveyed dataobtainedfromthetheatresaswellasagainstthresholdsofrelevant internationalcomfortstandards.Inlinewiththeresultsobtainedbypreviousthermalcomfortstudies,thisstudy


alsofounddiscrepanciesbetweenthemeasuredindicesandtheperceivedresults,aswellaswith the comfort standards’ thresholds. The PMV/PPDmodel overestimated the thermalperceptions of the respondents in both summer andwinter. This divergencemay not beunconnectedwith the situations of the dominant climatic conditions of the region understudy,whichwerefoundtobeoutsidetheacceptabilitylimitsofthecomfortstandards.Thetheatreswerefoundtobehotbasedontheresultsofthethermalindicesrecommendedbythestandards,howevertheuseofceilingfans(thoughoperatedat0.65m/sandbelow)wasfound tobeveryproductive, it raised theTmaxby2 °Cand therebyenhanced the thermalconditionsofthetheatres.Itishoweverbelievedthathigherairvelocitythanwhatthisstudyobtained can further enhance the thermal qualities of buildings in hot and dry tropicalregions,andceilingfanscanbeusedtoachievethat.

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Effects of ceiling fans on the thermal comfort of students in … · 2019. 9. 16. · Effects of ceiling fans on the thermal comfort of students in learning environments of Bayero