Effect of a superheating and sub-cooling heat exchanger to t(2)

Xietal.[19]presentedexperimentalstudiesonasolar-assistedgroundcoupledheatpumpsystemforspaceheating.Fouropera-tionmodesofthesystemwereinvestigatedthroughoutthecoldestperiodinwinter.Theheatpumpperformance,theboreholetemperaturedistributionsandthesolarcolletingcharacteristicsofthesystemwereanalyzed.Jeonetal.[20]analyzedtheperformanceofahybridcoolingsystemthatcombinesascrewwaterchillerwithagroundsourceheatpumpatvariouscoolingloads.

TheSHCHEisusedtosuperheatandsub-cooltherefrigerantinoutletofthecondenserandevaporatorofthevaporcompressionheatpumpsystem,respectively.Thesuperheatingandsub-coolingproceduresareappliedforimprovingthesystemef ciency.Intheliterature,availablestudiesonsub-coolingandsuperheatingeffectsofvaporcompressionrefrigerationcyclesareverylimited.Selbasetal.[21]obtainedoptimumheatexchangerareasandoptimumsub-coolingandsuperheatingtemperaturesundervariousoper-atingconditionsofvaporcompressionrefrigerationsystem.Theapplicationwasconsistedofdeterminingtheoptimumheatexchangerareaswiththecorrespondingoptimumsub-coolingandsuperheatingtemperatures.

Variousstudieshavebeencarriedoutbyresearchersinorderto

analyzetheperformanceoftheGSHPsystemsaroundtheworld.Theyperformedtheoreticalandexperimentalstudiesonthegroundsourceheatpumpsystemswithagroundheatexchangerandconcludedthatusingheatpumpsystemsarefeasible.

ThisstudyincludestheperformanceevaluationofthegroundsourceheatpumpsystemwithandwithoutSHCHEfortheclimaticconditionofErzurumhavingcoldclimateinTurkey.Anexperimentalset-up,describedinthenextsection,isconstructedandtestedonthebasisofauniversitystudy.Thecoef cientofperformanceoftheheatpumpandtheoverallsystemiscomputedfromthemeasurementsand,theenergyconsumptionofthegroundsourceheatpumpsysteminheatingseasoniscalculated.

2.Experimentalprocedureandmeasurement

ThegroundsourceheatpumpsystempresentedhereisatErzurum,Turkey,latitude:39.55 N;longitude:41.16 E;placedontheEastAnatolianRegionofTurkey.Table1givestheclimaticdataforErzurum.AschematicoverviewofthesystemisillustratedinFig.1.

Thecompressor(4)usedfortheheatpumpsystemisahermeticscrolltype,whichisdrivenbya2610Wattelectricalmotor.Theheatpumphasanevaporator(12)andcondenser(5);bothofwhicharewatercooledplatetypeheatexchanger.Theseequipmentshavebeeninsulatedentirelywith25mmthickrubberfoamagainstheatloss.

AsshowninFig.1,thesystemconsistsofaverticalgroundheatexchanger(3)locatedtothedepthof2Â53m,aheatpumpwithwater-to-refrigerantheatexchanger,watercirculatingpumps(1)andotherconventionalequipments.Inthesystem,theheattransfer uid(antifreeze-watermixtureof50%)whichcomesfromtheundergroundgoestothewater-sourceevaporatoroftheheatpumpwhereitreleasessomeenergy,andthen,itissenttothegroundheatexchangersbyawatercirculatingpump.However,duringtheday,theheattransferring uidthatcomesfromtheevaporatoroftheheatpumpissenttotheground.Refrigerant134awasusedasworking uid.Theverticalgroundheatexchanger(GHE)unitisasingleU-tubeplacedintwoverticalboreholesatthedepthof53m.ThepipesconnectingtheGHEtotheevaporatorwereinsulatedandburiedatthedepthof2mtominimizetheheatloss.

Inthepresentstudy,thetemperatures, owrates,pressuredrops,voltagesandcurrentsweremeasuredbyappropriateinstrumentsgiveninTable2.Thetestswereconductedon

the

Table1

ClimaticconditionofErzurumforlong-termaveragevalues.Climaticvalues

MonthsinheatingseasonNov.

Dec.Jan.Feb.Mar.Apr.AverageoutdoorÀ0.5À7.2À10.8À10.1À3.75.2temperature( C)MinimumoutdoorÀ6.8À12.6À16.9À16.7À9.8À0.9temperature( C)Maximumoutdoor6.9À1.6À4.4À3.12.611.8temperature( C)Averagerelative82.081.377.573.175.056.7humidity(%)Averagewind2.22.22.32.42.83.3velocity(m/s)Averagesunshine4.32.33.03.84.45.9duration(h)Averagesolar

8.8

7.0

8.9

12.6

16.0

17.0

radiation(MJ/m2.day)

998K.Bakirci,D.Colak/Energy44(2012)996e1004

7.Receiver8.Dryer

9. Sight glass 10. Solenoid valve 11. Expansion valve

18.Electroniccounter19.Controlpanel

20. NTC temperature indicator 21. Data acquisition card22. Computer and monitor

a

bc

Fig.1.a)Aschematicrepresentationofthesystem(withoutSHCHE)b)AschematicrepresentationoftheheatpumpwithSHCHEandc)Outsideviewsoftheheatpump.

groundsourceheatpumpsystemintheheatingperiodof2010.Themeasurementsweretakenfrom8.00a.m.to18.00p.m.withanintervalof30min.Thetemperaturesmeasuredbythethermo-couplesweremonitoredinacomputerandrecordedbyadata-acquisitioncardineachsecond,whichwaslaterusedforanalysis.

Theelectroniccounter(digitalpowermeter)wasusedtomeasurethepowerconsumptionofthecompressorand,thepowerconsumptionsofthepumpsweremeasuredbyawatt-meter.Allthemeasuringprocessesofthetemperaturesweremonitoredandcontrolledbyapersonalcomputer-baseddata-acquisitionsystem.

2.1.Uncertaintyanalysis

Experimentalerrorsanduncertaintiescanresultfrominstru-mentselection,instrumentcondition,instrumentcalibration,environment,observationandreadingandtestplanning.Theuncertaintyanalysisisneededtoprovetheaccuracyoftheexper-iments[22].AnuncertaintyanalysiswasperformedusingamethoddescribedbyHolman[23].

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