搜索
Elementsofelectricalpowerstationdesign=电站设计基础/M.V.Deshpande.─WheelerPublishing,1981Wherewaterresourcesareavailable,hydro-electriepowerstationareusedtosupplyelectricalenergytoconsumers.Thistypeofstation，however,cannotbelocatedeverywhere.First,theremustbeanamplequantityofwateratsufficienthard,andsuitablesitemustbeavailable.Streamflowdateforanumberofyearsshouldbestudied.Thepossibilityofconstructingadamatasuitablesitetostorewaterinthecatchmentareaandtheavailabilityofwaterthroughouttheyeararedecisivefactors.Theamountofpowerthatcanbedevelopeddependsonthequantityofwateravailable,therateatwhichitisavailable,thehead,etc.Hydro-electricprojectsinvolvealargeamountofcivilengineeringconstructionwork.Oftentheprojectisamultipurposeoneembracingirrigationandpower,floodcontrolandpower,orfloodcontrol,navigationandpower.Onemightimaginethathydro-electricpowershouldbeverycheapaswaterdoesnotcostanythingandthereisnofuelcost,This,however,isnottrue.Tostorewateratenoughhead,itisnecessarytohaveadamandcivilengineeringconstructionworks.Thiscostsmoney,andincreasesinvestmentcostsandfixedcostswhicharenotrequiredforothertypesofpowerplants,Wheneverahydro-electricprojectisconsideredforpowerpurposes,itisalwaysnecessarytoworkouttheeconomicscomparinghydro-electricpowercostswiththecostsforotherformsofstation.7.2HydrologyWhenconsideringthepossibilityofahydro-electricproject,thefirstrequirementistoobtaindataregardingthestreamflowofwaterthatwouldbeavailable,andtopredicttheyearlypossibleflowfromthedata,Thewatercycleingeneralconsistsofevaporationfromseasoroceansand/orotherwatersurfacesontheearthduetoheatfromthesun,theformationofmoistairandclouds,thecirculationinformofrain,hailorsnow.Determinationoftheamountofstreamflowanditsvariationinvolvesastudyofhydrography(orhydrology)whichdealswiththeoccurrenceandundertheearth'ssurface.Thescienceofhydrologyisbasedonmeteorology,geology,agriculturalphysics,chemistry,botanyanddataobtainedbyobservationandmeasurement.Itisnecessarytocalculatethemeanannualrainfallintheareaunderconsiderationfromarecordoftheannualrainfallforanumberofyears,say25to30,andalsotonotethefrequencyofdryyears.Theprecipitationorrainfallinaparticularareacanbemeasuredbyraingaugesatvariousplacesinthearea.Araingaugeconsistsofacollectingcylinderwhichexposesacirculargaugeconsistsofacollectingcylinderwhichexposesacircularsurfaceforcollectingtherainandastoragevesselinwhichthewaterisretaineduntilmeasured.Recordingraingauges,givingacontinuousrecord,areusewherepossible.Frequencyofprecipitationisalsocarefullynoted.Acurveofprecipitationinmillimetresplottedagainstpercentageoftimegivesarangeofableforanumberofyears,therangeofdistributionofmeanmonthlyprecipitationisobtainedthegeneralcharacteroftheperiodicdistributionoftherainfalltendstobesimilareachyearineachlocality.Thereissomevariationintheoccurrenceofrainfallfromyeartoyear,butwhentherecordofanumberofseasonsisavailable,itisusuallyfoundthattheoccurrencesofrainfallforagivenmonthinagivenlocalityaresimilar.Thefrequencyofoccurrenceofadryyearortwoconsecutivedryyearsisalsoseenwhenrecordsforalongperiodareavailable.Precipitationinaparticularlocalitydependstoacertainextentonaltitude,Thegeneralandfundamentaltendencyisfortheprecipitationtoincreaseasthealtitudeincreasesonthecoastalsideofamountainrange,ifthelocalityisnotfarfromthegeneralsourcesofwatervapour.Dateregardingmaximumrainfallinaparticularareaandthenatureofthedrainageareahelpinthestudyofpossiblefloodsinthearea.Knowledgeofthefrequencyofhighrainfallintheareisalsohelpfulintheabovestudies.Themeanprecipitationonadrainageareaoveraperiodoftimecanbeobtainedbyplottingisohyets,orlinesofequalprecipitation,fromtherecordsavailableandthenfindingaweightedaveragebyplanimeter.Ofthetotalprecipitation,somepartofthewaterislost.Sourcesoflossareevaporationfromthewaterare,soilevaporation,interceptionandtranspirationlossesfromlandarea.Thestreamflowisknownwhenthewaterlosseshavebeensubtractedfromtheprecipitation.Streamflowconsistsofsurfaceflowandpercolationthroughtheground.Thevariousfactorswhichaffectthedistributionofprecipitationaremeteorologicalconditions,drainageareacharacteristics,topographyofland,geologyoflandandvegetationinthearea.Evaporationiseffectedbyheatfromthesun.Temperatureandrelativehumidityaffectevaporation,andsotoodoeswindvelocity.Plantsabsorbmoisturefromtheground,givingitofftotheatmosphere,theprocessbeingknownastranspiration.Duringtheprocess,somewaterislost,thisisknownastranspirationloss.Draingeareacharacteristicsarethesizeandshapeoftheare.Asmallareatendstohaveahigherunitflooddischargethanalargearea.Thetopographyoflandandsurfaceslopesaffectsthesurfaceflowofwater.Steepslopestendtowardsarapidandlargesurfaceflow.Thesizeofthewaterareaaffectsthelosses.Alargepercentageofwaterareatendstoadiminishedrun-off.Thegeologicalcharacteristicsofthewatershedaffectrun-off.Evaporationvariesinverselywiththeporosityofsoil.Percolationandwaterlossesdependonwhetnerthesoilisperviousorimpervious.Alargesurfacerun-offisexperiencedwhenperviousgroundissaturatedwithrainwaterorwhenthegroundisofimperviousmateria