solar radiation measurement

Solar radiation measuring instruments

1. Clarityindexisdefinedastheratioofradiation
receivedonearthshorizontalsurfaceoveragiven
periodtoradiationonequalsurfaceareabeyond
theearthsatmosphereindirectionperpendicularto
thebeam.Itdependsupontheclarityof
atmosphereforthepassageofsolarbeam
radiation.Clarityindexcanbebetween0.1to0.7.
Concentration ratio (CR) is the ratio f solar power
per unit area of the concentrator surface to solar
power unit area on the line or point focus. For flat
plate collectors (CR=1). CR up to 100 can be
achieved by using parabolic trough collectors with
sun tracking in one plane.

2. Solarinsolation(Incidentsolarradiation)isdefined
asthesolarradiationreceivedonaflathorizontal
surfaceonearthataparticularinstantoftime.It
dependsonthefollwingparameters.
Dailyvariation
Seasonalvariation
Atmosphericclarity
Shadowsoftrees,tallstructures,adjacentsolar
panelsetc
Degreeoflatitudeofthelocation
Areaofexposedsurface,m^2
Angleoftiltofsolarpanel.

3. Generallysolarradiationismeasuredona
horizontalsurfacebutforsolarenergyapplications
andforthermalenvironmentaldesigns,the
knowledgeofradiationoninclinedsurfaceis
required.
Solarenergyreceivedonagivensurfacedependon
theorientationofthesurfacewithreferencetothe
sun.
Afullysuntrackingsurfacethatalwaysfacethe
sunreceivesthemaximum possiblesolarata
particularlocation.

4. Pyrheliometeritmeasuresonlythedirectsolar
radiationfluxatnormalincidence.thisinstrument
isusuallyattachedtoanelectricallydriveneqtorial
mountwhichtracksthesun.
PyranometeritmeasuresbothDirectanddiffuse
solarradiationarrivingfromthewholehemisphere.
PyrheliometerandPyranometerarebasicallya
radiometerwhichabsorbsthesolarradiationatits
sensor,transformsintoheatandmeasurethe
resultingamountofheattoascertainthelevelof
solarradiation.

35. Information contained in solar radiation data
•Whether it is measured or computed
•Whether it is direct, diffuse or global
•Whether it is daily, hourly or monthly
•What is the receiving surface orientation? Whether it is incident
on a horizontal or inclined surface.
•What is the azimuth of the surface?
•What is the time or time period of measurement?
•Whether it is averaged? If so, what is the time period over which
it is averaged?
•What type of instrument is used for measurement?

36. Solarradiationdataiscollectedforvariouslocations
intheworldonthebasisoffollowingfactors.
oSolarpowercalculationswithreferencetothe
movementofthesun,latitudeofthelocationetc.
oHourlymeasurementsofsolarradiationatthe
locationandcalculationofaverageglobalradiation
forthemonth,yearandfewyears.

37. Typical meteorological year (TMY) data sets
Itismadeupfromhistoricalweatherobservationsfora
setof12typicalmonthsatspecificlocations.these
datasetsaregivenintheformofchartsandtables.
Solarradiationatlasmap
Asolarradiationatlasmaphasbeendevelopedby
differentagencyandgovt.organizations.thismaps
areconstantlybeingupdated.solaratlasmaps
provideagraphicalviewofregionalaveragesolar
radiationlevelsandtheyareaquicksourcefor
findingmonthlyoryearlysolarradiationlevels.