Nishati AISPCA, Operation Manual

AISPCA!Operations!Manual!

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7!

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Solar'Energy'Basics'

Photovoltaic'(PV)'Systems'

PV!systems!are!designed!to!harvest!energy!directly!from!sunlight.!They!include!solar!panels,!batteries,!
power!control!electronics,!and!accessories!that!support!power!needs!for!many!applications.!

Solar'Insolation'Considerations'

Solar!irradiance!is!a!measure!of!the!power!per!unit!area!on!the!Earth’s!surface!produced!by!the!sun!
often!measured!in!Watts!per!square!meter!(W/m

2

).!

Although!the!energy!output!of!the!Sun!is!fairly!

constant,!solar!irradiance!varies!significantly!from!one!place!to!another!and!changes!throughout!the!
year.

!The!solar!radiance!is!an!instantaneous!power!density!in!units!of!kW/m

2

.!The!solar!radiance!varies!

throughout!the!day!from!0!W/m

2

!at!night!to!a!maximum!of!about!1000!W/m

2

.

Direct!solar!insolation!is!the!solar!radiation!energy!per!unit!area!during!a!given!time!often!measured!in!
Kilowatts!hour!per!square!meter!per!day![kWh/(m

2

Tday)].

The!amount!of!insolation!received!at!the!

surface!of!the!Earth!is!controlled!by!the!angle!of!the!sun,!the!state!of!the!atmosphere,!altitude,!
geographic!location,!and!time!of!year.

PV!cells!harvest!solar!rays!and!produce!direct!current!(DC).!PV!cells!are!combined!together!to!create!
modules!to!increase!the!total!power!output.!These!modules!are!most!productive!when!the!panels!face!
directly!into!the!sun!(i.e.!are!as!close!to!perpendicular!to!the!sun’s!rays!as!practical).!Two!factors!affect!
the!angle!at!which!the!sun’s!rays!strike!a!solar!panel:!

•

!

Direction!/!azimuth!

•

!

Tilt!angle!

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Diffuse'and'Direct'Solar'Radiation'

Solar!radiation!that!reaches!the!earth!is!composed!of!direct!and!diffuse!solar!radiation.!Direct!solar!
radiation!travels!on!a!straight!line!from!the!sun!down!to!the!surface!of!the!earth.!Diffuse!radiation!
describes!the!sunlight!that!has!been!scattered!by!molecules!and!particles!in!the!atmosphere!but!that!
has!still!made!it!down!to!the!surface!of!the!earth.!When!the!sky!is!clear!and!the!sun!is!very!high!in!the!
sky,!direct!radiation!is!around!85%!of!the!total!insolation!striking!the!ground!and!diffuse!radiation!is!
about!15%.!As!the!sun!goes!lower!in!the!sky,!the!percent!of!diffuse!radiation!keeps!going!up!until!it!
reaches!40%!when!the!sun!is!10°!above!the!horizon.!However,!this!ratio!can!vary!due!to!many!factors!
including:!

•

!

Air!molecules!

•

!Water!vapor!!!!

•

!Clouds!

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•

!Dust!

•

!

Pollutants! !

•

!Forest!fires! !

•

!Volcanoes! !

•!Latitude!

The!sum!of!the!diffuse!and!direct!solar!radiation!is!called!global!solar!radiation.!Atmospheric!conditions!
can!reduce!direct!radiation!by!10%!on!clear,!dry!days!and!by!100%!during!thick,!cloudy!days.!