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Solar energy fundamentals

Dr.S.SURESH

NIT

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Renewable Energy Sources

Alternate Energy Sources

or

Sustainable Energy Sources

orNew Energy

or

Non conventional Energy Sources

or

Nontraditional Energy Sources

OrClean energy

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1.Why this new energy sources?

2.What are the Types of renewable energy sources?

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The Energy Emergency!

Seem to be running out of the cheap sources of

energy.

No solution is immediately apparent.

The date of the oil peak is widely debated in

geological circles around the world.

Need to refocus our attention to the more practical

question of What next? rather than debatingWhen?

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Why Energy and Infrastructure?

Energy and Infrastructure are essential for economic development

These are man made long-life assets .

Investment is crucial to support a higher level of industrial growth.

Economic growth in India demands energy and development of its

infrastructure .

Huge investments are planned for these sectors.

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STRUCTURE OF THE SUN

T= 5,777 k

Energy production: By Fusion : H atoms combine to form Helium

Effective Black Body Temperature (as seen from earth) = 5,777 K

Diameter 1.39 x10^9 m ; Average distance between the sun and the earth =

1.15 x 10^11 m Dr.S.Suresh, NITT. 27-09-09

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Solar Radiation

The sun is a gaseous body composed mostly ofhydrogen .

Gravity causes intense pressure and heat at the coreinitiating nuclear fusing reactions

This means that atoms of lighter elements are combined

into atoms of heavier elements, which releases enormous

quantities of energy

Even when planet Earth is 93 million miles away, we still

received an amazing quantity of usable energy from the sun.

Considering 25% efficient PV modules, if we used 1% of

the surface of the earth we could meet 29 times our current

total energy demand.

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Solar Radiation

Dr.S.Suresh, NITT. 27-09-09

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Solar Radiation

Solar Spectrum most the energyreceived from the sun is

electromagnetic radiation in the form of

waves.

Electromagnetic Spectrum is the

range of all types of electromagnetic

radiation, based on wavelength.

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The electromagnetic spectrum is the range of all types of

electromagnetic radiation, which vary with wavelength.

Almost all of the energy received from the sun is electromagnetic

radiation.

Electromagnetic radiation is radiation in the form of waves withelectric and magnetic properties.

The waves vary in length depending on the source and energy level.

The wavelength determines the properties of the radiation.

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Extremely short wavelength (trillionths of a meter)

radiation takes the form of gamma rays, which is high-

energy radiation produced by sub-atomic reactions.Extremely long wavelength (millions of meters)

radiation takes the form of radio waves, which are

useful for transmitting data over long distances.

In between are X-rays, ultraviolet radiation, visible

light, and infrared radiation.

Solar radiation includes more types of energy than just

visible light.

A similar spectrum of solar energy reaching Earths

surface is studied to determine how atmospheric

effects have reduced the energy levels of certain

wavelengths.

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Solar radiation entering Earths atmosphere consists of direct,

diffuse

Solar radiation is absorbed, scattered,

and reflected by components of the

atmosphere, including ozone, carbon

dioxide, and water vapor, as well as

other gases and particles.

Cloud cover and local conditions such

as dust storms, air pollution, and

volcanic eruptions can also greatly

reduce the amount of radiationreaching the surface of Earth.

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Solar Radiation

Air Mass represents how much atmosphere the solar radiation

has to pass through before reaching the Earths surface.

We are specifically concerned with terrestrial solar radiation

that is, the solar radiation reaching the surface of the earth.At high altitudes or in a very clear days, Peak Sun may be

more than 1000 W/m^2 but it is a practical value for most

locations.

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Definitions

Beam radiation (Direct): Solar radiation propagatingalong the line joining the sun and the receiving

surface.

Diffused radiation: Radiation scattered by dust,

aerosols, molecules etc. No preferred directionTotal radiation =Beam + Diffused

Irradiance (

W/m

2

): The rate at which the radiantenergy is incident on a unit area of the surface

( incident radiant flux). Denoted by G.


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Emissive power: The rate at which the energy leaves asurface through emission.

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Solar Constant

The solar constant (ISC) can be defined as the rate at

which energy is received from the sun on a unit area

perpendicular to the Suns rays of the sun and located

outside the atmosphere , per unit of time and per unit

of area at the mean distance of the earth from the sun.

ISC = (1353 W/m^2)

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Solar irradiance is solar power per unit area

Solar irradiance is the power of

solar radiation per unit area.

Solar irradiance is commonly

expressed in units of watts per square

meter (W/m2) or kilowatts per square

meter (kW/

m

2

).Irradiance is measured with respect

to area as if the solar radiation is

striking an imaginary unit surface.

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Insolation

Another term for solar radiation energy is insolation.

Insolation is the solar irradiation received over a

period of time, typically one day. It is typically

expressed as kWh/m2

/day or equivalent peak sunhours.

Insolation is usually used to rate the solar energy

potential of a location by calculating the average

energy received on a surface per day.

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SunEarth Geometric Relationship

The amount and intensity of solar radiation reaching theEarths surface depends on the geometric relationship of the

Earth with respect to the Sun.

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Any location on Earth is described by two angles,

latitude () and longitude ().

The latitude corresponds to the elevation angle betweena hypothetical line from the center of Earth to any point on

the surface and its projection on the equator plane.

Latitude values fall between 90 < < 90;

latitude is zero at the equator, 90 at the northern pole,and 90 at the southern pole.

As for the longitude angle, imaginary lines extended

from pole to pole are called meridians; these lines are at

constant longitude.For each meridian crossing the equators circle, there is

an angle assigned.

Longitudes are measured from 0 to 180 east of the

Prime Meridian and 180 west (or 180).

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Earth coordinate system

North Pole

= 90

Lines of constant

latitudeLines of constant

longitude

Equator = 0

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Two major motions of Earth affect the apparent

path of the sun across the sky:

1. Its yearly revolution around the sun.

2. Its daily rotation about its axis.

These motions are the basis for solar timescale and the reason

why we have seasons, days and nights

Ecliptic Plane is the plane of Earths orbit around the sun.

Equatorial Plane is the plane containing Earths equator and

extending outward into space.

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Solar Radiation

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Solar Radiation

Solar Declination is the angle between the equatorial plane and

the ecliptic plane.

The solar declination angle varies with the season of the year,

and ranges between 23.5 and +23.5

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DeclinationThe declination is the angle between the earth-sun line(through their centers) and the plane through the equator.

Its value is given by

where n is the day of the year ( n=1 for January 1 ).

varies from +23.450 to -23.450

Declinations are positive in the northern hemisphere andnegative in the southern hemisphere.


!

365

2842sin45.23

nTH

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DERIVED SOLAR ANGLES


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Slope ()

The angle between the plane of the surface and the

horizontal, also called the tilt angle; 0 180 .


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Zenith Angle, z

The angle between the beam radiation and the vertical;


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Angle of Incidence

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SurfaceAzimuthAngle,

The deviation of the projection on a horizontal plane ofthe normal to the surface from due south, east is negativeand west is positive; -180 180


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Solar Azimuth Angle, s

The angle between south and the projection of the beamradiation on a horizontal plane, east is negative and westis positive.


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Solar Altitude Angle

Angle between the horizontal and the line to the sun (or thesuns rays), the complement of the zenith angle.

Solar Altitude Angle is the vertical angle between the sun

and the horizon added to the Zenith angle is equal to 90.


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Solar hour angleSolar hour angle is the angular displacement of the sun east or west of the

local meridian: morning negative, afternoon positive.

)()15( 10 noonsolarlocalfrohourshs v!

[


This angle is zero at solar noon and varies by 150 per hour from solar noon.

The solar angle is given by the equation

When the sun is due north ( southern hemisphere) ,the hour angle is zero

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Different Angles

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The equation of time is the difference between

apparent solar time and mean solar time, both takenat a given place (or at another place with the same

geographical longitude) at the same real instant of

time.

Apparent(or true) solar time can be obtained for exampleby measurement of the current position (hour angle) of

the Sun.

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Solar Time

Solar time is based on the mean solar day, butthe time that Sun reaches its highest point

each day (around noon) varies through out

the year.

The difference between noon at Greenwich

and when the sun is at its highest point (or

highest elevation angle) is call the Equation of

Time.

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Solar Time

The relation between the local solar time and local standardtime (LST) is given by

Solar Time = LST+ET +(Ist Ilocal) x 4 min / deg

where Ist is the standard time meridian and Ilocal is the local

longitude. The equation of time (ET) is given by

ET (minutes) = 9.87sin 2B-7.53 cos B -1.5 sin B

where

0

364

)81(360 !

nB


At local solar noon

s= 0 , s = 900- ( -) and s =0

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Th E i f Ti dj f i i i E h bi d

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The Equation of Time adjusts for variations in Earths orbit and

rotation that affect solar time.

Another difference between standard and solar time iscaused by small eccentricities in Earths rotation and orbit

around the sun.

The Equation of Time is the difference between solar time

and standard time at a standard meridian.This difference varies over the course of a year and can be

as much as +16 min or 14 min.

The Equation of Time is computed from a formula or

determined by looking up the date in a table or on a graph.

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Sunrise and Sunset Times

At sunrise or sunset, the sun elevation angle s is zero. The

local solar time for sunrise/sunset is computed with

-

v!

hournoonSunsetSunrise

deg/15

tantancos00.12/

1 H]O


For the tip of the sun at the horizon (apparent sunrise/sunset) subtract/add 4

minutes )

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Length of the day

The number of hours between the sunrise and the sunset are given by the

equation

where is the latitude and is the declination of the place.

)tantan(cos15

2 1 H! N


Variation of solar energy received with season and latitude Annual day length variation at different latitudes

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Daily Insolation


The daily insolation H is the total Solar Energy per unit

area received in one day

!

!

!ht

ht

GdtH

24

0

Units: J m-2 day-1

or

kWh m-2 day-1

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Hourly Variation of irradiance


On clear days , the irradiance varies as

}

N

tGG hh

'

max sinT

Where t is the time after sunrise and N is theduration of daylight for that particular day.

Integrating above eq. over the daylight period we

get for insolation

max2hh G

NH

}T

Where Hh is the total energy received on that particular day.

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Extraterrestrial radiation and clearness

index

Daily extraterrestrial (outside the atmosphere) radiation on a horizontal

surface is given by

where Gsc is the solar constant ( 1,366 W/m2).

This radiation is affected by the earths atmosphere and the clouds .

We define Clearness Index KTas the ratio of the solar radiation arriving atthe earths surface to extraterrestrial radiation

The monthly average clearness index is the ratio of monthly average dailysolar radiation at the surface to the monthly average daily extraterrestrialradiation. KT varies from place to place from about 0.3 for very overcastclimates to 0.8 for very sunny places.

)sinsinsincos(cos365

2cos033.0186400

H[[HTT

sssc nG


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Standard time organizes regions into time zones, where every

location in a time zone shares the same clock time.

Standard time is a timescale based on the apparent motion of the sun

crossing standard meridians.

A standard meridian is a meridian located at a multiple of 15 east or

west of zero longitude.

Zero longitude passes through Greenwich, England and is referred to as

the Prime Meridian.

Since Earth rotates 360 in approximately 24 hours, each 15 of

longitude is equal to one hour of solar time.

All standard time zones are at one hour multiples ahead of or behind the

time at the Prime Meridian, also referred to as Universal Time

Coordinated (abbreviated UTC).

The Equation of Time adjusts for variations in Earths orbit and

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The Equation of Time adjusts for variations in Earth s orbit and

rotation that affect solar time.

Another difference between standard and solar time is

caused by small eccentricities in Earths rotation and orbitaround the sun.

The Equation of Time is the difference between solar time

and standard time at a standard meridian.

This difference varies over the course of a year and can be

as much as +16 min or 14 min.

The Equation of Time is computed from a formula or

determined by looking up the date in a table or on a graph.

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Azimuth angle

s

s U

H[K

cos

cossinsin !

]H]H[U sinsincoscoscossin !s

]H]H[U sinsincoscoscossin ! ss


The solar azimuthal angle is given by

Where

s is the solar elevation angle

s is hour angle at present time

is the declination

Solar elevation angle s is given by

Where is the latitude

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Solar azimuth and altitude angles are used to describe

the suns location in the sky.

Two angles are used to define the suns position, relative to an observer on Earth.

The solar altitude angle is the vertical angle between the sun and the horizon.

During daytime, this angle varies between zero and 90 and complements the zenith

angle (the two added together always equal 90).

The solar azimuth angle is the horizontal angle between a reference direction (typicallydue south in the Northern Hemisphere) and the sun.

This angle varies between 180 and +180.

Sun position to the east of due south is generally represented as a positive azimuth

angle, and to the west as a negative azimuth angle.

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The solar window is the area of sky containing all possible

locations of the sun throughout the year for a particular location.

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Energy production at certain times of the year can be optimized by

adjusting the array tilt angle.

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Solar radiation at a tilted surface


The amount of solar

radiation

falling on the tilted

surface is the SModule

= Tilt angle, = Sun angle

Shorizontal= Radiation at a horizontal

surface Where = 90- +

Angles

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Solar Radiation

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Air mass is a representation of the amount of atmosphere radiation

that must pass through to reach Earths surface

The amount of solar radiation that is absorbed or

scattered in the atmosphere depends on how

much atmosphere it passes through before

reaching Earths surface.

When the sun is at zenith, the amount of

atmosphere that the suns rays have to pass

through to reach Earths surface is at a minimum.

Zenith is the point in the sky directly overhead a

particular location.

The zenith angle is the angle between the sun

and the zenith.

As the zenith angle increases (the sunapproaches the horizon), the suns rays must pass

through a greater amount of atmosphere to

reach Earths surface.

This reduces the quantity of solar radiation, and

also changes its wavelength composition.

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Solar Radiation Zenithis the point in the sky directly overhead a particular location as the

Zenith anglez increases, the sun approaches the horizon. AM = 1/Cosz

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Solar irradiance is solar power

per unit area.

Solar irradiance varies slightly as the sun goes

through normal cycles of maximum and minimum

activity. However, distance from the sun has a

much greater effect.

The inverse square lawis a physical law that states

that the amount of radiation is proportional to the

inverse of the square of the distance from the

source.

This means that at twice the distance of Earth to

the sun, solar radiation is only one-fourth theamount on Earth.

Likewise, moving three times the distance away

from a light source decreases the intensity by a

factor of nine.

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Even over the vast distance, an enormous amount of energy reachesEarth from the sun.

An astronomical unit (AU) is the average distance between Earth andthe sun (93 million mi) and is used as a measuring unit for other

distances in the solar system.Traveling at the speed of light (186,000 mi/s), radiation from the suntakes more than 8 min to reach Earths surface.

Earth receives approximately 170 million GW of power from thesun, which is a relatively tiny fraction of the suns total output, but ismillions of times greater than the maximum power demand of Earthsentire population.

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Solar irradiation equals the total solar irradiance

over time.

At the surface of Earth, the

magnitude of solar irradiance

changes throughout the day.

It begins at zero during nighttime,

increases as the sun rises, peaks

around noon, and decreases as the

sun sets.

In a plot of solar irradiance versustime, solar irradiation equals the

area under the irradiance curve.

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The two major types of radiation reaching the

ground are direct radiation and diffuse

radiation.

Total global radiation is all of the solar radiationreaching Earths surface and is the sum of

direct and diffuse radiation.

Peak sun hours is an equivalent measure of total solar irradiation in a

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Peak sun hours is an equivalent measure of total solar irradiation in aday.

Peak sun hours is the number of hours required

for a days total solar irradiation to accumulate atpeak sun condition.

An average day may have only one or two actual

hours at peak sun condition, but the total

irradiation for a day may be expressed in units of

peak sun hours by dividing by 1000 W/m2 (peak

sun irradiance).For example, a day with an average irradiance of

600 W/m2 over 8 hr may only reach peak sun

condition for an hour or less around noon.

However, the total irradiation of 4800 Wh/m2

(600W/m2 8 hr = 4800 Wh/m2) is equivalent to 4.8

peak sun hours

(4800 Wh/m2 1000 W/m2 = 4.8 peak sun hr).

P k h i i l t f t t l l i di ti i

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Peak sun hours is an equivalent measure of total solar irradiation in a

day.

Peak sun hours is the number of hours requiredfor a days total solar irradiation to accumulate at

peak sun condition.

An average day may have only one or two actual

hours at peak sun condition, but the total

irradiation for a day may be expressed in units ofpeak sun hours by dividing by 1000 W/m2 (peak

sun irradiance).

For example, a day with an average irradiance of

600 W/m2

over 8 hr may only reach peak suncondition for an hour or less around noon.

However, the total irradiation of 4800 Wh/m2 (600

W/m2 8 hr = 4800 Wh/m2) is equivalent to 4.8

peak sun hours (4800 Wh/m2 1000 W/m2 = 4.8

peak sun hr).

The atmosphere absorbs extraterrestrial radiation at certain

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wavelengths, resulting in an altered spectral distribution for

terrestrial radiation.

Besides its total power, terrestrial solar radiationalso differs from extraterrestrial solar radiation in

its spectral distribution, mainly due to the

absorption of radiation at certain wavelengths by

specific gases in the atmosphere.

One of the key atmospheric gases, ozone, plays

an important role in blocking harmful ultraviolet

radiation from reaching Earths surface.

Other constituents, such as water vapor and

carbon dioxide, also absorb solar radiation,principally in the infrared portions of the

spectrum.

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A pyranometer measures total global solar irradiance from

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the whole sky.

Solar irradiance is typically measured with a

pyranometer.

A pyranometer is a sensor that measures the

total global solar irradiance in a hemispherical

field of view.

Since pyranometers measure both direct and

diffuse radiation in a whole-sky view, they are

often used to monitor the solar radiation incidenton flat-plate type arrays.

These sensors are mounted adjacent to arrays,

in the same plane (facing the same direction) as

the arrays.

Solar irradiance values sampled at regular

intervals and stored by data acquisition

equipment can be used to determine the total

solar irradiation for the specific surface

orientation and site conditions.

Diff l i di b d b ddi h d i d i

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Diffuse solar irradiance can be measured by adding a shadowing device

to a pyranometer, which blocks the direct component of total irradiance.

Diffuse global radiation can be measured by

shading a pyranometer, which measures both

direct and diffuse radiation, from the direct

radiation component.

A shadow band pyranometer uses a metal strip

in the shape of an arc to shield the pyranometer

from direct radiation.To account for changing sun paths, this device

must be adjusted daily, either manually or

automatically.

Other shadowing devices may use a disk that

follows the sun to shadow the pyranometer at all

times of the day.

The direct radiation measurement is calculated

by subtracting the diffuse measurement from the

global measurement.

Handheld pyranometers use less precise sensors than precision

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Handheld pyranometers use less precise sensors than precision

pyranometers but are more suitable for field measurements.

Precision pyranometers use thermopile

sensors (thermocouple arrays that output a

voltage proportional to irradiance) and offer the

most accurate and consistent response across a

range of wavelengths.

Pyranometers that use silicon solar cell or

photodiode detectors are less precise, but are

also less expensive and offer the durability

required for most field measurements.

Some are even small handheld meters with an

easy-to-use digital readout of total solar

irradiance.

A pyrheliometer measures the direct component of solar irradiance

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A pyrheliometer measures the direct component of solar irradiance,

which is important when installing concentrating collectors.

Direct solar radiation is measured with a pyrheliometer.

A pyrheliometer is a sensor that measures only direct

solar radiation in the field of view of the solar disk (5.7).

It does not measure the diffuse radiation component.

Because pyrheliometers only measure the direct

radiation component, they must be pointed directly at the

sun and installed on sun-tracking devices to take

measurements of direct radiation over the course of the

day.

Most precision pyrheliometers use thermopile sensors.

Reference cells output a certain electrical current for eachit f l i di i d

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unit of solar irradiance received.

A reference cellis an encapsulated PV cell that outputs a

known amount of electrical current per unit of solar

irradiance.Since current output from a PV device varies linearly with

the incident solar irradiance, the output current can be

used to indirectly measure irradiance.

The calibration number is the conversion factor that

changes a reference cells current into solar irradiance

with respect to a certain air mass value.

Calibration numbers are usually expressed as

milliamperes per kilowatt per unit area (mA/kW/m2) or

amperes per kilowatt per unit area (A/kW/m2), both at

AM1.5.

The cell is usually encapsulated in an aluminum block

with an optical glass cover, and often includes a

thermocouple (temperature sensor) attached to the back

of the cell to measure and correct the output for

tem erature variations.

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Solar Radiation

Example problem of Peak sun hours per day:

If during the day we have 4 hours at 500 Wh/m^2 and 6 hours at 250 Wh/m^2 we should

compute the peak sun hours per day as follow:

First, multiply 4hs x 500 W/m^2 and add to it 6hs x 250 W/m^2 This will equal 3500 Wh/m^2

Second, we know that by definition Peak Sun is 1000 W/m^2, so if we divide the total irradiationfor the day by Peak Sun we will obtain Peak Sun hours. That is,

Peak Sun Hours = Total Irradiation [Wh/m^2] / Peak Sun [W/m^2] = Peak Sun hours

In our specific problem:

Peak Sun Hours = 3500 Wh/m^2 / 1000 W/m^2 = 3.5 Peak Sun hours

Note: most solar irradiation data is presented in Peak Sun Hours units

The equatorial plane is tipped 23 5 from the ecliptic plane As Earth

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The equatorial plane is tipped 23.5 from the ecliptic plane. As Earth

revolves around the sun, this orientation produces a varying solar

declination.

Solar declination is the angle between the equatorial plane

and the rays of the sun.

The angle of solar declination changes continuously as Earth

orbits the sun, ranging from 23.5 to +23.5 (positive when

the

Northern Hemisphere is tilted toward the sun).

The angle between the ecliptic and equatorial planes does

not change, but as viewed from the sun at different times of

the year, the equatorial plane appears to change in

orientation.

It appears to dip below the ecliptic plane (summer in the

Northern Hemisphere), become edge-on (fall), tip above the

ecliptic (winter), and return to edge-on (spring).

An analemma shows how sun position, at the same time of

d h h h h

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day, changes throughout the year.

At any given location on Earth, the suns

apparent position in the sky depends on

the latitude, the time of day, and the time

of year.

The position of the sun, as observedfrom the same location and at the same

time of day for a year forms a figure

eight.

An analemma is a diagram of solar

declination against the Equation of Time.

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The National Renewable

Energy Laboratory (NREL)

provides solar radiation data

for various locations, times of

the year, and south-facingarray orientations.

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NRELs HOMER software is

used to optimize the cost

effectiveness of systems with

multiple distributed-

generation sources

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Low Temp Solar Thermal

Residential Water Heating: used for

consumption, cooking, bathing, recreation

(hot tub, swimming pool), space heating and

cooling (radiant floor systems, ductedsystems), etc.

These systems collect or absorb the suns light,

which is then turned into heat, and that heatdirectly or indirectly heats water.

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Low Temp Solar Thermal System

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Flat Plate Collector

The flat plate collector is the most common

type of solar thermal collector.

They use the basic concept Horace de

Saussurre developed in 1767.

Flat plate collectors do well in almost any

environment and climate.

Can heat water to 160 or 180 degrees

Fahrenheit.

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Evacuated Tube Collectors

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Evacuated Tube Collectors

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Evacuated Tube System

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Evacuated Tube System

The vacuum in evacuated tubes helps reduceheat loss.

Evacuated tubes have an ability to heat water

to higher temperatures than other types ofcollectors, though over time a flat platecollector will typically produce more hot waterbetween 140 and 180 degrees F.

Evacuated tubes are only needed when watermust be heated above 180 degrees F.

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Batch Heater

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Batch Heater

Batch heaters work on the thermo-siphon

principle. As water is heated in the collector it

rises to the top and is channeled into the

holding tank. Cooler water moves into thebottom of the collector to take its place, which

continues the cycle.

Batch heaters can be susceptible to heat lossat night, reducing financial and energy gains.

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Unglazed Collectors

Unglazed collectors are commonly used for solarpool heating, but have recently been applied todomestic hot water systems.

They are made of a polymer (a type of plastic)

and are fairly inexpensive. They work best in Mediterranean climates and

contribute heated water in the warm/hotsummer months.

They dont do well in cold or windyenvironments.

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Unglazed Collector

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Unglazed Collector

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Solar Pool Heating

Solar pool heating can be done with flat plate

collectors, evacuated tubes, and unglazed

collectors.

Evacuated tube and flat plate collectors canthave chlorine in them, so a heat exchanger must

be used.

Chlorinated water can run directly throughunglazed collectors, making them more efficient

at heating water in specific temperature ranges.

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Characteristics of the moon

Spherical; made of rock

Has no atmosphere, no water, and no living things

Drastic temperature changes

Earths natural satellite

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Moon and its effects

One- fourth of the Earths diameter

revolves around the earth every 29 1/2 days

the gravity of the moon controls the tides (trans.)

Q ti

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Questions

What are threecharacteristics of the moon?

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Types of tides

Spring tides- gravity of the sun and moon work together

(tide is high)

Neap tides- gravity of the sun and moon PULL against

each other (tide is low)

Q ti

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Questions

What is the differencebetween a spring tide and a

neap tide?

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Eclipses

Solar eclipse- sunlight is blocked by the moon

lunar eclipse- Earth is between the sun and moon

moon is visible because of reflected sunlight

Q ti

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Questions

Why do we see the moon atcertain times of the

month? How are solar eclipses

different from lunareclipses?

Documents

solar energy 11-01-2011