UNIT II

Solar EnergyM.Tech. 1st Semester

By:

S. S. Joshi

Lecturer

Electrical Engg Dept.Electrical Engg Dept.

SOT PDPU

Sun Earth Relationships

Solar Constant• The solar constant is defined as the total energy received

from the sun per unit time on a surface of unit area kept

perpendicular to the radiation, in space just outside the

earth's atmosphere when the earth is at its mean distance

from the sun.

• Solar constant have approximately value is 1370 W/m2.• Solar constant have approximately value is 1370 W/m2.

Solar Radiation at the Earth’s surface

• The solar radiation that penetrates the earth's atmosphere

and reaches the surface differs in both amount and

character from the radiation at the top of the atmosphere.

• In the 1st place, part of the radiation is reflected back into

the space, especially by clouds.

• Further more the radiation entering the atmosphere is• Further more the radiation entering the atmosphere is

partly absorbed by molecules in the air.

• Oxygen and ozone(O3), formed from oxygen, absorb nearly

all the ultraviolet radiation and water vapor and carbon

dioxide absorb some of the energy in the infrared range.

Direct or Beam Radiation• Solar radiation that has not been absorbed or

scattered and reaches the ground directly

from the sun is called "direct(or Beam)

radiation".

• Diffuse Radiation:The diffuse radiation is that solar radiation received from the

sun after its direction has been changed by reflection and

scattering by the atmosphere.

• Total or Global Radiation:The total solar radiation received at any point on the earth's

surface is the sum of the direct and diffuse radiation.

Irradiance, W/m2 (G):• The rate at which radiant energy is incident

(occur) on a surface, per unit area of thesurface.

• The symbol G is used for solar irradiance, withappropriate subscripts for beam, diffuse ortotal.total.

Elliptical orbit of earth’s

revolution

Autumn

7% more radiation

Winter

Spring

Summer

Irradiation, J/m2 :The incident energy per unit area on a surface,

found by integration of irradiance over a

specified by time, usually an hour a day.

IRRADIANCE= POWER IRRADIANCE= POWER

(Wm2)

IRRADIATION= ENERGY

( Wh/m2)

Extraterrestrial radiationSolar radiation incident on the outer atmosphere of the earth is

called extraterrestrial radiation.

A circle of constant longitude passing through a given place on the earth's

surface and the terrestrial poles.

Terrestrial radiation

• Terrestrial radiation is heat that is radiated

from the earth, solar radiation is heat radiated

from the sun.

Beam radiation

• Solar radiation along the line joining the

receiving point and the sun is called beam

radiation.

Important terms

Air mass:

• A term called air mass (AM) is often used as a measure of

the distance traveled by beam radiation through the

atmosphere before it reaches a location on the earth's

surface. The air mass is the ratio of the path of the sun's

rays through the atmosphere to the length of path when the

sun is at zenith.sun is at zenith.

• The zenith is an imaginary point directly "above" a

particular location, on the imaginary celestial sphere

Basic Earth-Sun angles

Earth's Equator:

It is an imaginary great circle normal to the earth's axis,

dividing the distance between the earth's poles along its

surface into two equal parts. The equator divides the earth

into two hemispheres called Northern and Southern

hemispheres.

Basic Earth-Sun angles

Meridian:

It is necessary to select some reference location on the earth for

helping in locating a particular position. An imaginary great circle

passing through the point and the two poles, intersecting the

equator at right angle is called prime meridian. The location through

which the prime meridian is passing is Greenwich (0o Longitude,

England.)

Longitude:

It is the angular distance of location, measured east or west

from the prime meridian. For example longitude of Bhopal

is 77o30' E.

Latitude:

It represents the angular location north or south of the

equator, north positive. The latitude of a point on theequator, north positive. The latitude of a point on the

surface of the earth is its angular distance north or south of

the equator measured from the center of the earth. Denoted

by φ

Declination angle (δ):

• The declination is theangular distance ofthe sun's rays north(or south) of theequator. It is theangle between a lineextending from theextending from thecenter of the sun andthe center of theearth and theprojection of this lineupon the earth'sequatorial plane.

Hour angle:

The hour angle is the angular distance between the meridian

of the observer and the meridian whose plane contains the

sun. The hour angle is zero at solar noon (when the sun

reaches its highest point in the sky) and increases by 15oevery

hour, morning negative, afternoon positive. The hour angle ishour, morning negative, afternoon positive. The hour angle is

the angle through which the earth must turn to bring the

meridian of a point directly in Line with the sun's rays.

Solar Altitude angle:

• The angle between the horizontal and line to the sun.

• Denoted by Sα

Solar Zenith angle:

• Angle between the vertical and line to the sun, i.e.

complement of the solar altitude angle

• Denoted byZθ

Solar Azimuth angle:

• It is the angle between the projection of sun’s ray to the point

on the horizontal plane and line due south passing through

that point.

• Value of azimuth angle is taken +ve when it is measured from

south towards west.

• Denoted by γ• Denoted by Sγ

Solar altitude angle

Surface azimuth angle:

The deviation of the

projection on a horizontal

plane of the normal to the

surface from the local

meridian, with zero due

south, east negative andsouth, east negative and

west positive;

Denoted by ;

γ oo 180180 ≤≤− γ

Slop or Tilt angle:

• The angle between inclined slop and horizontal plane.

• Denoted by

β

SOLAR THERMAL TECHNOLOGIES

Solar collector

Evacuated tube collector

Thermosyphon water heater

Forced circulation system

SWH- An Israil scene

Anticipated Savings from Solar Water

Heating Systems

• It is estimated that about 15 billion units of

electricity could be conserved over a period

of 20 years @750 kwh/sq.m of collector area,

year through deployment 1 million sq meter

of collector area implying a life cycle saviof collector area implying a life cycle savi

ng of over Rs.7,500 crore @ Rs.5/- per unit

of electricity (or Rs.75,000/ sq.m collector

area over a 20 year life cycle).

Anticipated Savings…cont’d

• Furthermore, one million sq.m of collector

area is capable of providing a theoretical

maximum peak saving of 500 MW. 1 sq.m

collector solar area can save around 60 litres collector solar area can save around 60 litres

of diesel per year or 900 litres over a 15 year

life cycle or Rs.27,000 worth of diesel

@Rs.30/litre. The payback period for such

systems is estimated at about 4–5 years.

A typical domestic SWH savings

Region North East South West

No of

Days

200 200 250 250

Days

Elc’Y

saving

(Units)

950 850 1200 1300

Solar power plant- parabolic system

Solar Tower power generation

1.291 mirrored heliostats and a 54 story high

tower the World's largest solar power tower

plant located near Seville in Spain in now on line

generating 20 megawatts (MW) of electricity,

enough to supply 10,000 homes.

Solar Chimney

What is the area required for a solar PV

power plant, per MW? For solar CSP?power plant, per MW? For solar CSP?

Area required fpr solar power generation•About 5 acres/ MW for solar PV (crystalline) and for 7-12

acres

for solar CSP (depending on the type of technology used)

•Within solar PV, it is assumed at 4-5 acres for crystalline

silicon (c-Si) technology and 7-8 acres per MW for thin film

solar (a-Si or CdTe) technology.solar (a-Si or CdTe) technology.

In reality, it depends on other parameters like cost of land,

Ground Coverage Ratio (GCR) (to avoid inter array shading,

GCR can be 0.45 to 0.65 and generation will vary based on

GCR) and choice of sun tracking systems

(with sun trackers the land required will be about 6 acres per

MW for crystalline solar modules).

Cost of solar power plants

• The capital investment for solar PV ranges from Rs

14 cr to 16 cr per MW depending on the technology.

The capital costs have come down significantly in

the last few years, and this cost is expected to

decrease further with technological advancements.decrease further with technological advancements.

• Capex for solar CSP is about Rs 12-13 crores MW,

but this is an approximate number, as the estimate

can differ widely based on the technology used.

Portable solar cooker

Concentrating cookers

Comm’y concentrating cooker

World’s largest solar cooker at Shirdi

PV Cell

• The physics of the PV cell is very similar to that

of the classical diode with a pn junction.

• When the junction absorbs light, the energy of

absorbed photons is transferred to the electron–absorbed photons is transferred to the electron–

proton system of the material, creating charge

carriers that are separated at the junction.

• The charge carriers may be electron–ion pairs in a

liquid electrolyte or electron–hole pairs in a solid

semiconducting material.

PV effect converts the photon energy

into voltage across the PN junction

• The charge carriers in the junction region create a

potential gradient, get accelerated under the

electric field, and circulate as current through an

external circuit.external circuit.

• The origin of the PV potential is the difference in

the chemical potential, called the Fermi level.

• What is it?????

• When they are joined, the junction approaches a new

thermodynamic equilibrium. Such equilibrium can be

achieved only when the Fermi level is equal in the

two materials.

• This occurs by the flow of electrons from one

material to the other until a voltage difference is

established between them, which has a potential just

equal to the initial difference of the Fermi level.

• This potential drives the photocurrent in the PV

circuit.

Basic construction of PV cell with

performance-enhancing features

SPV Technology

MODULE AND ARRAY

Merits of PV system

• Use of clean, cheap., noiseless, safe, renewable energy to

produce electricity at the location of utilization.

• Suitable for remote loads away from main electrical

network and at places where other fuels are scare and

costly. Cost of distribution lines can be eliminated.costly. Cost of distribution lines can be eliminated.

• Suitable for portable mobile loads eg radios, cars, buses,

space crafts etc.

• Reliable service, long 15 years life.

• Modest maintenance.

Limitations of PV system

• Irregular, intermittent supply of solar energy.

• Need for storage battries.

• High capital cost (Rs/kW) due to larger number of PVcell, low output power, low efficiency and hightechnology involved.technology involved.

• Not economical for central power plants of MW ratingdue to very large area of PV panels and very large energystorage system.

• Require storage batteries in large amount and dieselgenerator sets in cloudy wheather.

• Installed at roof tops only…

• Low efficiency.

EQUIVALENT ELECTRICAL CIRCUIT

Rs = 0 (no series loss), and Rsh = ∞ (no leakage to ground)

Rs varies from 0.05 to 0.10 Ω and Rsh from 200 to 300 Ω.Rs varies from 0.05 to 0.10 Ω and Rsh from 200 to 300 Ω.

The open-circuit voltage Voc of the cell is obtained when the load current is

zero, i.e., when I = 0, and is given by the following:

Voc = V + Irsh

The diode current is given by the classical diode current expression:

where

ID = the saturation current of the diode; Q = electron charge = 1.6×10–19 C

A = curve-fitting constant, k = Boltzmann constant = 1.38×10–23 J/°K

T = Temperature on absolute scale °K

)1(

Rsh; IgnoreInitially

0 =−−⇒

−=

IeII

III

L

t

qV

ph

dphL

k

d

γ )1ln(γ

−+−

=⇒ IRI

II

q

tV LS

LphkL

Mathematical Proof

)1ln(

)()1ln(

)1(

)1(

0

0

)(

0

0

+−

=+⇒

+=+

−⇒

=−−⇒

=−−⇒

+

I

II

q

tIRV

t

IRVq

I

II

IeII

IeII

LphkLSL

k

LSLLph

L

t

IRVq

ph

Lph

k

LSL

γ

γ

γ

much.... that reduced

not is Voc halfedIph if

ln(Iph), Voc As

)1ln(

0 circuit open if

)1ln(

0

0

α

γ+=⇒

==>

−+=⇒

I

I

q

tV

I

IRIq

V

phkOC

L

LSL

Rsh; Including

Mathematical Proof

L

sh

SLLRIVA

ph

L

sh

DAV

ph

shdphL

IR

RIVeII

IR

VeII

IIII

SLL

D

=+

−−−⇒

=−−−⇒

−−=

+)1(

)1(

)(

0

0

Current vs. voltage (I-V) characteristic of the PV

module in sunlight and in the dark

Power vs. voltage (P-V) characteristic of

the PV module in sunlight

I-V characteristic of a 22-W PV module at

full and half sun intensities

THE PV I–V CURVE UNDER

STANDARD TEST CONDITIONS (STC)

The I –V curve and power output for a

PV module

The maximum power point (MPP) corresponds to the biggest

rectangle that can fit beneath the I –V curve. The fill factor (FF) is

the ratio of the area (power) at MPP to the area formed by a

rectangle with sides VOC and ISC.

Fill factors around 70–75% for crystalline silicon solar modules

are typical, while for multi junction amorphous-Si modules, it is

closer to 50–60%.

Physics of Shading

Effect of Shading

• Consider the case when the bottom n − 1 cells

still have full sun and still some how carry

their original current I so they will still produce

their original voltage Vn−1. This means thattheir original voltage Vn−1. This means that

the output voltage of the entire module VSH

with one cell shaded will drop to,

Hot Spot Heating

If the operating current of the overall series string

approaches the short-circuit current of the "bad" cell,

the overall current becomes limited by the bad cell.

Example

Bypass Diodes for Shade Mitigation

• Figure shows a typical situation.

• In Fig. (a) a solar cell in full sun operating in

its normal range contributes about 0.5 V to the

voltage output of the module, but in thevoltage output of the module, but in the

equivalent circuit shown in (b) a shaded cell

experiences a drop as current is diverted

through the parallel and series resistances.

In full sun a cell may contribute around 0.5 V to the module

output; but when a cell is shaded, it can have a large voltage drop

across it

Need of bypass diode

• The voltage drop problem in shaded cells couldbe to corrected by adding a bypass diode acrosseach cell, as shown in Figure.

• When a solar cell is in the sun, there is a voltagerise across the cell so the bypass diode is cut offrise across the cell so the bypass diode is cut offand no current flows through it—it is as if thediode is not even there.

• When the solar cell is shaded, however, the dropthat would occur if the cell conducted any currentwould turn on the bypass diode, diverting thecurrent flow through that diode.

Mitigating the shade problem with a bypass diode.

In the sun (a), the bypass diode is cut off and all the normal

current goes through the solar cell. In shade (b), the bypass diode

conducts current around the shaded cell, allowing just the diode

drop of about 0.6 V to occur

How improved in I – V curve

Showing the ability of bypass diodes to mitigate

shading when modules are charging a 65 V battery.

Without bypass diodes, a partially shaded module

constricts the current delivered to the load (b). With constricts the current delivered to the load (b). With

bypass diodes, current is diverted around the shaded

module.

Blocking Diodes

• Bypass diodes help current go around a shaded or

malfunctioning module within a string. This not only improves

the string performance, but also prevents hot spots from

developing in individual shaded cells.

• When strings of modules are wired in parallel, a similar• When strings of modules are wired in parallel, a similar

problem may arise when one of the strings is not performing

well.

• Instead of supplying current to the array, a malfunctioning or

shaded string can withdraw current from the rest of the array.

By placing blocking diodes (also called isolation diodes) at the

top of each string as shown in Fig., the reverse current drawn

by a shaded string can be prevented.

Blocking diodes prevent reverse current from flowing

down malfunctioning or shaded strings