4. Solar Photo-Voltaic Technology-1

Light energy  Electricity It is generated due to principle of photoelectric effect Let’s look at the process in some further detail:

• Photovoltaic energy is the direct conversion of light into electricity at the atomic level. • Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. • When these free electrons are captured, an electric current results that can be used as electricity.

Metal

Option of materials

Semiconductor

Deciding factor is capacity of material to excite electron

Electron is excited (by photon) from filled (valence band) to empty conduction band and then it participate in electricity conduction

Eg

Eg

Metal

Semiconductor

Insulator

Metal

• Bandgap is overlapping • Electric field will not be created in metals

Semiconductor

• Bandgap is significant • Best material and hence used in manufacturing

n-type Si Si

Group V donor atom eg, P, As or Sb) in the covalent bonding model of a Si crystal

Sb e--

Ec

Ed

Ed

Eg Ev

T = 0K

T > 50K

Donation of electrons from donor level to the conduction band

p-type Si Si

h +

Al

Group III acceptor atom (e.g. B or Al) in the covalent bonding model of a Si crystal

Ec Eg Ea

Ea Ev

T = 0K

T  50K

Acceptance of a valence band electrons by an acceptor level and the resulting creation of holes

• • • •

Elemental semiconductors: Si, Ge Compound semiconductors: GaAs, InP Ternary semiconductors: AlGaAs, HgCdTe Quaternary semiconductors: InGaAsP, InGaAlP Elemental

IV Compounds

Binary III-V

Binary II-VI

Si Ge As

SiGe SiC

AlP GaAs InP GaP

CdTe CdS ZnS CdSe

Video

Conduction band

Red photon

Green photon

Blue photon

Band gap, Eg

Valence band

λ red

<

Wavelength (λ)

λ green

λ blue

Recollect energy equation

E  hc / 

Recombination is reverse process of generation, it must be avoided in solar cells

represents loss of generated carriers

After recombination

Recombination can occur through several mechanism, trapassisted recombination (recombination via defects and impurities), is most common in solar cells.

12/24/2012

© IIT Bombay, C.S. Solanki

Types of recombination

• There are three main processes which cause recombination of excess carriers, shown below

Band to band recombination

Auger recombination Trap assisted recombination

Trap-assisted recombination (recombination via defects and impurities), is most common in solar cells.

Electric field is created

Electrons are transferred through external load



Solar cells are simply P-N junctions in which hole/electron pairs are created from photons.

 The holes/electrons diffuse into the junction, and are immediately swept to the other side.  The net charge gain is seen outside the cell as current and voltage, or power.

I total  I 0 (e

qV / kT

 1)  I L

Where IL is photo current (constant) IL = Isc

Shows Current (I) and Voltage (V) relation

Graphical representation

Short-Circuit Current (Isc) largest current which may be drawn from the solar cell

when the solar cell is short circuited

Isc

Vsc = 0

I total  I 0 (e At V=0  Itotal = -IL= Isc

qV / kT

 1)  I L

Open Circuit Voltage: Voc Maximum voltage that may be obtained from the solar cell when the solar cell is open circuit

Ioc=0

I total  I 0 (e

VOC qV / kT

 1)  I L

by setting Itotal = 0

kT I L Voc  ln(  1) q I0

Maximum power: Pm Isc

Pm

Im

X

Vm

Voc

Pm = Im x Vm Remember we get DC power from a solar cell… !!

Fill Factor: FF I

Ideal diode curve

Isc

Pm

Im

Vm

Voc

Graphically, the FF is a measure of the "squareness" of the solar cell

Max power from real cell Vm I m FF   Max power from ideal cell Voc I sc

Efficiency: η I

Isc

Im

Max. Cell Power  Incident light Intensity Vm I m  Pin

Pm X

Vm

Voc I sc FF  Pin

Voc

Depends on

Intensity Temperature

Efficiency

Uses

To compare solar cells To estimate actual output

Efficiency depends on material of PV Highest achieved efficiency of solar cell is 43% !!!

Where is the energy going?

Inherent Loses in Photovoltaic Cells :



photovoltaic cell has an open circuit voltage of 0.6V and a short circuit current of 250A/m2 at a cell temperature of 40ºC. Calculate the voltage and current density that maximizes the power of the cell. What would be the maximum power output per unit cell area ? 2 Ans: 123 W/m



What would be the corresponding maximum power output per unit cell area and the corresponding conversion efficiency if the global solar radiation incident on the cell is 850 W/m2 ? Calculate the cell area required for an output of 36 W

1st Generation

2nd Generation

3rd Generation

• silicon waferbased photovoltaic,

• Thin-film deposits of semiconductors

• Multi-junction solar cells • Dyesensitized nanocrystalline Gratzel solar cells • Organic polymerbased photovoltaic • Thermo photovoltaic solar cells.

• Single-crystalline and multicrystalline wafers

• amorphous silicon, cadmium telluride, copper indium gallium diselenide or copper indium sulfide

 Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon.  Hence most c-Si panels have uncovered gaps at the four corners of the cells.

 Dominates commercial market  Expensive due to high quality of Silicon required

 Polycrystalline thin-film cells are made of many tiny crystalline grains of semiconductor materials.  This means that they absorb slightly less solar energy and produce slightly less electricity per square metre.  On the plus side, the process of creating the silicon for a polycrystalline cell is much simpler, so these cells are generally cheaper per square metre.  Light direction changes and be reflected, and thus travels a greater distance within the cell then the cell thickness.

 The term “amorphous” commonly applied to non-crystalline materials prepared by deposition from gases.  Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon

 Can be deposited on a wide range of substrates, including flexible, curved, and roll-away types

 Copper Indium Diselenide consist of CuInSe2  Very good light absorber  Hetero-junction with cadmium sulfide (CdS) has been found to be more stable and efficient.

Research and development going on to improve efficiency and utility of Solar Cells



Multi-junction solar cells or tandem cells are solar cells containing several p-n junctions



Each junction is tuned to a different Wavelength of light, reducing one of the largest inherent sources of losses, and thereby increasing efficiency.



Traditional multi-junction cells have a maximum efficiency of 43% at lab scale



A dye-sensitized solar cell is a lowcost solar cell belonging to the group of thin film solar cells.



It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system.



it is simple to make using conventional roll-printing techniques, is semi-flexible and semi-transparent which offers a variety of uses not applicable to glassbased systems, and most of the materials used are low-cost.



A polymer solar cell is a type of flexible solar cell made with polymers, large molecules with repeating structural units, that produce electricity from sunlight by the photovoltaic effect.



Polymer solar cells include organic solar cells(also called "plastic solar cells").



They are one type of thin film solar cell, others include the currently more stable amorphous silicon solar cell.



Polymer solar cell technology is relatively new and is currently being very actively researched by universities, national laboratories, and companies around the world.

 Photovoltaic thermal hybrid solar collectors, sometimes known as hybrid PV/T systems or PVT, are systems that convert solar radiation into thermal and electrical energy. 

These systems combine a photovoltaic cell, which converts electromagnetic radiation (photons) into electricity, with a solar thermal collector, which captures the remaining energy and removes waste heat from the PV module.

Solar cells are connected electrically in series and/or parallel circuits to produce higher voltages, currents and power levels.

A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module.

Photovoltaic panels include one or more PV modules assembled as a pre-wired, fieldinstallable unit.

1.00

0.80 Current (A)

Current (A)

1.00

0.60 0.40 0.20 0.00 0.00

0.60 0.40 0.20

0.20

0.40

0.00 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

0.60

Voltage (V)

Voltage (V)

2.00

2.00

1.60

1.60

Current (A)

Current (A)

 Series connection adds the voltage,

0.80

1.20 0.80

 Parallel connection adds the currents

1.20 0.80

0.40

0.40

0.00 0.00

0.00 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

0.20

0.40 Voltage (V)

0.60

Voltage (V)

1. Photovoltaic modules were designed for charging batteries of 12 V 2. One PV module contains 36 c-Si solar cells 3. Commercial Si solar cells generally have an Voc = 0.55 V at 25 oC. 4. 36 cells in series , 36* 0.55 = 20 V output.

I I sc Im

5.

Voc ~ 1.1 Vm

The voltage at maximum power point would be about 20/1.1=18 V.

P X

m Vm Voc

6. Loss due to temperature rise Voltage of a solar cell decreases by about 2.3 mV/oC rise in temperature the module temperature under sunlight is in the range of 50 to 80 oC (for T ambient is 30 to 40 oC) 7. Thus the voltage from each cell at 70 oC and ambient temperature of 35oC would drop by 0.08 V. This will result in about 2.9 V drop in module voltage.

8. Thus at operating conditions the output voltage from a module would be in the range of 14 to 16 V (~15 V)

Wattage of modules 9. Wattage of the modules depends on the current generating capacity & voltage capacity of the module.

The current from a module is linearly proportional to its size (but voltage is independent of size) Current size of Si solar cells are of 12.5 x 12.5 cm2 and 15 x 15 cm2. These can be like 1. Pseudo square mono-crystalline cells 2. Truly square multi-crystalline cells. Current generating capacity of Si cells is about 30 mA/cm2 at 1000 W/m2 A cell of 15x15 cm2 will generate about 6.75 A current.

10. 36 solar cell module will produce power of about 15 x 6.75100 Wp. ‘p’ represents peak power output under standard test conditions (STC). Commercially PV modules are available 100 - 300 Wp power rating.

The cells should also be protected from dust, rain, mechanical shock etc. So the PV modules should be package by using 1. Glass at the front side - Low iron content, toughened and textured Glass - Higher transmitivity (over 90%) 2. Ethylene vinyl acetate (EVA) for encapsulation - high electrical resistivity (1014 Ω-cm) - very low water absorption ratio - good optical transmission 3. Solar Cell 4. Lower encapsulate layer

5. Rear layer (Tedlar – white colour) - back reflection of light 6. Outer frame (Al)

Components of Si wafer based PV module.

Video

PV modules are rated in terms of their peak power (Wp) output under Standard Test Conditions (STC). 80

4



3

 

60

40

STC

2

SOC

1



0 0

NOC Peak point

5

Power (Watt)

NOC (Nominal Operating Conditions) - Irradiation: 800 W/m2, - Ambient temperature: 20oC - Wind speed: 1 m/s - Mounting: open back side

5

Current (A)

STC (Standard Test Conditions) - Irradiation: 1000 W/m2, - AM1.5G global solar radiation - Cell or module temperature: 25oC - Wind speed: 1 m/s.

20 power

0 10

15

20

25

Voltage (V) SOC (Standard Operating Conditions ) - for more realistic figure for the possible power output from a PV Effect of different test conditions on the module I-V curve and peak power of a PV module.

Manufacturer specify Physical, Electrical, Environmental parameters Cells electrical characteristics

Nominal operating cell temperature (NOCT) It is the temperature reached by a cell in open circuited module under NOC conditions. It is used to give more realistic cell temperature of the module under operating conditions. NOCT lies usually between 42 to 50oC Module Temperature (Tmod )

Tmod

 NOCT  20   Tamb    * Pin 0.8  

Where Tamb - ambient temperature, Pin is the solar irradiation in kW/m2

I-V curve of a 75 Wp module The power output increase as the module voltage increases

6



Current

80

75 W

5

- and it drops as the voltage approaches to the open circuit voltage

4 3

40 Power

2

Power (W)

- it reaches to a peak (called the peak power )

Current (A)

60

20 1 0

0 0

5

10

15

20

25

Voltage (V)

The I-V and power curve of a 75 Wp module

The output power of a solar PV modules also depends on the temperature at which module is operating The current increase with temperature due to decrease in the band gap of Si.

Peak power decreases with increase in module temperature

3.5 3 Current in amps

The increased cell temperature results decrease in the open circuit voltage due to increase in reverse saturation current.

2.5

0 oC

2

25 oC

1.5

50 oC

1

75 oC

0.5 0 0

5

10

15

20

Voltage in volts

The effect of temperature on the I-V curve of solar PV module.

25

The current produced by a PV module is linear function of the radiation intensity

The power of a module decreases almost linearly with the decrease in intensity of solar radiation Solar irradiation available throughout the day is varying. Voltage of a module is logarithmic function of the radiation intensity, almost constant.

Current (A)

6 5

1000 W/m2

4

800 W/m2

3

600 W/m2

75W

  44W  28.7W  13.5W 59.5W

400 W/m2

2

200 W/m2

1 0 0

5

10

15

20

25

Voltage (V) Power output changes as radiation changes I-V curves and power variation of a 75 Wp solar PV module at 25oC as a function of variation in solar radiation intensity. Dotted line shows the maximum power point line.

• A photovoltaic system for supplying drinking water is installed in a village in Rajasthan as a part of the National Drinking water Mission. The water is pumped from a bore well, from depth of 48m. The solar cells are made from single crystal silicon and the array consists of 24 modules having following specifications Module Size

119.1 cm x 53.3 cm

Module weight

7.5 Kg

Cell Size

12.5 cm x 12.5 cm

Number of cells

36

Nominal Output

80 W

Nominal Voltage

12V

Maximum Voltage

17 V

Open Circuit Voltage

21.2 V

Short Circuit Current

4.9 A

Conversion Efficincy

12.5 %

• It is given that the inverter efficiency is 85% and the pump motor set efficiency is 45%. Calculate the water discharge rate at noon when the global radiation incident on the array is 945 W/m2. Answer Power output from array = Incident flux x cell area x conversion efficiency = 945 x ( 24x0.125x0.125x36) x 0.125 = 1594.7 Power available for lifting water = 1594.6 x 0.85x0.45 = 610 W Taking the density of water to be 996 kg/m3 , the water discharge rate = 610 /( 48 x 996x 9.81) = 0.0013005 m3/s = 4682 litres/ h



Photovoltaic effect



Working P-N junction



Types of solar cells



Characteristics of solar cells

ANY QUESTIONS