Solar Energy: The Ultimate Renewable

Resource

Xiangfeng Duan

What is Solar Energy?What is Solar Energy? Originates with the Originates with the

thermonuclear fusion thermonuclear fusion reactions occurring in reactions occurring in the sun.the sun.

Represents the entire Represents the entire electromagnetic electromagnetic radiation (visible radiation (visible light, infrared, light, infrared, ultraviolet, x-rays, ultraviolet, x-rays, and radio waves).and radio waves).

Radiant energy from Radiant energy from the sun has powered the sun has powered life on Earth for many life on Earth for many millions of years.millions of years.

Advantages and Advantages and DisadvantagesDisadvantages

AdvantagesAdvantages All chemical and radioactive polluting byproducts of All chemical and radioactive polluting byproducts of

the thermonuclear reactions remain behind on the the thermonuclear reactions remain behind on the sun, while only pure radiant energy reaches the sun, while only pure radiant energy reaches the Earth.Earth.

Energy reaching the earth is incredible. By one Energy reaching the earth is incredible. By one calculation, 30 days of sunshine striking the Earth calculation, 30 days of sunshine striking the Earth have the energy equivalent of the total of all the have the energy equivalent of the total of all the planet’s fossil fuels, both used and unused!planet’s fossil fuels, both used and unused!

DisadvantagesDisadvantages Sun does not shine consistently.Sun does not shine consistently. Solar energy is a diffuse source. To harness it, we Solar energy is a diffuse source. To harness it, we

must concentrate it into an amount and form that we must concentrate it into an amount and form that we can use, such as heat and electricity. can use, such as heat and electricity.

Addressed by approaching the problem through: Addressed by approaching the problem through:

1) collection, 2) conversion, 3) storage.1) collection, 2) conversion, 3) storage.

Solar Energy to Solar Energy to Heat Living Heat Living SpacesSpaces

Proper design of a building is for it to act as a solar Proper design of a building is for it to act as a solar collector and storage unit. This is achieved through collector and storage unit. This is achieved through three elements: insulation, collection, and storage.three elements: insulation, collection, and storage.

Solar Energy to Heat WaterSolar Energy to Heat Water

A flat-plate collector is A flat-plate collector is used to absorb the used to absorb the sun’s energy to heat sun’s energy to heat the water.the water.

The water circulates The water circulates throughout the closed throughout the closed system due to system due to convection currents.convection currents.

Tanks of hot water are Tanks of hot water are used as storage. used as storage.

PhotovoltaicsPhotovoltaicsPhotoPhoto++voltaicvoltaic = convert = convert lightlight to to electricityelectricity

Solar Cells BackgroundSolar Cells Background

1839 - French physicist A. E. Becquerel first recognized 1839 - French physicist A. E. Becquerel first recognized the photovoltaic effect.the photovoltaic effect.

1883 - first solar cell built, by Charles Fritts, coated 1883 - first solar cell built, by Charles Fritts, coated semiconductor selenium with an extremely thin layer of semiconductor selenium with an extremely thin layer of gold to form the junctions. gold to form the junctions.

1954 - Bell Laboratories, experimenting with 1954 - Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. Daryl with certain impurities was very sensitive to light. Daryl Chapin, Calvin Fuller and Gerald Pearson, invented the Chapin, Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful first practical device for converting sunlight into useful electrical power. Resulted in the production of the first electrical power. Resulted in the production of the first practical solar cells with a sunlight energy conversion practical solar cells with a sunlight energy conversion efficiency of around 6%.efficiency of around 6%.

1958 - First spacecraft to use solar panels was US 1958 - First spacecraft to use solar panels was US satellite Vanguard 1satellite Vanguard 1

http://en.wikipedia.org/wiki/Solar_cell

Driven by Space Applications in Driven by Space Applications in Early DaysEarly Days

The heart of a photovoltaic system is a solid-state The heart of a photovoltaic system is a solid-state device called a solar cell. device called a solar cell.

How does it workHow does it work

Energy Band Formation in SolidEnergy Band Formation in Solid

Each isolated atom has discrete energy level, with two electrons of opposite spin occupying a state.

When atoms are brought into close contact, these energy levels split.

If there are a large number of atoms, the discrete energy levels form a “continuous” band.

Energy Band Diagram of a Conductor, Energy Band Diagram of a Conductor, Semiconductor, and InsulatorSemiconductor, and Insulator

a conductor a semiconductor an insulator

Semiconductor is interest because their conductivity can be readily modulated (by impurity doping or electrical potential), offering a pathway to control electronic circuits.

SiliconSilicon

Si

Si

Si

Si

Si-

Si Si Si

Si

SiSi

Si

Si

Si

Shared electrons

Silicon is group IV element – with 4 electrons in their valence shell.

When silicon atoms are brought together, each atom forms covalent bond with 4 silicon atoms in a tetrahedron geometry.

Intrinsic SemiconductorIntrinsic Semiconductor At 0 ºK, each electron is in its lowest energy state so each covalent bond position is filled. If a small electric field is applied to the material, no electrons will move because they are bound to their individual atoms.

=> At 0 ºK, silicon is an insulator.

As temperature increases, the valence electrons gain thermal energy. If a valence electron gains enough energy (Eg), it may break its covalent bond and move away from its original position. This electron is free to move within the crystal.

Conductor Eg <0.1eV, many electrons can be thermally excited at room temperature.

Semiconductor Eg ~1eV, a few electrons can be excited (e.g. 1/billion)

Insulator, Eg >3-5eV, essentially no electron can be thermally excited at room temperature.

Extrinsic Semiconductor, n-type Extrinsic Semiconductor, n-type Doping Doping

Electron

-

Si Si Si

Si

SiSi

Si

Si

As

Extra

Valence band, Ev

Eg = 1.1 eV

Conducting band, Ec

Ed ~ 0.05 eV

Doping silicon lattice with group V elements can creates extra electrons in the conduction band — negative charge carriers (n-type), As- donor.

Doping concentration #/cm3 (1016/cm3 ~ 1/million).

Valence band, Ev

Eg = 1.1 eV

Conducting band, Ec

Ea ~ 0.05 eV

Electron

-

Si Si Si

Si

SiSi

Si

Si

B

Hole

Doping silicon with group III elements can creates empty holes in the conduction band — positive charge carriers (p-type), B-(acceptor).

Extrinsic Semiconductor, p-type Extrinsic Semiconductor, p-type doping doping

V

I

R O F

p n

p n

V>0 V<0

Reverse bias Forward bias

p-n Junction (p-n diode)p-n Junction (p-n diode)

A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact. In p-n junction, the current is only allowed to flow along one direction from p-type to n-type materials.

i

p n

V<0 V>0

depletion layer

- +

Solar Cells

Light-emitting Diodes

Diode Lasers

Photodetectors

Transistors

p-n Junction (p-n diode)p-n Junction (p-n diode)

A p-n junction is the basic device component for many functional electronic devices listed above.

How Solar Cells Work How Solar Cells Work

Photons in sunlight hit the solar panel and are absorbed by semiconducting materials to create electron hole pairs.

Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity.

p n

- +- +

- +

- +

- +hv > Eg

The Impact of Band Gap on The Impact of Band Gap on EfficiencyEfficiency

Efficiency, Efficiency, = (V = (VococIIscscFF)/PFF)/Pin in VVoc oc E Egg, I, Isc sc # of absorbed photons # of absorbed photons

Decrease EDecrease Egg, absorb more of the spectrum, absorb more of the spectrum But not without sacrificing output voltageBut not without sacrificing output voltage

hv > Eg

-30

-20

-10

0

10

20

30

Cu

rren

t D

ensi

ty (

mA

/cm

2 )

1.00.80.60.40.20.0

Voltage (volts)

Jsc

Voc

FF

Dark

Light

Jmp

Vmp

Fill Factor, FF = (VmpImp)/VocIsc

Efficiency, = (VocIscFF)/Pin

Cost vs. Efficiency Cost vs. Efficiency TradeoffTradeoffEfficiency 1/2

Long dHigh High Cost

d

Long dLow Lower Cost

d

decreases as grain size (and cost) decreases

Large GrainSingleCrystals

Small Grainand/orPolycrystallineSolids

Cost/Efficiency of Photovoltaic Cost/Efficiency of Photovoltaic TechnologyTechnology

Costs are modules per peak W; $0.35-$1.5/kW-hr

89.6% of 2007 Production 89.6% of 2007 Production 45.2% Single Crystal Si45.2% Single Crystal Si

42.2% Multi-crystal SI42.2% Multi-crystal SI

Limit efficiency 31%Limit efficiency 31% Single crystal silicon - 16-19% Single crystal silicon - 16-19%

efficiencyefficiency Multi-crystal silicon - 14-15% Multi-crystal silicon - 14-15%

efficiencyefficiency Best efficiency by SunPower Inc 22%Best efficiency by SunPower Inc 22%

Silicon Cell Average Efficiency

First Generation First Generation – – Single Junction Silicon CellsSingle Junction Silicon Cells

CdTe 4.7% & CIGS 0.5% of 2007 CdTe 4.7% & CIGS 0.5% of 2007 ProductionProduction

New materials and processes to New materials and processes to improve efficiency and reduce cost.improve efficiency and reduce cost.

Thin film cells use about 1% of the Thin film cells use about 1% of the expensive semiconductors compared to expensive semiconductors compared to First Generation cells.First Generation cells.

CdTe – 8 – 11% efficiency (18% CdTe – 8 – 11% efficiency (18% demonstrated) demonstrated)

CIGS – 7-11% efficiency (20% CIGS – 7-11% efficiency (20% demonstrated)demonstrated)

Second Generation Second Generation – – Thin Film CellsThin Film Cells

Enhance poor electrical performance while maintaining Enhance poor electrical performance while maintaining very low production costs.very low production costs.

Current research is targeting Current research is targeting conversion efficiencies of 30-conversion efficiencies of 30-60%60% while retaining low cost materials and manufacturing while retaining low cost materials and manufacturing techniques. techniques.

Multi-junction cells – 30% efficiency (40-43% demonstrated)Multi-junction cells – 30% efficiency (40-43% demonstrated)

Third GenerationThird Generation – – Multi-junction CellsMulti-junction Cells

Solar Home Systems

Space

Water Pumping

Telecom

Main Application Areas – Off-grid

Residential HomeSystems (2-8 kW)

PV Power Plants( > 100 kW)

Commercial BuildingSystems (50 kW)

Main Application AreasGrid Connected

Future Energy MixFuture Energy Mix

Renewable Energy Consumption Renewable Energy Consumption in the US Energy Supply, 2007in the US Energy Supply, 2007

http://www.eia.doe.gov/cneaf/solar.renewables/page/trends/highlight1.html

Top 10 PV Cell ProducersTop 10 PV Cell Producers

Top 10 produce 53% of world total

Q-Cells, SolarWorld - Germany

Sharp, Kyocera, Sharp, Sanyo – Japan

Suntech, Yingli, JA Solar – China

Motech - Taiwan

(in the U.S. in 2002)

1-4 ¢

2.3-5.0 ¢ 6-8 ¢

5-7 ¢

Production Cost of ElectricityProduction Cost of Electricity

0

5

10

15

20

25

Coal Gas Oil Wind Nuclear Solar

Cost6-7 ¢

25-50 ¢

Cos

t , ¢

/kW

-hr

Future Generation Future Generation – – Printable CellsPrintable Cells

Organic CellNanostructured Cell

Solution Processible Semiconductor

Organic Photovoltaics Convert Organic Photovoltaics Convert Sunlight into Electrical Power.Sunlight into Electrical Power.

n

Trans-polyacetylene (t-PA)

Sn

Polythiophene (PT)

N

Hn

Polypyrrole (PPY)

Nanotechnology Solar Cell Design

Interpenetrating Nanostructured Networks

-

metal electrode

transparent electrode

glass

+- 100 nm

---

metal electrode

transparent electrode

glass

+- 100 nm

Dye Sensitized Solar Cell