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Part B8: Photovoltaic power

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B8 Photovoltaic power

• www.scolar.org.uk • www.eere.energy.gov/solar/

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B8.1 Photovoltaic powerPhotovoltaic cells

Valence electrons

Tabs

Grid lines

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B8.1 Photovoltaic powerPhotovoltaic cells: Cross section

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B8.1 Photovoltaic powerPhotovoltaic cells: Parts

• Front contact (tabs and grid lines)– Collects current generated by the cell – negative

contact– Tab material generally copper with tin coating– Large tabbing – loses area: Small tabbing has

greater resistance• Anti reflective coating (~150 nm thick)

– Stops silicon reflecting ~1/3 of the light (reduces this to 5% - texturing reduces this to <2%) silicon monoxide is a common coating

• Texturing– Pyramids and cones, chemically etched onto the

surface

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B8.1 Photovoltaic powerPhotovoltaic cells: Parts

• n-type silicon (~300nm thick)– silicon doped with phosphorous forming the

negative side of the cell• p-n junction

– Where n and p type silicon meet– Sometimes called the depletion zone

• p-type silicon (250,000 nm thick)– silicon doped with boron forming the positive

side of the cell• Back contact

– The positive contact

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: the silicon atom

Valence electrons

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: atomic structure

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: doping

• Pure silicon is stable – electrons moved will just move to the next hole

• Introducing other materials into the silicon can create a net electric charge on the material – the process is called doping

• The most common dopants are:– Boron

• valence 3 – makes a positive charge• Doping is about 1 Boron to 10,000,000 silicon

– Phosphorous • valence 5 – makes a negative charge• Doping is about 1 Phosphorus to 1,000 silicon

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: doping

Boron doping Phosphorus doping

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: n&p layers

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: Band gap energy• The energy needed to release a valence

electron– Photons with an equivalent energy to the

bandgap energy displace electrons– Photons with higher energy displace electrons

and create heat– Photons with lower energy either pass through

the material or just heat it up slightly• Measured in electron Volts

– energy gained by an electron when it passes through a potential of 1 volt in a vacuum

• Determine the open circuit voltage of the cell– High band-gap = high voltage

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: Photon energy

10-9 10-8 10-7 10-6 10-5 10-4

1017 1016 1015 1014 1013 1012

103 102 101 100 10-1 10-2

Wavelength(m)

Frequency(Hz)

Photon Energy(eV)

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B8.2 Photovoltaic powerPhotovoltaic cells: How they work: Band gap energyMaterial Band gap

energy (eV)

at 25ºC

Crystal silicon 1.12

Amorphous silicon 1.75

Cadmium telluride 1.44

Gallium arsenide 1.43

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: “Standard” conditionsTemperature 25ºC

Insolation (Irradiance) 1000 W/m2

Air mass AM1.5

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: “Standard” conditions

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BeamradiationAir

mass1.0

Air mass1.5

B5.1 System designI rradiance: the atmosphere

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= efficiencyP= Power out (W)P= Power in (W)V= Voltage (V)I= Current (A)Gt= Irradiance on the surface

(W)A= Cell area (m2)

B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: Efficiency

out

in t

P VIP G A

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: measures• Peak power (Pmax)

• Open circuit voltage(Voc)

• Max power voltage(Vmp)

• Short circuit current (Isc)

• Max power current(Imp)

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve

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FF = Fill factorPmax= Maximum power out (W)Voc = Open circuit Voltage(V)Ics= Short circuit current (A)

B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve: Fill factor

max

oc sc

PFF

V I

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve: Fill factor

FF = 0.45

FF = 0.75

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve: Effect of temperature

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve: Effect of temperature• Power output falls as temperature

increases• Voltage falls ~0.0023V per ºC • Current rises (but only a bit – you can

really ignore it)

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve: Effect of insolation

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B8.3 Photovoltaic powerPhotovoltaic cells: Specifications: IV curve: Effect of insolation• Power output increases as insolation

increases• Voltage has a slight increase and can

be ignored• Current rises significantly in direct

proportion to insolation

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B8.4 Photovoltaic powerPhotovoltaic cells: Arrays

Cell Module Array

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B8.4 Photovoltaic powerPhotovoltaic cells: Arrays: Cells in parallel

Voltage from A to B = 0.5VCurrent through A = B = 3A

A

B

Voltage fro A to B = 0.5VCurrent through A = B = 6A

B

A

B

AVoltage fro A to B = 0.5VCurrent through A = B = 9A

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B8.4 Photovoltaic powerPhotovoltaic cells: Arrays: Cells in parallel

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B8.4 Photovoltaic powerPhotovoltaic cells: Arrays: Cells in series

Voltage from A to B = 0.5VCurrent through A = B = 3A

Voltage fro A to B = 1.0VCurrent through A = B = 3A

Voltage fro A to B = 1.5VCurrent through A = B = 3A

A B

A B

A B

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B8.4 Photovoltaic powerPhotovoltaic cells: Arrays: Cells in series

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B8.5 Photovoltaic powerPhotovoltaic cells: manufacturing process

• Crystal growing– Czochralski – Float zone– Ingot casting

• Sawing (wastes 20% of crystal)• Doping• Coating and contacts

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B8.5 Photovoltaic powerPhotovoltaic cells: encapsulation

Cover film

Solar cellEncapsulant

SubstrateCover film

SealGasket

Frame

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B8.5 Photovoltaic powerPhotovoltaic cells: encapsulation

• Electrical resistivity• Light transmission• Heat conduction• Thermal expansion• Durability• Weight (?)• Cost

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Single crystal silicon

Thickness 200 – 300m

Band gap 1.12 eV

Lab efficiency 24%

• Commercial production. Companies include: BP Solar, Siemens Solar Industries, University of New South Wales

• Main processes are Czochralski and Float zone• Make up the bulk of the PV market (>60%)• Most efficient (and most expensive) silicon cells

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Single crystal silicon

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• Commercial production. Companies include: Kyocera, Solarex (now BP Solarex)

• Main process is ingot casting • Make up a large part of the PV market (>30%)• Less expensive (and less efficient) than single

crystal cells – electrons and holes can recombine at crystal edges

B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Polycrystalline silicon

Thickness 200 – 300m

Band gap 1.12 eV

Lab efficiency 17.8%

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Polycrystalline silicon

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Amorphous silicon

Thickness 1-2m Band gap 1.75 eVLab efficiency 13% cells but 7-9%

for stable modules as they degrade during

the first month

• Commercial production. • Make up a significant part of the PV market (~4%) mainly

for low power applications such as watches and calculators• No crystal structure but hydrogen reduces recombination• High absorptivity (40 X single crystal)• p-i-n construction – intrinsic middle layer forms bulk of

material between doped layers• Cheap but slow production process

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Amorphous silicon

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: silicon: Amorphous silicon

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B8.6 Photovoltaic powerPhotovoltaic cells: Classification

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Polycrystalline thin films• Cheap and flexible manufacture

– series of vapour depositions – no crystal growth– Can be applied to cheap substrates– Can be applied to any surface shape

• Easily scaled from lab to manufacture• Tend to use“hetero-junctions” instead of

oppositely doped layers . The top layer has a very wide bandgap (>2.8eV) so is transparent

• Uses a thin layer of a transparent conducting oxide, such as tin oxide rather than a conducting grid

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Polycrystalline thin films

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Polycrystalline thin films

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Polycrystalline thin films

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Polycrystalline thin films: Copper Indium Diselenide (DIS)

Thickness 1-2m

Band gap 1.0 eV

Lab efficiency 17.1% cells but 11% for modules

• Commercial production. Companies include: Energy PV, International Solar Electric Technologies, Martin Marietta, Seimens Solar Industries, Solarex

• No degradation problem• Very high absorptivity (99%)

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Polycrystalline thin films: Cadmium Telluride (CdTe)

Thickness 1-2m

Band gap 1.44 eV

Lab efficiency 15.8% cells but 10.5% for modules

• Commercial production. Companies include: BP Solar, Golden Photon Inc., Matusushita

• Good bandgap• P-type highly resistive – tends to be intrinsic in an n-

i-p structure with p-type layer behind (such as zinc telluride (ZnTe)

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: Single crystalline thin films: Gallium arsenide (GaAs)

Thickness 1-2m

Band gap 1.43 eV

Lab efficiency 25.1% cells but 20% in commercial

production

• Gallium is a by-product of the smelting of other metals, notably aluminium and zinc - it is rarer than gold

• Very absorptive and with an ideal band gap• Not sensitive to heat and resistant to radiation damage• Alloys well so can give precise control over generation of

electrons and holes so efficiencies can approach theoretical limits

• Very expensive so used in critical applications such as spacecraft and concentrator systems

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: augmented: Multijunction cells• A stack (or cascade) of different cells with

different bandgaps – Highest bandgap top lowest bottom

• Photons not absorbed by top cell passed on to next

• Lab efficiencies of >35% possible (using gallium arsenide)

• Amorphous silicon can be good for the top cell and copper indium diselenide good for the bottom cell

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: augmented: Multijunction cells

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: augmented: Concentration• Highly efficient (but expensive) cells enclosed

in a light focusing device• Cell area is reduced per unit output due to

higher insolation levels• Usually parabolic concentrators• Needs tracking (not unlike concentrating solar-

thermal collectors)

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B8.6 Photovoltaic powerPhotovoltaic cells: Cell types: augmented: Concentration

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B8.7 Photovoltaic powerPhotovoltaic systems: Considerations

• Is the system going to be Standalone or Grid Connected?

• Are you going to use standard AC or will DC do?

• Are you using the system at night?• Energy audit

– How much energy do you use on a day to day basis?

– How much sun do you get at your location?

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B8.7 Photovoltaic powerPhotovoltaic systems: Storage

• Why?– Power is unavailable at night and may be

unreliable from hour-to-hour– Peak loads may be larger than panel power

• Reliability cost trade-off • Usually Deep cycle lead acid batteries

– Cycle efficiency 75-80%• In development

– Flywheels– Fuel cells– Super capacitors

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B8.7 Photovoltaic powerPhotovoltaic systems: Components

• Cell array• Regulator & shunt load• Storage

– Batteries– Flywheel

• DC distribution• Inverter (?)• AC distribution• Grid connection• Back-up generator

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B8.7 Photovoltaic powerPhotovoltaic systems: Components

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B8.7 Photovoltaic powerPhotovoltaic systems: Storage

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B8.7 Photovoltaic powerPhotovoltaic systems: Storage

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B8.7 Photovoltaic powerPhotovoltaic systems: Energy Audit

Appliance Amps (at

12V)

Hours /day

Amp hours/

dayFridge 10 12 120

Lights (8 x 13W) 9 5 45

….

….

….

Television 2 3 6

Total 500

Battery pack is usually 3-5 days power

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B8.7 Photovoltaic powerPhotovoltaic systems: Converters

• Converts power from one form to another– Voltage conversion

• DC-DC converter– DC-AC conversion

• Inverter

• Maximise array output– Peak power tracker– Dump load

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B8.7 Photovoltaic powerPhotovoltaic systems: Components: Peak power tracker

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid

• Keep it simple– Complexity lowers reliability and increases

maintenance cost. • Understand system availability

– Achieving 99+% availability with any energy system is expensive.

• Be thorough, but realistic, when estimating the load – A 25% safety factor can cost you a great deal of

money. • Cross-check weather sources

– Errors in solar resource estimates can cause disappointing system performance.

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid

• Make each connection as if it had to last 30 years – it does.

• Safety • Local and national building and electrical

codes. • Periodic maintenance• Life-cycle cost (LCC) to compare PV systems to

alternatives– LCC reflects the complete cost of owning and

operating any energy system.

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B8.7 Photovoltaic powerPhotovoltaic systems: On grid

• Grid can act as large battery (so no storage needed)– Sell to the grid (at wholesale rates)– Net metering

• Power must be delivered to the grid at the right frequency and in phase with the grid

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B8.7 Photovoltaic powerPhotovoltaic systems: On grid: Grid tied inverter

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B8.7 Photovoltaic powerPhotovoltaic systems: System costs

Component Cost

Single crystal silicon panels (23m2 - 3kW generating 0.5kWh/day*) £17,000

Regulator £75

30 x 12V deep-cycle batteries £1,500

Inverter (2000W modified sine) £300

£18,875

* British Photovoltaic association quotes ~ 750kWh/year/kW installed0.5kW/h/day = 2,190 kWh/year

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid system

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid system

• 24 75W panels• House wired for 24 V• Mounted south facing at 63º (15º more than

the latitude – better for winter sun but loses about 10% of summer insolation)

• 1400 Ah of storage in 6V batteries• Peak power tracker• Two inverters –large modified square wave and

small sine wave• Micro-hydro back-up

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid system

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid system

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid system

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B8.7 Photovoltaic powerPhotovoltaic systems: Off grid system

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B8.7 Photovoltaic powerPhotovoltaic systems: On grid system

Area covered 532 m2

Total installed power 73 kWp

Energy consumption 150,000 kWh

Energy generated 55,000 kWh

Total cost £900,000

www.portalenergy.com/caddet/retb/no162.pdf

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B8.7 Photovoltaic powerPhotovoltaic systems: On grid system

Area covered 26.5 m2

Total installed power 1.6 kWp

Energy consumption 1,042 kWh

Energy generated 1,189 kWh

Exported to grid 850 kWh

Rate paid 4p/kWh

Total cost £13,600

www.solarcentury.co.uk

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B8.7 Photovoltaic powerPhotovoltaic systems: Other applications

• Watches and calculators– Low power / low voltage – can operate even

under artificial light

• Satellites– Not a lot else out there – most initial

research was concentrated on space applications

• Stand-alone appliances– Path lights, street lamps (on remote

corners), train signals– No need for wiring which can offset cell cost

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B8.7 Photovoltaic powerPhotovoltaic systems: Other applications: Lighting

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B8.7 Photovoltaic powerPhotovoltaic systems: Other applications: Water pumping

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B8.7 Photovoltaic powerPhotovoltaic systems: Other applications: Grid augmentation

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B8.7 Photovoltaic powerPhotovoltaic systems: Other applications: Grid augmentation

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B8.7 Photovoltaic powerPhotovoltaic systems: Trends: Prices

Price /Wp ~ €27 in 1983, Cost of electricity generated is about 20 p/kWh in sunny climes (44 p/kWh in temperate)

5.2

5.4

5.6

5.8

6

6.2

6.4

Apr-01 Oct-01 Apr-02 Oct-02 Apr-03 Oct-03 Apr-04 Oct-04

Pric

e pe

r Wp

(€)

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B8.7 Photovoltaic powerPhotovoltaic systems: DTi photovoltaic demonstration programme March 2002

• 4 years, £20MM• Activities include

– Accrediting photovoltaic installers – Marketing to a wide range of customer types to

raise awareness of PV technology and its benefits

– Evolving strict approval guidelines to ensure grant funded applications offer the best performance possible

– Publishing a list of approved photovoltaic modules and inverters to ensure quality products

http://www.est.org.uk/solar/

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B8.7 Photovoltaic powerPhotovoltaic systems: DTi photovoltaic demonstration programme: Grants

• Small scale (0.5kWp - 5kWp)– Bolt-on systems £2,800/kWp or 50%. – Integrated systems £4,000/kWp or 50%– Grants are targeted at:

• Home owners • Small to medium sized enterprises (SMEs) • Public sector organisations (e.g. schools, local

authorities) • Charitable, Voluntary, Community groups

• Medium/large scale(<100kWp)– Grants of up to

• 50% of eligible costs to public bodies• 50% of eligible costs for small to medium sized

enterprises (SMEs) • 40% of eligible costs for large scale organisations

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B8.7 Photovoltaic powerPhotovoltaic systems: Trends: market size

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B8.7 Photovoltaic powerPhotovoltaic systems: Trends: market size

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B8.7 Photovoltaic powerPhotovoltaic systems: Trends: The UK PV industry• Worth £59 million• ~ 10% of world market• But only employs ~400 people

– Research and development (not including companies): 66

– Manufacturing of PV system components, including company R&D: 171

– All other, including within electricity companies, installation companies etc.: 166

• Most UK companies manufacture in Spain or Australia

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