Transcript solar-cells

Solar Cells
Basic Concepts
Materials and Device Structures
Technical Issues
System Design
Modified Lecture based on the
Original Presentation by J. M. Pearce, 2006
http://www.appropedia.org/Solar_Photovoltaic_Open_Lectures
What are Photovoltaics?
• Photovoltaic (PV) systems convert light energy
directly into electricity.
• Commonly known as “solar cells.”
• The simplest systems power the small calculators
we use every day. More complicated systems will
provide a large portion of the electricity in the near
future.
• PV represents one of the most promising means of
maintaining our energy intensive standard of living
while not contributing to global warming and
pollution.
A Brief History
Photovoltaic Technology
•
•
•
•
1839 – Photovoltaic effect discovered by Becquerel.
1870s – Hertz developed solid selenium PV (2%).
1905 – Photoelectric effect explained by A. Einstein.
1930s – Light meters for photography commonly
employed cells of copper oxide or selenium.
• 1954 – Bell Laboratories developed the first crystalline
silicon cell (4%).
• 1958 – PV cells on the space satellite U.S. Vanguard
(better than expected).
Things Start
To Get Interesting...
• mid 1970s – World energy crisis = millions spent in research and
development of cheaper more efficient solar cells.
• 1976 – First amorphous silicon cell developed by Wronski and
Carlson.
• 1980’s - Steady progress towards higher efficiency and many new
types introduced
• 1990’s - Large scale production of solar cells more than 10%
efficient with the following materials:
– Ga-As and other III-V’s
– CuInSe2 and CdTe
– TiO2 Dye-sensitized
– Crystalline, Polycrystalline, and Amorphous Silicon
• Today prices continue to drop and new “3rd generation”
solar cells are researched.
Types of Solar
Photovoltaic Materials
Photovoltaic Materials
Solar PV Materials:
Crystalline & Polycrystalline Silicon
• Advantages:
– High Efficiency (14-22%)
– Established technology
(The leader)
– Stable
• Disadvantages:
– Expensive production
– Low absorption coefficient
– Large amount of highly
purified feedstock
Amorphous Silicon
Advantages:
• High absorption (don’t need a lot
of material)
• Established technology
• Ease of integration into buildings
• Excellent ecological balance sheet
• Cheaper than the glass, metal, or
plastic you deposit it on
Disadvantages:
• Only moderate stabilized efficiency 710%
• Instability- It degrades when light hits it
– Now degraded steady state
How do they work?
The physics view
p-n and p-i-n Junctions
Vbi
Ef
Vbi
Ef
Barrier Changes
• Equilibrium means there
is no net current
• Reduced barrier height is
called forward bias
(positive voltage applied to
p-side)
– Result- increases current
through diode
• Increased barrier height is
called reverse bias.
– Result- decreases current to
a very small amount..
Electric Currents in p-n
Junction Under External Bias
Diode I-V Characteristics
Current in a Solar Cell
• Output current = I = Il-Io [ exp(qV/kT)-1]
–
–
–
–
Il=light generated current
q = electric charge
V = voltage
k = Boltzman’s constant = 1.3807 × 10-23 J/K
• When in open circuit (I=0) all light generated current passes
through diode
• When in short circuit (V=0) all current passes through
external load
2 Important points:
1) During open circuit the voltage of open circuit,
Voc = (kT/q) ln( Il/Io +1)
2) No power is generated under short and open circuit - but
Pmax = VmIm=FFVocIsc
I-V Curve for Solar Cells
Fourth quadrant (i.e., power quadrant) of the illuminated I-V
characteristic defining fill factor (FF) and identifying Jsc and Voc
Light Absorption by a
Semiconductor
•
•
•
Photovoltaic energy relies on light.
Light → stream of photons → carries energy
Example: On a clear day 4.4x1017 photons hit 1 m2
of Earth’s surface every second.
• Eph()=hc/ =hf
– h = plank’s constant = 6.625 x 10-34 J-s
–  = wavelength
– c = speed of light =3 x 108 m/s
– f = frequency
• However, only photons with energy in excess of
bandgap can be converted into electricity by solar
cells.
The Solar Spectrum
The entire spectrum is
not available to single
junction solar cell
Generation of Electron Hole
Pairs with Light
• Photon enters, is absorbed,
and lets electron from VB
get sent up to CB
• Therefore a hole is left
behind in VB, creating
absorption process:
electron-hole pairs.
• Because of this, only part
of solar spectrum can be
converted.
• The photon flux converted
by a solar cell is about 2/3
of total flux.
Generation Current
• Generation Current = light induced electrons across bandgap
as electron current
• Electron current:= Ip=qNA
– N = # of photons in highlighted area of spectrum
– A = surface area of semiconductor that’s exposed to light
• Because there is current from light, voltage can also occur.
• Electric power can occur by separating the electrons and holes
to the terminals of device.
• Electrostatic energy of charges occurs after separation only if its
energy is less than the energy of the electron-hole pair in
semiconductor
• Therefore Vmax=Eg/q
• Vmax= bandgap of semiconductor is in EV’s, therefore this
equation shows that wide bandgap semiconductors produce
higher voltage.
Direct vs Indirect Bandgap
• Everything just talked about, where all energy
in excess of bandgap of photons are
absorbed, are called direct-bandgap
semiconductors.
• More complicated absorption process is the
indirect-gap series
– quantum of lattice vibrations, of crystalline
silicon, are used in the conversion of a photon
into electron-hole pair to conserve momentum
there hindering the process and decreasing the
absorption of light by semiconductor.
The Solar Cell
•
Electric current generated in semiconductor is extracted by
contacts to the front and rear of cell.
Widely spaced thin strips (fingers) are created so that light is
allowed through.
•
–
these fingers supply current to the larger bus bar.
• Antireflection
coating (ARC) is
used to cover the
cell to minimize
light reflection
from top surface.
• ARC is made
with thin layer of
dielectric material.
Different Types of
Photovoltaic Solar Cells
Diffusion
Drift
Excitonic
Diffusion
• n-type and p-type are
aligned by the Fermilevel
• When a photon comes in
n-type, it takes the place
of a hole, the hole acts
like an air bubble and
“floats” up to the p-type
• When the photon comes
to the p-type, it takes
place of an electron, the
electron acts like a steel
ball and “rolls” down to
the n-type
Drift
• There is an intrinsic
gap where the
photon is absorbed
in and causes the
electron hole pair to
form.
• The electron rises
up to the top and
drifts downwards
(to n-type)
• The hole drifts
upwards (to p-type)
Excitonic Solar Cell
• Dye molecule
– electron hole pair
splits because it
hits the dye
– the electron shifts
over to the electric
conductor and the
hole shifts to the
hole conductor
Power
Losses in
Solar Cells
Recombination
• Opposite of carrier generation, where
electron-hole pair is annihilated
• Most common at:
– impurities
– defects of crystal structure
– surface of semiconductor
• Reducing both voltage and current
Series Resistance
• Losses of resistance caused by
transmission of electric current produced
by the solar cell.
• I-V characteristic of device:
• I = Il-I0 [exp(qV+IRs / mkT) – 1]
• m= nonideality factor
Other Losses
• Current losses- called collection efficiency,
ratio b/w number of carriers generated by
light by number that reaches the junction.
• Temperature dependence of voltage
– V decreases as T increases
• Other losses
– light reflection from top surface
– shading of cell by top contacts
– incomplete absorption of light
Minimize Recombination Losses
by Adapting the Device
Tandem Cells
Silver Grid
Indium Tin Oxide
p-a-Si:H
Blue Cell
i-a-Si:H
n-a-Si:H
p
Green Cell
i-a-SiGe:H (~15%)
n
p
Red Cell
i-a-SiGe:H (~50%)
n
Textured Zinc Oxide
Silver
Stainless Steel Substrate
Schematic diagram of state-of-theart a-Si:H based substrate n-i-p
triple junction cell structure.
• Tandem cellseveral cells,
– Top cell has
large bandgap
– Middle cell mid
eV bandgap
– Bottom cell
small bandgap.
Examples of
Photovoltaic Systems
Three Types of Systems
• Stand-alone systems - those systems which
use photovoltaics technology only, and are
not connected to a utility grid.
• Hybrid systems - those systems which use
photovoltaics and some other form of energy,
such as diesel generation or wind.
• Grid-tied systems - those systems which are
connected to a utility grid.
Stand Alone PV System
• Water pumping
Examples of Stand Alone
PV Systems
• PV panel on a
water pump in
Thailand
• PV powers stock
water pumps in
remote locations
in Wyoming
Examples of Stand Alone
PV Systems
• Communications facilities can be powered by
solar technologies, even in remote, rugged
terrain. Also, if a natural or human-caused
disaster disables the utility grid, solar
technologies can maintain power to critical
operations
Examples of Stand Alone
PV Systems
• This exhibit, dubbed
"Solar Independence", is
a 4-kW system used for
mobile emergency power.
• while the workhorse
batteries that can store up
to 51 kW-hrs of
electricity are housed in a
portable trailer behind the
flag.
• The system is the largest
mobile power unit ever
built
Examples of Stand Alone
PV Systems
• Smiling child
stands in front
of Tibetan
home that uses
20 W PV panel
for electricity
• PV panel on
rooftop of
rural residence
Hybrid PV System
Examples of Hybrid
PV Systems
• Ranching the
Sun project in
Hawaii
generates 175
kW of PVpower
and 50 kW of
wind power
from the five
Bergey 10 kW
wind turbines
Examples of Hybrid
PV Systems
• A fleet of small
turbines; PV
panels in the
foreground
Examples of Hybrid
PV Systems
• PV / diesel hybrid
power system - 12
kW PV array, 20 kW
diesel genset
• This system serves as
the master site for
the "top gun"
Tactical Air Combat
Training System
(TACTS) on the U.S.
Navy's Fallon Range.
Grid-Tied PV System
Examples of Grid Tied
Systems
• National Center
for Appropriate
Technology
Headquarters
Examples of Grid Tied
Systems
• The
world's
largest
residential
PV
project
Designing a PV System
1.
Determine the load (energy, not power)
•
2.
3.
4.
You should think of the load as being supplied by the stored energy
device, usually the battery, and of the photovoltaic system as a
battery charger. Initial steps in the process include:
Calculating the battery size, if one is needed
Calculate the number of photovoltaic modules required
Assessing the need for any back-up energy of flexibility for
load growth
Stand-Alone Photovoltaic Systems: A Handbook of
Recommended Design Practices details the design of
complete photovoltaic systems.
Determining Your Load
• The appliances and devices (TV's, computers, lights,
water pumps etc.) that consume electrical power are
called loads.
• Important : examine your power consumption and
reduce your power needs as much as possible.
• Make a list of the appliances and/or loads you are
going to run from your solar electric system.
• Find out how much power each item consumes
while operating.
– Most appliances have a label on the back which lists the
Wattage.
– Specification sheets, local appliance dealers, and the
product manufacturers are other sources of information.
Determining your Loads II
• Calculate your AC loads (and DC if necessary)
• List all AC loads, wattage and hours of use
per week (Hrs/Wk).
• Multiply Watts by Hrs/Wk to get Watt-hours
per week (WH/Wk).
• Add all the watt hours per week to determine
AC Watt Hours Per Week.
• Divide by 1000 to get kW-hrs/week
Determining the Batteries
• Decide how much storage you would like your battery bank
to provide (you may need 0 if grid tied)
– expressed as "days of autonomy" because it is based on the number
of days you expect your system to provide power without receiving an
input charge from the solar panels or the grid.
•
Also consider usage pattern and critical nature of your
application.
• If you are installing a system for a weekend home, you might
want to consider a larger battery bank because your system
will have all week to charge and store energy.
• Alternatively, if you are adding a solar panel array as a
supplement to a generator based system, your battery bank
can be slightly undersized since the generator can be operated
in needed for recharging.
Batteries II
•
Once you have determined your storage
capacity, you are ready to consider the
following key parameters:
–
•
Amp hours, temperature multiplier, battery size
and number
To get Amp hours you need:
1.
2.
daily Amp hours
number of days of storage capacity
( typically 5 days no input )
–
1 x 2 = A-hrs needed
–
Note: For grid tied – inverter losses
Temperature Multiplier
Temp oF
80 F
70 F
60 F
50 F
40 F
30 F
20 F
Temp oC
26.7 C
21.2 C
15.6 C
10.0 C
4.4 C
-1.1 C
-6.7 C
Multiplier
1.00
1.04
1.11
1.19
1.30
1.40
1.59
Select the closest multiplier for the average ambient winter
temperature your batteries will experience.
Determining Battery Size
• Determine the discharge limit for the batteries
( between 0.2 - 0.8 )
– Deep-cycle lead acid batteries should never be completely
discharged, an acceptable discharge average is 50% or a
discharge limit of 0.5
• Divide A-hrs/week by discharge limit and multiply
by “temperature multiplier”
• Then determine A-hrs of battery and # of batteries
needed - Round off to the next highest number.
– This is the number of batteries wired in parallel needed.
Total Number of Batteries
Wired in Series
• Divide system voltage ( typically 12, 24
or 48 ) by battery voltage.
– This is the number of batteries wired in
series needed.
• Multiply the number of batteries in
parallel by the number in series –
• This is the total number of batteries
needed.
Determining the Number of
PV Modules
• First find the Solar Irradiance in your area
• Irradiance is the amount of solar power
striking a given area and is a measure of the
intensity of the sunshine.
• PV engineers use units of Watts (or kiloWatts)
per square meter (W/m2) for irradiance.
• For detailed Solar Radiation data available for
your area in the US:
http://rredc.nrel.gov/solar/old_data/nsrdb/
How Much Solar Irradiance
Do You Get?
Solar Radiation
• On any given day the solar radiation varies
continuously from sunup to sundown and
depends on cloud cover, sun position and
content and turbidity of the atmosphere.
• The maximum irradiance is available at solar
noon which is defined as the midpoint, in time,
between sunrise and sunset.
• Insolation (now commonly referred as
irradiation) differs from irradiance because of the
inclusion of time. Insolation is the amount of
solar energy received on a given area over time
measured in kilowatt-hours per square meter
squared (kW-hrs/m2) - this value is equivalent to
"peak sun hours".
Peak Sun Hours
• Peak sun hours is defined as
the equivalent number of hours
per day, with solar irradiance
equaling 1,000 W/m2, that gives
the same energy received from
sunrise to sundown.
• Peak sun hours only make sense
because PV panel power output
is rated with a radiation level of
1,000W/m2.
• Many tables of solar data are
often presented as an average
daily value of peak sun hours
(kW-hrs/m2) for each month.
Calculating Energy Output of
a PV Array
• Determine total A-hrs/day and increase by 20% for
battery losses then divide by “1 sun hours” to get
total Amps needed for array
• Then divide your Amps by the Peak Amps produced
by your solar module
– You can determine peak amperage if you divide the
module's wattage by the peak power point voltage
• Determine the number of modules in each series
string needed to supply necessary DC battery
Voltage
• Then multiply the number (for A and for V)
together to get the amount of power you need
– P=IV [W]=[A]x[V]
Charge Controller
• Charge controllers are included in most PV systems
to protect the batteries from overcharge and/or
excessive discharge.
• The minimum function of the controller is to
disconnect the array when the battery is fully charged
and keep the battery fully charged without damage.
• The charging routine is not the same for all batteries:
a charge controller designed for lead-acid batteries
should not be used to control NiCd batteries.
• Size by determining total Amp max for your array
Wiring
• Selecting the correct size and type of wire
will enhance the performance and reliability
of your PV system.
• The size of the wire must be large enough to
carry the maximum current expected without
undue voltage losses.
• All wire has a certain amount of resistance to
the flow of current.
• This resistance causes a drop in the voltage
from the source to the load. Voltage drops
cause inefficiencies, especially in low voltage
systems ( 12V or less ).
• See wire size charts here:
www.solarexpert.com/Photowiring.html
Inverters
• For AC grid-tied systems you
do not need a battery or charge
controller if you do not need
back up power –just the
inverter.
• The Inverter changes the DC
current stored in the batteries
or directly from your PV into
usable AC current.
– To size increase the Watts
expected to be used by your AC
loads running simultaneously by
20%
Books for Designing
a PV System
• Steven J. Strong and
William G. Scheller, The
Solar Electric House: Energy
for the EnvironmentallyResponsive, Energy-Independent
Home, by Chelsea Green
Pub Co; 2nd edition, 1994.
• This book will help with
the initial design and
contacting a certified
installer.
Books for the DIYer
• If you want to do everything
yourself also consider these
resources:
– Richard J. Komp, and John
Perlin, Practical
Photovoltaics: Electricity from
Solar Cells, Aatec Pub., 3.1
edition, 2002. (A layman’s
treatment).
– Roger Messenger and Jerry
Ventre, Photovoltaic Systems
Engineering, CRC Press, 1999.
(Comprehensive specialized
engineering of PV systems).
Photovoltaics Design and
Installation Manual
• Photovoltaics: Design &
Installation Manual by SEI
Solar Energy International, 2004
• A manual on how to design,
install and maintain a photovoltaic
(PV) system.
• This manual offers an overview of
photovoltaic electricity, and a
detailed description of PV system
components, including PV
modules, batteries, controllers and
inverters. Electrical loads are also
addressed, including lighting
systems, refrigeration, water
pumping, tools and appliances.
Solar Photovoltaics
is the Future