The Physics of Photovoltaics: An
Analysis of Solar Panel Cost,
Effectiveness and Efficiency
Group 16: Angel Needham-Gilles,
Nico Mongillo and Nick Pauley
Project Overview
As a group we decided to go the analytical route.
The purpose of our project was to investigate the
physics of solar panels including their production
and how they convert light to energy. Also
investigated are their efficiency and efficacy as
long-term “green” solutions as well as some of
the political implications of their use.
The Raw Materials Required for Solar
Panels
•
There are several varieties of cells today that can be used to absorb sunlight and
convert those photons into electrical energy. These include:
–
–
–
–
•
CIS (Copper Indium Diselenide) Cells
CdTe cells (Cadmium Telluride) Cells
Organic Cells
Multi-junction Cells
Although multi-junction cells have the highest efficiency among solar cells
achieved in a laboratory setting (demonstrated by the following graph), Silicon
Cells (monocrystaline, polycrystaline and amorphous) due to their low cost and
decent efficiency are the most feasible for wide production and will thus be the
main interest of our study.
What’s in a Polycrystalline Si Solar
Cell?
-Polycrystalline cells are slightly
less efficient than monocrystalline
solar cells, but are cheaper to
manufacture in wafer form.
-The diagram below from How
Stuff Works illustrates the order of
the raw ingredients needed to
create a generic polycrystalline
silicon cell.
They include:
A. A layer of glass for protection
B. Antireflective coating
C. Contact grid
D. Highly purified N-Type Silicon
E. P-Type Silicon
F. Back Contact.
Raw Materials Continued
• There is no threat to the global supply of any
of the raw materials used to manufacture
silicon solar cells, even if their production
dramatically accelerated.
• Silicon is an abundant element in the earth’s
crust, and is in no way potentially threatened
with shortage.
• There is some concern about the supply of
materials needed for non-silicon based cells:
The development of CIGS cells “might be slightly
constrained by shortages of gallium and
selenium,” while, mass production of CdTe
cells may be hampered by tellurium availability
(Lynn, 208, 2010).
Generic Manufacturing of Silicon
Panels:
• Starting point for a polycrystalline cell is in molten
form, “cast in substantial blocks,” and then cut down to
smaller bricks and eventually into a thin wafer (Lynn,
2010).
• “As the molten silicon cools, crystallization occurs
simultaneously,” and these cells are soldered to a diode
that conducts electricity (Lynn, 2010).
• For a helpfully illustrative video on how a generic
silicon cell is produced click on the following link:
• Discovery Channel: Solar Panel Manufacturing
Forms of Solar Energy Gathering
• A single unit is
referred to as a cell
• A collection of cells is
a module
• A collection of
modules is an array
http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/
Note that while they may all be made up of the same type of cell a
module and an array will have different efficiencies due to the empty
spaces between each cell
How Solar Cells Work:
The Photoelectric Effect
• The basic principle of photovoltaics – the
branch of solid state physics which has a
focus of turning light into electrical energy
• It is a property of certain materials where
photons of light are absorbed and the
energy causes electrons to be “knocked
loose”
• Those materials are referred to as
semiconductors
Semiconductors
• Semiconductors can be
natural or created
through “doping” where
impurities are added to
give more or less
electrons
• Two kinds of
semiconductors used for a
single solar cells
* A Positive P-type (Electron
Poor)
* A Negative N-type
(Electron Rich)
http://www.cpushack.com/MakingWafers.html
Pure Silicon – while it is the
most commonly used
element for solar cells, it is
usually doped with other
materials to create the
specific type of
semiconductor needed
P-N Junction
• The electric potential
barrier between the two
semiconductors of a solar
cell
• Creates a low resistance
path for excited electrons
to flow through
• “Loose” electrons flow
from the rich end to the
poor one creating a
direct current
*This is called the
photovoltaic effect and
explains why the true
name for solar cells are
PV cells
http://express.howstuffworks.com/exp-solar-power1.htm
The Band Gap
http://mousely.com/encyclopedia/Band_gap/
Energy of a photon
• A property of the
atoms of the
semiconductor
• This is the energy gap
between two “bands”
of energy between
tow electron states in
a solid
• Only photon energy
which matches the
band gap energy of
the material can free
an electron from that
state.
Solar Cell Efficiency
• Considers how much
energy available and
compares it to how
much energy is used
productively
Solar cell efficiency ( ) depends on
- The total power light power
density (JV) on the cell
- The actual potential difference
of the system
- The actual current density of
the system
- The “fill factor” constant which
is the ratio between the actual values
and the maximum values
Efficiency – The Band Gap Problem
• Get image from book
Certain photon energy levels which are created by the sun get absorbed or reflected by
the atmosphere, which prevents solar cells from accessing that particular level for its
electrons. (Note: AM means Air Mass which is equivalent to the thickness of the
atmosphere)
(Picture Credit – Lynn 2010)
The Multijunction Solution
http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells
• Multijunction cells are
the most efficient solar
cells when tested
• Overcomes the issue of
a single band gap by
incorporating many
different materials into
a single cell, thus
adding more band gaps
• Multiple band gaps
allow for more of the
available light energy to
be used
The Best Solar Cell Research Efficiencies as Recorded by the
NREL (National Renewable Energy Lab)
(Picture Credit - http://www.nrel.gov/pv/thin_film/docs/kaz_best_research_cells.ppt)
Multijunction Concentrators
Three-junction (2-terminal, monolithic)
Two-junction (2-terminal, monolithic)
Crystalline Si Cells
Single crystal
Multicrystalline
Thin Si
Thin Film Technologies
Cu(In,Ga)Se2
CdTe
Amorphous Si:H (stabilized)
Emerging PV
ARCO
WestingOrganic cells
36
32
Efficiency (%)
28
24
20
Spectrolab
Spectrolab
Japan
Energy
NREL
NREL
Spire
Stanford
Georgia Tech
Varian
12
Kodak
Sharp
Georgia Tech
ARCO
8
Monosolar
Boeing
4
0
1975
Kodak
RCA
Boeing
AstroPower
RCA
Solarex
NREL
Cu(In,Ga)Se2
14x concentration
NREL
AstroPower
Boeing
NREL
United
Solar
United Solar
Photon
Energy
University
California
Berkeley
University
Konstanz
1985
NREL
NREL
Euro-CIS
Boeing
University
RCA
of Maine
RCA
RCA
RCA
RCA
1980
UNSW
NREL
AMETEK
Masushita
UNSW
NREL
University
So. Florida
Solarex
UNSW
UNSW
UNSW
No. Carolina
State University
Boeing
UNSW
Spire
house
16
NREL/
Spectrolab
1990
1995
Princeton
NREL
2000
Year
Other forms of Efficiency
• Two kinds of paybacks
* Energy Payback – how long does it take for
the solar cell to make the energy it took to
make the panel itself
* Cost Effectiveness – how long does it take
for the solar cell to generate the energy
equivalent to its cost
Current Statistics
• Energy Payback:
*It takes the average
silicon solar cell in the
United States between
two (for the lower half) to
four years (for the upper
half)
• Average life of a silicon
solar cell: 20-25 years
• Cost Effectiveness:
* It currently costs about
$7.00 per Watt but can go
as low as $4.30 per Watt
(This is a significant
decrease from the $300
per Watt cost during the
1970’s)
* Note that once installed
unless it has a sun
tracking system installed
to it, its only needed fuel
is sunlight
Data from Lynn 2010
Money Matters
• Solar cells now cost $3.50 per watt of production
capacity vs. $70 in the 1970s.
• However, the finished product of the actual panel
itself will run for at least $215. Highly efficient
ones will typically go for around $1000.
• Panels made from scrap solar cells (those broken
in manufacturing) can be purchased for a cheaper
price but are highly inefficient which seemingly
defeats the purpose.
The Vassar Switch:
Vassar Dorms
Using Solar Energy
As of 2008, Vassar was paying $16,000 a month for dorm electricity alone.
Based on some of the leading New York electric companies and New York’s
average solar irradiance of 4.47 kWh/sq m per day…
• 50% of Dorm Electricity
* Total Cost: $1,921,330 –
$3,537,151
* Total Area: 40,460 –
73,436 sq m
* Average Monthly Savings:
$8000
* Cost Payback: 15.00 –
23.09 years
• 100% of Dorm Electricity
* Total Cost: $3,903,911 $7,135,553
* Total Area: 80,921 –
146,873 sq m
* Average Monthly Savings:
$ 16,000
* Cost Payback: 15.4 –
23.22 years
Statistics Credit goes to Cooler Planet.com
Will Solar Energy REALLY reduce
dependency on other
resources?
Solar panels only generate electricity during daylight hours. In most cases this means that
it will only provide energy for half of the day. Obviously, electricity used at night is
gathered from energy stored during daylight collection hours.
Weather obviously also obstructs the absorption of photons. Temperature, however, has
little effect on the efficiency of solar panels.
Pre-existing pollution can interfere as well which will prove difficult in industrial areas and
in cities.
For these reasons, solar panels cannot, unfortunately, be the sole provider of energy for
the United States
Will solar power REALLY affect
importation of foreign fuels?
• 40% of the energy supply in the US comes
from petroleum
• 60% of this oil is imported. Because those
who control these foreign resources hold so
much power, there is little political will to
develop solar technology.
Government Incentives Explored
• In some instances the federal government
offers benefits to those who invest in solar
technology such as the Residential Federal Tax
Credit or Commercial Federal Grant.
• However, regulations are set on the state, NOT
federal levels. This can make it more difficult
to be eligible for programs that typically do
not reimburse consumers for more than 30%
of the total cost of their solar panel projects.
Developing Technology: Thin Film Solar Energy
•
A company in California called Nanosolar (2007) has recently been mass-producing a thin film capable of
producing high levels of energy from sunlight at a cost of one dollar per watt, competitive with coal.
Compared to usual solar cells that require glass, aluminum, copper and silicon, these cells are a thin film
consisting of five layers:
•
1. Aluminum foil for stability.
•
2. Molybdenum Electrode
•
3. CIGS absorber / semi-conductor: an ink made of a mix of copper, indium, gallium, and selenium.
•
4. As in the old solar model a P/N junction, a semi-conductor that doesn’t absorb light.
•
5. Lastly, a clear zinc oxide semi-conductor.
•
Check out the cool animation at the bottom of the page!
http://www.popsci.com/popsci/flat/bown/2007/green/item_59.html
Environmental impacts associated with
photovoltaic production
•
Coniff (2010) sites a report from the Scripps Institution of Oceanograpahy that
found the greenhouse gas NF3 (nitrogen tri-flouride), a common by product of
production of thin-film solar cells (an “economical and increasingly popular solar
power format”) has “17,000 times the warming potential of carbon dioxide,”
potentially undermining any environmental gain in the widespread use of the
developing technology (Yale e360, November, 2008).
Additionally, Cadmium is a heavy metal known used in CdTe solar cells
It is known to cause harmful long term effects when ingested or inhaled
such as kidney failure, lung damage and brittle bones
While dangerous, it is a by product of zinc mining and its application in solar
cells would be a much more productive use
When recycled and carefully disposed of it there is hardly any risk using it
Future Applications
• Constant trend of
increasing efficiencies
across all forms of solar
cells
• Inventive methods
currently being
considered include
*solar panels on
sattlelites which beam
the energy back to earth
in the form of
microwaves
*desert spanning solar
farms
*laser sunlight collectors
to focus sun rays right at
the solar cells
http://pneumaticaddict.wordpress.com/page/25/
http://www.maximumpc.com/article/news/solaren_quench_pges
_energy_thirst_with_spacebased_solar_power
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•
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