Download The Science of Renewable Energy Technology

How do photovoltaic cells produce
electricity?
Rob Snyder
Science and Engineering Saturday Seminar
April 11, 2015
Each renewable energy technology has a unique way of
transforming an energy resource into forms of energy that
heat interior spaces or power machinery and appliances.
The NGSS Standard HS-P53-3 suggests that a
study of renewable energy systems are to be
included in the High School STEM curriculum.
Design, build, and refine a device that works
within given constraints to convert one form of
energy into another form of energy.* [Clarification
Statement: Emphasis is on both qualitative and
quantitative evaluations of devices. Examples of
devices could include Rube Goldberg devices,
wind turbines, solar cells, solar ovens, and
generators. Examples of constraints could include
use of renewable energy forms and efficiency.]
[Assessment Boundary: Assessment for
quantitative evaluations is limited to total output
for a given input. Assessment is limited to devices
constructed with materials provided to students.]
The challenge is to determine when and to
what extent students can develop a genuine
understanding of how each renewable energy
technology transforms energy.
For Example: The similar NGSS Middle School
Performance Expectation is that “Students apply
scientific principles to design, construct, and test
a device that minimizes or maximizes thermal
energy transfer.”
Students have many experiences with sunlight
being transformed into thermal energy.
They would have little difficulty developing an
understanding of how solar thermal systems transform
energy.
A study of solar thermal systems also lends itself well to a
science class where student learn concepts that include:
• Nuclear fusion reactions in the sun transform matter
into energy in the form of photons.
• Photons of visible light have specific wavelengths,
frequencies and energies.
• Materials, such as glass, are transparent because they
absorb and reemit visible light photons with no
changes in wavelengths, frequencies or energy.
• Visible light photons can be absorbed by an opaque
materials in the interior of a structure and reemitted
as thermal energy (commonly referred to as heat).
• The amount of thermal energy gained in the interior
of a structure can be managed by controlling the
amount of heat loss by radiation, conduction and/or
convection.
Students can easily select materials to design,
construct and evaluate of the performance of a
solar thermal device.
• A cardboard box, transparent food
wrap, and tape can be used.
• Thermometers can be used to analyze
the performance of a student designed
solar thermal system.
• The design of the system can be
altered meet a variety of goals.
• Simple components can convert the
design to an active solar thermal
system.
A study of solar thermal systems also provides
an opportunity for students to conduct in-depth
studies of a renewable energy technology
A research report published by Schwartz, Sadler
and Tai concluded that “Students who reported
covering at least 1 major topic in depth, for a
month or longer, in high school were found to
earn higher grades in college science than did
students who reported no coverage In depth.
www.cfa.harvard.edu/.../articles/SE_Depth_versus.pdf
Photovoltaic cell kits can provide opportunities
for students to design and conduct investigations
that include:
• Determining the best arrangement of
cells to operate appliances
• Analyzing the affect of tilt and
direction on the performance of
photovoltaic cells
• Learning how daily and seasonal
changes in the position of the sun in
the sky affects electricity production
• Designing support structures for arrays
• Etc.
Students can also study the geometry of a tilted solar
array and locate suitable sites for a solar array.
Massachusetts Mathematic
Framework Standards include:
• Use informal arguments to establish
facts about the angle sum and
exterior angle of triangles, about
the angles created when parallel
lines are cut by a transversal, and
the angle-angle criterion for
similarity of triangles . (Geometry;
Grade 8: Page 68)
• Explain and use the relationship
between the sine and cosine of
complementary angles. . (High
School Geometry: Page 96)
10
However, it would be very difficult for students
to select materials, design, assemble and modify a
photovoltaic cell.
www.alternative-energy-tutorials.com
A photovoltaic cell is one example of the
many “Black Box” devices we use.
The term black box is used by scientists and
engineers to describe a device for which inputs to
and outputs of the device are understood but
processes that take place in the device are not
understood.
Many students can describe the energy input
to and energy output of a photovoltaic cell but
probably can not describe how the energy is
transformed in a photovoltaic cell.
What do you do if a student asks:
How do photovoltaic cells produce
electricity?
What learning experiences would
students need to have in order to
understand how photovoltaic cells
transform energy ?
Understanding how a photovoltaic cell transforms energy
requires a breadth of knowledge that includes:
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Nuclear fusion reactions in the sun transform matter into energy in the form of electromagnetic radiation.
Visible light consists of bundles of electric and magnetic fields (photons).
Photons of different colors of visible light have specific wavelengths, frequencies and energies.
The geometry of a crystal lattice structure is determined by the electron configuration of individual atoms.
Silicon is a semiconducting element used to produce many photovoltaic cells. Silicon atoms have 4 outer
shell electrons and form a tetrahedral crystal lattice structure. 4 covalent bonds are associated with each
silicon atom.
Chemical elements with 5 outer shell electrons are added to a layer of silicon creating an N-Type layer. This
results in free electrons atoms moving about in the N-Type layer .
Chemical elements with 3 outer shell electrons are added to a different layer of silicon creating a P-Type
layer. This results in vacancies (called “holes”) moving about in the P-Type layer.
Electrons migrate from the N-Type layer to the P-Type layer when a photovoltaic cell is manufactured
before the cell is exposed to sunlight until a maximum separation of charge is reached.
A separation of electric charge produces an electric field with a potential difference (voltage) across the P-N
junction.
An external electric circuit that connects the N-Type and P-Type layers provide a lower resistance pathway
for electrical charges (electrons) to move from the N-Type layer to the P-Type layer.
Electrons in the photovoltaic cell absorb energy as photons of visible shine on the surface of the
photovoltaic cell.
A continuous electric current flows through the external circuit when sunlight is available.
Appliances in the external circuit can transform electrical energy into other forms of energy.
Lets look at small groups of the concepts
associated with the energy transformation that
takes place in a photovoltaic cell.
• The concepts were selected provide a basic
understanding of how photovoltaic cells
produce electricity.
• The concepts would probably be studied in
a several different middle school and high
school science classes.
One set of concepts focuses on sunlight.
(The energy source for most renewable energy technologies)
• Nuclear fusion reactions in the sun transform matter
into energy in the form of electromagnetic radiation.
• Visible light consists of photons that have both wave
and particle characteristics.
• Photons of different colors of visible light have specific
wavelengths, frequencies and energies.
In what science classes might students use
spectrometers to study the properties of visible light?
When do students learn about the electron
configurations of atoms?
water.me.vccs.edu
These two concepts focus on the
“doping” of an N-Type layer of PV cell.
• Silicon is a semiconducting element used to produce many
photovoltaic cells. Silicon atoms have 4 outer shell
electrons and form a tetrahedral crystal lattice structure
with 4 covalent bonds associated with each silicon atom.
• In this “doping” process, atoms of an element with 5
outer shell electrons are added to a layer of silicon
creating an N-Type layer. The result is that one of the
outer shell electrons of each dopant atoms move about in
the tetrahedral lattice structure.
A closer look at the N-Type doping process
Red spheres in the diagram below left illustrate how silicon atoms
form 4 covalent bonds with neighboring atoms in the crystal lattice.
Yellow spheres are dopant atoms that have 5 valence electrons
The diagram below right illustrates that one of a dopant atom’s
outer shell electrons does not participate in the covalent bonding of
the tetrahedral crystalline structure.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html#c2
Is there a way for students to model this process?
The result of doping a layer of atoms silicon with
atoms of antimony that have 5 outer shell electrons is
that many free electrons are moving about in the NType Layer.
Does doping silicon with atoms that have 5 outer
shell electrons produce a N-Type layer that is:
• Negatively charged?
• Positively charged?
• Electrically neutral?
How would students use The Periodic Table of the
Elements to suggest what elements other than
Antimony could be used to “dope” an N-Type Layer?
www.universe-review.ca
This next concept focuses on the doping of a P-Type
layer of a common photovoltaic cell.
• In this doping process, an element with 3 outer shell
electrons is added to a different layer of silicon creating a
P-Type layer. This results in vacancies in the tetrahedral
lattice structure that are referred to as “holes” which
move about in the lattice structure.
ecee.colorado.edu
A Closer Look at the P-Type Doping Process
Red spheres in the diagram below left illustrate how
silicon atoms form 4 covalent bonds with neighboring
atoms in the crystal lattice. Yellow spheres are now
dopant atoms that have 3 valence electrons.
The diagram below right illustrates how a vacancy
(called a hole) forms.
How might students model the P-Type doping process?
The result of doping a layer of atoms silicon with
atoms of elements that have 3 outer shell electrons is
that many vacant spaces (holes) are moving about in the
P Layer.
Holes are moving about in the P-Type Layer
Does doping silicon with atoms that have 3 outer shell
electrons produce a P-Type layer that is:
• Negatively charged?
• Positively charged?
• Electrically neutral?
How can students use The Periodic Table of the
Elements to explore what elements other than Boron
could be used to “dope” a P-Type Layer?
www.universe-review.ca
Make a Prediction: What might happen when an N-Type
layer comes into contact with a P-Type layer to form a “P-N
Junction”?
Free electrons are moving about in the N-Type Layer
Holes are moving about in the P-Type Layer
Note: The two doping processes usually occur simultaneously in a
single sheet of silicon (or other semiconductor).
These two concepts focus on how a “built-in” voltage
develops when a photovoltaic cell is manufactured.
• Electrons tend to migrate from the N-Type layer to the PType layer when a photovoltaic cell is manufactured and
before the cell is exposed to sunlight.
• A separation of electric charge produces an electric field
with an electric potential difference across the P-N junction
(0.5 volts in a common silicon based photovoltaic cell).
What will be the electric charge of each layer of a
photovoltaic cell as a result of the migration of electrons?
When do students typically learn that electrons
can gain or lose energy?
en.wikipedia.org
Sunlight can really excite electrons!
If electrons absorb enough energy they can
overcome the attractive forces that exist
between positively charged nuclei and the
negatively charged electrons.
The result is the production of a large
number of mobile “conduction” electrons.
Note: This (as can be expected) is a
simplified explanation of a much more complex
process that occurs in a photovoltaic call.
These two concepts focus on what happens when
sunlight shines on a photovoltaic cell.
• Electrons in the photovoltaic cell absorb the energy of photons of
visible light that shine on the N-Type layer.
• An external electric circuit that connects the N-Type and P-Type
layers provides a pathway for electric charge (electrons) to move
from the N-Type layer to the P-Type layer.
The red arrows indicate the flow
of electrical charge (electrons).
What if a very curious student asks:
How do photons excite electrons?
The short explanation: Photons are electric and
magnetic fields that constantly change strength and
direction as they move through space. Energy
transfer occurs as magnetic and electric fields of
photons interact with negatively charged electrons.
Source: www.srh.noaa.gov
These two concepts focus on what continues to happen
as long as sunlight shines on a photovoltaic cell.
• Appliances in the external circuit can transform electrical
energy into other forms of energy.
• There will be a continuous electric current in the complete
circuit that includes the photovoltaic cell as long as sunlight
is available
What happens when there is no sunlight?
In the absence of sunlight, the flow of electrons
in the external circuit stops and the electric force
field with a “built in” voltage is re-established.
You have also been given a document with examples
of science concepts associated with common renewable
energy technologies.
This document is a work in progress and comments
would be appreciated – my email is included in the
document.
The document can also be used to compare
and contrast renewable energy systems.
• Determine which technologies use sunlight as
the energy source.
• Determine which technologies use a nuclear
reaction as the energy source.
• Determine which technologies rely on a device
that has the characteristics of a “black box”.
• Rate the renewable energy technologies from
the least complex to the most complex.
• Decide which renewable energy devices they
could construct.
• Etc