# PART constant (J·s) c – Speed of Light in

PART A. Formula based
Description of Photon Energy and its correlation with the Wavelength of Light.

Photon and Photon Energy Formula Description:

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v  A
photon is
related to as a theoretical particle used to explain light.

v  According
to theory of quantum mechanics, a light is composed of particles called photons.
They travel through a vacuum
at the speed of light, have no mass, but each is a packet of energy
related to wavelength of light.

v  The shorter the
wavelength, the larger the packet.

The formula for photon energy is
stated and described below:

Where:

E – Photon energy ( J)

h – Planck constant (J·s)

c – Speed of Light in
vacuum (constant in m/s)

– Wavelength of Light (?m)

Because
h and c are both constants,
photon energy changes with direct relation to
wavelength ?.

The
formulae represents an inverse
relationship between the energy of a photon (E) and the wavelength of the light
(?).

It means that light
consisting of high-energy photons has a short wavelength and light consisting
of low energy photons has a long wavelength.

Example calculation of Photon Energy of a Visible Violet light (at
the edge between violet and blue)

In visible spectrum Violet color ranges between 380
to 450nm. A maximum wavelength of 450nm of violet was chosen as shown on the
visible spectrum diagram above.

1 nm = 10-3 ?m

so

450 nm = 10 -3
× 450 nm = 0.45 ?m

Data Given:

h – 6.626  10 -34 joule·s

c – 2.998 × 108 m/s

– 0.45 ?m =

hc = 1.99 × 10-25 joules-m

Data Needed:

E = ? (in joules)

Define an
electron volt.  Explain the mathematical relationship
(formula) between photon energy in Joules and energy in electron volts. Include
an example calculation to convert energy in Joules, from (a) above, into
electron volts.

Electron
volt:

v  The
energy of individual photons of different wavelengths is expressed in electron
volts.

v  One
electron volt is the energy an electron acquires when it accelerates in a
vacuum across a potential of one volt.

v  This unit
of energy is commonly used by solar cell physicist since it is a convenient
size when considering individual electrons.

When dealing with “particles” such as
photons or electrons, a commonly used unit of energy is the electron-volt (eV)
rather than the joule (J). An electron volt is the energy required to raise an
electron through 1 volt, thus a photon with an energy of 1 eV =
1.602 × 10-19 J.

A photon with an energy of 1 eV has
1.602 × 10-19 J of energy.

Photon energy OF VISIBLE VIOLET LIGHT IN JOULES CAN
BE EASILY CONVERTED INTO ELECTRON VOLTS
UNIG THE FOLOWING CACULATION:

a.
Describe
the atmospheric effects, which reduce the power of solar radiation at the edge
of the earth’s atmosphere to the values received at the earth’s surface.

Atmospheric effects have several impacts on the
solar radiation at the Earth’s surface where a reduction in the power of the
solar radiation is one of the major issues.

Loss of Solar Power is an effect of absorption,
scattering and reflection in the atmosphere and these effect are described
below:

Absorption and scattering of light
due to air molecules and dust:

v  It is the major factor reducing

v  A reduction in power is caused due
to its dependency on the path length through the atmosphere.

v  It does not produce the deep

v  absorption of light by the air molecules
and dust reduces the visible spectrum uniformly
so the incident light appears white.

v  The longer path lengths the more
effectively higher energy light is more effectively absorbed and scattered.

Absorbtion of Gases:

v  Gases at the ede of the atmosphere,
like maily ozone (O3), carbon dioxide (CO2), and water vapor (H2O) absorb the
photons with energies close to the bond energies of these atmospheric gases.

v  When a  solar radiation passes through the
atmosphere, gasses, dust and aerosols absorb some of the photons

v  Gases have a rather minor impact
in overall power reduction

v  They  are changing the spectral content of the

PART D: Definitions & Formulas of Air Mass, Solar Altitude &
Azimuth Angle:

Air mass:

v  It is the path length which light takes through
the atmosphere normalized to the shortest possible path length (that is, when

v  It is a cell performance rating – To compare
performance of different solar cells, the cells are rated at specified amounts
and types of sunlight

v  Air Mass 0 (AM0) – The amount of sunlight
falling on a surface in outer space just outside the earth’s atmosphere (1.4
Kw/m2)

v  Air mass 2 (AM2) is a closer approximation to
usual sunlight conditions. The illumination is 800 W/m2.

v  Air Mass 1 (AM1) – It is the most common rating
for terrestrial cells.

It is the amount of sunlight falling on the
earth at sea level when the sun is shining straight down through a dry clean
atmosphere. A close approximation is the Sahara Desert at high noon.

The sunlight intensity is very close to 1
kilowatt per square meter (1Kw/m 2)

v  The Air Mass 1 Formulas can be
defined in multiple ways, each method has a different accuracy and purpose is
defined as:

Method 1: An approximation equation which doesn’t incorporate
the curvature of the earth is:

Method 2: An approximation equation, which
does not, incorporates the curvature of the earth and uses the shadow and
height of an object.

Where:

H – Object Height

Method 3:
A precise equation, which incorporates the curvature of the earth:

Where:

z – Solar Zenith Angle

Solar elevation/altitude angle:

v  The angle between the horizon and the centre of
the Sun’s disc.

v  Defines the altitude of the Sun.

v  It is represented on the neighbouring sketch.

v  Solar altitude angle (h) and Solar Zenith Angle
(z), which is the angle between the zenith and the centre of the sun’s disc are complementary which means that their sum is equal
90 °and the cosine of either one of them equals the sine of the other.

v  Both Solar Zenith angle and the Solar
elevation/altitude angle can be calculated using  the same formula:

Solar Azimuth angle:

v  A horizontal angle measured clockwise from true
north (The equator is at 90 °).

v  Defines the direction of the sun.

v  The angle is positive if the line is east of
south and negative if it is west of south.

v  It is represented on the neighbouring sketch.

v  It can be calculated to a good
approximation with the following formula:

NOTE: Angles should be interpreted with care because of
the inverse sine, which has multiple
solutions, only one of which will be correct.

Explain what is meant by the term
semi-conductor and describe the lattice arrangement of the atomic structure for
silicon including the concept of covalent bonds for electrons

Semiconductor :

v  substance, usually a solid chemical element or
compound

v  able to conduct electricity under certain
conditions making them a good medium for the control of electrical current.

v  Its conductance varies depending on the current
or voltage applied to a control electrode, or on the
intensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays

All illustrations below
represent a lattice arrangement of Silicon.

The first two
shows the tetrahedrical coordination of silicon atoms

The third illustration below represents a
lattice arrangement, which shows the presence of the valence electron. Each
atom of silica has 4 valence electrons.

Covalent bonds occur between
the atoms when a difference beteween electronegativity of each atom  is almost zero . Electronegativity is the
tendency of atoms to attract electrons

The covalent bond is formed
when valence electron of each atom are shared between each other.

b.    Explain what is meant by “doping” with regard
to semi-conductor materials and how this treatment can contribute to the
availability of free electrons.

Doping:

v  Doping is a deliberate process of adding a
known impurity to a pure semiconductor.

v  The material resulting in doping is an
ex-trinsic semiconductor.

v  It is a way to make the free electrons available

How to make
free electrons avaliable through doping?

When a small amount of atoms
of a certain element like phosphorus causing impurity is added to a silicon
crystal, the atoms of phosphorus which have 5 valence electrons will contribute
4 electrons and 4 levels to the valence bond and the extra electron be
liberated from its original atom and will occupy a level near the bottom of the
conduction band. It becomes a conduction electron.

v  Doping creates N-type semiconductor materials from group IV

are doped with materials from group V atoms.
This example is explained above.

v  N-type semiconductors increase the conductivity of a
semiconductor by increasing the number of available electrons

v  P-type materials are created when semiconductor materials
from group IV are doped with group III atoms.

v  P-type materials increase conductivity by
increasing the number of holes present.

Explain and describe how a pn junction is
created and explain how a pn junction acts as a diode.

PN
Junction:

v  It is the plane within a photovoltaic cell
where the positively and negatively doped silicon layers meet.

v  P-n junctions are formed by joining n-type and
p-type semiconductor materials. Since the n-type region has a high electron
concentration and the p-type a high hole concentration, electrons diffuse from
the n-type side to the p-type side.

v  Movement of electrons to the p-type side
exposes positive ion cores in the n-type side while movement of holes to the
n-type side exposes negative ion cores in the p-type side, resulting in an
electron field at the junction and forming the depletion region.

v  A voltage is a result from the electric field
formed at the junction.

v  junction pn
junctionis the foundation of solid state electronics

PN Junction
as a diode

v  A p-n junction diode also known as p-n junction
semiconductor device is two-terminal or two-electrode semiconductor device,
which allows the electric current in only one
direction while blocks the electric current in the reverse direction.

v  A basic pn junction creates a diode, which only allow unidirectional flow of current if
operated within a rated specified voltage
level because diode only blocks current in the
reversed direction

v  A pn justion is the simples form of the diode
which behaves at ideally a shortest circuit when it is in forward biased and
ideally open circuit when in reversed biased.

v  If the diode is forward biased, it
allows the electric current flow.

v  If the diode is reverse biased, it
blocks the electric current flow.

c.
Explain
and describe the concept of a band gap with regard to semi-conductors

Band gap

refers to the
energy difference in electron volts between the top of the valence band and the
bottom of the conduction band in insulatos
and semiconductors.

the minimum amount of energy
required for an electron to break free of its bound state.

1.      When the band gap energy is met, the electron is excited into a free
state, and can therefore participate in conduction.

2.      The band gap determines how much energy is needed from the sun for
conduction, as well as how much energy is generated.

3.      A hole is created where the electron was formerly bound. This hole also
participates in conduction.

The band gap of a semiconductor is the minimum energy required to excite
an electron that is stuck in its bound state into a free state where it can
participate in conduction.

The band structure of a
semiconductor gives the energy of the electrons on the y-axis and is called a
“band diagram”. The lower energy level of a semiconductor is called
the “valence band” (EV) and the energy level at which an
electron can be considered free is called the “conduction band” (EC). The band gap (EG) is the gap in
energy between the bound state and the free state, between the valence band and
conduction band. Therefore, the band gap is the minimum change in energy
required to excite the electron so that it can participate in conduction.

Once the electron becomes excited into the conduction band, it is free
to move about the semiconductor and participate in conduction. However, the
excitation of an electron to the conduction band will also allow an additional
conduction process to take place. The excitation of an electron to the
conduction band leaves behind an empty space for an electron. An electron from
a neighboring atom can move into this empty space. When this electron moves, it
leaves behind another space. The continual movement of the space for an
electron, called a “hole”, can be illustrated as the movement of a
positively charged particle through the crystal structure. Consequently, the
excitation of an electron into the conduction band results in not only an
electron in the conduction band but also a hole in the valence band. Thus, both
the electron and hole can participate in conduction and are called
“carriers”.

The concept of a moving “hole” is analogous to that of a
bubble in a liquid. Although it is actually the liquid that moves, it is easier
to describe the motion of the bubble going in the opposite direction.

d.
Describe
and explain–

·
How a
solar cell generates electricity by creating electron flow through an external
circuit

·
What is
meant by short circuit voltage, open circuit current, fill factor, solar cell
efficiency

e.
Describe
using a diagram the practical construction of a solar cell. The diagram should
demonstrate the pn junction, conducting fingers, substrate, rear electrical
contact.

f.
Describe
the typical semi-conductor materials used in commercial PV installations

g.
Carry out a laboratory
test on a pv module to determine output voltage and current, solar energy
input, solar cell efficiency at various loads. Include test results and
conclusions as an appendix to this report

h.
Analyse
the performance of the PV array located on the roof of the new social zone
building. This analysis should look into –

·
Site

·
Manufacturer’s
performance specification

·
Actual
performance

i.
Write a
brief guidance note on the use of PV in building services applications. The
guidance note is to be aimed at technically literate professionals from a
variety of construction backgrounds and should therefore explain in general
terms the validity (or otherwise) of specifying PV.

Hand in date – week 17,
Friday 20th January 2018

References/Book List

Energy Efficiency in Buildings: Guide F 2012

CIBSE ISBN 1903287340

Renewable Energy Resources for Buildings: TM 38 CIBSE ISBN 1903287731

Energy Assessment and Reporting Methodology: TM 22 CIBSE ISBN 190328760

Understanding Photovoltaics : TM25 CIBSE ISBN 1903287731

Sustainability without the hot air : David MacKay ISBN 978-0-9544529-3-3

from

Energy Management, Supply and Conservation Clive Beggs 2

nd

Edition ISBN:978-0-7506—8670-9

Renewable Energy Systems Dilwyn Jenkins ISBN 978-1-84971-369

http://www.pveducation.org/pvcdrom/properties-of-sunlight/energy-of-photon

http://philschatz.com/physics-book/contents/m42558.html