CLICK ON THIS LINKS FOR CISA REVIEW QUESTIONS

http://naijaskill.com/cisa2005/

http://naijaskill.com/cisa2006/

[center]LECTURE 1[/center]
[center][size=14pt]PROTECTECTION OF INFORMATION ASSESTS

SYSTEM ATTACK METHODS AND TECHNIQUES[/size][/center]
(A) MALICIOUS CODE
Malicious code, such as  VIRUS and WORMS, attacks a system in one of  many ways, either internally or externally. Traditionally, the VIRUS is an INTERNAL THREAT, while the WORM,  to a large extent,  is a threat from  EXTERNAL SOURCE.

Virus and Worms are related classes of malicious code, they are HOSTILE SOFTWARE. Both share  the PRIMARY OBJECTIVE OF REPLICATION. They  are different with respect to the techniques they use and their host system requirement.

HOSTILE SOFTWARE is a software that lies hidden from view until it  begins reproducing and wreaking havoc with the software and data.

CATEGORIES OF HOSTILE SOFTWARE ARE:  Virus, Worm, Bomb and trap door

SYMPTOMS OF A VIRUS - These includes:
(1) File Size Increase
(2) Change in update timestamp
(3) Sudden decrease of free space
(4) Unusual screen activity etc.

Effect of Hostile software can:
(1) Damage, Erase, Hide or Copy Application Software or Data files
(2) Disrupt processing by consuming computing time and disk space
(3) Take over a computer or screen display unexpectedly and display message that delay the user or cause needless worry.

TO READ MORE CLICK ON:
http://www.naijaskill.com/naijaskii_cisa_malicioucodes.html

[size=15pt]

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DATE:  18 DEC 2010 – 08 JANUARY 2011[/center][/size]

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[center][size=18pt]LEARN HOW TO DESIGN, CONSTRUCT AND INSTALL [/size]

[size=15pt]

** SOLAR POWER INVERTER

** WIND TURBINE

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** SOLAR CHARGE CONTROLLER

** SOLAR PANEL POWER BOSTER [/size]

[size=20pt]100% PRACTICAL[/size]

[size=15pt]NOTE: I WILL TRAIN YOU HOW TO DESIGN, BUILD OR CONSTRUCT THESE POWER SYSTEM LOCALLY, NO MORE IMPORTATION, [/size]

CALL DANIEL or OMOBAJESU: 08099423738[/center]



[center][size=26pt]DON'T WAIT APPLY NOW

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[size=24pt][center]OMOBAJESU IS BACK ONCE AGAIN FROM HIS TRIP

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POWER GENERATION THROUGH


WIND, NOISE, SOLAR AND POWER INVERTER[/center][/size]

[center][size=24pt]THIS TRAINING IS[/size][/center]

[center][size=60pt]FREE[/size][/center]

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TRAINING CENTERS : IYANA IPAJA / AGEGE / AGUDA – SURULERE  / IKEJA

EMAIL  ME: danieloshoetan@yahoo.com


CALL  LADE  ON:  2348035579246 OR DANIEL : 234-1-7938839, 234-1-7919283

[size=20pt][center]FREE PEACHTREE ACCOUNTING TRAINING MANUAL

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[/size]

see sample below

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POSTING YOUR WACKIEST, DIRTIES, URGLIEST[/size]

To Upload Your Wackiest, Urgliest, Dirtiest Photo click on the following links: www.nicedreamspropertis.com/uploadphoto.html

For further help: Call Daniel on: 234-1-4806843

[size=15pt]Note:

PHOTO SIZE

WIDTH: 150 PIXELS

HEIGHT: 100 PIXELS

RESOLUTION: 600

* Photo Size should not be more than 44KB,
If photo is more than this size, it will not be uploaded, so make sure you see your uploaded photo on photo box above, if photo appears in the photo box above, photo has been submitted into our database

FOLLOW THE PROCEDURE BELOW:

1. DOWNLOAD PHOTO ON " YOUR DESKTOP "
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7. CLIK ON OPEN ON THE MENU BAR
8. LOCATE PHOTO YOU SAVED
9. OPEN PHOTO YOU SAVED IN PAINT
10. CLICK ON IMAGE ON THE MENU BAR
11. CLICK ON RESIZE/SKEW
12. TYPE "20 FOR HORIZONTAL AND 20 FOR VERTICAL"
13. CLICK "OK BUTTON"
14. RE-SAVEYOUR PHOTO
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17. SELECT PHOTO
18. CLICK ON OPEN
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For further help: Call Daniel on: 234-1-4806843[/size]

[center][size=18pt]Single Phase vs. Three Phase Power Systems[/size][/center]

Single-phase meaning (2) power lines as an input source; therefore, only (1) primary and (1)secondary winding is required to accomplish the voltage transformation. However, most power is distributed in the form of three-phase A.C.

Therefore, before proceeding any further you should understand what is meant by three-phase power.

Basically, the power company generators produce electricity by rotating (3) coils or windings through a magnetic field within the generator . These coils or windings are spaced120 degrees apart. As they rotate through the magnetic field they generate power which is then sent out on three (3) lines as in three-phase power. Three-Phase transformers must have (3) coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage we need and maintain the proper phasing or polarity.

Frequency    

The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz. 


Impedance   

The impedance (or resistance to current flow) is important and used to calculate the maximum short circuit current which is needed for sizing, circuit breakers and fuses. Impedance is expressed as a percent.

This percentage represents the amount of normal rated primary voltage which must be applied to the transformer to produce full rated load current when the secondary winding is short circuited.

The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by the multiplying the reciprocal of the impedance timed the full load current . Thus, if a transformer has 5% impedance, the reciprocal of .05 is 20 and maximum short circuit current is 20 times the full load current. 
Insulation System and Temperature Rise   

All Federal Pacific FH class transformers are designed with 220 oC insulation systems. The standard units are rated X0 oC rise. The insulation system classification represents the maximum temperature permitted in the hottest spot in the winding when operated in a 40 oC maximum ambient.

The hotspot temperature is determined by adding the maximum value for each of the following:

40 oC maximum ambient
150 oC maximum average winding rise
30 oC maximum hot spot in winding
220 oC ultimate temperature at hot spot   

The temperature rise commonly associated with transformers is the temperature of the conductor inside the coil and does not apply to the outside surface. The wiring compartment is ventilated and cooled enough to permit the use of 60 oC cable for connections. Some customers will specify 220 oC insulation with 80 oC or 115 oC rise to get overloaded capability, better efficiency, and longer life.

These transformers are designed to operate with a lower rise per the following example at 80 oC rise 


40 oC maximum ambient
80 oC maximum average winding rise
30 oC maximum hot spot in winding
30 oC thermal overload 30%
220 oC ultimate temperature at hot spot
 
There may be differences between the voltage ratings of transformers and the rated voltage of some utilization equipment. Some equipment may be rated 230, 460, or 575 volts to allow drop due to impedance of wire, circuit breakers, ect. The respective transformer secondary voltages would be rated 240, 480, and 600 volts which are the system or source voltage . If you are asked to quote on a transfer rated 460V primary, 115/230V secondary would be proper to quote a transformer rated 480V primary, 120/240V secondary 

Basic Impulse Insulation Levels (BIL)   

Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. High voltage switches and circuit breakers can also create similar voltage surges when they are opened and closed. Both types of surges have steep wave fronts and can be very damaging to electrical equipment .

To minimize the effects of these surges, the electrical system is protected by lighting arresters but they do not completely eliminate the surge from reaching the transformer. The basic impulse level (BIL) of the transformer measures its ability to withstand these surges. All 600 volt and below transformers are related 10 KV BIL. The 2400 and 4160 volt transformers are rated 25 KV BIL. 

Transformer Sound/Noise   

A Humming is an inherent characteristic of transformers due to the vibration caused by alternating flux in the magnetic core. Sound levels will vary according to transformers due to the vibration caused by alternating flux in the magnetic core. Sound levels will vary according to transformer size.Attention to installation methods can help reduce any objectionable noise.

When possible ,locate the transformer in an area where the ambient sound will be equal or greater than the noise of the transformer sound level. Avoid locating units in corners.

Make connections with flexible conduits and couplings to prevent transmitting vibration to other equipment. Larger units should be installed on flexible mountings to isolate the transformer from the building structure. 

Sound Level In Decibles   
KVA
150 Degrees Celcius Rise K-1 Nema ST-20
Average
0-9 40
10-50 45
51-150 850
151-300 55
301-500 60
501-700 62
701-1000 64
 
We know that the air is thinner at higher altitudes which, in turn, reduces its ability to cool the transformer. Therefore, standard dry-type self cooled transformers are designed to operate with normal temperature rise at heights through 3300 feet above sea level. If the operation is at higher altitudes, the rating should be reduced 0.3% for each 330 feet above 3300 feet.

[center][size=19pt]DO YOU NEED PRACTICAL TRAINING ON SOLAR POWER INVERTER

GET THE SKILL, GET THE JOB

CALL DANIE: +2347037914178, +2347027255961[/size][/center]

[center][size=18pt]

FREE 1 DAY WORKSHOP ON DC - AC POWER INVERTER  +  COMPUTER ASSEMBLY (HARDWARE/SOFTWARE) [/size] [/center]
[size=15pt]
RESOURCE PERSON:   Engr. Kayode. O.S

DATE:                          Every Saturday

TIME:                           8AM - 6PM

LOCATIONS:                SURULERE / IPAJA / SANGO OTTA / ABUJA

NATURE:                      99% PRACTICAL[/size]

[size=16pt]COURSES:  

*** CONSTRCUTION OF POWER INVERTER

*** MANUFACTURING OF SOLAR STREET LIGHT

*** CONSTRCUTION OF DC CHARGER

*** ASSEMBLY OF COMPUTER ( HARDWARE / SOFTWARE )
[/size]     
[size=26pt]
[center]GET THE SKILL, GET THE JOB[/center][/size]
[size=18pt]
[center]YOU DON'T NEED A UNIVERSITY DEGREE OR POLYTECHNIC DIPLOMA, IF YOU CAN DIFFERENTIATE BETWEEN RED AND

BLACK, IF YOU HAVE AT LEAST ONE HAND AND FINGERS, AND YOU ARE TIRED OF POVERTY, THEN GRAP THIS

OPPORTUNITY, DON'T LET THIS OPPORTUNITY SLIP OFF YOUR HAND

IN THIS WORKSHOP / SEMINAR , I WILL TEACH YOU ALL YOU NEED TO KNOW ABOUT DC - AC POWER INVERTER, YOU

WILL START  CONSTRUCTION SAME DAY, IT IS EASY, EVEN IF U ARE A DUMMY DON'T WORRY, THE POWER MASTER

WILL TRAIN U, WITHIN 2 - 4 HOURS YOU WILL START CONSTRUCTION OF POWER INVERTER. THIS AUTOMATIC

POWER GENERATOR IS ALL YOU NEED FOR 24/7 POWER SUPPLY, NO POWER FAILURE[/center][/size]

[size=16pt]WHAT ARE THE ADVANTAGES OF THIS POWER SYSTEM:[/size]
[size=15pt]

**  NO MORE POWER  FAILURE

**  NOSE NOISE

** NO RUNNING COST

** NO FUEL

**  NO PULLING OF RIP CORD

**  IT IS ENVIRONMENTAL FRIENDLY

**  NO POLUTION

** IT IS AUTOMATIC

** YOU CAN USE WITH SOLAR CELL (PV) OR SOLAR PANEL[/size]
[size=15pt]

THIS IS AN OPPORTUNITY FOR YOU TO START MAKING MONEY BY CONSTRCUTING POWER INVERTER FOR MILLIONS OF

PEOPLE. I WILL ALSO TRAIN YOU ON PRACTICAL ASSEMBLY OF COMPUTER SYSTEM (HARDWARE / SOFTWARE)

LET US JOIN HAND TO TURN OUR ENVIRONMENT TO GREEN ZONE. NIGERIA GREAT PEOPLE, GREAT NATION [/size]

[size=15pt]HOW TO BE PART OF THE WORKSHOP / SEMINAR[/size]

1.  SEND A MAIL TO THE  RESOURCE PERSON ( kayodeoshoetan@yahoo.com)

WITH THE FOLLOWING DETAILS

2.  LOCATION PREFER: SURULERE / IPAJA / SANGO OTTA / ABUJA

3.  STATE YOU EDUCATION BACKGROUND

4.  YOUR  PHONE NUMBER

5.  PAY REGISTRATION FEE (COMPULSORY)


FOR WORKBENCH RESERVATION CALL:

[size=14pt]KAYODE ON: 070,27255961,07037914178, +23417938839, +23414748802 [/size]

[size=14pt]

INTRODUCTION TO DC - AC POWER INVERTER (VOL.1)

[center]WHAT IS DC - AC POWER INVERTER SYSTEM[/center]

[b]An AC/DC power inverter is a device used to convert AC, or alternating currency power, to DC, or direct currency

power. These types of devices are commonly used to convert energy from a car battery to allow electronic devices

to be powered.

In general, there are two types of energy:

1. Alternating current and

2. Direct current.

Alternating current flows between the negative and positive electrical poles, and direct current flows from the

positive to the negative electrical pole with no break. Direct current is naturally occurring and is re-created in

batteries. Alternating current is artificially generated and is used in the public utility system.

An AC/DC power inverter is used to allow AC-powered devices to work from a dc power source. A common example

of this is a car battery. The battery operates on a DC current, but people want to plug in devices such as music

players, seat warmers and satellite radios, all of which operate on AC power.

The AC/DC power inverter was invented to address this specific need. The power inverter is built into the car's

electrical system and works by modifying the wave type from alternating currency to direct current. This conversion

must be contained within a specially constructed container to ensure that the inversion does not impact on other

devices or systems.

There are two key components to an AC/DC power inverter: wave and wattage output. The wave is a description of

how electrical signals look on an oscilloscope. Sine waves or true sine waves appear as waves on a screen, and

square waves appear as squares.

An AC/DC power inverter that modifies the sine wave work much better than a square wave inverter. New AC/DC

power inverters can now work with either modified square or sine waves to create the required power switch. In

practical terms, the difference between the two types of waves are very minor and only impact on very high end

equipment with very specific requirements.

The wattage of the AC/DC power inverter is critical in determining how many devices can be used simultaneously

without risk of overloading the power inverter. Check the watts value listed on the product packaging before

making your selection. The difference in price for more watts is minor, and is outweighed by the benefits[/b].[/size]

CAN YOU WEAR THE CAP ? ARE YOU SELF DRIVEN TECHNOLOGICALLY ? ARE YOU A QUALIFIED ELECTRICAL/ ELECTRONICS / MECHANICAL ENGINEER ? DO YOU HAVE WHAT IT TAKES TO BE PART OF A TEAM ? CAN YOU BUILD NIGERIA FUTURE SOLAR CAR. SALARY VERY, VERY ATTRACTIVE.

THEN SEND YOUR CV TO OMOBAJESU

Email:kayodeoshoetan@yahoo.com

[size=19pt]  [center]Solar cell   [/center] [/size]

A solar cell or photovoltaic cell is a device that converts sunlight directly into electricity  by the photovoltaic effect . Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

Assemblies of cells are used to make solar panels

solar modules or photovoltaic arrays ,. Photovoltaics  is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy



A solar cell made from a monocrystalline

Timeline of Solar Energy

The term "photovoltaic" comes from the Greek  (ph?s) meaning "light", and "voltaic", meaning electric, from the name of the Italian physicist Volta after whom a unit of electrical potential, the volt , is named. The term "photo-voltaic" has been in use in English since 1849.

The photovoltaic effect was first recognized in 1839 by French physicist A. E. However, it was not until 1883 that the first solar cell was built, by Charles Fritts who coated the semiconductor  selenium  with an extremely thin layer of gold  to form the junctions. The device was only around 1% efficient.

Sven Ason Berglund had a number of patents concerning methods of increasing the capacity of these cells. Russell Ohl  patented the modern junction semiconductor solar cell in 1946[2], which was discovered while working on the series of advances that would lead to the transistor

The modern age of solar power technology arrived in 1954 when Bell Laboratories , experimenting with semiconductors, accidentally found that silicon  doped with certain impurities was very sensitive to light[citation needed ].Daryl Chapin, with Bell Labs colleagues Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful electrical power.

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent.The solar battery was first demonstrated on April 25, 1954.

The first spacecraft to use solar panels was the US satellite Vanguard 1 , launched in March 1958 with solar cells made by Hoffman Electronics . This milestone created interest in producing and launching a geostationary  communications satellite , in which solar energy would provide a viable power supply.

This was a crucial development which stimulated funding from several governments into research for improved solar cells.

In 1970 the first highly effective GaAs heterostructure  solar cells were created by Zhores Alferov  and his team in the USSR Metal Organic Chemical Vapor Deposition (MOCVD, or OMCVD) production equipment was not developed until the early 1980s, limiting the ability of companies to manufacture the GaAs solar cell. In the United States, the first 17% efficient air mass zero (AM0 ) single-junction GaAs solar cells were manufactured in production quantities in 1988 by Applied Solar Energy Corporation (ASEC).

The "dual junction" cell was accidentally produced in quantity by ASEC in 1989 as a result of the change from GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental doping of Ge with the GaAs buffer layer created higher open circuit voltages, demonstrating the potential of using the Ge substrate as another cell.

As GaAs single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed the first dual junction cells for spacecraft use in the United States, with a starting efficiency of approximately 20%.

These cells did not utilize the Ge as a second cell, but used another GaAs-based cell with different doping. Eventually GaAs dual junction cells reached production efficiencies of about 22%. Triple Junction solar cells began with AM0 efficiencies of approximately 24% in 2000, 26% in 2002, 28% in 2005, and in 2007 have evolved to a 30% AM0  production efficiency, currently in qualification.

Recent world record claims of efficiency for multiple junction solar cells are discussed in the Records section.

Three generations of solar cells

Solar Cells are classified into three generations which indicates the order of which each became important. At present there is concurrent research into all three generations while the first generation technologies are most highly represented in commercial production, accounting for 89.6% of 2007 production

First Generation

Crystalline silicon  and Vacuum deposition 

First generation cells consist of large-area, high quality and single  devices. First Generation technologies involve high energy and labor inputs which prevent any significant progress in reducing production costs. Single junction silicon devices are approaching the theoretical limiting efficiency of 33%and achieve cost parity with fossil fuel  energy generation after a payback period of 5-7 years.

Second Generation

Thin-film cell

Second generation materials have been developed to address energy requirements and production costs of solar cells. Alternative manufacturing techniques such as vapour deposition , electroplating , and use of Ultrasonic Nozzles  are advantageous as they reduce high temperature processing significantly.

It is commonly accepted that as manufacturing techniques evolve production costs will be dominated by constituent material requirements,whether this be a silicon  substrate , or glass cover

The most successful second generation materials have been cadmium telluride  (CdTe), copper indium gallium selenide , amorphous silicon  and micromorphous silicon .

These materials are applied in a thin film  to a supporting substrate such as glass or ceramics , reducing material mass and therefore costs. These technologies do hold promise of higher conversion efficiencies  particularly CIGS-CIS , DSC and CdTe  offers significantly cheaper production costs.

Among major manufacturers there is certainly a trend toward second generation technologies, however commercialisation of these technologies has proven difficult. In 2007 First Solar  produced 200 MW of CdTe solar cells making it the fifth largest producer of solar cells in 2007 and the first ever to reach the top 10 from production of second generation technologies alone.

Wurth Solar  commercialised its CIGS  technology in 2007 producing 15 MW. Nanosolar  commercialised its CIGS technology in 2007 with a production capacity of 430 MW for 2008 in the USA and Germany.Honda , also began to commercialize their CIGS base solar panel in 2008.

In 2007, CdTe production represented 4.7% of total market share, thin-film silicon  5.2% and CIGS 0.5%.

Third GenerationThird generation technologies aim to enhance poor electrical performance of second generation (thin-film technologies) while maintaining very low production costs.

Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques.They can exceed the theoretical solar conversion efficiency limit for a single energy threshold material, that was calculated in 1961 by Shockley and Queisser as 31% under 1 sun  illumination and 40.8% under the maximal artificial concentration of sunlight (46,200 suns, which makes the latter limit more difficult to approach than the former

There are a few approaches to achieving these high efficiencies including the use of Multijunction photovoltaic cells , concentration of the incident spectrum, the use of thermal generation by UV light  to enhance voltage or carrier collection, or the use of the infrared spectrum for night-time operation

High efficiency cells

High efficiency solar cells are a class of solar cell that can generate electricity  at higher efficiencies than conventional solar cells. While high efficiency solar cells are more efficient in terms of electrical output per incident energy (watt/watt), much of the industry is focused on the most cost efficient  technologies, i.e.cost-per-watt .

Many businesses and academics are focused on increasing the electrical efficiency of cells, and much development is focused on high efficiency solar cells

Records

Multiple junction solar cells
The record for multiple junction solar cells is disputed. Teams led by the University of , the Fraunhofer Institute ,and NREL  all claim the world record title at 42.8, 41.1, and 40.8 percent, respectively.

NREL claims that the other implementations have not been put under standardized tests and, in the case of the University of Delaware project, represents only hypothetical efficiencies of a panel that has not been fully assembled. NREL claims it is one of only three laboratories in the world capable of conducting valid tests, although the Fraunhofer Institute is among those three facilities.


Thin film solar cells

In 2002, the highest reported efficiency for solar cells based on thin films of CdTe is 18%, which was achieved by research at Sheffield Hallam University  although this has not been confirmed by an external test laboratory[citation needed

The US national renewable energy research facility NREL achieved an efficiency of 19.9% for the solar cells based on copper indium gallium selenide  thin films, also known as CIGS.

These CIGS films have been grown by physical vapour deposition in a three-stage co-evaporation process. In this process In, Ga and Se are evaporated in the first step; in the second step it is followed by Cu and Se co-evaporation and in the last step terminated by In, Ga and Se evaporation again.

Applications and implementations

Solar cells are often electrically connected and encapsulated as a module. PV modules often have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the semiconductor wafers  from the elements (rain , hail, etc.). Solar cells are also usually connected in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher current.

Modules are then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and current.

The power output of a solar array is measured in watts  or kilowatts . In order to calculate the typical energy needs of the application, a measurement in watt-hours , kilowatt-hours or kilowatt-hours per day is often used.

A common rule of thumb is that average power is equal to 20% of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.8 kWh per day (24 hours x 1kW x 20% = 4.8 kWh)

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the energy that is not needed immediately.

Solar cells can also be applied to other electronics devices to make it self power sustainable in the sun. There are solar cell phone chargers, solar bike light and solar camping lanterns that people can adopt for daily use.

Simple explanation

Photons  in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon
Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity .

Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. The complementary positive charges that are also created (like bubbles) are called holes  and flow in the direction opposite of the electrons in a silicon solar panel.
An array of solar cells converts solar energy into a usable amount of direct current  (DC) electricity

Photogeneration of charge carriers

When a photon  hits a piece of silicon, one of three things can happen:

The photon can pass straight through the silicon - this (generally) happens for lower energy photons,

The photon can reflect off the surface,

The photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap  value. This

generates an electron-hole pair and sometimes heat, depending on the band structure.

When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far.

The energy given to it by the photon "excites" it into the conduction band , where it is free to move around within the semiconductor.

The covalent bond that the electron was previously a part of now has one fewer electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice.

Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.

A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band.

However, the solar frequency spectrum  approximates a black body  spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth  is composed of photons with energies greater than the band gap of silicon.

These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons ) rather than into usable electrical energ


Charge carrier separation

There are two main modes for charge carrier separation in a solar cell:
drift of carriers, driven by an electrostatic field established across the device
diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential).

In the widely used p-n junction solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n-junction solar cells (typical of the third generation solar cell  research such as dye  and polymer solar cells ), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion


The p-n junction

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon.

In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).

If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction).

When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side.

The diffusion of carriers does not happen indefinitely however, because of an electric field  which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates.

The electric field established across the p-n junction creates a diode that promotes charge flow, known as drift current , that opposes and eventually balances out the diffusion of electron and holes.

This region where electrons and holes have diffused across the junction is called the depletion region  because it no longer contains any mobile charge carriers. It is also known as the "space charge region".


[size=16pt]Chosing Your Home Solar Electricity System[/size]

What You Should Know Before Purchasing a Solar Power System for your Home
Getting home solar electricity is much more affordable today than it was ten years ago.

In fact, many state governments are currently offering tax rebates to persons who choose to convert their systems to home solar electricity.

But, if installing a complete photovoltaic system to supply your home electricity is beyond your financial capabilities, there are also programs available which enable you to purchase your utilities from an accredited green energy supplier.
Green energy suppliers offer utilities which are fueled only by renewable resources, such as solar electricity, hydroelectricity, and wind-generated electricity.

If, however, you have decided to switch to utilizing a solar electricity system in your home, it's important to determine your electrical needs before you start shopping or planning.

How much power you need - The first thing to determine is how much power you will actually need for your home. Different arrays and solar panel set-ups offer different wattage outputs. Its important to take a the time to to determine what your average daily electricity usage will be.

You are also looking for a number that takes the entire year into account, and affords for the different amount of energy you will use during summer and winter months.

Most of us use more lights, heaters, air conditioners, etc., during different times of the year. So you will usually find that you use more electricity during some months, than you do during others.

The quality of the product - When shopping for the right photovoltaic system to supply your home solar electricity, make sure to research the quality of the product. A product which offers 13% efficiency, with a lifetime warranty, will be far more valuable than a panel offering 15% efficiency with a one-year warranty.

If you know of neighbors, friends, or even strangers who are currently using home solar electricity, ask them about it. Try to learn more about the differences amongst different types of systems.

You should also take some time to interview installers, and research different manufacturers as well.

Decide on your system – You will eventually need to make a final decision as to what type of system you will use, in order to supply your home with electricity. Generally three types of photovoltaic systems (which use solar panels) used to supply home solar power:

The first type is a grid-tied system. In this system, your solar panels are tied in with your existing power meter. You use both the electricity supplied by your panels, and, if and when needed, electricity from your electric company.
The second type of home solar power system is a grid-tie with battery backup. With this system you collect electricity from your solar panels, fill your batteries for emergencies, and send the excess back to the electric company through your power meter.

A stand-alone system – This is very similar to the grid-tied system with battery backup, except that you are not connected at all to any major power utility. This is the type of system which would be employed when living off-grid.
Once you have educated yourself on the different home solar power options that exist, you will probably be ready to start shopping.

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Remember, converting to home solar power is an investment. Financially, you will reap the rewards over time, 

You will also reduce the harmful impact of polluting fossil fuels on today's environment, which may be the most important benefit of all

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