Know everything about Solar Electricity

The electricity that is produced from sunlight is called solar electricity.

Let us understand the concept and the process of the generation of solar electricity.

What is electricity?

Electricity is the flow of electrons (charge) per unit of time.

All the matter in this universe is made up of atoms and the atom is made up of protons, neutrons, and electrons.

Structure of an atom

The name “Atom” is a Greek word that means indivisible because it was believed that atoms can’t be divided.

Later, it was found that atoms are made of sub-atomic particles called Electrons, Protons, and Neutrons.

The center of the atom is called the nucleus. It contains protons and neutrons. 

The protons are positively charged while neutrons are neutral. The nucleus is surrounded by negatively charged electrons.

This charge in electrons is equal and opposite to that of protons.

Also, the number of electrons in an atom is equal to the number of protons in its nucleus. This equal and opposite charge makes the atom neutral and stable.

When some outside force is applied, the balance between the electrons and protons gets upset.

This outside force causes the loss or gain of electrons.

When an electron is lost from an atom, it causes the free movement of the particular electron.

​This free movement of the electron constitutes an electric current.

And the energy for moving the electrons is provided by the sunlight.

Let us know how it happens.

Understand the source of energy

Light is electromagnetic radiation of a wide range emitted by the Sun.

And the part of it which can be detected by the human eye is called visible light.

(Visible light is the basic medium to see each other and perceive things around us).

Light is also a source of life on the earth.

It keeps our earth warm and helps plants in synthesizing their food which in return provides us with fresh oxygen to inhale.

But our human eyes can detect wavelengths of a very narrow range from 700 nano-meters (red) to 400 nano-meters (violet).

And this is called the visible band of the electromagnetic spectrum.

(1 nano-meter = 1 billionth of a meter)

In other words, we can say that visible light is a small subset of the wide electromagnetic spectrum.

What are electromagnetic radiations?

Electromagnetic radiations are a form of energy that is present all around us.

And sunlight is a form of electromagnetic radiation.
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Electromagnetic waves are produced when a charged particle says an electron is accelerated under the influence of the electric field.

This results in the production of the electric field and the magnetic mutually perpendicular to each other and also to the wave propagation.

Electric field, Magnetic field, and Direction of wave propagation all three are perpendicular to each other.

Properties of EM waves

• The EM waves can travel in a vacuum means they do not require any medium to propagate, unlike sound waves which require a medium to propagate as you will not hear any sound in a vacuum.
• The sunlight travels with a speed of 29,97,92,458 meters per second and reaches the surface of the earth in 8 minutes and 20 seconds.

Dual Nature of Light

The light has dual nature one is wave nature (due to the interference effect in light beams, implying that light is made up of waves).

And another is particle nature (the photoelectric effect supports the particle nature of light and the light may be viewed as packets of energy called photons).

However, to know the complete nature of light at any moment requires a detailed analysis which is seldom required while studying the photo-voltaic solar power system.
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Therefore, I am limiting my discussion about the dual nature of light to an introduction which is enough at this point.

The Electromagnetic Spectrum

The electromagnetic radiations occur over a wide spectrum from gamma rays having wavelengths as low as (100 picometers, one-trillionth of a meter) to radio waves in meters.

The spectral band adjacent to and above the violet region is called as Ultraviolet region. And the band below the red region of the visible band is called the infrared region of the electromagnetic spectrum.

Characteristics of light important for solar PV

However, there are certain characteristics of light that are required while studying and designing the solar power system and these are as follows:

Wavelength

All wavelengths falling on the surface of the panel cannot knock out electrons (from the valence band to the conduction band) and set them free to move and produce current.

So, solar panels perform under certain wavelengths of the electromagnetic spectrum.

The solar panels are made of silicon cells which perform well when the wavelength of the light around 1127 nm falls on the surface of the silicon cells.

The energy corresponding to the 1127 nm wavelength is sufficient enough to send electrons from the valence band to the conduction band by jumping the band gap.

While less energy may not be able to pluck the electrons while more energy than required results in heating and reduces the performance of the panel.

The minimum energy required by silicon cells

S.No	Electromagnetic radiations	Wavelength (nm)	Energy (eV)
1	Ultra Violet	290 – 400	4.27 – 3.099
2	Violet	400	3.1
3	Indigo	445	2.78
4	Blue	475	2.61
5	Green	510	2.43
6	Yellow	570	2.17
7	Orange	590	2.1
8	Red	650	1.9
9	Infrared	710 – 1500	1.75 – 0.826

A typical solar cell requires about 1.1 eV (The band gap of silicon is 1.1 eV).

It is the energy required by the electron to jump from the valence band to the conduction band.

The electrons absorb this energy, cross the band, and start the conduction of the electricity.

You can see in the table above that this amount of energy is present within the infrared region of light with a wavelength of around 1127 nm.

So, the infrared region of light with wavelength <= 1127 nm (Energy is >= 1.1 eV) is capable enough to make silicon cells start conducting and generating electricity.

The shorter wavelengths (< 1127 nm) carry higher energy.

Therefore, these are capable of extracting the electron from the valence band to the conduction band.

But higher energy does not necessarily mean an increase in the amount of the current flow. Generally, energy greater than 1.1 eV is passed out as heat in the solar panels.

What is Solar Electricity?

Solar electricity is the generation of free electrons when sunlight falls on a semiconductor material like silicon.

We can also say that solar electricity is the process of the conversion of solar energy into electrical energy.

Semiconductor material like silicon has the property to conduct an electric current when sunlight falls on it.

This process is also called the photovoltaic effect.

When the photons of a particular wavelength (around 1127 nm) carrying energy fall over the surface of the semiconductor material like silicon.

The cells absorb the energy which in turn excites the electrons to move out of their shells and constitutes the electric current.
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This is the basic principle behind solar electricity.  

When the multiple silicon cells or chips or wafers are connected together by soldering the wafers and then encapsulating the cells in toughened, high transmittance glass to form a module or the solar panel.

Solar Panels can run electrical appliances

This is the building block solar PV system that generates electricity from sunlight.

Any number of solar panels can be connected to get desired output of current. These PV modules can be taken at any place. Therefore, we have a mobile source of electricity.

However, the electric current generated by the module is direct current. Most of the household supply of current is alternating in nature i.e. the alternating current.

Therefore, an electrical device is required which can convert the direct current into alternating current.

This device is called an Inverter.

With the combination of solar panels and an inverter, one can get an alternating current that can be used to run electrical appliances like ceiling fans, CFLs, television, fridges, etc.

The PV panel generates electricity without harmful emissions and gases, unlike coal and petroleum.

Moreover, its operation is silent.

It requires very little maintenance.

Also, the life of the PV modules is in the range of 25 to 30 years.

The Efficiency of Solar Cells

All of the energy that a solar cell or panel receives is not totally converted into electricity but most of it gets dissipated & wasted in the form of heat.

This is because of the fact that a very specific amount of energy is required to extract the electron from the shell of the silicon atom.

Therefore, any energy above 1.1 eV most of the time is not converted into electricity but rather dissipated as heat into the surroundings.

And because of this very fact, the efficiency of solar cells is between 15% -22% that is they are able to convert only that percentage of light energy received by them into electricity.

How to improve efficiency?

Improving the efficiency of solar cells is the most challenging task for renewable energy scientists.

Today, the most efficient solar cell has an efficiency of over 22%

One way to improve the efficiency of the solar cell is to limit the reflected light from the surface of the cell.

This can be done by anti-refection coatings but these coatings cannot prevent the transmission of light and it passes through the cell straight forward and gets lost in the surroundings.

Another approach is light trapping in thin solar cells.

It prevents both the reflection and transmission of light, increasing the efficiency of the solar cells by light confinement (a photon bouncing back and forth many times within the cell) and increasing the absorption rate of the light within the cell.

However, different strategies are adopted to increase light confinement within the cell.

The simple approach could be to reduce the reflection of light falling on the surface of the cell by changing the texture in such a way that it enhances the bouncing of light within the cell rather than to the surroundings.

The textured surface makes the incident light bounce on the surface of the cell multiple times and hence increases the path length of the photon.

However, It is possible to enhance the path length up to 50 times the device thickness.

Indicating that the light bounces back and forth within the cell resulting in light confinement and increasing the efficiency of the solar cell.

You can see in the above picture that by altering the texture of the solar cell we can achieve multiple reflections within the cell, increasing the path length of the photon and hence increasing the efficiency of the solar cell.

The Peak Sun Hours

The variation in the sunlight falling on the solar panels affects their performance.

Solar radiation is blocked by moisture in the air and clouds. So, the panels perform less in cloudy weather and in moist climates.

The Tilt

The angle at which the solar radiations are falling on the panel in a day or throughout the year.

The radiations falling perpendicular to the surface of the panel will knock out more electrons and hence will generate more solar electricity than that of the obliquely falling radiations.

You will see an improved performance during the day when the sun is overhead and the rays are perpendicular to the surface and the output is low in the morning and the evening when the sun rays are grazing at an angle.

Watt and Watt-hour: The most power terms in Solar Electricity

The most common terms that you come across when you make up your mind about solar are power and energy.

• How much power will my solar panels generate?
• How much energy from the solar panels is required to meet my energy requirements?

Most of us get confused between these terms and it is very important to know the terms clearly before going for solar power installation.

​Let us solve this confusion with simple examples:

Watt in Solar Panels

When we talk about a watt, we are talking about power. It is the rate at which energy is consumed or produced while traveling or flowing from one point to another.

In simple words, the power is how quickly the energy is consumed or produced and the unit of power is "watt"

Let me clear this up with the help of a simple example:

Assuming, your office is on the 5th floor. As you are a health-conscious guy, you use stairs to climb to your office.

On Monday, it took you 5 minutes to reach your cabin while on Tuesday it took a bit longer, 6 minutes, to reach the same destination.

The energy consumption is the same in both cases but you used more power on Monday because the same energy is consumed more quickly as compared to that on Tuesday.
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A 100-watt bulb is consuming energy more quickly than a 40-watt bulb and hence shines brighter. 

Watt-hour

Watt-hour is the unit of energy.  

It measures the amount of work performed or energy consumed

You start from your home in the morning to meet your friend living in the nearby locality.

Whether you run or walk slowly to meet him, the amount of work done or the energy consumed is the same.
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Therefore, your electricity bill is based on the amount of energy consumed.

The Difference

Watt represents how fast the energy is consumed while Watt-hour/kW-hr tells the amount of energy consumed.

A 100-watt bulb when running for 2 hours (100 x 2 = 200-watt-hour) will consume equal energy to that of a 40-watt bulb when left on for 5 hours (40 x 5 = 200-watt-hour).

Power = Energy/ Time

What is the use in Solar?

The knowledge of watt and watt-hour is very important in sizing the solar panels and designing the overall solar power system.
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Say you have the following electrical appliances in your home with their quantity and power as follows:

The average daytime running time, in hours, of the above electrical appliances is as follows:

• Fans: 6 hours
• Bulb: 4 hours
• T.V.: 2 hours

The total energy consumed in the daytime is 4760 Watt-hours and is calculated as follows:

Therefore, we need a set of solar panels which can provide a total of 4760 watt-hours of energy in the daytime.

Conclusion

The process of production of electric current from the sunlight is Solar Electricity.

However, light of certain wavelengths and energy is responsible for this process.

a) If wavelength > 1127 nm, then Energy < 1.1 eV and there is no flow of current
b) If wavelength < 1127 nm, then Energy > 1.1 eV and the flow of current starts

However, it is not necessary that with the increase in the energy, falling on the silicon cells, there will be an increase in the current flow.

The extra energy > 1.1 eV is generally dissipated as heat into the surroundings.

Therefore, one cannot say that there is a linear relationship between energy and the flow of the current. 

Moreover, most of the daily electricity needs can be fulfilled with solar electricity.

There is an almost continuous decline in the prices of silicon PV cells from $ 76 per watt in 1977 to $ 0.3 per watt in the year 2015.

An impressive CAGR (Compound Annual Growth Rate) of around 15.7%.

Are you interested in installing a solar roof at your home and want to know the complete feasibility beforehand?

​Try my Solar Feasibility Spreadsheet, a very simple yet powerful tool for determining the technical and financial feasibility of your residential solar power system.

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