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Concepts of Solar Photovoltaic (PV) System & Associated Challeges

29 Jul 2021

Dr Rajesh Arora, Delhi Transco Limited
-Hitesh Thareja, Delhi Technological University(Formerly Delhi College of Engineering)
-AbhishekDhasmana, HMR Institute of Technology and Management
 
Abstract
The constant increase in population of our country has led to an increase in the demand of the natural resources. The conventional resources are being used extensively and there is no way to get these resources back once exhausted. Thus, we need to find alternative sources of energy to fulfil this increasing demand and try to make use of the available resources in more judicious way. One such method getting popular these days is the use of photovoltaic system (Refer figure 1). It is a system in which the components used make use of sunlight to produce electricity.
This article gives insight into the basic components of PV System and finally it’s application in different scenarios and types & associated challenges.
 
Keywords: Photovoltaic (PV) Cells, Charge Controller, Battery Bank, Inverter, Grid-Tied System, Grid-Tied with Battery Backup, Off-Grid.
 
1.0 Introduction
1.1 Origin and Growth
Solar Photovoltaic (PV) effect is the process of conversion of light energy, i.e. photons into electrical energy. The special cells called photovoltaic solar cells convert the light energy directly into electricity. These cells use semiconducting layers to perform this task (Refer figure 2).
 
The first observation of Solar PV effect was made in 1839 by Edmond Becquerel. While experimenting, he found that, a metal electrode in a solution produced a greater amount of electricity when exposed to sunlight. In 1960’s, the solar cells were practically used in satellite technology. Since then, the researchers and scientists have tried to obtain maximum efficiency from the technology. The picture of Solar PV used in the beginning of solar PV technology is shown in figure 3.
 
 
If we talk about India, the total installed capacity of generation is around 374.2 GW. The renewable energy sector constitutes only about 24.158% of the total installed capacity, which is around 90.399 GW.
Also, the contribution of solar energy to the total renewable sector is around 40.83% and its’ contribution to total installed capacity is around 9.864%, i.e. around 36.91 GW as depicted in figure 4 (All the above data is till November 2020).India aims to reach a target of 100 GW capacity only from solar generation by 2022.
 
2.0 Construction and Working of Solar PV System
A photovoltaic cell is made up of a semi conductive material mainly silicon, which has 2 layers: an upper n-type layer and a lower p-type layer. The combination of these two layers behaves as junction diode. Both n-type and p-type layers are connected to an electrode each. The current flows in the load with the help of these electrodes.
 
2.1Construction of Solar Cells
The structure of pure silicon as shown in the figure 5 below, it can be seen that a silicon atom has four valence electrons and each of the atom makes covalent bond with four other silicon atoms giving it a tetrahedral structure. This kind of bonding of silicon restricts the movements of electrons as they are bonded.
 
To make an effective use of silicon, we make use of a dopant.
Dopant is the impurity added to the silicon to make the material electrically conductive.The process of adding an impurity in the semiconductor is known as doping.
One of the wafers of silicon is added with a pentavalent impurity (dopant), such as phosphorus. Four electrons of each phosphorus atom get bonded to the neighbouring four silicon atoms, whereas the fifth valence electron of the phosphorus atom remains unbonded as depicted in figure 6. This fifth electron is loosely bonded to the parent (phosphorus) atom. Even a small amount of energy is enough for this electron to get disassociated from the parent atom. Since, this free electron is not bonded with any covalent bond, so, it does not generate any hole or get recombined with it. Phosphorus is known as donor, as it donates one free electron. The semiconductors doped by donor impurities are known as n-type or negative type semiconductor as there are plenty of free electrons which are negatively charged by nature.These free electrons are readily available to conduct electricity. Although, the silicon wafer has these free electrons, still the wafer is electrically neutral. Also, for making the current to flow in a closed path, we need to create some kind of potential difference.In other words, to make the random movements of the free electrons in a particular direction, we need some driving force. This driving force is obtained with the help of PN-Junction.
 
When the silicon wafer is doped with a trivalent impurity (dopant), such as boron, the 3-valent dopant is capable of catching an extra electron by leaving holes in the valence band of silicon atoms as shown in figure 7.
 
On joining the p-type and n-type layers, some electrons move from n-side towards p-side by diffusion. This creates a depletion region with n-side slightly positive charged and the p-side slightly negative charged. Thus, the direction of electric field formed is from positive to negative i.e. n-type to p-type as shown in figure 8.
 
2.2 Working Principle of Solar Cells
When light waves hit the top surface of the silicon solar cell, only wavelength in a specific range including visible range gets absorbed in the depletion layer. The ultraviolet rays are too short to enter the cell and infrared layers are too large that they pass the cell without being absorbed or get reflected back.
The light wave energizes the electron of the silicon atom present in the depletion region resulting in the separation of the electron from the silicon atom creating a hole.Now, due to the action of electric field, the electron moves towards the n-type region and the hole moves towards the p-type region. This process continues till the cell gets light from the sun. Once, the electrons and holes are separated, connecting a wire between n-type and p-type makes a pathway for the electrons to flow and current is produced (refer figure 9).
 
2.3 Types of Solar Panels
There are three major types of Solar Panels:
(1) Monocrystalline: These are the oldest type of solar panels and are most developed. They are made of pure silicon. Also, the silicon used to manufacture these is highest grade silicon and this is the reason why the efficiency of these panels is higher than others. They are generally black in colour and are octagonal in shape (Refer figure 10).
 
(2) Polycrystalline: They are also known as multi crystalline solar panels. They consist of several crystals of silicon in a single PV cell. Many fragments of silicon are melted together to produce these types of panels. Efficiency of these panels is less than mono crystalline panels. Their appearance is non-uniform type with a shade of blue colour and are rectangular in shape (refer figure 11).
 
(3) Thin-Film Solar Panel: These are an upcoming new development in the solar panel industry. The most unique feature of these panels is that sometimes some other materials are used for the production of these cells. They can be made from a variety of materials, including cadmium telluride (CdTe), amorphous silicon (a-Si), and Copper Indium Gallium Selenide (CIGS). These solar cells are created by placing the main material in the middle of thin sheets of conductive material. In addition a layer of glass is added for protection as shown in figure 12.
 
3.0 Components of Photovoltaic System
The Photovoltaic system mainly consists of six major components:
1. Solar Photovoltaic Array: A PV Array is formed when many panels are connected together in series or parallel as depicted in figure 13. The main function of the array is to generate DC electricity from the sunlight.
 
2.Charge Controller: The main purpose of charge controller or regulator is to regulate the voltage and current, and to prevent batteries from overcharging coming from the solar panel. “12 volt panel” gives about 16-20 volts; if there is no regulation in battery, overcharging may damage batteries as these get fully charged around 14-14.5 volts(Refer figure 14). Two types of charge controllers are:
a) Pulse Width Modulation (PWM)
b) Maximum Power Point Tracking (MPPT)
3.Battery Bank: This component is used to store the energy. If the amount of electricity being used is less than the amount being generated, there the batteries play a crucial role in storing the energy (Refer figure 14).
 
4. Inverter: Since most of our appliances work on AC, therefore the DC obtained from the panels is converted into AC with the help of inverters (Refer figure 15). Inverters use Power electronic devices such as mosfets and IGBTs for high switching frequency.
 
5. Utility Meter: This device takes in account of the amount of energy generated, used or stored and fed to the grid.
6. Existing Power Network: It is interconnected network for delivering extra generated solar power to the grid. If a house is connected to electric grid, then the power will be fed to the grid once the battery is full. Also, if the solar panels are not able to meet the energy needs of a house, then grid can supply the house (Refer figure 16).
 
4.0 Types of Solar PV System
There are three types of solar PV System:
4.1 Off-Grid System
An off-grid system is not connected to the electricity grid and requires battery storage system. The design of off-Grid system should be such that it can generate enough power throughout the year and should have enough capacity of the battery to meet the requirements.
Off-grid systems are expensive due to high cost of batteries and off-grid inverters. This system is needed where there is non-availability of electric grid (Refer figure 17).
 
4.2 Grid-Tied System
A grid-tied electrical system is also called tied to grid or grid tie system which is a semi-autonomous electrical generation or grid energy storage system which links to the mains so that the excess capacity is fed back to the local mains electrical grid. When electricity is not available in sufficient amount, electricity which is drawn from the mains grid can be utilized to meet the demand. In the same way, when excess amount of electricity is available, it is fed to the mains grid. When the Utility or network operator restricts the amount of electricity going into the grid, it is possible to prevent any input into the grid by installing Export Limiting devices. The homeowner is paid by the utility company for the power they provide to the electrical grid. Grid-tied solar is currently the most popular type of home solar power system in the market. The import-export of electricity is recorded by import-export meter as shown in the figure 18.
Direct Current (DC) electricity is passed into an inverter which is grid tied. The inverter monitors the alternating current mains supply frequency and generated electricity (solar PV). It must accurately match the voltage and phase of grid sine wave. When the grid fails to accept power during a "black out", most inverters are able to provide courtesy (emergency) power.
 
4.3 Grid-tied System with Battery Back-up
This system is also known as the grid-hybrid system.This system generates power in the same way as a common grid-tie system but use some additional components such as special hybrid inverters and batteries to store energy. Since, it stores energy, thus enables most hybrid systems to also operate as a backup power supply during a blackout (Refer figure 19).
 
5.0 Advantages, Disadvantages and Challenges of Solar PV System
The solar technology has it’s own benefits and demerits. Also, the challenges that would be faced are discussed below
5.1 Advantages:
i. Inexhaustible
ii. Low maintenance cost
iii. Reduces electricity bills
iv. Job Creation
5.2 Disadvantages:
i. The initial cost of installation is high.
ii. Weather dependent: Output decreases on cloudy days.
iii. Uses a lot of space.
 
5.3 Challenges:
1. India has a goal of 100 GW installation solar capacity by 2022 and only 36-37 GW has been installed as of November 2020. Trying to meet such a target in the situation of Covid-19 is itself a big challenge.
2. Since, there is a sudden shift to Non-Conventional Sources, efficiency of the components plays a key role in determining the overall performance. Thus, many researchers are trying to maximize the efficiency.
3. The cost of the setup is too high. So, there is a need to find out the ways to reduce the cost. 
4. Too much dependency on import of material poses a big challenge.
 
6.0 Conclusion
This article has provided the basics of solar PV system, it’s working and makes us realize that the energy from the sun is freely available. We are lucky enough to be present in a generation where the technology is at such an advanced level. Considering this, we should use this energy to the best possible extent. In fact, if we know the ways to harness the energy, then why to waste this opportunity. Solar technology is the upcoming technology after the fossil fuels. The brief/summary of the article is given below:
1. The first observation of Solar PV effect was made in 1839.
2. The contribution of solar energy to the total renewable sector is around 40.83% and its’ contribution to total installed capacity is around 9.86%, i.e. around 36.91 GW
3. Photovoltaic Cell is made of two layers n-type and p-type. In n-type layer phosphorus (pentavalent) is used as impurity (dopant) and in p-type layer boron (trivalent) is used as impurity (dopant).
4. There are three types of solar panels: Monocrystalline, Polycrystalline and Thin-Film Solar Panel.
5. There are three types of PV system: Off-Grid System, Grid-Tied System, Grid-tied system with battery back-up.
6. Apart from the benefits of solar PV system, there are some challenges also like India’s Installation goal by 2022, efficiency, cost, dependency on import etc.
 

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