Solar Energy and Photovoltaic Systems

Solar Energy and Photovoltaic Systems

Solar Energy and Photovoltaic Systems

Vaijayanthi Nayar

Solar Energy and Photovoltaic Systems

Vaijayanthi Nayar

ISBN - 9789361522925

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Preface

A system that is a collection of one or more solar panels combined with an inverter and some other mechanical, as well as electrical hardware, which is helping the energy from the sun to generate current or electricity, is called Photovoltaic system. The concept of the book is all about the PV systems and their operations. As we know, photovoltaics is the study of the combination of physics, photochemistry, and electrochemistry. The main concern is about how the sunlight is transferred as electricity through solar panels.

In addition to it, the subject will deal with the requirement of components and also the maintenance of them is very important. Today, the technology is developed, including the increase of the manufacturing scale and usability of current also been increased, so it is very easy to identify the changes, especially on the conversion of energy from sunlight to the solar panels. With every detailed information, this book will give out an exclusive valid subject to analyze them.

This is the book to help you out in how PV is the third renewable source in terms of global capacity after hydro and wind powers. Also, the drastic changes along with the growth of PV to be included in it with the coverage of global percentage demand. It is an opportunity to work in this industry and required to know the knowledge of how it could be done to perform well on the PV system.

The main thing to notice is the impact in the environment with the usage of PV systems. Let us get into it by taking some of the measures without changing the environment.

Table of Contents

1 Introduction to Photovoltaic 1

1.1 Introduction 1

1.2 Energy and Role of Photovoltaic 5

1.2.1 Solar Panel 6

1.2.2 Inverters 6

1.2.3 Racking 8

1.3 Photovoltaic in Energy Supply 10

1.3.1 Renewable Energy Sources 18

1.3.2 Types of Renewable Sources 20

1.4 Solar PV Production and Cost 24

1.4.1 Type of Inverter 26

1.4.2 Type of Mounting System 26

1.5 Exercise 27

2 Solar Cell Technologies 28

2.1 Solar Cell Technologies 28

2.1.1 Nameplate DC Rating 37

2.1.2 Diode and Connection Loss 37

2.1.3 Mismatch Losses 38

2.1.4 DC and AC Wiring 38

2.1.5 Sun-Tracking Loss 38

2.1.6 Shading Losses 39

2.1.7 Soiling Losses 39

2.2 Crystalline Cells 41

2.3 Thin Film Cells 48

2.4 CIGS Solar Cell 54

2.4.1 Metastability 57

2.4.2 Potential-Induced Degradation 58

2.4.3 Shading-Induced Damage 58

2.4.4 Comparative Cell Analysis 59

2.4.5 Band-Edge Effects 59

2.5 Exercise 60

3 Concentrators and PV Modules 61

3.1 Introduction 61

3.1.1 Fresnel Lens 63

3.1.2 Luminescent Concentrators 64

3.2 Series Resistance Optimization 70

3.3 Solar Photovoltaic Modules 76

3.4 Exercise 86

4 Solar PV Systems 87

4.1 Solar PV systems 87

4.2 Electrical Storage 92

4.3 Photovoltaic Systems 122

4.3.1 PV System Configuration 122

4.4 Grid Connection System 127

4.5 Exercise 136

5 Planning Procedure of System Capacity 138

5.1 Introduction 138

5.2 Module Selection and PV Generator 143

5.3 Selection and Sizing of Cables 151

5.3.1 Solar DC Cables 153

5.3.2 Mounting Systems 164

5.3.3 Sloped Roof Mounting Systems 164

5.3.4 Flat Roof Mounting Systems 166

5.3.5 Solar Shingles and BIPV 166

5.4 Exercise 167

6 Installation and Commissioning 168

6.1 Introduction 168

6.1.1 Commissioning 179

6.1.2 Earthing 182

6.1.3 Junction Box 184

6.1.4 Batteries 188

6.1.5 Commissioning Protocol 189

6.2 Instrumentation and Measurements 191

6.3 Economical and Financial Analysis 196

6.4 Exercise 202

Appendix 204

Glossary 206

Index 207

Chapter

1 Introduction to Photovoltaic

1.1 INTRODUCTION

A photovoltaic (PV) system is a system composed of one or more solar panels combined with an inverter and other electrical and mechanical hardware that use energy from the Sun to generate electricity. PV systems can vary greatly in size from a small rooftop or portable systems to massive utility-scale generation plants. Although PV systems can operate by themselves as off-grid PV systems, this article focuses on systems connected to the utility grid, or grid-tied PV systems.

Photovoltaics offer consumers the ability to generate electricity in a clean, quiet and reliable way. Photovoltaic systems are comprised of photovoltaic cells, devices that convert light energy directly into electricity. Because the source of light is usually the sun, they are often called solar cells. The word photovoltaic comes from photo, meaning light, and voltaic, which refers to producing electricity. Therefore, the photovoltaic process is producing electricity directly from sunlight. Photovoltaics are often referred to as PV.

PV systems are being installed by Texans who already have grid-supplied electricity but want to begin to live more independently or who are concerned about the environment. For some applications where small amounts of electricity are required, like emergency call boxes, PV systems are often cost-justified even when grid electricity is not very far away. When applications require larger amounts of electricity and are located away from existing power lines, photovoltaic systems can, in many cases, offer the least expensive, most viable option. In use today on street lights, gate openers and other low power tasks, photovoltaics are gaining popularity in Texas and around the world as their price declines and efficiency increases.

For more in detail by now, photovoltaic (PV) panels are familiar to most Americans. You have seen them on your hand-held calculator, on top of illuminated highway signs, and maybe even on your neighbors’ roofs. With PV systems becoming more common, perhaps you have been dreaming of making some homemade electricity. The dream is achievable, as long as you own a sunny patch of lawn or an unshaded south-facing rooftop, and as long as you have a bank balance of several thousand dollars.

A PV array is made up of rectangular modules (or panels) that measure between 2 and 5 feet on a side. The most common type of PV module has an aluminium frame and a glass cover protecting a collection of polycrystalline PV cells. When exposed to light, each PV cell produces 0.5 volt DC. So, if you add up the number of cells and divide by 2, you know the voltage of the module. The best performing commercially available PV cells are roughly 20% efficient at converting solar energy into electricity.

Unlike polycrystalline PV cells, Thin film (amorphous) PV products are manufactured on a flexible sheet. These Thin film PV products have many applications; for example, they are used to make PV roof shingles and peel-and-stick membranes designed for use on metal roofing. Thin film PV products have relatively low efficiencies, usually in the range of 10% to 12%, so they require almost twice the area required for a polycrystalline PV array with the same electrical output.

Both polycrystalline and Thin film PV arrays produce DC power. This DC electricity can be used directly to charge a battery; in most homes, however, the solar electricity is sent to an inverter that converts the DC power to AC. The inverter’s AC output can be used directly by the owners of the house or fed into the power grid.

Today, photovoltaic (PV) systems are widely known for their ability to generate electric power for homes and commercial buildings through the absorption of energy from the sun’s rays. In the movement toward increased sustainability, cleaner energy, and lower utility bills, they are becoming a more common sight, especially in regions of the country where sunlight is the strongest. However, there are still misunderstandings among building and design professionals and not to mention their customers also about the finer details of a PV system. Before specifying or installing a PV system, building and design, professionals should have a solid understanding of these systems and how they function. This article will go over the anatomy of PV systems and how they operate, factors that influence their performance, and how they contribute to LEED certification.

The most basic part of a PV system is the cell. Groups of cells are assembled in a series to form a module, which is enclosed between sheets of tempered glass or plastic to protect the cells. A group of modules wired together in one line is called a string, and a set of module strings installed together is called a PV array. The array may consist of one or more strings. An array may be designed to generate a specific amount of energy or to maximize the available roof area. The most common type of PV cell is crystalline silicon (c-Si). These can be fabricated from either a single silicon crystal (monocrystalline) or multiple silicon crystals cast in a single block (multicrystalline or polycrystalline). Thin film PV cells, however, use alloys, such as amorphous silicon (a-Si), copper indium gallium diselenide (CIGS), and cadmium telluride (CdTe). Thin film modules are made by depositing a very thin layer of semiconductor onto a substrate–usually glass, metal foil, or plastic. Flexible, Thin film modules are deposited onto flexible substrates (metal foil or plastic) and encapsulated in plastics to retain flexibility. Currently, these modules are less efficient than mono- or multi-crystalline-based modules, but also generally cost less.

The light from the Sun, made up of packets of energy called photons, falls onto a solar panel and creates an electric current through a process called the photovoltaic effect. Each panel produces a relatively small amount of energy but can be linked together with other panels to produce higher amounts of energy as a solar array. The electricity produced from a solar panel (or array) is in the form of direct current (DC). Although many electronic devices use DC electricity, including your phone or laptop, they are designed to operate using the electrical utility grid, which provides (and requires) alternating current (AC). Therefore, in order for solar electricity to be useful, it must first be converted from DC to AC using an inverter. This AC electricity from the inverter can then be used to power electronics locally, or be sent on to the electrical grid for use elsewhere.

Easily, we can say that PV cells convert sunlight directly into electricity without creating any air or water pollution. PV cells are made of at least two layers of semiconductor material. One layer has a positive charge, the other negative. When light enters the cell, some of the photons from the light are absorbed by the semiconductor atoms, freeing electrons from the cell’s negative layer to flow through an external circuit and back into the positive layer.

Fig 1.1 : Photovoltaic System

Image Source: energymodels.com

Link: http://energy-models.com/introduction-photovoltaic-systems

The flow of electrons produces an electric current. To increase their utility, dozens of individual PV cells are interconnected together in a sealed, weatherproof package called a module. When two modules are wired together in series, their voltage is doubled while the current stays constant. When two modules are wired in parallel, their current is doubled while the voltage stays constant. To get the desired voltage and current, modules are wired in series and parallel into what is called a PV array. The flexibility of the modular PV system allows designers to create solar power systems that can meet a wide variety of electrical needs, no matter how large or small.

In addition to the solar panels, there are other important components of a photovoltaic system, which are commonly referred to as the balance of system or BOS. These components (which typically account for over half of the system cost and most maintenance) can include inverters, racking, wiring, combiners, disconnects, circuit breakers and electric meters.

1.2 Energy and Role of Photovoltaic

Along with the PV array, the PV system is completed with equipment collectively called the Balance of System (BOS). The overall task of the BOS is to convert the electricity generated by the PV array into a form identical to the electricity provided by the building owner’s utility company and safely route it to the building’s existing electrical system for distribution. Additional functions involve maximizing energy harvested by the PV array, safety, maintenance, and monitoring of the array. In this article, we are focusing only on grid-connected buildings. The major components of the BOS include an inverter, conduit and wiring, disconnects, and fuses if needed. The BOS connects and interacts with a building’s existing electrical system to distribute power to loads in the building or to back feed the grid.

For best performance, terrestrial PV systems aim to maximize the time they face the sun. Solar trackers achieve this by moving PV modules to follow the sun. The increase can be by as much as 20% in winter and by as much as 50% in summer. Static mounted systems can be optimized by analysis of the sun path. PV modules are often set to latitude tilt, an angle equal to the latitude, but performance can be improved by adjusting the angle for summer or winter. Generally, as with other semiconductor devices, temperatures above room temperature reduce the performance of photovoltaic modules.

1.2.1 Solar Panel

A solar panel consists of many solar cells with semiconductor properties encapsulated within a material to protect it from the environment. These properties enable the cell to capture light, or more specifically, the photons from the sun and convert their energy into useful electricity through a process called the photovoltaic effect. On either side of the semiconductor is a layer of conducting material which collects the electricity produced. The illuminated side of the panel also contains an anti-reflection coating to minimize the losses due to reflection. The majority of solar panels produced worldwide are made from crystalline silicon, which has a theoretical efficiency limit of 33% for converting the Sun’s energy into electricity. Many other semiconductor materials and solar cell technologies have been developed that operate at higher efficiencies, but these come with a higher cost to manufacture.

1.2.2 Inverters

An inverter is an electrical device that accepts electrical current in the form of direct current (DC) and converts it to alternating current (AC). For solar energy systems, this means the DC from the solar array is fed through an inverter, which converts it to AC. This conversion is necessary to operate most electric devices or interface with the electrical grid. Inverters are important for almost all solar energy systems and are typically the most expensive component after the solar panels themselves. Most inverters have conversion efficiencies of 90% or higher and contain important safety features, including ground fault circuit interruption and anti-islanding. These shut down the PV system when there is a loss of grid power.

The primary job of a grid-tied inverter is to convert the initial direct current (DC) electricity generated by the PV array to alternating current (AC) electricity, allowing it to provide power to appliances in the building. Inverters are also responsible for conditioning the converted AC power to match that of the electrical power coming from the grid, allowing excess power to flow into the grid. Many utilities support a policy of net metering so that the utility bills are credited when electricity flows into the grid via the inverter. Grid-tie inverters are also designed to disconnect from the grid if it goes down quickly. This is a National Electrical Code (NEC) requirement that ensures that if a blackout occurs, the inverter will not be feeding any current into the grid, which could harm any linemen working on the grid. This also means that the home or building is unable to use any of the DC power being generated by the PV system while the grid is down. Inverters typically enable the PV array to be monitored for power output, usage, and other performance characteristics. They utilize a technique called Maximum Power Point Tracking (MPPT) to get the maximum power from the PV array. The inverter constantly monitors the PV array’s electrical production and adjusts the amount of current it draws from the array to get the maximum power.

Fig 1.2 : Role of Photovoltaic

Image Source: fsec.ucf.edu

Link: http://www.fsec.ucf.edu/en/consumer/solar_electricity/basics/how_pv_system_works.htm

1.2.3 Racking

Racking refers to the mounting apparatus, which fixes the solar array to the ground or rooftop. Typically constructed from steel or aluminium, these apparatuses mechanically fix the solar panels in place with a high level of precision. Racking systems should be designed to withstand extreme weather events such as hurricane or tornado level wind speeds and/or high accumulations of snow. Another important feature of racking systems is to electrically bond and ground the solar array to prevent electrocution. Rooftop racking systems typically came in two variations, including flat roof systems and pitched roof systems. For flat rooftops, it is common for the racking system to include weighted ballast to hold the array to the roof using gravity. On pitched rooftops, the racking system must be mechanically anchored to the roof structure. Ground-mounted PV systems, as shown in figure 4, can also use either ballast or mechanical anchors to fix the array to the ground. Some ground-mounted racking systems also incorporate tracking systems that use motors and sensors to track the Sun through the sky, increasing the amount of energy generated at higher equipment and maintenance costs.

Disconnect and Overcurrent Devices

The NEC requires disconnect devices (circuit breakers or switches) between PV equipment and all sources of power. For example, a disconnect device is needed between the inverter and the grid. This enables service and emergency personnel to ensure that no power is flowing in the system if