Fundamental Aspects Of The Hybrid Wind Engineering Essay

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Name: ALSHAMMARI FAHAD SHUMRAN

Student ID: 1091105465

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ACKNOWLEDGEMENT

First praise is to Allah, the Almighty, on whom ultimately we depend for sustenance and guidance. Second, apart from my efforts, the success of any project depends largely on the support and guidelines of many others who support me at every step I followed. I would like to show my greatest appreciation to my supervisor Dr. Naeem Mohamed Hannoon. I can't say thank you enough for his tremendous support and help. I feel motivated and encouraged at every time I attend his meeting. His knowledge, advices and patience have also got me overcome numerous difficulties encountered while doing the tasks.

I am also appreciated and thankful for my partner Mr. Omar Gonium, who has been always enthusiastic, supportive and diligent. From the starting of the project to the end we were facing the difficulties together and overcome thanks to the strong will and patience

ABSTRACT

This project covers the fundamental aspects of the Hybrid Wind & Solar Energy System for and deals with the modeling of an electrical energy system supplied with energy provided by renewable resource. The main objective is the study of hybrid systems behavior, which allows employing renewable and variable in time energy sources with a continuous supply.

table of contents

List of tables

List of figures

List of equations

Acronyms

WCED World Commission on Environment and Development

AC Alternative Current

DC Direct Current

EIA Energy Information Administration

PVS Photovoltaic System

CSP Concentrated Solar Power

HSWPS Hybrid Solar/Wind Power System

WES Wind Energy System

PMG Permanent Magnet Generator

AH Ampere per Hour

MOSFET Metal Oxide Semiconductor Field Effect Transistor

IGBT Insulated Gate Bipolar Transistor

LPSP loss of power supply probability

INTRODUCTION

Background

The energy conversion from one form to another have been existed since ancient times when primitive man was converting energy by burning wood for warming. Over time, the conversion of energy developed and divested until the boom in the world of energy conversion, when Scottish engineer James Watt (1736-1819) invented the steam engine in which the conversion of thermal energy from coal burning to boil the water and then to the rotational kinetic energy. This machine is used until today as a steam turbine to generate large percentage of the total electrical energy generated in the world. The industrial development persistent to evolve in this area until 1838 when British scientist called Michael Faraday invented the first electric generator in which the electricity is generated by using a copper disk that rotated between the poles of a magnet. The combination of those tow inventions produce Turning point in which the development of industrial, technological and informational progress have been accelerated. This development created the need for the growing demand for energy, which provides continuity of this progress. The electrical energy becomes a significant element of any country economic and security. Subsequently, the demand of the electricity is growing year after a year.

In the recent past, specifically in the beginnings of the industrial revolution at the middle of the seventeenth century in that era the main source and the only one to meet the needs of energy was fossil fuel. It had been used for many purposes such as running engines. Coal was utilized for this purposes and then, the diesel engine came after which was invented by a German engineer called Rudolf Diesel in 1892 to launch the era of oil engines. Even though, coal still widely utilized to operates thermal power plants so far. With scientific progress, Geologists found two important points that play a major role in utilizing of fossil fuels as a major source of energy production. The first point is the sustainability of these natural resources. The second point which is the most important is their impact on the environment.

The sustainable development is defined by (WCED, 1987) as meeting the needs of the present generations without compromising the ability of future generations to meet their own needs [1]. With this tremendous development witnessed by the world in various fields of energy consumption rate rose significantly as a Statistics indicate that he will grow by 2.4% in 2030 as shown in Table 1-1. It is noticeably clear, the heavy reliance on fossil fuels in the coverage of this growth. While, there are promising indicators in the growth of dependence on alternative energy. This is because the world head on already towards sustainable development.

Table ‎1: Electricity Balance [2].

Increasing energy demand coupled with a growing demand of natural resources which fossil fuels expected to reach about 86 % of global electricity production in 2030[3]. This percentage is distributed on oil 36.0%, coal 27.4%, and natural gas 23.0% [4]. This percentage is considered high comparing to the available reserve of fossil fuels which is shown in Table 1-2. The coal reserves in the world 64.99% followed by crude oil by 17.67% and then natural gas natural with 17.43% [5]. These resources always vulnerable to the increase or decrease in price same as other goods in the international economy. So that, they are important indicators in any country's economy either in importing or exporting. The crude oil consumption is the highest in the world's energy supply by 36.4% [4]. It is also subjected to the economic crises that affect its prices such as the war in Iraq and Iran and the Gulf War [6]. Sustainability of fossil fuel is an important issue but it is not the only issue that been considered to reduce the dependence on them. Plant health is another issue that associated with utilizing fossil fuel.

Table ‎1: fossil fuel reserves around the world in 2006[5]

As humans we do not have another place we can live better than our planet. This is due to the suitable climate for our lives. Nevertheless the use of fossil fuels may negatively affect the climate. This affect has three main issues. The first issue is the escape of carbon dioxide (CO2) from exhaust of power plants. The sum of carbon dioxide leads to its accumulation in the atmosphere of the planet and therefore, the heat entering the atmosphere cannot come out. Thus, heat accumulates too that what called global warming also it referred to greenhouse effect. That global warming has a negative impact on the climate as the following expression suggests:

→ Temperature Increase → Negative impacts

‎1.

The second issue is the occurrence of gas sulfur dioxide (SO2), which causes acid rain. Acid rain has impact on the environment. It increases the acidity of water springs, lakes and soil, harm plant life, effects and damage building material, and harms the human health through the negative impact of this rain on the environment. The third issue is particulate emissions (fly ash) which are harmful when it is breathed by human. The figure 1-1 shows the amount of waste of a power station with capacity of 2000MW in single day.

Figure ‎1: the amount of waste of a coal-fired power station in single day [7].

The concept of sustainable energy must be incorporated with the concept of clean energy because the conditions of present requirement need them together. If we look to sources of energy available now will see that the best sources to which it applies these concepts are solar and wind energy. But these two types of energy do not attain to feed large quantities of energy. For solar energy, the largest solar plant in the world located in Mojave Desert, California with production of 354MW [8]. While for wind power station, The Alta Wind Energy Center in in Kern County, California considered the largest wind frame with capacity of 1020 MW [9]. In order to raise the effectiveness of the use of these two types of energy, the idea of merging two types of energy known as hybrid. Hybrid is a system in which different types of energy incorporate together to charge a battery and the battery serve the load. But limitation batteries used now days not capable to store large quantities of power. Hybrid systems with renewable resources usually used for light loads. Such systems provide a solution for our energy's problems because of their sustainability and their less environment impacts.

Project Motivation

The motivation of doing this project is basically to build a standalone hybrid solar wind energy system for light load that supply different types of loads i.e. AC or DC with sufficient and economic manner. The solar and wind renewable energy sources become widely used in past few years because of their advantages over fossil fuel sources. The solar and wind renewable energy sources have a limitation in availability since they are related to the weather conditions. However, merging them together could overcome the issue of availability of these sources if a desired battery is used to store the energy. The load demand may vary during the day so that there are periods where the power delivered is more than load demand. The battery used to store that surplus energy. Also, there are a periods where the load demand is more than the power delivered the battery then cover up the shortage of energy. The optimal sizing of the system plays an important role in designing stage. It could effectively reduce the cost and increase the efficiency of the system.

Project Objectives

The analysis and design of this project are implemented in the laboratory with the aid of selected computer software; objectives can be listed as follow:

To study, analyze, and build namely a hybrid energy system solar and wind at prototype with full utilization of power output for light load.

To study, analyze, and build a sufficient Series-resonant inverter for light loads.

To provide an optimal sizing of hybrid solar wind energy system that finds the optimal size of PV panel and battery capacity based on given load curve.

Project Scope

The scope of this project is implementation of Solar/Wind hybrid system that supplies a given light load in reliable and efficient manner during a day with full utilization of the load. The reliability of the system is provided by increasing the reliability of the battery. The battery uses a charge controller which protects the battery from over-charging and over-discharging. Such protection can effectively increase the reliability of the battery. The efficiency of the system is done by making the battery in parallel with the load so that it can be categorized into three cases.

When the supply energy is more than the load demands the battery, the surplus energy is used to charge the battery.

When the supply energy is less than the load demand the battery, the battery is used as a third source to cover up the shortage of energy.

When the supply energy is equal to the load demand the battery, the battery remains in unchanged situation.

Then, these three cases are applied into optimal sizing calculations that find the efficient number of PV panels and the efficient battery's capacity to be used in order to supply a given variable load with reliable energy during a day. The utilization of the load is done by promote three types of the load which are 12VDC, 12VAC (by inverter), and 220V

Project Methodology

In order to complete this project within the specified duration, a methodology is undertaken as in Figure 1-2. It is known that well-planned organization of overall research progress is fundamentally important therefore the project can be finished efficiently in the giving time frame. Some Computer programs have been used in this project for purpose of calculations, implementation, and simulation tasks such as MATLAB, proteus, and mikroC.

Figure ‎1: Project Flow and research methodology

Thesis Organization

In order to make the reader grabs the flow and the contents of this project; the report is organized into five chapters as following:

Chapter 1 talks about the project motivations and issues that associated with the covenantal sources of energy. Research objectives are also presented. The scope of this project has been viewed as well. Chapter 1, also, illustrates the methodology presented in this report and the thesis organization.

Chapter 2 is the literature review of hybrid system that focuses on the background and basic information. Besides that, it shows the concept of various renewable energy sources. In this chapter, the reasons why the hybrid solar/wind system was chosen as a topic for this project is illustrated. The solar and wind energy systems have been studies individually. The charge controller of battery and battery concepts has been reviewed. It is also explain the operation principle of series resonant inverter. Finally, the principle of hybrid solar/wind system's optimal sizing using Loss of power supply probability (LPSP) has been reviewed.

Chapter 3 is the methodology of the project. This chapter essentially discusses in details how the project was built. A successful of project is all rely of this chapter. As a matter of fact a step by step trick for each section of the activity involves in doing the project will be deeply explained.

Chapter 4 is the data presentation and analysis. This chapter shows the experimental results and the theoretical results. The results are shown in form of data, graphs, and waveforms. It is also compares between the experimental results and theoretical results.

Chapter 5 will ultimately end the repot with discussions with a brief conclusion. Recommendations are also included for the development of the further work.

LITERATURE REVIEW

Introduction to Renewable Energy

Nowadays, debridement of world is heading towards renewable energy because of its many benefits that solve the problems of use fossil fuel resources for energy production. Many studies have proposed renewable energy as solution of climate change. (Sims et. al. 2003) studied how the adoption of renewable energy technologies could help to reduce carbon emissions by 8.7-18.7% by 2020[10]. (Jacobson and Masters, 2001) suggested that the U.S. can reduce carbon dioxide emissions by replacing 60% of coal-fired power station with 214,000-236,000 wind turbines rated at 1.5MW[11]. EIA defined the renewable energy as "Energy sources that are naturally replenishing but flow limited [12]. These limitations are not because of the limited renewable sources in terms of quantity, but are related to infrastructure used for renewable sources since the world over the past decades focused on fossil energy sources. Theoretically, renewable energy is enough to cover the need for electric power. (EIA, 2008) estimated the power required to fulfill all worldwide usage of power is about 12.5 TW trillion watts excluding losses in production and transmission [13].

The renewable sources such as; solar, wind, biomass, tidal, and geothermal which are delivered only electricity of 2 TW of the end-use total power [13]. While, the energy available in the renewable can be more than that value and also can be in many forms of energy as follow; Wind energy is the kinetic energy in the wind that been converted to mechanical energy in wind turbine, and then to electric energy by electric generator. (Sesto & Ancona, 1995) stated that the wind energy on the Earth is huge and enough to meet all the world's electricity needs. Essentially every country has sites with average wind speeds of more than 5 m/s measured at a height of 10 m [14]. Tidal energy is the movement of massive water masses in the oceans creates the tides involves huge amount of energy. Tidal energy is used by power stations on coasts with high tidal ranges. At high tide, water is stored into reservoirs and is prevented from flow back as the tide ebbs that create a potential difference between the collected water in reservoirs and water outside the reservoirs. The collected water is then released though turbines into the sea at low tide. The turbines rotate and produce electric energy. China is one of the richest countries in tidal energy, (Wang & Shi, 2008) estimated that the average power available from tidal energy in China exceeds 13,940MWwithout considering the un-investigated sea area that may experience strong currents [15]. Wave energy is captured from the surface waves or from pressure fluctuations below the surface. Waves are caused by the wind blowing the surface of the ocean. In many areas of the world, the wind blows with enough uniformity and force to provide continuous waves. There is huge energy in the ocean waves. Wave power devices extract energy directly from the surface motion of ocean waves or from pressure fluctuations below the surface [16]. In 2006, a 10 kW wave energy converter prototype was installed in Lysekil on the Swedish west coast results of this converter are available in (Thorburn & Leijon, 2006) [17]. Hydroelectric energy is the potential energy of water at a high level that utilized by water falling through turbines to produce electricity. Hydroelectric is the most popular form renewable energy because of its capability to store energy. (Ibrahim et. al., 2008) specified that pumped hydroelectric energy storage is the most utilized and developed large-scale energy storage technology available for electricity [18]. (Deane et. al., 2010) indicates there is over 7 GW of new pumped hydroelectric energy storage plants planned in the Europe Union only [19]. The geothermal energy is based on the fact that the earth's core contains a substantial amount of energy in the form of heat. This is referred to as geothermal energy and has the potential to generate geothermal power to provide huge amount of a renewable electricity supply [20]. (Mock & Tester, 1997) estimated the growth of the geothermal energy's usage with rate of about 8.5% per year since about 1920[21]. Biomass is carbon based and is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth and heavy metals. These metals are often found in functional molecules such as the porphyrins which include chlorophyll which contains magnesium [22]. (IPCC, 200) suggested large shares of biomass in the future energy system since it is play an important role in reducing greenhouse gas emissions [23]. Solar power is the conversion of sunlight into electricity, this conversion can be directly using photovoltaic (PV), or indirectly using concentrated solar power (CSP). Concentrated solar power systems use lenses and tracking systems to focus a large area of sunlight into a small beam. Photovoltaic convert light into electric current using the photoelectric effect [24].

From those diverge renewable energy sources the idea of Zero Energy Buildings is derived which defined by (Laustsen, 2008) as "Zero Energy Buildings do not use fossil fuels but only get all their required energy from solar energy and other renewable energy sources"[24]. Regularly, to ensure the optimal and efficient use of renewable energy, two renewable source of energy supply the load because renewable energy somehow related to climatic conditions based on the type of renewable energy used. So that, the need of such system required and that called as hybrid system.

Hybrid System

Hybrid systems combine two or more energy source, to be used in single device, so that; it overcomes limitations inherent in either. Hybrid systems can overcome limitations in terms of fuel flexibility, efficiency, reliability, emissions or economics. The case study chosen in this project is to use solar energy sources coupled with wind energy source to form a hybrid solar wind power system (HSWPS) for light load.

Hybrid Solar/Wind Power System

A hybrid solar wind power system (HSWPS) is important type of renewable energy systems. The hybrid combination of a photovoltaic system (PVS) and a wind energy system (WES) improves the overall energy output and reduces energy storage requirements [26]. HSWPS is based on the scenario that when there are no wind blowing the load still can be functional by solar energy and vice versa. The load is usually a battery bank to ensure the sustainability of energy supply. A basic HSWPS contain many parts as in figure 2-1 photovoltaic system (PVS), wind energy system (WES), energy storage, charge controller, and some other required circuit and components.

Figure ‎2: hybrid solar wind power system [27].

Photovoltaic System (PVS)

The term "photovoltaic" comes from the Greek word "phos" means light and the word "volt" named after Alessandro Volta. The term photovoltaic literally means light generating electricity. It is Converting photo (light) into voltaic (electrical voltage). The photovoltaic system is based on the photovoltaic effect in which the electrical potential developed between two dissimilar materials when their common junction is illuminated with radiation of photons as illustrated in figure 2-2. The photovoltaic effect is modeled by what so-called the Photovoltaic (PV) Panels. There are three basic types of PV panels though all of them use silicon which are mono-crystalline, polycrystalline and amorphous. Mono-crystalline cell is cut from a single crystal of silicon. They are a slice from a crystal. Mono-crystalline cell is the most efficient and the most expensive to be produced. Polycrystalline (or Multi-crystalline) cell is a slice cut from a block of silicon. It consists of a large number of crystals. Amorphous cell is made by placing a thin film of amorphous (non-crystalline) silicon onto a wide choice of surfaces. It is the least efficient and least expensive to be produced [29].

Figure ‎2: Photovoltaic effect converts photon energy into voltage across the p-n junction [28].

History of Photovoltaic System

The first utilization of solar energy was in 1767 by Swiss scientist named Horace-Benedict de Saussure made the first solar collector which is an insulated box covered with three layers of glass to absorb heat energy. Saussure's box reached temperatures of 230 degrees Fahrenheit [30]. In 1839 A French scientists called Edmond Becquerel discovered the photovoltaic effect using two electrodes placed in an electrolyte. After exposing it to the light, electricity increased [30]. In 1953, Calvin Fuller, Gerald Pearson, and Daryl Chapin, discovered the silicon solar cell. This solar cell produced enough electricity to run small electrical devices. The new York Times stated that this discovery was "the beginning of a new era, leading eventually to the realization of harnessing the almost limitless energy of the sun for the uses of civilization[31].In 1956, the first solar cells are available commercially. The cost was $300 for a 1 watt solar cell which was too expensive to consumers [31]. In the early 1960's satellites in the USA's and Soviet's space program were powered by solar cells and in the late 1960's solar power were the standard for powering space bound satellites[31]. In the early 1970's an efficient way to reduce the cost of solar cells was researched. This way carried the price down from $100 per watt to around $20 per watt. The research was organized by Exxon. Most of Exxon's off-shore oil rigs utilized the solar cells to power the waning lights on the top of the rigs [31]. In 1981, Paul Macready produced the first solar aircraft. The aircraft utilized more than 1600 cells, placed on the wings. The first flight of solar aircraft was from France to England [30]. Between 1986 and 1999, Evolution of large scale solar energy plants grew up. In 1999 the largest solar plant at that time was developed producing more than 20 KW.

The past few years have seen huge investment in utility-scale solar plants. In 2012, the largest solar energy plant in history have been introduced which is the Golmud Solar Park in China, with an installed capacity of 200 MW[30].

The Distribution of Solar Energy on Earth's Surface

The massive amount of energy provided by the sun encourage scientist to consider solar energy as alternative energy source. Earth receives 174,000 TW of solar radiation coming from the sun at the upper atmosphere (Smil, 1991) [32]. The figure 2-3 Shows About 29% of the solar energy that reaches at the top of the atmosphere is reflected to space by clouds, atmospheric particles, or bright ground surfaces like sea ice and snow. About 23% of incoming solar energy is absorbed in the atmosphere by water vapor, dust, and ozone, and the rest which is 48% passes through the atmosphere and is absorbed by the surface. Thus, about 71% of the total incoming solar energy is absorbed by the Earth system [33].

Figure ‎2: the percentage of reflected and absorbed solar energy at the earth's atmosphere [33].

From The figure 2-3, the maximum solar energy absorbed by the surface of earth is about. Nevertheless, that amount is not fixed in each meter square of the earth's surface. It is at variance from a place to another as shown in figure 2-4.

Figure ‎2: The annual mean net radiation balance from the Earth Radiation Budget Experiment (ERBE), 1985-1986[34].

Construction and Working Principle of Photovoltaic Panel

The photovoltaic cell is constructed by coupling two semiconductor materials n-type semiconductor material in top and p-type semiconductor material in bottom. The energy of incident light hits the n-type semiconductor material cause electrons displaces from their normal atomic orbits. Metal conductor strips that lane in the top layer of silicon capture those electrons. The cell converts them into a current. Another metal panel is attached to the bottom layer of silicon to feed electrons back to the cell. On the top of the cell, an anti-reflective coating is made to absorb as much light as possible by minimizing the reflection of light. A mechanical protection is provided by glass cover with a transparent adhesive Figure 2-5 shows basic construction of PV cell.

Figure ‎2: Basic construction of PV cell [28].

PV cell is the most basic building block in PV system. Its size is a few square inches and produces about one Watt of power. PV module is several of such cells are connected in series and parallel circuits on a panel of several square feet that produce a higher power. The solar array is a group of several modules connected in series-parallel combinations to produce even higher power [28].

Equivalent Electrical Circuit

The PV cell can be represented by the equivalent circuit shown in Figure 2.6. The output-terminal current (I) is equal to the light-generated current minus the diode-current and the shunt-leakage current. ( ) represents the internal resistance due to the current flow which depends on the P-n junction depth, the impurities and the contact resistance. The shunt resistance is inversely proportional to leakage current into the ground. In ideal case where (no series loss), and (no leakage into ground). The open circuit voltage is achieved when the load current is zero (I = 0), and is given by equation (2.1).

‎2.

The diode current is given by equation (2.2).

‎2.

Where

= the saturation current of the diode

Q = electron charge = 1.6 Coulombs

A = curve fitting constant

K = Boltzmann constant = 1.38 Joule/°K

T = temperature on absolute scale °K

Therefore the load current given by equation (2.3):

‎2.

The last term, the ground-leakage current in practical cells is small compared to and and can be ignored. Hence the diode-saturation current can be determined experimentally by applying voltage in the dark and measuring the current going into the cell. This current is regularly called as dark current or the reverse diode-saturation current.

Figure ‎2: The PV cell's equivalent circuit [28].

Photovoltaic Cell's Efficiency

The efficiency of photovoltaic cell is a measure of how much solar power input that strikes the photovoltaic cell is converted to electrical power in the load as in equation 2.4.

‎2.

At the moment maximum efficiency obtained of around 40% is using multi junction cells (multiple layers of silicon) each layer tuned to trap different frequencies of light. This type of cell is expensive to be produced. It is used in space where the efficiency is very important [29].

Photovoltaic Panel Tracking System

Solar trackers are device which is used to adjust photovoltaic panels toward the sun. As the sun's location in the atmosphere changes with the seasons and the time of day, tracker is used to align the collection system to maximize energy production. The photovoltaic panel tracking system is one of the methods to increase efficiency of photovoltaic panel. It keeps tracking of maximum sunlight beam to face the photovoltaic panel. There are two types of photovoltaic panel tracking system. The first type is single axis tracking system which follows the sunlight from east to west during the day (Kalogirou, 1996) [35]. The second type is multi-axes tracking system which follows the sunlight from east to west during the day and from north to south during the seasons of the year (Bakos, 2006) [36]. The performance of PV modules with daily two-position in the morning and in the afternoon resulting that an increasing of energy by 10-20% over a fixed PV system at an optimal angle (Tomson, 2008) [37]. Multi-axes (North-South, East-West, vertical) electromechanical sun-tracking system resulting an even higher energy increasing for about 30-45% compared to fixed PV system (Abu-Khadera et al, 2009)[38].the figure 2-7 Shows the different axes that PV tracking system is used.

Figure ‎2: North-South, East-West tracking system [39].

Modelling of Photovoltaic Panel

The modeling model of photovoltaic cells can be presented equation (2.5). it shows the power output of single photovoltaic panel in term of global irradiance incident on the plane. The photovoltaic panel efficiency is in term of many factors such as the PV panel tracking system efficiency which is perfectly is equal to one [40].

‎2.

Where

= the power output of single photovoltaic panel

= photovoltaic panel efficiency

= the area of a single panel

= the global irradiance incident on the plane

Wind Energy System (WES)

In some way, Wind is initiated from the sun. As the earth spins around the sun daily, light and heat is received. The majority of the heat is received at the equator and it is slowly reduced towards both poles of earth. The heat differences create wind. In hot regions of the earth the air becomes hot and consequently pressure is increased. Thus, the air starts to flow towards colder regions that have less temperature and less pressure. That phenomena cause the wind energy that can be converted to many other forms of energy. One of those forms is electrical energy that uses a wind turbine as prime mover coupled with electrical generator to produce clearly free and clean electrical energy.

History of Wind Energy

Wind power was used to sail ships for the first time at the Nile river in Egypt some 5000 years ago [28]. In early middle Ages, the first windmills were used in Sistan which is a region between Iran and Afghanistan. In 14th century, windmill had been utilized in Dutch to drain areas of the Rhine River delta. In 1887, the first utilization of wind energy to produce electrical energy was pioneered to Scottish Professor James Blyth by a wind generator which was used to light his holiday home in Marykirk [41]. In 1908, at United States there were 72 wind generators with rating of 5 kW to 25 kW and the largest machine was on 24 m tower with four-blade and 23 m diameter rotor [42]. In 1941, S. Morgan Smith Company commissioned the world's first megawatt-size wind turbine in Castleton, Vermont, USA. This wind generator delivered 1.25 MW [43]. In 1978, Teachers and students of school in Tvind at Denmark constructed the world's first multi-megawatt wind turbine [44]. Today, the world's biggest wind generator is commissioned by the German wind turbine producer Enercon with installed capacity of 7.5 MW. It has a hub height of 135 m, rotor diameter of 126 m and a total height of 198 m [45].

The Distribution of Wind Energy in the Earth

Universally, the wind energy are available over the world's land and ocean surfaces at 100 m is about 1700 TW if all wind at all speeds were used to power wind turbines[46]. Though, the wind power over land in locations where wind speed is equal to 7 m/s or higher is between 72-170 TW [47]. These locations regularly exist in the Great Plains of the U.S. and Canada, Northern Europe, the Gobi and Sahara Deserts, much of the Australian desert areas, and parts of South Africa. Cristina L. and (Mark Z, 2005) [48] estimated the average wind speed at 80 m height over all days of a at sounding locations with 20 valid readings each day is shown in the figure 2-8.

Figure ‎2: Wind speed at 80 m height [48]

The Working Principle of the Wind Energy System

The wind turbine captures the kinetic energy in wind through a rotor that consists of two or more blades mechanically coupled to a shaft an electrical generator. The shaft is connected then to an electrical generator that produces electric energy. However, in term of construction such a simple design is not sufficient enough to produce the required electrical energy. Some other components, specifications, design consideration, and site selection need to be considered while designing. For example, Wind turbine is attached into a tower to capture the supreme velocity of wind energy. The magnitude of the electric energy is proportional to the available power in the wind. The available power in the wind is given by equation 2.6.

‎2.

Where

ρ = density of air (1.201kg/)

A = swept area of blades ()

= mean air velocity (/s)

The three factors that influence the output of a wind energy system are: air density, swept area, and mean air velocity. Air density is the mass per unit volume of Earth's atmosphere which varies with height. Generally, 300 meters increase in height has a decrease of 3% in the air density. Temperature and humidity are also affect air density. Warmer air has less air density than colder air. However, humidity affect are usually negligible. Air density is not a factor that can be controlled. But, there are other key factors that can be considered. Swept area is the area of the circle that created when the blades are spinning. The longer blades make a larger swept area. Therefore, a larger swept area withdraws more energy captured from the wind. Theoretically, a double swept area doubles the output. Swept area given by equation 2.7[49].

‎2.

Where

r = length of blades (m).

The mean air velocity is the most important factor that affects the output of a wind turbine since the power obtained from the wind increases with the cube of wind speed. This factor is depends on the location of the wind energy system. In figure 2-9 shows an example of how the mean air velocity is affected when it's placed in top of a mountain. It is clearly seen the percentage is much higher in top due to collision of wind with foothill [49].

Figure ‎2: Effect of top-mountain location on Wind Speed [28].

The Components of Wind Energy System

Even though the basic operation idea of wind energy system is a simple, its design involves a high cost many other components that maximize its efficiency to provide the sufficient energy required. The main components that comprise the wind energy system are tower, wind turbine, the yaw system, the mechanical gear, the electrical generator, and a combination of speed sensors and controls as shown in figure 2-10.

Figure ‎2: The components of wind energy system [28].

The Tower

The wind tower supports the turbine and the nacelle that contains the mechanical gear, the electrical generator, the yaw system, and the other components. The main issue in the towers design is the dynamic structure. The tower vibration and cause undesirable resonance frequencies when wind speed fluctuates. The height of tower plays an important role to set the blades length which in turn it defines the swept area. For medium and large turbines, the tower should be marginally taller than the rotor diameter. While for Small turbines are usually fixed on the tower a few rotor diameters high. Steel and concrete towers are being used. Their construction can be tubular or lattice [28].

Wind Turbine

Wind turbine is an apparatus that collects kinetic energy from the wind to be carried out into wind generator. Wind turbine contains two or three blades which are usually made of high-density wood or fiberglass. There are two common types of wind turbines the vertical axis and horizontal-axis. The vertical axis is in the shape of an egg beater sometimes referred as Darrieus rotor. The horizontal-axis the rotor shaft and electrical generator located at the top of a tower in the direction of the wind. The horizontal-axis is the most common wind turbines being used today. The number of revolutions per minute of a wind turbine rotor ranges between 40 rpm and 400 rpm according to the corresponding wind speed. Wind turbines regularly use a gearbox transmission to increase the rotation of the generator's rotor to a speed that sufficient for desired electric energy production [28].

Yaw System

The yaw system continuously adjusts the rotor in the direction of the wind. The modern wind energy systems contain an automatically controlled yaw system. Automatic yaw system modeled by three main components which they are; yaw bearing, yaw drives and yaw brakes. Yaw bearing is a rotatable connection between the tower and the nacelle of the wind turbine. It rotates the nacelle whenever the direction of wind direction changed. The yaw bearing can handle very high loads since it must be capable to carry out weight of the nacelle and rotor. Yaw drives control a powerful motor that are in turn move the yaw bearing. The yaw drives are connected to a control system which determine the direction of speed and send a desired control signal in order to rotate the nacelle in that direction. The yaw brakes stabilize the yaw bearing from uncontrolled rotation such as oscillating due to the rotor rotation. Yaw brakes can be through in hydraulic mechanism or electric mechanism [28].

The Mechanical Gears

Mechanical gear is a mechanical device which consists of various gears in an enclosure in order to convert the rotational speed of an input shaft to a different speed on the output shaft. Wind turbine's blades usually have rotational speed smaller than required speed of the generator's shaft. Therefore, the gearbox must increase the rotor speed to become applicable for the generator's shaft. The rate in which the speed is transformed is called gear ratio. Gear ratio is the relationship between the numbers of teeth on two gears or even more gears which are matched. The friction between the gears produces a large amount of heat in the gearbox. This heat must be removed from the gearbox. Otherwise, the gears will be overheated and damaged. For this reason, lubricating oil is used to reduce friction and cool down the heat. Figure 2-11 shows a typical wind turbine gearbox.

Figure ‎2: A typical wind turbine gearbox [50].

The Electrical Generator

The conversion of the mechanical energy of the wind turbine into the electrical energy is accomplished many types of electrical machines. This electrical energy can be direct current (DC) or alternative current (AC), depending on the type of electrical machine used. The electrical machines work on the principles of the electromagnetic actions and reactions. Hence, the resulting electromechanical energy conversion is reversible. The same machine can be used for motoring such that it converts the electrical power into mechanical power, or for generating when, it converts the mechanical power into the electrical power. In wind power system, the criteria that choosing of electrical generator is based on many factors such as weight of active materials, operational characteristics, applicable type of semiconductor power converter, protection considerations, service and maintenance aspects, environmental considerations, and price[51]. Many types of generators can be used for HSWPS such as the direct current (DC) machine, the synchronous machine, or the induction machine [51].

The Permanent Magnet Generator

The permanent magnet generator is a generator where the excitation field is delivered by a permanent magnet as a replacement for a coil. The permanent magnet generator has an advantage over the other types of generators which is the property of self-excitation. This property allows operation at high power factor and efficiency and that's why the PMG is proposed as a wind turbine generator. However, large permanent magnet generator is costly so that it limited the economic rating of the machine. The PMG differs from the other generators in the magnetization which is provided by a permanent magnet pole on the rotor, instead of using excitation current from the armature winding terminals as that in the induction generator. Therefore the PMG's output frequency is in a fixed relationship to the shaft speed, while in the induction generator, the frequency is closely related to the network frequency, being related by the slip. The main disadvantage of the permanent magnet generator is that it does not provide a constant voltage, when the shaft speed and the load current vary which result a small voltage regulation (Mitcham & Grum, 1998) [51]. (Naoe, 1995)[52] Proposed a method of fitting series capacitors to improve voltage regulation.

PM generators are usually axial-flux or radial-flux generator as shown in figure 2-12. The axial-flux generator usually has slot-less air-gap windings. Since elimination of using slots simplifies the winding design. The magnets are in flat shape which is easy to manufacture. The length of the axial-flux generator is short compared to the radial-flux machine. The main disadvantage axial-flux generator is in maintaining a small air gap in a large diameter machine and the structural stability of the large diameter discs (Chalmers et al., 1996) [53].

(b)

Figure ‎2: (a) Radial flux machine construction (b) axial flux machine construction [54].

.

A Combination of Speed Sensors and Controls

A combination of sensors and controlling schemes is required in wind energy system. The controlling schemes fall into the following categories [25]:

• No speed control schemes: In this method, the turbine, the electrical generator, and the entire system are designed to withstand the extreme speed of wind.

• Yaw and tilt control schemes: in which the rotor axis is shifted out of the wind direction when the wind speed exceeds the design limit.

• Pitch control schemes: Pitch control changes the pitch of the blade with the changing wind speed to regulate the rotor speed.

• Stall control schemes: when the wind speed goes beyond the safe limit on the system, the blades are shifted into a position such that they stall. The turbine needs to be restarted after the gust has gone.

Energy Storage

The electrical energy is difficult to be stored in large scale. Practically most of electrical energy is generated upon demand. That might be satisfied for covenantal methods of electricity generation where the fuel is available whenever it's needed. Nevertheless, the hybrid solar and wind energy source cannot meet the load demand all of the time due its dependency on weather conditions. As a result, an energy storage mechanism is required. There are a plenty of technologies that being used for this purpose such as electrochemical battery, flywheel, compressed air, and superconducting coil.

Electrochemical Battery

Electrochemical battery is the most widely used device for energy storage. Electrochemical battery stores energy in the electrochemical form. Electrochemical Battery only store and deliver DC power. It has one-way conversion efficiency between 85% and 90%. There are two basic types of batteries the primary battery and the secondary battery. The electrochemical reaction in the primary battery is nonreversible; the battery is discarded after discharge. The secondary battery is also known as the rechargeable battery. The electrochemical reaction in the secondary battery is reversible, therefore after a discharge; it can be recharged by direct current from an external source. The electrochemical battery is made of many cells which are connected in a series-parallel combination for obtaining the desired operating voltage and current. The higher the number of cells in series indicates to higher the battery voltage. Figure 2-13 shows a lead acid internal construction of 12V battery that constructed by connected six cells of 2V each in series [55].

Figure ‎2: lead acid internal construction [56].

Lead Acid Battery

There are different types of rechargeable battery used for storing energy. Lead acid battery is the most common rechargeable battery type due to its maturity and high performance over cost ratio, although it has then least energy density by weight and volume. Under discharging of lead-acid battery, water and lead sulfate are formed, the water dilutes the sulfuric acid electrolyte, and the specific gravity of the electrolyte decreases with the decreasing state of charge. Under charging of lead-acid battery the reaction is reversed in the lead and lead dioxide is formed at the negative and positive plates, respectively, restoring the battery into its original charged state [28].

Capacity of Battery

Capacity of battery is the amount of electric charge that can be stored in the battery cells. The most common measure of battery capacity is Ampere per hour (AH) which is defined as the number of hours for which a battery can provide a current equal to the discharge rate at the nominal voltage of the battery [57].

Charge Controller

The main function of charge controller is to protect the battery from overcharge, over discharge, and provide load control functions. Lake of battery control might reduce battery performance and possibly cause a safety risk. In case of overcharge, controller will sense the battery voltage and stop the charging current when the voltage become over rated voltage of battery. Deep overcharging causes electrolyte to be boiled which cause damage of the battery. Charging controllers prevent excessive battery overcharge by discontinue the current flow from the source to the battery when the battery becomes fully charged [58]. Over discharge protection occurs when heavy electrical load usage and energy produced by the source is not be sufficient enough to keep the battery fully recharged. The fact that the load is generally not continuous and unpredictable which allow excessive battery over discharging imposes need of using charge controller. On the other hand, for a predictable and continuous doesn't require over discharge protection. The controller disconnects the battery from electrical load. When a battery has excessive discharged repetitively, it will loss the capacity and life eventually. Over discharge protection is accomplished by open-circuiting the connection between the battery and electrical load when the battery reaches adjustable low voltage load disconnect set point [49]. Two basic methods can be used to control charging of a battery from the sources which are they series and shunt regulation. Both of these methods can be effectively used. Each method may include a number of variations that adjust basic performance and applicability. Charge controller mechanism is characterized by choosing two different voltage thresholds. For example, 12 V batteries at higher voltage of 12.4 V, charge controller switches the load to the battery and disconnect the source. While, at lower voltage of 11.5 V, charge controller switches the load off.

Some Other Circuit and Components

Some other circuits and components are integrated with the system for purpose of signal processing and load utilization such as bridge rectifier, inverter, and step-up transformer. Bridge rectifier required for converting the AC power generated from wind energy system into DC power so that it can be processed in charge controller and charge the battery. Inverter is used for providing an AC output to be used in the load. It converts the DC power from battery charger to AC power. Transformer is used to step up the AC power to 220VAC for further utilization of the load.

Bridge Rectifier Circuit

Bridge rectifier circuit as shown in figure 2.14 consists of four diodes connected in a bridge topology. At any instant of time, only two diodes will be operated and other two will remain idle. In the positive half cycle, two diodes (D1 and D2) will be in forward bias mode and the other two diodes (D3 and D4) will be in reverse bias mode. In the negative half cycle, the other two diodes (D3 and D4) which were earlier in reverse bias mode will be in forward bias mode and the other two (D1 and D2) will be in reverse bias mode. The signal output of first stage shown in figure 2-14

Figure ‎2: Bridge rectifier circuit and its waveform [59].

The smoothing capacitor starts to charge up when the voltage increases during the first half of the voltage peaks from the rectifier. When the voltage decreases to zero in the second half of the peaks, the smoothing capacitor releases the stored energy to keep the output voltage as constant as possible. The efficiency of rectifier is the ratio of the dc output power to ac input power as described in equation 2.8. The maximum efficiency of a full wave bridge rectifier is 81.2% [59].

‎2.

Inverter

The function of an inverter is the opposite of the rectifier function. It converts a DC power to AC power. The inverter is the interface between the battery and load. The resultant output of inverter could be rectangle wave, square wave, quasi square wave or sine wave. And each output wave has its corresponding mechanism and circuitry to be produced. Inverters can be built using microprocessor circuits, 555 timers integrated with IGBT, MOSFET, or BJT transistors, and many other technologies. Many factors are used to evaluate inverters in hybrid system applications. First important factor is quality of the output voltage. Secondly, the inverter is supposed to have good transient response under transients of the load. Finally, Efficiency and cost are also important factors need to be considered [60]. The efficiency of Inverter is a ratio of AC power to DC power as in equation 2-9.

‎2.

The DC to AC conversion generally involves four stages in which PWM signals generated to inject a switching device that produce square wave . The square wave then subjected to the filter that filter out the harmonic and shape the signals. Finally, the filtered signals amplified using step-up transformer.

Voltage-Source Series-Resonant Inverters

One common inverter topology is Class D DC-AC resonant inverter which proposed by [Baxandall, 1959] (61). It utilized in many applications such as DC to AC inverters, DC-DC converter, high-frequency electric process heating, radio transmitters, and solid-state electronic ballasts for fluorescent lamps. Class D inverters are categorized into two main types which are Class D series (or voltage-source) inverters, and Class D parallel (or current-source) inverters. Class D series inverter consists of a voltage-fed half-bridge inverter with a series LCR circuit as shown in figure 2-15. In Class D series inverter, the current through the resonant circuit is sinusoidal and the currents through the switches are half-wave sinusoids. While, the voltages across the switches are square waves. Class D parallel inverter consists of a current-fed half-bridge inverter with a parallel LCR circuit as shown in figure 1xb. In Class D parallel inverter, the voltage across the resonant circuit is sinusoidal and the voltages across the switches are half-wave sinusoids. While, the currents through the switches are square waves.

Figure ‎2: Class D series inverter

Class D series inverter has an advantage over of Class D parallel inverter which is the low voltage across the semiconductor switches that is equal to the supply voltage. Therefore, Class D series inverter is preferable for low-voltage-rated inverters which increases their efficiency and decreases their cost (62).

Voltage-Source Series-Resonant Inverter that uses half-bridge topology as shown in Figure 1xa. It is constructed by two bidirectional switches S1 and S2 and a series resonant circuit LCR. The bidirectional switches consists of a transistor (power MOSFET, IGBT, or BJT) and an antiparallel diode. The switches can conduct either positive or negative current. If the transistor is OFF, the switch can conduct only a negative current that flows through the diode. If the transistor is ON, the switch can conduct only a positive current that flows through the transistor. Switches S1 and S2 are ON and OFF alternately with a duty ratio of 50%. The dead time is recommended to reduce the switching losses during the overlapping of voltage and current when the switch changes its state. The desired operation is when the switching devices turn on and off at zero current. The switching losses also can be reduced by adjusting the switching frequency above the resonant frequency and reducing the duty cycle below 50% (63).

Principle of operation

The operation series resonant inverter may be explained by three mode of operation in which the resonant frequency varied with respect to the switching frequency. Figure 2-16 shows the corresponding waveform of the three modes in term of Inductor current and capacitor voltage(64).

Figure ‎2: waveform of series resonant inverter with respect to (64)

Discontinuous Conduction Mode with < 0.5; Figure x1a shows the waveforms of and the resonant capacitor voltage in this mode of operation. The conduction will be as follow. From 0 to t1, Q1 conducts. From t1 to t2, the current in Q1 reverses its direction. The current flows through D1 and back to the supply source. From t2 to t3, both switches are in the off state. From t3 to t4, Q2 conducts. From t4 to t5, the current in Q2 reverses its direction. The current flows through D2 and back to the supply source. Q1 and Q2 are switched on under zero-current switching condition and they are switched off under zero-current and zero-voltage conditions. However, the switches are under high current stress in this mode of operation and thus have higher conduction loss (64).

Continuous Conduction Mode (CCM) with 0.5 < < ; Figure 16.24b shows the waveforms of inductor current and the resonant capacitor voltage in this mode of operation. The conduction will be as follow. From 0 to t1, the inductor current transfers from D2 to Q1. Q1 is switched on with finite switch current and voltage, having in turn-on switching loss. From t1 to t2, D1 conducts and Q1 is turned off softly with zero voltage and zero current. From t2 to t3, Q2 is switched on with finite switch current and voltage. At t3, Q2 is turned off softly and D2 conducts up until t4.

Continuous Conduction Mode (CCM) with ; Figure 16.24c shows the waveforms of and the resonant capacitor voltage in this mode of operation. The conduction will be as follow. From 0 to t1, inductor current transfers from D1 to Q1. Therefore, Q1 is switched on with zero current and zero voltage. At t1, Q1 is switched off with some voltage and current, causing in turn-off switching loss. From t1 to t2, D2 conducts. From t2 to t3, Q2 is switched on with zero current and zero voltage. At t3, Q2 is switched off. Inductor current transfers from Q2 to D1. The switches are turned on with ZVS (64).

However, there is a turn off switching loss in the transistor, while turn-on of the transistor and the diode are eliminated. The turn-off switching loss can be reduced using a dead time in the controlling signals. The ideal operation is when the switching frequency is equal to the resonant frequency in which the switching loss become zero and the output current is purely sinusoidal but this is difficult since the load is composed of inductive and capacitive components that effect the operation of the inverter. The switching topology used is called Push-Pull Follower.

Push-Pull Follower

Push-Pull Follower is a simple transistor circuit constructed by one NPN transistor and one PNP transistor as shown in figure 2-17. It is called Push Pull follower since NPN Transistor "push" drives the output voltage in a positive direction with respect to ground while PNP transistor "pull" drives the output voltage toward a negative direction with respect to ground. The resultant waveform is a square wave where it's positive cycle produced by NPN transistor and its negative cycle generated by PNP transistor.

Figure ‎2: Wavform of push-pull follower

BJT, or MOSFET can be used as switching device. (Kutkut, ) compared the efficiency of power switches as in table 1x. The MOSFETs have higher switching speed in which a zero switching loss can be achieved (66). However, the MOSFET has a delay at turn off (67). On the other hand, the BJT has an advantage when the voltage drop across the transistor is important, as a result they may be more efficient in some applications. Also, BJTs are not affected by electrostatic charges which can kill the MOSFETs (Floyd, 2002) (68).

Table ‎2: A compersion between BJTs and MOSFETs.

Series Resonant analysis

Resonance is the capability of a system to oscillate with greater amplitude at specified bandwidth of frequency. Series resonant filter is characterized by the load impedance being capacitive at low frequency and inductive at high frequency. The transition frequency between being capacitive or inductive is the resonant frequency at which the load circuit appears purely resistive and maximum power is transferred to the load. So that, it is classified according to the circuit's quality factor (Q), resonant frequency, and bandwidth BW. Table 2.2 shows list of equations that required evaluating the series resonant inverter

Characteristic

Equation

Unit

Time constant

Resonant angular frequency

Damping factor

Damping constant

Characteristic impedance

Quality factor

Bandwidth

Phase angle

Table ‎2: Equations required to evalute the filter.

To analyze the property of oscillation in series resonant inverter let's consider the circuit in figure 2.18.

Figure ‎2: series resonant filter.

By applying Laplace transform for solving the current through inductor and assuming zero initial condition we get equation 2.10.

‎2.

Let's the input voltage is unit step function, , then the current become as in equation 2.11.

‎2.

By solving the equation partial friction and plug the equations in table 2-2 we equation 2.12.

‎2.

Where

‎2.

When we apply inverse Laplace transform for we will get the current response of circuit in figure 2-18 as in equation 2.13

‎2.

When referring to equation 2.13 when is too small become less than zero and then we will ensure the current will oscillate.

The RMS voltage across the resistor R by voltage divider rule and then plug the equations in table 2-2 as given by equation 1.5.

‎2.

Where

is the RMS input to the filter

The frequency ratio terms in the equation 2.15 for the input phase angle φ shows that the resonant circuit is inductive (φ > 0, lagging current) when > and capacitive (φ < 0, leading current) when <.

Pulse Width Modulation (PWM)

Pulse width modulation (PWM) is a stream of pulses that is generated from DC input supply. Many topologies are capable to generate this kind of signal. One practical means of accomplishing pulse width modulation is to use a particular integrated circuit called the five fifty five. This name is taken from the type number NE555 which is named by its inventor Signetics Corporation (69). 555 timers configure in astable mode as free-running relaxation oscillator as shown in figure 2-19.

Figure ‎2: astable configuration of timer 555

The threshold input in pin 6 is connected to the trigger input in pin 2. The resistors,, and capacitor C formulate the timing of circuit which set the frequency and duty cycle. The capacitor of 0.01 is for decoupling and has no effect on the operation. The frequency of oscillation can be determined by equation 2.16.

‎2.

The duty cycle of the signal generated can be adjuster by selecting the values of and . Because C charges through and discharge only through. So By selecting the value of to be much smaller than a 50% duty cycle can be obtained as described in equation 2.17.

‎2.

Step-up Transformer:

Transformer is a very basic electrical machine which works based on faraday's law. A varying current in the primary winding creates a varying magnetic flux in the transformer's core and consequently a varying magnetic field through the secondary winding as illustrated in figure 2-20. The ratio that relates the number of turns in the secondary to the number of turns in the primary permits the step up or step down transformation. For step up transformers, the number of turns in primary winding has to be less than the number of turns in secondary winding.

Figure ‎2: An ideal transformer [70].

The equation 2-10: describe the voltage behavior of transformer at secondary winding.

‎2.

Where

Voltage at secondary winding

Voltage at primary winding

Number of turns at secondary winding

Number of turns at primary winding

So that, when the number of turns at secondary voltage is larger than the number of turns at primary windings the transformer r

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