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Advantages of Renewable Energy

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Chapter 1
INTRODUCTION AND BACKGROUND

1.1 Introduction

“In the last few years, the idea of having small-scale energy sources or micro-sources, distributed over a grid has gained a considerable interest. Innovations in the technology and changing economic and regulatory environment have been the main driver behind this growing interest in new distributed generation technologies.

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Distributed generation has offered a variety of benefits and has given the customers a choice for the electricity services best suited for them. An abundant amount of distributed generation technologies like micro-turbines and fuel cell are quite well known and being developed all across the world. However, there is another technology that has become viable only in last few years and is being developed to improve voltage control as well as the power quality. It is micro grid system technology [7].

A micro grid is a small-scale power supply network which is designed to provide power to individual consumers, small community or few building. Micro grids bear the promise of considerable environmental benefits, brought about by higher energy efficiency and by facilitating the integration of renewable sources such as wind turbines or photovoltaic arrays. By virtue of good match between generation and load, micro grids have a low impact on the electricity network, despite a potentially significant level of generation by intermittent energy sources. Although the ownership and operation issues for the micro grid concept have yet to be addressed, one possible way forward might be for a micro grid as intrinsically a local supportive project. In such systems, the consumers may be also the suppliers and so a more creative approach to load control may be possible in the joint interests of cost and efficiency [5]-[7].

Combining photovoltaics or wind turbine, fuel generator and a small battery requirement gives a microgrid that is independent of the national electricity network. In the short term, this has particular benefits for remote communities but more wide-ranging possibilities open up in the medium to long term. Microgrids could meet the need to replace current generation nuclear and coal fired power stations, greatly reducing the demand on the transmission and distribution network [5].

1.2 Motivation and Objectives

The aim of the research is working on RES (Renewable Energy Sources). Deeply understanding, knowledge and design of optimized small size micro grid power system from Renewable Energies including micro wind turbines, solar electricity and small hydro power system for micro generation of electricity to facilitate domestic homes or small businesses level.

Renewable Energies are the cheapest and reliable electricity generation sources in the world. We can increase benefits from RES due to increasing energy efficiency, reduce CO2 emissions for low carbon and climate change goals.

The objectives of the research include:

Research into renewable energy resources for small level micro generation of electricity for small businesses and domestic homes.
Investigate renewable energy resources and select economical, suitable and available source of energy for electricity micro generation in Scotland, UK.
Create optimized conceptual design of small scale micro grid system.
Control strategies which ensure the operation of micro grid power system capability to meet the requirements of specific load and controls.
Generation of electricity in the most reliable and efficient way with power system protection (grounding and safety issues).
Computing simulations of micro grid system design with controls (on-off load) and contrast the simulation results.

As a result the project’s effort and research into renewable energy resources on micro grid systems will contribute towards increasing energy efficiencies (on small businesses and domestic homes levels), low carbon economy as UK government plan & worldwide and energy cost effectiveness.

1.3 History of UK Electric Power System

In 1881 public electricity supply began in the UK but it was not properly established until 1926 with respect to growth and future direction. The electricity generation on large scale up to 2000MW usually coal fired power stations connected with high voltage transmission system network up to 400kV, distributed it on small scale or lower voltage feeders for supplying consumers at different voltages from 33kV-230V lower for domestic electricity users. A typical UK electricity transmission, distribution and public electricity supply system (adapted from R. Cochrane, Power to the people, CEGB/Newness Books, 1985) is shown below:

Figure 1.1: Electricity transmission and distribution system, to clarify on the example of UK public electricity supply (adapted from R. Cochrane, Power to the People, CEGB/Newness Books, 1985).

In 1989 the UK government introduced an Electricity Act for promoting competition and protect the consumers, also introduced an Office of Electricity Regulation (OFFER) with primary responsibilities, electricity industry was privatised and split into different parts under the new Electricity Act. The generating stations were privatised for selling their electricity on the open market; the privately owned managed transmission system was facilitated for the market with a number of Distribution Network Operators (DNOs). None of these ‘network owning’ companies bought or sold electricity their revenue came from the transmission or distribution of electricity.

Electricity Suppliers bought from the generators and resold to customers, the price being controlled by the market place. This situation existed, again in several forms, until today, when the New Electricity Trading Act (NETA) has altered pricing and charging methods [5].

1.4 Distributed Generation

Conventional power system is facing problems around the world due to gradual depletion of fossil fuel resources, global warming or environmental pollution and less energy efficiency. Due to having these problems, a new way of electricity generation locally at small scale or distribution voltage level by using more often non-conventional or renewable energy sources like wind power, solar photovoltaic (PV) cells, fuel cells, biogas, natural gas, micro-turbines, combined heat and power (CHP) systems, sterling engines and their integration utility distribution network. This type of electricity generation is termed as distribution generation (DG) and the energy sources are called distributed energy resources (DERs).

In another words we can say that distribution generation (DG) is an approach which employs small scale applications or technologies to generate electric power near to the consumer’s premises as shown in the diagram below [2]-[6]-[32].

Figure 1.2: Distributed generation system

The major issues about DG were investigated in late 1990s by the working groups of the International Council on Large Electric Systems (CIGRE) and the International Conference and Exhibition on Electricity Distribution (CIRED) in their review reports. According to several researches it shows that:

Distributed generation is normally smaller than 50 MW.
Distributed generation is not centrally planned by the power utility, nor dispatched.
The distributed energy resources or generators are mostly connected to the distribution system and usually voltage ratings are 230/415 V up to 145 kV [2].

In distributed generation mostly source of electric power is renewable energy sources (RES) which is one of the most important ways of reducing carbon dioxide emissions. There are also specific potential benefits using distributed generation, are under below:

Figure 1.3: Distributed generation benefits and services

Power system reliability increased using DG.
It is an emergency supply of electricity.
It reduces the peak power requirements.
Cost effective in the sense of generation, transmission and distribution.
Provision of ancillary services.
Improved power quality.
Cost reduction due to using very less land.
Improved infrastructure resilience and reduced vulnerability [83].

Consumers have become used to electrical power available on demand. They do not need to structure their load pattern, the entire responsibility for matching power and demand is placed upon the utilities, which must have enough generation available at all times. With more creative thinking about the way energy is supplied, used and controlled it may be possible to satisfy the demand for energy, but accommodate the fluctuating resources which are a feature particularly of renewable energy sources. This may be possible by ensuring a satisfactory mixture of sources and loads to enable the demand and supply to match [5].

1.5 Why Integration of Distributed Generation?

In front of several advantages provided by conventional power systems, the following benefits (technical, economical & environmental) by non-conventional or renewable energy sources have led to gradual developments and integration of distribution generation system:

Due to increasing load demand day by day the conventional power system brings about a continuous reduction of fossil fuel reserve. So, therefore most of the countries are paying attention on non-conventional or renewable energy resources for their alternate source of electricity demand.
Renewable energy resources are preferred over fossil fuels due to reducing environmental pollution and global warming. As a part of Kyoto Protocol, the UK, the Europe and many other countries representatives had meetings on reduction and cut down of greenhouse gases (carbon and nitrogenous) emissions [8]. Therefore they are working on renewable energy resources and integration of DERs would help to reduce greenhouse gases and generate environmental friendly clean power.
DG has more scope for utilizing CHP (Combined Heat and Power) plants to use waste energy to produce heat and electricity for domestic, commercial and industrial applications.
DG plants are often close to the consumer or on consumer sites at low voltage (LV), so it is a reduction of transmission and distribution capital investment and their (T&D) losses.
The power generation is on consumer sites; therefore the system has high power quality and reliability. So therefore more opportunities for DG integration, but there are shortage of electricity in some developing countries for meeting the load demand, so any form of electricity generation is encouraged [2]-[32].

1.6 Hybrid Power System

Hybrid power system is a combination of two or more form of electricity generation on renewable energy sources (RES) or mixed with storage batteries and diesel generator as a backup or stand-by for increasing load demand and the lack of electricity in rural, remote and difficult to reach areas [13]. Hybrid systems are usually in the power range from 1 kilowatt (kW) to hundreds kilowatt with best features of renewable energies and high power quality [9]. In hybrid systems generally using renewable energy sources are small wind turbine, solar photovoltaic (PV), fuel cells, biomass, micro-hydro and sterling engines [12]. Storage system (battery or fuel cells) are used to provide high level of efficiency and fossil fueled generator (diesel generator) are used to ensure the high level of security and reliability of the power system. A hybrid power system may or may not be connected to the main grid, because it depends on the consumer site.

Figure 1.4: Hybrid Power System [11]

The above hybrid power system figure considering on-site generation without connected to the main grid, using both renewable energy sources that are wind turbine and solar photovoltaic for electrifying remote locations or rural areas. Using both renewable energy sources, the hybrid system minimizes the weakness of each approach that are lacking of wind or sunlight.

Most hybrid power systems use wind turbine or solar photovoltaic with diesel generator set, because diesel generator provides more predictable electric power on demand and takes care of long term fluctuations. In some hybrid systems batteries are also used for more reliability and efficiency for meeting the daily load fluctuations [10]-[11]-[12].

Hybrid power system combines the benefits of both renewable (non-conventional) and conventional power conversion systems. Renewable energy sources in comparison with conventional energy sources offer an independence from fossil fuel and with that an independence from world fuels pricing and conterminously an increasing sustainability of the power supply. On the other hand conventional energy sources are self-reliant from environmental conditions for example wind velocity, solar irradiation, etc. They can support the renewable energy sources at the time of insufficient environmental conditions. Because of that, the reliability and efficiency of the hybrid power system can be increased [13].

The hybrid power system has received a lot attention over the past decade, because it is an effective solution of electricity generation as compared to the systems which totally rely on hydrocarbon fuel. Hybrid power system has also longer life cycle apart from mobility of the system. Especially, the integration approach of renewable energies makes a hybrid power system to be the most appreciate for remote locations or rural areas. For employing a system generating totally clean power, high capital cost is the main issue. However we can generate green power by using different renewable energy sources to diesel generator and batteries, which is also a hybrid power system. So this type of hybrid power system (renewable energies/diesel generator/batteries) compromises on investment cost, diesel fuel usage cost as well as on operation and maintenance (O&M) costs.

There are usually two types of hybrid power system configurations, which are below:

Hybrid power systems based mainly on diesel generators with renewable energy sources are used for reducing fuel consumption.
Hybrid power systems depending on renewable energy sources with a diesel generator used as a stand-by or back-up supply, especially when the load demand is high and renewable energy sources are less.

So, designing of a hybrid power system rely on site specification, load demand and available energy resources [12].

1.7 Why Microgrid Systems?

A microgrid is a small scale electricity supply network that is designed to electrify remote locations, rural areas, domestic homes, small scale businesses or a small community. A small community may range from a typical housing estate, isolated rural communities, to mixed suburban environments, academic communities such as schools or universities, to commercial or industrial sites, municipal regions, or trading estates. The key figures that differentiates this approach from conventional power system is that the generators are small mostly assigned to as micro-generators, of a similar size as the load within the microgrid, they are distributed and usually located near to the consumers. The small scale generators and load are controlled to attain a local electricity and power balance. So, the main task of microgrid operation is to supply high quality electric power with reliability regardless of any faults operating conditions [18]-[19].

The motivation behind the microgrid concept is to reduction of carbon dioxide (CO2) emissions, for the following reasons:

System overall efficiency increases due to both electricity and heat loads are close to the generation.
Having environmental advantages made possible by the use of zero or low emission generators including PV arrays and fuel cells.
Low impact on the electricity network, by virtue of good match between power generation and load demand [5].

Microgrids are usually designed and granted for operation by a single customer and group of customers with the key role of microgrids to reduce or minimize the environmental impacts. However, the main task is to reduce energy bill of electricity and heat users within the microgrid network. Microgrids can provide electricity at lower cost due to using of waste heat, no transmission and distribution losses, no customer services and other related costs unlike traditional power system. As compared to traditional energy system, microgrid has various advantages to minimize the cost of energy and also emerging distributed energy resources are quite promising in generating low cost clean power.

In microgrid economics with respect to UK scenario, the reduction of greenhouse gases (GHG) emissions is one of the major contributions towards environmental impacts. Combined heat and power (CHP) based micro generation is specially focused after the formation of Distributed Generation Co-ordination Group (DGCG) in the UK in early 2000. The potential of microgrid technologies, its contribution to the UK power system and major economic issues are also identified.

The potential advantages of microgrid economics are specified below:

Microgrid has high energy efficiency.
Reduction of transmission and distribution costs.
Lower energy losses.
Reduction in capital exposure and risk by small scale individual investment.
Low cost entry in open competitive market.
Matching capacity to the increasing demand growth.
Within the microgrid the micro-generators can share their energies without exporting energy to the public network at lower prices.
Additional security and ancillary services [2].

1.8 Issues to Consider

There are technical and regulatory issues that need to be considered before this concept can be applied on a wider scale. The principal issue to consider is how closely the energy supply within the microgrid can satisfy the local loads. The answer to this question will help decide how the microgrid interacts with the main utility and the nature of the connection to be determined. Indeed, it may even be desirable in some circumstances for the microgrid to be disconnected from the utility and operate as stand-alone.

The issues that must be resolved to permit this type of operation include:

Precise energy and power balance within the microgrid, on a time scale ranging from milliseconds to years. Over the short time scale, the power balance is linked to the question of control; over longer time scales, one needs to consider the relationship between energy supply, demand and storage. Similar arguments are used to design stand-alone power supplies, for example, photovoltaic or hybrid systems which power remote equipment or serve isolated rural communities across the world.
The nature of connection with the main utility. An arrangement which would permit the microgrid operator the choice to operate in the grid-connected or stand alone mode is an uncharted territory for conventional power utility engineers and issues remain both at the technical and regulatory level.
Energy storage. The conventional utility supply operates on the principle that power is generated when it is required. Energy storage introduces a novel component in a utility supply and broadens the design criteria. On a quantitative level, the size of the energy store is intimately linked to the energy balance and to the required security of supply provided by the microgrid.
Demand management. The temporal mismatch between generation and load can be alleviated by managing the demand. The shifting of load facilitates achieving the energy balance and helps reduce the size of energy storage. Whilst experience exists of demand-side management at industrial level and lessons can be learned from concepts such as storage heating, demand management at the domestic level is attracting much interest in the research community but further experience is needed before routine applications become commonplace [16].
Seasonal match between generation and load. Energy storage and demand management can be effective to achieve energy balance at the diurnal time scale. A sufficient energy must be available from the generators to ensure energy balance over longer time scales if a microgrid powered by renewable or other intermittent energy sources such as micro-CHP is to be capable of stand-alone operation. This can usually be achieved only by a diversity of generation methods appropriate to the load [5].

1.9 Layout of the Report

The layout of the report is as follows.

Chapter 1 gives a brief introduction and background followed by objective and motivation.
Chapter 2 explains the potential of renewable energies.
Chapter 3 explains about microgrid systems.
Chapter 4 provides information on designing the microgrid system.
Chapter 5 explains about design case studies.
Chapter 6 gives discussions, problems occurred, conclusions and future outwork.

Chapter 2
POTENTIAL OF RENWABLE ENERGY

2.1 Renewable Energy

2.1.1 Introduction

Renewable energy is the term used to describe energy that occur naturally and repeatedly in the environment and can be harnessed for human benefit. This includes energy from the sun, wind, biomass, hydro, waves or tides. Most of this comes from either the sun (which controls the earth’s weather patterns) or the gravitational effects of the sun and moon. This means that these energy sources are essentially endless. We also get renewable energy from trees and plants, rivers and even garbage [80]-[82].

Renewable energy encompasses many different types of technology at different stages of development and commercialization, from the burning of wood for heat in the residential sector (traditional and low-technology) to wind-generated electricity (widespread and technically proven) to processes such as biomass gasification for electricity generation (still under development although some plants are operating) [78].

The key issue is how to extract this energy effectively, and convert it into more useful forms of energy. For example, we can use energy from the sun to heat water or use mechanical devices such as wind turbines to convert the kinetic energy in wind into electricity [80]. Renewable energy sources emit lower levels of carbon dioxide emissions which is one of the main greenhouse gases (GHG) contribution to climate change than fossil fuel based generation plants [82].

2.1.2 Why Use Renewable Energy?
2.1.2.1 Climate Change

Climate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average. Climate change may be limited to a specific region or may occur across the whole Earth.

In recent usage, especially in the context of environmental policy, climate change usually refers to changes in modern climate. It may be qualified as anthropogenic climate change, more generally known as “global warming” or ‘anthropogenic global warming’ (AGW). Since 1900, the average temperature on the planet has increased by 0.74 degrees Celsius [77]-[81]. To help minimize the effects of climate change, we need to reduce the level of greenhouse gases (GHG) we are emitting. This means generating energy from sources that emit low or even zero levels of greenhouse gases, such as renewable energies [80].

2.1.2.2 General Pollution

As well as reducing climate change, using renewable energy can also help to reduce other forms of environmental and social damage that occur as a result of using fossil fuels, such as acid rain or air pollution [80].

2.1.2.3 Security of Supply

Another reason to use renewable energy is that fossil fuels are limited; they are going to run out. We need to ensure to have a reliable ongoing energy supply. In the UK, energy industry still relies largely on diminishing sources of coal, oil and gas. Renewable sources will reduce our dependence on these imported fossil fuels and help give us a diverse, secure mix of energy [80].

2.1.3 Potential of Renewable Energy in the UK

The UK has enormous wind, wave and tidal power; more than enough to meet all of our energy needs many times over. We could and should be global leaders in the field of renewable energies. We could be obtaining huge benefits from harnessing our native energy sources which use no fuel and will never run out. We could be obtaining industrial and economic advantages by being at the forefront of the fastest growing new technologies.

The total value globally of new wind power installed in 2006 was ?12 billion and the industry grows by an amazing 30 per cent or more a year. But the UK is only seizing a small percentage of that market and we are being left behind. Germany, Denmark, the US, Italy, Spain, China and India all have more wind capacity than us. Canada, France and Portugal are at about the same level or slightly less but last year they all grew faster than us.

To date, UK government has largely substandard the development of renewable technologies. They have been held back and undermined by weak policy, indecision, obstacles and the threat of nuclear power. When heat and transport energy is included, the UK ranks near the bottom of the EU league table for renewable energies development. With proper support, renewable energies can and must form the heart of our energy system [79].

2.1.4 UK Targets
The UK signed the Kyoto Protocol in December 1997 and it became legally binding in February 2005. As part of this, we pledged to reduce our greenhouse gas emissions by 12.5 percent between 2008 and 2012 and to seek to reduce emissions to 20 percent below 1990 levels by 2010.
The 2003 Energy white paper, pledged to cut current carbon dioxide (CO2) emissions in the UK by 60 percent by 2050. This target has now been increased to 80 percent.
In spring 2007, the UK agreed with other Member States to an EU-wide target of 20 percent renewable energy by 2020, including a binding 10 percent target for the transport sector. Member States have now signed up to the Renewable Energy Directive which includes a UK share of 15 percent of energy from renewable by 2020. This is equivalent to an eight-fold increase in renewable energy consumption from current levels. While such an increase is determined and will be challenging, we are fully committed to meeting our share of the target [8]-[80].

2.2 Wind Energy

2.2.1 Introduction

Wind is the airflow that consists of many gases in the atmosphere of the earth. Rotation of the earth, uneven heating of the atmosphere and the irregularities of the ground surface are the main factors that create winds. Motion energy of the wind flow is used by humans for many purposes such as water pumping, grain milling and generating electricity. Windmills that are uses for electricity generation are called wind turbines in order to distinguish them from the traditional mechanical wind power applications. Wind is a sustainable energy source since it is renewable, widely distributed and plentiful. In addition, it contributes to reducing the greenhouse gas emissions since it can be used as an alternative to fossil fuel based power generation [44]-[49].

2.2.2 Winds

The wind is the phenomenon of air moving from the equatorial regions toward the poles, as light warm air rises toward the atmosphere while heavier cool air descends towards the earth surface. Therefore cooler air moves from the North Pole toward the Equator and warms up on its way, while already warm air rises toward the North Pole and gets cooler and heavier, until it starts sinking back down toward the poles. Another phenomenon that is affecting global winds is caused by the “Coriolis force” which makes all winds on the northern hemisphere divert to the right and all winds from the southern hemisphere divert to the left [45].

Both of the above mentioned phenomena affect global winds that exist on the earth’s surface. Hence, as the wind rises from the Equator, there will be a low-pressure area close to ground level attracting winds from the North and South. At the poles, there will be high pressure due to the cooling of the air. In order to find the most suitable sites for wind turbines, it is crucial to study the geological data of the area since the wind’s speed and direction are highly influenced by the local topology. Surface roughness and obstacles not only will affect the speed of the wind but also affect its direction and overall power [49].

2.2.3 History of Wind Energy Harvesting

Wind turbines, as machines were used by ancient civilizations (Persians, Romans, etc.) mostly as vertical axle windmills with several blades. The primary use was to grind corn and wheat and for water irrigation systems [46]. Later on, windmills were developed mostly in the Netherlands, Denmark and Scotland for grinding mills. The first windmill that produced electricity was constructed by Professor James Blyth in Glasgow, Scotland in 1887 at Anderson’s College [47]. A great research contribution was made in Russia in 1931. A modern horizontal axis wind generator with 100 kW of power was mounted on a 30 m (100 ft) tower connected to the local 6.3 kV distribution system.

Ten years later in Castleton, Vermont, USA, the world’s first megawatt size wind turbine supplied the local electrical distribution system. The first wind machines were DC-type electrical machines. Later in the 1950s in Denmark, research and development led to the development of two-bladed wind turbines which utilized AC electrical machines instead of DC machines. At the same time a Danish group of researchers brought the concept of three-bladed wind turbine. After the first oil crisis in 1973, interest in wind energy increased in several countries such as Germany, Sweden, the UK and the USA [48].

Later developments in wind turbines were geared toward improving the 2 MW machine, creating wind power farms as group of wind turbines, establishing offshore sites for improved wind power and establishing new control techniques to increase the overall efficiency of the wind generators [49].

2.2.4 Wind Energy System

A wind turbine transforms the kinetic energy in the wind to mechanical energy in a shaft and finally into electrical energy in a generator. In simple words, a wind turbine converts wind energy into electrical energy.

The principal component of wind energy conversion systems is a wind turbine. This is coupled to the generator through a multiple ratio gearbox. There are generally two types of generator are used in the wind turbine conversion systems that are induction generators and synchronous generators, but most commonly induction generators are used in wind energy conversion systems. The main parts of the wind turbine are the tower, rotor and nacelle. The nacelle accommodates the transmission mechanisms and the generator. Rotor may have two or more blades. Wind turbine captures the kinetic energy of wind flow through rotor blades and transfers the energy to the generator side through the gearbox. The generator shaft is driven by the wind turbine to generate electric power. The function of the gearbox is to transform the slower rotational speeds of the wind turbine to higher rotational speeds on the induction generator side. Output voltage and frequency is maintained within specified range by using supervisory metering, control and protection techniques. Wind turbines may have horizontal axis configuration or vertical axis configuration, but most of the wind turbines we see now days are horizontal axis due to having high efficiency [2]-[29]-[30]-[31].

2.2.5 Basic Parts of Wind Turbines

The basic parts of wind turbine are shown in the figure 2.1 below:

Figure 2.1: Components of a typical wind turbine

The details of the main parts of wind turbine are follows.

2.2.5.1 The Tower

The main purpose of the tower is to support the nacelle and resist vibration due to the wind speed variations. The cables that connect the generator (on top of the tower, inside the nacelle) and transmission line (down, in the basement of the tower) are inside the tower. The tower is the main component that carries most of the other components such as the turbine, nacelle, blades and generator and so on.

The height of the tower is different for offshore and onshore turbines. The high towers are more appropriate for wind energy harvesting, since winds contain less turbulence in higher altitudes. However, stability issues limit the height of the tower. Onshore wind systems have higher towers than offshore turbines because the land has higher roughness than the water surface. On the water surface, there are almost no obstacles; hence the low tower length is sufficient to capture the wind. In onshore applications, there may be some objects around the tower that may block the wind speed. In areas with high roughness, high turbine towers are required to avoid the effect of wind blocking objects such as buildings, mountains, hills, trees and so on [2]-[49].

2.2.5.2 Yaw Mechanism

The yaw mechanism is composed of the yaw motor and the yaw drive. The yaw mechanism turns the whole nacelle toward the wind direction in order to face the wind directly. Regardless of the direction of the wind, the yaw mechanism can help the turbine face the wind by changing the direction of the nacelle and the blades. During the rotation of the nacelle, there is a possibility of twisting the cables inside of the tower. The cables will become more and more twisted if the turbine keeps turning in the same direction, which can happen if the wind keeps changing in the same direction. The wind turbine is therefore equipped with a cable twist counter which notifies the controller that it is time to straighten the cables [2]-[49].

2.2.5.3 The Nacelle

The gearbox, generator and the control electronics are all located inside of the nacelle. The nacelle is connected to the tower through the yaw mechanism. Inside the nacelle, two shafts connect the rotor of the turbine to the rotor of the electrical generator through the gearbox. The gearbox is the mechanical energy converter that connects the low-speed shaft of the turbine to the high-speed shaft of the electrical machine.

The control electronics inside the nacelle record the wind speed, direction data, rotor speed and generator load and then determine the control parameters of the wind operation system. If the wind changes direction, the controller will send a command to the yaw system to turn the whole nacelle and turbine to face the wind. The electrical generator is the main part of the nacelle. It is the heaviest part and produces electrical energy which is transferred through the cables. There are different types of generators that are used for wind turbines and depending on the type of generator; wind turbines can operate with either fixed or variable speed. Fixed speed (FS) turbines use synchronous machines and operate at fixed speed. These machines are not best solution for the wind turbines because the wind always changes its speed. Variable speed turbines use DC machines, brushless DC machines and induction machines. DC machines are not commonly used due to the maintenance problems with the brushes. Induction and DC brushless machines are more suitable for wind applications [2]-[49].

2.2.5.4 The Turbine

The turbine, also called ‘low speed rotor’, usually has two to six blades. The most common number of blades is three since they can be positioned symmetrically, maintain the system’s lightness and ensure the stability of the wind power system. Two-blade turbines have high stresses in cut-in speed; therefore the speed and power of the wind are insufficient for starting the rotation of the turbine and higher minimum wind speed values are required at the beginning. The radius of the blades is directly proportional to the amount of captured energy from the wind; hence and increased blade radius would result in a higher amount of captured energy.

The blades are aerodynamic and they are made of a composite material such as carbon or Plexiglas and are designed to be as light as possible. Blades use lift and drag forces caused by wind; therefore by capturing these forces, the whole turbine will rotate. The blades can rotate around their longitudinal axis to control the amount of captured wind energy. This called ‘pitch control.’ If the wind speed increases the pitch control can be used to change the effective blade surface, hence keeping the turbine power constant. The pitch angle control is usually used for wind speeds above the nominal speed [2]-[49].

2.2.6 Wind Turbines Based on Axis Position

Based on axis position, wind turbines are classified as the horizontal axis and vertical axis turbines.

Horizontal axis wind turbines (HAWTs) are more common than vertical axis wind turbines (VAWTs). The horizontal axis turbines have horizontally positioned shaft which helps ease the conversion of the wind’s linear energy into a rotational one.

VAWTs have a few advantages over the horizontal axis wind turbines. VAWTs electrical machines and gearbox can be installed at the bottom of the tower on the ground, whereas in HAWTs these components have to be installed at the top of the tower which requires additional stabilizing structure for the system. Another advantage of the VAWTs is that they do not need the yaw mechanism since the generator does not depend on the wind direction. The most famous design of VAWT is Darrieus type of turbine.

There are a few disadvantages that limit the utilization of the VAWTs. Due to the design of blades, the sweep area of VAWT is much smaller. Wind speed is low near the surface and usually turbulent; hence these wind turbines harvest less energy than horizontal axis ones. Additionally, VAWTs are not self-starting machines and must be started in motoring mode and then switched to generating mode [2]-[49].

2.3 Solar Energy

2.3.1 Introduction

Solar energy is one of the most important renewable energy sources that have been gaining increased attention in recent years. Solar energy is plentiful; it has the greatest availability compared to other energy sources. The amount of energy supplied to the earth in one day by the sun is sufficient to power the total energy needs of the earth for one year [50]. Solar energy is clean and free of emissions, since it does not produce pollutants or by-products harmful to nature. The conversion of solar energy into electrical energy has many application fields. Residential, vehicular, space & aircraft and naval applications are the main fields of solar energy [49].

2.3.2 History of Solar Energy Harvesting

Sunlight has been used as an energy source by ancient civilizations to ignite fire and burn energy warships using “burning mirrors.” Till eighteenth century, solar power was used for heating and lighting purposes. During the 1800s, Europeans started to build solar-heated greenhouses and conservatories. In the late 1800s, French scientists powered a steam engine using the heat from a solar collector. This solar-powered steam engine was used for a printing press in Paris in 1882 [51]. A highly efficient solar-powered hot air engine was developed by John Ericsson, a Swedish-American inventor. These solar-driven engines were used for ships [50]. The first solar boiler was invented by Dr. Charles Greely, who is considered the father of modern solar energy [52]. The first working solar cells were invented in 1883 by Charles Fritts [53]. Selenium was used to build these prototypes, achieving efficiencies of about 1%. Silicon solar cells were developed in 1954 by researchers Calvin Fuller, Daryl Chapin and Gerald Pearson. This accomplishment was achieved by following the fundamental work of Russel Ohl in the 1940s [54]. This breakthrough marked a fundamental change in the generation of power. The efficiency of solar cells increased from 6% up to 10% after the subsequent development of solar cells during the 1950s [55]; however, due to the high costs of solar cells commercial applications were limited to novelty items [49]-[54].

2.3.3 Solar Energy System

Solar photovoltaic (PV) is a simple and well designed method of harnessing the sun’s energy. PV devices as solar cells are unique in that they directly convert the incident solar radiation into electricity with no noise, pollution or moving parts, making them robust, reliable and long lasting. In another words, PVs are arrays of cells containing a material that converts solar radiation into direct current (DC) electricity.

Materials used today include amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride and copper indium selenide/sulfide. A material is doped to increase the number of positive (P type) or negative (N type) charge carriers. The resulting P and N type semiconductors are then joined to form a PN junction that allows the generation of electricity when illuminated. Photovoltaic’s can be mounted on roofs or combined into farms [22]-[23]-[24]-[25].

2.3.3.1 Solar Systems Application

Solar technologies are largely characterised as either passive solar or active solar depending on the way they capture, convert and distribute sunlight. Active solar techniques include the use of photovoltaic panels and solar thermal collectors while passive solar techniques include orienting a building to the sun, selecting materials with favourable thermal mass or light dispersing properties and spaces that naturally circulate air. The more commonly used solar system applications are as follows [43]:

I. Photovoltaic (PV) system: PV system convert sunlight directly to electricity by means of PV cell made of semiconductor materials.

II. Concentrating Solar Power System (CSP): CSP focus the sun’s energy using reflective devices such as troughs or mirror panels to produce heat then is used to generate electricity.

III. Solar Water Heating (SWH) System: SWH contain a solar collector that faces the sun and either heats water/ working fluid directly.

IV. Transpired Solar Collectors or Solar Walls: In this system solar energy is used to preheat ventilation air for a building [27]-[49].

2.3.3.2 Solar System Operation

The basic operation of a solar cell is light shining on the solar cell produces both current and voltage to generate electric power. This process requires firstly a material in which the absorption of light raises an electron to a higher energy state and secondly the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell. A variety of materials and processes can potentially satisfy the requirements for photovoltaic energy conversion but in practice nearly all photovoltaic energy conversion uses semiconductor materials in the form of PN junction, as shown in figure 3.1 below [2]-[22].

Figure 3.1: Cross section of a solar cell

2.3.5 PV System Advantages

Today solar photovoltaics are rapidly growing and increasingly important renewable alternative to conventional fossil fuel electricity generation. Their application and advantage to the remote power supply area was quickly recognized and encouraged the development of global photovoltaic’s industry. Small scale transportable applications such as calculators and watches were utilized and remote power applications began to benefit from photovoltaics.

The major advantages of PV systems include:

Sustainable nature of solar energy as fuel.
Minimum environmental impact.
Drastic reduction in customer’s electricity bills due to free availability of sunlight.
Long functional lifetime of over 30 years with minimum maintenance.
Silent operation.

Due to these benefits PV systems are recognized by governments, environmental organizations and commercial organizations as a technology with the potential to supply a significant part of the world’s energy needs in a sustainable and renewable manner. Moreover, due to the extensive improvement in inverter technologies, PV generation is now being preferred and deployed worldwide as DERs in a microgrid for expansion of local generation at distribution voltage level. It has been studied that small PV installations are more cost effective than larger ones which indicates the effectiveness of feeding PV generation directly into customer circuits at low voltage distribution networks. Hence, they can be potential contributors to a microgrid [2]-[22]-[24].

The UK government has recently committed ?10 million towards encouraging the installation of PV systems in the buildings as energy use. In the UK, PV systems are also being used for a long time to provide high reliability power for industrial use in remote and inaccessible locations or where the small amount of power required is more economically met from a stand-alone PV system than from mains electricity [2]-[8]-[20]-[21].

Chapter 3
MICROGRID SYSTEMS

3.1 Concept of Microgrid

The microgrid is a concept based small scale power generation system consisting of renewable energy sources together with fossil fuelled generators and local load. This small scale power system formed by integration of micro sources usually renewable energy resources/non-conventional DERs that are designed to provide electricity for domestic home, small business, individual consumer site, remote location or small community. Microgrid is more modern way for utilizing the available potential of DG, not only in remote area electricity development but also in overcoming the short-fall of electricity commercially. Most commonly available renewable energy resources used for the development of microgrids are Wind, Solar, Biomass, Micro-hydro, Fuel Cell etc. The micro sources must be equipped with power electronic interfaces (PEIs) and controls to provide required flexibility to ensure the operation. The control flexibility of microgrid allows itself to present a main utility power system which meets local energy requirements for reliability and security. So, the main task of microgrid is to obtain reliable and high quality electric power without any faults and abnormal operating conditions [2]-[3]-[4]-[18]-[19].

In technical terms microgrid is “a grouping of generating sources and loads operating semi-independently of the legacy power system”. Microgrids usually employ CHP (combined heat and power) equipment, due to more efficient energy strategy attempting to reduce energy demand through efficiency and smart controls while meeting these severity loads [14].

The differences between microgrid system and a conventional power system are:

In microgrid system, sources of generation are a lot smaller than the large scale generators in conventional power system.
Power generated by microgrid system at distribution voltage can be directly fed to load.
Micro sources are usually installed near to the consumer’s site so that the system could get benefits from negligible line losses, satisfactory voltage and frequency profile for the load.

Microgrid’s technical features make it suitable and reliable for electrifying domestic homes or remote locations where national grid supply is not available, any disturbances or repeatedly disrupted due to severe meteorological conditions [2].

There are few assumptions help conceptualize microgrid systems:

Electricity and heat are functionally integrated in a microgrid.
Power generation, conversion, heat recovery, renewable energy harvesting and excessive energy storage all operate at the same time within a microgrid.
A microgrid is an intelligent power system network that has the capability to sensibly prioritize loads so that most significant loads can be served during supply deficiency.
Microgrids calculatedly recycle energy and divert any extra either into storage systems or towards onto the grid.
Microgrid can operate as stand-alone (off the grid) or connected with main grid.
Excessive energy can be used for providing backup to other generators, providing power to less critical loads and charging energy storage batteries [14].

The concept of microgrid has made possible due to recent approaches in small scale reliable generators with power electronics and inverts the trend to large scale generation and bulk supply. In the review of microgrid’s key feature, there should be local electricity generation that matches the power requirements within the microgrid. Several types of micro generators can be considered for example photovoltaics and wind generators etc. So, if the environment is primarily residential the photovoltaic generators would be attractive for the main source of generation and for most remote locations wind generators are attractive.

The specific advantage of microgrid is that it facilitates with more inventive schemes for meeting the local requirements in elastic manner with small scale generators and consumers closely integrated. In some microgrid networks, the consumers can also be the suppliers, so a more inventive approach to load control may be possible in the mutual interests of cost and efficiency. In detailed, the microgrid system needs to inspire consumers to be involved in small scale cogeneration, photovoltaics and other renewable energy schemes. Metering and charging arrangements would be agreed locally within the microgrid and would have to reflect the market for power within the microgrid [5].

3.2 The Relationship between Microgrid and Local Electricity Utility

The intention is that the microgrid is self-sufficient, but for security of supply and flexibility it would almost certainly be connected to the local electrical utility network, or even to adjacent microgrids. These links may be bi-directional enabling the import or export of electricity, or, depending on commercial considerations, it might just be a unidirectional flow of power. From the point of view of the microgrid, the utility connection might be viewed just as another generator or load.

This raises the question as to whether or not the microgrid should be linked to other networks over a synchronous alternating current (AC) connection. The advantage of a synchronous link would be its simplicity, requiring only an electrical interconnection, circuit breakers and probably a transformer. Lasseter [17] has considered this possibility and shown that in principle it should be possible to run a microgrid with minimal central control of local generation which is able to operate connected to the utility, or, in the event of loss of the connection, move smoothly into stand-alone or island operation with no loss of power to the microgrid. What is perhaps less clear is how the synchronous connection would be re-synchronised once the utility was ready to re-establish the connection.

The alternative approach would be an asynchronous connection using a direct current (DC) coupled electronic power converter. This might be bi-directional, enabling import and export of power or simply a device to import power when local resources were inadequate. An advantage of this approach is that it isolates the microgrid from the utility as regards reactive power, load balance, etc. Only power is exchanged with the utility, the microgrid is entirely responsible for maintaining the power quality (frequency, voltage and supplying reactive power and harmonics) within its area.

With an asynchronous link the microgrid might be unusual that all its power will be supplied through electronic inverters. Some generators, such as photovoltaic cells are intrinsically sources of DC and hence need inversion to connect them to an AC network. Others, for example, micro-turbines or Stirling engines may generate AC but are not well suited to operate a synchronous generator because the frequency is unsuitable or variable. Voltage source inverters with suitable control schemes will be required to permit stable operation of the network with many small generators attached. Fortunately, advances in power electronics and digital controllers mean that sophisticated control strategies are possible and the cost need not be excessive. Which of these approaches is more appropriate may well depend on the size of the microgrid. It may also depend on the regulatory environment governing the interchange of power between the microgrid and the utility [5].

3.3 Internal Control of Microgrid System

Microgrids require wide range control to ensure security of whole power system, optimal operation, emission reduction and transferring form one operating mode to the other without violating system constraints and regulatory requirements. However, the technical problems of a microgrid must be managed for the concept to become a reality. The control of a microgrid is thoroughly tied with the energy and power balance in the microgrid, and the question of energy storage. There are three main parameters frequency, voltage and power quality that must be considered and controlled to acceptable standards whilst the power and energy balance is maintained [4]-[5].

3.3.1. Power Balance

A power system usually contains no significant energy storage; the generated and dissipated power must, therefore, be constantly kept in balance. This power balance must be maintained on a cycle-by-cycle basis if the system is to maintain its frequency. Too much generation and the system accelerate, too little and it slows; neither situation is acceptable. The permissible frequency deviation is defined by Statute and in the UK it is the responsibility for the NGC to ensure that this deviation is not exceeded. Since the whole of the UK is run as one synchronous system, any new generator means the disconnection of another or a rise in load, if the system frequency is to remain constant. Power balance in a microgrid is therefore essential for frequency control.

In a microgrid, frequency stability becomes critical; therefore, control is a major concern. There are a number of techniques used to restore the power balance and hence correct the frequency: the use of load shedding, increase in primary generation and recovery of stored energy. All of these are available within a microgrid, but because the system is small the problem is much more difficult to manage to the same standard as is normal in a utility system.

Short term storage of energy is needed to cope with the fluctuations in power demand or accommodate the sudden loss of some generation. A microgrid with many small generators will not be an intrinsically stiff system, unlike a national interconnected utility. The small generators will neither store significant energy in their mechanical inertia, nor will they necessarily respond quickly to sudden changes of load. Short term storage, probably distributed with the generators, will permit the inverters to follow the rapidly changing demand while giving time for the generators to respond, or extra generation to be brought on line or for generators to be closed down. This same storage could be used to help accommodate the diurnal variation of demand.

There are two related issues, firstly quite small power imbalances will produce large frequency excursions and secondly they will happen much more quickly. The first issue may also be an advantage for a microgrid since small energy stores will have significant effects. The second issue means that stored energy recovery must be fast and precise. Since the most probable store, in the near future is likely to be a battery with an inverter, this does not pose an insurmountable problem; such a system is quite fast enough to ensure adequate frequency control [5].

3.3.2 Frequency Control

Electric power system in the UK operates at frequency of 50Hz and definitely there are advantages adopting this frequency, whether there is to be a synchronous connection or not. The frequency limits are set it and operates by law, relatively tight and standards are not the same like other power systems. So, there are not any reasons or relaxation possible for not adopting these standards. There is a relaxation limit of +/- 0.5Hz acceptable, so frequency must be controlled within these limits.

In the UK electric power system the generally used frequency control method is by the control of rotational speed of synchronous machines supplying power. In large interconnected system with several synchronous generators, no single machine can control the frequency, so there would be flow of synchronising power into any machine which is slowed in order to keep it in synchronism. For altering the frequency and speed, large power imbalance required of the order of 5000 MW per Hz for the UK.

It would be less technical issue if there are few machines then the less stiff, system and frequency control. In such a system the machines must be able to respond quickly to load variations in respect to preserve the power balance at all times. It means fast detection of frequency change and accurate control of load generation or both.

In wind turbines, induction generators are often used and solar photovoltaics arrays are connected with inverters. So, for meeting the system’s operating demand these two require different frequency and load control. For controlling the frequency, inverters can be utilized and inverter frequency can be controlled independently, but inverters require various viewpoints due to not rotational devices (synchronous generators) [4]-5].

3.3.3 Voltage Control

The system voltage within a large multi generator system is controlled by initially the voltage of the machines but also by the reactive flow. In general, the reactive balance becomes more critical in a smaller system. For example, all reactive demand must be supplied from one generator in a single machine system. This is not strictly true, but adds significantly to cost and control problems if reactive demand has to be compensated by extra static plant.

A conventional distribution system is usually a feeder network, and there is little interconnection. Voltage drop along feeders becomes an issue, as it will vary with load and distance along the feeder. This dictates that any simple microgrid will have to be either small to be satisfactory or be specially designed as an interconnected network.

The voltage and its limits at consumer’s terminals are specified by law, but they are reasonably wide. With proper design, production of the correct voltage should not be an insurmountable problem [5].

3.3.4 Power Quality

Control of power quality will be the biggest issue for a microgrid. Voltage dips, flickers, interruptions, harmonics, dc levels, etc. will all be more critical in a small system with few generators. There will need to be a critical appraisal of both the effects and consequences of relaxing and/or enforcing standards in this area.

As has been discussed by Venkataramanan and Illindala [15], the distributed generation within the microgrid could enable better control of power quality. With electrical storage together with the distributed generation power quality could be maintained in much the same way as is achieved by Uninterruptible Power Supply (UPS) systems. The electronic inverters can not only supply power at the fundamental frequency, but also generate reactive power to supply the needs of reactive loads, cope with unbalanced loads and generate the harmonic currents needed to supply non-linear loads [5].

3.4 Energy Storage

There is no economic general purpose method for the storage of electricity per se in the quantities required for public utility use. There are off course methods involving capacitors and super conducting magnets; both of which are technically complex and with present knowledge, rather expensive but nevertheless used in specific situations. Because the direct storage of electricity is not very practical, the storage of energy by other methods for later use in electricity generation is employed. These are many and varied, depending upon the situation and the purpose for which the electricity is to be used.

It is likely that a microgrid will rely on chemical energy storage in the form of electric batteries. In the simplest of systems this will mean lead acid cells which are well developed, available, predictable and robust. For more sophisticated applications, redox batteries are becoming available and development will continue. In critical situations where cost is not an issue, the application of super conducting energy storage has been used. Again, continued development is expected to both reduce costs and to increase reliability.

The calculation of battery size (energy) and inverter rating (power) will depend on the size of the loads and generators within the microgrid as well as its topography. As an alternative to storing energy, the shedding of load is more likely to be used in a microgrid, rather than a large scale public utility because it is easier to identify those loads which are least critical. Where co-generation is used, some of this energy storage may well be in the form of heat. This storage could be in the form of domestic hot water or stored for use in space heating. Innovative control strategies can be developed to make use of this storage and if necessary the plant may be run to meet the electrical load when there is no demand for thermal energy [5].

Chapter 4
DESIGN OF MICROGRID SYSTEM

4.1 Software

A software package HOMER (Hybrid Optimization Model for Electric Renewable) is used for designing the microgrid system. The software details are specified below.

4.1.1What is HOMER?

The HOMER (Hybrid Optimization Model for Electric Renewable) software developed by the U.S. National Renewable Energy Laboratory (NREL) to assist in the design of micro-power systems and to facilitate the comparison of power generation technologies across a wide range of applications. HOMER models a power system’s physical behaviour and its life-cycle cost which is the total cost of installing and operating the system over its life span. HOMER allows the modeler to compare many different design options based on their technical and economic merits. It also assists in understanding and quantifying the effects of uncertainty or changes in the inputs.

HOMER simplifies the task of evaluating designs of both off-grid and grid-connected power systems for variety of applications. When a user designs power system, the user must make many decisions about the configuration of the system. HOMER performs the analyses to explore a wide range of design questions that are below:

What components does it make sense to include in the system design
How many and what size of each component should it use
Which technologies are most cost-effective
What happens to the project’s economics if costs or loads change
Is the renewable resource sufficient [1]-[26]
4.1.2How Does HOMER Work?

HOMER performs three main tasks, which are below:

I. Simulation

II. Optimization

III. Sensitivity analysis

Figure 4.1: Conceptual relationship between simulation, optimization and sensitivity analysis

The above figure 4.1 illustrates the relationship between simulation, optimization and sensitivity analysis. The optimization oval encloses the simulation oval to represent the fact that a single optimization consists of multiple simulations. Similarly, the sensitivity analysis oval encompasses the optimization oval because a single sensitivity analysis consists of multiple optimizations.

4.1.2.1 Simulation

HOMER simulates the operation of a system by making energy balance calculations for each of the 8,760 hours in a year. For each hour, HOMER compares the electric and thermal load in the hour to the energy that the system can supply in that hour. For systems that include batteries or fuel-powered generators, HOMER also decides for each hour how to operate the generators and whether to charge or discharge the batteries. HOMER performs energy balance calculations for each system configuration that we want to consider. It then determines whether a configuration is feasible i.e. whether it can meet the electric demand under the conditions that the user specifies and estimates the cost of installing and operating the system over the lifetime of the project. The system cost calculations account for costs such as capital, replacement, operation and maintenance (O&M), fuel and interest. A user can then view hourly energy flows for each component as well as annual cost and performance summaries.

4.1.2.2 Optimization

After simulating all of the possible system configurations, HOMER displays a list of feasible systems, sorted by lifecycle cost. We can easily find the least cost system at the top of the list or we can scan the list for other feasible systems.

4.1.2.3 Sensitivity Analysis

Sometimes we may find it useful to see how the results vary with changes in inputs, either because they are uncertain or because they represent a range of applications. We can perform a sensitivity analysis on almost any input by assigning more than one value to each input of interest. HOMER repeats the optimization process for each value of the input so that the user can examine the effect of changes in the value on the results. We can specify as many sensitivity variables as we want and analyze the results [1]-[26].

4.2 Description of Major Components

In a microgrid power system, a component is any part of a whole power system that generates, delivers, converts or stores energy. The microgrid comprises in four major components that are wind turbine or solar photovoltaics, generator, converter and storage batteries.

There are two intermittent renewable sources for electricity generation that are wind turbines and solar photovoltaics. Wind turbines convert wind energy into ac or dc electricity and PV modules convert solar radiation into dc electricity. Generator is a dispatch-able energy source, meaning that the system can control it as needed and it consumes fuel to produce AC or DC electricity. Converter is used to convert electrical energy into another form and it converts electricity from ac (alternating current) to dc (direct current) or from dc to ac. Finally, storage batteries are used for storing the DC electricity

4.2.1 Wind Turbine

There are some calculations and definitions regarding wind power as follows.

Cut-in Wind Speed

This is the minimum wind speed needed to start the wind turbine (which depends on turbine design) and to generate output power. Usually it is 3 m/s for smaller wind turbines and 5-6 m/s for bigger ones.

Cut-out Wind Speed

The cut-out wind speed represents the speed point where the turbine should stop rotating due to the potential damage that can be done if the wind speed increases more than that [49].

Rated Wind Speed

This is the wind speed at which the wind generator reaches its rated output. Note that not all wind generators are created equal even if they have comparable rated outputs.

Rated Output

This measurement is taken at an uninformed wind speed that the manufacturer designs for. It tends to be at or just below the governing wind speed of the wind generator. Any wind generator may peak at a higher output than the rated output. The faster you spin a wind generator the more it will produce until it overproduces to the point that it burns out. Manufacturers rate their generators at a safe level well below the point of self-destruction.

Peak Output

This figure may be the same as rated output, or it may be higher. Wind generators reach their peak output while governing, which occurs over a range of wind speeds above their rated wind speed [33].

Mean Wind Speed

The mean wind speed for a usual day of a month can be calculated by averaging all the recorded wind speeds for the month.

The mean wind speed is calculated using the equation below [36].

(1)

Where,

Vjobserved wind speed (m/s)

Njnumber of wind speed observation

Vimean wind speed (m/s)

Upgrading the Mean Wind Speed

The mean wind speeds are then upgraded to the hub height. Wind speeds increase with height [37]. The calculated mean wind speeds are speeds recorded near the ground surface. Since the hubs of wind turbine are usually more than ten meters high, the mean wind speeds at a particular height will be greater than Vi. Therefore, to obtain mean wind speeds, Vi has to be projected to the hub height. The projected Vi is calculated using the power-law equation shown [38]-[39].

(2)

Where,

VZ : mean wind speed at projected height Z

Vj : mean wind speed at reference height Zj (usually 10m)

Z: projected height, or hub height

Zj : reference height, usually 10m

X : power-law exponent

The power-law exponent, x depends upon the roughness of the surface. For open land, x is usually taken as 1/7.

Weibull Distribution

A random variable v can be expressed with a Weibull distribution by utilizing the probability density function (pdf) as given by Stevens and Smulders [34] and shown below:

(3)

Where c is a scale parameter with the same units as the random variable and k is a shape parameter.

Power Output

The electric power output of a wind turbine is primarily a function of wind speed [35] and as shown below:

(4)

Where,

Vi : the cut-in wind speed

Vr : the rated wind speed

Vo : the cut-out wind speed

Pr : the rated electrical power

As illustrated in Figure below:

Figure 4.2: Wind Turbine generator power curve

The average wind power output from a wind turbine is the power produced at each wind speed multiplied by the fraction of the time that wind speed is experienced and integrated over all possible wind speeds. The average power output of a turbine is a very important parameter for any wind power system since it determines the total energy production and hence the total income. It is a much better indicator of economics than the rated power, which can easily be chosen at too large a value.

The equation in integral form is as follows:

(5)

The formula of average wind power output can be obtained by substituting (3) and (4) into (5), which gives equation (6) below [9]:

(6)

The software HOMER models a wind turbine as a device that converts the kinetic energy of the wind into AC or DC electricity according to a particular power curve, which is a graph of power output versus wind speed at hub height. An example of power curve is shown in figure 4.3 below:

Figure 4.3: Wind turbine power curve

HOMER assumes that the power curve applies at a standard air density of 1.225 kg/m3 which corresponds to standard temperature and pressure conditions. Each hour, HOMER calculates the power output of the wind turbine in a four-step process. First, it determines the average wind speed for the hour at the anemometer height by referring to the wind resource data. Second, it calculates the corresponding wind speed at the turbine’s hub height using either the logarithmic law or the power law. Third, it refers to the turbine’s power curve to calculate its power output at that wind speed assuming standard air density. Fourth, it multiplies that power output value by the air density ratio, which is the ratio of the actual air density to the standard air density. Note that, HOMER calculates the air density ratio at the site elevation using the U.S. Standard Atmosphere [40] and assumes that the air density ratio is constant throughout the year.

In addition to the turbine’s power curve and hub height, the design engineer specifies the expected lifetime of the turbine in years, its initial capital cost in U.S. dollars ($), its replacement cost in dollars and its annual O&M (operation & maintenance) cost in dollars per year [26].

4.2.2PV Array

Power output of the PV array can be calculated using the equation below:

Where,

= PV derating factor

= Rated capacity of PV array (kW)

= Global solar radiation incident on the surface of PV array (kW/m2)

= 1 kW/m2 (The standard amount of radiation used to rate the capacity of PV array)

The rated capacity sometimes called the peak capacity of a PV array is the amount of power it would produce under standard test conditions of 1 kW/m2 irradiance and a panel temperature of 25oC.

The engineering software package HOMER is used for modelling the hybrid power system, in the software the size of PV array is always specified in terms of rated capacity. The rated capacity accounts for both the area and the efficiency of PV module, so neither of those parameters appears clearly in the software. The software itself calculate each hour of the year global solar radiation incident on the PV array using the Hay, Davis, Klucher, Reindl (HDKR) model of Duffie and Beckmann [28]. The derating factor is a scaling factor meant to account for effects of dust on the panel, wire losses, elevated temperature or anything else that would cause the output of the PV array to deviate from that expected under ideal conditions. The HOMER software does not account for the fact that the power output of a PV array decreases with increasing panel temperature but we can reduce the derating factor to (crudely) correct for this effect when modelling systems for hot climates.

In reality the output of a PV array does depend strongly and nonlinearly on the voltage to which it is exposed. The maximum power point (the voltage at which the power output is maximized) depends on the solar radiation and the temperature. If the PV array is connected directly to a dc load or a battery bank then it will often be exposed to a voltage different from the maximum power point and performance will suffer. A maximum power point tracker (MPPT) is a solid state device placed between the PV array and the rest of the dc components of the system that decouples the array voltage from that of the rest of the system and ensures that the array voltage is always equal to the maximum power point. By ignoring the effect of voltage to which the PV array is exposed, HOMER effectively assumes that a maximum power point tracker is present in the system.

To explain the cost of PV array the user specifies its initial capital cost in U.S. dollars ($), replacement cost in dollars and O&M (operating and maintenance) cost in dollars per year. The replacement cost is the cost of replacing the PV array at the end of its useful lifetime which the user specifies in years. By default the replacement cost is equal to the capital cost but the two can differ for several reasons [1]-[26]-[27]

4.2.3Generator

A generator consumes fuel to produce AC or DC electricity. The generator can be AC or DC and can consume a different fuel. The principal physical properties of the generator are its maximum and minimum electrical power output, its expected lifetime in operating hours, the type of fuel it consumes and its fuel curve which relates the quantity of fuel consumed to the electrical power produced. A generator can consume any of the fuels listed in the fuel library in the software package HOMER. A diesel generator is used for the microgrid system. The software assumes the fuel curve is a straight line with a y-intercept and uses the following equation for the generator’s fuel consumption:

F = F0Ygen + F1Pgen

Where F0 is the fuel curve intercept coefficient, F1 is the fuel curve slope, Ygen the rated capacity of the generator (kW) and Pgen the electrical output of the generator (kW). The units of F depend on the measurement units of the fuel. If the fuel is denominated in litres then the units of F are L/h. If the fuel is denominated in m3 or kg then the units of F are m3/h or kg/h respectively. In the same way the units of F0 and F1 depend on the measurement units of the fuel. For fuels denominated in litres the units of F0 and F1 are L/h.kW.

For a generator that provides heat as well as electricity, the design engineer also specifies the heat recovery ratio. HOMER assumes that the generator converts all the fuel energy into either electricity or waste heat. The heat recovery ratio is the fraction of that waste heat that can be captured to serve the thermal load. In addition to these properties, the modeller can specify the generator emissions coefficients, which specify the generator’s emissions of six different pollutants in grams of pollutant emitted per quantity of fuel consumed.

The design engineer can schedule the operation of the generator to force it ON or OFF at certain times. During times that the generator is neither forced ON or OFF, HOMER decides whether it should operate based on the needs of the system and the relative costs of the other power sources. During times that the generator is forced ON, HOMER decides at what power output level it operates which may be anywhere between its minimum and maximum power output.

The design engineer specifies the generator’s initial capital cost in U.S. dollars ($), replacement cost in dollars and annual O&M (operation & maintenance) cost in dollars per operating hour also. The generator O&M cost should account for oil changes and other maintenance costs, but not fuel cost because HOMER calculates fuel cost separately. As it does for all dispatch-able power sources, HOMER calculates the generator’s fixed and marginal cost of energy and uses that information when simulating the operation of the system. The fixed cost of energy is the cost per hour of simply running the generator without producing any electricity. The marginal cost of energy is the additional cost per kilowatt-hour of producing electricity from that generator.

HOMER uses the following equation to calculate the generator’s fixed cost of energy:

Cgen,fixed = Com,gen + Crep,gen/Rgen + F0YgenCfuel,eff

Where Com,gen is the O&M cost in dollars per hour, Crep,gen the replacement cost in dollars, Rgen the generator lifetime in hours, F0 the fuel curve intercept coefficient in quantity of fuel per hour per kilowatt, Ygen the capacity of the generator (kW) and Cfuel,eff the effective price of fuel in dollars per quantity of fuel. The effective price of fuel includes the cost penalties if any associated with the emissions of pollutants from the generator.

HOMER calculates the marginal cost of energy of the generator using the following equation:

Cgen,mar = F1Cfuel,eff

Where F1 is the fuel curve slope in quantity of fuel per hour per kilowatt-hour and Cfuel,eff is the effective price of fuel (including the cost of any penalties on emissions) in dollars per quantity of fuel [26].

4.2.4Battery Bank

Although renewable resources are attractive, they are not always dependable in the absence of energy storage devices. As a result, renewable resources are often used together with energy storage devices. However, in many cases, such systems are the least understood and the most vulnerable component of the system [56]. Among different types of energy storage devices, lead-acid batteries are still the most commonly used devices to store and deliver electricity in the range from 5V to 24V DC [57]-[58].

The battery bank is a collection of one or more individual batteries. The software package HOMER models a single battery as a device capable of storing a certain amount of dc electricity at a fixed round-trip energy efficiency with limits as; how quickly it can be charged or discharged, how deeply it can be discharged without causing damage and how much energy can cycle through it before it needs replacement. HOMER assumes that the properties of the batteries remain constant throughout its lifetime and are not affected by external factors such as temperature.

In HOMER, the key physical properties of the battery are its nominal voltage, capacity curve, lifetime curve, minimum state of charge and round-trip efficiency. The capacity curve shows the discharge capacity of the battery in ampere-hours versus the discharge current in amperes. Manufacturers determine each point on this curve by measuring the ampere-hours that can be discharged at a constant current out of a fully charged battery. Capacity typically decreases with increasing discharge current. The lifetime curve shows the number of discharge-charge cycles the battery can withstand versus the cycle depth. The number of cycles to failure typically decreases with increasing cycle depth. The minimum state of charge is the state of charge below which the battery must not be discharged to avoid permanent damage. In the system simulation, HOMER does not allow the battery to be discharged any deeper than this limit. The round-trip efficiency indicates the percentage of the energy going into the battery that can be drawn back out.

Figure 4.4: Kinetic battery model concept

To calculate the battery’s maximum allowable rate of charge or discharge, HOMER uses the kinetic battery model [41] which treats the battery as a two tank system as illustrated in the figure above. According to the kinetic battery model part of the battery’s energy storage capacity is immediately available for charging or discharging but the rest is chemically bound. The rate of conversion between available energy and bound energy depends on the difference in ‘height’ between the two tanks. Three parameters describe the battery. The maximum capacity of the battery is the combined size of the available and bound tanks. The capacity ratio is the ratio of the size of the available tank to the combined size of the two tanks. The rate constant is analogous to the size of the pipe between the tanks. . The kinetic battery model explains the shape of the typical battery capacity curve as shown in figure 4.5 below:

Figure 4.5: Capacity curve for deep-cycle battery model US-250 [42]

Modelling the battery as a two-tank system rather than a single-tank system has two effects. First, it means the battery cannot be fully charged or discharged all at once, a complete charge requires an infinite amount of time at a charge current that asymptotically approaches zero. Second, it means that the battery’s ability to charge and discharge depends not only on its current state of charge but also on its recent charge and discharge history. A battery rapidly charged to 80% state of charge will be capable of a higher discharge rate than the same battery rapidly discharged to 80%, since it will have a higher level in its available tank. HOMER tracks the levels in the two tanks each hour and models both these effects.

Figure 4.6: Lifetime curve for deep-cycle battery model US-250

The above figure shows a lifetime curve of a deep-cycle lead-acid battery. The number of cycles to failure (shown in the graph as the lighter-coloured points) drops sharply with increasing depth of discharge. For each point on this curve, one can calculate the lifetime throughput (the amount of energy that cycled through the battery before failure) by finding the product of the number of cycles, the depth of discharge, the nominal voltage of the battery and the aforementioned maximum capacity of the battery. The lifetime throughput curve as shown in the above figure as black dots typically shows a much weaker dependence on the cycle depth. HOMER makes the simplifying assumption that the lifetime throughput is independent of the depth of discharge. The value that HOMER suggests for this lifetime throughput is the average of the points from the lifetime curve above the minimum state of charge but the user can modify this value to be more or less conservative.

The assumption that lifetime throughput is independent of cycle depth means that HOMER can estimate the life of the battery bank simply by monitoring the amount of energy cycling through it, without having to consider the depth of the various charge-discharge cycles. HOMER calculates the life of the battery bank in years as:

Rbatt = min (NbattQlifetime / Qthrpt ,Rbatt,f )

Where,

Nbattnumber of batteries in the battery bank

Qlifetimelifetime throughput of a single battery

Qthrpt annual throughput (the total amount of energy that cycles through the battery bank in one year).

Rbatt,ffloat life of the battery (the maximum life regardless of throughput).

The user specifies the battery bank’s capital and replacement costs in U.S. dollars ($) and the O&M (operating & maintenance) cost in dollars per year. Since the battery bank is a dispatch-able power source, HOMER calculates its fixed and marginal cost of energy for comparison with other dispatch-able sources. Unlike the generator, there is no cost associated with operating the battery bank so that it is ready to produce energy; hence its fixed cost of energy is zero. For its marginal cost of energy, HOMER uses the sum of the battery wear cost (the cost per kilowatt-hour of cycling energy through the battery bank) and the battery energy cost (the average cost of the energy stored in the battery bank). HOMER calculates the battery wear cost as below:

Cbw = (Crep,batt / NbattQlifetime rt)

Where,

Crep,batt replacement cost of the battery bank

Nbatt number of batteries in the battery bank

Qlifetime lifetime throughput of a single battery (kWh)

rt round-trip efficiency

HOMER calculates the battery energy cost each hour of the simulation by dividing the total year-to-date cost of charging the battery bank by the total year-to-date amount of energy put into the battery bank. Under the load-following dispatch strategy, the battery bank is only ever charged by surplus electricity, so the cost associated with charging the battery bank is always zero. Under the cycle-charging strategy however, a generator will produce extra electricity (and hence consume additional fuel) for the express purpose of charging the battery bank, so the cost associated with charging the battery bank is not zero [26].

4.2.5 Converter

A converter is a device that converts electric power from DC to AC in a process called inversion and/or from AC to DC in a process called rectification. The software HOMER can model the two common types of converters that are solid-state and rotary. The converter size which is a decision variable refers to the inverter capacity, meaning the maximum amount of AC power that the device can produce by inverting DC power. The model design engineer specifies the rectifier capacity which is the maximum amount of DC power that the device can produce by rectifying AC power as a percentage of the inverter capacity. The rectifier capacity is therefore not a separate decision variable. HOMER assumes that the inverter and rectifier capacities are not surge capacities that the device can withstand for only short periods of time but rather continuous capacities that the device can withstand for as long as necessary.

The HOMER user indicates whether the inverter can operate in parallel with another AC power source such as a generator or the grid. Doing so requires the inverter to synchronize to the AC frequency, an ability that some inverters do not have. The final physical properties of the converter are its inversion and rectification efficiencies which HOMER assumes to be constant. The economic properties of the converter are its capital and replacement cost in U.S. dollars ($), its annual O&M (operation & maintenance) cost in dollars per year and its expected lifetime in years [26].

4.2.6 Domestic Load
4.2.6.1Load Profile

In electrical engineering, a load profile is a graph of the variation in the electrical load versus time. A load profile will vary according to customer type (typical examples include residential, commercial and industrial), temperature and holiday seasons. In the electricity generation sector, a load curve is a chart showing the amount of electricity customer’s use over a period of time. Generation companies use this information to plan how much power they will need to generate at any given time [61].

Load Profile is a broad term that can refer to a number of different forms of data. It can refer to demand and consumption data or it can be a reference to derived data types, such as Regression and Profile Coefficients. However, all these data types have one thing in common that they represent the pattern of electricity usage of a segment of supply market customers [60].

4.2.6.1.1 Load Factor

Load factor is the average power divided by the peak power, over a period of time. The peak may be a theoretical maximum, rather than a measured maximum [62]-[64].

A Peak Load Factor is defined as follows:

The ratio expressed as a percentage of the number of kWh supplied during a given period to the number of kWh that would have been supplied had the maximum demand been maintained throughout that period [60].

So for an Annual Peak Load Factor:

LF = [(Annual Consumption (kWh)) / (Maximum Demand (kW) * Number of Hours in the Year)] * 100

Note: 8760 hours or 8784 hours in a leap year

4.2.6.2 Domestic Load Profile Scenarios

There are two different types of domestic load profiles case studies scenarios for the simulations of microgrid system, which are as follows:

I. Domestic Load Profile in UK

II. Domestic Load Profile in Pakistan

4.2.6.2.1 Domestic Load Profile in UK

The load profile mostly depends on occupancy pattern so analyzing the load profile, it is necessary to identify the cluster of household. I worked on five most common cases or scenarios of UK domestic occupancy pattern due to having less information regarding household occupancy pattern, which are under below:

Scenario 1:

In this case unoccupied period is from 09:00 to 13:00. One of the occupants may have part time job in the morning in this type of household occupancy pattern.

Scenario 2:

Unoccupied period is from 09:00 to 16:00. The occupants in the house all have full time job.

Scenario 3:

Here unoccupied period is from 09:00 to 16:00. This type of household occupants may have a child to look after when school closed.

Scenario 4:

In this case the house is occupied all the time because this type of household occupants may have retired couples, children to look after and single

Scenario 5:

Unoccupied period is from 13.00 to 18.00. One of the occupants in this type of household may have a part time job in the afternoon session [59].

UK Domestic Typical Profile (Averge of All):

This is the average of above all five different scenarios of domestic load profile pattern in the UK at the present.

4.2.6.2.2 Domestic Load Profile in Pakistan

In Pakistan, the load profile depends on user electricity consumption and occupancy; so analyzing the load profile, it is necessary to identify the group of household. I worked on three most common scenarios of Pakistan household occupancy pattern depending on low consumption, medium consumption and high consumption electricity users. All three types of scenarios are calculated on assumption based with the average of all four seasons (spring, summer, autumn and winter), that are below:

Scenario 1:

There are two people living in one bed room house in this type of occupancy and are low electricity consumption users. The average of all four season’s graph is shown below.

Scenario 2:

This type of occupancy is under four persons living in three bed room house and they are medium electricity users.

Scenario 3:

In this scenario, there are five persons living in five bed room house. One of the occupants may have full time job, one school child and they are high electricity users.

Pakistan Domestic Typical Profile (Average of All):

This is the typical load profile which is the average of above three different scenarios of domestic load profile in the Pakistan.

Chapter 5
DESIGN CASE STUDIE

5.1 Introduction

There are two different case studies carried out in the two different places for the design of microgrid power system. The first one is wind/diesel/battery hybrid power system within the microgrid in the UK, and the other is photovoltaic/diesel/battery hybrid power system in the Pakistan. The both case studies are specified below:

I. Design Case Study 1

II. Design Case Study 2

5.2 Design Case Study 1

5.2.1 Introduction

The first case study for designing the microgrid system is carried out in the Isle of Arran, Scotland, UK. Arran is the seventh largest island in the Scotland and it is in the North Ayrshire unitary council area. Arran is located within the latitude and longitude of 55o 34’N, 05o 12’W. It has an area of 167 square miles (432 square kilometres), 874 meters height and 50 miles from Glasgow City. Temperatures are generally cool, averaging about 6 °C (43 °F) in January and 14 °C (57 °F) in July at sea level. The southern half of the island, being less mountainous has a more favourable climate than the northern half and the east coast is more sheltered from the prevailing winds than the west and south.

Figure 5.1: Isle of Arran [Source: Google Map®]

Wind energy is one the best renewable energy resource (RES) in Scotland and Isle of Arran is the best location due to fast winds blowing. Also RES’s are the most cost effective, reliable and environment friendly sources of electricity generation for the island areas [65]-[66]-[68].

It was decided to use available renewable energy sources based on hourly or daily energy consumption by implementing a small scale microgrid power system. This hybrid power system (wind/diesel/battery) combines into wind turbine, diesel generator, storage batteries, converter and some power electronic equipments. The HOMER (Hybrid Optimization Model for Electric Renewable) software is used for the modeling and simulations of the microgrid system.

5.2.2System Design

To verify the reduction in carbon emissions or green house gases (GHG) and economic viability, a microgrid hybrid power system is proposed to feed a typical house located in remote area of the Isle of Arran, Scotland. The model consists of wind turbine, diesel generator, battery bank, converter, domestic load and AC & DC busbars. The HOMER software is used for modeling the system design, simulation, economic analysis and calculation of green house gases (GHG). Monthly average wind resources data and average domestic loads are used as input parameters. The schematic diagram of the microgrid power system is modeled in the HOMER, which is shown in figure 5.2 below:

Figure 5.2: Schematic diagram of microgrid system

The average load profile of the UK domestic household or remote household is below in figure 5.3 and load parameters can be seen in the table 5.1 below:

Figure 5.3: Load profile

Table 5.1: Load Parameters

Baseline

Scaled

Average (kWh/day)

10.6

10.6

Average (kW)

0.443

0.443

Peak (kW)

1.10

1.10

Load Factor

0.402

0.402

Monthly average wind speed data of specific location ‘Isle of Arran’ is collected from weather underground [70]; figure 5.4 shows the average wind speed data as below:

Figure 5.4: Average wind speed

The table 5.2 shows the calculated Weibull distribution parameters (shape parameter and scale parameter) and figure 5.5 shows the Weibull distribution of wind speed as follows:

Table 5.2: Weibull distribution parameters

Shape parameter, k

Scale parameter, c

2.00

3.67

Figure 5.5: Weibull distribution of wind speed

The Skystream 3.7 1.8 kW turbine is used for wind power generation and it is suitable for powering rural domestic properties and many more applications. This wind turbine is intended for a range of conditions especially rural locations. The design life of the machine is 20 years and has been extensively tested by the US Government’s NREL organization [71]. Figure 5.6 shows the proposed 1.8 kW wind turbine with tower and figure 5.7 shows the particular wind turbine power curve.

Figure 5.6: Skystream 3.7 1.8 kW wind turbine [71]

Figure 5.7: Wind turbine power curve

Wind turbine specifications are as follows:

Rated power:1.8 kW

Rotor diameter: 3.7 m

Hub height: 25 m

Cut-in speed: 3.5 m/s

Cut-out speed: 27-33 m/s

Tip speed:9.7-63 m/s

Survival wind speed: 63 m/s

No. of blades:3

Rotor orientation:Downwind

Blade material: Fiberglass reinforced composite

Table 5.3 below shows the cost of microgrid components. In the generator, diesel is used as a fuel and its annual average price 0.753 $/litre [75] is assumed. Generator has 15,000 operating hours (lifetime), wind turbine has 20 years lifetime, converter has 15 years lifetime with 90% efficiency, 1 battery bank comprises in 8 batteries, each of nominal 6V & nominal capacity of 360 Ah.

Table 5.3: Cost of microgrid components

System Components

Capital Cost

in $

Replacement Cost

in $

O&M Cost

in $

Wind Turbine

1.8 kW

14000

7000

300 $/yr

Generator

1 kW

500

400

0.05 $/hr

Converter

1 kW

500

500

100 $/yr

Battery

(8 nos.)

300/Battery

300/Battery

20 $/yr

5.2.3Simulation Results

The figure 5.8 below shows the wind/diesel/battery hybrid power system’s simulation result summary. This is the main graph of system’s result which shows the monthly average electricity production and renewable fraction. It shows the contributions of wind turbine generation and generation by diesel generator for the microgrid system. The wind turbine contribution (renewable fraction) towards electricity generation is 27% (1177 kWh/year) and rest of the generation is 73% (3156 kWh/year) which is produced by generator, as can be seen in figure 5.9 and figure 5.10 below:

Figure 5.08: Monthly average electricity production

Figure 5.09: Simulation results

Figure 5.10: Electricity production summary

The figure 5.11 shows the results of reduction of green house gases (GHG) emissions by using wind/diesel/battery hybrid system.

Figure 5.11: Results of GHG emissions

The figure 5.12 and 5.13 below shows the cash flow summary of the each microgrid component and total cost of the complete system in U.S. Dollars ($). It shows 41,897$, the total cost of the system.

Figure 5.12: Cash flow summary

Figure 5.13: Cash summary of the complete system

Note:

In a politically supported drive towards a low-carbon economy, the UK government now provides grants of up to ?2500 per property towards the cost of installing low carbon micro generation technologies such as micro-wind turbines, solar electricity, solar water heating systems, small-scale hydro power systems, ground source heat pumps and biomass boilers etc [8]-[73].

So, as a result the total cost of the microgrid system will reduce. So, new total cost of the system would be:

New Total Cost ($) = 41,897 – 3,788

5.3 Design Case Study 2

5.3.1 Introduction

The second case study for microgrid system design is carried out in the Multan, Punjab, Pakistan. Multan is among the big cities in the Punjab Province of Pakistan, capital of Multan District and at almost the exact centre of Pakistan. The closest major city is Sahiwal. Multan is located within the latitude and longitude of 30o 15’N, 71o 36’E. It has a total area of 1436.7 square miles (3721 kilometers). The area around the city is a flat plain and is ideal for agriculture. Multan features an arid climate with very hot summers and mild winters. The city witnesses some of the most extreme weather in the country. During the summers, temperatures reach approximately 54 °C (129 °F) and in the winter -1 °C (30.2 °F) has been recorded [67]-[68].

Figure 5.14: Multan [Source: Google Map®]

Solar energy is one of the most promising renewable energy sources in Pakistan and Multan is in the best locations for harnessing the solar power in the Pakistan. So that is why this location is selected for photovoltaic (PV) generation. Solar energy is more predictable than wind energy and less vulnerable to changes in seasonal weather patterns than hydropower. Solar energy can produce power at the point of demand in both rural and urban areas [69].

The hybrid power system (photovoltaic/diesel/battery) combines into PV modules, diesel generator, storage batteries, converter and some power electronic equipments. The HOMER software is used for design and simulations of the microgrid system.

5.3.2System Design

Reducing the green house gases (GHG) emissions and economic viability, a microgrid hybrid power system is planned to provide electricity for a typical house located in remote area in Multan, Pakistan. The model consists of photovoltaic arrays, diesel generator, battery bank, converter, domestic load and AC & DC busbars. The HOMER software is used for modeling the system design, simulation, economic analysis and calculation of green house gases (GHG). Monthly average solar radiations and average domestic loads are used as input parameters. The figure 5.15 below shows the schematic diagram of the microgrid system.

Figure 5.15: Schematic diagram of microgrid system

The average load profile of the Pakistan domestic household is below in the figure 5.16 and load parameters can be seen in the table 5.4 below:

Figure 5.16: Load profile

Table 5.4: Load Parameters

Baseline

Scaled

Average (kWh/day)

24.3

24.3

Average (kW)

1.01

1.01

Peak (kW)

1.70

1.70

Load Factor

0.597

0.597

The average monthly solar radiations data of particular location ‘Multan’ is collected from the Science Direct research paper “Prospects for secure and sustainable electricity supply for Pakistan” [69]. The figure 5.17 below shows the average solar radiations data.

Figure 5.17: Average solar radiation

The figure 5.18 below shows the solar panels mounted on the house roof.

Figure 5.18: Solar panels mounted on the roof

The table 5.5 shows the cost and approximate area required for mounting the solar panels on the house roof.

Table 5.5: PV module and approximate area

PV-Module (Watts)

Approximate Roof Area

Required (m2)

Approximate Cost

in $/Watt

2000

18.58

4 to 5

Table 5.6 below shows the cost of microgrid components. In the generator, diesel is used as a fuel and its annual average price 0.753 $/litre [75] is assumed. Generator has 15,000 operating hours (lifetime), PV array capital cost is 8000$ which I got it from “free sun power” [76] and it has 20 years lifetime, converter has 15 years lifetime with 90% efficiency, 1 battery bank comprises in 8 batteries, each of nominal 6V & nominal capacity of 360 Ah.

Table 5.6: Cost of microgrid components

System Components

Capital Cost

in $

Replacement Cost

in $

O&M Cost

in $

PV Module

2 kW

8000

6000

20 $/yr

Generator

1 kW

500

400

0.05 $/hr

Converter

1 kW

500

500

100 $/yr

Battery

(8 nos.)

300/Battery

300/Battery

20 $/yr

5.3.3Simulation Results

The figure 5.19 below shows the photovoltaic/diesel/battery system simulation result summary. It shows the contributions of photovoltaic generation and generation by diesel generator for the microgrid system. The photovoltaic’s contribution (renewable fraction) towards electricity generation is 42% (4371 kWh/year) which is quite good and rest of the generation is 58% (5991 kWh/year) which is produced by generator, as can also be seen in figure 5.20 and figure 5.21 below:

Figure 5.19: Monthly average electricity production

Figure 5.20: Simulation results

Figure 5.21: Electricity production summary

The figure 5.22 shows the results of reduction of green house gases (GHG) emissions by using photovoltaic/diesel/battery hybrid system.

Figure 5.22: Results of GHG emissions

The figure 5.23 and 5.24 below shows the cash flow summary of the each microgrid component and total cost of the complete system in U.S. Dollars ($). It shows 44,069$, the total cost of the system.

Figure 5.23: Cash flow summary

Figure 5.24: Cash summary of the complete system

So, as a result the total cost of the photovoltaic/diesel/battery microgrid system for Multan, Pakistan is 44,069$ or ?29,080 [72].

Chapter 6
RESULTS

6.1 Discussions

From the study of this project it can be summarizing that the simulation results of energy production by small scale generators (conventional and non-conventional) in close proximity to the energy users, integrated into microgrid, can manage to feed the load efficiently with quality clean or green power. The system generates power with the reduction of harmful green-house gases (GHG) emissions as compared to pure conventional power system, which makes global warming. The system is efficient enough to meet the domestic load requirements, and the system can be more efficient and eco-friendly if the wind turbine and solar photovoltaics generate more electricity to make the system more greener or environmental friendly.

6.2 Problems Occurred

First of all, at the beginning of the project I had difficulties to find proper software for modeling and simulating the microgrid system. I found various types software’s for designing hybrid system, but they were not actually suitable for my proposed system design. After searching a lot I found HOMER software for designing, modeling and simulating the microgrid system with the help of Prof. Chengke Zhou.

Finding wind resource data was another issue to concern because the main weather forecast department ‘Met-office’ charges for giving annual wind resource or solar radiations data. But at last I found the required annual wind resources data from weather underground website.

Designing the microgrid system on HOMER was the main issue because I never used this software before or neither designed any hybrid system. So, it was a challenge for me to deal with. Because making reliable, economical and efficient microgrid system; the right specification of the each component had to be considered. But working on HOMER software for designing microgrid system has been very useful with great experience.

Designing two different microgrid systems for two different remote locations was a big challenging task because a lot research and design work was involved and it was lengthy as well due to two different case studies.

6.3 Possible Achievements

I have achieved and learned a lot of conceptual and broad knowledge related to this project like software skills, system design calculations, impotency of renewable energies and designing small scale electric power system for electrifying remote locations or individual consumers.

6.4 Conclusions

The study highlighted the increasing requirement for the combination of renewable energy systems at the distributed generation level. Small scale wind turbines and PV modules have found applications in numerous sectors including domestic.

From overall project, designed microgrid power systems for both case studies contribute their part to reduce greenhouse gasses emissions (GHG) which makes the world warming; also designed systems meet the requirements of remote load efficiently. Case study 1 (wind/diesel/batter hybrid system) in the Isle of Arran, Scotland, UK has 27% renewable fraction which is quite good, the case study 2 (photovoltaic/diesel/battery hybrid system) in Multan, Punjab, Pakistan has 42% renewable fraction which is very good result and the overall system is also economical because Pakistan has a lot potential in solar energy.

As a result, the total cost of case study 2 is higher than the case study 1 because it has more domestic load rather than case study 1 and had to design system with 2 kW PV module, also there is not governmental support included in the system because government of Pakistan does not contribute towards installing small scale renewable energy system, but overall case study 2 contributes more towards reduction of carbon emissions.

Overall, the principal conclusion is that microgrid systems do have real potential to make a major contribution to reducing GHG emissions from individual or domestic locations. This will only happen if there are major changes to the electricity market and regulatory structure.

6.5 Future Outwork

After done this project there are some recommendations for further work in order to improve or efficient the power system:

If the designed wind turbine cut in speed is less or minimum rather than 3.5 m/s (the used Skystream wind turbine) then the wind turbine would produce more power, or if we install the system in a particular area where the wind speeds are really high then the system would generate more power.
If the PV module quality can be improved then we can get more power by photovoltaic system, so have to work on PV module material.

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