Electrical Power Generation Started Engineering Essay

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Introduction

Since the first public power distribution system was developed, in 1882, by the famous Thomas Edison, our modern life style started to shape. Electricity made a shift for human history, bringing all life's modern luxuries into being. (Chapman S. J., p.66). Before electricity became available over 100 years ago, houses were lit with kerosene lamps, food was cooled in iceboxes, and wood-burning or coal-burning stoves warmed rooms. In other words, Electricity has changed that and become a key driver in our modern life development.

Electrical power generation started in the form of cool power plants using Steam turbines to drive Direct Current generators. That was followed by huge developments in electrical power generation methods. Combined cycle power plant, Nuclear Power Plant and Hydroelectric Power Plant are the latest forms of power generation methods. Although those types of power plants are considered to have high reliability and low loss of load probability (LOLP) fraction, they still suffer from many major issues threatening the globe indirectly, by increasing Green House Gases (GHG), and increasing the availability of some types of fuel, which might not be available for all nations, either now or in the future.

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Waldau A. J. et al (2011) mentioned that "besides the increasing pressure on the supply side of energy by the increasing world energy demand, environmental concerns shared by a majority of the public and add to the list of weaknesses of fossil fuels and the problems of nuclear energy. These concerns include the societal damage caused by the existing energy supply system, whether such damage is of accidental origin (oil slicks, nuclear accidents, methane leaks) or connected to emissions of pollutants". (Baker J. A., 2004) added that generating electricity has made major damages to the environment which might, in the end, cause global catastrophes. Green House Gases (GHG) and CO2 emissions in particular cause environmental damages. Global warming or the expansion of the Ozone hole, which could lead to the melting of more ice in Antarctica and increase the water level in the seas, represent clear examples of the danger of GHG. Such Issues have led scientists to search for other alternatives, which might balance the scales.

Renewable Power Generation is being strongly considered. The technology offers a free fuel energy that is free of GHG emissions. Solar Power Generation has a long history and a promising future. Generally, Photovoltaic Power Systems helped to supply electricity to many rural places but since 1991, this case has changed. In Aachen, Germany (1991), the first installation of building-integrated photovoltaic's (BIPV) was realized.

In addition, the energy market in the UK is growing, according to many market analysts. In December 1997, the European Council and the European Parliament adopted the "White Paper for a Community Strategy and action Plan". In this paper, the aims are described as follows, "Renewable energy sources may help to reduce dependence on imports and increase security of supply. Positive effects also anticipated in terms of CO2 emissions and job creation. Renewable energy sources accounted in 1996 for 6% of the Union's overall gross internal energy consumption. The Union's aim is to double this figure by 2010" (European Commission, 1997). The UK government is stating policies to support renewable projects. Subsequently, seeking sustainable and cleaner energy to provide a secure energy level of consumption is an international concern.

Residential Buildings contribute in a large way to the total GHG and CO2 emissions. In the UK, residential CO2 and GHG emissions are 14% and 12% respectively. The commercial institutions contribute in 3.8% and 3.2%. (European Commission, 2010). Figure.1 illustrates GHG and CO2 emissions by each sector. As well, Domestic and household consumption of electricity represents 32% of the total electricity generation, while the commercial sector consumes 19% of the total electricity produced. (DECC, 2010). Refer to Figure 2.

Figure

Figure Consequently, it is important to consider better solutions for residential sector electricity production. If the nineteenth century was the age of coal and the twenty century is the age of oil, then definitely the twenty first century is the age of sun and solar power. Building Integrated PV system (BIPV) is considered to be one of most efficient solutions. PV system integration in buildings can overcome all the above problems and achieve most of the required objectives to a certain extent.

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Furthermore, PV systems on roofs or on facades have become more challenging. The UK market faces a short drop in demand, due to reduction on feed in tariff (FIT) by the UK government. As a result, most of the PV panels prices dropped dramatically. On the other hand, installation still costs almost the same. Competitiveness among the markets contractors has increased, which opened the market for cheaper installation prices.

On the other hand, UK electricity production, using solar cells has increased dramatically. The total production in 2005 was 10.9 MW, this number has jumped in 2011 to 975.8 MW. (Dukes chapter 6 XX)This is an indication of how promising the photovoltaic market is. The total consumption of electricity from photovoltaic in 1999 used to be only 1000 MWh this number has increased after a decade to be 11000 MWh in 2007. Figure XX shows the increase along nine years. (European Commission gross electricity XX)

Finally, it is important to note that the UK photovoltaic market now is under uncertain conditions due to the change of the incentives and low feed in tariff. All the calculations of the financial part have been done under this assumption, which might decrease the figure. On the other hand, when the market returns to a stable situation the figures are expected to increase.

Figure dada from xx

Solar Radiation Received by the Earth's surface

- Solar System:

It is obvious that the Photovoltaic system is related to the sun and the earths movement around it, thus, studying this movement and the way the radiation will fall into the earths surface has great importance, in order to achieve the highest possible performance. In addition, it is important to understand the geometric relationships between a planet relative to the earth at anytime and the incoming radiation. This will make it possible to find the power output for any system intended to be installed.

The sun is a sphere containing hot gaseous matter and has a diameter of 1.39 x 109 m. On average, the earth is 1.5 x 1011 m away from the sun. This distance equals about 12000 times the earth's diameter. The earth revolves around the sun in an elliptical unusual orbit that varies the distance between the sun and the earth by 1.7%. The day of the closest approach in the northern hemisphere is known as Perihelion and occurs on the 2nd of January, whist on 2 of July, the earth is at its greatest distance from the sun, this distance is known as Aphelion. (The European solar radiation atlat xx) It has an effective blackbody temperature of 5777 K. The radiation emitted by the sun and its spatial relationship to the earth result in a nearly fixed intensity of solar radiation outside the earth's atmosphere, often referred to as extraterrestrial radiation. The extraterrestrial radiation's values, referred to as solar constant, found in the literature vary slightly due to the measurement techniques or assumptions for necessary estimations. The World Radiation Center (WRC) has adopted a value of 1367 W/m2, with 1% uncertainty. (IEA xx, photovoltaic i buildings)

Figure The European Solar radiation atlas xx

The Solar Radiation outside the earth's atmosphere changes throughout the year due to the change in the distance from the sun and the rotation of earth around its axis. The solar radiation outside the atmosphere is then calculated depending on the eccentricity correction factor () and the day of the year. (Handbook XX). According to (beckman xx, 2006), depends on the distance of the earth from the sun, which will vary by ± 1.7% of its mean value , which is equal to 1.495Ã-1011 m. A simple equation for engineering proposes combines the change in the day and distance and defines the solar radiation outside the earth's atmosphere as following:

Equation

Where

Gsc: solar constant, 1367 W/m2.

n: is the day number of the year.

- Geometrical considerations:

To put a formula to find the radiation received on the system's surface, tilted surface, by only knowing the total radiation on the horizontal surface. It is important to know the direction from which the beam or the diffused radiations are received. The geometrical properties should be studied. The next definitions and equations are used in the calculation later in this paper.

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The Declination angle is the key input for the solar geometry. It is defined by (xx European solar radiation atlas) as "the angle between the Equatorial Plane and the line joining the centre of the Earth's sphere to the centre of the solar disk. The axis of rotation of the Earth about the poles is set at an angle to that so called Plane of the Ecliptic. "The angle varies along the Julian days between 23.45Ëš and -23.45Ëš. The following equation relates to the declination angle and the day number n, along the year.

Equation

Solar Hour Angle , according to (beckman xx, 2006), is the angular displacement of the sun east and west of the local meridian. It changes 1Ëš for each minute and 15Ëš each hour. It changes 15Ëš each hour after the solar noon and -15Ëš each hour before the solar noon. The solar noon corresponds to the moment when the sun is at the highest point in the sky. So the solar noon does not depend on the local time but on the solar time. The solar time can be found as following:

Equation

Where Lst is the standard meridian for the local time zone, Lloc is the longitude of the specific location in degree. E is the equation of time in minutes which equals to:

Equation

The Latitude angle , it is the angular location north of the equator as positive and south of the equator as negative. It's values range between -90Ëš and +90Ëš.

The Sunset Hour angle, according to (RETScreen xx, 2011), is the angle of the sun at the sunset solar hour. It can be found using the following equation:

Equation

Slope Angle , this is the tilt angle where the Photovoltaic panel or array is tilted from the horizontal. Generally, as a rule of thumb, to collect maximum annual energy, a surface slope angle should be adjusted to be equal to the latitude angle. For the summer maximum energy gain, slope angle should be approximately 10Ëš to 15Ëš less than the latitude and for the winter, maximum energy gain can be acquired when the angle is adjusted to be 10Ëš to 15Ëš more than the latitude. (beckman xx, 2006)

Surface Azimuth angle , this is the deviation of the projection, on a horizontal plane, of the normal to the surface from local meridian. It is equal to zero when it is pointed to the south, negative to the east and positive to the west. It ranges between .

Angle of Incident this is the angle between the beam radiation on a surface and the normal to that surface. It can be calculated as follows:

Equation

Zenith Angle , is the angle between the vertical of the sun and the incident solar beam. Its value must be between 0Ëš and 90Ëš. For a horizontal surface the zenith angle can be calculated using the following equation.

Equation

The following figure XX illustrates the angles on a tilted surface. Please note that the previous equations will be implemented in a manual calculation for the total power output of the proposed system, later in this paper. The calculation will be done using Microsoft Excel.

Figure Solar Geometry Angles xx

- Solar Radiations reaches a specific tilted surface:

The directions from which solar radiation reaches a specific tilted surface are a dependent on conditions of cloudiness and atmospheric clarity. (Beckmanxx, 2006) Those radiations are considered to be distributed over the sky dome. In general, the data of cloudiness and clarity are widely available.

In this paper radiations have been dealt with as three parts; Beam radiation, Diffused radiations and Ground reflected or what is known as Albedo. The beam radiations are the amount of radiations that have been received on a specific surface without scattering; it will be represented as Hb. The diffused radiations are those radiations, which their direction have been changed before they receive a specific surface. Finally, the ground reflected radiations are the radiations received on a specific surface after they have been reflected from the ground.

-Clearness Index:

The Clearness index gives a measure of atmospheric transparency. It shows the relation between solar radiation at the Earth's surface and extraterrestrial radiation. It is related to the path of which the solar radiations have been received on earth's surface, which will be illustrated in a later section, referred to as atmospheric AM value. It also represents the composition and the cloud content of the atmosphere. (Handbook xx, 2011) Thus, the Clearness Index is defined as:

Equation

Where, is the monthly average daily solar radiation on a horizontal surface and is the monthly average extraterrestrial daily solar radiation, which can be found from the following equation:

Equation

- Calculating of Hourly Global and Diffused Irradiance:

To calculate the hourly irradiances, a developed method by Erbs et al and introduced by (Beckman xx, 2006), was used. It is obvious that the amount of the diffused radiations will be a function of Kt, thus the theory developed the monthly average diffused fraction correlation. Equations for these correlations are as following, for:

Equation

For :

Equation

The average daily irradiance is now broken into hourly values. To do so, the equation developed by Collares-Pereira is used in the calculations. The formulas are as following:

Equation

Where is:

Equation

Where a and b values can be found as follows:

Equation

Equation

Note that the values of sunset angle and the hour angles are in radians. Then the values of both the diffused and the Beam irradiances can be calculated as follows:

Equation

Equation

can be found using this equation:

Equation

The calculation of the total hourly irradiance is a combination of the three irradiances values; the beam irradiance, diffused irradiance and the ground reflectance. This equation was developed upon an Isotropic Model, which had been derived by Jordan and Liu in 1963. (Beckman xx, 2006). The equation equals to:

Where:

Moreover, is the average diffused ground reflectance, Albedo.

Building Integrated PV (BIPV) systems

Examining Photovoltaic modules for building integration, produced as a standard building product that fit into standard facade and roof structures. (IEA, 1996). Since the first integration for Photovoltaic into buildings, it has become one of the fastest growth market segments in photovoltaic. (Benemann J. et al, 2001)

There are several reasons for the great interest in PV systems in buildings. Its image as a high-tech and its futuristic technology makes it more interesting for engineers, architect and consumers. As well, integration of PV is technically simple to install compared with other solar technologies such as solar thermal. (Fieber A., 2005). Furthermore, the price of PV panel integration in building is economically attractive where its profit expectation is promising.

A roof or facade element with photovoltaic can be used in all kind of building's structures, curtain wall facade (with isolating glass), rear vented curtain wall facade, structural glazing and tilted facade. It is expected from the photovoltaic system to cover day lighting, reduce the noise and produce electricity. (Benemann J. Et al, 2001). While Thomas R. And Fordham M. argued (date ?) that the reasons of why Photovoltaic is attractive technology is that using it includes supplying all, or most likely the largest portion, of the annual electricity requirement of a building, making a contribution to the environment, making a statement about innovative architectural and engineering design and using them as a demonstration or educational project. (Thomas R. and Fordham M., 2001).

To integrate a PV system in any building, many considerations must be taken into account by the designer and engineers. One of the crucial points is the orientation of the building and tilt angle of the PV panel, Solar irradiations and the electrical system used including the proposed inverter and control methods.

In general, any BIPV system consists of Photovoltaic panel(s), inverter(s) and accessories, which are usually referred to as Balance of System (BOS) and switchgears. PV panels are the main component used to convert the energy carried by the photons, particles that exist in sunlight, into electrical power. The inverter will convert the produced DC electrical power by the PV panels to an AC usable electrical power. The BOS includes kWh meter(s), cables, fuses, combiners, fittings, grounding connections, switchgear and strings, DC and AC switches and connectors.

The PV system integrated into a building would not need a storage system, batteries; since the storage system is normally used to supply the load during the night hours or when there is not enough radiation to produce electricity into the PV panels. In this case, the national grid will act as a storage system. (Handbook) Figure XX illustrates a basic grid connected (On-Grid) schematic of PV system. More details about each component of the system are presented later; specifically on PV cell, module and array and on the conditioning system (inverter).

Figure handbook page 846 (pdf876) grid connected pv system

To explain how the solar system does work, it is important to describe the nature of the sun light and the radiations that fall on earth's surface. As well, a short introduction about the sun and earth position should be presented to be able to elucidate sunlight, radiation analysis and solar system.

System Components

- Solar Cell Basics:

The Solar cell is a solid-state device that absorbs light and converts part of its energy- directly into electricity. The process is done within the solid work structure; the solar cell does not have any moving parts (practical photovoltaic).

The photovoltaic cell is manufactured by combining two layers of semiconductors differently doped, a p-type and an n-type layer. The combination will result of a matching between holes and electrons which will lead to creating a potential layer. This is why the solar cells are usually referred to as "Photovoltaic cells", the photovoltaic effect. Photovoltaic effect is the electrical potential, developed between the two dissimilar materials. When the two dissimilar material's common junction, or what is called the depletion layer is illuminated with radiation of photons, thus an electrical potential gradient will be created. (Wind and Solar Power Systems (Patel) xx, 1999)

Each photon, if it has enough energy, is capable of releasing an electron, which has a negative charge, or creating a hole, which has positive charge. The accumulated process will result in a current and potential difference on both cells sides, the p-type and the n-type. The released electrons will be accelerated because of the resultant gradient, which is called Fermi level, and can then be circulated as a current through an external circuit. (wind and solar power systems xx, )

Figure Schematic of a solar cell. The solid white lines indicate the conduction and valence bands of the semiconductor layers; the dotted white lines indicate the Fermi level in the dark. XX

- Light characteristics:

All electromagnetic radiations can be viewed as being composed of particles called Photons. According to the theory of quantum, the photons are particles that travel in vacuum with the speed of light and have no mass. Each photon carries specific amounts of energy as a packet, referred to as an electron volt (ev). The amount of energy is related to the proton's source spectral properties. The shorter the wavelength the larger the packet. (practical xx)

The sunlight spectral is divided into three regions see figure XX. The first region has a wavelength between 400 to 700 nanometres. At 700 nanometres, the visible spectrum appears red and on the shorter end of 400 nanometres it appears violate. All other colours appear in between. Our eyes are most sensitive to the spectrum around 500 nanometres. At 400 nanometres and less, the spectrum is called Ultraviolet (UV) wavelength and most of it is filtered or absorbed by the Ozone or the transparent material before it reaches the earth's surface. Our skin perceives the spectrum as radiant heat spectrums above 700 nanometres, which is referred to as Infrared. practical xx) The water vapour, CO2 and other substances in our atmosphere absorb most of the Infrared spectrums. On the other hand, Most of those absorptions become longer wavelengths than the wavelengths the solar system uses. While the solar system effectively collects wavelengths less than 2000 nanometres, thus its efficiency is not significantly affected. (solar engineering of thermal processes, duffie XX). Photon energy can be calculated as follows:

Equation

Where is the wavelength, is Plank's constant () and is the speed of light ( m/s).

As well as this, the energy held by a photon is affected by Air Mass. The Air Mass is the path length which light takes through the atmosphere normalized to the shortest possible path length (the shortest path is when the sun is directly overhead). The Air Mass quantifies the reduction in the energy of light as it passes through the atmosphere and is absorbed by air, dust, ozone (O3), carbon dioxide (CO2), and water vapour (H2O) with the last three having a high absorption for photons that have energies close to their bond energies. The air mass (AM) is defined using the following equation (noting that is defined later in this paper):

Equation

Figure

Electrical Characteristics of a PV-Cell:

A PV cell equivalent circuit is similar to that of the diode, since they have similar structures. A photovoltaic cell is considered as a current generator and can be represented by the equivalent circuit of Figure XX. The current I at the outgoing terminals is equal to the current generated through the PV effect IPV by the ideal current generator, decreased by the diode current Id and by the ground leakage current Ish. The resistance in series Rs represents the internal resistance to the flow of generated current and depends on the thickness of the junction P-N, the present impurities and on contacts resistances.

The shunt resistance Rsh takes into account the current to earth under normal operational conditions. In an ideal cell the values of Rs is zero while the value of Rsh is maximum. On the contrary, in a high-quality silicon cell the typical value of Rs is around five milliohm and the shunt resistance is around 285 ohm. The conversion efficiency of the PV cell is greatly affected also by a small variation of Rs, whereas it is not affected by the variation of Rsh too much.

Figure

The no-load voltage Voc, open circuit voltage, occurs when the load does not absorb any current, i.e. IL equals zero, thus according to ohms law, the open circuit voltage will be the current passing through the shunt resistance, times the shunt resistance Voc =IshRsh. (Handbook xx, 2011)

In addition, the diode current is given by the classical formula for the direct current:

Equation

Where: ID is the diode's saturation current, Q is the charge of the electron (1.6Ã-10-19 C), A is the identity factor of the diode and it depends on the recombination factor between the holes and electron inside the diode itself (for crystalline silicon it is about 2). K is the Boltzmann constant (1.38Ã-10-23 J/K). Finally, T is the absolute temperature in Kelvin degree. Therefore, the current supplied to the load is given by:

Equation

The final term, the ground-leakage current, in practical cells is small compared to Iph and ID, thus it can be ignored. The diode-saturation current can be determined experimentally by applying the open circuit voltage Voc in the dark (when Iph is zero) and measuring the current going into the cell. This current is usually referred to as the dark current or the reverse diode-saturation current. Wind and Solar Power Systems (Patel) xx, 1999)

The voltage-current characteristic curve of a PV module is shown in Figure xx. The generated current is at its highest under short-circuit conditions (Isc), whereas with the circuit open, the voltage (Voc=open circuit voltage) is at the highest. Under the two of those conditions, the electric power produced in the module is equal to zero, whereas under all the other conditions, when the voltage increases, the produced power rises too; at first, it reaches the maximum power point (Pm) and then it falls suddenly near to the no-load voltage value.

Figure Voltage-Current characteristics example (Photovoltaic Plants ABB xx, 2010)

In summary, the electrical characteristics needed to be known about for a photovoltaic module is as follows:

Isc short-circuit current;

Voc no-load voltage;

Pm maximum produced power under standard conditions (STC);

Im current produced at the maximum power point;

Vm voltage at the maximum power point;

FF filling factor: this is a parameter which determines the form of the characteristic curve V-I. It can be defined as the actual maximum power divided by the ideal power value; the ideal power is that value that would be obtained under ideal conditions. i.e. when the voltage is equal to the open voltage and the current is equal to the short circuit current. The filling factor is:

Equation

It should be pointed that all those data can be found in the manufacturer data sheet. Most of the information is experimentally distinguished. There are some methods to calculate the series resistance value but it will not be needed in this paper, thus it will not be presented.

Voltage and Current in PV Plant:

PV modules generate a current from 4 to 10 A at a voltage from 30 to 40 V. To achieve the projected peak power, the panels are electrically connected in series to form the strings, which are connected in parallel. The trend is developing strings constituted by as many panels as possible, given the complexity and cost of wiring, in particular of the paralleling switchboards between the strings. The maximum number of panels which can be connected in series (and therefore the highest reachable voltage) to form a string is determined by the operational range of the inverter and by the availability of the disconnection and protection devices suitable for the voltage reached. In particular, the voltage of the inverter is bound, due to reasons of efficiency, to its power. Generally, when using inverters with power lower than 10 kW, the voltage range most commonly used is from 250V to 750V, whereas if the power of the inverter exceeds 10 kW, the voltage range usually is from 500V to 900V. (Photovoltaic Plants ABB xx, 2010)

Shading:

Taking into consideration the area occupied by the modules of a PV plant, part of them (one or more cells) may be shaded by trees, fallen leaves, chimneys, clouds or by PV panels installed nearby. In the case of shading, a PV cell consisting in a junction P-N stops producing energy and becomes a passive load. This cell behaves as a diode, which blocks the current produced by the other cells connected in series, thus jeopardizing the whole production of the module. Moreover the diode is subject to the voltage of the other cells which may cause the perforation of the junction due to localized overheating (hot spot) and damages to the module. In order to avoid that one or more shaded cells prevent the production of a whole string, some diodes which by-pass the shaded or damaged part of module are inserted at the module level. Thus, the functioning of the module is guaranteed even if with reduced efficiency. In theory, it would be necessary to insert a by-pass diode in parallel to each single cell, but this would be too onerous for the ratio costs/benefits. Therefore, by-pass diodes are usually installed for each module. See figure xx

Figure

Inverter and Control:

Maximum Power Point Tracking (MPPT):

A maximum Power Tracker is a device that keeps the impedance of the circuit of the cells at levels corresponding to best operation. It also converts the resulting power from the PV array, so its voltage is that required by the load. There is some power losses associated with the power tracking process

Any PV array, however its size or sophistication, is only capable of producing Direct Current (DC) power, thus for the system to be integrated into the building it is necessary to have a methodology to convert the produced DC power into the building integrated AC power system. The DC to AC Inverter, sometimes referred to as converter, is used to achieve this function. The System might require more than one inverter depending on the system size and sophistication.

For many systems, a three-phase inverter is used. In addition, in some cases, single phase inverter is only needed with a final decision taken by knowing whether the grid supply is single or three phase; this is because the system should be coupled with the electrical grid. The system can be connected to the inverters with three deferent methods depending on the rating of both the PV Generator and the inverter.

The first method is a single inverter plant, which might consist of single or several strings; a string is a connection of many modules to form one DC output, positive wire and negative wire. The single inverter plant implies that the rating of both the PV generator and the inverter required is relatively small. This method has many advantages in terms of lower investment cost and low maintenance; but on the other hand, using one inverter will reduce the reliability of the system since a total stoppage of power production will occur in case of inverter failure. In addition, this solution is not suitable for increasing the size of the system, since this increases the problems of protection against over currents and the problems deriving from different shading, that is when the exposition of the panels is not the same in the whole plant.

The second method is to have many strings with an inverter for each string. In this layout, the blocking diode will prevent the source direction from being reversed; it is usually included in the inverter. The diagnosis on production is carried out directly by the inverter, which in addition can provide protection against the over-current and under-voltage on the DC side. Moreover, having an inverter on each string will reduce the coupling problems between the modules and inverters and the reduction of the performances caused by shading or different exposition. Again, in different strings, modules with different characteristics may be used, thus increasing the efficiency and reliability of the whole plant.

Finally, the last method is to have a combination of large-size plants, the PV field is generally divided into more parts (subfields), each of them served by an inverter of one's own to which different strings in parallel are connected. In comparison with the layout previously described, in this case there is a smaller number of inverters with a consequent reduction of the investment and maintenance costs. However it maintains the advantage of reducing the problems of shading, different expositions of the strings and of those due to the use of modules that are different from one another, if subfield strings with equal modules and with equal exposition are connected to the same inverter. Besides, the failure of an inverter does not involve the loss of production of the whole plant (as in the case of single- inverter), but of the relevant subfield only. It is advisable that each string can be disconnected separately, so that the necessary operation and maintenance verifications can be carried out without putting the whole PV generator out of service. When installing a parallel switchboard on the DC side, it is necessary to provide for the insertion on each string of a device for the protection against over-currents and reverse currents so that the supply of shaded or faulted strings from the other ones in parallel is avoided. Protection against over-currents can be obtained by means of either a thermo-magnetic circuit breaker or a fuse, whereas protection against reverse current is obtained through blocking diodes. With this configuration, the diagnosis of the plant is assigned to a supervision system, which checks the production of the different strings.

Inverter selection:

The selection of the inverter, depending on size, is carried out according to the PV array rated power that the inverter should manage. The size of the inverter can be determined starting from 0.8 to 0.9 for the ratio between the active power delivered to the network and the PV generator. This ratio is to consider the power under real operational conditions (working temperature, voltage drops on the electrical connection...etc in addition to the efficiency of the inverter itself.

The choosing of the correct size of the inverter must be done by taking the following considerations:

- DC Side:

rated power and maximum power;

rated voltage and maximum admitted voltage;

variation field of the MPPT voltage under standard operating conditions;

- AC Side:

rated power and maximum power which can be continuatively delivered by the conversion group, as well as the field of ambient temperature at which such power can be supplied;

rated current supplied;

maximum delivered current allowing the calculation of the contribution of the PV plant to the short circuit current;

maximum voltage and power factor distortion;

maximum conversion efficiency;

efficiency at partial load and at 100%.

Electrical Power Output:

The electrical power output of the system will depend on three values, the total hourly irradiance, the efficiencies of the electrical components used and the total area of the panels. The values of total hourly irradiance will be found as described previously in this thesis.

The efficiency of the Photovoltaic's arrays will be characterised by the average module temperature Tc. Thus, the efficiency will depend on the ambient temperature. (RETscreen xx, 2011)The efficiency equation using the calculation for this study purpose is as follows:

Equation

Where is the temperature coefficient for the module efficiency and and are the efficiency and the temperature of the panel under the Standard Testing Conditions (STC). Normally the testing temperature is equal to 25CËš. In addition, the standard testing conditions will define the Nominal Operating Cell Temperature NOCT. NOCT values normally ranges from 42CËš to 46CËš. (handbook xx, 2011). The average module temperature Tc is related to the mean monthly ambient temperature through the following equation, which had been developed by Evans in 1981 (Beckman xx,2006):

Equation

Furthermore, the equation above is valid when the tilting angle is equal to the latitude angle minus the declination angle, when the tilt angle is different, then the right side of the equation has be multiplied by a correction factor defined as Cf. (RETscreen xx, 2011) It can be found using the following equation:

Equation

Where sM is equal to the latitude angle minus the declination angle and s is the current tilt angle.

On the other hand, STC efficiency will vary for each type of module. In general, the efficiency values range between 5%, for example for a module of a-Si type, up to about 15%, for example a mono-crystalline silicon module.

Finally, the power output of the PV generator can be defined as the total reached irradiances multiplied by the final efficiency and the total area used S. The equation can be shown below:

Equation

To calculate the electrical power delivered by the PV generator, which is received by the building or the grid, the EP must be multiplied by the inverter efficiency and the electrical losses due to the wiring. As well, other miscellaneous losses of the BOS should be deducted from the total power production. (RETscreen xx, 2011)

In later sections, a method to calculate the power output will be presented and illustrated systematically giving one example of the whole system. The codes and work sheet of the manual model can be found in the appendix xx.

Project Model and Feasibility Study:

The approach to decide whether the system will be feasible or not will be taken through a manual calculation and system simulation using PVsyst. The first section will describe the manual calculation of the system. The calculation will use one day as an example to demonstrate the method. The simulation will be produced using PVsyst, as quoted from the user help booklet of the program, "PVsyst is a PC software package for the study, sizing and data analysis of complete PV systems. It deals with grid-connected, stand-alone, pumping and DC-grid (public transport) PV systems, and includes extensive meteorological and PV systems components databases, as well as general solar energy tools." Comparison between both calculations will be presented.

Manual Calculation:

The manual calculations have been done using a Microsoft Excel data sheet. All the data and the codes can be found in appendix xx. The calculations have been done for the days in the same method, thus one-hour example will be presented to demonstrate the methodology of the power output calculation followed.

Power output calculation for a specific site will predict the system performance on the long term. Many methods have been developed to calculate the power output of a PV system. (Beckman xx, 2006), demonstrated many theories, the reader can refer to this reference for further reading. For the purpose of the project one method had been selected to follow with occasional adjustment from other theories. The following table presents the data of location Table XX

ADD A TABLE OF THE DATA FOR THE LOCATION