Photovoltaic System Integration

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Photovoltaic System Integration for

Roehampton Vale Campus, Kingston

University London

Omar Hamdan

Supervised by: Dr. Paul Wagstaff

MSc Renewable Energy Engineering

October 2012

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Omar Hamdan | Kingston University London

Synopsis

This thesis is divided in a way to permit the reader to follow the content in a logical sequence.

The main objective of this thesis is to design a photovoltaic system to be optimally integrated with the electrical system in Kingston University London, Roehampton Vale campus. This system main objective is to supply the electrical demand of the facility.

The thesis presents a piece of work to calculate the power output of the photovoltaic system in hand calculations and software simulation.

The thesis will evaluate the location of the installation by means of radiation falls on the location, construction of the photovoltaic system, sizing the system by evaluate the options according to area available and capital cost.

The hand calculation will present a model develop on excel to calculate the power output by calculating the solar irradiances on a tilted surface, converting the irradiances to electrical power and considering the effect of temperature on photovoltaic cells.

The simulation part will present an entire design of the system by means of calculating the power output, losses associated with the conversion process and connection, shade study and result analysis

The sizing of the system was carried out through hand calculation, simulation and economical analysis. Finally an economical evaluation for many models will be presented.

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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, 2005). 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.

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, (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. The 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.

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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, 2010). 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).

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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 one century is the age of sun and solar power. Building Integrated PV system (BIPV) considered being one of most efficient solutions. PV system integration in buildings can overcome all the above problems and achieve most of the required objectives to certain extend.

Furthermore, PV systems on roof or on Façade become more challenging. The UK market faces a short drop in demand, due to reduction on feed in tariff (FIT) by UK government. As a result, most of the PV panels prices dropped dramatically. On the other hand, installation cost still almost the same. For sure, the competitiveness among the market contractors has increased which opened a space for less installation prices.

On the other hand, the UK electricity production using solar cell has increased dramatically. The total production in 2005 is 10.9 MW; this number has jumped in 2011 to be 975.8 MW (DECC, 2012). This is an indication on how promising is the photovoltaic market is. The total consumption of electricity from photovoltaic in 1999 used to be only 1000 MWh this number has increase along the decade to be 11000 MWh in 2007. Figure (3) shows the increase along nine years (European Commission, 2010)

Finally, it is important to note that the UK photovoltaic market now is under uncertainty conditions due to the change of the incentives and feed in tariff low. 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 2: Electricity consumptions by Sector. (DECC, 2009)

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Figure 3: Electrical PV generation (European commission, 2010)

1.2 Building Integrated PV System

Examining Photovoltaic modules for building integration, produced as a standard building product that fit into standard façade 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 façade element with photovoltaic can be used in all kind of building's structures, curtain wall façade (with isolating glass), rear vented curtain wall façade , structural glazing and tilted façade . 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 (2001) 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

1 1 3

4 ₃ 4

8 10

11

1999 2000 2001 2002 2003 2004 2005 2006 2007

Gross Electricity Generation of

Photovoltaic GWh

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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 (Luque and Hegedus, 2011). Figure (4) 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).

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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.

1.3 Solar Radiation and Solar Constant

It is obvious that the Photovoltaic system is related to the sun and the earth's movement around it, thus, studying this movement and the way the radiation will fall into the earth's 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 meter 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, whilst on 2nd of July, the earth is at its greatest distance from the sun, this distance is known as Aphelion, see Figure (5) (Scharmer, 2000). The sun 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 Centre (WRC) has adopted a value of 1367 W/m2, with 1% uncertainty (IEA, 1996).

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 (Luque and Hegedus,

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2011). According to (Duffie and Beckman, 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 1.495×1011 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 1 Extraterrestrial Radiation Where

Gsc: solar constant, 1367 W/m2.

n: is the day number of the year.

Figure 5: Earth Positions around the sun (Scharmer, 2000)

1.4 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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1.4.1 The Declination Angle :

It is the key input for the solar geometry. It is defined by (UNESCO and NELP, 1978) 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 23.45˚ and -23.45˚. The following equation relates to the declination angle and the day number n, along the year.

Equation 2 Declination Angle

1.4.2 Solar Hour Angle :

According to (PEN, 2012), is the angular displacement of the sun east and west of the local meridian. It changes 1˚ each for minutes and 15˚ each hour. It changes 15˚ each hour after the solar noon and -15 each hour before the solar noon. The solar noon corresponds to the moment when the sun 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 3 Solar Time

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 4 E value.

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1.4.3 The Latitude angle :

It is the angular location north of the equator as positive and south of the equator as negative. Its values range between -90˚ and +90˚.

1.4.4 The Sunset Hour angle:

According to (RETScreen International, 2005) is the angle of the sun at the sunset solar hour. It can be found using the following equation:

Equation 5 Sunset Hour Angle

1.4.5 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 10˚ to 15˚ less than the latitude and for the winter, maximum energy gain can be acquired when the angle is adjusted to be 10˚ to 15˚ more than the latitude. (Duffie and Beckman, 2006).

1.4.6 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 .

1.4.7 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:

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1.4.8 Zenith Angle :

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

Equation 7 Zenith Angle

The following figure (6) illustrates the angles on a tilted surface. Please note that the previous equations will be implemented in a hand calculation for the total power output of the proposed system, later in this paper. The calculation will be done using Microsoft Excel.

Figure 6: Solar Geometry Angles (Duffie and Beckman, 2006).

1.5 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 (Duffie and Beckman, 2006). Those radiations are considered to be distributed over the sky dome. In general, the data of cloudiness and clarity are widely available.

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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.

1.5.1 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 (Luque and Hegedus, 2011). Thus, the Clearness Index is defined as:

Equation 8 Clearness Index

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 9 global Hourly irradiance on horizontal surface

1.5.2 Calculating of Hourly Global and Diffused Irradiance

To calculate the hourly irradiances, a developed method by Erbs et al and introduced by (Duffie and Beckman, 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 :

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Equation 10 Diffused Radiation Ratio ws ≤ 81.4˚

For :

Equation 11 Diffused Radiation Ratio ws > 81.4˚

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 12 Average Daily irradiance Where is:

Equation 13 rt ratio

Where (a) and (b) are values can be found as follows:

Equation 14 constant a

Equation 15 constant b

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:

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Equation 17 Beam irradiance can be found using this equation:

Equation 18 rd ratio

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 (Duffie and Beckman, 2006). The equation equals to:

Equation 19 Total irradiance on tilted surface

Where:

Equation 20 Rb Value

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2.1 System Components 2.1.1 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 (Richard J. K., 1995).

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 (Mukund R. P., 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 cell's 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, see figure (7) (Mukund R. P., 1999).

Figure 7: 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.

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2.1.2 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 of the proton, the larger the packet (Richard J. K., 1995).

The sunlight spectral is divided into three regions see figure (8). 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 (Clark and Eckert, 1975). 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 (Duffie and Beckman, 2006). Photon's energy can be calculated as follows:

Equation 21 Photon Energy

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

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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 22 Atmospheric Mass

Figure 8: Light wavelength ranges

2.1.3 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 (9). 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

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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 9 Equivalent circuit of Photovoltaic

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 (Luque and Hegedus, 2011)

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

Equation 23 Diode current

Where: ID is the diode's saturation current, Q is the charge of the electron

(1.6×10-19 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 (1.38×10-23 J/K). Finally, T is the absolute temperature in Kelvin degree. Therefore, the current supplied to the load is given by:

Equation 24 current delivered by the photovoltaic panel

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

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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. (Mukund R. P., 1999).

The voltage-current characteristic curve of a PV module is shown in Figure10. 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. (Sera, D et al, 2007)

Figure 10 Voltage-Current characteristics example (ABB, 2010)

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

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 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 25 Filling Factor

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.

2.1.4 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. (ABB, 2010)

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2.2.0 Electrical Power Output:

The electrical power output of the system will depend on three values, the total hourly irradiance, and 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 International, 2005). The efficiency equation using the calculation for this study purpose is as follows:

Equation 26 Cell temperature effect on the cell Efficiency

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 25C˚. In addition, the standard testing conditions will define the Nominal Operating Cell Temperature NOCT. NOCT values normally ranges from 42C˚ to 46C˚ (Luque and Hegedus, 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 (Duffie and Beckman, 2006):

Equation 27 Ambient temperature relation with the cell temperature

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 International, 2005). It can be found using the following equation:

Equation 28 tilt angle correction factor for the cell temperature

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

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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 29 Energy supplied to the building and the electrical grid

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 International, 2005).

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 hand model can be found in the appendix A.

2.3.0 Components Selection PV panel

In order to optimise the system for the best conditions, it is highly required to choose the most suitable component in the system. Reliable, high efficient and low cost components are the optimal components to choose. In the following, the detailed process for the main component selection is presented.

There are many kinds of photovoltaic panels which vary in material used, technology, manufacturing process and size. Looking into the features of each panel then comparing it with its price and its installation cost can be a very difficult process, especially if the life time of the PV panel, warranty, market availability and efficiency are taken into account as well. Therefore, the selection process can be narrowed by specifying the priority features needed in the panel.

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2.3.1 PV Panel Selection Methodology

The selection of the PV panel for this project was based on three aspect as priority features; the efficiency of the photovoltaic panel, the panel price and the market availability. In addition to those characteristics, an additional facet took priority when the economical evaluation had been completed. The project life-time needed to be increased because the payback and the breakeven level of output, was found to be longer than 20 years. Hence, the PV panel life-time and the entire project studies have been extended to 25 years.

2.3.2 Chosen Panel

The panel which has the highest efficiency is mostly mono-crystalline, thus the panel's types have been narrowed by only mono-crystalline panels. One of the most established, experienced brands in the market of manufacturing panels is SHARP, when the panel specifications have been studied, and only the panels with life-time of 25 years are used. They had a higher level of efficiency was compared to the other panels in the market.

The panel is mono-crystalline which has 14.14% efficiency and lower sensitivity to the variation of the temperature, the voltage variation is only a decreasing of 104 mV/˚C. The peak power of the panel is 185 W P. The voltage at maximum power point is 24 while the current is 7.71 Amp. The filling factor is 71.75%. The Nominal Operation Cell Temperature (NOCT) is 47.5 ˚C. The Panel dimensions as show in figure 11 is 1.318×0.994 m. the panel has a bypass diodes which, as mentioned before, will minimise the loss in output when shading occurs. The panel behaviour with different irradiances is shown in figure 12.

Additional data about the panel which might be useful for the installer:  156.5 mm × 156.5 mm mono-crystalline solar cells

 48 cells in series

 2,400 N/m2 mechanical load-bearing capacity (245 kg/m2)  1,000 V DC maximum system voltage

 IEC/EN 61215, IEC/EN 61730, Class II (VDE: 40021391)

Finally, a vital point need for economical evaluation purposes; a full performance of the panel is guaranteed for five years, a 90% of the full performance

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for ten years and an 80% for twenty five years. Therefore, it will be possible to extend the project life time to twenty five years. (A detailed data sheet is attached in appendix C for the reader to refer to if needed).

Figure 11 Selected Panel Dimensions

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2.4.0 Inverter and Control

2.4.1 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.

2.4.2 Connection of Inverter to Array

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 phases; 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 these 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 (Esram and Chapman, 2007).

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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. (Esram and Chapman, 2007).

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 are 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 (ABB, 2010).

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2.4.3 Inverter, process and Functions

Inverters take a great role in photovoltaic electrical production; the inverter makes it possible to convert the DC power to an AC power used in the building's systems. It will add a great flexibility with dealing with the produced power since the dealing with DC power can often be difficult and dangerous.

It is important to present a brief introduction about the inverter in order to be able to understand the basic methodology of how the inverter works. The circuit used in the inverter is usually a three phase bridge inverter. This circuit is used to convert the DC power to three phase AC power, which will make it easy to connect, and to, integrate, the whole new photovoltaic system into the existing system in the building. Moreover, after integrating both systems together; it is possible to connect their integration to the electrical grid through a bidirectional kWh meter to calculate the spending and selling. The typical circuit used in the inverter can be seen in figure 13.

Figure 13 typical circuit used in PV inverters.

The process of inverting the DC power to an AC power inside the inverter is done using mostly a Pulse Width Modulator PWM to great a sinusoidal AC output. The process can be explained using figure 13. The battery in the figure represent the PV panels production, they are connected to the inputs of three legs, two transistors, and are protected from the reverse current by a diode connected in parallel with each of them. The DC voltage should be converted to a three phase, lines, AC output, therefore, each transistor, of the six transistors, will be triggered sequentially by a controller. The controller has a reference PWM wave, Sinusoidal

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form. The controller will trigger one of the transistors in a process and will form three phase AC power. Each of the phases is shifted by 120˚ electrical degree.

Furthermore, in this process the inverter is able to vary the voltage and frequency at the output. For the case of building integration, the frequency required to be fixed to either 50 Hz in the UK or 60 Hz in other places. On the other hand, the inverter need to be able to cope with the variation in the voltage level, hence the PV generator, relatively, does not have a fixed voltage. The voltage variation can be due to the change in the temperature of the cell or due to the voltage drop caused by the resistance of wiring.

The output wave ought to be filtered to lower the effect of any ripples or harmonics, which might be caused during the conversion process.

2.4.4 Component Selection, Inverter

In conjunction with the photovoltaic panel, the selection of an optimal inverter to use for the project can be a difficult process since there are many issues to be considered. One of the main issues when selecting an inverter is to consider the Maximum Power Point Tracking MPPT voltage range which might affect the final performance of the system. Any inverter with MPPT will be able to optimally decrease the effect of shadowing.

In order to select a suitable inverter to be used in the system, some aspects should be considered. The capability of the inverter to cope with the variation in voltage is an important matter. The system size is determined according to much iteration to evaluate the system technically and economically. According to the system size the inverter rated power will be distinguished. Therefore, the options to choose an inverter will be limited to a certain level. It is better to choose an inverter rating half the system size. In this way, two inverters will be installed instead of one. The main purpose of this is to increase the reliability of the system. When one of the inverters is out of service only half of the system is lost. Under certain circumstances, one inverter could be selected, especially if the system rating is low.

The inverter type chosen for this project is going to be able to handle the whole system solely, since the system size is relatively small compared other projects. After considering many aspects the system's voltage and current have been

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stated. Hence, the voltage rating is also known. The system voltage is ranging between 540 V and 576 V, 576 V to be the maximum power point voltage. Now, the options to choose an inverter, are narrowed, due to the fact that only the inverters have voltages around this range. ±25% of the maximum and the minimum voltage are considered as a good estimation because a margin of variation above or below the maximum or the minimum level must be considered. Finally, the market availability, quality guarantee and the cost should be taken into account.

2.4.5 Summary

In summary, 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, from 0.8 to 0.9 for the ratio between the active power delivered to the network and the PV generator. This ratio considers the power under real operational conditions (working temperature, voltage drops on the electrical connection...etc) in addition to the efficiency of the inverter itself.

Finally, the choice of correct size, for 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 tracking 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;

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 Efficiency at partial load and at 100%. - Other:

 Market availability  Life time

 Cost

The chosen inverter is a tri-phased Sinvert 60 M from Siemens with 50 Hz frequency and has a nominal power of 65 kVA (apparent). The inverter has a minimum MPP voltage of 450 V and a maximum MPP voltage of 750 V. The power conditioning unit consists of Isolated Gate Bipolar Transistor (IGBT) inverter, DC/AC distribution, isolating transformer and a controller number SIMATIC S7. It also has a MPP tracking for optimum utilisation of PV field power. In addition, it has an optional Voltage Ampere Reactive (VAR) controller for three-phase network. The unit comes with a control panel with display of operating states and actual values for the user to interface in order to set the parameters of the inverter.

Furthermore, it does have a switch-over in both manual and automatic mode by integrated key-switch. Moreover, the following features are included in the unit, isolation monitoring with selective fault allocation and safety disconnection; visualization and service software Power Protect solar; interface for process visualization and an optional integration in management systems via Ethernet, cabinets for floor mounting, forced ventilation by fan, air intake through lower cabinet front and cabinet bottom, air discharge through the cabinet roof; cable entry at base from. Figure 14 shows the component's internal combination. Figure 15 shows the process in PWM to convert the DC power to an AC power.

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Figure 14 inverter combination

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2.5.0 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 (Seung-Ho and Eun-Tack, 2002). 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 (Kajihara and Harakawa, 2005). See figure 16

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3.1.0 Project Demand and Hand Calculations

This approach is to decide whether the system will be feasible or not. The feasibility study will be done through a hand calculation and system simulation using PVsyst. The Use of both methods; hand and simulation will make it easier to decide the system size and specification. In addition, the hand calculation will present a power output at an hourly pace which then can be compared with hourly demand if available. The hand calculation will make it easier to predict the hourly share of the proposed system to set an economical plan for the building.

The first section will describe the hand calculation of the system. The calculation will use one day, as an example to demonstrate the method. Starting with only one monthly value, for each month, the hand calculation will find the hourly estimated power output throughout the day, then, the estimated daily total kWh that can be produced. It is important to note that the hand calculation does not consider the system losses, since it is going to be considered in the simulation more precisely. 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."(PVsyst., 2012) Comparison between both calculations will be presented.

3.1.1 Process of progression:

The system to be proposed will be installed in Kingston University London, Roehampton campus. The campus is positioned at 15° 26ˈ and -00° 15ˈ latitude and longitude. The system proposed is to integrate a photovoltaic system with the existing electrical system.

The main objective of the project is to, fully or partially; supply the facility’s electrical demand throughout the year. This will be done by:

 This chapter:

- Calculating the system demand and electric system review, - Calculating the hourly solar radiation on the system,

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- Hand Calculation the hourly electrical power produced annually,

- Optimising the area used and evaluating the available options, system sizing,

 Next Chapters:

- Calculating the monthly power production, losses and shading effect using a simulating program,

- Electrical consideration and power layout, - Economical Evaluation of the project,

- Comparison of the hand calculation and the simulation results,

The integration of a PV system with a building will be carried out while carefully considering all the aspects.

3.1.2 Overview, System Demand and Electrical System Review:

The site consists of a main building, which is composed of a library, lecture rooms, and engineering laboratories, and a secondary building, which mainly consists of lecture rooms. The building’s total area is approximately 4730 m2

, the total useful area, which is above the main building, is around 3030 m2. The electrical load main consumptions are air chillers, laboratories machines, wind tunnel; which is used for experimental purposes and derived by two main electrical motors, lights, personal computers and printers.

Similar to all the building's systems in the UK, the facility has 240 Volt/ 50 Hz electrical systems, which will be supplied through a three phase main incomer connected on the main board with electrical meters. The main electrical board is located at the east gate of the building. This main incomer is supplying both buildings through two sub-boards and connected to monitoring system.

The calculation of the electrical demand of the facility had been carried out based on electrical monthly bill readings over a period of three years. This monthly bills had been converted from kWh consumption to kW consumption. The final monthly demand is shown in figure 17 and table 1. For the sake of comparison, the electrical demand is kept in kWh in later sections.

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Figure 17 system demand in kW Table 1 System Demand kW

Month Electrical Demand (kW) January 345.4943727 February 324.6823017 March 264.0997798 April 200.4881069 May 182.0981522 June 152.2148372 July 156.0801467 August 164.6080031 September 219.5328399 October 287.221441 November 355.4871879 December 341.3481184

The data in both the figure and the table above shows the variation of the electrical demand from one month to another. It can be shown that the highest demand had been consumed in November while the least consumption takes place in June. This situation, unfortunately, contradicts with the incident solar radiations during each month, i.e. the highest measured incident solar radiation occurs in June and July when the lowest electrical consumption takes place, and the lowest measured incident solar radiation occurs in November, December and January. This is why system sizing should be carried out with cautious consideration of all the aspects, since the main purpose of the project is only to supply the facility consumption. Not taking this into account would lead to an unnecessary increase of the investment cost. This point shall later be discussed in detail.

0 100 200 300 400 Jan ua ry Febr ua ry Mar ch A pr il May _Jun e Jul y A ug us t Septem ber O cto ber N o vem ber D ec em ber Monthly Average System Demand

Photovoltaic System Integration