Engr. Tanveer ul Haq Chairman PEN Community MS Electronics Scholar in GIKI B.Sc Electrical Engg. From UCE&T BZU
Lecture 01:- Energy From Sun Lecture 02:- Introduction to Solar Cells Lecture 03:- Electronic Structure of Semiconductor Lecture 04:- How solar cells work? Lecture 05:- Typical Device Structure Lecture 06:- Losses in Solar Cell Lecture 07:- Silicon Solar Cell Technology Lecture 08:- Typical Cell Fabrication Process
Lecture 09:- Structure of a Photovoltaic System Lecture 10:- Photovoltaic Engineering Lecture 11:- Power Conditioning and Control Lecture 12:- Sizing of Photovoltaic System Lecture 13:- Concentrating Photovoltaic Bibliography And References
Energy From Sun Lecture 01 Engr. Tanveer-ul-Haq
Solar Power Solar Constant Irradiance Aerosols Solar Radiation in Atmosphere Air Mass Solar Spectrum
Solar Power The luminosity of the Sun is about 3.86x10^26 watts. This is the total power radiated out into space by the Sun. Most of this radiation is in the visible and infrared part of the electromagnetic spectrum, with less than 1 % emitted in the radio, UV and X-ray spectral bands.
Solar Power The sunâ€™s energy is radiated uniformly in all directions. Because the Sun is about 150 million kilometers from the Earth, and because the Earth is about 6300 km in radius, only 0.000000045% of this power is intercepted by our planet. This still amounts to a massive 1.75x10^17 watts.
Solar Constant For the purposes of solar energy capture, we normally talk about the amount of power in sunlight passing through a single square meter face-on to the sun, at the Earth's distance from the Sun. The power of the sun at the earth, per square meter is called the solar constant and is approximately 1370 watts per square meter (W/m^2). 7/22/2012
Irradiance The total power from a radiant source falling on a unit area is called Irradiance.
Aerosols When the solar radiation enters the Earthâ€™s atmosphere, a part of the incident energy is removed by scattering or absorption by air molecules, clouds and particulate matter usually referred to as aerosols. 7/22/2012
Solar Radiation in Atmosphere • • • •
Direct or Beam Raidiation Diffuse Radiation Albedo Global
Direct or Beam Radiation The radiation that is not reflected or scattered and reaches the surface directly in line from the solar disc is called direct or beam radiation.
Diffuse Radiation The scattered radiation which reaches the ground is called diffuse radiation.
Albedo Some of the radiation may reach a receiver after reflection from the ground is called Albedo.
Global The total radiation consisting of these three (Direct, Diffuse & Albedo) components is called global.
Air Mass A concept which characterises the effect of a clear atmosphere on sunlight is the air mass, equal to the relative length of the direct beam path through the atmosphere. One clear summer day at sea level, the radiation from the sun at zenith corresponds to air mass 1(abbreviated to AM1); at other times, the air mass is approximately equal to 1/cosÎ¸z, Where Î¸z is zenith angle. 7/22/2012
Solar Spectrum The extraterrestrial spectrum, denoted by AM 0, is important for satellite application of solar cell. AM 1.5 is a typical solar spectrum on the Earthâ€™s surface on a clear day which, with total irradiance of 1KW/m2, is used for the calibration of solar cells and modules.
Introduction to Solar Cells Lecture 02 Engr. Tanveer-ul-Haq
What are Solar Cells? Solar cells represent the fundamental power conversion unit of a photovoltaic system. They are made from semiconductors, and have much in common with other solid-state electronic devices, such as diodes, transistors and integrated circuits. For practical operation, solar cells are usually assembled into modules.
Different type of solar cells • Monocrystalline Solar Cell • Polycrystalline Solar Cell • Amorphous Solar Cell
Crystalline Solar Cell Crystalline silicon hold the largest part of the market. To reduce the cost, these cells are now often made from multicrystalline material, rather than from the more expensive single crystal. Crystalline silicon cell technology is well established. The modules have a long life (20 years or more) and their best production efficiency is approaching 18%. 7/22/2012
Monocrystalline Solar Cell
• Monocrystalline silicon is the most efficient • Works in low light condition • Absorbs 18% of available sun light • Most expensive type of solar cell 7/22/2012
Polycrystalline Solar Cell
• Most affordable in the market today • Made of small silicon crystal mashed together • It is durable and can be used for moderate purposes • Absorbs 15% of sun light available to it 7/22/2012
Amorphous Solar Cells Amorphous technology is most often seen in small solar panels, such as those in calculators or garden lamps, although amorphous panels are increasingly used in larger applications. They are made by depositing a thin film of silicon onto a sheet of another material such as steel. The panel is formed as one piece and the individual cells are not as visible as in other types. 7/22/2012
Amorphous Solar Cells The efficiency of amorphous solar panels is not as high as those made from individual solar cells, although this has improved over recent years to the point where they can be seen as a practical alternative to panels made with crystalline cells. Their great advantage lies in their relatively low cost per Watt of power generated. This can be offset, however, by their lower power density; more panels are needed for the same power output and therefore more space is taken up. 7/22/2012
Amorphous Solar Cells
• • • •
Cheapest and lightest Absorbs 10% of light available Used for vehicles like boats Work best in intense sun light
Some Other Types of Solar Cells A variety of compound semiconductors can also be used to manufacture thin-film cells ( for example, cadmium telluride or copper indium diselenide). These modules are now beginning to appear on the market and hold the promise of combining low cost with acceptable conversion efficiencies.
Some Other Types of Solar Cells A particular class of high-efficiency solar cells from single crystal silicon or compound semiconductors (for example, gallium arsenide or indium phosphide) are used in specialised applications, such as to power satellites or in systems which operate highintensity concentrated sunlight.
How Solar Cell Works The solar cell operation is based on the ability of semiconductors to convert sunlight directly into electricity by exploiting the photovoltaic effect. In the conversion process, the incident energy of light creates mobile charged particles in the semiconductor which are separated by the device structure and produce electric current.
Electronic Structure of Semiconductor Lecture 03 Engr. Tanveer-ul-Haq
Semiconductor Physics The principle of semiconductor physics are best illustrated by the example of silicon, a group 4 elemental semiconductor. The silicon crystal forms the so-called diamond lattice where each atom has four nearest neighbours at the vertices of a tetrahedron. The four fold tetrahedral coordination is the result of the bounding arrangement which uses the four outer electron of each silicon atom.
This crystal structure has a profound effect on the electronic and optical properties of the semiconductor.
Band Structure According to the quantum theory, the energy of an electron in the crystal must fall within well defined bonds. The energies of valence orbitals which form bonds between the atom represent just such a band of states, the valance band. The next higher band is the conduction band which is separated from the valence band by the energy gap or bandgap.
The width of the bandgap Ec - Ev is a very important characteristic of the semiconductor and is usually denoted by Eg. This table gives the bandgaps of the most important semiconductors for solarcell applications. Material
Energy gap (eV)
Type of gap
Doping A pure semiconductor (which is called intrinsic) contains just the right number of electrons to fill the valence band, and the conduction band is therefore empty. Electrons in the full valence band cannot move - just as, for example, marbles in a full box with a lid on top. For practical purposes, a pure semiconductor is therefore an insulator.
Semiconductors can only conduct electricity if carriers are introduced into the conduction band or removed from the valence band. One way of doing this is by alloying the semiconductor with an impurity. This process is called doping. As we shall see, doping makes it possible to exert a great deal of control over the electronic properties of a semiconductor, and lies in the heart of the manufacturing process of all semiconductor devices.
Suppose that some group 5 impurity atoms (for example, phosphorus) are added to the silicon melt from which the crystal is grown. Four of the five outer electrons are used to fill the valence band and the one extra electron from each impurity atom is therefore promoted to the conduction band. For this reason, these impurity atoms are called donors. The electrons in the conduction band are mobile, and the crystal becomes a conductor. Since the current is carried by negatively charged electrons, this type of semiconductor is called n type.
A similar situation occurs when silicon is doped with group 3 impurity atoms (for example, boron) which are called acceptors. Since four electrons per atoms are needed to fill the valence band completely, this doping creates electron deficiency in this band. The missing electrons - called holes - behave as positively charged particles which are mobile, and carry current. A semiconductor where the electric current is carried predominantly by holes is called p-type.
Semiconductor junctions The operation of solar cells is based on the formation of a junction. The important feature of all junctions is that they contain a strong electric field. To illustrate how this field comes about, let us imagine the hypothetical situation where the p-n junction is formed by joining together two pieces of semiconductor, one p-type and the other n-type.
In separation, there is electron surplus in the n-type material and hole surplus in the p-type. When the two pieces are brought into contact, electrons from the n region near the interface diffuse into the p side, leaving behind a layer which is positively charged by the donors. Similarly, holes diffuse in the opposite direction, leaving behind a negatively charged layer stripped of holes.
The resulting junction region then contains practically no mobile charge carriers, and the fixed charges of the dopant atoms create a potential barrier acting against a further flow of electrons and holes.
The potential barrier of a junction permits the flow of electric current in only one direction - the junction acts as a rectifier, or diode. This can be seen in our example where electrons can only flow from the p region to the n region, and holes can only flow in the opposite direction. Electric current, which is the sum of the two, can therefore flow only from the p-side to the n-side of the junction (remember that it is defined as the direction of flow of the positive carriers!).
I-V characteristic of a diode
Light absorption by a semiconductor Photovoltaic energy conversion relies on the quantum nature of light whereby we perceive light as a flux of particles called photons. On a clear day, about 4.4 x 1017 photons strike a square centimentre of the Earth's surface every second. Only some of these photons - those with energy in excess of the bandgap - can be converted into electricity by the solar cell.
When such photon enters the semiconductor, it may be absorbed and promote an electron from the valence to the conduction band. Since a hole is left behind in the valence band, the absorption process generates electron-hole pairs.
Each semiconductor is restricted to converting only a part of the solar spectrum. The spectrum is plotted here in terms of the incident photon flux as a function of photon energy. The shaded area represents the photon flux that can be converted by a silicon cell - about two-thirds of the total flux.
The nature of the absorption process also indicates how a part of the incident photon energy is lost in the event. Indeed, it is seen that practically all the generated electron-hole pairs have energy in excess of the bandgap. Immediately after their creation, the electron and hole decay to states near the edges of their respective bands. The excess energy is lost as heat and cannot be converted into useful power. This represents one of the fundamental loss mechanisms in a solar cell.
How solar cells work? Lecture 04 Engr. Tanveer ul Haq
This diagram shows a typical crystalline silicon solar cell. The electrical current generated in the semiconductor is extracted by contacts to the front and rear of the cell. The top contact structure which must allow light to pass through is made in the form of widely-spaced thin metal strips (usually called fingers) that supply current to a larger bus bar. The cell is covered with a thin layer of dielectric material - the anti-reflection coating, ARC - to minimize light reflection from the top surface.
Current in p-n junction under illumination â€˘ This diagram shows a typical silicon solar cell â€˘ Note the two possible electron energy bands: LOW (black)- known as the valance band HIGH (white)- known as the conduction band
When light falls on the solar cell, energy from the photons generates electron-hole pairs on both sides of the p-n junction.
• Electrons diffuse across the p-n junction to a lower energy level. • Holes diffuse in the opposite direction • New electron-hole pairs continue to be formed while light falls on the solar cell.
â€˘ As electrons continue to diffuse, a negative charge builds up in the emitter. â€˘ A corresponding positive charge builds up in the base. â€˘ The p-n junction has separated the electrons from the holes and transformed the generation current between the bands into an electric current across the p-n junction.
â€˘ If an electrical circuit is made between the emitter and base, a current will flow. â€˘ The current continues to flow while the solar cell is illuminated.
Solar cells are essentially semiconductor junctions under illumination. Light generates electron-hole pairs on both sides of the junction, in the n-type emitter and in the ptype base. The generated electrons (from the base) and holes (from the emitter) then diffuse to the junction and are swept away by the electric field, thus producing electric current across the device. Note how the electric currents of the electrons and holes reinforce each other since these particles carry opposite charges. The p-n junction therefore separates the carriers with opposite charge, and transforms the generation current between the bands into an electric current across the p-n junction.
A more detailed consideration makes it possible to draw an equivalent circuit of a solar cell in terms of a current generator and a diode. This equivalent circuit has a current-voltage relationship.
â€˘ In solar cell applications this characteristic is usually drawn inverted about the voltage axis, as shown below. The cell generates no power in short-circuit (when current Isc is produced) or open-circuit (when cell generates voltage Voc). The cell delivers maximum power Pmax when operating at a point on the characteristic where the product IV is maximum. This is shown graphically below where the position of the maximum power point represents the largest area of the rectangle shown.
Efficiency of Solar Cell The efficiency (n) of a solar cell is defined as the power Pmax supplied by the cell at the maximum power point under standard test conditions, divided by the power of the radiation incident upon it. Most frequent conditions are: irradiance 100 mW/cm2 , standard reference spectrum, and temperature 25 0 C. The use of this standard irradiance value is particularly convenient since the cell efficiency in percent is then numerically equal to the power output from the cell in mW/cm2.
Typical Device Structure Lecture 05 Engr. Tanveer ul Haq
High Efficiency Silicon Solar Cell The passivated emitter solar cell has been developed at the University of New South Wales in Australia for operation under ordinary sunlight. The point contact cell of Stanford University USA, has been designed for optimum operation under concentrated sunlight.
Gallium Arsenide Solar Cells These are usually intended for operation on satellite or in concentration systems.
The structure and band diagram of Gallium Arsenide Solar Cell
Amorphous Silicon Solar Cell
The structure of amorphous Silicon p-i-n Solar cell
Tandem Cell Solar cells containing several p-n junctions are called Tandem Cell. Each junction is tuned to a different wavelength of light, reducing one of the largest inherent sources of losses, and thereby increasing efficiency.
The structure and spectral contribution of the tandem cell
Power Losses in Solar Cell
Fundamental Losses Carrier generation in the semiconductor by light involves considerable dissipation of the generated carrier energy into heat. In addition, a considerable part of the solar spectrum is not utilised because of the inability of a semiconductor to absorb the below-bandgap light.
Can these losses be reduced? Yes, but not with a simple structure that we have in mind at the moment. Such a device is called a tandem cell and represents a stack of several cells, each operating according to the principles that we have described. The top cell must be made of a high bandgap semiconductor, and converts the short-wavelength radiation. The transmitted light is then converted by the bottom cell. This arrangement increases considerably the achievable efficiency.
Recombination An opposite process to carrier generation is recombination when an electron-hole pair is annihilated. Recombination is most common at impurities or defects of the crystal structure, or at the surface of the semiconductor where energy levels may be introduced inside the energy gap. These levels act as stepping stones for the electrons to fall back into the valance band and recombine with holes as shown in next figure.
Defect-assisted recombination of electron-hole pair
An important site of recombination are also the ohmic metal contacts to the semiconductor.
What measure can one take to minimise the recombination losses? Surface recombination and recombination at contacts which are considerable in the conventional silicon cell can be reduced by adapting the device structure of high efficiency silicon cells. The external surfaces of the semiconducter are here protected by a layer of passivating oxide to reduce surface recombination. The top layer of GaAlAs in the GaAs cell has similar purpose.
The contacts are surrounded by heavily-doped regions acting as â€˜minority-carrier mirrorsâ€™ which impede the minority carriers from reaching the contacts and recombination. Recombination reduces both the voltage and current output from the cell.
Collection Efficiency The current losses can be grouped under the term of collection efficiency, the ratio between the number of carriers generated by light and the number that reaches the junction. Consideration of the collection efficiency affect the design of the solar cell. In crystalline materials, the transport properties are usually good, and carrier transport by simple diffusion is sufficiently effective. In amorphous and polycrystalline thin films, however, electric fields are needed to pull the carriers.
Other Losses Other losses to the current produced by the cell arise from light reflection from the top surface, shading of the cell by the top contacts, and incomplete absorption of light.
Losses in Solar Cell Lecture 06 Engr. Tanveer ul Haq
Fill Factor The short-circuit current and the open-circuit voltage are the maximum current and voltage respectively from a solar cell. However, at both of these operating points, the power from the solar cell is zero. The "fill factor", more commonly known by its abbreviation "FF", is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc.
Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve. The FF is illustrated below.
Series Resistance The transmission of electric current produced by the solar cell involves ohmic losses. These can be grouped together and included as a resistance in the equivalent circuit.
Series Resistance It is seen that the series resistance affects the cell operation mainly by reducing the fill factor.
The power losses in Solar Cell
Temperature Effect This has an important effect on the power output from the cell. The most significant is the temperature dependence of the voltage which decreases with increasing temperature (its temperature coefficient is negative). The voltage decreases of a silicon cell is typically 2.3mV per ËšC. The temperature variation of the current or the fill factor are less pronounced and are usually neglected in the PV system design.
Temperature dependence of I-V characteristic of solar cell
Irradiance Effect The light generated current is proportional to the flux of photons with above bandgap energy. Increasing the irradiance increases, in the same proportion, the photon flux which, in turn, generates a proportionately higher current. Therefore, the short circuit current of a solar cell is directly proportional to the irradiance. The voltage variation is much smaller (it depends logarithmically on the irradiance), and is usually neglected in practical application.
Irradiance dependence of the I-V characteristic of a solar cell
Summery The solar cell is a semiconductor device that converts the quantum flux of photons into electric current. When light is absorbed, it first creates electron-hole pairs. These mobile charges are then separated by the electric fields at the junction. The electrical output from the cell is described by the I-V characteristic whose parameters can be linked to the material properties of the semiconductor.
Summery Various solar-cell structures have been discussed in relation to the principal power losses in a solar cell. In addition to the fundamental losses associated with light absorption, other losses, including recombination and losses dependent on the structure of the device, have been analysed in some detail.
SILICON SOLAR CELL TECHNOLOGY Lecture 07 Engr. Tanveer ul Haq
INTRODUCTION The technology based on crystalline silicon is the most reliable and most developed photovoltaic technology at the present time. It is not simple, however, and require the use of sophisticated equipment and complex technological process. Four major stages need to be followed to make photovoltaic modules from sand 1. From sand to pure silicon 2. Growth of silicon crystals 3. From wafer to solar cell 4. From cell to module
FROM SAND TO PURE SILICON
GROWTH OF SILICON CRYSTALS Silicon is first melted at 1400ËšC. A small silicon crystal properly cooled is used as a seed to start the crystallization process. As the seed is pulled out silicon solidifies at the interface with the melt and, if the pulling is slow enough, the silicon atoms arrange themselves according to the crystallographic structure of the seed. This yields an ingot of single crystal silicon.
THE BASICS OF CRYSTAL GROWTH
The degree of purity improves during the growth process since impurities tend to segregate towards the liquid phase. A controlled amount of boron (or phosphorus) is usually added to the melt to dope the silicon p- or n-type.
METHODS OF GROWING SILICON CRYSTALS There are various methods of growing silicon crystals. ď‚˘ The one that resembles most closely the basic description is the Czochralski method which is also the most common in industrial use. ď‚˘ The second method is float zone process. The purest silicon is obtained by this process.
CZOCHRALSKI METHOD ď‚˘ The
cylindrical ingots are typically 1 m long, 15 cm in diameter and 40 kg in weight. The growth rate is about 0.1-0.2 cm/min. To increase the throughput, the crucible can be continuously replenish with molten silicon in some machines. Ingots up to 3.5 m long and 150 kg in weight have been grown this way.
CZOCHRALSKI METHOD Another recent development is the use of magnetic fields to reduce the interaction between the molten silicon and the crucible, thus reducing the usual carbon and oxygen contamination from the latter. The state of art for solar cells made from CZ silicon is 18% efficiency for 100 cmÂ˛ industrial cell, and 19% efficiency for a 49 cmÂ˛ laboratory cell with a laser-grooved metal grid.
CZOCHRALSKI METHOD Even though the CZ process is commonly used for commercial substrates, it has several disadvantages for high efficiency laboratory or niche market solar cells. CZ wafers contain a large amount of oxygen in the silicon wafer. Oxygen impurities reduce the minority carrier lifetime in the solar cell, thus reducing the voltage, current and efficiency. In addition, the oxygen and complexes of the oxygen with other elements may become active at higher temperatures, making the wafers sensitive to high temperature processing.
FLOAT ZONE METHOD In this process, a molten region is slowly passed along a rod or bar of silicon. Impurities in the molten region tend stay in the molten region rather than be incorporated into the solidified region, thus allowing a very pure single crystal region to be left after the molten region has passed.
Typical Cell Fabrication Process Lecture 08 Engr. Tanveer ul Haq
Typical Cell Fabrication Process To transfer a silicon wafer into a solar cell, the wafer is subjected to several chemical, thermal and deposition treatments. The cross section of a silicon cell shows the different layers that need to be formed.
Main steps of Fabrication Process 1. 2. 3.
Surface texturing p-n junction formation Possible back P+ region formation Front and back metal contacts Antireflection layer deposition
Surface Texturing Surface texturing, either in combination with an anti-reflection coating or by itself, can also be used to minimise reflection. Any "roughening" of the surface reduces reflection by increasing the chances of reflected light bouncing back onto the surface, rather than out to the surrounding air.
Surface Texturing Surface texturing can be accomplished in a number of ways. A single crystalline substrate can be textured by etching along the faces of the crystal planes. The crystalline structure of silicon results in a surface made up of pyramids if the surface is appropriately aligned with respect to the internal atoms. One such pyramid is illustrated in the drawing below.
Surface Texturing An electron microscope photograph of a textured silicon surface is shown in the photograph below. This type of texturing is called "random pyramid" texture, and is commonly used in industry for single crystalline wafers.
Electron microscope photograph of a textured silicon surface.
Surface Texturing Another type of surface texturing used is known as "inverted pyramid" texturing. Using this texturing scheme, the pyramids are etched down into the silicon surface rather than etched pointing upwards from the surface.
Electron microscope photograph of a textured silicon surface.
Surface Texturing Multicrystalline wafers cannot be textured by using either of the above methods. However, multicrystalline wafers can be textured using a photolithographic technique.
Electron microscope photograph of a textured multicrystalline silicon surface.
p-n junction formation The wafers are usually p-type. The p-n junction is then formed by thermal diffusion of n-type impurity, usually phosphorus atoms diffuse into silicon at a temperature of 900ËšC or higher. Figure shows a quartz diffusion furnace.
P+ region formation Although it is not absolutely necessary and might be considered irrelevant for lowefficiency cell, a back P+ region may be formed to improve the cell performance. This feature creates a back surface field that decreases the chances of carriers recombining at the back surface. The easiest way to form it is by depositing an aluminum layer and alloying it at about 800ËšC, or even diffusing it at about 1000ËšC.
Front and back metal contacts Electrical contacts are usually formed by screen printing. This technology is inexpensive, simple and can be automated. The screen consists of a mesh of wires imbedded in am emulsion. This emulsion is photographically patterned and removed from the places where metal is to be deposited. A paste containing the metal is squeezed through the screen onto the wafer. Upon firing the organic solvents evaporate and the metal powder becomes a conducting path for the electric current.
Screen printing Close up of a screen used for printing the front contact of a solar cell. During printing, metal paste is forced through the wire mesh in unmasked areas. The size of the wire mesh determines the minimum width of the fingers. Finger widths are typically 100 to 200 Âľm.
Screen printing Close up of a finished screen-printed solar cell. The fingers have a spacing of approximately 3 mm. An extra metal contact strip is soldered to the bus bar during encapsulation to lower the cell series resistance.
Screen printing Front view of a completed screen-printed solar cell. As the cell is manufactured from a multicrystalline substrate, the different grain orientations can be clearly seen. The square shape of a multicrystalline substrate simplifies the packing of cells into a module.
Screen printing Rear view of a finished screen-printed solar cell. The cell can either have a grid from a single print of Al/Ag paste with no back surface field(BSF), or a coverage of aluminium that gives a BSF but requires a second print for solderable contacts.
Antireflection layer deposition A thin layer of a transparent material which acts as antireflection coating can be deposited before or after the formation of the metal contacts. This dielectric material has an optimum value of refractive index between those of silicon and glass. An antireflection of silicon nitride is typically deposited using chemical vapour deposition process (CVD). Older cell designs use titanium dioxide (TiO2), which provides a good antireflection coating and is simpler to apply but does not provide surface or bulk passivation.
Antireflection layer deposition Wafers being deposited with silicon nitride antireflection coating giving a blue color.
Lecture 09 Engr. Tanveer ul Haq
The photovoltaic system consists of a number of parts or subsystem. a. The photovoltaic generator with mechanical support and possibly a sun tracking system. b. Batteries (storage subsystem). c. Power conditioning and control equipment, including provision for measurement and monitoring. d. Back up generator.
The choice of how and which of these components are integrated into the system is governed by various considerations. There are two main categories of systems, 1. Grid connected 2. Stand alone
It consists simply of a photovoltaic generator alone which supplies DC power to a load whenever there is adequate illumination. This type of system is common in pumping applications. In other instances, the system will usually contain a provision for energy storage by batteries. Some form of power conditioning is then frequently also included, as is the case when AC current is required at the output from the system. In some situation, the system contains a back-up generator.
Grid connected systems can be subdivided into those in which the grid merely acts as an auxiliary supply (grid back-up) and those in which it may also receive excess power from the PV generator (grid interactive). In PV power stations, all the generated power is fed into the gird.
Grid-Interactive systems use the light available from the sun to generate electricity and feed this into the main electricity grid. If at a particular moment in time more power is being produced than is required in the house, the extra power is sent back onto the grid to be used by neighbouring households. At night or when there is insufficient power being produced to supply the households needs, electricity is drawn from the grid in the same manner other households do.
The heart of the system is the photovoltaic generator. It consists of photovoltaic modules which are interconnected to form a DC power-producing unit. The physical assembly of modules with supports is usually called an array.
Most frequently, the cells in a module are interconnected in series. The reason comes from the electrical characteristics of an individual solar cell. A typical 4-inch diameter crystalline silicon solar cell, or a 10cmĂ—10cm multicrystalline cell, will provide between 1 and 1.5 watts under standard conditions, depending on the cell efficiency. This power is usually supplied at a voltage 0.5 to 0.6 V. Since there are very few appliance that work at this voltage, the immediate solution is to connect the solar cell in series.
The number of cells in a module is governed by the voltage of the module. The nominal operating voltage of the system usually has to be matched to the nominal voltage of the storage subsystem. Most of photovoltaic module manufacturers therefore have standard configurations which can work with 12 volt batteries. Allowing for some overvoltage to charge the battery and to compensate for lower output under less-than perfect conditions, it is found that a set of 33 to 36 solar cells in series usually ensures reliable operation.
The power of silicon modules thus usually falls between 40 and 60 W. The module parameters are specified by the manufacturer under the following standard conditions: Irradiance 1kW/m² Spectral distribution AM 1.5 Cell Temperature 25˚C Indeed, they are the same conditions as are used to characterise solar cells. The nominal output is usually called the peak power of a module and expressed in peak watts, W.
The three important electrical characteristics of a module are the short-circuit current, open circuit voltage and the maximum power point as functions of the temperature and irradiance.
The temperature and irradiance dependence of the module I-V characteristic
Temperature is an important parameter of a PV system operation. The temperature coefficient for the open circuit voltage is approximately equal to -2.3mV/ËšC for an individual cell. The voltage coefficient of a module is therefore negative and very large since 33 to 36 cells are connected in series. The current coefficient on the other hand, is positive and small, about +6ÎźA/ËšC for a square centimeter of the module area.
Accordingly only the voltage variation with temperature is allowed for in practical calculation, and for an individual module consisting of nc cells connected in series in set equal to: dVoc/dT= -2.3Ă—nc mV/ËšC
It is important to note that the voltage is determined by the operating temperature of the cells which differs from the ambient temperature.
As for a single cell, the short circuit current Isc of a module is proportional to the irradiance, and will therefore vary during the day in the same manner. Since the voltage is a logarithmic function of the current, it will also depend logarithmically on the irradiance. During the day, the voltage will therefore vary less than the current. In the design of the PV generator, it is customary to neglect the voltage variation and to set the short circuit current proportional to irradiance: Isc(G)=Isc(at1kW/mÂ˛)Ă—G(in kW/mÂ˛)
The operation of the module should lie as close as possible to the maximum power point. It is a significant feature of the module characteristic that the voltage of the maximum power point, Vm, is roughly independent of irradiance. The average value of this voltage during the day can be estimated as 80% of the open-circuit voltage under standard irradiance conditions. This property is useful for the design of the power conditioning equipment.
The characterisation of the PV module is completed by measuring the Normal Operating Cell Temperature (NOCT) defined as the cell temperature when the module operates under the following conditions at open circuit: Irradiance 0.8 kW/m² Spectral distribution AM 1.5 Cell Temperature 20˚C Wind speed > 1m/s
NOCT (usually between 42ËšC and 46ËšC) is then used to determine the solar cell temperature Tc during module operation. It is usually assumed that the difference between Tc and the ambient temperature Ta depends linearly on the irradiance G in the following manner: Tc-Ta= (NOTC-20)G(kW/mÂ˛) /0.8
Determine the parameters of a module formed by 34 solar cells in series, under the operating conditions G=700 W/m² and Ta=34˚C. The manufacturer’s values under standard conditions are: Isc=3A; Voc=20.4; Pmax=45.9 W; NOCT=43˚C.
1. Short circuit current Isc(G)=Isc(at1kW/m²)×G(in kW/m²)=3×0.7=2.1A
2. Solar Cell temperature Tc-Ta= (NOTC-20)G(kW/m²) /0.8 Tc=Ta+ (NOTC-20)G(kW/m²) /0.8 =34+(43-20)0.7/0.8=54.12˚C 3. Open circuit voltage dVoc/dT= -2.3×nc mV/˚C Voc(54.21)=20.4-0.0023×34×(54.12-25)=18.1
4. We shall now determine the maximum power point using the simplifying assumption that the fill factor is independent of the temperature and the irradiance: FF=45.9/3×20.4=0.75 Pmax(G,Tc)=2.1×18.1×0.75=28.5W Thus, noting the manufacturer’s value of Pmax we see that the module will operate at about 62% of its nominal rating.
Lecture 10 Engr. Tanveer ul Haq
A schematic diagram of a PV generator consisting of several modules is shown. In addition to photovoltaic modules, the generator contains bypass and blocking diodes.
The module are connected in series to form strings, where the number of modules Ns is determined by the selected DC bus voltage, and the number of parallel strings Np is given by the current required from the generator. Analysis assume that all the modules are identical. In practice, the module are not identical, and their parameters exhibit a certain degree of variability for two reasons:
The solar cells and modules vary in quality as a result of the manufacturing process. In general, the current produced by commercial modules suffers a high degree of dispersion than the voltage. 2. Different operating conditions may exist in different part of the PV array. For example one must allow for different cleanness of different parts of the PV generator, or some modules may be obscured by a cloud which is covering only a part of the array. 1.
This variability of component parameters has two important effects: Firstly, the output power of the generator is less than the sum of values corresponding to all the constituent modules. This gives rise to mismatch losses. These losses can be minimised by forming series strings from modules with similar values of short-circuit current.
Secondly, there is a potential for overheating the ‘poorest’ cell of a series string. In some circumstances, a cell can operate as ‘load’ for other cells acting as ‘generators’. Consequently, this cell dissipate energy and its temperature increases. If the cell temperature rises above a certain limit(85-100˚C) the encapsulating materials can be damaged, and this will degrade the performance of the entire module. This is called hot spot formation.
This effect is illustrated in figure, which shows a cell in a string which does not produce current, this can happen, for example, when the cell is shaded. The shading of one cell converting it into a diode under reverse biasâ€“ therefore eliminate the current produced by the entire string.
Furthermore, the shaded cell will dissipate all the power produced by the illuminated cells in the string which can be considerable if the string is large. The common technique used to alleviate this effect is to employ by-pass diodes which are connected across a block of several cell in a string. This limits the power which is dissipated in this block and provides a low-resistance path for the module current.
An important problem that confronts the designer of an array is whether the modules are to be mounted at fixed positions, or their orientations will follow the motion of the sun. In most arrays, the modules are supported at a fixed inclination facing the equator. This has the virtue of simplicity, no moving parts and low cast. The optimum angle of inclination depends mainly on the latitude, the proportion of diffuse radiation at the site and the load profile.
By mounting the array on a two-axis tracker, up to 40% more of the solar energy can be collected over the year as compared with a fixed-tilt installation. But this increases complexity and result in lower reliability and higher maintenance costs. Single axis tracking is less complex but yields a smaller gain. Where labour is available, the orientation may be manually adjusted to increase the output. It has been estimated that, in sunny climates, a flat plate array moved to face the sun twice a day and given a quarterly tilt adjustment can intercept nearly 95% of the energy collected with a fully automatic two-axis tracking.
Tracking is particularly important in systems which operate under concentrated sunlight. The structure of these systems ranges from a simple design bases on side booster mirrors, to concentration systems which employ sophisticated optical techniques to increase the light input to the cell by several orders of magnitude. These systems must make allowance for an important fact that concentrating the sunlight reduces the angular range of rays that the system can accept for conversion. Tracking becomes necessary once the concentration ratio exceeds about 10 and the system can only convert the direct component of solar radiation.
Some energy storage systems Energy Stored
1. Pumping water 2. Compressed air 3. Fly wheel
Electric current in Superconducting ring
1. Batteries 2. Hydrogen production
Although a variety of energy storage methods are under consideration, the majority of stand alone PV systems today use battery storage. The batteries in most common use are lead acid batteries because of their good availability and cost effectiveness. Nickel cadmium batteries are used in some smaller applications where their ruggedness, both mechanical and electrical, is considered essential. However, their high cost per amount of energy stored has prevented their wider use in photovoltaic's.
POWER CONDITIONING AND CONTROL Lecture 11 Engr. Tanveer ul Haq
The Blocking Diode We know that a solar cell in the dark behave as a diode. Without special precautions, this type of night-time operation of the photovoltaic generator will provide a discharge path for the battery. The simplest solution is to separate the generator and battery by blocking diode. When the voltage at the battery exceeds the voltage at the generator, the diode becomes reverse-biased and prevents the battery discharge.
Effect of Blocking Diode During daytime operation, however, there will be a voltage drop across the blocking diode which should be taken into account when designing the system. In system using modern PV modules where the series resistance is low and the I-V characteristic approaches the ideal curve, the battery discharge current via the PV generator at night can be very small. The power dissipated at the blocking diode during daytime operation may exceed the nighttime discharge losses. For this reason, the blocking diode is sometimes omitted from the circuit design.
Charge Regulator Measures must be taken to prevent excessive discharge and overcharging of batteries. Various type of charge regulators are available that fulfill this role. In small applications (up to 100W), a shunt regulator can be used to dissipate the unwanted power from the generator. A common implementation is to use a transistor in parallel with the PV generator which is set to conduct and divert current from the battery at a certain threshold voltage value.
Charge Regulator In larger applications, it is advisable to disconnect the battery from the generator by means of series regulator. This can be an electromechanical switch (for example a relay) or a solid state device (bipolar transistor, MOSFET, etc) . The former devices have the advantage that they do not dissipate energy but their reliability can be a problem in locations with high dust or sand occurrence.
Charge Regulator The battery may be protected against excessive discharge by a charge limiter. This device is introduced between the load and the battery and acts as a switch which opens when the battery charge reaches a minimum acceptable level.
DC/DC Converter The variability of the power output from the PV generator will often operate away from its maximum power point. The associated losses can be avoided by the use of maximumpower-point tracker which ensures that there is always a maximum energy transfer from the generator to the battery.
DC/DC Converter The principles of the MPP tracker are demonstrated in figure for the situation when the PV generator feeds power to a resistive load.
DC/DC Converter The I-V characteristic of the generator and the load, together with constant power curves P=VI=constant is shown. It is seen that at the operating point 1 the delivered power is significantly below Pmax, the maximum power of PV generator.
DC/AC converter (inverter) This is standard item of electronic equipment which is used in many different applications. The input power is the DC power from the photovoltaic generator or battery and the output is AC power used to run AC appliances or fed into the utility grid. The efficiency of the inverters usually depends on the load current being a maximum at the nominal output power.
DC/AC converter (inverter) The majority of inverter for PV application can be classified into three main categories. First one are variable frequency inverters. These are used for stand-alone drive/shaft power applications, almost exclusively in PV pumping system.
DC/AC converter (inverter) â€˘ Second are Self commutating fixed frequency inverters. These are able to feed an isolated distribution grid and, if equipped with special paralleling control, also a grid supplied by other parallel power sources.
DC/AC converter (inverter) Third are Line-commutated fixed-frequency inverters. These are able to feed the grid only where the grid frequency is defined by another power source connected in parallel. The inverter will not work if such external frequency reference is lacking.
DC/AC converter (inverter) The advantages and draw back of these two inverter types are summarised.
Alarms, Indicators and monitoring equipment The system electronics should include some indicators which display the state of the system, or at least its main parameters. The main indicators should display the low charge state for batteries and the over charge. In some instances, the user should be warned about the state of the system by an alarm.
Engr. Tanveer ul Haq
Sizing of a PV system, particularly a stand-alone one, is an important part of its design. Since the capital equipment cost is the major component of the price of solar electricity, oversizing the plant has a very detrimental effect on the price of the generated power. Undersizing a stand alone system, on the other hand, reduces the supply reliability. 7/17/2012
Engr. Tanveer ul Haq
The sizing of a system requires a knowledge of the solar radiation data for the site, the load profile and the importance of supply continuity. In addition, other constrains on the design (for example economic) must also be known, The sizing procedure then recommends the size of the photovoltaic generator and battery capacity that will be optimum for the application. It will also allow the nominal characteristics of the electronic components to be specified. 7/17/2012
Engr. Tanveer ul Haq
The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows: 1.1 Calculate total Watt-hours per day for each appliance used. Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which must be delivered to the appliances. 1.2 Calculate total Watt-hours per day needed from the PV modules. Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get the total Watt-hours per day which must be provided by the panels. 7/17/2012
Engr. Tanveer ul Haq
Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider â€œpanel generation factorâ€? which is different in each site location. For Thailand, the panel generation factor is 3.43. To determine the sizing of PV modules, calculate as follows:
Engr. Tanveer ul Haq
2.1 Calculate the total Watt-peak rating needed for PV modules Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get the total Watt-peak rating needed for the PV panels needed to operate the appliances. 2.2 Calculate the number of PV panels for the system Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available to you. Increase any fractional part of result to the next highest full number and that will be the number of PV modules required 7/17/2012
Engr. Tanveer ul Haq
An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery. For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. 7/17/2012
Engr. Tanveer ul Haq
In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting. For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation. 7/17/2012
Engr. Tanveer ul Haq
The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows: 4.1 Calculate total Watt-hours per day used by appliances. 7/17/2012
Engr. Tanveer ul Haq
4.2 Divide the total Watt-hours per day used by 0.85 for battery loss. 4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge. 4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage. 4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you need the system to operate when there is no power produced by PV panels) to get the required Ampere-hour capacity of deep-cycle battery. Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy (0.85 x 0.6 x nominal battery voltage) 7/17/2012
Engr. Tanveer ul Haq
A house has the following electrical appliance usage: • One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day. • One 60 Watt fan used for 2 hours per day. • One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours. The system will be powered by 12 Vdc, 110 Wp PV module.
Engr. Tanveer ul Haq
Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours) = 1,092 Wh/day Total PV panels energy needed = 1,092 x 1.3 = 1,419.6 Wh/day.
Engr. Tanveer ul Haq
2.1 Total Wp of PV panel capacity needed =1,419.6 / 3.4 = 413.9 Wp 2.2 Number of PV panels needed = 413.9 / 110 =3.76modules
Actual requirement = 4 modules So this system should be powered by at least 4 modules of 110 Wp PV module. 7/17/2012
Engr. Tanveer ul Haq
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size. The inverter size should be about 190 W or greater.
Engr. Tanveer ul Haq
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours) Nominal battery voltage = 12 V Days of autonomy = 3 days Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3 (0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah So the battery should be rated 12 V 600 Ah for 3 day autonomy. 7/17/2012
Engr. Tanveer ul Haq
PV module specification Pm = 110 Wp Vm = 16.7 Vdc Im = 6.6 A Voc = 20.7 A Isc = 7.5 A Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40A at 12 V or greater. 7/17/2012
Engr. Tanveer ul Haq
Engr. Tanveer ul Haq
Lecture 13 Engr. Tanveer ul Haq
Principle In Concentrating Photovoltaics (CPV), a large area of sunlight is focused onto the solar cell with the help of an optical device.
By concentrating sunlight onto a small area, this technology has three competitive advantages: 1. Requires less photovoltaic material to capture the same sunlight as non-concentrating pv. 2. Makes the use of high-efficiency but expensive multi-junction cells economically viable due to smaller space requirements. 3. The optical system comprises standard materials, manufactured in proven processes. Thus, it is less dependant on the immature silicon supply chain. Moreover, optics are less expensive than cells.
Concentrating light, however, requires direct sunlight rather than diffuse light, limiting this technology to clear, sunny locations. Despite having been researched since the 1970s, it has only now entered the solar electricity sector as a viable alternative. Being a young technology, there is no single dominant design. The most common classification of CPV- modules is by the degree of concentration, which is expressed in number of "suns". E.g. "3x" means that the intensity of the light that hits the photovoltaic material is 3 times than it would be without concentration.
Low Medium High concentration concentration concentration Degree of 2 - 10 concentration Tracking?
10 - 100
No tracking necessary
Dual axis 1-axis tracking tracking sufficient required
No cooling required
Active cooling Passive cooling reuqired in sufficient most instances.
Photovoltaic High- quality Material silicon
Fresnel Lens A Fresnel lens, named after the French physicist, comprises several sections with different angles, thus reducing weight and thickness in comparison to a standard lens.
Fresnel lenses can be constructed ď‚— in a shape of a circle to provide a point focus with concentration ratios of around 500, or ď‚— in cylindrical shape to provide line focus with lower concentration ratios. With the high concentration ratio in a Fresnel point lens, it is possible to use a multi-junction photovoltaic cell with maximum efficiency. In a line concentrator, it is more common to use high efficiency silicon.
Parabolic Mirrors Here, all incoming parallel light is reflected by the collector (the first mirror) through a focal point onto a second mirror.
Parabolic Mirrors The second mirror, which is much smaller, is also a parabolic mirror with the same focal point. It reflects the light beams to the middle of the first parabolic mirror where it hits the solar cell. The advantage of this configuration is that it does not require any optical lenses. However, losses will occur in both mirrors. SolFocus has achieved a concentration ratio of 500 in point concentratorshape with dual axis- tracking.
Reflectors Low concentration photovoltaic modules use mirrors to concentrate sunlight onto a solar cell. Often, these mirrors are manufactured with silicone-covered metal. This technique lowers the reflection losses by effectively providing a second internal mirror.
Reflectors The angle of the mirrors depends on the inclination angle and latitude as well as the module design, but is typically fixed. The concentration ratios achieved range from 1.5 - 2.5. Low concentration cells are usually made from monocrystalline silicon. No cooling is required. The largest low-concentration photovoltaic plant in the world is Sevilla PV with modules from three companies: Artesa, Isofoton and Solartec.
Luminescent Concentrators In a luminescent concentrator, light is refracted in a luminescent film, and then being channelled towards the photovoltaic material.
This is a very promising technology, as it does not require optical lenses or mirrors. Moreover, it also works with diffuse light and hence does not need tracking. The concentration factor is around 3. There are various developments going on. For instance, Covalent are using an organic material for the film, whilst Prism Solar use holographic film. Furthermore, this concentrator does not need any cooling, as the film could be constructed such that wavelenghts that can not be converted by the solar cell would just pass thru. Hence, unwanted wavelenghts would be removed.
Cooling Most concentrating pv systems require cooling. Passive Cooling: Here, the cell is placed on a cladded cermaic substrate with high thermal conductivity. The ceramic also provides electrical isolation. Active Cooling: Typically, liquid metal is used as a cooling fluid, capable of cooling from 1,700째C to 100째C.
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PALZ, W., ed. European Solar Radiation Atlas, Volumes 1 and 2 2nd edn. Verlag TUV Rheinland, Cologne, 1984. GREEN, M. A., Solar Cells, Prentice Hall, Englewood Cliffs, NJ, 1982. HERSH, P. and ZWEIBEL, K., Basic Photovoltaic Principles and Methods, U.S. Government Printing Office, Washington, DC, SERI/SP-290-1448, 1982. PULFREY, D. L., Photovoltaic Power Generation, Van Nostrand Theinhold, New York, 1978. VAN OVERSTRAETEN, R. And MERTENS, R., Physics, Technology and Use of Photovoltaics, Adam Hilger, Bristol 1986.
GREEN, M. A., Solar Cells, Prentice Hall, Englewood Cliffs, NJ, 1982. HERSH, P. and ZWEIBEL, K., Basic Photovoltaic Principles and Methods, U.S. Government Printing Office, Washington, DC, SERI/SP-290-1448, 1982. PULFREY, D. L., Photovoltaic Power Generation, Van Nostrand Theinhold, New York, 1978. VAN OVERSTRAETEN, R. And MERTENS, R., Physics, Technology and Use of Photovoltaics, Adam Hilger, Bristol 1986.
• BOGUS, K. Space photovoltaics– present and future, ESA Bulletin 41, 70-77. • HILL, R., Applications of Photovoltaics, Adam Hilger Bristol, 1988. • MARKVART, T. Radiation damage in solar cell, Journal of Materials Science: Materials in Electronics 1 (1), 1990:1-8. • MAYCOCK, P-D. and STIREWALT, E. N., A Guide to the Photovoltaic Revolution, Emaus, PA., 1985. • BLOSS, W.H.,PFISTERER, F., KLEINKAUF, W., LANDAU, M., WEBER, H. and HULLMAW, H. Grid-connected solar houses, In: proc. 10th European Photovoltaic Solar Energy Conf., Lisbon, 1991: 1295-1300.
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