Lecture 6:Photovoltaic Power Generation

Lecturers:

Syafaruddin & Takashi Hiyama

syafa@st.eecs.kumamoto-u.ac.jp

hiyama@cs.kumamoto-u.ac.jp

Time and Venue:

Wednesdays: 10:20 – 11:50, Room No.: 208

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http://www.cs.kumamoto-u.ac.jp/epslab/APSF/

Contents

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1. Principles• Energy Gap Model• Conductor, Semiconductor &

Insulator• Conduction Mechanism of

Semiconductor• Photo Effect• P-N Junction• Photovoltaic Effect

2. Technical Description• Photovoltaic cell and module• Further system components• Grid independent systems• Grid connected systems• Energy conversion chain, losses &

characteristics of power curve

3. Economic Analysis• Investment• Operation costs• Power production costs

4. Environmental Analysis• Construction• Normal operation• Malfunction• End of generation

Energy Gap Model

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•Besides the positively charged protons positively charged protons and the uncharged neutronsuncharged neutrons inside the nucleus an atom is composed of the negatively charged electrons that assume discrete energy levels (such as "shells" or "orbitales") around the nucleus. •There is a limited number of electrons that can occupy a certain energy level; according to the so-called Pauli exclusion principle Pauli exclusion principle any possible energy level may only be occupied by a maximum of two electrons. •These two electrons are only allowed if they again differ from each other by their "spin"(i.e. self angular momentum).

with increasing energy level, so that outer bands overlap.

•If several atoms form a crystal, the different energy levels of the individual atoms overlap each other and stretch to form energy bands. •Between these "allowed“ energy bands there are energy gaps energy gaps (i.e. "forbidden" bands). •There are narrow permitted bands for the inner electrons, closely bound to the nucleus, and wide permitted bands for the outer electrons. •The width of the forbidden width of the forbidden bandsbands varies in the opposite way; forbidden bands are wide close to the nucleus and decrease with increasing energy level, so that outer bands overlap. •The energetic distances of permitted bands and the width of energy gaps, respectively, and the distribution of electrons to the permitted bands determine the electric and optic properties of a crystal.

Energy Gap Model… cont

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•Also within these bands the number of energy levels to be occupied by electrons is limited (i.e. the number of spaces is restricted). •There is thus a "finite energy state density". •The inner shells of atoms and the energy bands of solids with low energy levels, respectively, are almost entirely covered with electrons. •The electrons are unable to move freely hereunable to move freely here; they are only able to change places.•These electrons do not produce any conductivity. •The most energy-rich energy band, fully occupied with electrons, is referred to as valence valence bandband; the electrons it contains determine the chemical bond type of the material.

•A solid with electrical conductivity solid with electrical conductivity requires freely moving electrons. •However, electrons are only able to move freely if they are located in an energy band that is not fully occupied. •For energy reasons, this is only true for the energy band located above the valence band. •This energy band is thus referred as the conduction bandconduction band.

Energy Gap Model…cont.

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•The energy gap Eg between the valence band and conduction band is termed as "band gap“ •This energy gap exactly equals the minimum amount of energy required to transfer one electron from the valence band into the conduction band.

Conductor, Semiconductor & Insulator

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Conductors: Conductors: (metals and their alloys) two different conditions might occur:− The most energy-rich band (i.e. conduction band) occupied by electrons is not entirely occupied.− The most energy-rich band fully occupied with electrons (i.e. valence band) and the conduction band located on top overlap, so that also a partly covered band (conduction band) is formed.

•Current is thus transferred by freely moving electrons, abundantly available within the crystal latticecrystal lattice regardless of the respective temperature of the material.•Due to this, electrical conductors (like metalsmetals) are characterized by a low specific resistance. •With rising temperature, the increasing thermal oscillation of the atomic cores impedes the movement of the electrons. •This is why the specific resistance of metals increases with a rising temperature.

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Insulators:•(e.g. rubber, ceramics) are characterized by a valence a valence band fully band fully filled with filled with electronselectrons, a wide energy gap (Eg > 3 eV) and an empty conduction band. •Hence, insulators possess virtually no freely moving electrons. •Only at very high temperatures (strong "thermal excitation") are a small number of electrons able to overcome the energy gap. •Thus, ceramics, for instance, show conductivity only at very high temperatures.

Conductor, Semiconductor & Insulator…cont.

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Semiconductors:•(e.g. silicon, germanium, galliumarsenide) are insulators with a relatively narrow energy gap (0.1 eV < Eg < 3 eV).•Therefore, at low temperatures, a chemically pure semiconductor acts as an insulator.•Only by adding thermal energythermal energy, electrons are released from their chemical bond, and lifted to the conduction band. •This is the reason why semiconductors become conductive with increasing temperatures. •This is the other way round compared to metals, where conductivity decreases with rising temperatures. •Regarding specific resistance, semiconductors are insemiconductors are in--between conductors and insulatorsbetween conductors and insulators.•Within the transition area between semiconductors and conductors, in case of very narrow energy gaps (0 eV < Eg < 0.1 eV), such elements are also referred to as metalloids or semi-metals as they may show similar conductivity as metals.•However, unlike "real" metals they are characterized by a reduced conductivity with decreasing temperatures.

Conductor, Semiconductor & Insulator…cont.

Conduction mechanisms of semiconductors

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Intrinsic Intrinsic conductivity:conductivity:•Semiconductors are conductive beyond a certain temperature level as valence electrons are released from their chemical bonds with increasing temperatures and thus reach the conduction band (intrinsic conductivity).•They become conduction electrons that are able to move freely through the crystal lattice (i.e. electron conduction).

•On the other hand, also the resulting hole inside the valence band can move through the semiconductor material, since a neighboring electron can advance to the hole. •Holes thus contribute equally to conductivity (hole conduction). •Since every free electron creates a hole within undisturbed pure semiconductor crystals both both types of charge carriers types of charge carriers equally existsequally exists.

Energy gap model

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Intrinsic conductivity…cont.Intrinsic conductivity…cont.•Intrinsic conductivity is counteracted by recombinationrecombination, namely the recombination of a free electron and a positive hole. •Despite this recombination the number of holes and free electrons remains equal since at a certain temperature level always the same number of electron-hole-pairs are formed as recombine. •For every temperature, there thus exists an equilibrium state with a certain number of free holes and free electrons. •The number of free electron-hole-pairs increases with rising temperature.

Conduction mechanisms of semiconductors

Energy gap model

If an external voltage is applied to such a crystal lattice from outside, electrons move to the positive pole while the holes move to the negative pole.

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Conduction mechanisms of semiconductors

Extrinsic conduction:•In addition to the – low – intrinsic conduction of pure crystal lattices extrinsic conduction is created by intentional incorporation of foreign atoms ("doping"). •Such impuritiesimpurities are effective if their number of valence electrons differs from that of the base material.

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•The valence electron number of the incorporated impurities exceeds that of the lattice atom (e.g. in pentavalent arsenic (As) incorporated into tetravalent silicon (Si) lattice, the excess electron is only weakly bound to the impurity atom. •It thus separates easily from the impurity atom due to thermal movements within the lattice and increases the conductivity of the crystal lattice as a freely moving electron. •Such foreign atoms which increase the number of electrons are referred to as donor atomsdonor atoms. •By this the number of electrons exceeds by far that of the holes. •In this case electrons are called majority carriers, whereas the holes constitute the minority carriers. •Since conductivity is mainly created by negatively charged particles, this type of conduction is referred to as n-conduction.

Conduction mechanisms of semiconductors

Conduction mechanisms of semiconductors

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•If the impurities incorporated into the semiconductor material are by contrast provided with less valence electrons (e.g. trivalent boron (B) or aluminium (Al) incorporated into tetravalent silicon (Si), these doping atoms tend to absorb one additional electron from the valence band of the basic material. •Such foreign atoms are thus referred to as acceptor acceptor atomsatoms. •They increase the number of holes (quasi positive charge carriers) and create pcreate p--conductivityconductivity. •Under these conditions deficit electrons (i.e. holes) are majority carriers whereas electrons act as minority carriers.

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Conduction mechanisms of semiconductorsWhy we need to dope the material?•Within the undoped semiconductor material: a certain equilibrium concentration of free mobile charge carriers is formed due to the recurrent processes of electron-hole-pair creation and recombination.

During this process the number of holes and electrons are equaequal. •Besides temperature, temperature, charge carrier charge carrier density at equilibrium concentration density at equilibrium concentration is determined by the minimum energy required to release one electron from the valence band, and is thus described by the energy gap Eg. •For example for Ge the energy gap amounts to 0.75 eV and to 1.12 eV for Si. •Since electron and hole densities remain relatively low, also conductivity of the undopedsemi-conductor material is low.

By doping the semiconductor with acceptors (p-doping) and donors (n-doping) conductivity of semiconductor materials can be controlled across several orders of magnitudes. However, the product of electron density and hole density of a the product of electron density and hole density of a certain material certain material is a temperature-dependent material constant. Hence, if for instance the electron density is increased by the incorporation of donorsthe electron density is increased by the incorporation of donors, the hole density is automatically reduced. Nevertheless, the conductivity increases. However, both kinds of doping must not be applied simultaneously, since the effects of acceptors and donors cancel each other.

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•Semiconductors are further distinguished into "direct" and "indirect" "direct" and "indirect" semiconductors.semiconductors.•While for direct semiconductors only energy is required to transfer charge carriers from the valence band to the conduction band, for indirect semiconductors also a momentum needs to be transferred to the charge carrier. •This is mainly due to the band structure and has a tremendous impact on the appropriateness of semiconductor materials for solar cells. •While for a direct semiconductor absorption absorption of an incident photon of an incident photon is sufficient to lift the charge carrier up to the conduction band (i.e. exclusive energy transfer by the photon), for indirect semiconductors an appropriate momentum additionally needs to be transferred. •This process requires three particles: the charge carrier (1st particle) simultaneously receives sufficient energy quantities from the photon (2nd particle) and the required momentum from a phonon (3rd particle); quanta of a crystal momentum are referred to as phonons. •Only if all three particles meet simultaneously (i.e. three particle process), charge carriers are lifted into the conduction band. In comparison to direct semiconductors (two body process) these conditions are much rarer.•This is why in case of indirect semiconductors the photon inside the semiconductor material has to travel a much longer distance until it is absorbed.

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Crystalline silicon is such an indirect semiconductor, and silicon cells must thus be relatively thick and/or contain an appropriate light-trapping scheme to generate a prolonged optical path length. Amorphous silicon, CdTe or CIS are in contrast direct semiconductors.

Solar cells made of these materials can thus have a thickness clearly below 10 μm, while the thickness of crystalline silicon solar cells typically stretches from 200 to 300 μm.

Thinner crystalline silicon cells are under development, but must be provided with the discussed optical properties, resulting in increased manufacturing expenditure.

Conduction mechanisms of semiconductors

Photo effect

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The term "photo effect" refers to the energy transfer from photons (i.e. quantum of electromagnetic radiation) to electrons contained inside material. Photon energy is thereby converted into potential and kinetic energy of electrons. The electron absorbs the entire quantum energy of the photon defined as the product of Planck's quantum and the photon frequency. External External and internal photo effect and internal photo effect is distinguished.

External:External:•If electromagnetic radiation hits the surface of a solid body within the ultraviolet range, electrons can absorb energy from the photon.•Then they are able to surmount the required work function to escape from the solid body, provided that there is sufficient photon energy.

Internal:Internal:•The internal photo effect describes also absorption of electromagnetic radiation within a solid body. •The electrons are in this case not detached from the solid body. •They are only lifted from the valence band up to the conduction band. •Therefore, electron-hole-pairs are created which enhance the electric conductivity of the solid body

PV effect however, additional boundary layer:e.g: a metal-semiconductor junction, a p-n-junction or a p-n-heterojunction (i.e. an interface between two different materials with different types of conductivity

basis for the photovoltaic effect andsolar cell.

P-N Junction

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•If p- and n-doped materials brought into contact, holes from the p-doped side diffuse into the n-type region and vice versa. •First a strong concentration gradient is formed at the p-n-junction, consisting of electrons inside the conduction band and holes inside the valence band. •Due to this concentration, gradient holes from the p-region diffuse into the n-region while electrons diffuse from the n- to the p area.•Due to the diffusion, the number of majority carriers are reduced on both sides of the p-n-junction. •The charge attached to the stationary donors or acceptors then creates a negative space charge on the p-side of the transition area and a positive space charge on the n-side.

19Creation of a depletion layer within the p-n-junction

P-N JunctionAs a result of the equilibrated concentration of free charge carriers an electrical field is built up across the border interface (p-n-junction). The described process creates a depletion layer in which diffusion flow and reverse current compensate each other. The no longer compensated stationary charges of donors and acceptors define a depletion layer whose width depletion layer whose width is dependent on the doping concentration

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p-n-junction within a solar cell (ideal condition)

Simplified, it has been assumed that majoritycarrier density is negligible over the entire space-charge region and that depletionlayer density remains constant up to the edges of the depletion zone (c)

This also implies that the respective doping concentration is constant up to the p-n-border; p-n-transition is thus abrupt. (d) shows the correspondingpotential curve of a positively charged particle and the diffusion voltage createdwithin the depletion layer. (e). Photovoltaic effect…..next

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•(e): If photons, the quantum's of light energy, hit and penetrate into a semiconductor, they can transfer their energy to an electron from the valance band•If such a photon is absorbed within the depletion layerwithin the depletion layer, the region’s electrical field directly separates the created charge carrier pair The electron moves towards the n-region, whereas the hole moves to the p-region. •If, during such light absorption, electron-hole-pairs are created outside of the depletion outside of the depletion region within the pregion within the p-- or or n region n region (i.e. outside of the electrical field), they may also reach the space-charge region by diffusion due to thermal movements (i.e. without the direction being predetermined by an electrical field). •At this point the respective minority carriers (i.e. the electrons within the p-region and the holes in the n-region) are collected by the electrical field of the space-charge region and are transferred to the opposite side. •The potential barrier of the depletion layer, in contrast, reflects the respective majority carriers.

Finally, the p-side becomes charged positively while the n-side is charged negatively. Both, photons absorbed within, and outside, of the depletion layer contribute to this charging. This process of light-induced charge separation is referred as pp--nn--photo effect or as photo effect or as photovoltaic effect.photovoltaic effect.

Photovoltaic Effect

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•Thus, the photovoltaic effect only occurs if one of the two charge carriers created during light absorption passes the p-n-junction. •This is only likely to occur when the electronthe electron--holehole--pair are generated within the depletion pair are generated within the depletion layerlayer. ••Outside of this Outside of this electrical field electrical field there is an increasing likelihood that charge carrier pairs created by light get lost by recombination. •This is more likely the greater the distance is between the location of the generation of the electron-hole-pair and the depletion layer. •This is quantified by the "diffusion length" of the charge carriers inside the semiconductor material. •The term "diffusion length" refers to the average path lengths to be overcome by electrons or holes within the area without an electrical field before recombination takes place. •This diffusion length is determined by the semiconductor material and, in case of the identical material, highly depends on the impurity content – and thus also on doping (the more doping the lower the diffusion length) – and on crystal perfection. •For silicon the diffusion length varies from approximately 10 up to several 100 μm.• If the diffusion length is less than the charge carrier’s distance to the p-n-junction most electrons or holes recombine •To achieve an effective charge carrier separation the diffusion length should be a multiple of the absorption length of the solar radiation incident on a photovoltaic cell.

Photovoltaic effect…cont

Photovoltaic Effect…cont.

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•Due to the charge separation during irradiation, electrons accumulate within the n-region, whereas holes accumulate in the p-region. •Electrons and holes will accumulate until the repelling forces of the accumulated charges start to impede additional accumulation; i.e. until the electrical potential created by the accumulation of electrons and holes is balanced by the diffusion potential of the p-n junction.•Then the openopen--circuit voltage circuit voltage of the solar cell is reached. •The time to achieve these conditions is almost immeasurably short.

If p- and n-sides are short-circuited by an external connection, the short-circuit current is measured. In this operating mode the diffusion voltage at the p-n junction is restored. According to the operating principle of a solar cell, short circuit current increase is proportional and almost linear to solar irradiance

Photovoltaic cell and module

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Structure:•The basic structure of a photovoltaic cell consisting of p-conducting base material and an n-conducting layer on the topside. •The entire cell rear side is covered with a metallic contact while the irradiated side is•equipped with a finger-type contact system to minimize shading losses. •Also full cover, transparent conductive layers are used. •To reduce reflection losses the cell surface may additionally be provided with an anti-reflecting coating. •A silicon solar cell with such construction usually has a blue color. By the incorporation of inverse pyramids into the surface reflection losses are further reduced. •The inclination of inclination of the pyramid surfaces the pyramid surfaces is such that photons are reflected onto another pyramid surface, and thus considerably enhance the possibility of photon penetration into the crystal. •Absorption of the solar light by these cells is almost complete, the cells appear blackblack.

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Structure of a typical solar cell and an equivalent circuit diagram

Current-voltage characteristic and equivalent circuit

•An illuminated solar cell ideallyideally can be considered as a current source provided with a parallel diode. •The photocurrent IPh is assumed to be proportional to the photon flow incident on the cell. •The Shockley equation for ideal diodes describes the interdependence of current and voltage (current-voltage characteristic) of a solar cell.

The Shockley equation for ideal diodes

I : current flowing through the terminalsIPh : photocurrentI0 : saturation current of the diode e0 represents the elementarycharge (1.6021 10-19 C)U : cell voltage k : Boltzmann constant(1.3806 10-23 J/K)Θ: the temperature.

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Under realistic Under realistic conditions:conditions:•Without irradiation, the solar cell is equal to an ordinary semiconductor diode whose effect is also maintained at the incidence of light.

This is why diode D has been diode D has been connected connected in parallel to the photovoltaic cell in the equivalent circuit diagram.

•Each p-n-junction also has a certain depletion layer capacitancecertain depletion layer capacitance, which is, however, typically neglected for modeling of solar cells.

At increased inverse voltage the depletion layer becomes wider so that the capacitance is reduced similar to stretching the electrodes of a plate capacitor. Thus, solar cells represent variable capacitances whose magnitude depends on the present voltage. This effect is considered by the capacitor C located in parallel to the diode.

•Series resistance resistance RRSS consists consists of the resistance of contacts and cables of the resistance of contacts and cables as well as of the resistance of the semiconductor material itself.

To minimize losses, cables should be provided with a maximum cross-section.•Parallel or shunt resistance Rshunt resistance RPP includes includes the "leakage currents" at the photovoltaic cell edges at which the ideal shunt reaction of the p-n-junction may be reduced.

However, for good mono-crystalline solar cells shunt resistance usually is within the kΩ region and thus has almost no effect on the current-voltage characteristic.

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•At the intercept points of the curve and the axes the short-circuit current ISC is supplied at U = 0 and the open-circuit voltage UOC at I = 0.•Starting with the short-circuit current, the cell current is at first only slightly reduced and decreases over-proportionally shortly before reaching open-circuit voltage when increasing the cell voltage continuously.

•Typical shape of a I-V curve for various operating modes (i.e. changing irradiation and temperature).

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Electric power is defined as the product of voltage and current. a certain point of the characteristic curve the maximum power of the solar cell isReached MPP (Maximum Power Point).the MPP = f(solar radiation and the temperature of the photovoltaic cell)− Photo-current or short-circuit current increases almost linearly with increasingirradiance of the photovoltaic cell. Also, open-circuit voltage is increased however, the increase is logarithmic This correlation is only true if the temperature of the solar cell is kept constant.− If the temperature is increased the diffusion voltage within the p-n-junction isreduced. The open-circuit voltage of a silicon solar cell is, for instance,changed by approximately -2.1 mV/K.

In parallel, the short-circuit current increases by approximately 0.01 %/K due to the enhanced mobility of charge carriers within the semiconductor.

Thus, at increased temperatures the characteristic current-voltage curve of a commercially available silicon solar cell is characterized by a slightly increasing short-circuit current and relatively strong decreasing open-circuit voltage.Cell Cell power power is therefore reduced with increasing temperatures.

Further explanation of I-V curve

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The relation between the maximum power (product of current IMPP and voltageUMPP within the MPP) and the product of open-circuit voltage UOC and short-circuitcurrent ISC is referred to as fill factor FF

Fill factor FF

The fill factor serves as an index for the "quality" of the photovoltaic cell. Highvalues are achieved with good rectifying properties of the p-n-junction (i.e. for alow saturation/latching current I0, at a low series resistance RS and a high parallelresistance RP).

Efficiencies and Losses

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Theoretical efficiencies of various types of simple solar cells under average conditions

Efficiencies and Losses…cont.

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Cell types

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•Due to the energy gap, crystalline silicon is not regarded as an ideal semiconductor material for photovoltaic cells. •Furthermore, silicon is a so-called indirect semiconductor whose absorption coefficient for solar radiation shows relatively low values. •Solar cells made of such semiconductor material must thus be relatively thick; a conventional crystalline silicon cell of simple planar structure, must have a layer thickness of at least 50 μm to nearly completely absorb the incident sunlight. •High layer thickness implies high material consumption and thus high costs.

Nevertheless, crystalline silicon crystalline silicon is commonly used for photovoltaic cellsis commonly used for photovoltaic cells. The main reason: that silicon is the semiconductor material that shows the widest market penetration, that has been theoretically best understood, and that is most easily controlled.

Cell types…cont.

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•Already in the sixties of the last century, a multitude of research and development activities have been conducted to develop cost-efficient thin film solar cells.•For this purpose "direct" semiconductors are required. •This substance category mainly includes II-VI, III-V and I-III-VI2 compounds. •Also amorphous silicon (a-Si), discovered in the 1970's within the scope of photovoltaic projects, is a direct semiconductor.

•It is characterized by good absorption properties and seemed suitable as base material for thin film solar cells.

•Yet, due to still unresolved problems with regard to the competing semiconductor materials or technologies, crystalline silicon (includes crystalline silicon (includes crystalline and polycrystalline technologies) crystalline and polycrystalline technologies) will continue to be predominantly used as base material within the years to come.

Cell types…cont.

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•Since solar cells are (still) relatively expensive, there is a tendency to concentrate solar radiation and thus to reduce the required surface of the photovoltaic cells. •Furthermore, efficiencies of photovoltaic cells tend to increase with increased irradiance – if cell temperature remains constant. •For concentrating systems, more expensive but more efficient solar cell technologies may be applied cost-efficiently.

For instance, mirror and lens systems are used to concentrate solar radiation. •But under these circumstances tracking systems are additionally needed, helping to enhance the energy yield per unit surface. •Such concentrating systems are most suitable for direct radiation (only direct radiation can be focused) and thus for regions throughout the world where the solar radiation is determined by direct irradiance (like in deserts).

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Scheme of the manufacturing steps of a silicon solar cell manufactured accordingto the silk-screen process printing (FSC front side contact; RSC rear side contact

With regard to crystalline cell manufacturing three steps are distinguished:− production of high-purity silicon as base material,− manufacture of wafers or thin films and− solar cell production.

•Silica sand (SiO2) serves as base material for high-purity silicon. •By means of a specific reduction method (melting electrolysis) silica sand is transformed into "metallurgical grade silicon" characterized by a maximum purity of 99 %. •However, this purity is still insufficient for solar cell production.

*Polycrystalline Si*Polycrystalline Si*Monocrystalline Si*Monocrystalline Si*Thin film solar cells made of crystalline silicon(Different process)(Different process)

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Thin-layer amorphous silicon (a-Si:H) solar cells

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Thin film solar cells based on chalcogenides and chalcopyrits, particularly CdTeand CuInSe2 ("CIS").

Layer order of a CdS/CdTe Layer order of

a CdS/Cu(In,Ga)Se2

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Thin film solar cells with integrated serial circuit

Integrated serial connection of the cell strips of a thin film solar cell on the substrate (i.e. the serial connection links the bottom electrode of a cell strip to the top electrode of the next strip)

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Solar cells for concentrating photovoltaic systems:•Solar cells of concentrating photovoltaic systems are illuminated up to 500 times more at standard test conditions (STC) compared to fixed mounted cells.• However, at higher radiation concentrations the serial resistance constitutes a major problem due to the high currents.•This is why concentrator cells must be especially highly doped and be provided with low-loss contacts•Terrestrial concentrating photovoltaic systems are almost exclusively provided with silicon-based solar cells, whose structure is similar to that of the highly efficient silicon solar cells mentioned above. •On a laboratory scale, they reach electrical efficiencies of up to 29 % at 140-fold concentration of the radiation.

For such concentrator systems it is of particular importance to avoid high temperatures, which will cause power losses. Further more it has to be taken into consideration that high concentration factors, in the range of several 100's, do need a two axis tracking system and only direct radiation can be used.

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Dye solar cells made of nano-porous titan oxide (TiO2):

Solar Module

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Solar Module…cont.

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Connection of solar modules inside a photovoltaic generator

Further System Components

• Inverter

• Mounting System

• Battery and Charge Controllers

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Inverter

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Requirement for island inverter:•High efficiency•Low self-consumption•Stable operation behaviour•Sine-shaped output voltage without direct current bias•Coverage of the entire voltage range

Favorable and unfavorable characteristic efficiency curves of island inverters

Other one: Grid connected inverter

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Requirement for Grid connected inverter:− The output current is synchronous to the mains. − The output current should be of sine shape. − Feed-in current and grid voltage should not show any phase shift (cos φ = 1), to prevent reactive power from oscillating between the grid and the inverter that may cause additional losses. − In case of abnormal operating conditions (such as missing or excessive mains voltage, strong deviations from the target frequency, short-circuits or isolation errors) the inverter must automatically disconnect from the grid. − Further safety components such as isolation or earth leakage protection switches suitable for AC and DC currents must be provided in accordance with the inverter concept.− Ripple control signals integrated into the grid voltage by the power supply companies must not be distorted by the inverter nor disturb its operation.− The entry side should be well adapted to the solar generator, e.g. by Maximum Power Point Tracking (MPPT). − Input voltage fluctuations (voltage ripple) should be low (< 3 %) for single-phase inverters that feed energy into the grid at 50 Hz to enable inverter operation at the optimal operating point. − Excess voltages, for instance caused by idle solar generators at low temperatures and high solar radiation, but also by distant lightning strikes, must not cause any defects.− Inverters are generally designed for a slightly lower nominal power than that of the photovoltaic generator (e.g. factor varying from 0.8 to 0.9). − Grid-connected inverters should be supplied with energy by the solar generator itself, so that no energy from the grid is consumed at night-time. − High conversion efficiency should already be achieved for small capacities (> 90 % at 10 % nominal power). approximately 95 and 97 % for larger inverters.− Grid-connected inverters should be provided with integrated self-monitoring systems equipped with user-friendly displays and interfaces to a communication system, if required.

Grid-independent systems

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Schematic of a photovoltaic system for supply of a direct current load or directcurrent consumer application (PV photovoltaic generator, DC direct current)

Block diagram of a hybrid system with direct current and alternating current busbar(PV photovoltaic generator, MG motor generator, WG wind generator, DC direct current,AC alternating current)

photovoltaic systems with battery storage

photovoltaic systems with battery storage and additional power generator (so called hybrid systems).

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Grid-independent Systems…cont.

Grid-connected systems

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Roof mounted photovoltaic generator directly feeding into the public grid

Grid-connected systems…cont.

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Concepts for grid-connected photovoltaic plants

Energy conversion chain

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Energy conversion chain of photovoltaic power generation

Losses

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Energy flow of a photovoltaic generator under Central European conditions(solar cell losses have been assumed as minimum losses under standard test

conditions (STC)

Economic and environmental analysis

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The economic and environmental assessment:*stand-alone photovoltaic systems is much more difficult and largely depends on the respective site conditions.*hybrid systems are largely influenced by the distribution of the generated power among the individual power generating units.

(there is no fixed easily determinable comparison facilitating the economic and environmental comparison)

*grid-connected systems, the parameters mentioned above are generally comparedto power plant alternatives.Reasons: the assessment of stand-alone systems is much more difficult:light generated by photovoltaic solar home systems is often used in replacement of candles, kerosene lamps and lead batteriesstand-alone systems are generally only applied in regions where they have an economic edge over grid extension.

The economic assessment quickly reveals that not so much the power production costs but rather the power distribution cost account for the major share of the consumer end price for this reason, not further discussed.

Economic analysis

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Currently grid-connected photovoltaic power generation:•Mainly roof-mounted systems (3 kWe, installed on a slanting roof) •a system located on a horizontal roof of an industrial building (installed capacity of 20 kW).• To cover the overall market, the analysis will additionally be performed for a 2,000 kW photovoltaic power plant mounted on a steel frame on the ground.

Site I: 800 h/a for sites in North to Central Europe, Site II: to approximately 1,000 h/a for sites in Central to South Europe, Site III: to approximately 1,200 h/a for promising sites in South Europe to North Africa.

Costs

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The major share of the expenditures accounts for module module costs costs : (55 to 65 % of theoverall investments)•Monocrystalline modules: 2,000 and 3,300 €/kW. •Multi-crystalline modules are slightly below (between 1,900 and 3,200 €/kW)

Power Generation Costs

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Parameter variation of the main influencing variables on the specific power production costs of the 20 kW photovoltaic multi-crystalline generator at Site 1

Average specific power production costs of current multi-crystalline photovoltaicgenerators assuming Middle European radiation conditions (Site 1)

Environmental analysis

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Construction: Productions of solar cellIn recent years, they have been discussed primarily in the context of consumption of scarce mineral resources and toxicity.

MonoMono--crystallincrystalline and multimulti--crystallinecrystalline as well as amorphous silicon solar cellsamorphous silicon solar cellsare generally characterized by a low consumption of scarce resources, Cadmium Cadmium telluride (telluride (CdTeCdTe) ) and CISCIS cell technologies show medium mineral resource consumption.

The application of germanium (Ge) appears to be particularly problematic for amorphous silicon cell production; the same applies to indium (In) with regard to CIS cells and tellurium for CdTe cells According to current knowledge only limited quantities of these elements are available on earth

In terms of toxicity:only low environmental effects are expected low environmental effects are expected for crystalline silicon technologies. However, CdTe and CIS cell technologies are considered more problematic due to their high content of cadmium (Cd), selenium (Se), tellurium (Te) and copper (Cu).

In addition, during manufacture of CIS modules gaseoustoxic substances (e.g. hydrogen selenide (H2Se)) may be produced which are generally associated with a certain environmental hazard potential.

Environmental analysis…cont.

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Normal operation:no noise is created and no substances are releasedno major impacts on the locale climate have to be expected.Ground mounted photovoltaic generators inhibit the use of the ground for other purposes. However, only a very small part of the ground is lost for other purposesDue to the relatively large covered surfaces and the highly divergent absorption and reflection conditions when compared to the agricultural cropland impacts on the microclimate are possible in case of intensive photovoltaic utilizationOperation of photovoltaic generators is also related to the transmittance ofelectromagnetic radiation (aspect of electromagnetic compatibility) extensive direct current (DC) cabling

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Environmental analysis…cont.Malfunction:To prevent hazards to humans and the environment due to operational malfunctions of photovoltaic generators, generator failures and inadmissible fault currents must be reliably identified and signalized. The inverter and photovoltaic plant design must allow for power disconnection detection and auto shut-off.

*Fires in buildings, causing solar modules and the building envelope to burn, may cause evaporation of certain components contained in the solar cells.“harmful cadmium (Cd) concentrations to the surrounding air masses can only be reached from plant capacities of 100 kW onwards*Injury hazards due to falling solar modules, improperly mounted onto roof panels or facades, or in consequence of electrical voltages between electrical connections

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End of operation:recycling of solar modules is possible highly sophisticated chemical separation processes are required*Amorphous frameless modules are best suited for recycling, as they may be transferred to hollow glass recycling without any pre-treatment. *Possible recycling methods suitable for "classic" photovoltaic modules include acid separation of solar wafers from the bond, transfer of frameless modules into ferrosilicon suitable for steel production, as well as complete separation of the modules into glass, metals and silicon wafers*Cadmium tellurium (CdTe) and CIS technologies need to be further assessed in order to determine whether their heavy metal content precludes or requires further processing

Environmental analysis…cont.