PV Conversion Theory PDF

Renewable Energy Technologies PV CONVERSION THEORY Bibliography: - documents in moodle - https://www.pveducation.org/  Basics Peripheral electrons may be grouped as: Conduction electrons : Electricity carriers in the conduction...

Renewable Energy Technologies PV CONVERSION THEORY Bibliography: - documents in moodle - https://www.pveducation.org/  Basics Peripheral electrons may be grouped as: Conduction electrons : Electricity carriers in the conduction band. They are free to move and in presence of an electric field they contribute in generating the electric flow. Valence electrons : Limited mobility (just close to the equilibrium) Forming chemical bonds with other atoms Conduction band Electron energy Energy gap Valence band Valence electrons Schematic representation of energy bands for electrons in a solid The band gap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound state into a free state where it can participate in conduction. Electrons are free to move Strong bonds: no electron Some bonds are absent: (conduction) able to move (i.e. no Few electrons and holes condution) Energy of electons Metal: overlapping Insulator: distance Semiconductor: reduced bands between bands distance between bands  Energy gap Conduction band Conducting materials Valence Conduction band and valence band overlap band Conduction band Semiconductors Energy gap is around 1-2 eV Valence band Conduction Insulating materials band Conduction band and valence band are far away one from the other Valence band Conductive materials present a very small bandgap and, in some cases, the energetically possible bands overlap: already at room temperature, numerous electrons occupy the conduction band. Other elements have a very wide bandgap (4 ÷ 5 eV, but also 10 eV) which, at room temperature, does not allow the existence of conduction electrons. For this reason they are defined as insulators. For semiconductors, the bandgap is in the range of 1 ÷ 1.5 eV (1 eV is the work necessary to move 1 electron through a voltage difference of 1 V) It is therefore possible that a valence electron, having received an appropriate amount of energy, exites and hops to the higher energy level, where it is then free to move under the action of any applied electric field. The hole left in the valence band by the electron which passed to the conduction band can be considered to all intents and purposes a positive charge, as it can be also free to move in the presence of an electric field. In semiconductors, therefore, both the electrons in the conduction band and the holes in the valence band participate in the phenomenon of electrical conduction. In this case the concentration of the negative charges (conduction electrons) is equal to the concentration of the positive charges (conduction holes) and is called the intrinsic carriers concentration (ni). In intrinsic semiconductors, that is absolutely pure, electrons and conduction holes are formed only due to the breaking of the bonds; in terms of the band diagram, the energy required to break a bond corresponds to the width of the bandgap or energy gap Eg. Material Energy [eV] Material Energy [eV] ENERGY Si 1.12 A-Si 1.76 GAP Ge 0.62 GaInP 1.88 CuInSe2 1.05 Cu2O 2.1 InP 1.22 Se 2.2 GaAs 1.42 GaP 2.25 CdTe 1.45 CdS 2.4 Experimental data show that the concentration of intrinsic carriers ni increases very rapidly with increasing temperature (and therefore with the increase in the vibrational energy of the lattice) and that, for a given temperature, ni decreases very rapidly with increasing bandgap. The dependence of the concentration of intrinsic carriers on Eg and on temperature follows the following relationship: Concentration of intrinsic carriers (effect of temperature) where the activation energy Ea is approximately equal to Eg / 2, k = 1.38 10-23J / K is the Boltzman constant T is the absolute temperature. At 300 K: the concentration of intrinsic carriers in silicon ni = 1.5 1010cm-3; in gallium arsenide ni = 1.8 106cm-3. Let us make this calculation: Concentration of silicon atoms in the crystalline solid  Doping By adding impurities to the crystal, the semiconductor can increase the amount of electrons in the conduction band or holes in the valence band. Hence we get extrinsic semiconductors of type “n” (one more valence electron) and type “p” (one less valence electron) and the conduction is due to the presence of impurities. When silicon crystals are doped with materials such as phosphorus and boron: the phosphorus, inserted in the crystal lattice, leads to one more valence electron while boron has one less valence electron. Phosphorus belongs to group V of the periodic system of elements; its atoms are called donors. Boron belongs to group III of the periodic system of elements; its atoms are called acceptors. silicon extra electron hole bond phosphorus boron ionized acceptor ionized donor Schematic representation of valence bonds: a) in a cristal of pure silicon; b) in a cristal of doped n type silicon; c) in a cristal of doped p type silicon In the case of phosphorus-doped silicon, the ionization energy of the phosphorus in the silicon is much smaller than the energy to be supplied to the silicon to allow an electron to pass in the conduction band exceeding the energy gap It is the same for boron-doped silicon Bands diagram and electron-hole distribution in semiconductors silicon extra electron hole bond phosphorus boron ionized acceptor ionized donor  Energy level Probability that a certain energy level E is occupied by an electron at temp T  f ( E )  1 1  exp( E  E f ) / kT  Distribution function by Fermi-Dirac where k is Boltzman's constant, T is the absolute temperature, while the parameter Ef, called Fermi energy level, is the energy at which the probability that an electron occupies this energy state is equal to ½. In an intrinsic semiconductor the number of energy states is approximately the same for the two possible bands, moreover the number of electrons in the conduction band is equal to the number of holes in the valence band. This means that, since the Fermi - Dirac function is symmetric with respect to the Fermi level, Ef must be in the middle of the bandgap. By doping with "n" type atoms, the concentration of electrons in the conduction band is increased which, however, has the same density of energy states as the starting semiconductor. There is, therefore, an increase in the Fermi energy level and with this an upward shift of the entire distribution function. In the case of a “p” type semiconductor, Ef and the distribution function moves downwards. In conditions of thermodynamic equilibrium, two semiconductors, one of type "p" and the other of type "n" assume the same Fermi energy level.   f ( E )  1 1  exp( E  E f ) / kT  Distribution function by Fermi-Dirac Probability that a certain energy level E is occupied by an electron at temp T intrinsic n-type p-type Distribution function by Fermi-Dirac for intrinsic, n-type and p-type semiconductor  Conduction band Valence band Temperature Fermi energy level in silicon vs. temperature at various concentration of impurities  p-n junction formation and resulting band structure If a semiconductor material (for example silicon) incorporates dopant atoms of the "p" type (boron) on one side and "n" type atoms (phosphorus) on the other side, we get a junction. The two layers of material, originally electrically neutral, through the contact form an electric field. Due to the charge concentration gradient in the region near the junction, there is a simultaneous diffusion of electrons in "p" and holes in "n". The diffusion current creates a potential barrier between the two regions by charging "p" negatively and "n" positively The resulting voltage difference tends to oppose the motion of the charges until a condition of electrostatic equilibrium is reached. As a result of the opposite flows of the charges, the energy levels of the valence and conduction bands of the region doped with atoms of type "p" rise with respect to those in the material "n", since the energy level of Fermi, which originally was higher in the “n” type material, must remain constant across the junction in a condition of thermodynamic equilibrium. The result is a distortion of the bands along the junction. The internal electric field produces the diode effect p-type n-type This figure shows the band diagram of a p-n junction in equilibrium and when an external voltage is applied to the diode: a. with no voltage applied, there is no current across the junction; b. If a potential difference is applied across the junction in a condition of direct polarization, the electric field is cancelled and there is an exponential increase in the current; c. if, on the other hand, a potential difference is applied across the junction in a condition of reverse polarization, the internal electric field becomes more intense and no current flows (except for those few electrons which, when thermally excited, are able to overcome the barrier constituted by the junction and the applied voltage). The junction therefore acts as a diode. n n p p a. b. c. I-V characteristic curve of a diode I  I O expqV kT   1 I0 dark saturation current, q electron charge, V voltage, k Boltzmann constant, T absolute temperature When a photon reaches the semiconductor, if it has enough energy it can be absorbed and makes possible that an electron exites passing from the valence band to the conduction band. Since a hole remains in the valence band, the absorption process generates electron-hole pairs. Generation of electron-hole pair by light About 4.4 x 1017 photons reach 1 cm2 of terrestrial surface each second Photon energy E=hc/ Electron energy It must be E > Eg h: Planck constant 6.6 x 10-34 J s c: light velocity 3 x 108 m/s  Photon energy with wavelength  = 1,1 mm = 1,1 · 10-6 m: Being: 1 eV = 1,6·10-19 J E = h c /   6,6 · 10-34 · 3·108 / 1,1·10-6 = 1,8·10-19 = 1,12 eV In the case of silicon, wavelength of solar radiation must be lower or equal to 1,1 mm (max) Wavelength required to reach the energy gap Material Energy [eV]  max [mm] Material Energy [eV]  max [mm] Si 1.12 1.1 A-Si 1.76 0.70 Ge 0.62 2 GaInP 1.88 0.66 CuInSe2 1.05 1.18 Cu2O 2.1 0.59 InP 1.22 1.01 Se 2.2 0.56 GaAs 1.42 0.87 GaP 2.25 0.55 CdTe 1.45 0.65 CdS 2.4 0.52  Photon energy Only photons with E > Eg are able to convert solar energy into electric energy in the PV cell. Hence the semiconductor can convert only part of solar energy into electric energy. photon energy available energy in Photon energy [eV] curve the photoelectric E=hc/ effect of silicon lost energy E>Eg lost energy E Eg. Once generated the exceeding energy is transferred into heat and cannot be converted into electricity. This is the main mechanisms of energy loss in a PV cell. Generation of electron-hole pair by light  Preliminary evaluation of electric power Considering ideal conditions, the electric current which can be generated IL can be calculated as: IL = q N A N number of photons with energy higher than energy gap A exposed area q charge of electron For the whole solar spectrum on Earth JL = IL /A = 1.6 10-19  4.4 1017 = 70 mA/cm2 An upper limit value can be given for the voltage generated by a cell V = Eg / q  The solar cell A solar cell consists by the junction between a semiconductor material of type p and one of type n which produces an internal electric field capable of separating the electrical charges created by the absorption of light. ANTI-REFLECTIVE LAYER Type n material SUNLIGHT Junction Type p material REAR METALLIC CONTACTS FRONT CONTACTS Picture of a solar cell On the left: material type p (base) is negatively charged (–) On the right: material type n (emitter) is positively charged (+) - + The sunlight generates the electron- hole pair on both sides of the junction The generated carriers are swept away by the electric field, thus producing current across the device. Note how the electric current of electrons and holes reinforce each other since these particles carry opposite charges. The p-n The p-n junction separates FERMI ENERGY the carriers with opposite LEVEL charge and transforms the generation current IL between the bands into an electric current across the p-n junction. LIGHT GENERATED CURRENT I-V characteristic of a solar cell compared to a diode I Note that, under open circuit (I = 0), all the light-generated current passes through the diode. qV I = IL - ID = I = IL - I0 exp -1 k T I0 dark saturation current Note that, under open circuit (I = 0), all the light-generated current passes through the diode (IL = ID ). Under short circuit (V= 0) on the other hand, all this current passes through the external load (IL = I ). Open-circuit voltage Voc is obtained by setting I = 0: Voc = k T ln IL +1 q I0  Electric characteristic of the solar cell Currents in a p-n junction under illumination The cell can be drawned as an equivalent circuit. The generation of the current IL by light is represented by the current generator in parallel with the diod (p-n junction). The resulting current I can be calculated as: I = IL - ID equivalent circuit Rs: Series resistances Ohm resistances in metal and semiconductor + contacts resistances Rc: shunt resistance (manufacturing defects)  The IV curve Without illumination a solar cell has the same When light shines on the cell, the IV curve electrical characteristics as a large diode shifts as the cell begins to generate power The greater the light intensity the The solar cell curve is flipped by greater the amount of shift convention Characteristic curve I-V and maximum power generated (peak power Pp) Isc shortcut current (with V=0) Peak power generated by the cell: PP = I M VM PP = FF Isc Voc Voc open-circuit voltage (with I=0) Fill-Factor FF: FF = PP /( Isc Voc ) The fill-factor FF is a parameter which may explain the efficiency of the cell. It is the maximum available power related to the ideal power. For silicon cells FF in standard conditions is about 0.75-0.80 For a common silicon cell the peak power in standard conditions is at circa 0.5 V. Photovoltaic solar cell: real performance Main unefficiencies in solar cells: Series resistance and its effect on the I-V characteristic Impurities or defects in the structure (recombination) which may annihilate the pair electron-hole Surface reflection and shading of the top contacts Series resistances: a. the movement of current through the emitter and base of the solar cell; b. the contact resistance between the metal contact and the silicon; c. the resistance of the top and rear metal contacts. The main impact of series resistance is to reduce the fill factor, although excessively high values may also reduce the short-circuit current. Efficiency of a solar cell: Curve I-V of a solar cell and Pp Input: Solar radiation G Solar cell: area A Output: Electric power (Pp) Efficiency: (Power output) / (Power input ) η = (VM IM ) / (G A) To compare different cells there is the need to define standard conditions, in order to consider the same reference input (same denominator): η = (VM IM )STC / (GSTC A)  Conversion efficiency of a cell Monochromatic radiation [W/(m2 mm)] Wavelength [mm]  Direct gap and indirect gap semiconductors Optical absorption properties of direct- and indirect-gap semiconductors: fraction of light with E>Eg absorbed  absorption coefficient = 1 – exp(-  s) Absorption constant of semiconductor materials Effect of temperature and solar radiation  Solar cell efficiency Power losses in silicon solar cells. The figures are per square centimetre for production cells and (in brackets) for laboratory cells  Peak power and cell efficiency P= VM IM η = VM IM / (GG A) η stc= PSTC / (GSTC A) The reference standard conditions (STC) to define peak power and cell efficiency: irradiance equal to 1000 W/m2, cell temperature equal to 25 °C spectral irradiance as defined by the standard IEC 60904-3 (AM 1,5) NOCT – Nominal Operating Cell Temperature It is defined as the temperature reached by the cells in a module operating in open circuit conditions, in the following standardized operating mode: Global irradiance on the cell surface equal to 800 W/m2 External air temperature: 20 °C Wind speed (parallel to the PV module plane): 1 m/s All sides exposed to the wind Under the same mounting conditions, but under different operating conditions, the cell temperature can be calculated with the equation (Ross): NOCT  20 Tcella  Taria  G cell air 800 Temperatures in [°C] Irradiance in [W/m2]  Solar conversion in a single junction cell A single junction solar cell is a type of solar cell that is made from a single layer of semiconductor material, such as silicon. Solar cells are devices that are able to convert sunlight into electricity through the photovoltaic effect. Single junction solar cells are able to absorb light over a wide range of wavelengths, but they are most efficient at converting light in a specific range of wavelengths. As a result, they are typically able to convert between 15% and 20% of the energy in sunlight into electricity, depending on the specific material and design of the cell. Single junction solar cells are relatively simple to manufacture and are widely available. They are an important part of the global renewable energy industry and are used in a variety of applications around the world. A single junction gallium arsenide (GaAs) solar cell is a type of solar cell that is made from a single layer of gallium arsenide, a semiconductor material. GaAs solar cells are able to absorb and convert light over a wide range of wavelengths, but they are most efficient at converting light in the near- infrared (NIR) range of the electromagnetic spectrum. There is the possibility to absorb and convert light over a wider range of wavelengths trying to convert more energy in the infrared (IR) range of the electromagnetic spectrum.  Solar conversion in a multi-junction cell Multi-junction solar cells are a type of tandem solar cell, meaning they are made of stacked materials that are optimized to absorb different frequencies of sunlight. The photovoltaic effect can be summarized in three main steps: Light is absorbed by solar cells and electrons in the semiconducting material are knocked loose Loose electrons flow through the p-n junction between semiconductor layers, creating an electrical current The current is captured and transferred to wires Efficiency about 32% today (possible increasing efficiency). High costs: used in aerospace Triple junction cell (GaInP/GaAs/Ge) Multijunction nanowire (InxGa1-xN/Si/SixGe1-x Heterojunction The first practical solar cell - 1954 Market of PV Sharing of PV cells by type in 2008 Source: Photon International, 2008 Sharing of PV cells by type in the last 30 years Crystalline silicon Polysilicon is refined into either single crystal polysilicon (Mono-Crystalline polysilicon) or Multi-Crystalline (also called polycrystalline) polysilicon.  Multicrystalline silicon for the production of monocrystalline silicon using the Czochralski method (lenght 10 cm, diam 4 cm) Monocrystalline silicon rod for ingot growth with process Czochralski (lenght 14 cm, diam 1.2 cm) Process Czochralski to produce monocrystalline silicon Silicon ingot (single crystal) From here, wafers are obtained with an average thickness of 300 µm and with a diameter of up to 10” (≈25 cm). Costs and energy sharing in Silicon PV Costs of PV Source: NREL Market of PV Different approaches to PV CPV – Concentrating photovoltaics  Thin film technologies microcrystalline silicon (m-Si) amorfous silicon (a-Si) cadmium telluride (CdTe) copper indium gallium selenide (CIS / CIGS) Thin film cells  Emerging technologies Perovskite Solar Cells A perovskite solar cell is a type of solar cell which includes a perovskite-structured compound. A perovskite is any material with a crystal structure similar to the mineral called perovskite, which consists of calcium titanium oxide (CaTiO3). Perovskite materials are cheap to produce and simple to manufacture. While the currently used silicon solar panels are about 180 mm thick, perovskite solar cells are about 1 mm thick. Cells in this material can be made by spreading the pigment on a glass or metal sheet, using a few other layers of materials to facilitate the movement of electrons through the cell. Solar cell efficiencies of laboratory-scale devices using these materials have increased to 25% in 2020 in single-junction architectures, and, in silicon-based tandem cells, to 29%, exceeding the maximum efficiency achieved in single-junction silicon solar cells. Perovskite solar cells are therefore a fast- advancing solar technology. With the potential of achieving even higher efficiencies and very low production costs, perovskite solar cells have become commercially attractive. Core problems and research subjects include their short- and long-term stability.  Emerging technologies Quantum Dot Cells A quantum dot solar cell (QDSC) is a solar cell design that uses quantum dots as the absorbing photovoltaic material. It attempts to replace bulk materials such as silicon, copper indium gallium selenide (CIGS) or cadmium telluride (CdTe). Quantum dots have bandgaps that are tunable across a wide range of energy levels by changing their size. In bulk materials, the bandgap is fixed by the choice of material(s). This property makes quantum dots attractive for multi- junction solar cells, where a variety of materials are used to improve efficiency by harvesting multiple portions of the solar spectrum. In their simplest form they are based on a silicon substrate covered with a very thin layer of quantum nanocrystals (or quantum dots). Nanostructures composed of silicon quantum dots of different sizes - each of which interacts with a certain frequency of the solar spectrum - can make it possible to exploit a large portion of the spectrum of sunlight with great efficiency, similar to multi-junction cells.  PV Technologies www.nrel.gov/pv/cell-efficiency.html PV Technologies  Renewable energy technologies Design of photovoltaic systems Dipartimento di Ingegneria Industriale Università di Padova Design of PV systems grid connected stand alone Site analysis Energy consumption analysis Calculation of user needs Sizing of the PV generator Expected production and necessary surface System diagram Choice of the inverter Sizing of the connection cables Safety devices Charge control and regulation Interface with the network Storage system Design of grid-connected systems  Site analysis: inspection and shading  Sizing of the PV generator Determine peak power Select modules Determine voltage  Expected production  System diagram (type of conversion)  Choice of the inverter  Sizing of the connection cables  Protection devices  Interface with the network Grid-connected systems Low Voltage Network counter from network  The system is counter from network connected to the counter distribution network.  It is necessary to interface with a bidirectional energy counter. 4 Grid-connected systems When the solar system produces more energy than is needed at that time, the surplus enters the electricity grid of the local electricity distributor and is measured by a second dedicated meter. Counter of Bidiretional PV modules 3 produced counter 1 2 Inverter energy 5 Electric line < FV Energy from PV Energy from grid 4 User 5 Grid-connected systems When the solar system does not produce (at night) or produces little (bad weather) or in any case the current required by the user is greater than that which can be supplied at that moment by the PV system, the current from the network is used, as given by the dedicated counter. Contatore Contatore Moduli 3 energia bidirezionale 1 fotovoltaici 2 Inverter prodotta 6 Electric line < FV Energy from PV 4 Utenze Energy from grid locali 6 Site inspection Having assessed the customer's willingness to install a system of given power, it is necessary to carry out a detailed economic analysis based on the characteristics of the site. For the site to be "efficient" it must be verified that this is oriented between SOUTH - EAST and SOUTH – WEST Or there is the possibility of installing two photovoltaic fields arranged relative to WEST and EAST Tools needed: Compass Site plan 7 Estimation of incident solar energy  Approximate data  Estimation of the energy incident on an inclined plane based on the latitude of the site and data in the standards (UNI 10349, UNI / TR 11328-1)  Based on geographic-astronomical calculations  Based on historical data 8 Estimation of incident solar energy (isoradiative curves) 9 Estimation of incident solar energy Transposition factors Coefficients to be applied to solar radiation on a horizontal surface to obtain solar radiation on differently oriented and inclined surfaces (Northern Italy: lat.44 ° N) 10 Estimation of incident solar energy Transposition factors Central Italy: lat. 41°N Southern Italy: lat. 38°N 11 Shading Shading is a percentage of direct solar energy that cannot be exploited due to the presence of natural elements (mountains and hills) or artificial elements (buildings) The shading of the modules has a double negative effect: - it reduces the irradiation on a part of the photovoltaic array and therefore its production - it causes asymmetry phenomena, in modules and between strings 12 Shading To identify any shading you must have: - a compass with which the direction of the obstacle is determined with respect to the south (angle ) - a clinometer to measure the angle  with respect to the horizontal - a diagram of the solar path Shading The solar diagram indicates the position of the sun (angle  of elevation on the horizontal and angle  with respect to south) during the hours of the day Shading Shading Once the possible shading has been identified, the arrangement of the modules must be established to: - minimize shadows - configure the system scheme to minimize the effect of unavoidable shading (for example, connect strings with the same irradiation conditions to the same inverter). System power and required surface area The choice of the nominal power of the system depends on: -system destination -economic resources -available area It is necessary to add some area to the surface occupied by the modules (net surface) to avoid shading (gross surface) Choice of modules and voltage The choice of modules includes: - identification of the type of material (crystalline or amorphous silicon, CdTe, CIS, etc) - identification of the type of module The cost of the modules available on the market affects the choice of the module itself. Large modules (e.g. 250 W) mean fewer anchoring points and fewer cabling, but also imply less flexibility The power of the system determines the number of modules needed The choice of the system voltage identifies the number of modules that make up the single string Criteria to choose the voltage - regulatory aspects (may be better low voltage) - containment of energy dissipation (better high voltage) - adaptation to the electrical characteristics of the other system components Centralized conversion String conversion Yearly production PV generator, nom power 1 kWe Irradiance < 1000 W/m2 Dirt on the modules Energy losses of Effect of temperature Shading BOS DC-side losses Balance of System (mismatch, wires) Defects in tracking the maximum power point Inverter losses Other AC-side losses Yearly production ηBOS = Ereal / Eideal Eideal = Irad A ηmod E (kWh), I (kWh m-2) ηBOS = 0.7 – 0.85 kWh / kWp ηmod = Pnom/Prad = Pnom(kW) / [1 (kW/m2)A(m2) ] = Pnom(kW)/A(m2) Thus: Pnom(kW) = ηmod A(m2) From definition of BOS: Ereal = ηBOS Eideal = ηBOS Irad A ηmod Ereal(kWh)/Pnom(kWp) = ηBOS Irad(kWh m-2) Example: Photovoltaic system located in Brindisi (40.65 ° N) connected to the grid http://re.jrc.ec.europa. eu/pvgis/index.htm Energy production Stand-alone systems Stand alone PV systems are ideal for remote rural areas and applications where other power sources are either impractical or are unavailable to provide power for lighting, appliances and other uses. In these cases, it is more cost effective to install a single stand alone PV system than pay the costs of having the local electricity company extend their power lines and cables directly to the home as part of a grid connected PV system.  Service: pumping, powering repeaters, battery chargers for boats, campers, small electrical appliances in DC...  Isolated users: domestic users in DC or AC with the inclusion of an inverter  Mini grids: moderate power (tens of kW) AC system, also three-phase and with a diesel-electric backup (hybrid groups). Small distribution (islands).  Aerospace systems for satellite power supplies  Sites where the electric grid cannot be connected (natural parks...) Stand-alone systems PHOTOVOLTAIC GENERATOR DC STORAGE BATTERY DC CHARGE CONTROL DC (CHARGE REGULATOR, MPPT) AC ALTERNATING CURRENT LOAD DIRECT CURRENT LOAD Stand-alone systems Stand-alone systems Batteries – Batteries are an important element in any stand alone PV system. Batteries are used to store the solar-produced electricity for night time or emergency use during the day. Depending upon the solar array configuration, battery banks can be of 12V, 24V or 48V and many hundreds of amperes in total. Charge Controller – A charge controller regulates and controls the output from the solar array to prevent the batteries from being over charged (or over discharged) by dissipating the excess power into a load resistance. Fuses and Isolation Switches – These allow PV installations to be protected from accidental shorting of wires allowing power from the PV modules and system to be turned “OFF” when not required saving energy and improving battery life. Inverter – Inverters are used to convert the 12V, 24V or 48 Volts direct current (DC) power from the solar array and batteries into an alternating current (AC) electricity and power of either 120 VAC or 240 VAC for use in the home to power AC mains appliances such as TV’s, washing machines, freezers, etc. Wiring – The final component required in and PV solar system is the electrical wiring. The cables need to be correctly rated for the voltage and power requirements. Design of stand-alone systems  Calculation of user needs  Sizing of the PV generator Production and surface estimation Choice of nominal voltage Electrical configuration of the generator  Charge control and regulation  Cable sizing  Storage system Electricity requirement of the user Electric loads Daily use (hours) Daily consumption (Wh) Analysis of the loads of a residential user Design steps Analysis of the loads Definition of the PV surface area based on the lowest irradiation Choice of nominal voltage Electrical configuration of the PV generator Charge control and regulation Storage system Charge control and regulation Charge control is a fundamental element for the long- term reliability of isolated PV systems Function: protect the accumulators from deep discharge phenomena and from overload phenomena Methods: - ON/OFF regulation - PWM regulation - MPPT regulation Pulse-width modulation (PWM) is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. Along with maximum power point tracking (MPPT), it is one of the primary methods of reducing the output of solar panels to that which can be utilized by a battery. Charge control devices control the energy coming from solar panels. It adjusts the current and the voltage then send it to batteries. A charge control device prevents over charge and over discharge of the batteries. Therefore, it protects the system. Each solar energy system requires a charge control device. Charge control devices divided into two as PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). The main difference between PWM and MPPT charge control devices is that the MPPT devices are more efficient. MPPT charge control devices have 30 % more efficient in charge efficiency according to PWM type. Storage system Lead acid batteries for solar use: with these batteries, we must be careful to avoid: - excessive voltage in the charging phase (to avoid corrosion phenomena) - too low voltage during discharge (risk of corrosion) - deep discharge (sulfation) - prolonged periods without a full charge (sulfation) In order to prevent excessive battery discharge, it is necessary to limit the depth of discharge PDmax which depends on the type of battery. The actual capacity of the storage Cu will be lower than the nominal one Cb Cu = Cb * PDmax Lead batteries PD = 0,5 – 0,7 Lithium batteries PD = 0,8 The actual capacity also depends on the discharge time: C20 is capacity measured over 20 hours of discharge time as rules of thumb C100 =1,25*C20 ; C40 =1,14*C20 Electric storage for photovoltaic systems Energy can be exchanged with the grid, feeding it when the system produces, then withdrawing it when the user requests electricity. The mechanism that allows all this is called “Scambio sul posto (exchange on the spot)": however, in this "exchange" operation, energy loses its economic value compared to when it is consumed without passing it through the network. One way to increase self-produced solar electricity is to have an energy storage system, that is, a battery or electrochemical storage system. Generally (rule of thumb), the percentage of self-consumption from a PV system is around 30%, but with the integration of a battery system it can even reach up to 80%. How to install the storage DC SIDE AC SIDE Benefits of storage - electricity consumption profile This is an «average» ideal case In practice: - the daily trend in consumption may differ from day to day - then there is a seasonal variation A numerical example follows in the next slides Profile of electricity Consumption PV power consumption PV production Storage size Average season consumption production from the grid to the grid Day of March consumption production from the grid to the grid Profile of electricity consumption Day of March – storage size 2 kWh consumption production from the grid to the grid Day of March – storage size 3,7 kWh consumption production from the grid to the grid Profile of electricity consumption Day in November consumption production from the grid to the grid Day in July consumption production from the grid to the grid Energy input into the grid / energy absorption from the grid consumption production from the grid to the grid kWh Savings on the bill Yearly consumption [kwh/year] Yearly savings [Euro/year] kWh Size of the storage system The costs for lead-acid batteries are between € 400-700 per kWh of capacity. Prices are expected to remain stable over the next few years. The costs for lithium batteries can be estimated in a range of € 900-1,600 per kWh of capacity, but they will have a drop in costs in the coming years. Renewable Energy Technologies PV COMPONENTS  Panel multiple modules assembled in a common structure String Cell Module set of panels / modules connected in series Photovoltaic generator set of strings connected in parallel to obtain the desired power PV technology Photovoltaic module Photovoltaic module Photovoltaic module with monocrystalline with polycrystalline in amorphous silicon silicon cells silicon cells  = 5-8 %  = 24-25 %  = 20-23 %  Polycrystalline silicon: wafer of 230 mm – 350 mm. During solidification phase crystals arrange randomly and this has as effect the iridescent aspect. Thickness 230-350 mm. Dimensions: 125 mm by 125 mm; 156 mm by 156 mm; 210 mm by 210 mm Monocrystalline silicon: wafer of 200 mm – 300 mm. They are shaped so as to diminish the unused space and maximize the amount of cells installed in a module. Thickness 200-300 mm. Dimensions: 100 mm by 100 mm; 125 mm by 125 mm; 156 mm by 156 mm Amorphous silicon: Messy distribution of the structure. In this case there are no cells, but the material covers wide surfaces (larger than the usual cells). Possibility to create different shapes. Thickness 1 mm. Dimensions: 2 m by 3 m Thin film: Composed of photon-absorbing material layers deposited over a flexible substrate. Thickness 2 mm.  Crystalline silicon Mature and reliable production technology with an image of safe product towards the market Very low electrical performance degradation over time (many manufacturers guarantee 80% performance over 25 years of useful life) Conversion efficiency of 23% (single crystal on the market) Ability to create "custom" modules (i.e. tailored to the customer) using commercial cells Poor cost reduction margins for the automation of the production process Aesthetic appearance of the module not entirely satisfactory for architectural use Generally rigid or semi-flexible modules with weight not always insignificant  Amorphous silicon It is possible to obtain modules on multiple substrates (also flexible) Aesthetic appearance of the product suitable for architectural use with the possibility, in the case of polymeric substrates, of having low weight products Possibility of creating photovoltaic elements to replace typical elements of the construction sector (tiles, corrugated sheets) Cost reduction prospects for both the automation of the production process and its refinement Possibility of obtaining uniform “photovoltaic” windows with 20-40- 60% transparency Degradation of electrical performance in the first 100 hours of exposure of about 10-15% Module efficiency around 6% after initial degradation. Useful life not yet tested for a period of more than 20 years An amorphous Si photovoltaic generator occupies about twice the surface area of an equivalent crystalline Si system Efficiency Comparison of Technologies Best Lab Cells vs. Best Lab Modules Source: Fraunhofer ISE Current efficiencies and power commercial PV modules Sorted by technology  Energy pay-back time in PV systems Years  Energy pay-back time of PV systems Years Energy pay-back time in PV systems EPBT:the lower, the better Irradiation: the higher, the better Grid efficiency: the higher, the better Source: Fraunhofer ISE annual report 2024  Typical cross section of a photovoltaic module Aluminu m frame Glass Seala nt Seala nt Photovoltaic cell  Glass or other material with high transmission (low ferrous content) and as little reflection as possible. Internal texturing increases efficiency Ethylene vinyl acetate (EVA) encapsulant to make the PV cells adhere to the front and back of the module. It must be The back must have low thermal stable at high resistance, and be impermeable temperatures and to moisture and dust. In some optically transparent double-sided modules it is optically transparent The photovoltaic modules are composed of various components that surround the solar cells, whose function is to protect them from external agents. Such is the case of the EVA encapsulant. EVA is ethylene vinyl acetate, a material that has good radiation transmission and low degradability to sunlight. This is a thermoplastic polymer, which is used in solar modules as an encapsulating agent since, by applying heat to the assembly, it forms a sealing and insulating film around the solar cells. It prevents the entry of air and the formation of moisture, lets the sun’s energy go through while being resistant to sunlight degradation over time. If the EVA is of poor quality or the lamination process was not done correctly, the silicon wafer will come into contact with water or air (both very rich in oxygen) and the panel will begin to oxidize, looking yellow or brown depending on the case. The panel has lower efficiency, which will reduce the efficiency of the entire system.  I-V characteristic: effect of irradiance I As the solar radiation decreases, there is a significant decrease in the maximum deliverable current The open-circuit voltage is affected to a lesser extent To null the open circuit voltage it is necessary to completely darken the cell 15 I-V characteristic: effect of temperature The open circuit voltage decreases by 2,3 mV/°C Simultaneously the short circuit current increases by 0,2%/°C These two phenomena (with opposite sign) lead to a decrease of performance when the cell temperature increases 16  PV module Current [A] Voltage [V] I-V characteristic curve at 40 ° C of a 50 Wp commercial module consisting of 36 monocrystalline silicon cells connected in series Connection of PV modules  Mismatch for Cells Connected in series As most PV modules are series-connected, series mismatches are the most common type of mismatch encountered. Of the two simplest types of mismatch considered (mismatch in short-circuit current or in open-circuit voltage), a mismatch in the short-circuit current is more common, as it can easily be caused by shading part of the module. This type of mismatch is also the most severe. For two cells connected in series, the current through the two cells is the same. The total voltage produced is the sum of the individual cell voltages. Since the current must be the same, a mismatch in current means that the total current from the configuration is equal to the lowest current. Variation of the operating parameters of a mc-Si photovoltaic module as the temperature varies Temperature increases and related energy losses for typical photovoltaic module installations Efficiency as a function of cell temperature (irradiance and air mass are constant) NOCT – Nominal Operating Cell Temperature It is defined as the temperature reached by the cells in a module operating in open circuit conditions, in the following standardized operating mode: Global irradiance on the cell surface equal to 800 W/m2 External air temperature: 20 °C Wind speed (parallel to the PV module plane): 1 m/s All sides exposed to the wind Under the same mounting conditions, but under different operating conditions, the cell temperature can be calculated with the equation (Ross): NOCT − 20 Tcella = Taria + G cell air 800 Temperatures in [°C] Irradiance in [W/m2] Efficiency as a function of air mass (irradiance and cell temperature are constant) Efficiency as a function of irradiance (air mass and cell temperature are constant) Example of datasheet 26  25 °C CELL TEMPERATURE 25 °C EXTERNAL AIR TEMPERATURE  5 °C EXTERNAL AIR TEMPERATURE 25 °C EXTERNAL AIR TEMPERATURE  Panel multiple modules assembled in a common structure String Cell Module set of panels / modules connected in series Photovoltaic generator set of strings connected in parallel to obtain the desired power  Shaded solar cells One of the factors which affect the output and efficiency is the fully or partially shading of solar panels due to clouds, trees, leaves, building etc. In this case, some of the photovoltaic cells are not able to generate power as they are not exposed to the direct sunlight. In this scenario, the affected cells act as a load and may be damaged due to hot-spot. That is the reason why we need a bypass diode in a solar panel.  Solar panel junction box https://www.electricaltechnology.org/2019/10/blocking-bypass-diode-solar-panel-junction-box.html  Shading: bypass diodes Bypass diode in a solar panel is used to protect partially shaded photovoltaic cells array inside solar panel from the normally operated no shading photovoltaic string. The faulty panel or string is bypassed by the diode with one bypass diode for 18 cells which provides alternative path to the flowing current from solar panels to the without bypass diode load.  Shading of a photovoltaic system Each module is a series of elementary photovoltaic cells. The PV field is made up of strings, i.e. modules connected in series The photovoltaic cells that make up a string are connected in series. The darkening of a cell involves a diode inversely polarized on the series The obscurity of a single cell strongly penalizes the behavior of an entire string 38  Shading Once the possible shading has been identified, the arrangement of the modules must be established to: minimize shadows configure the system scheme to minimize the effect of unavoidable shading (for example, connect strings with the same irradiation conditions to the same inverter).  Temporary shading Horizontal arrangement Vertical arrangement  Arrangement of modules SHADO W a) half module is excluded b) both modules are excluded  Connection of modules 4 a) the shadow affects only the string 3 b) the shadow affects only the string C  Inclined modules on parallel rows Sun at solar noon on 21 December Distance D between rows of modules such as to avoid shadow at midday on December 21st (winter solstice) Inclined modules on parallel rows  PV generator Blocking diode Load Battery Photovoltaic system with electrochemical storage Blocking diode in a solar panel is used to prevent the batteries from draining or discharging back through the PV cells inside the solar panel as they act as load in night or in case of fully covered sky by clouds.  PV generator Load Battery Photovoltaic system with battery and DC/DC converter A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. INVERTER Output power conditioning and control system capable of converting a continuous (DC) to alternating (AC) (power) signal by controlling the 'power quality' (voltage, current, frequency, harmonic content). A solar inverter or PV inverter, is a type of electrical converter which converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical balance of system (BOS) component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. DC AC There are several existing technologies, differentiated in the electronics used, depending on the final use: car inverter (50 W) … inverter for power conversion (5000 W) INVERTER – Application to PV Need to adapt the generator and load through the instant-by-instant identification of the pair of I and V values ​of the generator that allows you to work at maximum power (Maximum Power Point Tracking). The inverter is equipped with a chopper (DC regulator) tracker that identifies the points of tangency between the I-V characteristic curve of the photovoltaic generator and the hyperbola with constant I-V product. The MPPT devices identify the point of V x I = cost maximum power on the I-V characteristic curve of the generator causing small load I variations at regular intervals that Icc determine deviations in the voltage and current values, evaluating whether the new I-V product is higher or lower than the previous one. If there is an increase, the load conditions continue to vary in the direction considered. Otherwise, the conditions are changed in the opposite direction V0 V Due to the required performance characteristics, the inverters for stand-alone systems and for systems connected to the distribution network must have different characteristics: in the stand alone systems, the inverters must be able to supply an AC voltage as constant as possible as the generator output and load demand vary; in the grid-connected systems, the inverters must reproduce, as faithfully as possible, the grid voltage, while trying to optimize and maximize the energy production of the photovoltaic modules. Characteristics of commercial inverters Efficiency of a PWM inverter as a function of the load - The inverters or power converters don’t operate always at their maximum efficiency, but according to an efficiency profile as function of the Power. - The "European Efficiency" is an averaged operating efficiency over a yearly power distribution corresponding to middle-Europe climate. This was proposed by the Joint Research Center (JRC/Ispra), based on the Ispra climate (Italy), and is now referenced on almost any inverter datasheet. - The value of this weighted efficiency is obtained by assigning a percentage of time the inverter resides in a given operating range. - If we denote by "Eff50%" the efficiency at 50% of nominal power, the weighted average is defined as: Euro Efficiency = 0.03 x Eff5% + 0.06 x Eff10% + 0.13 x Eff20% + 0.1 x Eff30% + 0.48 x Eff50% + 0.2 x Eff100%. - Now for climates of higher irradiations like US south-west regions, the California Energy Commission (CEC) has proposed another weighting, which is now specified for some inverters used in the US. CEC Efficiency = 0.04 x Eff10% + 0.05 x Eff20% + 0.12 x Eff30% + 0.21 x Eff50% + 0.53 x Eff75%. + 0.05 x Eff100%. https://www.pvsyst.com/help/index.html?inverter_euroeff.htm The so-called Euro efficiency is determined as the weighted average of the partial load efficiencies: euro = 0,035% + 0,0610% + 0,13 20% + 0,1030% + 0,4850% + 0,20100% Fronius Fronius solar configurator SMA Sunny design https://www.pvsyst.com/ https://www.ise.fraunhofer.de/en/publications/st udies/photovoltaics-report.html Renewable energy technologies PV APPLICATIONS Dipartimento di Ingegneria Industriale Università di Padova  BIPV Building Integrated PhotoVoltaics BIPV technologies are bringing new added values to the building while respecting design consideration. Especially when BIPV products are fully integrated. The aim of BPIV is the adaptation of the PV technology to become 100% part of the building. Least integrated More integrated Fully integrated (Open (Close roof (Direct-mounted BIPV, rack-mounted PV) rack-mounted PV) multifunctional) https://solstis.ch/fr Potential applications Low rise buildings: in low rise buildings the roof is the most diffuse application, due to the www.alamy.com relevance of the available area High rise buildings: the façade of Skyscrapers can be significant, compared to roof-top only applications in these cases. Moreover, usually the roofs on skyscrapers are already occupied by other technical infrastructures. Positive points Electric grid energy balance: in Spain in some regions with lot of RES installed outside urban areas have problems to deliver electrical energy during peak power production. If PV is installed in urban areas the contemporary production and consumption on site will diminish the load on the electric grid Heat reduction: in regions where ambient temperature and inside cooling is an issue, the white PV modules are not only bringing the advantage of power generation but also on reducing the building’s temperature by means of 20-30 K less. This also reduces the problem of Urban Heat Island effect.  Improved integration in building envelopes www.solaxess.ch www.glasstopower.com Ground-mounted PV Italy - GSE Germany - Fraunhofer  AgriVoltaics (Agrivoltaics, agrophotovoltaics, agrisolar, or dual-use solar) Agrivoltaics is the simultaneous use of areas of land for both solar photovoltaic power generation and agriculture. Solar panels and crops need a sharing of light, hence the design of agrivoltaics facilities may require trading off such objectives as optimizing crop yield, crop quality, and energy production. However, in some cases crop yield increases due to the shade of the solar panels mitigating some of the stress on plants caused by high temperatures and UV damage. Fish culture Large Livestock Spot or applications applications greenhouses Checkerboard Island arrays Linear arrays Vertical pattern and corridors installation arrays (bifacial PV) Efficiency of APV is the combination of: agricultural yield with agrovoltaics/agricultutal yeld without agrivoltaics energy delivered with the agrovoltaics/ energy delivered without the agrovoltaics Advantages Solar panels work best at lower temperatures. Transpiration, or the movement of water from plants into the atmosphere, cools the air around plants. This means agrivoltaics can improve solar panel efficiency by cooling off the microclimate around solar arrays. Agricultural land is the most suitable for solar farms in terms of efficiency: the most profit/power can be generated by the solar industry by replacing farming land with fields of solar panels, as opposed to using barren land In countries with low or unsteady precipitation, high temperature fluctuation and fewer opportunities for artificial irrigation, such systems are expected to beneficially affect the quality of the microclimate In experiments testing evaporation levels under solar panels for shade resistant crops cucumbers and lettuce watered by irrigation in a California desert, a 14–29% savings in evaporation was found, and similar research in the Arizona desert demonstrated water savings of 50% for certain crops. A study was done on the heat of the land, air and crops under solar panels for a growing season. It was found that while the air beneath the panels stayed consistent, the land and plants had lower temperatures recorded. Constraints: Agrivoltaics will only work well for plants that require shade and where sunlight is not a limiting factor. Shade crops represent only a tiny percentage of agricultural productivity. Agrivoltaic greenhouses may be inefficient if not properly design: in some case studies on greenhouses crop output was reduced by 64% and panel productivity reduced by 84% It requires a large investment, not only in the solar arrays, but in different farming machinery and electrical infrastructure. The potential for farm machinery to damage the infrastructure can also drive up insurance premiums as opposed to conventional solar arrays Requires shade tolerant species. Agrivoltaics works best with shade tolerant crops. Lettuce, for example, is a prime candidate for dual use planting. Other crop species adapted to understory growth can also work well, including chiltepin peppers, some types of tomatoes, berries. Cereal grains, on the other hand, prefer full sun and are unlikely to perform as well with APV.  Floating PV Current percentages  PVGIS Freeware software for energy performance of PV systems https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#PVP