ELECTRONICS, PHOTONICS AND MICROSYSTEMS Andrzej DZIEDZIC, Piotr MARKOWSKI Autonomous Power Supplying Systems Topic 11. Photovoltaic and thermophotovoltaic microgenerators 1. Photovoltaics, photovoltaic effect – basic information 2. Parameters of solar cells and solar modules 3. Examples of photovoltaic microgenerators 4. Thermophotovoltaic conversion 5. Elements of thermophotovoltaic systems 6. Thermophotovoltaic (TPV) power microgenerators The forecast of global use of energy sources Remaining renewable Annual energy consumption [EJ/a] Solar thermic Solar energetic Wind Biomass Water energy Nuclear energy Gas Coal Petroleum Source: solarwirtschaft.de Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels comprising a number of cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CuInxGa1-xSe – CIGS). 0.25 0.20 m] a) ultraviolet 0.15 2 b) visible radiation c) infrared 0.10 P [mW/cm 0.05 0.00 Spectrum of solar radiation: 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 [m] (1) – radiation of excellent black-body for T = 6000 K (2) – solar radiation reaching high layer of earth atmosphere (3) – solar radiation at the sea level (with losses caused by presence in the atmosphere of O2, O3, H2O and CO2) The photovoltaic effect is the creation of a voltage (or a corresponding electric current) in a material upon exposure to light - the generated electrons are transferred between different bands (i.e. from the valence to conduction bands) within the material, resulting in the buildup of a voltage between two electrodes. It refers to photons of light knocking electrons into a higher state of energy to create electricity. The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode. In most photovoltaic applications the radiation is sunlight and for this reason the devices are known as solar cells. In the case of a p-n junction solar cell, illumination of the material results in the creation of an electric current as excited electrons and the remaining holes are swept in different directions by the built-in electric field of the depletion region. Photovoltaic effect - simple explanation 1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon (except of this lower energy photons can pass straight through solar cell or the photon can reflect off surface of semiconducting material) 2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. 3. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. p-n homo- or heterojunction - charge carrier separation 1. drift of carriers, driven by an electrostatic field established across the device (dominant mode in thin film cells as a-Si or CIGS) 2. diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration, following a gradient of electrochemical potential (dominant mode in mono- or multicrystalline silicon solar cells) The equation for total current I flowing in semiconductor is as follow: qU qU I I S1 exp 1 I S 2 exp 1 kT 2kT where IS1 i IS2 - diffusive and recombination part of diode dark saturation current, respectively: Dn n p0 D p p n0 n I Sq I Sq i Wv N S1 L L S 2 2 th t n p n p where W – width of active layer of junction: 2 Na Nd W VD q Na Nd and ε – electrical permeability of semiconductor, Na, Nd – density of proper trap states (acceptor and donor ones), VD – diffusive voltage. I-V characteristics (black) and load characteristics (red) of typical illuminated solar cell with marked characteristic parameters of such device qV I [exp( OC ) 1] I 0 S AkT Parameters of solar cells (1) ISC - Short Circuit Current VOC - Open Circuit Voltage - can be calculated from Shockley equation (for I = 0) AkT I VOC ln(1 ) q I S where I – photogenerated current (dependent on solar flux insolation, independent on voltage), V – voltage cross the output terminals, k – Boltzmann’s constant , T – junction temperature (in Kelvin), A – diode ideality factor. A is from the range 12 and shows the participation of recombination current and diffusion current in total photocurrent. Diode ideality factor can be calculated from the formula q V V A 1 2 kT 1 ln( x) (assuming that is voltage change for current change by an order of x), at T = 300 K = 0.026 V. Parameters of solar cells (2) • (Vnom, Inom) – nominal power point Pnom • (Vm, Im) – maximal power point Pmax (Pmax = ImaxVmax) • Fill Factor, FF: I V FF max max I SC VOC Efficiency, η – ratio of maximal power point Pmax to the power of solar irradiation (Pin): I U max max 100% Pin • Series resistance, RS • Shunt resistance, RSH All parameters of photovoltaic cell are strongly dependent on solar radiation and ambient temperature. qVD VD I I I S exp 1 AkT RSH DC equivalent circuit of a solar cell – single diode equivalent (SEM) model qV D 1 I I IS exp AkT V D RSH and VD V I RS DC equivalent circuit of a solar cell – double diode equivalent (DEM) model qVD I I I S1 exp 1 kT qVD VD I S 2 exp 1 2kT RSH and VD V I RS Cell temperature Reverse saturation current Effect of temperature on Effect of reverse the I-V characteristics of saturation current on the a solar cell I-V characteristics of a solar cell Series resistance Shunt resistance Effect of series resistance Effect of shunt resistance on the I-V characteristics on the I-V characteristics of a solar cell of a solar cell Example - parameters of ST40 CIGS module - DEM model (1) Arrhenius plot for IS1 – diffusive part Arrhenius plot for IS2 – of dark current of ST 40 CIGS module recombination part of dark current in a wide range of total insolation of ST 40 CIGS module in a wide Gipoa and module working range of insolation Gipoa and temperature Tm module working temperature Tm Example - parameters of ST40 CIGS module - DEM model (2) Temperature dependence of series Temperature dependence of shunt resistance of ST40 CIGS module in resistance of ST40 CIGS module in a wide range of total insolation Gipoa a wide range of total insolation Gipoa Example - parameters of ST40 CIGS module - DEM model (3) Efficiency η of ST40 CIGS module in Fill factor FF of ST40 CIGS module a wide range of its working in a wide range of its working temperature Tm temperature Tm Basic structure of a silicon based solar cell and its working Band diagram of a silicon solar cell mechanism Polycrystalline photovoltaic Solar cell made from a cells laminated to backing monocrystalline silicon wafer material in a module Sunlight Metallization Antireflective layer Conductive layer Buffer layer Junction (n semiconductor) OVC layer Absorber layer (p semiconductor) Ohmic contact Substrate Schema of cross-section of thin-film CIGS cell/module Cross-section through CIGS structure with marked places of module dividing onto many photovoltaic cells and roads of electrical current flow (SGL - …., Mo - …., CIGS - …., ZAO - … Best laboratory efficiencies obtained for solar cells with various materials and technologies PV cell interconnection and module fabrication Solar cells are rarely used individually – cells with similar characteristics are connected and encapsulated to form modules (panels) which, in turn, are the basic building blocks of solar arrays. The module must be able to withstand such environmental conditions as dust, salt, sand, snow, humidity and hail, as well as maintaining performance under prolonged exposure to UV light. I-V characteristics of solar module (1) a) composed of identical cells (N cells in series × M cells in parallel) q(Vtot /N) Itot MI MI 0[exp( )1] AkT b) composed of non-identical cells • mismatched cells connected in parallel: same voltage Cell 2 has lower output caused by: - manufacturing defects V - degradation - partial shading - higher temperature I-V characteristics of solar module (2) An easy method of calculating The effect on current output the combined open circuit of mismatched cells voltage (VOC) of mismatched connected in parallel cells connected in parallel I-V characteristics of solar module (3) 2 1 An easy method of calculating I the combined short circuit Series connected current (I ) of mismatched mismatched cells and the SC cells connected in series effect on their voltage output PV modules – thermal considerations It is desirable for modules to operate at as low temperature as possible because: • cell output is increased at lower temperatures, • thermal cycles and stress are reduced, • degradation rates approximately doubles for each 10oC increase in temperature. The Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open circuited cells in module under the following conditions: • irradiance on cell surface – 800 W/m2, • ait temperature – 20oC • wind velocity – 1 m/s • mounting – open back side The following approximate expression for calculating