Photo Voltaic Systems_ Solar Energy Based Technology

Photovoltaic systems: solar energy based technology

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http://dev.emcelettronica.com/print/51991

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Photovoltaic systems: solar energy based technology By arag61 Created 10/13/2008 - 21:36

Technology photovoltaic cell photovoltaic system solar energy technology solar photovoltaic cell Photovoltaic systems uses solar energy technology. The silicon atom has 14 electrons, 4 of these are valence electrons ; in a silicon pure crystal every atom is joined in covalent manner to 4 other electrons: every valence electron join to another valence electron. The transition from the valence band to the conduction band takes place transmitting to the electron a proper amount of energy. During this transmission the electron leaves behind it a hole called “gap” that can be occupied from another electron. The electrons movement involves, as a consequence, the “gaps” movement. The conversion of the solar energy in electric energy occurs utilizing the effect caused from a luminous flow that hits a “doped” semiconductor material. Every photon, that has enough energy, according to the connection E = h • λ (where h is the Plank constant and λ is the radiation wavelenght), can release inside the P-N junction an electron – gap couple. Using the silicon as semiconductor, the minimum energy required to release an electron – gap couple is equivalent to the highest wavelenght of the luminous radiation (1.15µm). Anyway, as the wavelenght decreases, the photons have a more and more higher energy and this energy is excessive compared to the one needed to release the electron – gap couple. As a consequence, the solar energy percentage that can be converted in electric energy do not exceed 44 %, the remaining 56 % is converted in heat. The main used semiconductors are : - Silicon (Si); - Germanium (Ge) - Gallium arsenide (GaAs) - Cadmium sulfide (CdS) - Copper sulfide (Cu2S) - Cells at multiple junction (Tandem) Equivalent circuit of a photovoltaic cell The efficiency of the photovoltaic silicon cells, in the laboratory tests also, is very far from the theoretical 44 % value since further inefficiencies arise: 1 Many, but not all the photons that hit the photovoltaic cell penetrate inside it, some are reflected and some are intercepted from the frontal electrode (Rs resistance – see the following drawing) 2 Some electron – gap couples join again among themselves before they can be separated from the electric field that is inside the junction (purity silicon level) 3 Some quantity of the potential energy transferred to the cell is not sufficient to release the electron – gap couple (diode – see the following drawing)

21.10.2008 21:36

Photovoltaic systems: solar energy based technology

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http://dev.emcelettronica.com/print/51991

Below there is an equivalent circuit of a photovoltaic cell.

In the following drawings we have the graphics related to: 1. voltage – current properties 2. electric property related to the temperature 3. electric property related to the solar radiation 4. I-V property and power trend of a solar cell

21.10.2008 21:36

Photovoltaic systems: solar energy based technology

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21.10.2008 21:36

Photovoltaic systems: solar energy based technology

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http://dev.emcelettronica.com/print/51991

Typology of the photovoltaic cells The majority of the photovoltaic cells, that are in the market at the moment, is constituted by silicon semiconductors for the following reasons: • Endless availability practically (planet resources) • • Great use in the electronic industry (technological processes of refining, processing and doping well refined) • • • Possibility of recycling the electronic industry scraps since the photovoltaic industry allows impurity concentrations typically of 10-5÷10-6 (compared to the 10-8 ÷ 10-9 values related to the electronic industry) Here in succession the several types of silicon cells, that are in the market at the moment, will be listed. Cells at monocrystalline silicon (µ ≈ 13,5%) • Gemmation and crystalline growth – The silicon at single crystal is obtained through a process named melting, starting from silicon crystals of great purity that, after being fused, are leaved to solidify in contact with a crystal seed. The silicon makes solid with the shape of a cylindrical ingot, constituted by a single crystal with a diameter of 13 ÷20 cm and with a length of about 200cm; • • Cut – The ingot is “sliced”, using particular saws, in wafers with a thickness of 250 ÷350µm (extreme ingot exploitation against an extreme wafers fragility) Cells at polycrystalline silicon (µ ≈ 9.0%) • Shape – The polycrystalline silicon is characterized from the presence of several crystals aggregated among themselves with different shapes, dimensions and positioning • • Low costs – (compared to the monocrystalline silicon) Cells at shapeless silicon (µ ≈ 6.0%) • Shape – The semiconductor, under gas shape, is deposited in layers of 10µm over every kind of surface (thin films technique ); • • Electric performances instability – (There is no a clear answer at the moment) • • • Multiple junction technique – Through a different doping of several silicon layers, serial connected, cells with different sensibility to the solar spectrum can be obtained. The result turns in a bigger performance and a bigger energy return. • • • • Low costs – (compared to the polycrystalline silicon) The electric connection among photovoltaic cells is achieved through two metallic contacts, one on the exposed face and the other one on the opposite face. Generally these metallic contacts are obtained by vacuumized evaporation of metals with very low electric resistance and consecutive thermal treatments are implemented in order to ensure the wanted

21.10.2008 21:36

Photovoltaic systems: solar energy based technology

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http://dev.emcelettronica.com/print/51991

adherence to the cell surface. While the rear metalization covers the face totally, the front one, that is exposed to the light, must have a geometric conformation which allows a good compromise between transparency to the incident radiation and maximum collection of free electrons in the conversion process. Photovoltaic systems A photovoltaic system is constituted by many photovoltaic cells, connected among themselves in modules or panels. Every single photovoltaic cell has small dimensions and generally produces a power between 1 and 3 watts and 0,5Volts, at the standard test conditions (STC) of 1000W/m². To get a bigger power and voltage, it is necessary to connect several cells among themselves to create bigger units called modules. The modules, in their turn, can be connected to constitute panels, called photovoltaic fields too, that produce the power wanted from the application. The dimensioning of a whole photovoltaic system is called “system balancing” (BOS) and through it the best equipment configuration, according to the load, is studied. Integral parts of the system balancing are a series of components as: charge regulators, inverters, accumulators to store the electricity, cables etc. Therefore the photovoltaic system is a unification of mechanical, electric and electronic components that contribute to capt and transform the available solar energy, making it usable from the usership in electric energy. The photovoltaic system structure can be very different. Talking about the module support structures, we have: • Systems with fixed inclination - (fixed supporting structure) • • Systems with active tracking - single/double axis tracking systems (characterized by step by step motors and control electronics) • • • Systems with passive tracking – (functioning principle based on the pressure difference that it is created in two cylinders, each one containing particular materials, for ex. freon and oil) From the electric point of view we have: • Lonely systems or “stand alone” • • Network connected systems or “grid connected” The photovoltaic field The photovoltaic field is an union of photovoltaic modules, serial and parallel properly connected, to realize the wanted operative conditions. In the design phase of a photovoltaic field some choices, that influence its functioning, must be done: • Series - parallel configuration of the field modules (mismatch effect due to the unhomogeneity of their electric peculiarities ex.: in a series of modules the current is limited from the module that supplies the lower current; in a parallel of modules the voltage is limited from the module that supplies the lower voltage) • • Choice of the working voltage • • • Choice of the support structures • • • •Minimum space among the panel arrays in order do not to have shading Below there is the photovoltaic field layout.

21.10.2008 21:36

Photovoltaic systems: solar energy based technology

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http://dev.emcelettronica.com/print/51991

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21.10.2008 21:36