Technical Application Papers No.10 Photovoltaic plants - ABB

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1 Generalities on photovoltaic (PV) plants 8 Photovoltaic plants Technical Application Papers 1.3 Main components of a photovoltaic plants 1.3.1 Photovoltaic generator The elementary component of a PV generator is the photovoltaic cell where the conversion of the solar radiation into electric current is carried out. The cell is constituted by a thin layer of semiconductor material, generally silicon properly treated, with a thickness of about 0.3 mm and a surface from 100 to 225 cm 2 . Silicon, which has four valence electrons (tetravalent), is “doped” by adding trivalent atoms (e.g. boron – P doping) on one “layer” and quantities of pentavalent atoms (e.g. phosphorus – N doping) on the other one. The P-type region has an excess of holes, whereas the N-type region has an excess of electrons (Figure 1.7). Figure 1.7 - The photovoltaic cell Silicon doped BORON Atom Si Si Si Hole B Si P Si Si Si Free electron PHOSPHORUS Atom Depletion region Junction +5 +5 +5 +3 +3 +3 +5 +5 +5 +3 +3 +3 +5 +5 +5 +3 +3 +3 +5 +5 +5 +3 +3 +3 +5 +5 +5 +3 +3 +3 +5 +5 +5 +3 +3 +3 In the contact area between the two layers differently doped (P-N junction), the electrons tend to move from the electron rich half (N) to the electron poor half (P), thus generating an accumulation of negative charge in the P region. A dual phenomenon occurs for the electron holes, with an accumulation of positive charge in the region N. Therefore an electric field is created across the junction and it opposes the further diffusion of electric charges. By applying a voltage from the outside, the junction allows the current to flow in one direction only (diode functioning). When the cell is exposed to light, due to the photovoltaic effect 2 some electron-hole couples arise both in the N region as well as in the P region. The internal electric field allows the excess electrons (derived from the absorption of the photons from part of the material) to be separated from the holes and pushes them in opposite directions in relation one to another. As a consequence, once the electrons have passed the depletion region they cannot move back since the field prevents them from flowing in the reverse direction. By connecting the junction with an external conductor, a closed circuit is obtained, in which the current flows from the layer N, having higher potential, to the layer N, having lower potential, as long as the cell is illuminated (Figure 1.8). Figure 1.8 - How a photovoltaic cell works Photons Luminous radiation Electron flow Hole flow N-type silicon P-N junction P-type silicon Electric current Load 2 The photovoltaic effect occurs when an electron in the valence band of a material (generally a semiconductor) is promoted to the conduction band due to the absorption of one sufficiently energetic photon (quantum of electromagnetic radiation) incident on the material. In fact, in the semiconductor materials, as for insulating materials, the valence electrons cannot move freely, but comparing semiconductor with insulating materials the energy gap between the valence band and the conduction band (typical of conducting materials) is small, so that the electrons can easily move to the conduction band when they receive energy from the outside. Such energy can be supplied by the luminous radiation, hence the photovoltaic effect.
The silicon region which contributes to supply the current is the area surrounding the P-N junction; the electric charges form in the far off areas, but there is not the electric field which makes them move and therefore they recombine. As a consequence it is important that the PV cell has a great surface: the greater the surface, the higher the generated current. Figure 1.9 represents the photovoltaic effect and the energy balance showing the considerable percentage of incident solar energy which is not converted into electric energy. Figure 1.9 - Photovoltaic effect 1 Separation of the charge 2 Recombination 3 Transmission 4 Reflection and shading of the front contacts 1 2 1 P-N region Positive contact 1 P layer 3 100% of the incident solar energy - 3% reflection losses and shading of the front contacts - 23% photons with high wavelength, with insufficient energy to free electrons; heat is generated - 32% photons with short wavelength, with excess energy (transmission) - 8.5% recombination of the free charge carriers - 20% electric gradient in the cell, above all in the transition regions - 0.5% resistance in series, representing the conduction losses = 13% usable electric energy Under standard operating conditions (1W/m 2 irradiance at a temperature of 25° C) a PV cell generates a current of about 3A with a voltage of 0.5V and a peak power equal to 1.5-1.7Wp. 4 Negative electrode N Layer On the market there are photovoltaic modules for sale constituted by an assembly of cells. The most common ones comprise 36 cells in 4 parallel rows connected in series, with an area ranging from 0.5 to 1m 2 . Several modules mechanically and electrically connected form a panel, that is a common structure which can be anchored to the ground or to a building (Figure 1.10). Figure 1.10 Several panels electrically connected in series constitute an array and several arrays, electrically connected in parallel to generate the required power, constitute the generator or photovoltaic field (Figures 1.11 and 1.12). Figure 1.11 Cell Module Photovoltaic generator assembly of arrays connected in parallel to obtain the required power Figure 1.12 Panel several modules assembled into a single structure Array assembly of panels connected in series Photovoltaic plants 9 1 Generalities on photovoltaic (PV) plants
Page 1: Technical Application Papers No.10
Page 4 and 5: 2 Index Technical Application Paper
Page 6 and 7: Introduction 4 Introduction Photovo
Page 8 and 9: 1 Generalities on photovoltaic (PV)
Page 20 and 21: 2 Energy production 18 Photovoltaic
Page 22 and 23: 2 Energy production 20 2.4 Nominal
Page 28 and 29: 3 Installation methods and configur
Page 36 and 37: 4 Connection to the grid and measur
Page 42 and 43: 5 Earthing and protection against i
Page 44 and 45: 6 Protection against overcurrents a
Page 52 and 53: 7 Feed-in-Tariff 50 Photovoltaic pl
Page 54 and 55: 8 Economic analysis of the investme
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9 ABB solutions for photovoltaic ap
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Annex A: New panel technologies 84
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Annex B: Other renewable energy sou
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Annex C: Dimensioning examples of p
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Technical Application Papers No.10 Photovoltaic plants - ABB

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