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SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice Analysis of renewable energy sources utilization Ing. Martin Bačko, Ing. Anna Hodulíková Dept. of Theoretical Electrical Engineering and Electrical Measurement Electronics, FEI TU of Košice, Slovak Republic [email protected], [email protected], Abstract—This article deals with possibilities of using renewable energy sources especially wind and solar energy for gradual replacement of traditional energy gained from fossil fuels which will be depleted relatively soon. Renewable energy sources are available in much higher amount than the whole world needs and their usage is imperative for keeping the ecological equilibrium. electron moves from valence to conductive zone. Electron can freely move on the crystal lattice. When electron leaves the valence zone, the free space is known as the hole, or so called defective electron. Creation of these defective electrons is responsible for inner semiconductors conductivity. Electrones and holes are always in pairs, so in other words there is the same amount of electrons and the holes. Keywords—photovoltaics, renewable sources, solar energy, wind energy I. INTRODUCTION In term renewable sources of energy we understand ecologically clean direct or indirect form of solar energy, which can be transformed by suitable technological solution to electrical energy. Burning of fossil fuels probably caused accumulation of harmful carbon dioxide emissions, which are responsible for greenhouse effect and climatic changes. Direct form of solar energy can be used for direct water heating, or direct transformation to electrical energy using the photovoltaics principle. Indirect form of solar energy is in the form of water and wind energy which again can be with suitable technical solution used for generating the electrical energy. II. PHOTOVOLTAICS A. Basic principle Photovoltaic systems use the principle of semiconductors. Semiconductors are elements from IV group of periodic table such as silicon (Si), germanium (Ge), tin (Sn) which have 4 valence electrons on sphere. Semiconductor systems consist of 2 elements, for example III-V semiconductor galliumarsenic (GaAs) and II-VI semiconductor cadmium-tellurium (CdTe). Silicon is the most common material in photovoltaics. It is the second most common element in Earth´s crust, but it is not possible to find it in chemically clean form. It is the fundamental semiconductor of IV group of periodic table, therefore it have 4 valence electrons. Because it wants to keep the most stable electron configuration, 2 electrons from neighboring atoms in crystal lattice form a pair connection. Such pairing (covalent bond) with 4 neighboring atoms provides silicon with stable electron configuration similar to rare gas argon (Ar). From energetical point of view, the valence zone is fully taken and the conductive zone is empty. By providing additional energy, for example from light or heat source Fig.1 Photovoltaics solar cell Current flowing through PN crossing can be formulated as algebraic sum of balanced heat flows of electric charge carriers. In state of equilibrium is the sum of the heat flows zero. Sums of electron and hole currents passing through the PN crossing are also zero. Absolute values of electron and hole currents from the N type semiconductor can be marked I nN and I pN , from semiconductor type P can be marked as as I nP and I pP . In equilibrium state: N n N P P - I + I p + In - I p = 0 (1) N N (2) P P (3) - In + I p = 0 + In - I p = 0 Illumination will cause the increase in concentration of minority carriers. It will create the If current which flows through the PN crossing. When illuminated, Fermi´s level shatters to quasilevels for electrons and holes. Their , which was difference φ resembles the voltage Uf e created as the result of illumination. In stationary state, the current flowing through the PN crossing equals to zero. SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice I f I nN I pN I nP I pP 0 Majority carriers currents (4) I nN and I pP will change because of illumination. Energetic levels are mutually shifted and levels of potential barriers are changed: I nN I nP exp( I pP I pN exp( kT kT ) (5) ) (6) describes exactly the solar cells behavior, especially in cases where different operational conditions have to be considered. Charge carriers in real solar cell show a voltage loss when passing through the PN crossing. Serial connected resistor RS allows representation of this voltage loss. Parallel resistor RP represents inner loses in cell. RS value for real cells is about few milliohms (fig. 3) and RP value is mostly higher than 10 Ω (fig. 4). Using the equations (4), (5), (6) and after adjustments we get: I f I s [exp( kT ) 1] 0 (7) For photoelectromotoric force: Uf e I kT ln( f 1) e Is (8) If PN crossing is connected to circuit where current I is flowing, using the (7), (8) equations we get: I f I I s [exp( Fig.2 Solar cell equivalent scheme ) ] kT (9) Uf I I kT ln( f 1) e Is (10) If PN crossing is connected to resistor R Uf I , equation (9) will be: If In Uf R I s [exp( the case I Is[exp( kT kT of Fig.3 V-A Characteristics of Rs resistor ) 1] small (11) external resistors when: ) 1] we get I f I . In the case of big external resistors when I 0 , we get I f I s [exp( kT ) 1] 0 . If we connect the source of voltage to PN crossing we get: If U f U R I s [exp( kT ) 1] (12) For the solar cell power we use this formula: P=U.I For the maximum output: eU d (UI ) e Ik Is Is exp m 0 dU kT kT (13) For idle connection: U0 I kT ln( k 1) e Is (14) B. Solar cell equivalent scheme, V-A characteristics One diode enhanced model Simple equivalent circuit is sufficient for most applications. The difference between measured and calculated values is only few percents. Only enhanced model (fig.2) Fig.4 V-A Characteristics of Rp resistor C. Photovoltaics on KTEEM – module and program For measurements on department the photovoltaics cell QX6926 (fig.6) and SFH203 infra diode are used. We can calculate the power P [W] because of 4Ω resistor which is connected to cell and simulates the load. Program (fig.5) was written for the measurement, which can collect the voltage or current values simultaneously from 4 devices. The results are daily written to *.csv file (fig.7) and are sent to specified ftp server at midnight, where they can be further evaluated (fig.8). Figures 9 and 10 shows the classic bigger photovoltaic module which is commonly used for household or industrial applications and its technological parameters. SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice Fig.5 Measuring program main screen Fig.9 Photovoltaic panel Fig.6 Solar cell QX6926 Fig.10 Photovoltaic panel – technical parameters Fig.7 Example of output file in *.CSV format Fig.8 Example of daily graph - Voltage Fig.11 Example of solar cell system SCYR 2010 - 10th Scientific Conference of Young Researchers – FEI TU of Košice III. WIND ENERGY Wind energy is the indirect form of solar energy. Solar irradiation causes temperature differences on Earth and these are the origin of winds. Wind can achieve much higher energy concentration than solar radiation (10 kW/m2 during violent storm and more than 25 kW/m2 during hurricane, compared to maximum value of solar irradiation 1 kW/m2). Slow wind speed about 5 m/s however has energy concentration about only 0,075 kW/m2. B. Wind/solar energy system A. Wind from energetic point of view Kinetic energy W in wind with speed v is equal to: W 1 m.v 2 2 (15) Fig.12 Example of combined solar/wind system Power P of the wind with constant speed v, is: P W 1 m.v 2 2 Density ρ and content V of air determine its weight: m .V (17) Weight of air with density ρ, which flows through the area S with speed v on trajectory š, can be calculated using this equation: m .V .S .š (18) Power P of the wind will be: P 1 .S .v 3 2 (19) Wind density ρ is changing due to air pressure p and temperature υ It is directly proportional to pressure nex to temperature. Ratio between wind power taken by turbine PT and total wind power P0 is called power coefficient CP: CP PT 1 v 2 .1 P0 2 v1 v 22 .1 2 v1 (20) Betz calculated ideal wind ratio which is: v2 1 v1 3 (21) After using the equation (20) we get so called Betz power coefficient: C P _ Betz 16 0,593 27 (22) If wind turbine slows the wind from initial speed v1 to one third v1 (v2 = (1/3).v1), then it is theoretically possible to achieve the maximum power which in the case of the wind turbine is 60%. Real wind generators cannot achieve this theoretical optimum, however good systems have CP coefficient between 0,4 a 0,5. Ratio between used wind power PT and ideal power Pid defines efficiency η of the wind generator. PT CP Pid C P _ Betz IV. CONCLUSION (16) (23) Solar and wind energy can be considered as a real alternative because systems which utilize it already exist (fig.11,12) and achieve good results. Hopefully it will be a matter of few decades until they achieve a wide range utilization in every sphere of industry and household applications, as they are the only way how to get clean and harmless energy. Acknowledgment The paper has been prepared by the support of Slovak grant projects VEGA 1/0660/08, KEGA 3/6386/08, KEGA 3/6388/08 REFERENCES [1] QUASCHNING, Volker: Understanding Renewable Energy Systems, London, 2005, 260s, ISBN 1-84407-128-6 [2] TWIDELL, John - WEIR, Anthony: Renewable energy Resources, London, 2006, 597s, ISBN 9-78-0-419-25330-3 [3] PIMENTEL, David: Biofuels, Solar and Wind as Renewable Energy Sources, USA, 2008, 504s, ISBN 978-1-4020-8653-3 [4] FRERIS, Leon - INFIELD, David: Renewable Energy in Power Systems, 2008, West Sussex, 2008, 283s, ISBN 978-0-470-01749-4 [5] KOVÁČOVÁ, Irena - KOVÁČ, Dobroslav: Non-harmonic power measuring. In: [6] [7] Acta Electrotechnica et Informatica. Vol. 8, No. 3 (2008), pp. 3-6. ISSN 13358243. 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