MODELING AND SIMULATION OF RENEWABLE HYBRID ENERGY SOURCES USING MATLAB/SIMULINK ENVIRONMENT

Transcription

1 The 4 th edition of the Interdisciplinarity in Engineering International Conference Petru Maior University of Tîrgu Mureş, Romania, 009 MODELING AND SIMULATION OF RENEWABLE HYBRID ENERGY SOURCES USING MATLAB/SIMULINK ENVIRONMENT Cristian-Dragoş DUMITRU #, Adrian GLIGOR #, Adrian-Vasile DUKA #3 # Department of Electrical Engineering and Computer Science, Petru Maior University of Tg. Mureş No., N. Iorga St., Tg. Mureş, Romania cristian.dumitru@ing.upm.ro adrian.gligor@ing.upm.ro 3 adrian.duka@ing.upm.ro ABSTRACT The using on a large scale of renewable energies brought a series of new problems in power energy generation and distribution systems. This represents one of the main reasons for study the impact of renewable energy sources on power systems. The paper proposes an effective study on the impact of renewable energy sources on power systems by using simulation models and the results obtained from these models. In the present paper are conceived Matlab/Simulink simulation models by considering mathematical models for hybrid systems based on renewable energy sources applied in the central region of Romania. The models can be implemented in any other Matlab SimPowerSystems simulation and study models and can be used in real simulated systems functioning using Matlab. The experimental results obtained from simulated models and conclusions resulted from the study of the impact of renewable energy hybrid systems on power systems are also presented. Keywords: photovoltaic system, wind energy system, hydroelectric system, renewable energy systems modeling, renewable energy systems simulation. Introduction Hybrid power systems consist on a combination of renewable energy sources such as: photovoltaic (PV), wind generators, hydro, etc., to charge batteries and provide power to meet the energy demand, considering the local geography and other details of the place of installation. These types of systems, which are not connected to the main utility grid, are also used in stand-alone applications and operate independently and reliably. The best applications for these systems are on remote places, like as example, rural villages, telecommunications, etc. Nowadays renewable energy technologies offer important benefits compared to those of conventional energy sources. Many of them are significant related to the environmental benefits obtained, such as []: - no air pollution or production of hazardous waste; - clean and inexhaustible source of energy; - don t require liquid or gaseous fuels to be transported or combusted; - PV sources can generate power in all weather and climates since their modules resist under the worse environmental conditions; - PV systems have clean, safe, reliable and quiet operation; - easy maintenance; - energy security and efficiency; - installed PV-wind hybrid system generate power continuously with minimal operating costs; - PV modules and battery bank are modular so you can double or increase the PV systems anytime; - reduction of dependence on foreign and/or decentralized sources of energy; - PV-wind hybrid systems are usually placed close to where the electricity is used, so they require much shorter power distribution lines than those needed to bring power in from the utility grid; - consumers can produce the energy they need within their own borders; - operating time of over 30 years; 44

2 - reduction of utility energy bills; - operates reliably for long periods of time with virtually no maintenance; - power from the PV-wind hybrid system is transferred to the utility company. The owner of any grid-connected PV-wind hybrid system can buy and sell electricity to the utility company. The importance of hybrid systems has grown as they appeared as the right solution for a clean and distributed energy production. We must mention that a new implementation of a hybrid system requires special attention on analysis and modelling. One issue is determined by the variable and unpredictable supply from renewable sources. As available tools are quite limited, the paper intends to present a variant of a software application useful in the decisional process, for choosing optimal solutions, as well as in educational purposes.. Modeling and simulation of the solar photovoltaic system The model of the solar cell can be realized by an equivalent circuit that consists of a current source in parallel with a diode (Fig. ) [], [3]. In Fig. R S, R P and C components can be neglected for the ideal model. increases logarithmically according to the Shockley equations () and (), which describe the interdependence of current and voltage in a solar cell [], [4]. qu k T I I I e PV 0 () kt I I PV U ln q I 0 () where: - k - Boltzmann constant ( J/K); - T reference temperature of solar cell; - q elementary charge ( As); - U solar cell voltage (V); - I 0 saturation current of the diode (A); - I PV photovoltaic current (A). Fig. presents the Matlab Simulink model of PV module. Fig. Matlab Simulink model of PV module. Fig. Equivalent circuit diagram of a solar cell The p-n junction has a certain depletion layer capacitance, which is typically neglected for modeling solar cells. At increased inverse voltage the depletion layer becomes wider so that the capacitance is reduced similar to stretching the electrodes of a plate capacitor. Thus solar cells represent variable capacitance whose magnitude depends on the present voltage. This effect is considered by the capacitor C located in parallel to the diode. Series resistance R S consists of the contact resistance of the cables as well as of the resistance of the semiconductor material itself. Parallel or shunt resistance R P includes the leakage currents at the photovoltaic cell edges at which the ideal shunt reaction of the p-n junction may be reduced. This is usually within the kω region and consequently has almost no effect on the currentvoltage characteristic []. The diode is the one which determines the current-voltage characteristic of the cell. The output of the current source is directly proportional to the light falling on the cell. The open circuit voltage Solar system model consist of three Simulink blocks: solar model block, PV model block and energy conversion modules. Solar model block implements the mathematical model of solar radiation. This is done by using standard Simulink and Matlab modules and functions. This block allows selecting different type of patterns for solar radiation [5]. PV model implements the equivalent circuit of a solar cell presented in Fig.. Here standard functions and block of Matlab and Simulink were used to obtain this model. Its structure is presented in Fig 3. Fig. 3 Matlab Simulink implementation of the PV module. The output of the PV module is processed by an 45

3 energy conversion block implemented with an PWM IGBT inverter block from standard Simulink/ SimPowerSystems library. 3. Modeling and simulation of the wind energy system Modeling the wind energy converter is made considering the following assumptions []: - friction is neglected; - stationary wind flow; - constant, shear-free wind flow; - rotation-free flow; - incompressible flow (ρ=. kg/m 3 ); - free wind flow around the wind energy converter. On the above condition the maximum physical achievable wind energy conversion can be derived using a theoretical model that is independent of the technical construction of a wind energy converter. The flow air mass has certain energy. This energy is obtained from the air movement on the earth s surface determined by the difference in speed and pressure. This is the main source of energy used by the wind turbines to obtain electric power. The kinetic energy W taken from the air mass flow m at speed v in front of the wind turbine s pales and at the back of the pales at speed v is illustrated by equation (3). W mv v (3) The resulted theoretical medium power P is determined as the ratio between the kinetic energy and the unit of time and is expressed by equation (4) W m V P v v v v (4) t t t where: - V- air mass volume; - t - time; - ρ - air density. Assuming the expression of the mean air speed v med v v the mean air volume transferred per unit time can be determined by (5): V V Av med med (5) t The equation for the mean theoretical power is determined using equation (5) (Fig. 4), by: 3 Av v v P A v v v v (6) 4 4 v v We can conclude that an adequate choice of ratio leads to a maximum power value taken by the wind converter from the kinetic energy of the air masses, as shown by equation (7). 8 3 P Av max (7) 7 This power represents only a fraction of the incident air flow theoretical power given by (8): Av 3 Pwind (8) Fig. 4 Flow through a wind energy converter Equations (7) and (8) lead to (9) P A v Av 0, 59 P wind C max p (9) 7 where: C P represents the mechanical power coefficient which expresses that the wind kinetic energy cannot be totally converted in useful energy. This coefficient, meaning the maximum theoretical efficiency of wind power, was introduced by Betz [5]. The electrical power obtained under the assumptions of a wind generator s electrical and mechanical part efficiency is given by (0). 3 Pel CeAv (0) where: C e represents the total net efficiency coefficient at the transformer terminals [6]. In Fig. 5 is shown the Matlab Simulink model of wind generator module. Fig. 5 The Matlab Simulink model of the wind generator module The wind system model consists of three Simulink blocks: the wind model block, the wind generator model block and energy conversion modules. The wind model block implements the mathematical model of the air mass flow. This is done by using standard Simulink and Matlab modules and functions. This block allows the selection of different patterns for the air mass flow and the equations mentioned above were used in the design of this model. The resulted Matlab-Simulink model for the wind generator is a particular case of a DC generator general model and is presented in Fig 6. The output of the wind energy generator module is processed by an energy conversion block implemented with a PWM IGBT inverter block from the standard Simulink/ SimPowerSystems library. 46

4 Fig. 6 Matlab-Simulink model of the small wind generator 4. Modeling and simulation of the hydroelectric system Small hydroelectric power plants harness the falling water kinetic energy to generate electricity. Turbines transform falling water kinetic energy into mechanical rotation energy and then, the alternator transforms the mechanical energy into electricity. Water flows within a river from a higher geodesic site to a lower geodesic site due to gravitation. This is characterized by different particular kinetic and potential energy at both sites. The correct identification of the resulting energy differences of the out-flowing water can be assumed by considering a stationary and friction-free flow with incompressibility. The hydrodynamic Bernoulli pressure equation applied in such conditions is written according to equation (). p watergh watervwater. const () where: p hydrostatic pressure; water water density; g acceleration of gravity; h the water height; v water velocity of the water flow. Equation () can be transformed so that the first term expresses the pressure level, the second term the level of the site and the third term the water velocity level by (). p vwater h. g g const () water v The term water refers to the dynamic height g and is defined as the height due to the speed of water flow and can be identified by the term of kinetic water energy. The usable head h util of a particular section of river can be determined by considering: the difference in pressure, the geodesic difference in height and the different flow velocities of the water, using equation (3). It must be mentioned that the equation is used to analyze an ideal case and does not consider the losses due to the friction of the individual water molecules among each other and the surrounding matter. pup p vwater, up vwater, hutil ( hup h) waterg g (3) where: - p up upstream hydrostatic pressure; - p stream hydrostatic pressure; -h up upstream geodesic water height (headwater); -h stream geodesic water height (tailwater); - v water, up upstream water velocity; - v water, stream water velocity; Considering equation (3), the power of a water supply P water can be determined using (4). Pwater watergqwaterh (4) util where: q water is the volume-related flow rate. According to equation (4), the power of a water supply is determined by the volume-related flow rate and usable head. The water flow assumes high values in lowland areas, while large heads can be achieved in mountain areas. Considering two specific points of a river, the theoretical power of the water P water,th, can be calculated based on (5). P gq ( h h ) (5) water, th water water up where q water represents the volumetric flow rate through a hydroelectric power plant. In the real case, considering the energy balance between two specific points of a river, and also the energy losses, the hydrodynamic Bernoulli pressure equation can be written according to equation (6). pup vwater, up p v water, hup hav g g g g water, up water, vwater, const. g (6) where:- p - hydrodynamic pressure energy; water g - h - potential energy of the water; - v water - kinetic energy of the water; g v water - - energy losses; g - ξ - loss coefficient. The energy losses are represented by the part of the rated power which is converted into ambient heat by friction and cannot be used technically. In the turbine, pressure energy is converted into mechanical energy. The conversion losses are described by the turbine efficiency η turbine. Equation (7) describes the part of the usable water power that can be converted into mechanical energy at the turbine shaft P turbine. Pturbine turbine watergq waterh (7) util where h util is the usable head at the turbine, and the term g q h represents the actual usable water water util 47

5 water power []. The water model described by the equations mentioned above was introduced in a Matlab- Simulink model of the hydroelectric system. This model is shown in Fig. 7 and it encapsulates the model of the hydroelectric plant connected to the water model. Measurement of power and voltage is also provided by this model. In order to implement a real hybrid system a theoretical preliminary study is required. Such study can be performed on simulation models. A simulation model is presented in Fig. 9. Fig. 7 The Matlab Simulink model of the hydroelectric system The model of the hydroelectric plant (generator) has the same form as the one of the wind generator and also an equivalent diagram as the one we considered for the wind generator (Fig. 6). 5. Modeling and simulation of the hybrid renewable energy system Considering the above models, by using Matlab- Simulink environment, an application useful for study of hybrid renewable energy system connected to a local grid was developed. The purposes of the application reside in scientific studies and, also didactical ones, concerning renewable hybrid solar-wind-hydro systems. The structure of the application is based on solar, wind, hydro, energy conversion, transport and consumer modules shown in Fig. 8. Fig. 9 Simulation model of a hybrid renewable energy system. By using the presented simulation several functioning studies of solar-wind-hydro hybrid system can be performed. Different patterns of solar, wind and hydro models and also different type of loads can be selected. Fig. 0 illustrates the voltage waveform measured at the bus bar. It can be seen a voltage waveform distortion caused by electronic devices inverters - used for energy conversion. In Fig. is shown the variation of the voltage caused by variation of the primary energy source. PV WG HG EC EC EC Fig. 0 Voltage waveform at the bus bar T B L Legend: PV photovoltaic module WG wind generator HG - hydrogenerator EC energy converter modules T energy transport module B batteries L load (consumer) Fig. 8 Architecture of the hybrid solar-windhydro system model. Fig. Voltage variation caused by primary energy variation 48

6 The developed model can be used not only for study of energy conversion but also for study of primary energy source influence on consumers. 6. Conclusions In this paper is presented a rural street lighting The paper presents the modeling of a solar-windhydro hybrid system. Based on presented mathematical models a Matlab/Simulink application was developed. The application is useful for analyze and simulate a real hybrid solar-wind-hydro system connected to a local grid. Application is built on modular architecture to facilitate easy study of each component module influence. Blocks like wind model, solar model, hydroelectric model, energy conversion and load are implemented and the results of simulation are also presented. With the proposed application many situations can be studied. An important study is the behavior of hybrid system which allows employing renewable and variable in time energy sources while providing a continuous supply. Application represents a useful tool in research activity and also in teaching. References [] Dumitru, C. (007), The Development of Local DC and AC Distribution Networks Supplied with Energy Provided by Renewable Resources, Conferinţa de Inginerie Energetică CIE 007, Băile Felix, iunie 007, Analele Universităţii din Oradea, Fascicula Energetică; [] Kaltschmitt, M., Streicher, W., Wiese, A. (007), Renewable Energy, Technology, Economics and Environment, Springer-Verlag, Berlin Heidelberg; [3] Markvart, T., Castaner, L. (003), Practical Handbook of Photovoltaics, Fundamentals and Applications, Ed. Elsevier, Oxford, UK; [4] Patel, M. R. (999), Wind and solar power systems, CRC Press LLC, Boca Raton, Florida; [5] Dumitru, C.D., Gligor, A. (008), Power Quality Analysis of a System Based on Renewable Energy Supplying a Local Distribution Network, Acta Electrotehnica, Special Issue: Proceedings of the nd International Conference on Modern Power Systems MPS 008, -4 nov. 008, pag. 4-6, Cluj Napoca, România; [6] Golovanov, N., Postolache, P., Toader, C. (007), Eficienţa şi calitatea energiei electrice, Ed. AGIR, Bucureşti, Romania 49