2018 China International Conference on Electricity Distribution Tianjin, 17-19 Sep. 2018 Operation planning of wind-PV-battery hybrid system Abdulla Ahmed, Member, IEEE and Tong Jiang State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, School of Electrical and Electronic Engineering, North China Electric Power University, Beijing - China Abstract—This paper explains the operation principles of two renewable energy sources (RES) hybrid system consists of solar photovoltaic (PV) and wind power (WP) which are connected to the battery energy storage (BES) system and load demand. The system has been designed for the city in Sudan and the results are presented for variable wind speeds and different solar radiations. From the results, it can be seen that the maximum power is being extracted for all wind speeds from rated speed to average speed. The location of the solar power station is suitable to install near to the wind farm station since the sun and wind powers usually complement each other, this could prove to be a good combination. In addition, most importantly the solar energy and wind power generation systems also produce no harmful waste and deplete no resources. The proposed system is important for the backup system during the changes in solar radiation and wind speed under the assumption that the load demand is fixed as well as helpful for power system management and designing. Index Terms— Solar energy, wind energy, battery energy storage system, modeling and design. I. INTRODUCTION I N the world, some regions such as Sudan has a great wealth of RES, especially solar and wind energies. These energies do not utilize to compensate for the increasing demand for electrical power. Also, the burden on transmission network is increasing at an unexpecting manner so that the transmission network becomes economically cheap. Furthermore, the depletion of fossil fuels and the rampant increase in the price of these fuels have resulted in increased interest to include RES for power productions. As a result some areas far from the Manuscript received July 15, 2018. This work was financially supported by the Science and Technology Project of State Grid Corporation of China No.SGHE0000KXJS1700086. Abdulla Ahmed was with the Department of Electrical and Electronics Engineering, Faculty of Engineering Science, University of Nyala, Nyala – Sudan. He is now with the School of Electrical and Electronic Engineering; North China Electric Power University, Beijing-China. (Corresponding author phone: 008613241448448; e-mail: [email protected]). Tong Jiang is a professor with the School of Electrical and Electronic Engineering; North China Electric Power University, Beijing-China. (Phone: 008613691338016; e-mail: [email protected]). CICED2018 Paper No. 201805280000367 978-1-5386-6775-0/18/$31.00 ©2018 IEEE national electricity network there is a great need for including RES like wind and solar sources as major contributors to the electrical power system. Control and power management of a hybrid system that consists of WP/PV and subsystems and BSS connected to AC residential load and the power grid (PG) which is used as a backup source are presented to manage the electricity between its different generation sources and the system load demand profile [1],[2]. An optimization system aiming to obtain the overall costs, which including fuel costs and maintenance costs by taking an independent micro-grid of a certain school as a case study for the design is introduced in the work [3]. Two algorithms are presented for optimal operation of the hybrid system consist of wind/PV and BESS in the grid-connected mode of a microgrid. The first one is called sources sizing algorithm to obtain the optimal sizes of RES and the second one is called battery sizing algorithm to obtain the optimal capacity of BESS [4]. For the Stand-alone mode of a microgrid, hybrid system configuration consist of wind/PV/diesel/storage based techno-economic analyses to plan the optimal scheduling and solve the problem for the future development planning of island power supply is presented in [5],[6]. An artificial intelligent method to design the stand-alone energy containing PV and WP with BESS is pretested for reducing the yearly operating costs while meeting the basic requirements of the stand-alone energy system design [7]. The performance analysis of a PV-connected grid system with compressed air energy storage system (CAES) is proposed and investigated in terms of thermodynamic and exergy and the results show that the efficiencies of the CAES system are improved from 35% to 65% during the operation period in the year [8]-[10]. The main contribution of this work is to model and design the distributed hybrid generation system based on PV system and WP farm connected to the BESS. Implementation of distributed generation will reduce the CO2 emission as well as minimize the generation cost. The rest of the paper is organized as follows: Section II presents a general description and mathematical modeling of the system. The case study is dealt in Section III. Section IV describes the results and discussions. Finally, the conclusion is presented in Section V. Page1/4 2655 2018 China International Conference on Electricity Distribution II. GENERAL DISCRIBTION AND MATHEMATICAL MODELING OF THE SYSTEM The proposed system is constituted of two kinds of RES, the first is WP with rectifiers and the second is PV with DC/DC converter system; the system load demand and lead-acid battery are placed in order to mitigate the variability of the of RES as shown in figure 1. Fig.1. Schematic diagram of the proposed system. A. Modeling of Solar System The solar cell is basically a p-n junction fabricate in a thin wafer or layer of semiconductors. The radiations of solar energy can be directly converted to electricity through the photovoltaic effect. There are many types of solar cell models. This paper will discuss the ideal photovoltaic model and photovoltaic model with Single-diode and series resistance. 1- Ideal Photovoltaic Model The ideal equivalent circuit of a PV cell is a current source in parallel with a single-diode. The configuration of the simulated ideal solar cell with single-diode is shown in figure 2. Tianjin, 17-19 Sep. 2018 𝐼 = 𝐼𝑃𝑉 − 𝐼0 [exp ( 𝑉 𝐴𝑉𝑇 ) − 1] Where 𝐼𝑃𝑉 is the current generated by the incidence of sunlight; 𝐼0 is the diode reverse bias saturation current and 𝑉𝑇 is the thermal voltage of a PV module and can be expressed by: 𝑉𝑇 = 𝑁𝑆 ∗𝑘∗𝑇 𝑞 Where Ns is cells connected in series; q is the electron charge; k is the Boltzmann constant; T is the temperature of the p-n junction and A is the diode ideality factor. A PV cell can at least be characterized by the short circuit current (ISC ), the open circuit voltage (VOC ) and the ideality factor A. The output of the current source is directly proportional to the light falling on the cell. For the same irradiance and p-n junction temperature conditions, the short-circuit current (ISC ) is the greatest value of the current generated by the cell. The short current is given by: For V=0, ISC = I = IPV Likewise, for the same irradiance and p-n junction temperature conditions, the open circuit voltage (𝑉𝑂𝐶 ) is the greatest value of the voltage at the cell terminals and it can be written as: 𝑉 = 𝑉𝑂𝐶 = 𝐴 ∗ 𝑉𝑇 ln [1 + 𝐼𝑆𝐶 𝐼0 ] In Eq. 2, for I = 0. Thus, ISC = IPV And at the same conditions, the output power is given by: Fig. 2. Ideal PV cell with single-diode. 𝑉 The output current of an ideal solar cell can be expressed by: 𝑃 = 𝑉 {𝐼𝑆𝐶 − 𝐼0 [𝑒𝑥𝑝 ( 𝐼 = 𝐼𝑃𝑉 − 𝐼𝐷 2. Photovoltaic Model with Single-diode and Series Resistance To build the photovoltaic model with single-diode and series resistance can only adding a series resistance to the previous model to be more accuracy and complexity. The circuit of this model is shown in figure 3. The current passed through the diode (𝐼𝐷 ) can be given by: 𝐼𝐷 = 𝐼0 [exp ( 𝑉 𝐴𝑉𝑇 ) − 1] 𝐴∗𝑉𝑇 ) − 1]} By substituting Eq. 2 into Eq.1, The output current of an ideal solar cell is expressed as: CICED2018 Paper No. 201805280000367 Page2/4 2656 2018 China International Conference on Electricity Distribution Tianjin, 17-19 Sep. 2018 The power in moving air is the flow rate of kinetic energy per second. Therefore: 𝑝𝑜𝑤𝑒𝑟 = 1 𝑑𝑚 2 𝑑𝑡 𝑣2 The power extracted by the blades is customarily expressed as a fraction of the upstream wind power as: 𝑝0 = Fig. 3. PV circuit model with single-diode and series resistance. For the same irradiation and temperature conditions, the inclusion of a series resistance in the model implies the use of a recurrent equation to determine the output current in function of the terminal voltage. The I-V characteristics of the solar cell are given by: 𝐼 = 𝐼𝑃𝑉 − 𝐼0 [exp ( 𝑉+𝐼∗𝑅𝑆 𝐴∗𝑉𝑇 The short circuit current 𝐼𝑃𝑉 is given by: 𝐼𝑠𝑐 = 𝐼 = 𝐼𝑃𝑉 − 𝐼0 [exp ( 𝐴∗𝑉𝑇 ) − 1] For V = 0 and ISC = IPV Normally the series resistance is small and negligible. The open circuit voltage 𝑉𝑂𝐶 can be given by: 𝑉 = 𝑉𝑂𝐶 = 𝐴 ∗ 𝑉𝑇 ln [1 + 𝐼𝑆𝐶 𝐼0 ] For I = 0, and the output power is given by: 𝑃 = 𝑉 {𝐼𝑆𝐶 − 𝐼0 [exp ( 𝑉+𝐼∗𝑅𝑆 𝐴∗𝑉𝑇 ) − 1]} B. Modeling of Wind Power Wind energy can be defined as the process by using the wind turbines to convert the kinetic energy in the wind into mechanical power. Once a wind turbine has converted the kinetic energy in the wind into rotational mechanical energy, that rotational energy is usually converted by a generator into electricity. The kinetic energy in the air of mass “m” moving with speed v is can be given by: 𝐾𝑖𝑛𝑒𝑡𝑖𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 = CICED2018 1 2 2 𝑚𝑣 2 Paper No. 201805280000367 𝜌. 𝐴 . 𝑉 3 . 𝐶𝑝 Where p0 is the output power (KW); ρ is the air density (Kg/m3 ); A is the area swept by the rotor blades (m2 ) and V is the velocity of the air (m/s 2 ). 𝐶𝑝 is the fraction of the upstream wind power, which is captured by the rotor blades. The remaining power is discharged or wasted in the downstream wind. The factor 𝐶𝑝 is called the power coefficient of the rotor or the rotor efficiency and can be determined by: ) − 1] 𝐼𝑃𝑉 ∗𝑅𝑆 1 𝑉 𝐶𝑝 = 𝑉 2 (1+ 𝑉0 )(1−( 𝑉0 ) ) 2 For a given upstream wind speed, the value of 𝐶𝑝 depends on the ratio of the downstream to the upstream wind speeds, that is (𝑉0 /V). The maximum value of 0.59 when the (𝑉0 /V) is one-third. The maximum power is extracted from the wind at that speed ratio when the downstream wind speed equals one-third of the upstream speed. Under this condition, the power output can be written as: 𝑝𝑚𝑎𝑥 = 1 2 𝜌. 𝐴 . 𝑉 3 . (0.59) The theoretical maximum value of 𝐶𝑝 is 0.59. In practical designs, the maximum achievable 𝐶𝑝 is below 0.5 for high-speed. III. CASE STUDY This work is a part of project developing in Sudan. We used data from the developing wind farm and solar PV system in the small city in Sudan as the case study. The ministry of electricity and dams of the Republic of Sudan is developing a wind farm with a final capacity of 20 MW and PV power station with final capacity of 5MW. These plants are planned to be integrated into an existing isolated Diesel power station. A wind speed and solar radiations measurements has been finished and identified areas of high wind speed and solar radiations and investigated the feasibility of generation electricity by wind and solar energies. The mechanical conversion efficiency of the wind turbine is taken as 95% and the electrical conversion efficiency of the generator is taken as 90%. Page3/4 2657 2018 China International Conference on Electricity Distribution IV. RESULTS AND DISCUSSIONS For the PV system, the power generated is a function of the temperature and the sunshine values of the site where the solar cell is placed. This power produced can increase or decrease according to any variations in the temperature and/or the shining. In this system, the temperature is considered as constant and equal to 25°C, and the sunshine changed from zero to 1000𝑤/𝑚2 . It can be seen that from the table I, the maximum power rating is 300wp for the one section and the maximum input power into the inverter is 600kwp as shown in Table III. For the wind power, the value of the power generated depends on the performance coefficient CP and the available wind speed. To maximize this output power, and as the wind speed is varying from time to time, the performance coefficient must be maximized and controlled. Table II shows the results of the specific parameters of the wind turbine and power rated is 850kw. For the battery energy storage system conditions, the battery system should not be discharged to the minimum limit and charged to the maximum limits; because maximum charging and minimum discharging will cause a decrease in the life cycle of the battery system. Thus, in order to overcome this problem, the SOC should be controlled. Therefore, SOC must be lies between 0.3<SOC<0.8. The initial value of SOC is considered to be equal to 0.3. For the load system demand conditions, the load is manifested by the current it needs from the source and both the voltage of the wind turbine and the solar panels should be higher than the voltage of the battery system. TABLE I SIZING INFORMATION RELATED TO THE PV FARM Power rating Module efficiency Rated voltage Rated current open circuit voltage Short circuit current Max reverse current Max system voltage 300wp 15.15% 37.03V 8.12A 44.98V 8.46A 20A 1000V TABLE II POWER GENERATED FROM THE VARIABLE WIND SPEED Wind Turbine Parameter Value Cut-in wind speed 34m/s Cut-out wind speed 32 m/s Rated power 850kw-1MW Type 3 blade upwind Rotor diameter 54m Rotor swept area 2123.7 m2 Nominal wind speed 6.5m/s CICED2018 Paper No. 201805280000367 Tianjin, 17-19 Sep. 2018 TABLE III INPUT PARAMETERS FOR THE INVERTOR Max input power 600kwp DC voltage range.mpp 450 to 750V Max DC voltage 900V Max DC current 1145A Voltage rippl ˂ 3% V. CONCLUSION The work chooses a small city in Sudan as the case study to designs a hybrid system containing solar PV and wind power connected with BESS aiming to reduce the operation cost. 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