Modelling and Simulation Hybrid Wind Solar Energy System Using MPPT
Abstract
With the emerging technology, demand for more energy makes us look for new energy sources. The most significant application field is renewable energy. Wind and solar energy have been popular ones owing to profuse, ease of availability and convertibility to the electrical energy. This topic covers the understanding of a hybrid renewable energy system for a domestic application.
The main objective of this project is to boost the power transfer capability of grid interfaced hybrid generation system. This hybrid system is a combination of solar and wind energy systems. In order to get maximum and constant output power from these two renewable energy systems at any time, this project proposed the idea of maximum power tracking techniques. The main concept of this maximum power point tracking controller is used for controlling the DC to DC boost converter. Finally, the performance of this MPPT based Hybrid system is observed by simulating using Matlab/Simulink.
Proposed Hybrid Energy System
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Figure 1 show the structure of the hybrid system based on solar and wind energy systems. A rotor in the wind turbine captures the wind’s kinetic energy. It consisting of two or more blades mechanically attached to an electrical generator 4. The mechanical power taken from wind by a wind turbine can be formulated as
Pm =0.5?ACp V3
0.59 is the hypothetical maximum value power coefficient, It is based on two variables the pitch angle tip speed ratio, with respect to longitudinal axis turbine blades are aligned at an angle that is the pitch angle. The linear speed of the rotor to the wind speed is tip speed ratio. Wind turbine “Cp, max vs ?” curve is shown in figure 2. In practical applications, 0.4 to 0.5 is the maximum range for high-speed turbines and for low-speed turbines the range is around 0.2 to 0.4. At ?opt its maximum value (Cpmax) is shown in figure 2 which results in optimum efficiency and maximum power is captured from wind by the turbine.
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In photovoltaic (PV) system, a solar cell is the main component. PV array is nothing but solar cells are connected in series or parallel to achieve required current, voltage and power. Each Solar cell is similar to a diode with a p-n junction formed by semiconductor material 5. It produces the currents when light absorbed at the junction by the photovoltaic effect. Figure 4 shows an insulation output power characteristic curves for the PV array. It can be observed that maximum power point exists on each output power characteristic curve. Figure 4 shows the (I-V) and (P-V) characteristics of the PV array. The equivalent circuit of a solar cell is the current source in parallel with a diode of a forward bias, Load is connected to the output terminals. The current equation of the solar cell is given by 6 7
I = Iph – ID – Ish
I = Iph – Io exp (q(V+RsfAkBT) – 1 – V+RsfRshcenter553802700
http://myelectrical.com/notes/entryid/225/photovoltaic-pv-electrical-calculationscenter-671567
Figure4. Output Characteristics of a PV array
Battery Energy Storage System (BESS)
The conversion of AC to DC is done by Battery Energy Storage System (BESS). It is power electronic devices control system and batteries. We use batteries for storing purpose, the working of the battery is the conversion of electrical energy into chemical energy. Batteries are charged and discharged by DC power. Bidirectional power electronic devices are for regulating power flow between batteries and energy systems 8. Based on the type of battery it’s having various advantages and drawbacks like cost, weight, size, power, and energy capability. Lithium-Ion, Lead-Acid, Nickel-Cadmium, Nickel Metal Hydride are the important types of energy storage technologies. High discharge rates are attained by Lead-Acid batteries. Long cycle life, high energy density, charge or discharge efficiency is high are the qualities of sodium sulfur batteries. Nickel Cadmium (NiCd) batteries are better in all qualities and low maintenance requirements than the lead-acid batteries 9 10. But these batteries cost is high when compared to lead-acid battery. Nickel Metal Hydride (NiMH) batteries are used in hybrid electric vehicles and tele-communication applications because these are compact batteries and light in weight. The highest energy density among all types of batteries are Lithium-Ion batteries. They are currently used in cellular phones, computers, etc. and development of this technology is used in distributed energy storage applications. Because of its sizes availability small, medium and large scale renewable energy systems and high rate of progress in development it is commanding the electronics market. During coupled operation, changes in the outputs of wind and solar PV generation 11 12 will change in the output of BESS and BESS must neutralize by quick changes in output power. Rate variation control or ramp rate control is applied for an associated coupled system to smooth their real power fluctuations. The information is processed by the battery energy system controller. It estimates the State of Charge (SoC) and capacity of each battery cell and protects all the cells to operate in the designed SoC range. On a smaller scale the economic and technical merits of energy storage systems are as follows
• Electrical supply quality and reliability are improved.
• For critical loads it supplies backup power.
Maximum Power Point Tracking
The efficiency of wind turbine and solar panel is improved by MPPT when they are set to operate at point of maximum power. In different techniques of MPPT the most popular techniques are incremental conductance method, perturb and observe, fuzzy logic, neural networks. Initial photovoltaic array reference voltage and the initial rotor speed reference for the wind turbine are to be adjusted if the two systems output powers are does not match to their maximum powers 13. We need to adjust the initial reference values in direction of increasing manner of output power and vice-versa. Until the wind turbine and photovoltaic array reach the maximum power points same process repeats. The characteristic power curve for a PV array is shown in figure 4. If MPPT techniques considered it as a problem then it finds the voltage VMP or current I and automatically under a given temperature and irradiance the PV array should get the maximum output power PMP 14.
Simulation Results
left376110500The complete system design (hybrid energy system) is simulated using SIMULINK. A 10-KW wind/PV/BESS hybrid system was considered. The simulation study of system parameters are presented below and to predict their actual characteristics. Three energy sources are modeled accurately in SIMULINK. Figure 5 show the simulation diagram for hybrid system with solar and wind systems.
Simulated Graphs
• The load demand to fulfill is 10 KW throughout the time scale except at 4 to 5 sec when it increases to 14 KW.
• Solar energy drops its irradiance to 15 % from 2 sec.
• Wind turbine initially rotating at 5m/s excels to base speed 12m/s after 0.5 sec. Its rotating speed is decreased to 25 % of its base speed.
• All these conditions are clearly observed in the below graph.
• The maximum voltage of PV array is observed at around 640 V. The curve below explains that the varying irradiance is the deciding factor of the maximum voltage derivations.
• Figure 6 shows the simulation result for output voltage across load terminals. From this result we observed that the voltage changes with respect to the changes occur in either wind or solar plants.
• Figure 7 show the simulation result of output current through the load. If the load is changed or suddenly extra load applied to the system then changes occur in the load current. In this thesis we suddenly applied the load during the time of 1 to 2 sec and then in this period the current rises.
• Figure 8 shows the wave form for powers which are obtained from the solar plant and wind energy system and with this the line power is depended. Figure 9 shows the simulation result for wind turbine output voltage.
• Figure 10 show the simulation result of output power from the battery system.
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Conclusion
Output from solar and wind system is converted into AC power by using inverter. In the given time additional load of 5 KW is connected by using circuit breaker. Under all operating conditions to meet the load the hybrid system is controlled to give maximum output power. Battery is supporting to wind or solar system to meet the load and also simultaneous operation for the same load.
