一种基于单片机控制的新型光伏电池毕业论文外文翻译.doc

【精品文档】如有侵权，请联系网站删除，仅供学习与交流一种基于单片机控制的新型光伏电池毕业论文外文翻译.精品文档.附录A Microprocessor-Controlled New Class of Optimal Battery Chargers for Photovoltaic ApplicationsAbstractA simple, fast and reliable technique for charging batteries by solar arrays is proposed. The operating point of a battery is carefully for cednear the maximum power point of solar cells under all environmental (e.g., insolation, temperature, degradation) conditions. Optimal operation of solar arrays is achieved using the Voltage-Based Maximum Power Point Tracking (VMPPT) technique and the charger operating point is continuously adjusted by changing the charging current. An optimal solar battery charger is designed, simulated and constructed. Experimental and the oretical results are presented and analyzed. The main advantages of the proposed solar battery charger as compared with conventional ones are shorter charge time and lower cost.Index TermsCharger, microprocessor, maximum power point tracking (MPPT), photovoltaic.I.INTRODUCTIONThe field of photovoltaic systems is quite broad with many stand-alone and grid-connected configurations. Applications of solar energy include water pumping , refrigeration and vaccine storage, air conditioning, light sources, electric vehicles ,PV power plants ,hybrid systems , military and space applications.Reference8has divided photovoltaic applications into four categories: large-scale grid connected systems, small remote photovoltaic plants, low power stand-alone systems, and a combination of solar systems with other alternative energy sources. These categories may also be viewed in terms of load characteristics. There are three load types:a DC load, a “dead” AC load, and a “live” AC load(e.g.,a utility system).Most of these applications use batteries as backup energy systems and/or matchingdevices for balancing their energy flow during peak load or poor environmental conditions (e.g., low insolation, high temperature or high degradation).The main drawbacks of PV systems are high fabrication cost, low energy conversion efficiency, and nonlinear characteristics. For increasing conversion efficiency, many Maximum Power Point Tracking (MPPT) techniques have been proposed and implemented. They can be categorized as:A)“Look-up table”methods 12,13The nonlinear and time varying nature of solar cells and their great dependency on radiation and temperature levels as well as degradation(aging,dirt,snow) effects, make it difficult to record and store all possible system conditions.B)“Perturbation and observation(P&O)” methods 14,15 Measured cell characteristics (current, voltage and power)are employed along with an on-line search algorithm to compute the corresponding maximum power point which is dependent on insolation, temperature or degradation levels. Problems with this approach are undesirable measurement errors(especially for current)which strongly affect tracker accuracy.C)“Computational” methods 2,1619The nonlinear V-I haracteristics of a solar panel are modeled using mathematical equations or numerical approximations, and maximum power points are computed for different load conditions as a function of cell open-circuit voltages or cell short-circuit currents. In the literature, many battery charging techniques are investigated and proposed2024.These methods use avariety of battery characteristics(voltage and temperature) to achieve a safe and fast charging process. However, two well-known charging methods employing photovoltaic sources are the constant current charging, and the direct connection of solar panel to battery and load(e.g., battery tied solar systems).In this paper, a simple and fast variable-current charging tech-nique, based on “computational” methods, is proposed for photovoltaic applicationswhere photovoltaic charger and battery are matched with respect to voltage and current. Online measurements of panel open-circuit voltage are used to detect the maximum power point of a solar panel. Battery charge rate is continuously adjusted such that the system operating point is forced near the detected maximum power point of solar panels. The oretical and experimental analyses are used to demonstrate the reliability and validity of the proposed technique.II.MODELING OF PROPOSED FAST SOLAR BATTERY CHARGERElectrical models for solar panel, maximum power point tracker, battery and battery charger will be used to simulate the proposed solar charging technique.A.Solar Panel ModelUsing the equivalent circuit of solar cells(Fig.1),the radiation and temperature dependent V-I haracteristics of m parallel strings with n series cells per string is （1）where is the cell short-circuit current(representing in solation level), is the reverse saturation current,is the series cell resistance and is a constant coefficient which depends on the cell material and the temperature T.For the silicon solar panel(,)used for theoretical and experimental analyses of this paperTable I, manufactured by the Iranian Optical Fiber Fabrication Co.(OFFC),(1)can be written as at T=25 （2）Equations(3a)and(3b)are evaluated for one OFFC panel at T=70 and T=-20, respectively. Computed and measured V-I as well as P-I characteristics for the OFFCpanel are shown in Fig.2 for two insolation levels. This figure illustrates the variations of cell maximum power points (e.g., maxima of P-I curves)with respect to insolation levels. (3a) (3b)Eqs.(2)and(3)along with Fig.2 depict the strong nonlinear dependency of the Maximum Power Point(MPP)with respect to insolation and temperature levels and justify for any high efficient PV system an accurate MPP tracker.B.Voltage-Based Maximum Power Point Tracking to determine operating points corresponding to maximum power for different insolation and temperature levels,(2)and(3) are commonly used2,17to compute the partial derivative ofpower with respect to cell voltage.Instead of finding the maximum via derivative,18and19employ numerical methods to show a linear dependency between “cell voltages corresponding to maximum power ”and“ cell open circuit voltages” (4)This equation characterizes the main idea of the Voltage-Based Maximum Power Point Tracking (VMPPT) technique. Is called the“voltage factor”and is equal to 0.74 for the OFFC silicon cells18,19.Equation(4)is plotted in Fig.3 together with the computed(almost linear)dependency of with respect to(shown by “+” signs ). C. Nonlinear Battery Model Most battery models ignore the presence of nonlinear electro chemical characteristics27,28.For the theoretical and experimental analyses of this paper, we propose a new nonlinear model for Ni-Cd batteries as shown in Fig.4. Measurements show linear variations of, and nonlinear characteristics of with respect to charge rate: （5）where is the charging current and R , Cs and Co are parameter values at biasing current level. For one cell of the 7 Ah Ni-Cd battery used for theoretical and experimental analyzes of this paper, the constants of(5)are obtained from measured characteristics(Table II)at charge rates of,and C. Computed and measured battery characteristics are compared in Fig.5. D.The Proposed Solar Charger For the optimal solar charger,an appropriate combination of the MPPT algorithm and battery charging technique must be selected. For the tracker, the simple and reliable voltage-based MPPT technique is used requiring very few components for sensing the solar-panel, open-circuit voltage. For the charging technique, variable-current charging is selected. This will allow the tracker to continuously adjust battery-charging rate and force the system operating point near the maximum power point of solar panels. Other tracking techniques could also be used. However, they require more components(for sensing panel short-circuit current and/or simultaneous panel voltage and current measurements)resulting in lower overall efficiency.III.SIMULATION OF PROPOSED SOLAR BATTERY CHARGERSimulink software and its facilities are used to model the proposed solar battery charger(Fig.6).We have created a block called“PV Source”to simulate the nonlinear V-I characteristics of one OFFC solar panel(2)employing cell short-circuit current as a measure of insolation level Fig.6(b).Saturation and delay functions are introduced to limit the fast response of the “controlled voltage source”and to improve convergence. The output of this block is the panel operating voltage.To simulate voltage-based maximum power point tracking, a block called “VMPPT” is introducedFig.6(c)that usesand to generate desired duty cycles for the charge unit.The panel open-circuit voltage is calculated,thereafter the panel voltage corresponding to maximum power(4)is computed and compared with and the error is amplified through a proper transfer function to generate the desired duty cycle.The charger unit consists of a DC/DC buck converter(chopper and output filter)and a LC input filter. The chopper includes a fast switch and a schottky diode. A block called“Battery Parameter Calculation” computes battery parameters Fig.4 and(5)corresponding to the system operating point.IV.CONSTRUCTION OF PROPOSED SOLAR BATTERY CHARGERFig.7 shows the constructed battery charger, which consists of the following parts:Silicon Solar Panelone OFFC silicon solar panel with maximum output power of about 35 W(Table I)is used togenerate solar energy. MicroprocessorThe 8085 Micro Controller Unit(MCU)is used to record and process measured voltage and current waveforms and to compute required signalsfor control and drive circuits. The 1524 IC employed to generate the required PWM command(e.g.,at 50 kHz)and voltage/current signals for the charger unit. Thevoltage-based MPPT for the solar panel is implemented by MCU under different environmental and output operating conditions. Note that the panel open-circuit voltage is continuously measured at a slower rate (e.g., every minute).Fig.8 shows the main functions of the MCU. If multiple solar panels with similar characteristics are used, a reference panel could be relied on to sense the open-circuit voltage. Any shadowing effects caused by dust, snow or clouds will result in power-current characteristics with several maxima. This will complicate MPPT.Charger UnitA chopper circuit is used to properlyconnect and disconnectbased on PWM signalssolarpanel from battery and load. Input and output filters are employed to suppress electrical noise at the output of the solar panel and at the input of the battery.Input and output current and voltage sensors are relied on for signal measurements. Battery and LoadFive units of 7 Ah Ni-Cd batteriesare connected in series to store electrical energy. Resistors serve as loads during discharging and charging modes, respectively. In discharge mode, the solar panel is partially or totally inactivated by shadow or eclipse effects.V.ANALYSIS OF EXPERIMENTAL AND THEORETICAL RESULTSThree charging methods are investigated: the proposed variable-current charging (method 1), direct connection of battery and load to solar panel(method 2),and constant-current charging(method 3). Battery (full) charging state is detected using the approach (e.g., using magnitude and slope of battery voltage as a function of time)of24.Experiments are performed for the following three operating conditions.Case A:Operation at an Incidence Angle of about Measured and computed time functions for battery current and voltage as well as solar panel power and voltage are shown in Fig.9 for normal operating condition(e.g.,normal insolation and temperature).As expected,fine tracking of solar maximum output power is achieved throughout the charging process when the proposed charging technique is usedFig.9(c),method 1).Charging time for the proposed method is only 3 hours which is about 73%and 52%of the required charging times for methods 2 and 3,respectively. In method 1,panel voltage(corresponding to maximum power)which is determined by(4)is slightly higher at 11 A.M.due to lower environmental temperature.In method 2,panel voltagee.g.,in Fig.9(d)and its operating point is dominated by battery voltage.This causes panel output power to decrease from 29 W(for method 1)to about 20 W(for method 2).In method 3,panel voltageabout 17 V in Fig.9(d)is determined by panel current which is proportional to the constant battery current (e.g.,0.2 C).This rate of charge is used to determine panel operating points for the simulation as outlined in Fig.6.The comparison of computed(X)and measured results forsome selected operating points is shown in Fig.9.Case B:Operation at an Incidence Angle of about Similar experiments are performed for a change in angle of incidence(Fig.10).At 12:30 P.M.the solar panel is rotated forward(in the direction of sun)such that the angle of incidence is changed from about to about. During the first 105 minutes, the charging processes of the three methods are normal and results are similar to Fig.9. At the start of changing the angle of incidence from about to about, maximum panel output power is decreased to about 25 WFig.10(c),method 1.Our detailed measurements show that under all operating conditions (e.g., before and after changing the angle of incidence),method 1 continues to adjust panel operating point near the maximum power point of the V-I characteristics. The angle of incidence of about increases charge time of method 1 to about 3.2 h. Methods 2 and 3 are not able to completely charge the battery since their operating points are not optimally selected. Note the inherent small voltage regulation of method 1,caused by the increasing slope of battery voltage. This is not true for methods 2 and 3 where fast voltage drops e.g., at 12:30 P.M. and 13:45 P.M.in Fig.10(b)occur. The measured characteristics of method 3 are interesting: constant-current charging continues for some time after changing the angle of incidence from to,this is so because the battery requires about 20 W of power Fig.9(c),method 3.At 13:45 P.M.,the solar panel is no longer able to produce the required power since its maximum power is decreased to about 20 W. The converter duty cycle is forced to unity, causing direct connection of panel, battery and load. Therefore, measured characteristics of methods 2 and 3 become similar.Case C: Operation with Eclipse This environmental operating condition is essential in satellite and spacecraft applications .Cease of insolation along with considerable temperature drop makes panel V-I characteristics very different before and after eclipse. We have generated this effect(Fig.11)by completely covering solar panel from 12:00 to 12:30 P.M. and decreasing its temperature from 24 C to 12 C As expecte