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This paper dealt with the optimization of the performance of a photovoltaic mill system operating on the sun race. Depending on the characteristics of the powered load which is a DC motor driving a grain mill and on the weather conditions (temperature and illumination), we noted a very big difference between the potential maximum power and that actually transferred to the load. In order to improve the overall efficiency of the system, we use an adaptation circuit consisting of a boost converter controlled by a numerical MPPT (Maximum Power Point Tracking) command. With the Perturb & Observe (P & O) algorithm, the MPPT control measure the photocurrent, the photo tension and the power released by the photovoltaic generator. From this result, the MPPT control adjusts the duty cyclic of the converter to bring the system to the optimum operating point. Hence, using MATLAB/Simulink software, we did the modeling and the simulation of the system which is composed by a PV generator, a boost converter, a Pulse Width Modulation and a DC motor.

Currently, solar photovoltaic is no longer limited to lighting or powering domestic appliances; but it also supplies income generating activities, particularly solar grain mill [

The use of a photovoltaic generator (PVG) must be subjected to some requirements:

The system should be as simple as possible;

The photovoltaic generator must be optimized with a correct efficiency;

The system must operate automatically and reliably.

The simplest system we can have in this case is to directly couple the PV generator to a DC load (DC motor-mill group for example). But generally, the operating point of the DC load does not match the optimum operating point of the PVG [

That’s why we are interested to sizing of a solar system operating mill running over the sun. In order to optimize the system performance, we use a matching circuit consisting of a boost converter controlled by MPPT (Maximum Power Point Tracking) control.

In our work, we proposed a modeling study under MATLAB/Simulink for the various components of the PV system and we presented the simulation results which are discussed.

The system studied is composed of a PV generator, a boost converter, a PWM controller with MPPT technology and a DC motor-mill group. The following

The matching circuit is a boost converter inserted between the PV field and the DC load and which maximizes the power supplied to the load for any level of illumination and temperature thanks to the digital MPPT control.

The power delivered by a photovoltaic cell is not enough to supply a DC load like motor-mill group. It is necessary to associate multiple solar cells in series

and in parallel to have a solar panel and attain the desired power. Similarly, the interconnection of several solar panels in series and in parallel enables to obtain a power higher than that of the solar panel; that is why the notion of PV generator is created [

If we set

The relationship between the current I_{pv} (A) and the voltage V_{pv} (V) at the output of the PVG constituted by several panels connected in series and in parallel is modeled in the literature by [

where:

The equation of the photocurrent (

where,

^{2});

^{2});

The saturation current of the diode is given by the following equation:

From these equations, the model of PV generator is made using Simulink software.

The boost converter is an elevator DC-DC converter inserted between the PVG and the DC load. Its typical application is to convert the input voltage to a higher output voltage [_{r} which takes two states (closed (u = 1) and open (u = 0)), an inductor L and an output capacitor Cs. The basic scheme of the boost converter is shown in

When the transistor T_{r} is closed (on mode), the photovoltaic source charges the inductance L, meanwhile, the capacitor C_{s} maintains the output voltage of the converter using the energy previously stored. When the position of the transistor T_{r} change (off mode), the DC source and the energy stored in the inductance go together supply the load, resulting in an increase of the output voltage [

The modeling of the boost converter can be obtained by applying the fundamental laws governing its operation [

and

During the hash period, the transformation ratio is, by calling α duty cyclic (i.e. ratio of the time during which the transistor is closed [

With α taking values comprised between 0 and 1.

For a given incident power, the optimum power transferred to the load is maximum only for a well-defined duty cyclic.

In this study, we used the Perturbation and Observation (P & O) method. This choice is due to the fact that this is a widespread approach in seeking the MPP (Maximum Power Point); in addition it is simple to use and requires only measurements of current and voltage of the PV generator (

The reference voltage thus generated by the MPPT control is then compared with a triangular (or sawtooth) signal in order to provide an adequate duty cyclic. This principle is called PWM (Pulse Width Modulation) controller [

A comparator makes it possible to generate at its output a rectangular voltage modulated in impulsion width. (

When the voltage V_{ref} increases (decreases), the duty cyclic increases (decreases). The variations of the voltage, V_{ref}, induce, for a given sunshine and temperature, a move of the operating point on the P_{pv}/V_{pv} characteristic curve.

DC Motors are used for the training of electrical machines. In our study it is a grain mill. The motor shaft is connected to the hammers of mill via a polished- belt system. The block diagram of such DC load is presented in

The DC machine can be modeled through electrical, electromagnetic and mechanical equation. These three groups of equations describe the real operation of the DC motor:

Ø Electrical equation [

where:

The electromotive force (E) is related by the rotational speed of the motor by the relation:

where:

Ø Electromagnetic equation [

When a current I_{a} circulates in the armature of the motor, there appears an electromagnetic torque T_{em} (N·m) created by the Laplace forces which are exerted on the conductors of the armature. This torque is related to the inductor flux

If the armature presents an electromotive force E, while it is traversed by a current of intensity_{em}:

The rotor turns at the angular velocity Ω. Hence the power P_{em} can also be written as:

Ø Mechanical equation :

The mechanical equation of the electric motor described the ratio between the moment of inertia J (kg·m^{2}), the rotational speed Ω (rad/s) and the torque T (N·m). By equating the motor torque to the electromagnetic torque (true to a constant: friction torque), this equation is given in [

The results obtained after simulation of the PV generator are shown in

These results show the nonlinearity characteristics of the PV generator. Indeed, on the characteristic curve I-V (or P-V), there is a point where the power delivered by the PV generator is maximum (

In order to know the effect of the temperature on the performance of the PV system, we presented in the

We can see that the increase of temperature causes a small drop in the power available at the terminals of the PV generator.

For to know the effect of solar radiation on the I-V and P-V characteristics curves of the PV generator, we set the temperature at^{2}. We get the following results: (

These results show that the current produced by the PV generator is highly

dependent on solar radiation, but the voltage varies slightly. Then the maximum power point (MPP) of PV generator varies with decreasing sunlight.

The simulation using the MATLAB/Simulink software made it possible to have the

results presented in

In

These results show that the output voltage of the converter is higher than that the input. So the DC-DC converter performs its role properly.

The simulation results for the direct connection between the PVG and the DC load are shown in

We can observe that the PV generator is poorly operated and does not provide the maximum potential power. The difference being wasted as heat dissipated in the PVG. This can be explained by the fact that the nominal operating voltage of the load is different from the optimum voltage V_{opt} of PVG.

With the measured values of the PVG characteristics (

Direct connection was simulated output power and gives as results:

In this case, there is an efficiency of:

The obtained results are represented in

These results show that the operating point of the system converges to the maximum power point of the PV generator.

The optimized system gives as results:

Hence the efficiency of the system is:

In this paper, we modeled and simulated all the components of a solar mill system: PV generator, boost converter, MPPT control “Perturbation and Observation” and the load (DC motor-mill group). The simulation results and discussions for direct connection PVG-load and indirect connection controlled by a DC/DC converter are presented. From these results, we noted that:

The performances of PVG degrade with fluctuations of weather conditions (temperature and illumination);

The boost converter and the MPPT control (P & O) properly perform their role. The boost converter provides an output voltage higher than the input voltage; and the MPPT control adjusts the PV generator to the load: There’s a maximum transfer of the available power to the terminals of the PV generator.

Gaye, T.A., Dieng, B., Mbodji, S., Sow, O. and Sissoko, G. (2017) Study and Optimization of a Photovoltaic Mill System Functioning on the Course of the Sun. Energy and Power Engineering, 9, 260-272. https://doi.org/10.4236/epe.2017.94018