Contemporary Maximum Power Point Tracking Methods of Solar Photovoltaic Modules

Abstract

This paper presents a detailed analysis of different maximum power point tracking approaches for solar photovoltaic (PV) modules from traditional techniques. This paper also details the design of different sections of solar PV simulation for research purposes. The benefits and limitations of different approaches brief the researchers to propose novel techniques for extracting maximum power point under different operating conditions. The results presented in this paper will give a brief about the upcoming approaches in the considered research problem solutions. Novel artificial intelligence-based controllers and their implementation on the maximum power point tracking of a solar photovoltaic has been discussed which will abridge the research gap between the existing MPPT methods and the novel algorithms.

1 Introduction

Renewable energy sources are the most sought-after areas because of their availability and affordability. Non-renewable energy sources get easily exhausted, unlike renewable energy available in large amounts. Conventional energy sources expel out carbon and sulphur emissions into the atmosphere, which can cause damage to the environment as well as human beings. Solar energy is one of the cleanest sources of energy, i.e. there are no carbon or sulphur emissions, requires less maintenance and is an economical energy source compared with other energy sources [1]. PV systems are used to convert solar energy to electrical energy. PV systems are divided into two groups based on their applications, i.e. stand-alone PV systems (electric cars, street lights, battery charging, etc.) and utility shared PV system (hybrid power system, power plant, etc.) [2, 3].

The devices that are used for PV conversion is called solar cells. When solar irradiation falls on solar cell, it is directly converted into DC current. Major advantages are that there is no moving part, it requires less maintenance, works quite adequately with beam or diffused irradiation and can be adapted for various power requirements. But the limitation is that solar cells have minimal efficiency and require storage, and the initial cost of solar cells is high. A single solar cell cannot be used alone for the generation of energy where high power is required, like powering a household. The output of a single solar cell is very small, i.e. only 0.5 V, and the solar cell requires protection to protect it from moisture, mechanical shock, dust and harsh outdoor conditions. Solar cells are connected in series and parallel in a solar module. They are connected in series to build up the voltage and in parallel in order to supply the required amount of current. A group of cells connected in series is called a solar cell string. Most commercial modules have a connection of 32 or 36 silicon cells, making them capable of charging a 1-V battery. The power output from the installed solar PV modules also plays key role in its effective utilization. There are several methods proposed by the researchers [2,3,4,5], and further research is recommended for improved performance characteristics.

The paper is organized as, design aspects are listed in Sect. 2, MPPT techniques are presented in Sect. 3, and Sect. 4 provides different configurations of solar PV and finally conclusion and references.

2 Design Aspects of Solar PV Module

2.1 Solar Panel

The PV model consists of solar arrays composed of several solar panels connected in series and parallel. They are laminated with plastic to protest against environmental damage. And the PV array is composed of several photovoltaic panels and modules to generate power. Based on standard test conditions (STC), the maximum power output for a PV array is determined. Generally, the PV modules used are very safe and there are very few chances of failure. As well it has a long lifetime of 20 or more years. Depending upon our description of necessity, we choose the desired PV modules.

2.2 Buck Boost DC/DC Converter

The PV model uses a buck boost DC/DC converter. As we can derive by its name, it combines the function of a buck converter (steps down DC voltage) and a boost converter (steps up DC voltage). The converter comprises of capacitor, diode, inductor and a switching MOSFET. Based upon the duty cycle, voltage value can be incremented or decremented. The inductor stores energy in its magnetic field when the MOSFET is turned on. No current flows through load due to the reverse biasing of the diode (Fig. 1).

Fig. 1

Buck boost converter

The capacitor functions when the diode is reverse biased, the inductor is removed from the source, and the MOSFET is switched off. Reverse biasing causes the current to decrease instantaneously. The step-down duty ratio is below 50% and the step-up duty ratio is above 50%. This converter is used in PV system because when there is un-uniform illumination, the output voltage produced by the solar panel varies. So, with the help of buck boost converter, we may increase or decrease its value to our desired output value. The output voltage equations can be given by Eq. 1.

$${\text{Vo}} = - \frac{{{\text{DV}}i}}{1 - D}$$

(1)

2.3 PWM Modulator

PWM stands for pulse width modulation. This modulator is a switch connected between solar array and battery bank. This modulator decreases the current supplied to the battery, depending upon its charged state and reduces power supplied as it reaches its fully charged state. This increases the battery life and reduces duty cycle. It keeps batteries in floating condition, i.e. fully charged during the working period. It is used in solar PV system so that the charge stored does not again flow back into the solar array from the batteries during the night or a cloudy day because the output voltage is zero. Its efficiency lies in the range of 65–85%. It is especially useful when employed for small systems. Its operation is unaffected by the solar array size and works at constant efficiency.

2.4 MPPT Controller

It is a charge controller with an in-build MPPT also which sees maximum power being drawn into the battery from the solar panels. This is mostly useful in off-grid solar systems such solar pumps, solar home system, etc. It helps in studying the solar V–I characteristics. It is used in PV system such that it maintains the voltage close to the maximum power point value, so that the system always draws maximum power. It simplifies the system and increases efficiency. Its output value directly controls the DC–DC converter. It regulates the voltage difference and provides optimization for DC load. It is most suitable for large systems with comparatively larger output voltage than battery voltage.

2.5 Capacitors

The MPPT value is affected by the value of the variable capacitance of a capacitor. We assume that the capacitor is connected in parallel with the PV array, so the transfer function for the buck boost converter is derived and reduces source impedance.

2.6 Filters

Usually, low pass filters are used in PV systems that only pass the waveform's fundamental component and appear in the output component. It also prevents the passage of components consisting of harmonics. We need to use a resonant filter if we use an inverter that works for a single fixed frequency and supplies power for that particular frequency.

2.7 Inverter

They are used to convert the DC current generated at solar panels to AC. This current can be used for household purposes or in an electrical grid. If the power generated finds application in grid, it needs a special inverter called a grid-tie inverter. This grid tie works by matching the phase of generated current and the current required by grid. Else if this condition is not satisfied, it can get quite dangerous for the workers fixing the power lines. As soon as there is a power cut, it shuts down the supply in solar panel.

2.8 Series and Parallel Combination of Solar PV

To increase the voltage required, the solar PV modules are connected in series. The voltage of a single PV module is very small, and after connecting n number of PV modules we get desired voltage required by power plant. The open circuit voltage of the array is given by algebraic sum of individual open circuit voltages. We connect the solar cells in parallel to generate a large power, since increasing the voltage doesn’t satisfy the purpose, so we have to increase the current. In parallel combination, the current of individual PV modules gets added. While designing a PV array, we combine parallel- and series-connected solar modules to get maximum power because power is a product current and voltage.

2.9 PWM Pulse Signals

The buck boost converter generates the pulse with modulation signals. An inverter is used to convert DC voltage generated by PV module to a variable or fixed AC voltage of the required frequency and magnitude. We can get variable AC voltage output by keeping the inverter gain constant and changing the DC value. Or if we keep the input DC value constant and want a variation AC output voltage, we have to vary the gain constant of the inverter. Sinusoidal output waveform is desired, but practical inverters fail to achieve it due to harmonics present in low and medium power applications. So, we receive square wave or quasi square wave voltage for these applications. We need nearly sinusoidal waves with the least distortion in high-power applications. This is achieved by switching techniques that cuts away the harmonic components in the output voltage wave.

2.10 Bypass Diodes

These diodes allow one directional flow of current and it is generally connected in parallel to the solar array. Its function is to present the generated power to flow from healthy solar cells to non-illuminated solar cells and not burn it in the process. These are designed to carry the short circuit current smoothly. There are two types of bypass diodes. They are Schottky barrier diode and PN-junction silicon diode. Their designing matches that of blocking diodes, but its function makes it different. Blocking diode is connected in series and its function is to allow the current to flow from PV array to the load and prevents the current to flow back into the array. We use this diode when we have several parallel branches of PV arrays, so we connect a blocking diode in each branch to prevent charged batteries from discharging at nightfall. Generally, manufacturers make both types of diodes in wide ranging current capacities.

2.11 Load

The term load refers to the devices that consumes the generated energy. The PV system comprises of variable load. The MPPT draws maximum power during operation but if this much power is not absorbed by load, then MPPT automatically adjusts at operating point. The buck boost converter enables us to match the load demand with the solar module with the help of a transformer. If the PV works under constant power factor and is connected to a grid, then the MPPT doesn’t change with load change.

3 Maximum Power Point Tracking of Solar Cell

The solar radiation falling on the solar PV module will release free electrons which will result in the generation of voltage across the module, resulting in the flow of correct through the closed circuit. The maximum power point of a solar cell is the point on the power curve (I–V curve) at which the highest value of the maximum net power output can be obtained. Different techniques are used to track the MPP to improve solar panel efficiency [3,4,5,6,7].

The offline methods that allow the PV system to work around its estimated MPP are:

• Fractional open circuit voltage method

• Fractional short circuit current method

These types of offline techniques determine the MPP on the basis of appropriate data. The PV panel is disconnected from the load, so offline parameters such as open circuit voltage and short circuit current can be noted. Because the PV panel is isolated from the rest of the system at the time of measurement, these are called offline maximum power point trackers. Here, continuous tracking of voltage and current is not carried out. The data usually consists of typical current–voltage curves for different temperatures and irradiances. The main advantage here is the requirement of a fairly lesser number of current and voltage sensors (Fig. 2).

Fig. 2

Variation of solar PV module power and current w.r.t. voltage

3.1 Fractional Open Circuit Voltage Method

This method makes use of the fact that the relationship is almost linear between the open circuit voltage VOC and the PV array voltage, corresponding to the maximum power point VMPP for different irradiance and temperature levels.

$$V_{{{\text{MPP}}}} = K_{{{\text{PV}}}} V_{{{\text{OC}}}}$$

(2)

Here, KPV is a constant and is termed as voltage factor. KPV value is usually provided in the datasheet of panels. It typically lies between 0.7 and 0.9. This value can also be determined using panel V–I curves. The main aim here is to estimate the value of voltage at MPP. To do so, the VOC is found out periodically by isolating the panel from the load at certain time intervals and it is then multiplied with the voltage factor, i.e. KPV. A precise estimation of the voltage at maximum power point can be made with the help of VOC measurement, because the value of voltage factor given by VMPP/VOC does not change much with the temperature and irradiance. By opening the switch at regular instants, the VOC is sampled periodically. The duration of the process of sampling and its frequency determines the accuracy of estimation of VMPP. If high frequency is there and duty cycles are large, the quality of estimation is improved. But this comes at a cost, high frequency of sampling results in increased power loss. The FOCV method faces two major challenges, first that the panel can never really operate at true MPP and second, preventing the occurrence of power loss. At every time interval, the load is disconnected from the panel and the open circuit voltage is measured at those instants. The value of VOC is then multiplied with voltage factor K which gives the voltage at MPP. The PI controller then compares the actual PV operating voltage when it is connected to the load with the voltage at MPP, treats the difference between these two values as error and tries to reduce it by adjusting the duty cycle of converter.

3.2 Fractional Short Circuit Current Method

By observing the current–voltage curve of a solar module, we get that the current corresponding to maximum power point and the panel's short circuit current lie near each other. So, the main idea here is to find the ISC at different temperature and irradiance levels and then multiply it by voltage factor K to find IMPP. This way, we can approximate the value of IMPP and make the solar panel deliver power at higher efficiency.

Mathematically, this can be written as,

$$I_{{{\text{MPP}}}} = K * I_{{{\text{SC}}}}$$

(3)

The voltage factor K is mentioned in the datasheet of panels and usually ranges between 0.8 and 0.92.

At the start of the operation, to measure the ISC, the switch between the panel and converter is closed, so that the panel gets short circuited and no power gets delivered to the load. The current sensor at this point measures the ISC. After that, the switch is opened and current starts flowing to the load. The same current sensor now measures the IPV. The difference between IPV and IMPP is that the PI controller calculates the error and then the required duty cycle is computed and fed to the DC–DC converter. Now according to the duty cycle fed to the converter, it starts delivering power to the load. This way, the error is rectified every few minutes and the solar panel is made to operate somewhat near the maximum power point.

3.3 Advantages of Offline MPP Tracking

The methods discussed above yield approximate MPP tracking, yet they have some advantages as mentioned below.

The fractional open circuit voltage method requires only one voltage sensor and the fractional short circuit current method requires only one current sensor. This way, this technique becomes economical to implement. They are easier to implement when compared with other MPPT methods. The speed of their convergence is fast. Oscillations around maximum power point is not faced in these methods.

3.4 Disadvantages of Offline MPP Tracking

These methods have been limited to use because of several disadvantages. Because these are approximated techniques, using them, we cannot make the solar panel operate at the exact MPP because they are not suitable for highly efficient PV system applications. During the course of a day, the temperature and irradiance level varies a number of times. So, periodic measurements of VOC and ISC have to be taken quite a number of times. The PV panel gets isolated from the system and power is cut off from the load many times. This consequently results in significant power loss. This particular drawback is being worked upon by a few improvement measures such as making the intermittent measurement process not time-based. The measurements of offline parameters are taken only when the difference between the stored value (at the starting time of the system) and the instantaneous values crosses a certain value. In this way, the amount of power loss can be reduced. So, these were the conventional offline techniques to track the MPP of a solar panel. They are economical and easy to implement, but are a bit low in terms of efficiency. There are some other techniques in which the maximum power point (MPP) is captured online where a steady operating area is set by designing its control strategy. The duty cycle is kept constant when the operating point is within the stable operating area, i.e. aperiodic perturbation, and when the operating point is outside the stable operating area, maximum power point capturing is triggered to capture a new MPP with a different steady operating area. Some online techniques [4, 5] of MPPT are:

• Perturb and observe method

• Incremental conductance.

3.5 Perturb and Observe Algorithm for Maximum Power Point Tracking

It is one of the most basic methods of tracking maximum power point. It is also known by two other names like hill-climbing and two point power comparison method. This method can only track the maximum power point when the irradiance is uniform on the solar PV panel. This method is not applicable when the irradiance on the solar PV panel is non-uniform, i.e. when it is partially shaded.

Let us suppose the solar irradiance is uniform and we draw a PV curve, this curve will have only one maximum power point which is both the local and global maxima (for now, we're keeping aside the partial shading condition).

The graph for power versus voltage for perturb and observe algorithm is shown in Fig. 3.

Fig. 3

Change in power magnitude corresponding the changes in voltage of a solar PV module

From the graph, we can see that on the left-hand side of the MPP, the change in power with respect to voltage is positive, so when the voltage in this region is increased, the power increases and when it is decreased the power decreases. But on the right-hand side, the change in power with respect to voltage is negative, so if the voltage is increased the power decreases, and if it is decreased the power increases. At the MPP stage, the change in power with respect to voltage is zero, which is the condition for maxima.

As a result, the perturb and observe algorithm checks the power to voltage slope. If it is positive that means we're on the left-hand side of MPP, we're on the right side of the MPP if it is negative and we're at MPP if it is zero, i.e.

$${\text{d}}P/{\text{d}}V = 0\;{\text{At}}\;{\text{MPP}}$$

(4)

$${\text{d}}P/{\text{d}}V > 0\;{\text{Left}}\;{\text{side}}\;{\text{of}}\;{\text{MPP}}$$

(5)

$${\text{d}}P/{\text{d}}V < 0\;{\text{Right}}\;{\text{side}}\;{\text{of}}\;{\text{MPP}}$$

(6)

This logic can be used to make a flowchart in which the voltage of the panel is perturbed and the corresponding change in power is observed. The flowchart is shown in Fig. 4. From the flowchart, we can observe that if the perturbation results in power increase, then the perturbation is repeated in the same direction, otherwise it is repeated in the opposite direction. This perturbation is done repeatedly during each sampling period, until the slope is zero and MPP is reached. Finally, we save the value of the voltage and power at which MPP was reached. Now, this voltage is compared with the voltage of the solar panel to calculate the error which is the difference in the two voltages. The controller receives this error and converts it to the duty ratio ‘D’, the PWM generator receives this duty ratio and generates a PWM signal. In this hill climbing method, the duty ratio is directly perturbed. In order to increase the panel’s voltage, the duty ratio is decreased and vice versa when it is increased. We can stop perturbing the duty ratio and set it to one value as MPP is reached.

Fig. 4

Flowchart of perturb and observe algorithm

Some advantages of P&O method are that it has low hardware and software complexity, reasonable dynamic, analog and digital implementation. Some disadvantages of the P&O method are power oscillations, does not work under partial shading conditions and sudden change in lighting conditions results in drifting of the MPP to another point.

We cannot neglect the effect of weather condition for considering the efficiency of PV developed power  [7, 8]. The generated power is mainly influenced by irradiance and temperature. The graph is different in shading conditions. As we can see from the Fig. 5, when there are shading conditions, the PV graph will have many local maxima and one global maxima, so while tracking we may get stuck at a local maxima. Hence, this method fails for shading conditions and we go for other advanced techniques.

Fig. 5

PV characteristics of solar photovoltaic module under partial shading conditions

3.6 Incremental Conductance Maximum Power Point Tracking Algorithm

The incremental conductance (IC) method helps in overcoming the shortcomings of perturb and observe method. The P&O method cannot properly track the maximum power under quick varying environmental conditions, which can be overcome by the incremental conductance method. The IC method ensures that the MPPT works at its maximum efficiency and helps control the buck boost converter duty cycle. It improves the tracking time and produces more energy for vast irradiance change in environment. The algorithm of incremental conductance is obtained by differentiating the power of PV array/module with respect to voltage and equating the result to zero.

$${\text{d}}P/{\text{d}}V = 0$$

(7)

$${\text{d}}\left( {V * I} \right)/{\text{d}}V = 0$$

(8)

$$I + V{\text{d}}I/{\text{d}}V = 0$$

(9)

By rearranging the solution, we get:

$${\text{d}}I/{\text{d}}V = - I/V$$

(10)

where dI/dV is the incremental conductance.

So, at MPP the above equation should satisfy and depending upon the relationship between dl/dV and − I/V, we will obtain the MPPT operating point.

Hence,

$${\text{d}}I/{\text{d}}V = - I/V\;\;\;{\text{at}}\;\;{\text{MPP}}$$

(11)

$${\text{d}}I/{\text{d}}V > - I/V\;\;\;{\text{left}}\;{\text{of}}\;{\text{MPP}}$$

(12)

$${\text{d}}I/{\text{d}}V < - I/V\;\;\;{\text{right}}\;{\text{of}}\;{\text{MPP}}$$

(13)

If dI/dV >  − I/V, the PV array operating point is towards the left of the maximum power point, so we need to increase the PV array voltage to reach the MPP. If dI/dV <  − I/V, the PV array operating point is towards the right of the maximum power point, so we need to decrease the PV array voltage to reach the MPP. The flowchart of incremental conductance maximum power point tracking is shown below.

We obtain the maximum power through this method and directly control the PV’s extracted power. The advantages of this method are that it can determine when the MPPT has reached the maximum power point, whereas P&O oscillates around the MPP. It is more accurate in determining the increase or decrease in irradiance level when compared with P&O method, it is highly responsive and power output can be controlled.

This method assumes that the ratio of change in output conductance equals the negative of the instantaneous conductance. The disadvantage of this method is the complexity and cost of implementation is higher. So now we work on obtaining a condition such that (dI/dV) + (I/V) equals zero. The PWM control signal of the DC–DC boost converter is regulated by MPPT until the condition: (dI/dV) + (I/V) = 0 is obtained. This algorithm depends on PV curve slope and depends upon solar irradiance level and load resistance.

4 Solar PV System Configurations

The solar PV system configurations are of mainly,

• Stand-alone SPV systems

• Grid-interactive PV system

• Small systems for consumer applications

• Centralized PV station or PV power station

• Hybrid system.

4.1 Centralized PV Power Station or PV Power Station

This accounts for wide ranging applications in supplying power from solar PV system to the electric grid. It resembles conventional central power plant. It finds applications in solar farm, solar plant, etc. The peak value of solar energy is usually at 1 MWp. Many big PV power stations are owned and operated by self-sustaining producers. The world’s largest solar park is Bhadla Solar Park (2020) and has a storage of 2245 MW and is occupied over 5700 ha (14,000 acres) in Jodhpur district of Rajasthan.

4.2 Distributed PV System

This type of configuration is used for small-scale power generation such as residential loads (2–10 kW) and commercial loads (20–200 kW). They are usually set-up on the top of the roofs of houses or mounted on ground if space is available. These are mainly generated for personal use of owners, but if produced in large quantity it is sold. They are further classified as:

• (a) Stand-alone PV system

• (b) Grid-interactive PV system

• (c) Small systems for consumer applications.

4.3 Stand-Alone PV System

This PV system is not dependent on the electricity grid; the generated energy is stored in batteries. This is mainly produced in areas that do not have easy access to electric grid. It also comprises an inverter that converts the DC power produced by the PV modules to AC as used in household or industrial purposes.

The functions of some of its components are that the voltage and current characteristics enable the MPPT to set its operating point to extract maximum power and the DC/AC converter converts the DC energy received from the solar PV module to AC and feeds it into the load. When the battery is fully charged and has more charge, it is transferred to dump load. With the help of inverter, the battery provides power to load during night time. The battery does not get overcharged when the charger is removed, because it is discharged by diode DB. When the sun has set the diode, DA does not allow the current to flow into the array through the battery by isolating it. Mode controller receives all system signals and keeps a check on the charging or discharging of batteries. Thus, it acts as a centralized controller.

4.4 Grid-Interactive PV System

It functions along with the electric utility grid. It supports power generation in the range of hundreds of watts from one PV array to tens of megawatts from a large set-up. If power generated becomes excess, it is directed to electric grid and we don’t require a dump load. The battery is not needed for power stored at night time.

MPPT studies the current and voltage characteristics in this system and sets its operating point to withdraw maximum power. It consists of a power conditioner that converts the DC current to AC and removes the harmonics before feeding it into the electric grid. Some grid-interactive systems employ a battery to be used at times of power shortage or at low illumination conditions.

4.5 Hybrid Systems

Hybrid systems are composed of two of more than two types of generating stations. It provides more storage and employs advanced technologies compared with any individual energy generating station. One of its applications lies in mining, where diesel power plant is integrated with solar and wind power plants. This reduces the transportation as well as material cost considerably.

As we know, since the past few years, solar and wind costs have greatly decreased. And now the solar energy is utilized at great per cent and the wind mills have become taller which is an advantage to this power system. So, generally we combine diesel plants with renewable energy sources. Solar wind hybrid power systems are used widely nowadays, because we know the peak power for individual power systems occurs at different times of the day and when employed together, they provide less fluctuations in energy generations. It provides power back up of 6–7 h when there is power cut. And this system does not support more than one air conditioner.

5 Conclusion

This paper provides detailed description in the solar photovoltaic system simulation and analysis research area. The basic modelling of different components involved in simulations has been demonstrated. Contemporary methods on maximum power point tracking methods starting from offline and online methods with their advantages and disadvantages have been described. The connection of solar photovoltaic modules for the different applications have been presented in this paper. The paper presents the scope of the work in the solar PV maximum point tracking and the application of solar energy. Hence, the future scope of the paper is to adopt numerous state-of-the-art techniques in extracting maximum power from the existing solar PV modules in the industries and household applications.