MPPT or PWM??? Which is better??

Maximum Power Point Tracking (MPPT)

Maximum Power point tracking or an MPPT solar charge controller is a charge controller embedded with MPPT algorithm to maximize the amount of current going into the battery from the Photo Voltaic or PV module. MPPT uses DC-DC converter topology, which takes DC input from the solar panel, changes it into AC and converting it back to a different DC voltage and current in order to exactly match the PV module to the battery. Buck converter and Boost converter are the examples of DC-DC converter.

Fig 1. Block Diagram of MPPT controller

 

If the PV input voltage is less than the battery voltage in the system, we use a Boost converter. Boost converter or a step up converter is a DC-DC converter which steps up voltage from its input to its output.

If the PV input voltage is greater than the battery voltage in the system, we need to step down the voltage, to exactly match the PV module to the battery. In that case we use a step down converter or a Buck converter. Buck converter or a step down converter is a DC-DC converter which steps down voltage from its input to its output.

If the battery system voltage is less than or equal to 48V, buck converter is useful. On the other hand, if the battery system voltage is greater than 48V, Boost converter should be chosen.

For any application in which the PV module is the source of energy,  we use MPPT solar charge controller used to correct the variation in the current-voltage characteristics of  solar cell.

In any solar power system, MPPT solar charge controller is necessary to extract maximum power from PV module. MPPT charge controller forces PV module to operate at voltage close to maximum power point to draw maximum available power.

Maximum power point tracking (MPPT) uses a high efficiency DC-DC converter technology compared to other controllers like Pulse Width Modulation (PWM) technology. MPPT is most effective under conditions like cold weather.  In cold weather condition, the sun hours are low and we need the power to recharge the batteries the most. MPPT is an effective controller in that condition which extracts maximum power from the PV module.

 

COMAPARISON OF MPPT AND PWM CONTROLLER

Pulse Width Modulation or PWM solar charge controller only utilizes the power generating from the solar panels and charges the battery. That is, it does not step up or step down the power to match the PV module power and battery system power.

MPPT solar charge controller extracts maximum power from the solar panels and charges the battery efficiently.

Efficiency of PWM solar charge controller is 70% which means it uses 70W of power out of 100W solar panel. Remaining 30W power is waste.

Efficiency of MPPT solar charge controller is 96% which means it efficiently utilizes 96W of power out of 100W solar panel.  So, MPPT solar charge controller produces 26W power extra than the PWM solar charge controller.

 

Fig 2. Block Diagram of PWM Solar charge controller

 

The PWM solar charge controller utilizes the available power generated from the 100W solar panel.

It can be expressed by using a simple formula:

                                                         

The maximum voltage of 100W solar panel is 17V.

Power output from the PWM solar charge controller,

 

Thus a PWM solar charge controller utilizes only 70.5W of power from a 100W solar panel. And the remaining 30W power is waste.

To understand the difference between MPPT solar charge controller and PWM solar charge controller, let’s substitute the PWM solar charge controller by MPPT solar charge controller in the above figure.

 

Fig 3.  Block Diagram of MPPT Solar charge Controller

Consider, the MPPT solar charge controller boost the current output from the solar panel to 30%. As a result MPPT solar charge controller provides an 8A current to the battery. 

Let’s substitute these values to the formula.

                                                                                                  

MPPT solar charge controller output power is 96W which is 26W more than that of PWM solar charge controller. MPPT charge controller has 96% efficiency.

If we are using PWM solar charge controller, we need to install 7kW solar panel to run 5kW load. Because efficiency of PWM solar charge controller is 70%. Remaining power is wasted. So to give 5kW power to battery, we need to install a solar panel of 7kW.

If we replace the PWM solar charge controller using MPPT solar charge controller, we can efficiently run 5kW load using a 5kW solar panel. Thus, a low power panel is needed for MPPT solar charge controller compared to PWM solar charge controller.

 

ADVANTAGES OF MPPT SOLAR CHARGE CONTROLLER

 

* MPPT solar charge controller can directly runs DC load.

* It extracts maximum power from the solar panel even in the cloudy weather condition.

* So, a maximum utilization of solar power is possible by installing MPPT solar charge controller.

 

MPPT ALGORITHMS

 

i. Perturbation and Observation Algorithm

 

Perturbation and Observation (P and O algorithm) algorithm is widely used in MPPT system because of its simple structure and easy implementation. In P and O algorithm, the perturbation variable is the reference value for the PV panel terminal voltage, PV panel output current or the duty cycle of the MPPT controller. If the output voltage of the PV panel is perturbed, and dp/dv>0, the operating point is on the left side of the Maximum Power Point (MPP).

 

Fig 4. Graph represents P and O algorithm for MPPT

 

Then the P and O algorithm increase the PV panel reference voltage to move the operating point towards MPP. If dp/dv<0, then the operating point is on the right side of MPP. Then the P and O algorithm decrease the PV panel reference voltage to move the operating point towards MPP.

 

Here, the perturbation variable is the voltage. Then perturbation in PV output current is accomplished by either decreasing or increasing the reference voltage by a small value. So the major function of Perturbation and Observation algorithm is to determine the perturbation direction. The figure below represents the flowchart for Perturbation and Observation algorithm.

 

Fig 5. Flow chart of P and O algorithm

 

ii. Incremental Conductance Algorithm

 

The main advantage of this algorithm is its fast power tracking process. The derivative of power with respect to voltage or current becomes zero at MPP because the MPP is the maximum point of the power curve. Refer the below given formula.

 

This algorithm starts the cycle by obtaining the present values of I(k) and V(k) and at the end of the cycle the corresponding values is stored in the variables I(k-1) and V(k-1).

 

Fig 6. Flow chart of Incremental Conductance algorithm

 

When the new value is read in to the program, it calculates the previous value compare with the new one, and then determines the voltage differential is zero or not. When the voltage differential is zero, the current difference can be determined zero or not. If voltage differential and current differentials are zero, it means, reference does not need to change because the operating point remains at the MPP. Two other checks are included in the algorithm to check whether a control action is required or not.

If the voltage differential is zero, but the current differential is not zero, it shows that the solar power has changed.  When the difference of the current values is greater than zero, duty ratio will increase, when the difference of the current value is less than zero the duty ratio will decrease.

If the voltage differential is not zero determine it whether satisfy the equation 1 or not, when the equation 1 is satisfied the slope of the power curve will be zero that means the system is operating at MPP, if the variance of conductance is greater than the negative conductance values, it means the slope of the power curve is positive and the duty ratio is to be increased, otherwise it should be decreased.