Choice of charge control technique: Active or TSS Passive MPPT?
Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) are the two techniques which have been used in off-grid solar charge controllers to charge batteries from a solar array. Both these technologies have been widely used in the off-grid solar industry and are both great options for charging your battery. The choice of PWM or MPPT regulation is not merely based on which charging method is “better” than the other. Rather, it encompasses the determination of which type of controller will work best in a specific system’s design.
PWM charge control
The PWM charge controller is a switch which connects the PV module to the battery. When in PWM regulation, the current from the solar array tapers according to the battery’s condition and recharging needs. When the switch is closed, the PV module and battery will be at nearly the same voltage and the solar module operational voltage is given by the following formula:
Since the solar modules operate at the same voltage window (battery voltage), rather than a higher watt peak rating, it is more important for a solar module to be able to produce more current at that voltage window when used with a PWM controller.
Active MPPT charge control
The maximum power point tracking algorithm based charge controller functions as a typical DC to DC converter which can step-down or step-up the voltage available from the PV module by making the input power to be equivalent to the output power. Excluding a small loss in the conversion process, the amount of power available at the output doesn’t change. Thus, if the output voltage is higher than the input voltage, the output current will be lesser than the input current such that Power (= Voltage · Current) is maintained constant.
Effect of solar module temperature on power extracted
Let us consider two scenarios of solar module temperature by maintaining the type of solar module and irradiance as constant.
At standard test conditions (STC):
Note: STC stands for standard test conditions which is at a solar module temperature of 25°C. The solar module temperature is much higher than the ambient temperature. As a rule of thumb, the difference in temperature is approximately 20 °C .
Ppwm: For a 24 V PWM based system, considering the average battery charging voltage to be 27.45 V and a voltage drop of 10% (worst case), the PV module operational voltage will be 30.2 V. From the above figure, it can be defined that at a PV voltage of 30.2 V, the PV operational current will be 10.75 A and the PV power output will be 324.58 W.
Pmpp: The maximum power that can be extracted from the PV module with a MPPT based controller is 355.32 W.
Thus, MPPT seems to be extracting about 9 % more power in comparison to PWM based controller at a low cell temperature of 25 °C.
At solar module temperature of 80 °C:
From the above figure, it can be seen that at a PV voltage of 30.2 V, the PV operational current will be 10.4 A and the PV power output will be 314.1 W which is the power output in a PWM based system.
At the same cell temperature, the maximum power that can be extracted by an MPPT based system is also found to be 314.1 W.
Thus, at higher solar module temperatures , the advantages of the MPPT over a PWM is negligent as the power output from a PWM based system is equal to that of MPPT based system. The MPP is not actively tracked, but the MPP operation is obtained with a PWM controller due to matching module voltage to system voltage. This can be achieved by using PWM optimized solar modules. The combination of PWM charge controller and PWM optimized solar modules provides a passive MPP regulation that TSS is providing in their solid solar solutions.
It can be inferred that the choice of Passive MPPT based system or Active MPPT based system lies in the smart interplay between the various factors that need to be incorporated while designing a specific system. At TSS, we believe the choice for PWM in combination with PWM optimized solar modules is still the best in remote industrial environments with high temperatures since it provides the highest reliability and energy security. From a costing point of view, both the technologies are similar at high temperature conditions. For locations with lower insolation, the benefit of MPPT based on the amount of solar modules is reduced even further since the cost of the system with MPPT controllers is higher compared to a system with PWM controllers. This has been tested and verified in association with TU Delft, The Netherlands.