Choice of charge control technique: Active MPPT or TSS Passive MPPT?
Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT) are the two most common techniques used in off-grid solar charge controllers to charge batteries from a photovoltaic (PV) array. Both technologies have been widely used in the off-grid solar industry and both are 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 design.
PWM charge control
A PWM charge controller is a switch which connects the PV module to the battery. When in PWM regulation, the current from the PV array tapers according to the battery’s status and recharging needs. When the switch is closed, the PV module and battery will be at nearly the same voltage and the PV module operational voltage is given by the following formula:
Since the PV modules operate at the same voltage window (battery voltage), rather than a higher watt peak rating, it is more important for a PV module to be able to produce more current at the battery voltage window when used with a PWM controller.
Active MPPT charge control
A MPPT algorithm-based charge controller sweeps the I-V curve of the PV module to pin-point the Maximum Power Point (MPP) of the PV module in all conditions. The power conversion is done with a DC-DC converter that converts the higher DC voltage from PV modules to a lower voltage required to charge the batteries:
Effect of PV module temperature on power extracted
An often forgotten aspect of PV modules is the change of parameters at different temperatures. Let us consider two scenarios of PV module temperature by maintaining the type of PV module and irradiance as constant.
For a 24V PWM based system, considering the average battery charging voltage to be 27.45V and a voltage drop of 10% (worst case), the PV module operational voltage will be 30.2V. From the above figure, it can be seen that at a PV voltage of 30.2V, the PV power output will be 325.3W.
The maximum power that can be extracted from the PV module with an MPPT-based controller is 359.8W. In this case MPPT extracts about 10.6% more power in comparison to a PWM controller at a PV cell temperature of 25°C.
Above case is considered at STC. STC stands for Standard Test Conditions which is at a PV module temperature of 25°C. In reality, the PV module temperature is much higher than the ambient temperature. As a rule of thumb, the difference in temperature is approximately 20°C. Hence, it would be more realistic to consider a cell temperature of around 50°C.
From the above figure, it can be noticed that at a PV voltage of 30.2V, the PV power output has reduced slightly to 322.4W in a PWM based system, only 0.9% reduction compared to STC. The maximum power that can be extracted by a MPPT based system is 328.8W, a reduction of 8.8% compared to STC. Reducing the difference in power output to only 1.9%.
Hence, at the actual temperatures a PV module will operate, the advantages of the MPPT over a PWM becomes negligible as the power output from a PWM based system is very close to that for an MPPT based system.
TSS passive MPPT charge control
In essence, with a PWM system the MPP is not actively tracked, but MPP operation is obtained by matching the module voltage to the system voltage. This can be achieved by using TSS PWM optimized PV modules. The combination of a TSS PWM charge controller and TSS PWM optimized PV modules provides a passive MPP operation which we call the TSS passive MPPT charge control and is provided in our solid solar only solutions.
As seen in the earlier section, at lower temperatures, the MPPT controller can extract the same power as PWM from a lower number of PV modules. This leads to slightly lower number of PV modules for a MPPT system. To partly overcome this challenge, TSS uses PWM system optimized PV modules in order to extract the maximum power out of the PV system at any given ambient temperature.
Ease of system sizing
MPPT is the more advanced style charge controller as they will sweep the I-V curve throughout the day to maximize the energy harvest from PV array. Consequently, sizing MPPT based charge controllers is more complex as there are many considerations to take into account in comparison to a straightforward PWM based system sizing.
Controller de-rating with temperature
An overlooked fact is often that all equipment is installed inside enclosures where operational temperature will be higher than the ambient temperature. This difference can be up to 20°C to 30°C and additionally controller efficiency comes into play.
With a typical efficiency of a MPPT controller being 97%, its’ dissipated power will have a large effect on the enclosure temperature. Additionally, the complex electronics will demand a power derating at higher temperatures. Risking not being able to extract enough energy to supply the load and also charge the battery.
The TSS PWM Controller on the other hand has an outstanding efficiency of 99.97%. This keeps the enclosure temperature at a lower level compared to MPPT. Besides this a PWM controller does not demand for derating at higher temperatures.
A MPPT based system consists of more complex electronic DC-DC converter circuits to cater the MPP tracking. A PWM controller is of a way less complicated nature. It only has to switch the PV modules on and off, making it a more reliable solution.
From a costing point of view, Solar Energy Systems are implemented in two main climates:
- High temperature: similar system cost for both MPPT and PWM
- Lower insolation: larger number of PV modules require larger number of MPPT Controllers, this reduces the benefit of MPPT since the cost of MPPT controllers is vastly higher compared to PWM controllers
It can be concluded that the choice for a TSS Passive MPPT based system or an 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 a TSS passive MPPT solution is still the best solution for remote industrial environments, since it provides the highest reliability and energy security. All at the same or lesser cost.