PWM vs MPPT: Principles and peculiarities of charging methods in PV systems

Published : 04/28/2017 15:21:27
Categories : SOLAR CHARGERS

MPPT and PWM are the two main types of charge controllers used in the photovoltaic industry for the charging management of the batteries connected to the PV system and during the planning phase of a stand-alone (which means it’s not connected to the grid) or on-grid PV system, the right choice of the method of charging and the corresponding controller, is a key component for a smart and conscious setup of your own photovoltaic system.

Our comparison, however, cannot start without the necessary presentation of the two distinct types of regulator.

Following a chronological order, it makes sense to introduce for first the PWM (acronym of the English term Pulse Width Modulation) technology, introduced on the market when the interest in photovoltaics was addressed solely to off-grid and stand-alone applications.

In this sector, in fact, the "classics" photovoltaic modules features a 36-cell structure with an open circuit voltage equal to about 18 / 20V, designed for charging of the typical 12V batteries even in case of panel overheating conditions as a result of which it can in fact occur a decrease in the voltage output by the module itself.

Precisely because the fact that the normal photovoltaic modules deliver current at a voltage which is normally higher than the one at which the storage system operate, the principle of operation of the PWM regulators can be imagined as a switch that operates a rapid series of connections and disconnections between panel and battery in order to protect and control the battery voltage since this is never connected for excessive periods of time to the panel.

Therefore on a practical level, during the charging phase (we assume that the battery is initially discharged) the panel voltage is reduced depending on the closing and opening time of the PWM controller switch, so that the panel voltage is the same of the battery. As the battery keeps charging, the charging voltage continues to increase. Finaly, when the battery voltage absorption threshold is reached, to protect it from overcharging problems, the regulator switch is constantly open and closed. It is precisely from this mechanism that takes name the PWM class regulators.

This mechanism makes several aspects clear, such as: PWM controllers are simple switches and NOT "DC to DC" converters since, to protect the storage system, they are forced to give up part of the additional power that the panel could generate because they can not convert it to amperes at the correct voltage. On the other hand, as the system is brought to the operating standard of the batteries, they undergo less thermal and electrical stress during the charging phase, thus extending life and allowing a virtually permanent state of float (ie maximum charge). It should also be kept in mind that being this the oldest technology, PWM controllers are generally cheaper and more reliable, especially when it comes to the complexity of internal electronics, compared to the MPPT counterparts.

Changing side, we come now to talk about the most recent of the two photovoltaic recharge technologies, introduced when the photovoltaic industry was also beginning to take up on photovoltaic systems connected to the national grid.

Here, however, the panel standard changes and from modules of 30/36 cells it passes to 60 or more cells, with (much) higher open voltages than 30V. In dealing with these panels but always using common 12V batteries, it immediately appears clear that using a PWM controller would waste at least half the power that the panel could deliver from its operating voltage.

It is therefore necessary to have a more complex energy collection mechanism that takes into account all the electrical characteristics of the system: from panel to battery.

These features and their correlations can easily be understood by using graphs describing the Ohm law P = V*I on a Cartesian plane Voltage (Vx)/Current (Ay), in which the power of the panel is expressed as the curve shape:

Then the same curve is matched with a second graph placed on a Cartesian plane Power (W)/Voltage (V) always observing the Ohm law P = V*I and deducted by the inverse derivation of the first (of which it represents the slope, that is the graphical representation of the derivative concept), which allows us to understand under what conditions we can obtain as much energy as possible from the panel at a given operating voltage, i.e. the MPP (Maximum Power Point).

The Determination of the Maximum Power Point within this graph occurs between two extremes: the case where the panel does not produce energy since it short circuit (0*I = 0) and the case where there is no load applied to the panel (V*0 = 0). Within this range, you can locate an area underneath the power curve, which reaches its maximum area just when the MPP meets the panel power curve:

Leaving aside further mathematical or geometric considerations on the identification of this point, we can state that the peculiarity of an MPPT regulator lies in its ability to detect the amperage and working voltage of the panel and convert them to the battery amperage and voltage, which makes these appliances real power converters.

In practice, an example of this kind can be helpful: assuming a 3A panel current, with a conventional PWM regulator this current (already calibrated by the controller for a 12V charging system) would be directly transferred to the battery.
An MPPT regulator analyzes the power generated by the panel (P = V x I, as mentioned before), and therefore considers the voltage as well: if we assume that this voltage is at that moment equal to 17V, the power delivered by the panel is 17V x 3A = 51W.
This means that if the battery charge voltage is 13V, considering the maximum power output of 51W, the charge current that will be transmitted to the battery is 51W/13V = 3.9A, which is about 30% more than what can bring the PWM controller.

Another remarkable feature of MPPT technology is its wide intercompatibility: which means that it is possible to use panels designed to work at high voltages also to charge storage systems that work at significantly lower voltages. This in turn leads to another advantage: a lower power loss along the cables. In fact by using high voltage panels, even considering long cable sizes from the panel to the controller, the loss of power at such high voltages (36V, 48V or more) is irrelevant to what would be on 12V systems.

Given their sofisticated technology, it must be noted that MPPT regulators generally have more advanced charging control functions: they often have systems that, by disconnecting panel and battery, prevent the inversion of the flow of current, a phenomenon that can occur at night when the panel does not produce electricity.

Given all these peculiarities, however, they can not miss some "disadvantages", which do not make the MPPT controller the only possible choice when looking for a charge controller for photovoltaic.

First of all, there is the purely economic factor: all the technology incorporated in the MPPT controllers often makes it double the prices, if not more, than the PWM counterparty with the same dispensable amperage, which is not an indifferent aspect for those who are planning a small-size system and therefore could decide to invest more on the panel than on the controller. Also, for low power systems an MPPT controller would be almost wasted as the incentive given by the MPPT mechanism becomes relevant for systems of at least 170W, below this threshold PWM and MPPT provide almost the same efficiency.

Also, always in relation to the complex internal components, MPPT controllers are more prone to failures or malfunctions than simple PWMs.

Finally, there is another important factor mentioned before that could adversely affect the performance of MPPT regulators (but also PWMs): the temperature of the surrounding environment.

In fact, an MPPT controller expresses its maximum potential in cold or myriad climates, under which a panel manages to deliver current at a much higher amperage than in hot or sunny ambient conditions where the voltage can decrease by 15/20%, although the current delivered remains the same (see graph below).

This percentage, if it's not lost due to the rise of the outside temperature which leads to a drop in the MPP, can be fully exploited by the MPPT controller to be converted into additional current to be supplied to the battery. However, if case of high panel temperature (especially 75 °C upwards) the only way to improve the performance of an MPPT controller and continue to make it work more efficiently than the PWM counterpart, is to increase the voltage of the panel by adding additional cells in parallel (a solution that would not bring benefits to a PWM controller as it would lower the voltage of these cells to that of the batteries connected and also reduce the overall performance at mild temperatures). Finally, another small advantage of MPPT on PWM regards also the case of partial shading: under such conditions there is a decay of the charging voltage towards the battery, which with a PWM controller would lead to a consequent reduction in the voltage of the panel, resulting in even lower power output.

As you can guess from here, there are really many aspects to consider when choosing the charge controller that best suits your needs. Other, more marginal aspects than those reported include the choice between monocrystalline panels compared and polycrystalline panels (generally polycrystalline panels have a slightly lower nominal voltage than monocrystalline), the placement of panels (raised rather than attached to a surface) or even the thermal insulation level of the back of the panels.

Therefore, making an accurate choice may seem difficult, but just following the brief guidelines below, you can make a fairly clear idea of the conditions of advantage and disadvantage between the two types of regulators.

 

PWM Controller 

MPPT Controller

String Voltage

String and battery voltages must correspond (or are corrected by the controller, resulting in panel power leakage)

The string voltage can be higher than the battery voltage

Battery Tension

The panel operates at the same voltage as the battery: so the efficiency is better under hot temperatures and/or when the battery is almost full charged

It works also when the panel voltage is higher than the battery voltage: in this way it can provide additional charge in low temperatures or until the battery is low charged.

System dimension

 

Recommended for use in smaller systems where the benefits of MPPT are minimal

Recommended for systems with power greater than 150W - 200W; to best take advantages of MPPT

Off-Grid system or Grid-Tie systems

It is necessary to use off-grid photovoltaic modules with Vmp ≈ 17/18 Volt for each 12V nominal voltage battery pack

It allows the use of low cost or grid-tie photovoltaic modules, which allow to reduce the overall cost of the photovoltaic system

String Dimension

Ampere-sized photovoltaic system (based on the current produced when the PV module operates at battery voltage)

Watt-sized photovoltaic system (based on the product between the maximum charge current supported by the controller and the battery voltage)

Cost of the controller

Lower, as technology is more dated

High, due to the more expensive interior components; Low cost versions can also be fraudulent

BEWARE OF FRAUDS: HOW TO RECOGNIZE A TRUE MPPT REGULATOR

As it often happens to be available for sale on various online stores cheap versions of MPPT controllers, it is very important to have an idea of how is an MPPT controller internally made. In this way, we will be able to verify in person whether, aside of what is advertised on the site or on the product packaging itself, the controller purchased is actually an MPPT or a mere PWM sold for what is not.

The following picture shows the inside of a Chinese-manufactured controller advertised as MPPT (as you can see from the writing at the center of the electronic board), basically composed of a PCB plate to which are connected the screen (in the picture it was taken off for convenient reasons), processors, resistors, condensers and all other electronic components typical of suchdevice:

Below is portrayed the image of the internal components of two true MPPT controllers, in which is possible to clearly distinguish the missing components on the first device, including: the large capacitors needed for high voltage current accumulation and especially the massive Inductor required for accumulation and conversion of current through its magnetic field.

Given these considerations, we can say that the first model shown does not correspond to any MPPT regulator, as it lacks most of the electrical components that distinguish it.

Cases of this kind may happen for products from unknow brands or otherwise from the price that is too advantageous for the product category to which they belong. It is therefore necessary to be aware of the risks that arise by deciding to purchase such appliances and above all, to be able to evaluate the product regardless of how it is advertised.

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