PWM vs. MPPT Solar Charging Controller for Solar Streetlight

PWM vs. MPPT: Which one is better?

We have learned about the features of both controllers (PWM & MPPT) in previous chapters. We well noted that PWM does not convert extra voltage into currents, which results in low power conversion rates. In other words, PWM doesn’t transfer all energy that is collected by solar panels to batteries, but MPPT is always tracking the maximum power point from the panels and adjusting its currents and voltage accordingly so that it can transfer all energy collected by solar panel to battery.

PWM vs. MPPT   A concrete example will explain this clearly:

The basic physical formula:

Power (watts) = V (Volts) x I (Amps)

If we use a nominal 12V, 100W solar panel to charge a 12V battery system, the actual Vmp is 17V, and we can calculate its current output:

I = Power / V

I = 100 / 17 = 5.88 amps

Now we know the panel output is 17V and 5.88A.

Scenario 1: The photovoltaic system is with PWM solar charge controller.

PWM will drag the voltage down to battery charging voltage – approximate 14V. After going through the PWM, the solar energy only remains 14V and 5.88A.

That is:

P = V x I = 14 x 5.88 = 82.32 W

Scenario 2: The photovoltaic system is with the MPPT solar charge controller.

The MPPT not only drags the voltage down to 14V, but also increases the current, so that the power almost equals to power out.

So, if the voltage decreases by 17/14 = 1.21

Then the current to battery increases by 1.21, we get

5.88 x 1.21 = 7.11A

Total power out

P = 14 x 7.11 = 99.54 W

In this example, the power wasted by PWM is

99.54 – 82.32 = 17.22W

Almost 20% energy was not converted to battery chemical energy. If we consider the scenario in a large solar array, the loss could be tremendous.

So, it’s better to use MPPT for large solar array.

6.2 The strengths of MPPT

a) High conversion efficiency

If your photovoltaic system comes with a large solar array, MPPT would be the best choice to boost the solar energy conversion, especially in cold weather, since the panel voltage will rise as the temperature drops. The MPPT conversion rate could rise from 20% to 40%. That’s green and free energy that really saves money on your bill.

Solar panel array in distance

b) Lower energy loss in cables or lower cost for purchasing cables.

Please remember Ohm’s law formula

V (Volts) = R (Ohm) x I (Amps)

Output power  P_O (Watts) = V (Volts) x I (Amps)

So

Resistance loss  P_R(Watts) = R (Ohm) x  I² (Amps)

Then, if your PV panels are installed a long distance from your battery bank, the power loss of cable resistance is considerable ( P_R = R x I²  ). Here R represents the resistance of cables. R increases as cable length increases:

Cable resistance formula

But if we double the Voltage of the solar array by wiring them in more series, according to P_O  = V x I, there is no change of total power outputP_O , so the current through cable I should be half.

Finally, the resistance P_R(Watts) = R (Ohm) x  I² (Amps) will be a quarter than before.

In fact, with MPPT, you could raise the voltage of the solar array even higher to reduce the current flow.

In this case, we increase the panel’s voltage to reduce the resistance loss through cables, and since we are using MPPT, which always tracks to harvest maximum power from panels, we have no voltage waste as PWM may have.

We could review this topic from another aspect. If you cannot raise the panels voltage, then you have to find out some solution to reduce cable resistance, as resistance = resistivity × length / area, it seems the only way is to use cables with large transverse areas, and that will be another huge sum of money to spend.

To recap, when it comes with small systems, PWM is a good solution since it is inexpensive, but for large systems, in order to improve conversion rates and not waste the solar panel capacity of harnessing solar energy, MPPT is preferable. MPPT would always be applied to higher power systems.

6.3 Pros and cons

Learning knowledgeable information from the previous content is necessary before making the decision to purchase a solar charge controller for your PV system. A comparison table, listing the difference between PWM and MPPT, is also suggested. So, we put their pros and cons together to make it more convenient for you to review.

6.4 Does every solar PV system need a charge controller?

The answer is no.

Generally, if your solar panel is less then 5 watts for every 100 amp hours battery, then you do not need a solar charge controller.

Here is a formula we can use:

Quotient = Battery Capacity (Amp Hour) / Imp of solar panel (Amps)

If the quotient is larger than 200, you don’t need a controller; otherwise, you’d better install a controller.

For example, if you have a 200AH battery and 20W panel, the quotient would be 200/1.18=169.5; in this case, you need a controller.

If you have 400AH battery and 10W panel, the quotient would be 400/0.59=677.9; in this case, you do not need a controller.
 

PWM Charge Controllers

PWM stands for Pulse-Width Modulation. This comes into play when the batter bank is full. As it is charging, the solar charge controller lets in as much current as the PV panel can generate in order to achieve the target voltage for the charge stage of the controller. As the battery reaches its target voltage, the controller will then switch between the battery banks to the panel array. The battery bank will then be disconnected, which regulates the battery voltage and makes it constant. This quick switch method is called PWM, and what this does is it makes sure that your battery bank is charged while preventing the process of overcharging.

The PWM controller will usually operate slightly above the maximum power point. Figure shows the operating range for PWM controllers.

MPPT Charge Controllers

MPPT or the Maximum Power Point Tracking provides an indirect connection between the battery bank and the PV array. This indirect connection includes a DC/DC voltage converter that takes extra PV voltage and transforms it into an additional current at a lower voltage without necessarily losing power.

MPPT controllers are more costly than PWM controllers. However, the benefit to using MPPT is the fact that it is more efficient under particular conditions. If you want to purchase an MPPT, it would be better to check first if the professional function of the MPPT can be applied at the given site conditions. If you find out that there the MPPT’s functions aren’t applicable in your chosen site, then it would be more economical for you to choose the PWM controller.
MPPT controllers are able to pull this off because of an adaptive algorithm that follows the MPPT of the PV array, which then adjusts the incoming voltage in order to maintain the most efficient level of power for the system in place.