What is the advantage of PWM over MPPT?

The charge controller is a key component of a solar power system and specifying the best one for the system requires some analysis. Below is a quick overview.

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The two types of charge controllers most commonly used in today&#;s solar power systems are pulse width modulation (PWM) and maximum power point tracking (MPPT). Both adjust charging rates depending on the battery's charge level to allow charging closer to the battery&#;s maximum capacity as well as monitor battery temperature to prevent overheating.

&#; Comparing the Two

If maximizing charging capacity were the only factor considered when specifying a solar controller, everyone would use a MPPT controller. But the two technologies are different, each with it&#;s own advantages. The decision depends on site conditions, system components, size of array and load, and finally the cost for a particular solar power system.

&#; Temperature Conditions

An MPPT controller is better suited for colder conditions. As solar module operating temperature goes down, the Vmp1 increases. That&#;s because the voltage of the solar panels operating at their peak power point at Standard Testing Conditions (STC is 25C°) is about 17V while the battery voltage is about 13.5V. The MPPT controller is able to capture the excess module voltage to charge the batteries.  As a result, a MPPT controller in cool conditions can produce up to 20 &#; 25% more charging than a PWM controller.

In comparison, a PWM controller is unable to capture excess voltage because the pulse width modulation technology charges at the same voltage as the battery. However, when solar panels are deployed in warm or hot climates, their Vmp decreases, and the peak power point operates at a voltage that is closer to the voltage of a 12V battery. There is no excess voltage to be transferred to the battery making the MPPT controller unnecessary and negating the advantage of an MPPT over a PWM.

&#; Array to Load Ratio

In a scenario where the solar array is large relative to the power draw from the batteries by the load, the batteries will stay close to a full state of charge. A PWM controller is capable of efficiently maintaining the system without the added expense of an MPPT controller.

&#; Size of the System

Low power systems are better suited to a PWM controller because:

• A PWM controller operates at a relatively constant harvesting efficiency regardless of the size of the array
• A PWM controller is less expensive that a MPPT, so is a more economical choice for a small system
• A MPPT controller is much less efficient in low power applications. Systems 170W or higher tickle the MPPT&#;s sweet spot

&#; Type of Solar Module

Stand-alone off-grid solar modules are typically 36-cell modules and are compatible with both PWM and MPPT technologies. Some grid-tie solar modules on the market today are not the traditional 36-cells modules that are used for off-grid power systems. For example, the voltage from a 60-cell 250W panel is too high for 12-Volt battery charging, and too low for 24-Volt battery charging. MPPT technology tracks the maximum power point (thus MPPT) of these less expensive grid-tie modules in order to charge the batteries, whereas PWM does not.

&#; Cost

MPPT controllers are typically more expensive than PWM&#;s but are more efficient under certain conditions, so they can produce more power with the same number of solar modules than a PWM controller. One must then analyze the site to verify that the MPPT can indeed perform more efficiently when used in that system&#;s given set of conditions.

When specifying one technology over the other, the cost of the controller becomes less important than the total cost of the system. To specify a controller technology simply based of cost, be sure to perform a close analysis of realized efficiencies, system operation, load and site conditions.

SUMMARY OF COMPARISON

 

PWM Charge Controller

MPPT Charge Controller

Array Voltage

PV array & battery voltages should match

PV array voltage can be higher than battery voltage

Battery Voltage

Operates at battery voltage so it performs well in warm temperatures and when the battery is almost full

Operates above battery voltage so it is can provide &#;boost&#; in cold temperatures and when the battery is low.

System Size

 

Typically recommended for use in smaller systems where MPPT benefits are minimal

&#; 150W &#; 200W or higher to take advantage of MPPT benefits

Off-Grid or Grid-Tie

Goto KINGSUN to know more.

Must use off-grid PV modules typically with Vmp &#; 17 to 18 Volts for every 12V nominal battery voltage

Enables the use of lower cost/grid-tie PV Modules helping bring down the overall PV system cost

Array Sizing Method

PV array sized in Amps (based on current produced when PV array is operating at battery voltage)

PV array sized in Watts (based on the Controller Max. Charging Current x Battery Voltage)

At Solarcraft, when we select one type of charge controller over another we assess its advantages in the overall system cost. The goal is to power a system efficiently and continuously while preserving the health of the battery bank. To learn more about the solar power systems we design and build by giving us a call at 877-340-.

1 The Vmp (maximum power voltage) is the voltage where the product of the output current and output voltage (amps * volts) is greatest and output power (watts = amps * volts) is maximized. Module wattage ratings (e.g. 100W, 205W) are based on Pmp (maximum power) at Vmp under standard test conditions (STC).

MPPT vs PWM | The two major types of solar charge controllers are:

As shown in the chart below, PWM controllers tend to be smaller and they operate at battery voltage, whereas MPPT controllers use newer technology to operate at the maximum power voltage. This maximizes the amount of power being produced which becomes more significant in colder conditions when the array voltage gets increasingly higher than the battery voltage. MPPT controllers can also operate with much higher voltages and lower array currents which can mean fewer strings in parallel and smaller wire sizes since there is less voltage drop.

PWM controllers need to be used with arrays that are matched with the battery voltage which limits what modules can be used. There are many 60 cell modules with maximum power voltage (Vmp) equal to about 30V, which can be used with MPPT controllers but are simply not suitable with PWM controllers.

To answer the question: Which is better, PWM or MPPT? All things being equal, MPPT is a newer technology that harvests more energy. However, the advantages of MPPT over PWM controllers come at a cost, so sometimes a less expensive PWM controller can be the right choice, especially with smaller systems and in warm climates where the MPPT boost is not as significant.

PWM vs. MPPT Solar Charge Controller Comparison

PWM Controllers MPPT Controllers Array voltage is &#;pulled down&#; to battery voltage Convert excess input voltage into amperage Generally operate below Vmp Operate at Vmp Suitable for small module configurations Suitable for large module configurations that have a lower cost per watt Often chosen for very hot climates which will not yield as much MPPT boost Provide more boost than PWM, especially during cold days and/or when the battery voltage is low

Every Morningstar PWM and MPPT solar charge controller is listed on the Morningstar Product Series page. Each listed product is hypertext linked to its product page that includes datasheets, operation manuals, and other helpful information.

PWM Charging

Traditional solar regulators featuring PWM (Pulse Width Modulation) charging operate by making a connection directly from the solar array to the battery bank. During bulk charging when there is a continuous connection from the array to the battery bank, the array output voltage is &#;pulled down&#; to the battery voltage. The battery voltage adjusts slightly up depending on the amount of current provided by the array and the size and characteristics of the battery.

MPPT Charging

Morningstar MPPT controllers feature TrakStar technology, designed to quickly and accurately determine the Vmp (maximum power voltage) of the solar array. TrakStar MPPT controllers &#;sweep&#; the solar input to determine the voltage at which the array is producing the maximum amount of power. The controller harvests power from the array at this Vmp voltage and converts it down to battery voltage, boosting charging current in the process.

Why Choose PWM Over MPPT

The preceding discussion of PWM vs. MPPT may cause some to wonder why a PWM controller would ever be chosen in favor of an MPPT controller. There are indeed instances where a PWM controller can be a better choice than MPPT and there are factors which will reduce or negate the advantages the MPPT may provide. The most obvious consideration is cost. MPPT controllers tend to cost more than their PWM counterparts. When deciding on a controller, the extra cost of MPPT should be analyzed with respect to the following factors:

1. Low power (specifically low current) charging applications may have equal or better energy harvest with a PWM controller. PWM controllers will operate at a relatively constant harvesting efficiency regardless of the size of the system (all things being equal, efficiency will be the same whether using a 30W array or a 300W array). MPPT regulators commonly have noticeably reduced harvesting efficiencies (relative to their peak efficiency) when used in low power applications. Efficiency curves for every Morningstar MPPT controller are printed in their corresponding manuals and should be reviewed when making a regulator decision. (Manuals are available for download on the Morningstar website).

2. The greatest benefit of an MPPT regulator will be observed in colder climates (Vmp is higher). Conversely, in hotter climates Vmp is reduced. A decrease in Vmp will reduce MPPT harvest relative to PWM. Average ambient temperature at the installation site may be high enough to negate any charging advantages the MPPT has over the PWM. It would not be economical to use MPPT in such a situation. Average temperature at the site should be a factor considered when making a regulator choice

3. Systems in which array power output is significantly larger than the power draw of the system loads would indicate that the batteries will spend most of their time at full or near full charge. Such a system may not benefit from the increased harvesting capability of an MPPT regulator. When the system batteries are full, excess solar energy goes unused. The harvesting advantage of MPPT may be unnecessary in this situation especially if autonomy is not a factor.

Why Choose MPPT Over PWM

Increased Energy Harvest:

MPPT controllers operate array voltages above battery voltage and increase the energy harvest from solar arrays by 5 to 30% compared to PWM controllers, depending on climate conditions.

Array operating voltage and amperage is adjusted throughout the day by the MPPT controller so that the array&#;s power output (amperage X voltage) is maximized.

Less Module Restrictions:

Since MPPT controllers operate arrays at voltages greater than battery voltage, they can be used with a wider variety of solar modules and array configurations. Moreover, they can support systems with smaller wire sizes.

Support for oversized Arrays

Unlike PWM controllers, MPPT controllers can support oversized arrays that would otherwise exceed the maximum operating power limits of the charge controller. The controller does this by limiting the array current intake during periods of the day when high solar energy is being supplied (usually during the middle of the day).

While energy from the array is capped or shaved off during the middle of the day, the oversized array is able to provide more power during teh early and late part of the day compared to smaller non-oversized array.

Download Our PWM vs MPPT White Paper

Please click here to download the Traditional PWM vs Morningstar&#;s TrakStar&#; MPPT Technology white paper. Morningstar&#;s MPPT charge controllers use the TrakStar advanced control MPPT algorithm to harvest maximum power from a Solar Array&#;s peak power point. It is generally accepted that even the most basic MPPT controller will provide an additional 10&#;15% of charging capability, when compared to a standard PWM regulator. Besides this extra charge capability, there are several other important differences and advantages between MPPT and PWM technologies that are outlined in this whitepaper.

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