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What is Power Point Tracking and is it Worth the Expense?

by Mike Kocsmiersky

The output of a PV module is characterized by a performance curve of voltage versus current, (I-V curve). See Figure 1. The maximum power point of a PV module is the point along the I-V curve that corresponds to the maximum output power possible for the module. This value can be determined by finding the maximum area under the current versus voltage curve. The maximum power point for standard test conditions of 1000W/m2 and 25C with air mass of 1.5 is shown in figure 1 to have about 17.4 volts and 2.5Amps.

For crystalline modules, the current remains fairly constant as the voltage moves up and down throughout the typical battery voltage ranges. In PV systems that charge a battery, the moduleÕs output voltage is determined by the battery to which it is connected. Should the battery be at a low state-of-charge, the output voltage of the PV module will be reduced in voltage, and hence the module output wattage is reduced. (See V discharge on figure 1) With a battery discharged to 11.0V, a corresponding module current of 2.6A can be realized, which is only 66% of the available module power.

Maximum power point tracking enables a PV module, or array, to operate at its maximum power point while charging a battery at a lower voltage, in this instance the module can produce 43.5W instead of 28.6W. There are several factors that will influence the amount of power gain one can expect; these factors are cell temperature, conversion losses, amount of available sunlight, cell structure, battery voltage, and blocking diodes. Some power is lost in the conversion from the voltage at the maximum power point to battery voltage. The efficiency of most maximum power point tracking units is usually around 93%.

Some maximum power point trackers, like the Fire Wind and Rain unit, will have an automatic bypass that will allow the charge controller to use the battery voltage as the module output voltage if the conversion takes more power than is being gained by using maximum power point tracking.

For crystalline modules, voltage will drop about 2.4mV per degree C per cell. A 36-cell module on a typical summerÕs day in Kingston, NY will have a cell temperature of 45C during peak sun hours. This yields a voltage drop of 1.73V, and shifts the I-V shown in Fig 1 thus lowering the maximum power point closer to the battery voltage.

As sunlight diminishes from the standard test condition of 1000W/m2, the voltage corresponding to the maximum power point drops slightly, but the main component in the decrease of available power is the decrease in available current.

In the case of amorphous silicon modules, the I-V curve will change current more dramatically as the voltage changes throughout the battery voltage and maximum power point ranges. This will translate into less gain seen by using the maximum power point tracker.

Battery voltage will also play a major role in the amount of increased watt-hours one can expect from a module or array using a maximum power point tracker. If the battery bank is mostly near a full state-of-charge, then the voltage of the battery bank will be closer to the maximum power point voltage and very little gain will be seen using the maximum power point tracker. The use of blocking diodes will also mandate that the module be .3 to .7 volts higher than the battery voltage, thus lessening the difference between battery voltage and voltage at the maximum power point.

When does using a maximum power point tracker make sense? The typical wattage gain using a maximum power point tracker is 10 to 13%. Therefore, for systems under 300W, it is usually more cost effective to buy another module than to buy a maximum power point tracker. However, for systems above 300W the additional cost of the maximum power point tracker can more than pay for itself with increased watt-hour output from the array. Also, the percentage gain is greater in the winter, when the air temperature is colder (thus colder cell temperature and higher max power point voltage), and this is when the added array output is most needed. Recently there have been several manufacturers that are marketing several relatively inexpensive units that will find a home in many PV designs to come.

Mike Kocsmiersky is a Project Engineer at SunWize Headquarters in Kingston, NY

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