Benefits of using MPPT at module level instead of at the string level

There are far more benefits. 1) no bypass diodes, saving their cost and the silver in the home runs; 2) can use Grade B cells, which are just as efficient but lower cost, 3) can build a high internal voltage, low internal current panel, thus can connect cells with aluminum instead of silver (too much to explain here, but a quarter cell puts out slightly more power than 25% of a full cell), 4) and as mentioned, the ability to mix headings and each panel is at it max in a single inverter; the string is not degraded. Read the while paper at the web site. The prototype panels were made with 100% quarter Calisolar Grade B rejected cells and outperformed a standard panel, and verified that a partially shaded cell gets *cooler*, not hotter, due to no reverse conduction.

Even if you were to select identical panels to begin with,the mismatch can occur due to partial shading of some of the panels in the array.The shape of the final I-V curve of the partially and non-shaded panels with a single global maxima and maybe several local maximas becomes important.The MPPT algorithm built into the string inverter needs to take care of this.The panel optimizer will check and make each module operate at its local maxima which may not be desirable when integrated with other panels into an array.

- - - -> by: Ravi

The module mismatch can occur due to different current and voltage characteristics of each module.Since the modules are in series,I guess the final current will be an average of the individual module currents with the voltages adding up.So the final power output of the array will be lower.
But the moot point is about installing a PV power optimizer for each module(?) which maximises power for each module.Is this a desirable option when modules are connected to form an array?I thought the MPPT tries to find the global maxima of an array which may conflict with the several local module maximas.The global maxima of an array may not be the sum of several individual local maximas?I am confused.

- - - -> by: Akella

You have an interesting question. I'd like to throw back some questions:

a) When you connect two modules in series, what parameter is increased?

b) Case example: the series modules are used to charge a 24 volt battery. Note that output current of a pv module is dependent on irradiance and system voltage. If let's say irradiance is 1 kW/m2 and temperature is 25 degrees C (Standard Test Condition of PV modules), what is the expected current in this string (string is the term used when modules are connected in series)?

The answer to your question is to look at the IV curve of the two modules (from data sheets). How would you draw the IV curve when modules are in series? You'll make your own discovery when you try this.

- - - -> by: Jun

I personally would not entertain power optimizers for various reasons but the way forward if you have to mix panels is to use Enphase microinverters, as long as you keep within the voltage, 60 cell and wattage parameters set by Enphase this is your best option. You will get a standard 20 year warranty and the monitoring software is remarkable.

But the answer to your question is you cannot missmitch panels on a string inverter as people have commented, the weakest panel will bring the whole string down.

- - - -> by: Grant