How increased efficiency and cell design lead to higher charge controller performance

Photovoltaic modules are continuously improving thanks to R&D efforts that contribute to increased efficiency and lower costs. The industry directly reaps these benefits every year. Every once in a while, a technological change takes a bigger and more significant step that is worth taking note of.

One such change was the introduction of passive and backside cell (PERC) technology, which significantly increased cell current and voltage, and, in effect, maximum module voltage. In addition, there has been a large influx of half-cell modules into the market. These half-cell modules include an innovative cell arrangement that can provide even higher performance than traditional full-cell modules.

With a focused look at how solar power electronics can take advantage of these changes, we can make smarter system sizing decisions and leverage these new developments more effectively in off-grid solar systems, where every watt matters.

How do the new PERC photovoltaic modules differ?

Half-cell modules have made a comeback

PERC photovoltaic cell technology has rapidly become the dominant cell technology in the market.

Many manufacturers also offer these PERC modules in a half-cell configuration. Unlike small modules that use half-cells to achieve a smaller overall form factor and meet the low power needs of the market, these new configurations are the same size as whole-cell modules, but contain twice as many to achieve the same power, but with some unique advantages. The new relative configurations are shown below:

• 36 whole cells ~ 72 half cells
• 60 whole cells ~ 120 half cells
• 72 whole cells ~ 144 half cells

Half-cell modules have slightly higher efficiencies due to the lower current in each cell and have higher efficiencies than full-cell modules.

In addition to these new cell configurations, the Vmp and Voc voltages of PERC modules are 5 to 15% higher than those of traditional modules. than that of traditional modules. Table 1 shows a comparison of the nominal voltages between the traditional traditional modules and the new PERC modules under standard test conditions.

Partial shading: advantages of the half-cell module arrangement

Partial shading of solar modules has always been a major concern for solar PV systems. In off-grid systems it is especially important to achieve the highest possible load to maintain loads and extend battery life. This is where the use of half-cell modules can really make a difference.

• Half cells have the same voltage as full cells but 50% of the current.
• The modules consist of 2 parallel strings of half-cells, which is similar to 2 modules connected in parallel.
• Shading one side of the panel only affects 50% of the module current, allowing the module to operate at full voltage with at least 50% of the current.
• Partially shaded half-cell modules can provide sufficient voltage to support charge controllers where full-cell modules do not.
• An MPPT controller can allow full current at lower voltage with some bypass diodes activated within limits.
• Better MPPT tracking can be obtained with half-cell modules with higher array voltage levels.

Partial shading with a whole-cell module acts as a bottleneck by limiting the current of the entire panel. With partial shading, the half-cell module can still operate at full voltage and at least 50% of the module current of the unshaded half of the panel. This can be much better than if the partial shading affects 100% of the current.

As these points demonstrate, the ability of half-cell modules to operate at full voltage and at least half the current can make a significant difference in allowing higher load when partial shading is present. This additional power can be critical for systems that can have partial shading that lasts longer.

How does this affect the sizing of arrays with solar controllers?

Meeting the voltage requirements of a PV field is one of the most important considerations when calculating string size. The voltage rating of the new PERC modules, 5 to 15% higher, can have a big effect on this.

• The 36-cell and 72-cell PERC modules have a higher voltage rating than what was considered the standard with traditional voltage rated modules in the past.
• The voltage ratings of the 60-cell PERC modules are now only a few volts lower than those of the traditional 72-cell modules.

36- and 72-cell "nominal voltage" PERC modules

One of the advantages of using MPPT controllers instead of PWM controllers is the "MPPT Boost": the extra power achieved by operating at the maximum power voltage (Vmp) compared to a PWM controller operating in the lower battery voltage operating range.

• The 36-cell and 72-cell PERC modules can still be used at nominal voltage with an equivalent battery voltage match of 12V or 24V respectively, with both PWM and MPPT controllers.
• MPPT Boost gains are now even more significant compared to a PWM controller.
  • MPPT Boost vs. PWM is ~5-30% with traditional modules.
  • MPPT Boost vs. PWM is ~15-55% with PERC modules.

Although it is possible to use PWM controllers with the new 36-cell and 72-cell PERC modules, the increase in MPPT associated with the PERC modules makes the MPPT controllers even more economically attractive than before. The graphs below illustrate the additional MPPT increase obtained with the new of the new modules compared to the older ones.

The above graphs represent a traditional module operating at its rated voltage Vmp. It can be seen that the power shown at the Vmp voltage of 18V achieved by an MPPT charge controller is higher than that generated by a PWM controller with an operating voltage range of 10-15V.

The figure above illustrates the performance of a new PERC PV module operating at its nominal Vmp voltage of 21 volts.

The MPPT increase with these new 36 and 72 cell PERC models is significantly higher than what we have seen before. The voltage versus power graph shown here has a higher MPPT voltage of 21V compared to 18V in the previous graphs. At these higher voltages, the MPPT increase will be approximately 15 to 55%, which is a significant gain over traditional modules. The higher the Vmp value, the higher the MPPT increase.

Array sizing with the new 60-cell PERC modules

As noted in Morningstar's technical advice on sizing 60-cell PV modules, traditional 60-cell modules are not well suited to meet the nominal voltage matrix requirements for PWM controllers. This is because the Vmp voltage can drop too low at higher PV cell temperatures, causing marginal performance and preventing the battery from fully charging. Some customers may be tempted to consider using 60-cell PERC modules instead of 72-cell modules in nominal size systems, especially with PWM controllers. Use extreme caution and be aware of the potential consequences before deciding to use a 60-cell PERC module with a PWM controller.

The following factors reduce the risk of marginal performance when sizing a 24V or 48V PWM system with 60-cell PERC modules:

• Higher nominal Vmp values.
• Lower maximum regulation voltages; for sealed, AGM or GEL batteries; not for flooded batteries.
• Climates with lower average maximum temperatures.
• Systems that deploy lithium batteries that can tolerate extended periods in a partial state of charge.

The following figure shows the IV and power curve plots of a 60-cell PERC module operating at its nominal Vmp voltage of 33 volts. The Vmp voltage is not much higher than the highest operating voltage of the PWM controller. When the module voltage is reduced due to high temperatures, the Vmp of the array may drop below the voltage set points required by the controller and the load may temporarily stop during this period.

Insufficient voltage will be more of a problem with MPPT controllers. You can see from this graph that the lower Vmp voltage results in little or no MPPT boost. In addition, MPPT controllers are DC-DC Buck converters and require slightly higher input voltages than PWM controllers for charging to occur, which increases the risk of marginal performance in warmer conditions.

Based on the above recommendations, it is recommended to use 60-cell PERC modules with MPPT controllers as long as there are enough modules in series to fully charge the battery bank:

• For 24V battery banks, this means that at least two 60-cell modules are needed in series.
• For 48 V battery banks, it means that three or more modules are needed in series.

Conclusions

In summary, PERC cell technology, with its new cell configuration options, provides more voltage to charge controllers to greatly increase energy harvest. This provides more overall gains in maximum power point tracking systems and provides PWM controllers with higher input voltage to improve their charging performance in hot conditions. Power Point Tracking and provides PWM controllers with a higher input voltage to improve their charging performance at elevated temperatures. In addition, the higher cell count modules, with their two-in-one design, mitigate the impact of partial shading, so the modules can still provide most of their rated power during these conditions. We hope this technology overview, and the sizing examples we have included, will help you take advantage of the benefits of PERC technology in your off-grid system designs.

Source: "PERC Cell Technology & Half Cell Solar Modules: How Efficiency Gains and Cell Layout Yield Greater Charge Controller Output", whitepaper de Morningstar