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'Mismatch' in Solar Power Systems: Ways to Mitigate Its Impacts

A solar panel functions normally only when all its interconnected cells work together seamlessly. Likewise, only when all solar panels operate as expected can the entire array perform flawlessly.

However, in the real world, it is not uncommon that ‘mismatch’ occurs between either cells or panels of the solar power systems, posing negative impacts to the performance.

This article will shed light on ‘mismatch’, including its causes, classifications, impacts as well as mitigation strategies. Industry practices related to mismatch issues will also be explored in the discussion.

Install Solar Panel Array on Rooftop

What Is 'Mismatch'? What Are the Causes?

In solar power systems, ‘mismatch’ refers to the phenomenon where the electrical characteristics of individual cells or panels with a solar array differ from one another.

This discrepancy can directly result in significant power losses and reduced overall efficiency of the solar power system.

Here are some common causes of mismatches:

  • Shading: Nearby trees, buildings or other obstructions will create partial shading on solar panels, which subsequently cause a mismatch in the current generated by the shaded and unshaded cells.
  • Soiling: The shading effect can also be formed by the accumulation of dust, dirt, droppings and other debris on the surface of panels. Similarly, this effect causes electrical discrepancies between cells or panels.
  • Installation Variations: This is another external cause of mismatches. Differences in orientation, positioning and tilt angles of panels can give rise to varying levels of sunlight absorption and resulting performance among the panels in the array.
  • Manufacturing Variations: Variations in electrical characteristics can be found not only between panels from different manufacturers but also between those from the same manufacturer and batch. These differences can lead to mismatch losses when the panels are interconnected.
  • Aging & Degradation: Over time, solar cells and panels may degrade due to both chemical and physical processes (e.g., light-induced degradation, mechanical stress, overheating, etc.) at different rates, leading to variations in their electrical characteristics.
  • Electrical Configuration: Faulty wiring, differences in string lengths, connector problems or inverter issues can also contribute to mismatch losses.

One thing to note is that temperature variations in cells or panels can be frequently concurrent with the onsets of the causes above. This type of variation exerts influences on electrical characteristics as per wiring configurations and temperature coefficients.

What Are the Different Types of Mismatches in Solar Power Systems?

While mismatches can be categorized by their causes as explained above, a more practical approach is to classify them by their electrical characteristics. This will provide a more tangible understanding of how mismatches affect performance.

Current Mismatch

Also known as amperage mismatch, current mismatch happens when the output currents of the cells or panels within a string are not identical. This impact is particularly pronounced when the string is configured in series where the current of the entire string is limited by the cell or panel with the lowest current output.

Voltage Mismatch

A voltage mismatch occurs when there are differences in the voltage output of solar panels connected in series within a string.

This type of mismatch is generally less severe than current mismatch in a series connection as the overall current is determined by the lowest current output whilst overall voltage is the sum of the individual voltages of the individual cells or panels.

Wattage Mismatch

Wattage mismatch directly correlates to voltage and/or current mismatch.

A wattage mismatch refers to differences in the power output of individual cells or panels which can be due to variations in their voltages, currents or both. This mismatch affects the overall energy production and reduces the efficiency of the entire string or system.

What Are the Impacts of Mismatches?

In addition to inducing reductions in efficiency and output at the panel, array or system level, mismatches can also lead to several negative impacts:

Stresses on Components

Mismatched voltage and current characteristics among cells or panels can bring about localized overheating, causing hotspots that accelerate material degradation at varying rates. This uneven degradation can exacerbate mismatch issues over time and create a vicious cycle of declining efficiency.

On the other hand, significant mismatches can make it difficult for maximum power point tracking (MPPT) devices to find and maintain the optimal point. Constantly adjusting to fluctuating outputs due to mismatches can place additional dynamic stresses on these devices.

Furthermore,  uneven load distribution across accessories such as cables and connectors can result in localized heating and potential overloading, increasing the risk of faults or fires.

Operational Challenges

Mismatches can challenge the capabilities of the inverter's MPPT algorithms. This can lead to suboptimal energy conversion. If mismatches persist, they can ultimately affect the reliability of the solar power system, increasing maintenance needs and potential downtimes.

Financial Impacts

Reduced efficiency and energy production of the system directly translates to lower revenue generated, lower savings on energy bills as well as a longer payback period for the investment.

Rising maintenance and component replacements can drive up operational expenses, eroding the project’s ROI.

Are There Any Types of Solar Panels That Have Fewer Mismatch Issues?

It is an incontestable fact that ‘mismatch’ is a common problem in every type of solar panel. However, there are certain types of panels that have fewer mismatch issues than the others.

Monocrystalline Panels

Fabricated with high-purity and premium materials, monocrystalline solar panels tend to exhibit a more uniform performance compared to their polycrystalline counterparts. This uniformity leads to fewer mismatch issues.

Bifacial Solar Panels

These panels can capture sunlight from both sides. By producing more power, they reduce the relative effect of mismatches and can compensate for partial shading more effectively.

Solar Panels With MLPEs

Microinverters and power optimizers are both module-level power electronics.

In contrast to string inverters, the former mitigates mismatch issues by optimizing the power output of each panel individually, reducing the impact of differing panel performance within an array. While the latter helps reduce the adverse effects of shading, soiling and panel degradation on the overall string or system, causing fewer mismatch issues.

Thin-Film Panels

Attributed to particular properties of their materials and composition, thin-film panel technologies such as CdTe have higher tolerance to shading and temperature variations. This feature can reduce mismatch issues in certain applications.

How to Mitigate Mismatches?

Besides selecting the premium panel products introduced above, several other strategies can be adopted for the mitigation of mismatches.

Bypass Diodes

Solar Panel String With Bypass Diodes
Pendem, S. R., & Mikkili, S. (2018). Modeling, simulation and performance analysis of solar PV array configurations (Series, Series–Parallel and Honey-Comb) to extract maximum power under Partial Shading Conditions. Energy Reports, 4, 274-287

Despite that many modern panels are integrated with bypass diodes in their design, many older-generation products don’t have these parts.

Incorporating bypass diodes into existing strings can allow currents to bypass shaded or underperforming panels. This prevents the entire string's performance from being dramatically dragged down by a single underperforming panel.

Binning Strategy

Grouping solar panels with electrical characteristics (e.g., same model, production batch and manufacturer) can reduce inherent mismatch losses.

Specifically, this involves sorting and pairing panels based on their performance metrics to ensure uniformity within strings.

Reconfiguration

For existing systems, grouping shaded panels into separate strings or rearranging panels to place higher-rated replacements in shaded sections can help to optimize overall performance.

Integration With MLPEs

Many solar projects rely on string inverters, but when mismatch issues become severe, it's worth considering a retrofit to integrate microinverters or power optimizers, leveraging their MPPT technology to mitigate mismatches.

Compared to using panels with MLPEs for new projects, upgrading existing projects necessitates a comprehensive evaluation of the compatibility of these devices for current systems. 

Maintenance

Regular cleaning and maintenance of solar panels can reduce mismatch losses due to soiling and debris. Ensuring that panels are free from obstructions and/or shading can help maintain uniform performance across the array.

Monitoring and Diagnostics

Regular monitoring and diagnostics can help identify issues such as micro-cracks, faulty bypass diodes, hotspots and other defects that can cause mismatches. Early detection contributes to early addressing of these issues and can avoid severe mismatch problems. MLPE devices, if installed, can effectively facilitate the detection.

Future Trends and Research

Mismatches present a threat to the performance, longevity and reliability of systems.

First of all, the industry is carrying out enhanced quality control processes, more standardized panel characteristics and tighter manufacturing tolerances to ensure a more consistent panel performance.

Commercialization of innovative technologies involving premium materials and structures, such as tandem cells is gaining momentum. More and more panels with self-cleaning functionality are being rolled out to address mismatch issues caused by shading and soiling.

The industry also strives to improve MLPEs by developing more advanced MPPT algorithms using machine learning and AI that adapt in real-time to changing conditions.

Internet of Things (IoT) devices and advanced sensors are being increasingly used to offer more accurate, real-time monitoring. The concept of digital twins is also being widely implemented to model solar farms, predict and identify mismatch issues, and optimize operations in real time.

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