Solar array mismatch losses: What are they, How to prevent them?
A PV array is made up of a number of PV modules that are connected in series and/or parallel. These modules ideally would have the same characteristics (e.g. Voc, Isc, Imp, Vmp, and Pmax).
However, in real life, PV modules are not perfectly matched. This mismatch results in power losses that can be significant in large arrays.
The solar array mismatch loss is the difference between the power that would be produced by an array of perfectly matched modules and the actual power output of the array. The mismatch loss can be due to many factors, such as manufacturing differences, shading, soiling, and temperature.
The solar array mismatch loss is expressed in the percentage of the total array output. For example, if an array has a mismatch loss of 0.5% and the total array output is 1000 W, then the mismatch loss would be 0.005 kW (0.5% x 1000W).
The solar array mismatch loss can be a significant contributor to overall system losses. Therefore, it is important to quantify and mitigate these losses where possible.
What are the reasons for solar array mismatch losses?
There are many reasons for solar array mismatch losses. Here are some of the most common reasons:
Manufacturing mismatch
The two modules in a PV system are not identical. Even if they are from the same batch and have the same specs, their characteristics will differ slightly. These manufacturing differences can lead to solar array mismatch losses.
Manufacturers usually sort modules at the end of the production line based on the characteristics obtained from a flash test. Each module will have its own set of flash test results. You can always ask the manufacturer for the flash test results to see how well your module matches the ideal values specified in the datasheet.
Typically, most modules will have a 1.5 to 2% mismatch loss as a result of the flash test.
However, the flash-test apparatus used for this sorting is not perfect. Their precision cannot be greater than +/-3 percent. As a result, the actual dispersion between modules may be much greater.
Given the flash test's inherent inaccuracies, a PV system with a mismatch loss of less than 2% is considered to have very good matching.
Thermal gradients
Panels on the array's periphery receive more airflow and operate at a lower temperature than panels in the array's core.
Because most crystalline silicon panels lose 0.44% of their power output for every degree Celsius difference in temperature, a 10°C difference in temperature can result in a power loss of nearly 5%.
Failed bypass diodes
Bypass diodes are designed to shunt current around a PV module when it is not producing power. If one or more of the bypass diodes fails, the current will flow through the module, creating a power loss.
Bypass diodes most often fail when the panels are operating, reducing the voltage by one-third (in a 3-diode panel.) Because this represents only about 2-3% of a string's total voltage, measurement equipment has a difficult time detecting it.
Shading
Partial shading can result in significant energy losses for solar PV arrays. Trees, buildings, roof obstructions, power lines, dust, and bird droppings can all cause shading.
Additionally, when the PV modules are installed at different orientations or tilt the mismatch loss between modules can be significant.
When using series-connected strings, the current of the solar array is only as good as the worst-performing panel. This means that a single shaded panel can significantly reduce the power output of an entire array. The amount of power lost depends on the severity and location of the shading.
Soiling
Soiling is the accumulation of dirt, dust, or snow on the PV module. Because the efficiency of a PV module drops as the light hitting its surface is scattered, soiling can cause a significant power loss.
The amount of soiling that a panel can withstand without any loss in power production is called the soiling factor.
The soiling factor varies depending on the type of dirt, dust, or snow that is accumulating. For example, dry desert dust has a low soiling factor, whereas sticky tree sap has a high soiling factor.
In order to minimize the effects of soiling, PV modules should be maintained regularly. This can be done by hosing off the array and using a soft brush to remove any accumulation.
Voltage drop
When current flows through a conductor, it creates a voltage drop. This voltage drop is due to the resistance of the conductor. The voltage drop increases as the length of the wire increases. In a PV system, this can result in significant power losses if not properly accounted for.
To minimize mismatch losses that may be caused by voltage drops, it is important to use a wire gauge that is appropriate for the current being carried. It is important to use as short a wire as possible while using a larger gauge wire. This will minimize the voltage drop and prevent potential mismatch losses.
Variable degradation
As a PV module ages, its power output decreases. Several factors, including environmental conditions and the module's electrical load, are responsible for this degradation.
Due to many inherent inaccuracies, the actual amount of power lost due to degradation can be difficult to determine. Some modules will degrade more rapidly than others, while others may degrade very slowly.
However, it is generally agreed that crystalline silicon PV modules degrade at a rate of 1% to 4% per year. However, degradation rarely pursues a linear trend. Thus, it is not uncommon for degradation to accelerate or decelerate over time.
Accumulated wear and tear
All PV modules are subject to wear and tear. This can be caused by environmental factors, such as wind, rain, and hail. Additionally, the modules can be damaged by debris or objects that come into contact with them.
Damage to the PV modules can cause a significant power loss, especially if it is not repaired quickly. In order to avoid this, it is important to regularly inspect the modules and repair any damage that is found.
Solar PV systems may not show any visible sign of wear and tear while having significant energy losses. Although the human eye is a very good tool to detect any problems early on, it is important to have a system in place that can accurately measure and track solar array performance.
For this reason, periodic maintenance and inspections should be made using more sophisticated tools. Voltage and current measurements, as well as spectral analysis, can help identify any problems that may be occurring in the solar array.
Using infrared thermography, it is also possible to detect any hot spots that may be present on the modules. These hot spots can indicate a number of problems, including partial shading, poor insulation, or high electrical loads.
By understanding the different causes of solar array mismatch losses, system owners and operators can take steps to mitigate these losses and improve the performance of their PV systems.
LID (Light-induced degradation)losses
When light shines on a solar cell, some of the energy is converted into electricity. The remainder of the energy is lost as heat. This heat can cause the solar cell to increase in temperature, which can lead to a loss in power output.
LID losses occur when the light-absorbing layer of the PV cell is damaged. This can be caused by a number of factors, including UV exposure, humidity, and high temperatures.
LID losses are typically irreversible and result in a permanent reduction in power output. In order to avoid these losses, it is important to protect the PV cells from exposure to damaging elements.
Since not all PV modules are created equal, they will not all experience the same degree of LID losses. The type of PV cell, as well as the manufacturing process, will play a role in the level of LID losses that a module will experience.
It is important to note that LID losses are not always immediately apparent. They may occur slowly over time and may not be noticeable until the power output of the module has decreased significantly.
Since LID losses are caused by damage to the light-absorbing layer, they can be detected using spectral analysis. By measuring the reflectance of the PV cell at different wavelengths, it is possible to determine if there is any damage to the light-absorbing layer.
PID (Potential-induced degradation)losses
PID losses occur when there is a potential difference between the PV cell and the ground. This can happen when the solar array is not properly grounded or when there is a difference in the voltage between different modules.
PID losses can be caused by a number of factors, including exposure to moisture, high temperatures, and electromagnetic fields. PID losses are typically irreversible and result in a permanent reduction in power output.
In order to avoid PID losses, it is important to properly ground the solar array and keep the PV cells clean and dry. Additionally, it is important to avoid exposure to high temperatures and electromagnetic fields.
PID losses can be detected using voltage and current measurements. By measuring the voltage and current at different points in the system, it is possible to determine if there is any difference in voltage between modules.
The effects of PID losses are usually not immediately apparent. They may occur slowly over time and may not be noticeable until the power output of the module has decreased significantly. The extent of the PID losses will depend on the size of the voltage difference and the number of modules in the system.
Since PID losses are caused by a difference in voltage between modules, every module in the system will have different PID losses. The difference in PID losses between modules will likely cause mismatch losses in the system.
How to minimize solar array mismatch losses?
Solar array mismatch losses will happen in any PV system, but there are a number of steps that can be taken to minimize these losses.
Use identical PV modules
By using identical PV modules, it is possible to minimize the difference in power output between modules. Identical modules are manufactured in the same way and will have very similar electrical characteristics although there may be some slight variation between modules.
Manufacturers may change their manufacturing techniques over time, which can cause some variation in the power output of modules. So while identical modules are not always available, it is important to try to use modules from the same manufacturer and batch.
Use bypass diodes
Bypass diodes are used to bypass the current around a damaged or shaded PV cell. Bypass diodes can help to minimize the power losses caused by shading and can also help to prevent hot spotting.
Bypass diodes are not always available on all modules and they can add cost and complexity to the system. So it is important to weigh the benefits and drawbacks of using bypass diodes before deciding to add them to the system.
Minimize the effects of shading
Shading can have a significant impact on the performance of PV systems. By minimizing the effects of shading, it is possible to improve the power output of the system and reduce solar array mismatch losses.
One way to minimize the effects of shading is to use a microinverter or DC optimizer. These devices allow for the individual optimization of each PV module in the system.
By using a microinverter or DC optimizer, it is possible to minimize the effects of shading and improve the overall performance of the system.
Choose products with low LID losses
There are a number of PV modules on the market that have low LID losses. These modules have been designed to minimize the amount of damage that can be done to the light-absorbing layer. By using a PV module with low LID losses, it is possible to reduce the level of mismatch losses in the system.
LID losses can be reduced by using a PV module with an anti-reflective coating. This coating helps to reduce the amount of light that is reflected off of the surface of the module.
Check the voltage and current at different points in the system
By checking the voltage and current at different points in the system, it is possible to detect any differences in voltage between modules. This can help to identify the sources of PID losses and help to minimize their effects.
The effects of PID losses can be minimized by using identical PV modules, bypass diodes, and microinverters or DC optimizers. By minimizing the effects of PID losses, it is possible to improve the performance of the system and reduce solar array mismatch losses.
Avoid exposure to damaging elements
It is important to avoid exposure to damaging elements such as salt spray, hail, and high winds. These elements can damage the PV modules and cause a loss in power output.
By avoiding exposure to these elements, it is possible to reduce the level of damage to the PV modules and prevent mismatch losses in the system.
Use of an MPPT (Maximum Power Point Tracker)
Using an MPPT charge controller will minimize the effects of mismatch losses in the system. By tracking the maximum power point of each module, the MPPT can ensure that all modules are operating at their optimal power output.
This will minimize the losses due to mismatched modules and will also increase the overall power output of the system.
Module-level monitoring
Module-level monitoring means that each module in the system is monitored individually. This allows for the identification of modules that are not performing as well as others.
By monitoring each module, it is possible to identify modules that are experiencing LID or PID losses and replace them with new modules. This will help to minimize the losses due to mismatched modules and will ensure that the system is operating at its maximum efficiency.
Using a String Inverter
String inverters are designed to work with multiple modules in parallel. By using a string inverter, it is possible to connect multiple modules in series and minimize the effects of mismatch losses.
String inverters have the ability to automatically adjust the voltage of the modules to match the voltage of the system. This will minimize the losses due to mismatched modules and will improve the overall efficiency of the system.
Ensure that all modules are properly grounded
Grounding is important for any electrical system. By ensuring that all modules are properly grounded, it is possible to reduce the level of voltage fluctuations in the system.
If not properly grounded, modules can experience a loss in power output due to voltage fluctuations. By ensuring that all modules are properly grounded, it is possible to reduce the level of mismatch losses in the system.
Keep the PV cells clean
It is important to keep the PV cells clean. Dust, dirt, and debris can collect on the surface of the cells and reduce their ability to absorb light. By keeping the cells clean, it is possible to improve the efficiency of the PV modules and reduce solar array mismatch losses.
Plan your PV system with compatibility in mind
When planning your PV system, it is important to take into account the compatibility of the modules. By using modules that are compatible with each other, it is possible to reduce the level of mismatch losses in the system.
Also, avoid combining 12V and 24V modules in the same system. If you will have a 12V system, use 12V modules. If you will have a 24V system, use 24V modules. Choose your batteries and inverters wisely considering the same.
Choose products that come with a performance warranty
Many manufacturers offer performance warranties on their products. This warranty guarantees that the product will perform as advertised for a certain period of time.
By choosing products with a performance warranty, it is possible to ensure that the modules will perform as expected and minimize solar array mismatch losses.
Choose products that are manufactured to tight specifications
PV modules that are manufactured to tight specifications will have less variation in their electrical characteristics. This means that there will be less difference in the power output of the modules and less solar array mismatch losses.
Use a module-level power optimizer
Module-level power optimizers are devices that are attached to each PV module. These devices allow for the individual optimization of each module in the system.
This allows for the maximum power output of each module to be extracted and will minimize the effects of mismatch losses. By using a module-level power optimizer, it is possible to improve the overall efficiency of the system.
Install a monitoring system
A monitoring system can be used to track the performance of PV systems. By monitoring the performance of the system, it is possible to identify issues that are causing solar array mismatch losses.
By installing a monitoring system, it is possible to identify and correct problems that are causing solar array mismatch losses. This will improve the performance of the system and reduce the amount of energy that is lost due to mismatched modules.
Solar array mismatch losses can have a significant impact on the performance of PV systems. By taking steps to minimize these losses, it is possible to improve the efficiency of the system and reduce the amount of energy that is lost.
Conclusion
Solar array mismatch losses are caused by a number of factors that can lead to a decrease in power output.
Some of these losses are sourced from the PV cells themselves, while others are sourced from the module and system design. It is very important to note that usage and environment can play a significant role in the degree of mismatch losses that are seen in a system.
In order to avoid these losses, it is important to properly design and install the solar array. Additionally, it is important to protect the PV cells from exposure to damaging elements.
By taking these steps, it is possible to reduce the amount of solar array mismatch losses that are seen in a system. This will improve the overall efficiency of the system and help to reduce the amount of energy that is lost.
