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Cell To Module (CTM) losses in solar PV systems

Solar cells are the building blocks of solar PV systems. These cells are connected together in series and parallel to form modules, which act as the basic unit of a photovoltaic system.

The modules are then combined into arrays, which are also connected in series and parallel to form a solar PV system.

Solar PV systems are only efficient as the energy produced by their modules. Module efficiency depends on many factors, including but not limited to cell conversion efficiencies, cell to module (CTM) losses, and other system-related losses.

Despite the fact that cell efficiency has improved considerably in recent years, such improvements have not always been reflected at the module level. Today it is very common to see cell-to-module power losses of around 2% to 3% for most solar panels on the market.

What are “Cell to module” (CTM) losses?

Since cells come together to form modules, the output of the modules should be the combined output of the cells. But, this is only true in theory. In reality, there are “Cell to module” (CTM) losses that result in modules not being able to deliver the combined output of their cells.

“Cell to module” (CTM) losses refer to the decrease in output power from a solar cell to an assembled module. In other words, the output power of the module will be less than the combined output power of each individual cell.

Why do (CTM) losses happen?

Cell to module (CTM) losses happen in solar PV systems for a variety of reasons including:

Reflection at subsequent interfaces

Reflection at the multiple interfaces between cells and modules, namely air-glass, glass-encapsulant, and encapsulant-solar cells, leads to the loss of incident light energy.

These interfacial reflections contribute to cell-to-module (CTM) losses by creating additional boundaries that will eventually result in lower power output.

Shadowing (shading) between cells and modules

Shadowing (shading) at the cell-to-cell boundaries also leads to cell-to-module (CTM) losses.

If the cells are not exactly aligned with one another, an outer cell will only receive light that can pass through a smaller fraction of its area than an inner cell. Since the outer cells are not able to produce as much power, these losses contribute to lower module output.

Hot-Spot Losses

In certain circumstances, the operation of the module may result in a module overheating in a small location. This is generally due to an incorrect solar cell current caused by local shade or a faulty solar cell.

Because the unaffected solar cells seek to keep the current high, they push the shaded or damaged solar cell to function in reverse bias, and the current in a reverse-biased solar cell is often quite localized.

This results in extremely high current densities and, as a consequence, very high temperatures. These localized high temperatures might seriously damage the PV module.

Because of this, all PV modules contain bypass diodes, and all solar cells are tested for reverse bias current densities (if these currents are very high, the solar cells would not be used to build PV modules).

Snail Trails

Snail Trails are caused by a break in the solar cell’s backsheet, which allows moisture to seep into the PV module and reach the solar cell’s front via the crack.

The moisture in the air reacts with the silver and EVA, producing a “snail trail.” Although snail trails are not extremely detrimental to PV module performance, they are not desirable from an aesthetic standpoint.

Path length mismatch

Path length mismatch occurs when one cell absorbs more photons than another. If the second cell receives fewer incident photons, it produces less power and contributes to cell-to-module (CTM) losses.

This mismatch can be due to manufacturing variances, cell degradation over time, or other reasons.

Impurities in the cell materials

Impurities in the cell materials are another cause of cell-to-module (CTM) losses. When cells are manufactured, impurities enter the materials at various stages in the manufacturing process and impact their performance.

As a result, the cells lose power when they are assembled into modules and contribute to cell-to-module (CTM) losses.

Intercell resistance

Intercell resistance is the loss of power due to the resistance that cells experience between each other. This is caused by the different doping levels in cells of the same type (i.e., cells with identical doping levels will generally not contribute to cell-to-module (CTM) losses).

String interconnection losses

String interconnection losses are the result of the resistance between PV cells in a string. The more strings there are, the higher these losses will be. These losses can be attributed to wire resistance between modules and string-to-string connections.

Mismatch in cell electrical parameters 

Cells with different electrical parameters may not perform equally in the same module.

Cells that are more sensitive to environmental conditions can produce less power when used with other cells that are more temperature or irradiance-tolerant. This mismatch contributes to cell-to-module (CTM) losses as well.

Soiling losses and EVA discoloration

Soiling loss refers to the loss of output power caused by the deposition of snow, dust, dirt, and other particles on the surface of PV modules.

Because the performance of a module is determined by the amount of sunlight that reaches the cells, these small environmental particles immediately reduce power output.

Given the importance of module care after installation, the PV industry is developing a variety of technologies to keep modules clean for longer periods of time.

The breakdown of the EVA is another factor for decreasing PV module performance during continuous operation. Some EVA compositions are not fully UV stable, resulting in browning of the EVA.

This is not just an aesthetic issue, but it also causes a decline in module performance due to an increase in EVA film absorption.

How to reduce CTM losses? 

CTM losses are unavoidable, but there are several ways to reduce them.

Manufacturers can improve the quality of cells and modules by increasing their accuracy during manufacturing processes, which would result in more high-quality solar cells being used for PV modules. Here are the quality checks and the best practices that can be done in the manufacturing process to reduce CTM losses:

Perform Electrical Testing

Perform electrical testing on cells after they are manufactured. This will help manufacturers identify poorly performing cells before they are assembled into PV modules, which reduces CTM losses caused by poor-performing cells.

Cells that fail this test may have either high intercell resistance or mismatched electrical parameters.

Perform Temperature Testing

Perform a temperature test on cells after they are manufactured to identify cells that have high intercell resistance, which can contribute to CTM losses in PV modules.

If the cell junction temperature is higher than expected when exposed to certain environmental conditions (e.g., irradiance), it may be due to the low quality of materials or impurities.

Perform Impedance Matching Test

This test checks the cells’ response to certain voltages and currents at different frequencies, which helps to determine the cells’ performance under certain conditions. This is especially important for cells that are used in outdoor applications because they will be affected by environmental conditions.

Perform Thermal Cycling Tests

It is very important to determine the cells’ performance under extreme temperatures that are common in certain climates, such as in desert areas.

If cells are not able to withstand these conditions, they may have high intercell resistance and break down during module operation.

Use the same type of cells for all modules in a system

Using the same type of cells ensures that each module will have a similar performance. Use bypass diodes for modules with different electrical parameters

Bypass diodes allow current to flow around areas of a solar PV system that have higher electrical resistance. This minimizes power loss due to these resistances. Have multiple bypass diodes in a solar PV system

Multiple bypass diodes provide a wider current path for electricity to flow. This minimizes power loss due to electrical resistance even more so than single bypass diodes.

Use high-quality glass and encapsulants

Using high-quality glass and encapsulants prevents reflection at the cell-to-module (CTM) boundaries. This minimizes power loss due to these reflections.

Reduce the number of cell-to-module (CTM) boundaries

The fewer interfaces, the lower power loss. Reducing the number of cell-to-module (CTM) boundaries reduces the amount of power lost due to reflection, shadowing, path length mismatch, and impurities.

Better design cell-to-module connection system

The cell-to-module (CTM) connection system is key to ensuring that module electrical parameters remain constant. Better-designed systems also minimize power loss due to electrical resistance.

Manufacturing companies can reduce losses by utilizing high-quality cells, as well as designing the best cell-to-module connection system to ensure that the module’s output is as close to that of its cell input.

Use anti-reflective (AR) coating

Using an anti-reflective coating minimizes power loss due to reflection at the cell-to-module boundary. AR coatings work by minimizing the reflection and increasing light transmission.

Conclusion

Manufacturers can take steps to reduce cell-to-module (CTM) losses by performing electrical testing on cells, using the same type of cells for modules in a system, and reducing the number of CTM boundaries.

They should also use high-quality glass and encapsulants, bypass diodes where necessary, multiple bypass diodes when possible, thermal cycling tests to ensure performance under extreme conditions that are common in certain climates like deserts.

Finally, manufacturing companies should utilize anti-reflective coatings on the CTM boundaries to minimize reflection and increase light transmission.

Cell to module (CTM) losses in solar PV systems can be minimized by using high-quality cells, designing the best cell-to-module connection system to ensure that module electrical parameters remain constant and making use of anti-reflective coatings.