Simply put, a solar PV module is a packaged and connected assembly of multiple solar cells. When the PV module is exposed to sunlight, it produces electricity because of the photovoltaic effect

Types of PV modules

There are two main types of PV modules: crystalline silicon PV modules and thin film PV modules.

Crystalline Silicon PV Modules

Crystalline silicon PV modules are very popular and contribute almost 90% to the total PV modules produced all over the world. They are further classified as monocrystalline PV modules and polycrystalline PV modules.

Monocrystalline PV modules are made from a single crystal or monocrystalline solid in which the crystal lattice is continuous and unbroken to the edges with no grain boundaries. The absence of the defects associated with grain boundaries give monocrystalline PV modules unique properties, especially mechanical, optical, and electrical.

Polycrystalline PV modules are made from polycrystalline materials which are composed of many crystallites of varying sizes and orientation.

Thin Film PV Modules

Thin film PV modules are PV modules made of thin film solar cells. As their name suggests, these thin film solar cells are made by depositing one or more thin films of photovoltaic material ona substrate.

Depending upon the type of substrate used, thin film PV modules are categorized in two main types:

  1. Rigid thin film PV modules
  2. Flexible thin film PV modules.

Depending upon the type of photovoltaic material used, thin film PV modules are categorized in four main types:

  1. Amorphous silicon (a-Si) and other thin film silicon (TF-Si)
  2. Cadmium telluride (CdTe)
  3. Copper indium gallium selenide (CIGS)
  4. Dye-sensitized solar cells (DSC) and other organic solar cells

Market Share

Global Annual PV Installation by technology

The figure above shows the market share of various types of PV modules from 2000 till 2010. As can be seen, polycrystalline PV modules had a 45% market share,monocrystalline PV modules had a 40% market share, while thin film PV modules had a 14% market share in 2010.

Market share of thin film technologies

Efficiency of PV modules

Depending on their construction, the efficiency of PV modules varies greatly. The variation is because different types of PV modules are able to generate electricity from different frequencies of light – infrared frequencies, visible frequencies, and ultraviolet frequencies – to varying degrees. Usually a particular type of PV module can generate electricity from one or more frequencies but not all. As a result, much of the solar energy that is incident on the PV modules is wasted, which is why their efficiency is in the 7% to 20% range. It is worth noting that even in the case of highest efficiency PV modules, 80% of the solar energy that is incident on the PV modules is wasted!

Therefore, one design concept is to split the light into different frequencies and direct it onto solar cells that are tuned to generate electricity from those frequencies. This concept has the potential to generate PV modules with 50% conversion efficiency. The theoretical upper limit on the conversion efficiency is 70%.

However, it is not just the efficiency that matters. Finally, it has to be rationalized with cost as well. If a PV module has 40% efficiency, it is double the efficiency as compared to a PV module which has 20% efficiency. However, if the cost of production of the former is three times that of the latter, then the latter actually represents a better value for money despite having a lower efficiency.

That is why although thin film PV modules having efficiency of only 7% could, and do, represent a better value for money in many cases.

Currently, the PV modules of different types have the following efficiencies:

  • Monocrystalline PV modules have the highest efficiency ranging from 14% to 17%.
  • Polycrystalline PV modules come in next with efficiency ranging from 12% to 15%.
  • Thin film PV modules have the lowest efficiency ranging from 7% to 12%. However, some of the latest thin film PV module technologies have fairly high efficiencies. First Solar has reported cell efficiency of 20.4% for their prototype CdTe solar cells.

Typically, solar cell efficiencies are the highest, prototype PV module efficiencies are lower, and production PV module efficiencies are the lowest.

PV Module Performance and Warranty

PV modules are rated based on the DC power that they can generate under Standard Test Conditions, or STC in short. For example, if a PV module generates 300W under STC, it will be rated as 300Wp. The ‘p’ stands for ‘peak’ and it is mentioned in the rating to signify that it is the power that the PV module will generate under STC. In many ways, it is the peak power that the PV module can generate because STC (or conditions very close to it) are hardly encountered in real life; in actual operating conditions, in most cases, the same PV module might generate a lot less power compared to this number.

STC stands for Standard Test Conditions and consists of three parameters:

  1. Solar Irradiance: This is the intensity of the solar radiation that is incident on the PV module, and it must be 1000W/m2.
  2. Temperature: The temperature is the cell temperature and not the ambient temperature, and it must be 25°C.

Air mass:Air mass, to put it simply, is the amount or thickness of air through which the solar rays travel before reaching the PV module. Air mass varies at various points on the Earth’s surface because the Earth is spherical thus causing the solar rays to strike the PV modules at varying angles of incidence. Air mass of 1 would be when the sun is directly overhead. As the sun goes away from this vertical, the air mass increases.

Characteristics(Module 1-5 Graph 240W)

The figure above shows two graphs for a PV module of rating 240W: I-V characteristics (in the top part of the figure) and the power characteristics as a function of the voltage (in the bottom part of the figure). The small circles in both the graphs signify the maximum power points.

I-V Characteristics

Some observations about the I-V characteristics of PV modules are as follows:

  • The I-V characteristics of a PV module plots the module current (I) vs the module voltage (V).
  • The current is plotted on the Y-axis and measured in Amperes, whereas the voltage is plotted on the X-axis and measured in Volts.
  • The extreme point on the Y-axis represents the short-circuit current (ISC), whereas the extreme point on the X-axis represents the open circuit voltage (VOC).
  • The various curves are for different irradiation levels. The topmost curve is for the highest irradiation level, while the lowermost curve is for the lowest irradiation level.
  • ISCincreases in direct proportion (or linearly) with the radiation level. The VOC, on the other hand, increases slightly (or sub-linearly)with the radiation level.

 

Power Characteristics

Some observations about the power characteristics of PV modules are as follows:

  • The power characteristics of a PV module plots the module power (P) vs the module voltage (V).
  • The power is plotted on the Y-axis and measured in Watts, whereas the voltage is plotted on the X-axis and measured in Volts.
  • Power at both ends of the curve is zero. In short-circuit condition, the current is maximum but the voltage is zero, which is why the power is zero. In open circuit condition, the voltage is maximum but the current is zero, which is why the power is zero again.
  • As can be seen in the I-V characteristics, as we go from the short towards the open circuit condition, the voltage increases steadily but the current decreases only slightly. After a certain point however it starts falling sharply. This inflection point represents the maximum power point. It is shown in the curves with circles.
  • The various curves are for different irradiation levels. The topmost curve is for the highest irradiation level, while the lowermost curve is for the lowest irradiation level.

 

Power Output in Normal Conditions

For the module above, the temperature coefficients are as follows:

  • Temperature coefficient of ISC: (0.06±0.01)%/K
  • Temperature coefficient of VOC: -(78±10)mV/K
  • Temperature coefficient of peak power: -(0.5±0.05)%/K

So the power output decreases as the temperature increases with respect to the temperature specified in STC. This is because the current increases slightly as temperature increases, but VOC decreases quite a bit as temperature increases. The exact opposite happens if the temperature decreases with respect to the temperature specified in STC.

To reiterate, the temperature specified in STC is the cell temperature and not the ambient temperature. Further, the cell temperature of the PV module is almost 15°C to 20°C higher than the ambient temperature. Why? The PV module converts only about 7%-20% of the solar energy that is incident on it into electricity. The remaining energy is dissipated as heat energy which is why the cell temperature of the PV module is much higher than the ambient temperature.

So for the cell temperature to be equal to that specified in STC – 25°C – the ambient temperature would have to be around 5°C. Many places on Earth are a lot hotter than that during the day which is when the PV module produces electricity. The places that are on the cooler side and where the ambient temperature is likely to be close to the desired temperature, the radiation levels are much lesser than those specified in STC since these places are far away from the equator.

Therefore, PV modules almost never produce their peak power in normal operating conditions.

 

Warranty

PV modules come with a workmanship warranty of anywhere from 5 to 10 years.

Most PV modules also come with a power warranty for 25 years the specifics of which are as follows:

  • 90% of the rated power for the first 10 years
  • 80% of the rated power for the next 15 years

Some PV module manufacturers give a slightly better power warranty the specifics of which are as follows:

  • 95% of the rated power for the first 5 years
  • 90% of the rated power for the next 5 years
  • 80% of the rated power for the next 15 years

 

PV Module Aging and Damage/Degradation

Aging

PV modules age over time and their power generation capacity goes on reducing with every passing year. The typical degradation is 0.5% per year. However, the actual degradation for a particular PV module can vary from one to the other; please check the manuals for the actual degradation numbers.

Damage/Degradation

Apart from the aging described above, which is normal, PV modules are also susceptible to degradation due to various mechanisms which are described.


Mechanical Damage

Mechanical Damage

Hail can damage PV modules which can eventually lead to delamination. This is more likely to affect thin film PV modules; most crystalline PV module manufacturers use toughened glass made specially for PV modules and can therefore withstand hailstorms.

However, this mechanism of mechanical damage is not an issue where there are no or very infrequent hailstorms.

Hot Spots

Hot Spots

This film PV modules are more likely to suffer from cracks and/or glass breakages. If that happens, it also leads to creation of hot spots along the cracks which increases the risk of overheating of the polymer materials. In unfavourable conditions, this can also cause fire.

Since crystalline PV modules are less likely to suffer from glass cracks and glass breakages, they are less likely to have hot spots.


TCO Corrosion

TCO Corrosion

The Transparent Conductive Oxide (TCO) of some of the thin film PV modules corrodes. The damage of this electrically conductive layer on the inside of the cover glass cannot be repaired and leads to substantial power loss.

TCO corrosion is specific to thin film PV modules and does not affect crystalline PV modules.

Potential Induced Degradation

Potential induced degradation (PID) happens in crystalline silicon PV modules due to what are called stray currents, and can lead to substantial power loss of as much as 30%.


Potential Induced Degradation

PID occurs most commonly in the PV module closest to the negative pole. The potential of the PV cells here ranges from -200V to -350V depending on the length of the string and on the device type of the inverter used. The frame of the PV module, however, is at 0V; it has to be grounded for safety reasons.

The huge potential difference between the cells and frame can cause some electrons from the materials used in the PV module to come loose and discharge through the grounded frame. This causes a polarization that can adversely alter the I-V characteristics of the PV module, which can be seen in the figure below.

PID Fig 2

If the current of just this one module decreases, it acts a “current limiter” and brings down the current of the entire PV module string. If the current loss is 30%, the power of the entire string, and not this module alone, will come down by 30%.

However, the polarization mentioned above is generally reversible and thankfully so. Therefore, it has to be distinguished from irreversible degradation mechanisms like corrosion and normal deterioration.

One way to avoid the above situation is to ground the negative pole of the PV module string which will bring its potential at the same level as the frame, thus avoiding PID. However, grounding the negative pole of the PV module string has another problem if the inverter is transformerless; it can transfer the ground faults on the input side to the grid since there is no galvanic isolation between the input and output side.

However, most crystalline PV module manufacturers today have made their modules immune to PID.

Glass Breakage

Glass Breakage

The figure above shows the various ways in which glass breakage can occur in thin film PV modules. These glass breakages are specific to thin film PV modules; crystalline PV modules seem to be quite immune to such breakages.