Concerns regarding to selection of PV module

   Photovoltaic module occupies highest cost sharing in the overall cost of the project. Return on investment for investors are largely depends on performance of solar module after certain years. It is the only asset that generates real money for the investor. Obviously it has been getting highest attention on procurement of PV panels.

    Since solar market is still emerging worldwide, ideas about its development are untested; every year comes with new issues it may be regarding to overcapacity or the securitization of domestic market. Especially in India it is immature in nature. Many developers are never exposed to such situations, they are lagging in planning and commissioning of the project, procuring and testing of the equipment. In that background selection of PV panels becomes hard task.

    Conventionally developers’ buys solar panel based on warranty, Insurance and company’s balance sheet or past records. But recently those criteria seem to be falling down as many plants are raising issues with their PV panels. A warranty associated with PV panels does not cover most of the field failures, insurance can backup warranties but suitable insurance are highly expensive for the projects. Another important concern is that to claim for warranty after 10-15 years, manufacturing company should be exist on land to provide you money as day by day manufacturing unites are shutting down.

    Many buyers start their quality assessment when the modules arrive on plant which arises few problems, primarily Due to that when result revels any default in the modules at that time most of the panels were installed already.

       Increasing demand and failing manufacturing companies are triggering this concern about the selection criteria of PV module at the time of procuring. We are expecting more caution in this area in near future

Performance standards for PV technology part_4

Thermal test:-

1) Bypass diode:-

This test is a critical component determining the thermal behavior of the module under hot- spot conditions and therefore also directly affecting reliability in the field. The test method requires attaching a thermocouple to the diode (s) body, heating the module up to 75°C ± 5 °C and applying a current equal to the short circuit current Isc measured at STC for 1h.

Failures of bypass diode test still occur either by overrating of the diode manufacturer, or incorrect electrical configuration with respect to the module’s Isc by the module manufacturer.

2) Hot spot endurance:-

(This part was covered in diagnostic tests)

Irradiance check:-

1) Outdoor exposure:-

The purpose is a preliminary assessment of the module’s ability to withstand exposure to outdoor conditions. However, it only involves exposure for a total of 60 kWh/m2, which is a rather short period of time to make any judgments about module’s lifetime. IEC 61215 requires degradation of maximum power Pmax not to exceed 5% of the initial value. While for IEC 61646 Pmax not to be lower than the marked “Pmax – t%“.

2) UV preconditioning:-

This test is carried out to identify materials that are susceptible to ultra- violet (UV) degradation before the thermal cycle and humidity freeze tests are performed. IEC 61215 requires to subject the module to a total UV irradiation of 15 kWh/m2 in the (UVA UVB) regions (280 nm – 400 nm), with at least 5 kWh /m2.

But in the laboratory test condition very low UV irradiation is apply to the module than real exposures during the life time of the module.

Environmental test:-

1) Thermal cycling TC200:-

IEC 61215 requires the injection of a current within ±2% of the current measured at peak power (Imp) when the module temperature is above 25°C.this test is carried out to simulate thermal stresses on materials as a result of changes of extreme temperatures. The module is subjected to the cycling temperature limits of –40°C ± 2°C and 85°C ± 2°C with the below profile.

Failure rates for T C200 can be as high as 30- 40%. It is lower for thin-film.

2) Humidity freeze:-

 In this test module is subjected to 10 complete cycles as per harmonized profile below (IEC 61646) to determine the module’s ability to withstand the effects of high temperatures combined with humidity, followed by extremely low temperatures. Failure rates of this test remain in the range 10- 20%.

3) Damp heat DH1000:-

This test is carried out to determine the ability of the module to withstand long-term exposure to penetration of humidity by applying 85°C ± 2°C with a relative humidity of 85% ± 5% for 1000 hours. This test is known for highest failure rate test which is 40-50% for both technologies.

Mechanical test:-

1) Robustness of terminations:-

This test is carried out to determine the robustness of the module’s terminations, which can be wires, flying leads, screws, or as for the majority of the cases: PV connectors. The terminations undergo a stress test that simulates normal assembly and handling through various cycles and levels of tensile strength, bending and torque tests as referenced in another standard, IEC 60068-2-21.

2) Mechanical load test:-

This test comes after Damp Heat and therefore done on a sample that has undergone a severe environmental stress. This test is carried out to investigate the ability of the module to withstand wind, snow, static or ice loads. If the module is to be qualified to withstand heavy accumulations of snow and ice, the load applied to the front of the module during the last cycle of this test is increased from 2400 Pa to 5400 Pa.

An issue remains with this test is that if module failed then it may due to structural problems, or because of an inappropriate mounting technique.

If the module is to be qualified to withstand heavy accumulations of snow and ice, the load applied to the front of the module during the last cycle of this test is increased from 2400 Pa to 5400 Pa.

 Successful completions of IEC61215/61646 tests means

• The product has met a specific set of requirements
• Those modules that have passed the qualification test are much more likely to survive in the field and not have design flaws that lead to infant mortality.
• They suffer almost no degradation in power output from the test sequence.

Limitations of IEC61215/61646

• It does not identify and quantify wear-out mechanisms.
• It doesn’t differentiate between products that may have long and short lifetimes
• Not to address all failure mechanisms in all module designs
• Not to address failure mechanisms for all climates and system configuration

Performance standards for PV technology part_3

Diagnostic tests:-

1) Visual inspection:-

The purpose is to detect any of the “major visual defects” defined above by checking the module in a well illuminated area (1000 lux).

It is frequently occur throughout the procedure.

2) Hot spot endurance:-

Technically hot spot occur due to the operating current of the module exceeds the reduced short –circuit current of a faulty cells. This will force the cell(s) into a reverse bias condition when it becomes a load dissipating heat. Serious hot spot phenomena can be as dramatic as outright burns of all the layers, cracking, or even breakage of the glass. This test is carried out to determine the module’s ability to withstand localized heating caused by cracked, mismatched cells, interconnection failures, partial shadowing or soiling.

Electrical test:-

1) Insulation resistance:-

This test is carried out to determine whether a module has a sufficient electrical insulation between its current-carrying parts and the frame. A dielectric strength tester is used to apply a DC Voltage source of up to 1000 V plus twice the maxim um system voltage. After the test, there shall be no breakdown, nor any surface tracking. F or modules with an area larger than 0.1 m2, the resistance shall not be less than 40 MΩ   for every square meter.

2) Wet leakage current test:-

In this test the module is submersed in a shallow tank to a depth covering all surfaces except cable entries of junction boxes not designed for immersion. A test voltage is applied between the shorted output connectors and the water bath solution up to the maximum system voltage of the module for 2 minutes.

To pass this test the insulation resistance shall be not less than 40 MΩ   for every square meter for modules with an area larger than 0.1 m2. There is not any IEC standard for PV connectors, but there is a harmonized European standard (EN 50521) for it.

The wet leakage current test is ranked as one of the most reoccurring failures during PV qualification at the testing laboratories

 Performance parameters tests:

1) Maximum power Pmax:-

It is common practice among PV laboratories to perform it at 1000W/m2, 25°C cell temperature, with a reference solar spectral irradiance called Air Mass 1.5 (AM1.5), as defined in IEC 60904-3. A correct and traceable Pmax measurement to the World PV Scale is of critical importance. Not only is it one of the pass/fail criteria, but the measured values can also be used by the end users as a performance indicator for power yield evaluations.

Due to its importance it is perform frequently in the process.

2) Temperature coefficients:-

This test is carried out to determine the temperature coefficients of short- circuit current Isc (α), open-circuit voltage Voc (β) and maximum power Pmax (δ) from module measurements. over an interval of 30°C (for instance, 25°C-55°C), and at every 5°C intervals, the sun simulator takes an I - V measurement (Isc, Voc, Pmax are not reflected, but measured during the I-V sweep) including Isc, Voc and Pmax. The values of Isc, Voc and Pmax are plotted as functions of temperature for each set of data. The coefficients α, β and δ are calculated from the slopes of the least squares fit straight lines for the three plotted function.

3) Nominal Operating Cell Temperature (NOCT):-

NOCT can be used by the system designer as a guide to the temperature at which a module will operate in the field and it is therefore a useful parameter when comparing the performance of different module designs. The test setup requires data logging and selection for irradiance (pyronameter), ambient temperature (temperature sensors), cell temperature (thermocouples attached on the back side of the module corresponding to the two central cells), wind speed (speed sensor) and wind direction (direction sensor).All these quantities shall be within certain intervals in order to be acceptable for the calculation of NOCT.

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Performance standards for PV technology part_2

1)  Major visual defects (IEC 61215/61646):-

• A crack in a cell the propagation of which could remove more than 10% of that cell's area from the electrical circuit of the module

• Broken, cracked, or torn external surfaces, including superstrates, substrates, frames and junction boxes

• Bent or misaligned external surfaces, including superstrates, substrates, frames and junction boxes to the extent that the installation and/or operation of the module would be impaired

• Bubbles or delaminating forming a continuous path between any part of the electrical circuit and the edge of the module

• Loss of mechanical integrity, to the extent that the installation and/or operation of the module would be impaired

            Separately for IEC 61646

• Visible corrosion of any of the thin film layers of the active circuitry of the module, extending over more than 10% of any cell

• Module markings (label) are no longer attached, or the information is unreadable

2) ’Pass/fail’ criteria:-

• the degradation of maximum output power does not exceed the prescribed limit after each test nor 8% after each test sequence

• no sample has exhibited any open circuit during the tests

• the insulation test requirements are met after the tests

• there is no visual evidence of a major defects

• specific requirements of the individual tests are met

• the wet leakage current test requirements are met at the beginning and the end of each sequence and after the damp heat test

Conclusion from “pass/fail” tests

	CONDITION	RESULT
1)	Two or more samples failed in test criteria	Fail
2)	One sample failed If one or both of these new samples also fail  If both samples pass the test sequence	Remaining  2 Start from beginning Result  “Fail” Result  “Pass”

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Performance standards for PV technology

Under the JNNSM, MNRE has mentioned certain technical requirements for selection of PV module for grid connected power plant

Crystalline Silicon Solar Cell Modules     IEC 61215

Thin Film Modules                                      IEC 61646    

Concentric PV modules                              IEC 62108

In addition for safety qualification test    IEC 61730

         For Highly corrosive atmosphere    IEC 61701

But what exactly IEC stand for?

What kinds of tests are carried out for what purpose?

Do those standards also define ‘reliability’ of the module?

 Are IEC standards sufficient for comparison of module ‘quality’?

As a part of emerging market many similar questions are unclear in the industry, though due to the custom or branding it is followed by manufacturer and developers. But on the long term scenario those questions should be clear.  

The design qualification is responsible to represent the ‘performance’ capability of the module under standard climate condition. So despite of having defined climate condition if someone is considering its module performance future under the basis of IEC standards results then it may prove wrong. 

There is another concern about modules ‘reliability’ which is ambiguously related to the ‘quality’. But in reality both are quite different term, in fact there isn’t any standard introduced to claim modules reliability. Reliability is neither defined, nor covered by the existing IEC standards. Experts from manufacturers, testing institutes and standardization bodies are coming together in an effort to elaborate the basis for a PV reliability standard.

IEC 61215 has been designed based on crystalline silicon (c-si) technology, while IEC 61646 is on amorphous silicon (a-si) technology such as CIGS, CdTe etc. Both standards require that samples for testing be taken at random from a production batch (accordingly IEC 60410)

In the following articles we will explain important tests and its requirements Along with few insights about other accelerated stress tests, qualification tests like JPL block buys tests and limitations of IEC 61215/61646

  

  Tests carried out under the Performance standards IEC 61215/61646

·       Diagnostic: Visual inspection, Hot spot.

·       Electrical: Insulation resistance, Wet leakage current

·       Performance: Pmax at STC, Temperature coefficients,  NOCT, Pmax at low irradiance.

·       Thermal: Bypass diode test, Hot spot.

·       Irradiance: Outdoor exposure, UV exposure, Light soaking.

·       Environmental: Temperature cycles, Humidity freeze, Damp heat.

·       Mechanical: Mechanical load, Robustness of terminations, Hail impact.

       

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