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In the race for cost reduction are you thinking about long term performance of your PV projects?

In the race for cost reduction are you thinking about long term performance of your PV projects?

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In the race for cost reduction are you thinking about long term performance of your PV projects?

Source solar panels with the right bill of materials to help ensure solar PV systems produce power reliably for their expected lifetime

 – Rahul Khatri, Technical Manager, DuPont Photovoltaic Solutions, South Asia

India set a record in 2016 in solar capacity addition as well as release of new tenders and the market is set to ramp up solar installations in 2017 with tariffs expected to fall even further. While projects are being won at record breaking tariffs, there is an increasing uncertainty in the industry, especially from lending community, on the ability of these projects to generate expected power over 25 years of lifetime of a solar PV system. The extremely low tariffs are based many a times on impractical and lower than prevailing market prices of the modules. With additional factors, such as high cost of financing and insurance, there is a constant push on module manufacturers to reduce module prices. Module suppliers seeking short term gains and to manage cash flows agree on unrealistically low prices, but in the process quality is compromised by use of inferior and unproven materials, which increases failure risks. This is impacting risk perception of solar projects in India which is not helping in reducing lending rates and due to the same reason, foreign insurers are either reluctant to extend their services to India or charge hefty premiums. The cost down approach for generatingsolar poweris driven by lower product quality instead of focusing on better quality product and management of risks during operation. As a wise buyer, one needs to be aware of the materials used in solar modules, the required properties, field experience, and then ensure use of right materials.

Prevailing risk mitigation tools are not effective

Besides insurance, the tools currently used to manage risks associated with PV modules are based primarily on three aspects – tier rating of module manufacturers, IEC certifications, and warranties which are not very effective in mitigating risks. Tier ratings change every six months, are mostly based on manufacturing capacities and financial health of the company, but hardly reflect quality of the product being supplied. IEC certification / extended IEC testing to evaluate field performance of modules have not been proven effective enough. Warranties associated with PV modules are good to have but O&M companies who have dealt with warranty claims, know how unpleasant an experience it can be to enforce warranties. Moreover, settling a claim takes at least few months which is a lost time and loss of power generation and impacts revenue for investors risking business for module manufacturers. Considering these business threatening situations, robust quality standards are critical for a product which needs to operate for 25 years in challenging weather and environmental conditions.

Specification of ‘Bill of Materials’ – key to effectively manage performance risks

Central and state governments, driving majority of the projects, need to make quality a key selection criterion for solar panels to restrict use of low quality modules by strengthening technical requirements which are limited to IEC certification right now. Private developers and lenders, who are financing solar projects, are at maximum risk and therefore need to go a few steps further and incorporate detailed specifications in their module selection criteria. PV mature regions such as China, US, and Europe have experienced failures due to material degradation and focused on specification of ‘Bill of Materials’ to enhance and control quality of panels. Though two modules may look exactly same and have same nameplate rating, but their field performance will depend entirely on materials used inside them.

‘Bill of Materials’ – what should be specified?

Front glass and backsheet are the outermost layers of a PV module and provide first line of defense to solar cells from environmental stresses. Both materials provide electrical insulation and are the two most important components for long term performance and safety of the modules. Front glass is also responsible for mechanical strength and structural rigidity of the module.

One of the key issues detected in front glass is its breakage which results in module not generating power. Dynamic wind loads and hot spots (resulting in high thermal stresses) are common causes of glass breakage in the field. Typically, toughened / tempered glass, known for its superior strength, is used in PV modules. Besides toughness, glass thickness is also an important parameter and determines its capability to withstand wind loads in the field. Larger the area of the module, higher will be the wind force and thicker should be the glass. Use of 4 mm thick glass for 72-cell modules, and 3.2 mm thick for 60-cell module is recommended.

Backsheet of a crystalline silicon PV module typically has a three-layer structure. The outer layer, which is directly exposed to environment, needs to withstand harsh stresses to protect the cells and materials inside.  Inner layer should enable excellent adhesion with EVA encapsulant and protect the core or middle layer.  The core layer is responsible for mechanical and electrical insulation properties of the backsheet. Thickness of backsheet is critical for electrical insulation and sand abrasion (particularly in desert areas). DuPont in its global field studies has found visual defects in backsheets (9% of 1.5 million inspected modules demonstrated visual issues in the backsheet). Typical issues include outer layer yellowing and cracking, inner layer yellowing and cracking, outer layer melting due to hot spots, faster abrasion of outer layer, etc.– these can either result in power loss or safety risks or both. It is critical to specify material type and thickness for each layer to prevent degradation and premature failure.It is highly recommended to use PVF film based backsheet as only PVF backsheets have more than 30 years of proven field experience in different climates. Most other backsheets have less than 8 years of field experience with multiple cases of field failures (refer figure 1).

EVA encapsulant is responsible for laminating cells between glass and backsheet. Browning of the front EVA placed above cells results in reduction of its light transparency and reduces power generated by the cell (due to less sunlight reaching it), and delamination from glass/cell/backsheet are commonly observed field issues. Quality of EVA adhesion, after lamination process, is determined by the gel content of the laminated EVA;therefore, gel content should be specified for modules. EVA browning, which happens due to prolonged UV exposure, can be prevented by using UV resistant EVA containing UV additives.

Module edges are a potential area of moisture ingress and require to be sealed with a material which not only has excellent sealing properties but also demonstrates good resistance to harsh climatic conditions. Silicone sealant is a preferred choice due to its proven long term experience for this purpose. Double sided tape, which has been found to be used mainly to save some costs, is not at all recommended.

Solar cellsare the most important and active component in the module and are quite prone to develop serious issues in the field such as cracks and hot spots. Micro-cracks and cracks can occur due to multiple reasons and at multiple stages from module production to installation. In fact, most of the cracks in cells are developed during installation and maintenance phase (due to improper handling), and therefore any power loss is not covered under warranty claim. While precautions can be taken at every stage to minimize occurrence of these cracks, it’s extremely difficult to control them – especially when the cell thickness is constantly being reduced. Silicon wafer is a brittle material and the probability of development of micro-cracks increases with reduction in thickness. Developers should define and specify cell thickness for better control on micro-crack development particularly during stages of installation & maintenance that are under their scope along withspecification of 100% EL inspection to eliminate cracks from manufacturing.

Table 1 summarizes key field issues associated with each material in a PV module and what developers should specify to mitigate risks of their occurrence in projects.

Material Typical Defect Modes Properties / tests to be specified Recommended Specs.
Solar Cells ·      Micro-cracks during manufacturing, transportation & handling ·      Thickness (lower the thickness, higher are the chances of development of micro-cracks) ·   Thickness: 180 – 200 um
·      Electroluminescence (Post lamination) of 100% modules
Glass ·      Breakage due to wind loading and mechanical impact ·      Glass thickness ·   Type: Low iron tempered

·   Thickness: 4mm for 72 cell module; 3.2 mm for 60 cell module

·      Reliability of Anti-reflective coating (ARC) ·      Abrasion and salt spray test
Encapsulant (EVA) ·      UV / thermal induced yellowing ·      UV resistant capability of EVA (UV exposure test) ·   UV blocking (≤340 nm) & UV resistant

·   VA content: 28-33%

·   Gel content (cross-lined EVA): >75%

·      Delamination from Glass / Backsheet ·      VA content

·      Gel Content

·      Snail Trail ·      Long Term (3-6 months) Outdoor exposure test
Backsheet (3-layer structure) ·      Cracking of outer layer (Air Side) ·      Material type based on its field experience Three-layer structure having:

·   Outer layer: PVF Film; ≥25 um

·   Middle Layer: PET Film; ≥190 um thick

·   Inner layer (cell side): PVF / EVA / Polyethylene / Polyolefin / Fluoro-polymer coating thickness > 10 um to effectively block UV

·   Total thickness: ≥300 um

·      Abrasion of outer layer especially in desert regions ·      Outer layer thickness

·      Abrasion resistance (falling sand test)

·      Cracking of inner layer (EVA side) ·      Material type based on its field experience

·      UV resistance (UV exposure test)

·      Electrical insulation failure ·      Thickness of middle layer

·      Total back-sheet thickness

Sealant ·      EVA delamination and cell corrosion due to moisture ingress from edge sealant ·      Material ·      Silicone

Conclusion

Reverse bidding needs to embrace a robust quality control system to prevent use of low quality modules that can hamper long-term sustainability of India’s solar mission. Lack of strong technical specifications in tenders is allowing wide variations in PV modules in terms of design, construction, and the ‘bill of materials’, critical for long term performance. Government bodies need to upgrade technical requirement for modules in tenders and incorporate standards and specifications for materials and manufacturing process. Investors including lenders and developers should be aware of the right materials and manufacturing practices and ensure their useformodule selection.

 

Anand Gupta Editor - EQ Int'l Media Network

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