Loughborough University
Leicestershire, UK
LE11 3TU
+44 (0)1509 222222
Loughborough University

Centre for Renewable Energy Systems Technology (CREST)

Applied Photovoltaics

PV Modules: Production, Durability & Failure Mode Analysis

PV Module Production

In conducting PV research every relevant variable must be controlled. One of the most important variables involved is the manufacturing process of the samples. In CREST we are able to manufacture crystalline silicon flat PV modules, from small-area samples for experiments on materials to standard commercial-size modules for final product testing.

By controlling the variables of manufacturing process, we can also assess their impact on performance and long-term reliability.

These are the main steps of PV module manufacturing process:

Soldering of cells:  Copper-based flat wires are soldered to each side of the cell to carry out current. Then the cells are electrically connected in series, forming strings of cells.

String interconnection: The strings are electrically connected using copper-based flat wire, according to the required design.  The PV module positive and negative contacts are built to allow the connection to the rest of PV system.

Module layout:  All layers are stacked up according to this structure: glass / encapsulant / electric circuit with cells / encapsulant / backsheet.

Lamination: The materials are bonded together and the encapsulant is transformed into a transparent and durable material which guarantees the long term reliability of the PV module.

Lamination is a key process to control to achieve a long life of the product. We have an industrial laminator which allows us replicate the same lamination conditions used by industrial PV manufacturers, investigate the optimum variables and develop processes for new materials. The laminator has the following characteristics:

-       Size of modules up to 1.6 x 1 m

-       Homogeneity of temperature of 2°C

-       Cooling system

Junction box attatchent: Consists of electrical connection system to module contacts, inclusion of bypass diodes to protect the cells, and cables with connectors.

Framing:  Improves the mechanical strength and facilitates fixing to supporting structure of the system.

 

PV Module Quality Assurance

High quality packaging is crucial to ensure the long-term performance and reliability of photovoltaic (PV) modules. There are several indexes used in the PV industry to check the quality of module lamination such as the adhesion strength between different layers, gel content of EVA, yellowing index of EVA, etc. CREST has set up a series of techniques which are used to check the quality of module production and also the degradation study of encapsulation materials used in PV modules. These quality assurance setting up include Peel Tester, Differential Scanning Calorimeters (DSC), Solvent Extraction kit, Fourier Transform Infrared Spectroscopy (FTIR).

Peel Tester

The Chatillon LTCM-500 peel tester is a machine to test the adhesion strength of different interfaces within PV modules. Both 90o and 180o peel test can be conducted through fixed grips or a 90omoving table. The speed range is 0.2 to 20 inches/min (5 to 500 mm/min). Crosshead travel is 750mm. The machine has a capacity of 2500N.

A dial is used to set the speed requirement and a digital display indicates the speed selected to 0.1 resolutions. An Up and Down switch is used to control crosshead travel direction. An emergency stop is located on the console for easy access. The LTCM-500 may also be used for tensile, compression, and flexural testing when combined with a proper force gauge.

Differential Scanning Calorimeters (DSC)

Differential Scanning Calorimeters (DSC) measures temperatures and heat flows associated with thermal transitions and chemical reactions when heating up a material. The RCS90 cooling accessory allows operation over the temperature range from -90oC to 550°C with a temperature precision of +/- 0.01°C. It is mainly used to characterise the glass transition, the melting point, the crystallisation, the product stability of different encapsulant materials such as EVA, PVB and Ionomers. It also offers a quick testing of the curing state of EVA.

Fourier Transform Infrared Spectroscopy (FTIR)

PerkinElmer 100 series FTIR offers an IR analysis of the polymers within PV modules. It is used to investigate the chemical changes of the encapsulant materials of PV modules during degradation testing. This system gives an inspection range of 7800cm-1 to 370cm-1 with a resolution of 0.5cm-1. With the Universal ATR Accessory (UATR), a wide range of samples from liquid to solid can be tested. As it is sensitive to hydroxyl groups, it is also used to detect moisture absorption within encapsulant.

Solvent Extraction Kit

A condensation extraction kit is assembled in CREST to test the gel content of EVA. Xylene is used as the solvent. 120 mesh stainless steel sieve works as a sample container. The non-crosslinked part of EVA can be extracted into xylene while the crosslinked part remains in the sieve. This can be utilized to calibrate the curing state results from DSC.

 

PV Module Reliability

Issues of PV module reliability play an important role in total energy yield of any PV module or system during its lifetime. Indoor accelerated ageing test facilities enable the study of PV module reliability and investigation of module degradation modes due to different stresses. Environmental stresses such as irradiation, temperature and humidity can be applied indoors in a controlled manner that leads to faster degradation compared to modules operating outdoors.

CREST has four environmental chambers, one weather-ometer and one temperature-irradiance stress chamber on site to implement accelerated ageing tests.

The temperature-light stress chamber can accommodate 8 mini-modules of 10cm*10cm. It can be used for warm and cold light soaking and annealing tests.