Accelerated Testing of Modules and Components for Photovoltaic Reliability
To conduct accelerated testing of modules and components, NREL subjects photovoltaic (PV) components and materials to stressors such as thermal cycling, heat, moisture, and ultraviolet light to provide early indication of potential failure modes. New accelerated test and diagnostic techniques are developed to meet specific needs, especially those associated with understanding new devices and materials.
NREL capabilities for accelerated testing for photovoltaic reliability involve two types of testing:
Accelerated Testing of Modules
To conduct tests on accelerated testing, NREL maintains and operates a collection of environmental chambers for application of humidity, heat, electrical bias, white light, and ultraviolet (UV) light. The chambers accommodate modules of variable sizes. NREL's use of these chambers is designed to develop new tests that quantitatively correlate with field failure rates.
Quantitative Module Testing
Qualitative tests have been used to identify design failures in PV modules, greatly enhancing their reliability. The same tests that are useful in identifying early failures can also be useful for more quantitative predictions. NREL is studying quantitative accelerated testing of modules using a test-to-failure protocol. A key element of this testing is to add system-voltage bias during the damp-heat test. The application of the bias voltage simulates the actual exposure in the field, but the combined effects of 85% relative humidity, 85°C, and system voltage have caused failures that have not been observed in the field. The goal of NREL's study is to identify the best way to stress the modules and how to use the results to predict performance in the field. More information can be found in this recent publication.
Accelerated Testing of Components and Materials
NREL maintains and operates hardware for testing small test components and materials. These can apply up to ~40 suns of UV intensity while controlling the temperature and humidity and up to ~1,000 suns of UV in an uncontrolled environment. Rapid thermal cycling of small components with variable electrical bias is also available. Below are specific tests performed at NREL.
Concentrating Photovoltaic Cell-Heat Sink Bond
Concentrating photovoltaic (CPV) cell assemblies have been observed to fail at the point of attachment between the cell and the heat sink. If even small voids form between the cell and the heat sink, catastrophic failure may occur, as has been observed in the field. Many companies have raised a concern about the current qualification test. The objective of this activity is to develop new tests for stressing this bond, both to improve the current qualification test and to reduce the time needed for testing of new designs. More information can be found in this recent publication.
Degradation of Fresnel Lens Materials
The type of optical materials acceptable for use in CPV is a subject of controversy. Each company must develop its own accelerated test, slowing product development. NREL is collaborating with a number of companies to study the causes of failure and the related methods of accelerated testing. Our emphasis is to communicate more broadly about what is already known and to gain new information where there is a lack of knowledge. Our goal is to identify useful test procedures and help the industry move to a consensus about which materials meet the needs of CPV.
Measuring Moisture Ingress into Flexible Thin-Film PV Technologies
PV industry representatives have expressed substantial interest in developing flexible thin-film products, but the development of these products is impeded by the sensitivity of these modules to moisture. We have been developing a technique that can measure moisture ingress at water-vapor transport rates as low as 10-5g/cm2/day. We welcome the opportunity to apply this test to new moisture-barrier concepts.
Evaluation of Creep Rate in Thermoplastic Materials
As manufacturers have striven to reduce costs while retaining performance, they are investigating replacement of ethylene vinyl acetate (EVA) and back-sheet materials. A potential lack of thermo-mechanical and dielectric stability of these materials in the field could easily lead to safety issues. We are working with PV community members to better understand the risks and how to test for them. One aspect of this work is better quantifying the thermal-use environment, as described in a recent publication.
Stressing of Encapsulant Materials
There is general consensus that the UV dose applied to encapsulant materials during testing falls far short of field exposure. Accelerated UV exposure, either indoors or outdoors, can increase the dose. A method for applying concentrated UV light and the resulting damage caused to some encapsulants is described in a recent publication. Some CPV systems have shown degradation of the encapsulant material, but the cause is not well understood (i.e., UV, heat, strain?). The goal of this activity is to understand the failure mechanisms of CPV encapsulants and to improve our ability to test all encapsulants, enabling development of an appropriate test protocol. A recent publication helps to define the optical fluxes within many CPV optical designs.