John H. Wohlgemuth

Buried contact solar cells

Solarex has applied the buried contact solar cell technology for one-Sun and 18-Sun linear focus concentrator applications. This paper presents results for large area cells fabricated on FZ, CZ and polycrystalline silicon. Chemically textured FZ yields the highest efficiency, with one-Sun cells exceeding 18%. Chemically and mechanically textured CZ cells have exceeded 16% at one-Sun and nearly 19% under 18 Suns concentration. Large area mechanically textured polycrystalline solar cells have been fabricated with efficiencies up to 14.7%. All of these results have been achieved using a dicing saw to cut the grooves.<<ETX>>

Long Term Reliability of Photovoltaic Modules

BP Solar has utilized long term module exposure data and field return data to determine module lifetimes, expected failure rates and to identify failure mechanisms. While outdoor testing is a must for understanding PV reliability, it takes much too long to be of use in determining the effects of changes in materials, processes or equipment. This paper describes how BP Solar utilizes accelerated stress testing to verify the robustness of its new PV products

Reliability testing beyond Qualification as a key component in photovoltaic's progress toward grid parity

This paper discusses why it is necessary for new lower cost PV modules to be tested using a reliability test sequence that goes beyond the Qualification test sequence now utilized for modules. Today most PV modules are warranted for 25 years, but the Qualification Test Sequence does not test for 25-year life. There is no accepted test protocol to validate a 25-year lifetime. This paper recommends the use of long term accelerated testing to compare now designs directly with older designs that have achieved long lifetimes in outdoor exposure. If the new designs do as well or better than the older ones, then it is likely that they will survive an equivalent length of time in the field.

Reliability and performance testing of photovoltaic modules

In recent years there has been a paradigm shift in photovoltaic module testing from reliability to qualification testing. This has raised several questions as to whether or not reliability testing is still required. This paper discusses why reliability testing is still a necessary and integral part of product development and deployment. It also discusses new test procedures that have been developed or are currently under development to improve and enhance reliability testing.

The effect of cell thickness on module reliability

This paper addresses the issue of the reliability of modules using ultra-thin crystalline silicon cells. Do thin cells have a greater likelihood of cracking during production, transport, installation or use and if so does this result in long term degradation of the power? The present qualification test sequence, IEC 61215 does not adequately address this issue, The only mechanical test in IEC 61215 is a static mechanical load test consisting of three load cycles that are performed after the accelerated stress tests. So even if this mechanical load test does break cells, it is unlikely to result in significant power loss. BP Solar has developed a test sequence for evaluation of cracked cells in PV modules. The test sequence incorporates a dynamic mechanical load test performed before the 50 thermal cycles/10 humidity freeze cycles of IEC 61215. Usually there is no significant power loss after the dynamic load testing. However, the subsequent thermal cycling opens up the cracks that propagated during the dynamic load test resulting in significant power loss. Having a test sequence that identifies damaged cells allows us to develop processes and handling procedures to minimize or eliminate damage to the ultra-thin cells, resulting in reliable modules.

Reliability of EVA modules

The issue of the long term reliability of photovoltaic (PV) modules made with ethylene vinyl acetate (EVA) encapsulant is discussed, particularly in reference to the yellowing or darkening observed after long term exposure to high temperature and high illumination levels. Observations of the results of long term exposure of Solarex modules at a number of field sites is reported. Laboratory results on high temperature exposure of EVA and long term exposure of the cell metallization to acetic acid is presented. None of these tests explain the fill factor loss observed in the Arco modules at Carrisa Plains. Detailed examination of a number of Carrisa modules has shown that a major contributor to the power loss is failure of the back interconnects, independent of any EVA degradation.<<ETX>>

Using accelerated testing to predict module reliability

Long-term reliability is critical to the cost effectiveness and commercial success of photovoltaic (PV) products. Today most PV modules are warranted for 25 years, but there is no accepted test protocol to validate a 25-year lifetime. The qualification tests do an excellent job of identifying design, materials, and process flaws that are likely to lead to premature failure (infant mortality), but they are not designed to test for wear-out mechanisms that limit lifetime. This paper presents a method for evaluating the ability of a new PV module technology to survive long-term exposure to specific stresses. The authors propose the use of baseline technologies with proven long-term field performance as controls in the accelerated stress tests. The performance of new-technology modules can then be evaluated versus that of proven-technology modules. If the new-technology demonstrates equivalent or superior performance to the proven one, there is a high likelihood that they will survive versus the tested stress in the real world.

Evaluation of high-temperature exposure of rack-mounted photovoltaic modules

Photovoltaic (PV) modules operate in an extreme environment and are exposed to radiation, humidity, and hot and cold thermal extremes. This paper focuses on polymeric-material degradation during PV-module operation at high ambient temperatures, high solar irradiance and low wind speed. The 2004 version of the IEC 61730 specification requires all polymeric materials used in a photovoltaic module to have a Relative Thermal Index (RTI) or Relative Thermal Endurance Index (RTE) at least 20°C greater than the maximum material temperature measured during the temperature test conducted at 40°C ambient. There is currently an international debate regarding this requirement. This paper explores the thermal exposure of photovoltaic modules in the field as a technical basis for this debate. For the hottest cities, the thermal exposure is found to be equivalent to aging at a constant temperature of 42–53°C, with maximum temperatures of 75°C.

Photovoltaic Module Qualification Plus Testing

Reliability is a critical element in the continued growth of the photovoltaic (PV) industry. Design qualification tests such as IEC 61215 and IEC 61646 have been key to mitigating infant mortality, but do not address many of the module failures now observed in the field. Qualification Plus has been created to fill an immediate need by providing a well-defined set of accelerated stress tests that correlate with the field performance of PV modules. The tests in Qualification Plus include module-level tests like those in the qualification test sequences, as well as material and component level tests like those in the module safety standard. This paper will describe the details of Qualification Plus including the rationale for the required tests, the selection of samples, and the requirements for the Quality Management System.

How can we make PV modules safer

Safety is a prime concern for the photovoltaics (PV) industry. As a technology deployed on residential and commercial buildings, it is critical that PV not cause damage to the buildings nor harm the occupants. Many of the PV systems on buildings are of sufficiently high voltage (300 to 600 Volts dc) that they may present potential hazards. These PV systems must be safe in terms of mechanical damage (nothing falls on someone), shock hazard (no risk of electrical shock when touching an exposed circuit element), and fire (the modules neither cause nor promote a fire). The present safety standards (IEC 61730 and UL 1703) do a good job of providing for design rules and test requirements for mechanical, shock, and spread of flame dangers. However, neither standard addresses the issue of electrical arcing within a module that can cause a fire. To make PV modules, they must be designed, built, and installed with an emphasis on minimizing the potential for open circuits and ground faults. This paper provides recommendations on redundant connection designs, robust mounting methods, and changes to the safety standards to yield safer PV modules.