Jennifer E. Granata

Differentiating series and parallel photovoltaic arc-faults

The 2011 National Electrical Code® requires PV DC series arc-fault protection but does not require parallel arc-fault protection. As a result, manufacturers are creating arc-fault circuit interrupters (AFCIs) which only safely de-energize the arcing circuit when a series arc-fault occurs. Since AFCI devices often use the broadband AC noise on the DC side of the PV system for detection and series and parallel arc-faults create similar frequency content, it is likely an AFCI device will open in the event of either arc-fault type. In the case of parallel arc-faults, opening the AFCI will not extinguish the arc and may make the arc worse, potentially creating a fire. Due to the fire risk from parallel arc-faults, Tigo Energy and Sandia National Laboratories studied series and parallel arc-faults and confirmed the noise signatures from the two arc-faults types are nearly identical. As a result, three alternative methods for differentiating parallel and series arc-faults are presented along with suggestions for arc-fault mitigation of each arc-fault type.

Long-term performance and reliability assessment of 8 PV arrays at Sandia National Laboratories

In the last decade, c-Si module degradation rates of ≪1%/year have been reported [1–3]. It is unclear if this degradation rate extends directly to the string level and what is the expected statistical spread of degradation rates. Nine photovoltaic (PV) arrays totaling nearly 100 kW at Standard Reporting Conditions are currently being used at Sandia National Laboratories (SNL) primarily for inverter testing. The measured power degradation of these arrays at the string level varied from no change over three years within measurement error to greater than 25% in three years. This paper outlines the methodology used to test the DC output, outlines analysis techniques used to evaluate the array performance, provides a current reliability assessment, presents the comparative data for up to five years of use and exposure, and discusses the methods used to track down the causes of unexpected string-level degradation.

Improved test method to verify the power rating of a photovoltaic (PV) project

This paper reviews the PVUSA power rating method [1–6] and presents two additional methods that seek to improve this method in terms of model precision and increased seasonal applicability. It presents the results of an evaluation of each method based upon regression analysis of over 12 MW of operating photovoltaic (PV) systems located in a wide variety of climates. These systems include a variety of PV technologies, mounting configurations, and array sizes to ensure the conclusions are applicable to a wide range of PV designs and technologies. The work presented in this paper will be submitted to ASTM for use in the development of a standard test method for certifying the power rating of PV projects.

Photovoltaic-reliability R&D toward a solar-powered world

The continued exponential growth of photovoltaic technologies paves a path to a solar-powered world, but requires continued progress toward low-cost, high-reliability, and high-performance PV systems. High reliability is an essential element in achieving low-cost solar electricity by reducing operation and maintenance (O&M) costs and by extending system lifetime and availability, but these attributes are difficult to verify at the time of installation. Utilities, financiers, homeowners, and planners are demanding this information in order to evaluate their financial risk as a prerequisite to large investments. Reliability research and development (R&D) is needed to build market confidence by improving product reliability and by improving predictions of system availability, O&M cost, and system lifetime. Universities, industry, National Labs, and other research entities can be most effective by working together and in complementary ways. The Department of Energy supports a variety of research projects to improve PV-system reliability. These projects and current reliability issues for each PV technology are surveyed.

Thin-film photovoltaic radiation testing and modeling for a MEO orbit

A radiation test plan for thin-film photovoltaic technologies focused on a MEO flight experiment is outlined. The radiation response of thin film, triple junction amorphous Si solar cells, with and without a space coating, is presented. The degradation of the photovoltaic output under 2 MeV proton irradiation is measured and analyzed. Irradiations performed both at room temperature, in the dark, and at open circuit and at elevated temperature, under illumination, and under load were performed. The experimental data are presented and analyzed. These data will form the basis for an on-orbit prediction model as applied to a high-radiation MEO orbit.

PV inverter performance and reliability: What is the role of the IGBT?

The inverter is still considered the weakest link in modern photovoltaic systems. Inverter failure can be classified into three major categories: manufacturing and quality control problems, inadequate design, and electrical component failure. It is often difficult to deconvolve the latter two of these, as electrical components can fail due to inadequate design or as a result of intrinsic defects. The aim of the current work is to utilize the extensive background in both inverter performance testing and component reliability found at Sandia National Laboratories to assess the role of component failures in PV performance and reliability. Although there is no consensus on the least reliable component in a modern inverter system, the IGBT is often blamed for failures and hence this was the first component we studied. A commercially available 600V, 60A, silicon IGBT found in common residential inverters was evaluated under normal and extreme operating conditions with DC and pulsed biasing schemes. Although most of the sample devices were robust even under extreme conditions, a few of the samples failed during operation well within the manufacturer-specified limits. Additionally, we have begun in situ monitoring of IGBTs as well as other components within an operating 700 W, single-phase inverter. The in situ testing will guide future device-level work since it allows us to understand the conditions that are experienced by inverter components in a realistic operating environment.

Lifetime testing of metallized thin film capacitors for inverter applications

In order to understand the degradation mechanisms and failure precursors of metallized thin film capacitors (MTFC) used in photovoltaic (PV) inverters, we have carried out accelerated testing on MTFCs. By understanding the degradation mechanisms and precursors of imminent catastrophic failure, implementation of a prognostics and health management (PHM) plan can be used to optimize PV array operations and maintenance (O&M), decreasing cost per watt towards the US Department of Energy goals.

Product Reliability and Thin-Film Photovoltaics

Despite significant growth in photovoltaics (PV) over the last few years, only approximately 1.07 billion kWhr of electricity is estimated to have been generated from PV in the US during 2008, or 0.27% of total electrical generation. PV market penetration is set for a paradigm shift, as fluctuating hydrocarbon prices and an acknowledgement of the environmental impacts associated with their use, combined with breakthrough new PV technologies, such as thin-film and BIPV, are driving the cost of energy generated with PV to parity or cost advantage versus more traditional forms of energy generation. In addition to reaching cost parity with grid supplied power, a key to the long-term success of PV as a viable energy alternative is the reliability of systems in the field. New technologies may or may not have the same failure modes as previous technologies. Reliability testing and product lifetime issues continue to be one of the key bottlenecks in the rapid commercialization of PV technologies today. In this paper, we highlight the critical need for moving away from relying on traditional qualification and safety tests as a measure of reliability and focus instead on designing for reliability and its integration into the product development process. A drive towards quantitative predictive accelerated testing is emphasized and an industrial collaboration model addressing reliability challenges is proposed.

Impacts of Humidity and Temperature on the Performance of Transparent Conducting Zinc Oxide

The impact of humidity and temperature on a zinc oxide based transparent conducting oxide (TCO) was assessed under accelerated aging conditions. An in situ electroanalytical method was used to monitor the electrical properties for a conducting zinc oxide under controlled atmospheric (humidity, temperature and irradiation) conditions. A review of thin film photovoltaic (PV) literature has shown one major failure mode of cells/modules is associated with the ingress of water into modules in the field. Water contamination has been shown to degrade the performance of the TCO in addition to corroding interconnects and other conductive metals/materials associated with the module. Water ingress is particularly problematic in flexible thin film PV modules since traditional encapsulates such as poly(ethyl vinyl acetate) (EVA) have high water vapor transmission rates. The accelerated aging studies of the zinc oxide based TCOs will allow acceleration factors and kinetic parameters to be determined for reliability purposes.

The comparison of three photovoltaic system designs using the photovoltaic reliability and performance model (PV-RPM).

Most photovoltaic (PV) performance models currently available are designed to use irradiance and weather data and predict PV system output using a module or array performance model and an inverter model. While these models can give accurate results, they do so for an idealized system. That is, a system that does not experience component failures or outages. We have developed the Photovoltaic Reliability and Performance Model (PV-RPM) to more accurately model these PV systems by including a reliability component that simulates failures and repairs of the components of the system, as well as allow for the disruption of the system by external events such as lightning or grid disturbances. In addition, a financial component has also been included to help assess the profitability of a PV system. In this report we provide some example analyses of three different PV system designs using the PV-RPM.