Sai Tatapudi

Evaluation of 12-year-old PV power plant in hot-dry desert climate: Potential use of field failure metrics for financial risk calculation

This paper provides a metric definition for the safety failures, reliability failures and degradation loss for the PV modules. These metrics are then used in real power plant evaluations to calculate the distribution among these three metrics which in turn could objectively be used to perform financial risk calculations. The results obtained on 2352 and 1280 modules in two of the evaluated power plants, aged 12 and 4 years, in a hot-dry desert climate are analyzed using these defined metrics. The results indicate that the mean and median degradations, respectively, are 0.95 and 0.96 %/year for the 12-year old, and 0.96%/year and 1%/year for the 4-year old power plants. The distribution between safety failures, reliability failures and durability loss is determined to be 7%, 42% and 51%, respectively for the 12 year old power plant.

Potential induced degradation of pre-stressed photovoltaic modules: Effect of glass surface conductivity disruption

Potential induced degradation (PID) due to high system voltages is considered as one of the possible degradation mechanisms of PV modules in the field. In the previous studies carried out at ASU-PRL, the surface conductivity of the entire glass was obtained using either conductive carbon layer (covering the entire glass surface and extending it to the frame) or humidity inside an environmental chamber. This study investigates the influence of disruption of glass surface conductivity on the PID. In this study, the conductive carbon layer was applied on the modules glass surface but without extending it to the frame and hence the surface conductivity was disrupted (no carbon layer) at 2 cm distance from the periphery of frames inner edges. This study was carried out on the modules of different manufacturers under dry heat conditions at multiple stress temperatures and voltages. To replicate closeness to the field-aged modules, half of the selected modules for the PID investigation were pre-stressed under damp heat for 1000 hours and the other half under thermal cycling for 200 cycles. When the surface continuity was disrupted, the degradation was found to be absent or negligibly small even after 35 hours of negative bias at elevated temperatures. This preliminary study appears to indicate that the modules could become immune to PID losses if the continuity of the glass surface conductivity is disrupted at the inside boundary of the frame. The surface conductivity of the glass, due to water layer formation in a humid condition, close to the frame could be disrupted just by applying a transparent hydrophobic layer near the inner edges of the frame or by attaching the frameless laminate with the conductivity disrupting mounting methods such as glue-on rail on the backsheet.

Determination of Dominant Failure Modes Using FMECA on the Field Deployed c-Si Modules Under Hot-Dry Desert Climate

The failure and degradation modes of about 5900 crystalline-Si glass/polymer modules fielded for six to 16 years in three different photovoltaic (PV) power plants with different mounting systems under the hot-dry desert climate of Arizona are evaluated. Based on the results of this evaluation, failure mode, effect, and criticality analysis, a statistical reliability tool that uses risk priority number is performed for each PV power plant to determine the dominant failure modes in the modules by means of ranking and prioritizing the modes. This study on PV power plants considers all the failure and degradation modes from both safety and performance perspectives and, thus, comes to the conclusion that solder bond fatigue/failure with/without gridline contact fatigue/failure is the most dominant failure/degradation mode for these module types in the hot-dry desert climate of Arizona.

Failure and degradation modes and rates of PV modules in a hot-dry climate: Results after 16 years of field exposure

This study evaluates the frameless modules of same type (model B) in two 16-year old photovoltaic power (PV) systems to ascertain degradation rates, reliability failure modes and safety failure modes which occur in a hot-dry climate. Each system is composed of 1512 modules. The average degradation rate is determined to be 0.85%/year for the best modules and 1.1%/year for all the modules (excluding the safety failed modules). Primary safety failure mode is the backsheet delamination though it is small (less than 1.7%). Primary degradation mode and reliability failure mode may potentially be attributed to encapsulant browning leading to transmittance/current loss and thermo-mechanical solder bond fatigue (cell-ribbon and ribbon-ribbon) leading to series resistance increase. Under the typical 20/20 warranty terms, 0.5-1.7% of the modules qualify for the safety returns, 73-76% of the modules qualify for the warranty claims and 24-26% of the modules are meeting the warranty terms.

Effect of tilt angle on soiling of photovoltaic modules

Soiling on PV modules is known to reduce PV system performance, mainly in dry arid climatic conditions. Cleaning with water or other means may become an expensive solution to the problem. For the highest annual energy production from a fixed tilt PV system, the modules are typically installed at tilt angle close to latitude angle of the systems location. Soiling loss is an interplay between terrain of the installation, tilt angle, rain frequency and rain intensity. For un-cleaned arrays in certain dusty locations, it would be better to optimize the tilt angle for maximizing transmitted radiation to the cells by adjusting the tilt angle slightly higher which would in turn minimize the soiling loss. In this work, we have developed an inexpensive soiling station which evaluates soiling loss at different tilt angles (0°, 5°, 10°, 15°, 20°, 23°, 30°, 33°, 40°). For Mesa, Arizona (a hot-dry climate), the 0° tilt angle showed a 2.02% loss whereas 23° and 33° showed soiling loss close to 1% during the first three months of 2011.

Interlaboratory Study to Determine Repeatability of the Damp-Heat Test Method for Potential-Induced Degradation and Polarization in Crystalline Silicon Photovoltaic Modules

To test reproducibility of a technical specification under development for potential-induced degradation (PID) and polarization, three crystalline silicon module types were distributed in five replicas each to five laboratories. Stress tests were performed in environmental chambers at 60 °C, 85% relative humidity, 96 h, and with module nameplate system voltage applied. Results from the modules tested indicate that the test protocol can discern susceptibility to PID according to the pass/fail criteria with acceptable consistency from lab to lab; however, areas for improvement are indicated to achieve better uniformity in temperature and humidity on the module surfaces. In the analysis of variance of the results, 6% of the variance was attributed to laboratory influence, 34% to module design, and 60% to variability in test results within a given design. Testing with the additional factor of illumination with ultraviolet light slowed or arrested the degradation. Testing at 25 °C with aluminum foil as the module ground was also examined for comparison. The foil, as tested, did not itself achieve consistent contact to ground at all surfaces, but methods to ensure more consistent grounding were found and proposed. The rates of degradation in each test are compared, and details affecting the rates are discussed.

Influence of soiling layer on quantum efficiency and spectral reflectance on crystalline silicon PV modules

Reduction in transmittance of glass superstrate due to soiling on PV modules is studied by various groups throughout the world. However, the soil layer changes not only the irradiance level but also changes the spectral distribution of the incoming solar radiation. In this work, we have selected three solar cells within a commercial module covered with different thickness of soil layer. These soiled solar cells within a module are used to study the spectral reflectance and quantum efficiency changes at various wavelengths. Heavily soiled solar cell (~74.6gm/m2) showed a very high reflectance loss and very low quantum efficiency at all wavelengths with respect to after-cleaning. The three soiled solar cells were then cleaned using three different cleaning techniques - 60psi compressed air clean, brush assisted 30psi compressed air clean and water cleaned. Short circuit current, spectral reflectances and quantum efficiencys dependence on wavelengths is studied before and after each cleaning steps.

Degradation and failure modes of 26-year-old 200 kW power plant in a hot-dry desert climate

The primary focus of this paper is the reliability / failure evaluation of a 26-year-old photovoltaic (PV) monocrystalline silicon (c-Si) system located in Phoenix, Arizona (a hot-dry desert condition). This 4000-frameless-module bipolar system was originally installed in August 1985 with a rated capacity of 175 kW (17° fixed south tilt). This paper presents only the degradation and failure modes, and the degradation rate and its determination of this power plant is presented in another/twin paper of this conference. The results obtained from the extensive visual inspections and other non-intrusive tests made on the entire plant were used to determine the degradation and wear-out failure modes. The encapsulant browning, cell gridline blossoming, cell-ribbon solder bond degradation, ribbon/metal-strip (copper) attachment degradation/break outside the modules, cell cracks and backsheet delamination have been determined to be the degradation and/or wear-out failure modes of these 26 years old modules in this specific hot-dry desert climate.

Angle of incidence effects on soiled PV modules

The transmission level of the incident light on the photovoltaic (PV) modules depends on the angle of incidence (AOI) and air/superstrate interface. The AOI dependence for the air/glass interface has already been well established. When the glass superstrate is covered by a soil/dust layer, the air/glass interface is altered and thereby changes the AOI dependence to air/soil/glass interface. In this work, PV modules retrieved from the field that had different dust densities have been measured for the dependence of the AOI curves on the dust gravimetric densities. It was determined that the AOI curve is inversely related to the soil density. The critical AOI for the air/glass interface is about 57° and it shifts dramatically as the soil gravimetric density (g/m2) increases. The measured AOI curves were then fitted and validated with the analytical/empirical models reported in the literature.

Potential induced degradation of pre-stressed photovoltaic modules: Influence of polarity, surface conductivity and temperature

Potential induced degradation (PID) is one of the factors that could contribute to the long-term degradation of PV modules. It is, therefore, essential to carry out the PID test for different open-air conditions as well as both voltage polarities for modules that maybe designated by the manufacturer to operate in either polarity. This paper evaluates the influence of PID test temperature, humidity-based and carbon-based surface conductivity of glass, and system voltage polarity on the degree of PID effects on fresh and pre-stressed (thermal-cycling or damp-heat) mono- and poly-crystalline silicon modules. Irrespective of PID test temperature (85°C or 60°C) and pre-history (fresh or pre-stressed in damp-heat or thermal-cycling), this work indicates that the positive-voltage has little or no PID effect on all the tested modules when humidity or carbon is used for the glass surface conductivity. In contrast, irrespective of PID test temperature (85°C or 60°C) and pre-history (fresh or pre-stressed in damp-heat or thermal-cycling), all the tested modules appear to be susceptible to the negative-voltage when conductive carbon (carbon layer) is used for the glass surface conductivity. However, when humidity is used for the surface conductivity instead of carbon, only the fresh and damp-heat stressed modules, excluding thermal-cycling stressed modules, appear to be susceptible to the negative voltage irrespective of PID test temperature (85°C or 60°C). It is also concluded that the humidity based approach may be a better replication of the field degradation (if any) pattern as compared to the metallic/carbon conductive layer approach.