Degradation in Field-aged Crystalline Silicon Photovoltaic Modules and Diagnosis using Electroluminescence Imaging

Degradation phenomena observed in field-aged crystalline silicon photovoltaic modules include EVA browning, delamination between the glass-encapsulant and the cell-encapsulant interfaces, degradation of the anti-reflective coating, corrosion of busbars and contacts, cracks, humidity ingress, etc. The type and severity of the defects observed vary significantly between cells, modules and installations as affected by a number of both internal and external parameters. This study presents mild to severe degradation effects observed in crystalline silicon PV modules operating outdoors for different periods of time and investigated through non-destructive testing techniques including I-V characterisation, UV fluorescence, IR thermography and Electroluminescence (EL) Imaging. The identification and diagnosis of defects and further correlation to the electrical degradation of the module is achieved through the complementary contribution of these techniques. Severe electrical degradation and mismatch between the cells are identified through IR thermography and EL imaging. Diagnosis of rather uniformly degraded modules is enhanced through EL Imaging by which shunts, higher resistance regions, cracks, broken metallization are identified, while the module may appear to operate reliably. Signs of early degradation are further diagnosed through UV fluorescence and EL Imaging, allowing to monitor the evolution of defects and evaluate module reliability

An effective simulation model to predict and optimize the performance of single and double glaze flat-plate solar collector designs

This paper outlines and formulates a compact and effective simulation model, which predicts the performance of single and double glaze flat-plate collector. The model uses an elaborated iterative simulation algorithm and provides the collector top losses, the glass covers temperatures, the collector absorber temperature, the collector fluid outlet temperature, the system efficiency, and the thermal gain for any operational and environmental conditions. It is a numerical approach based on simultaneous guesses for the three temperatures, Tp plate collector temperature and the temperatures of the two glass covers Tg1, Tg2. A set of energy balance equations is developed which allows for structured iteration modes whose results converge very fast and provide the values of any quantity which concerns the steady state performance profile of any flat-plate collector design. Comparison of the results obtained by this model for flat-plate collectors, single or double glaze, with those obtained by using the Klein formula, as well as the results provided by other researchers, is presented

Degradation effects in sc-Si PV modules subjected to natural and induced ageing after several years of field operation

This paper presents ageing effects observed in sc-Si PV modules operating in field conditions for 18 and over 22 years. The effects of both natural ageing processes and induced ageing by external agents, causing partial or total shading of cells for a prolonged period of time, are examined. Optical degradation effects observed through visual inspection include discoloration of the EVA, degradation of the AR coating, degradation of the interface between the cell and encapsulant, corrosion of busbars and fingers, and tears, bubbles and humidity ingress at the back surface of the modules. Thermal degradation effects examined via IR thermography reveal the existence of hot cells, hotspots on the busbars, and colder bubbles. Modules' power and performance degradation is assessed through I-V curve analysis. Results show naturally aged modules to exhibit milder ageing effects than modules subjected to induced ageing, an outcome also supported by their power degradation ratio

Thermal modelling and experimental assessment of the dependence of PV module temperature on wind velocity and direction, module orientation and inclination

A theoretical and experimental analysis of PV module temperature under various environmental conditions is presented in relation to module inclination, wind velocity and direction. The present experimental study, makes use of hourly PV temperature data collected from a double-axis sun-tracking PV system and environmental parameters monitored for a period of one year. The f coefficient which relates the PV module temperature with the intensity of the global solar radiation on the PV plane and the ambient temperature, is assessed in relation to the angle of PV inclination, the wind velocity and the angle of incidence of the wind stream on the PV surface, either front or back. The f coefficient is evaluated both experimentally and theoretically through thermal modelling based on the energy balance equation. The simulation model developed in this study considers heat convection by natural and air forced flow, the flow pattern either laminar or turbulent, the relative geometry of the PV module with respect to the wind direction, and the radiated heat by the PV module. Various expressions for the forced heat convection coefficient available in the literature are tested within the thermal model with reference to the windward and leeward side of the PV module, and their applicability to PV thermal analysis is experimentally assessed in terms of the agreement shown with measured data. The values of the f coefficient provided by the simulation model lie very close to the experimental data for the entire range of PV inclination angles, wind velocities and wind directions tested

Intelligent energy buildings based on RES and Nanotechnology

The paper presents the design features, the energy modelling and optical performance details of two pilot Intelligent Energy Buildings, (IEB). Both are evolution of the Zero Energy Building (ZEB) concept. RES innovations backed up by signal processing, simulation models and ICT tools were embedded into the building structures in order to implement a new predictive energy management concept. In addition, nano-coatings, produced by TiO2 and ITO nano-particles, were deposited on the IEB structural elements and especially on the window panes and the PV glass covers. They exhibited promising SSP values which lowered the cooling loads and increased the PV modules yield. Both pilot IEB units were equipped with an on-line dynamic hourly solar radiation prediction model, implemented by sensors and the related software to manage effectively the energy source, the loads and the storage or the backup system. The IEB energy sources covered the thermal loads via a south façade embedded in the wall and a solar roof which consists of a specially designed solar collector type, while a PV generator is part of the solar roof, like a compact BIPV in hybrid configuration to a small wind turbine

On the relationship factor between the PV module temperature and the solar radiation on it for various BIPV configurations

Temperatures of c-Si, pc-Si and a-Si PV modules making part of a roof in a building or hanging outside windows with various inclinations were measured with respect to the Intensity of the solar radiation on them under various environmental conditions. A relationship coefficient f was provided whose values are compared to those from a PV array operating in a free standing mode on a terrace. A theoretical model to predict f was elaborated. According to the analysis, the coefficient f takes higher values for PV modules embedded on a roof compared to the free standing PV array. The wind effect is much stronger for the free standing PV than for any BIPV configuration, either the PV is part of the roof, or placed upon the roof, or is placed outside a window like a shadow hanger. The f coefficient depends on various parameters such as angle of inclination, wind speed and direction, as well as solar radiation. For very low wind speeds the effect of the angle of inclination, β, of the PV module with respect to the horizontal on PV temperature is clear. As the wind speed increases, the heat transfer from the PV module shifts from natural flow to forced flow and this effect vanishes. The coefficient f values range from almost 0.01 m2°C/W for free standing PV arrays at strong wind speeds, vW>7m/s, up to around 0.05 m2°C/W for the case of flexible PV modules which make part of the roof in a BIPV system

TiO2-based nanocoating with self-cleaning and anti-reflective properties: effects on PV performance

Photovoltaic modules operating in field conditions exhibit a significant reduction in their power output due to dust accumulated on their surface. Depending on the amount of dust accumulated the reduction in peak power has been reported in the range of 5-15%. The accumulated dust is linked to meteorological and environmental parameters such as humidity, precipitation, solar radiation, ambient temperature, dusty winds, air pollution, etc., but also to the location and surroundings of the installation and the period for which the PV modules have been left without cleaning. To reduce the effect of dust, research has been recently focused on coatings with self-cleaning properties that may be applied on PV glass surface. Also, coatings with spectral selective properties have been investigated to enhance PV performance. The purpose of this study is to examine the effect of a nanocoating with self-cleaning and anti-reflective properties on the performance of a PV module when applied on its glass surface. Particular interest is given to its anti-reflective properties which are assessed for angles-of-incidence of solar radiation greater than 40o, where reflectance is generally higher. The performance of two same PV modules one with and one without the coating is compared

Self-cleaning properties of TiO2/palygorskite and TiO2/halloysite nanocomposite coatings

Tubular halloysite and microfibrous palygorskite clay mineral combined with nanocrystalline TiO 2 are involved in the preparation of nanocomposite films on glass substrates via sol-gel route at 450°C. The synthesis employing nonionic surfactant molecule as pore directing agent along with the acetic acid-based sol-gel route without addition of water molecules. Drying and thermal treatment of composite films ensure elimination of organic material lead to the formation of TiO 2 nanoparticles homogeneously distributed on the palygorskite and halloysite surfaces. Nanocomposite films without cracks of active anatase crystal phase on palygorskite and halloysite surfaces are characterized by microscopy techniques, UV-Vis spectroscopy, and porosimetry methods in order to examine their structural properties. The composite palygorskite- TiO 2 and halloysite/ TiO 2 films with variable quantities of palygorskite and halloysite were tested as photocatalysts in the photo-oxidation of Basic Blue 41 azo dye in water. These nanocomposite films proved to be most promising photocatalysts and highly effective to dye’s decoloration in spite of small amount of palygorskite/ TiO 2 or halloysite/ TiO 2 catalyst immobilized onto glass substrates

PV cell and module degradation, detection and diagnostics

With crystalline silicon photovoltaic (PV) modules being on the market for over 3 decades, investigation into usual causes and extent of module degradation after prolonged exposure in field conditions is nowadays possible. Degradation phenomena vary significantly between cells, modules and installations, giving rise to different power degradation rates reported. The main defects that have been observed in field aged PV modules, include EVA browning, degradation of the anti-reflective coating, delamination between the glass-encapsulant and the cell-encapsulant interfaces, humidity ingress, corrosion of busbars and contacts, shunt paths, cracks/ micro-cracks in the cell, damage of the glass and the back sealing, and bypass diode failure. This study presents severe degradation effects observed in PV modules operating outdoors for over 20 years. In many of the cases investigated different defects were seen to coexist within the same cell or module, leading to more severe effects of optical/physical, thermal, and electrical degradation phenomena significantly reducing the PV power output. Other modules which exhibited extensive optical/physical degradation showed milder degradation in performance. Detection of module degradation was carried out in this study first through visual inspection and I-V curve analysis. Further non-destructive diagnostic techniques were used such as infrared thermography for the identification of hot spots, that were seen to be mainly linked to resistive busbars and contacts, and electroluminescence imaging for the identification of shunts and other defects. The detection, diagnosis and monitoring of such defects is of great importance for a deeper understanding of the complex ageing mechanisms that take place after prolonged PV exposure in field conditions, and the identification of underlying causes, assisting the early identification of defects and the extension of the energy life of PV systems