A COMPREHENSIVE ASSESSMENT METHODOLOGY BASED ON LIFE CYCLE ANALYSIS FOR ON-BOARD PHOTOVOLTAIC SOLAR MODULES IN VEHICLES

This dissertation presents a novel comprehensive assessment methodology for using on-board photovoltaic (PV) solar technologies in vehicle applications. A well-to-wheels life cycle analysis based on a unique energy, greenhouse gas (GHG) emission, and economic perspective is carried out in the context of meeting corporate average fuel economy (CAFE) standards through 2025 along with providing an alternative energy path for the purpose of sustainable transportation. The study includes 14 different vehicles, 3 different travel patterns, in 12 U.S. states and 16 nations using 19 different cost analysis scenarios for determining the challenges and benefits of using on-board photovoltaic (PV) solar technologies in vehicle applications. It develops a tool for decision-makers and presents a series of design requirements for the implementation of on-board PV in automobiles to use during the conceptual design stage, since its results are capable of reflecting the changes in fuel consumption, greenhouse gas emission, and cost for different locations, technological, and vehicle sizes. The decision-supports systems developed include (i) a unique decision support systems for selecting the optimal PV type for vehicle applications using quality function deployment, analytic hierarchy process, and fuzzy axiomatic design, (ii) a unique system for evaluating all non-destructive inspection systems for defects in the PV device to select the optimum system suitable for an automated PV production line. (iii) The development of a comprehensive PV system model that for predicting the impact of using on-board PV based on life cycle assessment perspective. This comprehensive assessment methodology is a novel in three respects. First, the proposed work develops a comprehensive PV system model and optimizes the solar energy to DC electrical power output ratio. Next, it predicts the actual contribution of the on-board PV to reduce fuel consumption, particularly for meeting corporate average fuel economy (CAFE) 2020 and 2025 standards in different scenarios. The model also estimates vehicle range extension via on-board PV and enhances the current understanding regarding the applicability and effective use of on-board PV modules in individual automobiles. Finally, it develops a life cycle assessment (LCA) model (well-to-wheels analysis) for this application. This enables a comprehensive assessment of the effectiveness of an on-board PV vehicle application from an energy consumption, Greenhouse Gas (GHG) emission, and cost life-cycle perspective. The results show that by adding on-board PVs to cover less than 50% of the projected horizontal surface area of a typical passenger vehicle, up to 50% of the total daily miles traveled by a person in the U.S. could be driven by solar energy if using a typical mid-size vehicle, and up to 174% if using a very lightweight and aerodynamically efficient vehicle. In addition, the increase in fuel economy in terms of combined mile per gallon (MPG) at noon for heavy vehicles is between 2.9% to 9.5%. There is a very significant increase for lightweight and aerodynamic efficient vehicles, with MPG increase in the range of 10.7% to 42.2%, depending on location and time of year. Although the results show that the plug-in electric vehicles (EVs) do not always have a positive environmental impact over similar gasoline vehicles considering the well-to-wheel span, the addition of an on-board PV system for both vehicle configurations, significantly reduces cycle emissions (e.g., the equivalent savings of what an average U.S. home produces in a 20 month period). The lifetime driving cost ($per mile) of a gasoline vehicle with adding on-board PV, compared to a pure gasoline vehicle, is lower in regions with more sunlight (e.g., Arizona) even of the current gasoline price in the U.S. ($4.0 per gallon) assuming battery costs will decline over time. Lifetime driving cost ($per mile) of a plug-in EV with added PV versus pure plug-in EV (assuming electricity price 0.18$/kWh) is at least similar, but mostly lower, even in regions with less sunlight (e.g., Massachusetts). In places with low electricity prices (0.13 $/kWh), and with more sunlight, the costs of operating an EV with PV are naturally lower. The study reports a unique observation that placing PV systems on-board for existing vehicles is in some cases superior to the lightweighting approach regarding full fuel-cycle emissions. An added benefit of on-board PV applications is the ability to incorporate additional functionality into vehicles. Results show that an on-board PV system operating in Phoenix, AZ can generate in its lifetime, energy that is the equivalent of what an American average household residential utility customer consumes over a three-year period. However, if the proposed system operates in New Delhi, India, the PV could generate energy in its lifetime that is the equivalent of what an Indian average household residential utility customer consumes over a 33-year period. Consequently, this proposed application transforms, in times of no-use, into a flexible energy generation system that can be fed into the grid and used to power electrical devices in homes and offices. The fact that the output of this system is direct current (DC) electricity rather than alternative current (AC) electricity reduces the wasted energy cost in the generation, transmission, and conversion losses between AC-DC electricity to reach the grid. Thus, this system can potentially reduce the dependency on the grid in third world countries where the energy consumption per home is limited and the grid is unstable or unreliable, or even unavailable

NASA Tech Briefs Index, 1977, volume 2, numbers 1-4

Announcements of new technology derived from the research and development activities of NASA are presented. Abstracts, and indexes for subject, personal author, originating center, and Tech Brief number are presented for 1977

Metrology and Characterisation of Defects in Thin-Film Barrier Layers Employed in Flexible Photovoltaic Modules

Flexible thin-film photovoltaic (PV) modules based on copper indium gallium selenide (CIGS) materials are one of the most recent developments in the renewable energy field, and the latest films have efficiencies at or beyond the level of Si-based rigid PV modules. Whilst these films offer significant advantages in terms of mass and the possibility of building-integrated photovoltaic (BIPV) applications, they are at present highly susceptible to long term environmental degradation as a result of water vapour transmission through the protective encapsulation layer to the active (absorber) layer. To maintain the PV module flexibility and to reduce or eliminate the water vapour permeability, the PV encapsulation includes a barrier layer of amorphous aluminium oxide (Al2O3) material of a few nanometres thickness deposited on a planarised polyethylene naphthalate (PEN) substrate. The highly conformal barrier layer of the Al2O3 is produced by atomic layer deposition (ALD) methods using roll-to-roll (R2R) technology. Nevertheless, water vapour permeation is still facilitated by the presence of micro and nano-scale defects generated during the deposition processes of the barrier material, which results in decreased cell efficiency and reduced unit longevity. The state of the art surface metrology technologies including: optical microscopy, white light scanning interferometry (WLSI), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were extensively deployed in this project as offline surface characterisation methods to characterise the water vapour barrier layer defects, which are postulated to be directly responsible for the water vapour ingress. Areal surface texture parameters analysis based on wolf pruning, area pruning and segmentation analysis methods as defined in ISO 25178-2; allow the efficient separation of small insignificant defects from significant defects. The presence of both large and small defects is then correlated with the barrier films functionality as measured on typical sets of Al2O3 ALD films using a standard MOCON® (quantitative gas permeation) test. The investigation results of the initial analysis finishes by drawing conclusions based on the analysis of the water vapour transmission rate (WVTR), defects size, density and distribution, where it is confirmed that small numbers of large defects have more influence on the deterioration of the barrier films functionality than large numbers of small defects. This result was then used to provide the basis for developing a roll-to-roll in process metrology device for quality control of flexible PV barrier films. Furthermore, a theoretical model approach was developed in this thesis based on the water vapour diffusion theory to determine the cut- off level between large significant defects and small insignificant defects. The results of the model would seem to reveal that, in order to build up in process, non-contact optical defect detection system for R2R barrier films, the critical spatial resolution required for defect detection need not be less than 3 μm laterally and 3Sq nm (Sq= root mean square surface roughness deviation of non-defective sample area) per field of view (FOV) vertically. Any defect that has dimensions less than this appears to have a significantly lower effect on the PV barrier properties and functionality. In this study, the surface topography analysis results and the theoretical model approach outcomes, both provide the basis for developing a R2R in process metrology device for PV barrier films defect detection. Eventually, the work in this thesis reports on the deployment of new (novel) in-line interferometric optical sensors based on wavelength scanning interferometry (WSI) designed to measure and catalogue the PV barrier films defects where they are present. The sensors have built-in environmental vibration compensation and are being deployed on a demonstrator system at a R2R production facility in the UK

Solar cell degradation : the role of moisture ingress

Moisture ingress is one of the key fault mechanisms responsible for photovoltaic (PV) devices degradation. Moisture and moisture induced degradation (MID) products can attack the solar cell and the PV module components which can lead to solar cell degradation (e.g., microcracks), corrosion, optical degradation, potential induced degradation (PID), etc. These MID mechanisms have dire implications for the performance reliability of PV modules. Understanding the influence of moisture ingress on solar PV device’s degradation will boost the interest in investing in solar PV power installations globally, especially in the Nordics. In this thesis, the effect of moisture ingress on 20-years old field-aged multicrystalline silicon (mc-Si) PV modules is investigated. The defective areas in the PV modules were identified using visual inspection, electroluminescence (EL), ultraviolet fluorescence (UV-F), and infrared thermal (IR-T) techniques. Scanning electron microscopy and energy dispersive Xray spectroscopy (SEM-EDS) analyses were used to elucidate the role of moisture on the observed degradation mechanisms. In addition, temperature coefficient profiling is used as a diagnostic tool to characterize different moisture induced defects. The ethylene vinyl acetate (EVA) front encapsulation was found to undergo optical degradation and the extracted cells show dark discolored Tedlar®/Polyester/Tedlar® (TPT) backsheets. Corrosion at the solder joint was dominant and is attributed to the dissolution of lead and tin (main components of solder) and the Ag grids in moisture and acetic acid due to galvanic corrosion. Degradation of the EVA encapsulation produces acetic acid, carbon dioxide, phosphorus, sulfur, fluorine, and chlorine. It was observed that under the influence of moisture ingress, leached metal ions e.g., Na, Ag, Pb, Sn, Cu, Zn, and Al migrate to the surface of the solar cells. This led to the formation of oxides, hydroxides, sulfides, phosphates, acetates, and carbonates of silver, lead, tin, copper, zinc, and aluminum. Also, other competing reactions led to the formation of stannates of copper, silver, sodium, and zinc. Similarly, migration of silver and aluminum to the surfaces of the TiO2 antireflection coating (ARC) nanoparticles (NPs) lead to the formation of titania-alumina and silver-titania complexes. Formation of these titania-metal complexes affects the opto-electrical efficiency of the TiO2 ARC in the PV module. Additionally, in the presence of moisture and acetic acid, Pb is preferentially corroded (to form lead acetate complexes) instead of the expected sacrificial Sn in the solder. In the EL and UV-F images, these degradation species appear as dark spots, and as hot spots in IR-T images. More importantly, these MID defects and fault modes lead to parasitic resistance and mismatch losses, and hence, degradation in the current-voltage (I-V) characteristics, temperature coefficients, and maximum power (Pmax) of the field-aged PV modules. The observed temperature sensitivities are characteristic of different moisture-induced defects. Taken together, this work has expounded on the understanding and detection of MID phenomenon in field-deployed solar PV modules.publishedVersio

Fracture Mechanics of Silicon: From durability of photovoltaic modules to the production of thin film solar cells

Nowadays the photovoltaic research is focused on increasing the performances, the durability and reducing the cost of production of solar cells, such as PV modules. These are the paromount fields to make photovoltaics more attractive for the energetic market. In this dissertation two of these aspects are investigated: the durability and the cost reduction issues. The fracture mechanics of the Silicon, the standard material used for the solar cells, is the main subject of the presented study. In the recent and next years the relevance of the durability studies is expected to increase more and more because of the developing of a new segment of PV, the building integrated Photovoltaic (BIPV). These new products incorporating PV modules in the building materials are curtains, walls, windows, sloped roofs, flat roofs, facades, shading systems and roofing shingles. In the new generation of BIPV systems, PV modules replace parts of the building structure, providing functional considerations and lowering costs. In this market the thin-film PV is the most promising technology because of its superior flexibility, minimal weight, and the ability to perform in variable lighting conditions. The issues of this particular PV market are not only the energy production but also the structural safety and performance in addition to architectural specifics as the shadowing. In this framework the durability, the degradation and new technology to achieve a cost reduction are of fundamental importance. In this thesis, experimental diagnostic techniques and interpretative models based on linear and nonlinear fracture mechanics for studying the phenomena of fracture in Silicon are presented. In particular the development and the use of techniques for the quantitative analysis of electroluminescence signals, for the detection of cracks in Silicon caused by thermo-elastic stresses, have been developed. The experimental results have been obtained during an extensive experimental campaign conducted at Politecnico di Torino. For the interpretation of the experimental evidence it has been proposed an original onedimensional electrical model for predicting the eect of cracks on the distribution of electric current. Subsequently, the electric field has been coupled to the mechanical, introducing an electric resistance located at the level of the crack and dependent on the crack itself. In parallel, a numerical analysis has been carried out, using the finite element codes FRANC2D and FEAP, on the phenomenon of peeling in mono-crystalline Silicon induced by thermoelastic stresses. This study, which can be very important in applications because it may allow the production of ultra-thin solar cells with a significant saving of material, is carried out in collaboration with the Institute for Solar Energy Research (ISFH), Hamelin, Germany. This process exploits the thermo-mechanical stresses due to the contrast between the elastic proviii perties of Silicon and Aluminium in line with earlier studies of the school of Harvard. It has been proposed a broad campaign experimental and numerical in order to optimize the process

Survey of maintenance management for photovoltaic power systems

The sustainability of the global energy production systems involves new renewable energies and the improvement of the existing ones. Photovoltaic industry is growing thanks to the development of new technologies that increase the performance of photovoltaic systems. These systems are commonly subject to harsh environmental conditions that decrease their energy production and efficiency. In addition, current photovoltaic technologies are more sophisticated, and the size of photovoltaics solar plants is growing. Under this framework, research on failures and degradation mechanisms, together with the improvement of maintenance management, becomes essential to increase the performance, efficiency, reliability, availability, safety, and profitability of these systems. To assess maintenance needs, this paper presents a double contribution: an exhaustive literature review and updated survey on maintenance of photovoltaic plants, and a novel analysis of the current state and a discussion of the future trends and challenges in this field. An analysis of the main faults and degradation mechanisms is done, including the causes, effects, and the main techniques to detect, prevent and mitigate them

Compressed sensing current mapping spatial characterization of photovoltaic devices

In this work a new measurement technique for current mapping of photovoltaic (PV) devices is developed, utilising the compressed sensing (CS) sampling theory. Conventional current mapping measurements of PV devices are realised using the light beam induced current (LBIC) measurement method. For its realization, a light beam scans a PV device and the induced current is measured for every point, generating the final current map of the device. Disadvantages of the LBIC method are the low measurement speed, the complicated and usually expensive measurement layouts and the impractical application of the method on PV modules. With the development of CS current mapping in this work, the above issues can be mitigated. Instead of applying a raster scan, a series of illumination patterns are projected onto the PV sample, acquiring fewer measurements than the pixels of the final current map. The final reconstruction of the current map is achieved by means of an optimisation algorithm. Spatially resolved electrical simulations of CS current mapping demonstrate that theoretically the proposed method is feasible. In addition, it is shown that current maps can be acquired with even 40% of the measurements a standard LBIC system would require, saving a significant amount of measurement time. The performance of CS current mapping is the same, regardless of the features a sample may contain and measurements can be applied to any type of photovoltaic device. The ability of the method to provide current maps of PV modules is demonstrated. The performance of several reconstruction algorithms is also investigated. An optical measurement setup for CS current mapping of small area PV devices was built at the National Physical Laboratory (NPL), based on a digital micromirror device (DMD). Accurate current maps can be produced with only 40% of the measurements a conventional point by point scan would need, confirming simulation results. The measurement setup is compact, straightforward to realise and uses a small number of optical elements. It can measure a small area of 1cm by 1cm, making it ideal for current mapping of small research samples. A significant signal amplification is achieved, since the patterns illuminate half of the sample. This diminishes the use of lock-in techniques, reducing the cost for current mapping of PV devices. Current maps of an optical resolution up to 27μm are acquired, without the use of any demagnification elements of the projected pattern that the DMD generates. v A scale up of this new current mapping method is demonstrated using Digital Light Processing (DLP) technology, which is based on DMD chips. A commercial DLP projector is utilised for building a proof of concept CS current mapping measurement system at the Centre of Renewable Energy Systems Technology (CREST). Current maps of individual PV cells in encapsulated modules can be acquired, something that is extremely difficult to achieve with conventional LBIC systems. Direct current mapping of a PV module with by-pass diodes is successfully applied for the first time. Specific shading strategies are developed for this purpose in order to isolate the cell under test. Due to the application of compressive sampling, current maps are acquired even if the signal-to-noise-ratio levels are so low that a point by point scan is not possible. Through the above implementations of CS current mapping of this work, the proposed technique is studied and evaluated. The results demonstrate that this novel method can offer a realistic alternative approach for current mapping of PV cells and modules that can be cost effective and straightforward to implement. In addition, this work introduces the application of the CS theory and DLP technology to PV metrology in general

Quantitative electroluminescence measurements of PV devices

Electroluminescence (EL) imaging is a fast and comparatively low-cost method for spatially resolved analysis of photovoltaic (PV) devices. A Silicon CCD or InGaAs camera is used to capture the near infrared radiation, emitted from a forward biased PV device. EL images can be used to identify defects, like cracks and shunts but also to map physical parameters, like series resistance. The lack of suitable image processing routines often prevents automated and setup-independent quantitative analysis. This thesis provides a tool-set, rather than a specific solution to address this problem. Comprehensive and novel procedures to calibrate imaging systems, to evaluate image quality, to normalize images and to extract features are presented. For image quality measurement the signal-to-noise ratio (SNR) is obtained from a set of EL images. Its spatial average depends on the size of the background area within the EL image. In this work the SNR will be calculated spatially resolved and as (background independent) averaged parameter using only one EL image and no additional information of the imaging system. This thesis presents additional methods to measure image sharpness spatially resolved and introduces a new parameter to describe resolvable object size. This allows equalising images of different resolutions and of different sharpness allowing artefact-free comparison. The flat field image scales the emitted EL signal to the detected image intensity. It is often measured through imaging a homogeneous light source such as a red LCD screen in close distance to the camera lens. This measurement however only partially removes vignetting the main contributor to the flat field. This work quantifies the vignetting correction quality and introduces more sophisticated vignetting measurement methods. Especially outdoor EL imaging often includes perspective distortion of the measured PV device. This thesis presents methods to automatically detect and correct for this distortion. This also includes intensity correction due to different irradiance angles. Single-time-effects and hot pixels are image artefacts that can impair the EL image quality. They can conceivably be confused with cell defects. Their detection and removal is described in this thesis. The methods presented enable direct pixel-by-pixel comparison for EL images of the same device taken at different measurement and exposure times, even if imaged by different contractors. EL statistics correlating cell intensity to crack length and PV performance parameters are extracted from EL and dark I-V curves. This allows for spatially resolved performance measurement without the need for laborious flash tests to measure the light I-V- curve. This work aims to convince the EL community of certain calibration- and imaging routines, which will allow setup independent, automatable, standardised and therefore comparable results. Recognizing the benefits of EL imaging for quality control and failure detection, this work paves the way towards cheaper and more reliable PV generation. The code used in this work is made available to public as library and interactive graphical application for scientific image processing

A global/local approach for the prediction of the electric response of cracked solar cells in photovoltaic modules under the action of mechanical loads

AbstractA numerical approach based on the finite element method to assess the impact of cracks in Silicon solar cells on the electric response of photovoltaic modules is proposed. A global coarse-scale finite element model of the composite laminate is used for carrying out the structural analysis. The computed displacements at the edges of each solar cell are passed via a projection scheme as boundary conditions to a 3D local fine-scale finite element model of the cells which accounts for cohesive cracks. The evaluated crack opening displacements along the crack faces are finally used as input to an electric model characterizing the grid line/solar cell ensemble. The identification of the relation between the localized electric resistance due to cracks and the crack opening, to be used as a constitutive model of cracks, is finally discussed in reference to experimental tests performed in the laboratory