Thesis (M.S.)--Tennessee Technological University, 2009. / Title from title page screen (viewed on July 26, 2010). Includes bibliographical references.

University of Macau / Faculty of Science and Technology / Department of Electromechanical Engineering

Solar cells allow the energy from the sun to be converted into electrical energy; this makes solar energy an environmentally friendly, sustainable alternative to fossil fuel energy sources. Solar cells are connected together in a photovoltaic (PV) module to provide the higher current, voltage and power outputs necessary for electrical applications. However, the performance of PV modules can limited by the degradation and defects. PV modules can be characterised using various opto-electronic techniques, each providing information about the performance of the module. The current-voltage (I-V) characteristic curve of a module being the most commonly used characterisation technique. The I-V curve is typically measured in outdoor, fully illuminated, conditions. This allows performance parameters such as short circuit current (ISC), open circuit voltage (VOC) and maximum power (PMAX) to be determined. However, it can be difficult to determine the root cause of the performance drop from the I-V curve alone. Electroluminescence (EL) is a module characterisation technique that allows defects and failures in PV modules to be successfully identified. This study investigates the characterisation of solar cells and photovoltaic modules using EL. EL occurs when a solar cell or module is forward biased and the injected electron-hole pairs recombine radiatively. The intensity of the emitted EL is related the applied voltage and the material properties. EL imaging is a useful characterisation technique in identifying module defects and failures. Defects such as micro-cracks, broken contact fingers and fractures are detected in EL images as well as material features such as grain boundaries. The common defects in crystalline silicon are catalogued and the possible causes are discussed. An experimental setup was developed in order to systematically take a high resolution EL image of every cell in the module and record the applied voltage and current. This produces a very detailed, clear, image of each cell with a pixel size in the micrometre range. This process is time consuming to acquire an EL image of an entire module so alternatively a different setup can be used and an EL image of a whole module can be captured in a single frame with an increased pixel size in the millimetre range. For EL imaging a silicon charge-coupled device (CCD) camera was used because it has very good spatial resolution however this sensor is only sensitive to wavelength in the range of 300-1200 nm. There is an overlap in wavelengths from about 900 to 1100 nm allowing the EL emitted from silicon solar cells to be detected. In conjunction with the high-resolution EL system an image processing program was developed to crop, adjust and align the images so only the relevant cell was included. This program also automatically detects certain defects that have a regular shape. Micro-cracks, broken fingers and striation rings are automatically identified. The program has an adjustable sensitivity to identify small or large defects. Defective cells are distinguished from undamaged cells by comparing the binary images to the ideal, undamaged cell. The current-voltage curves and the performance parameters of modules were compared with the EL images in order to discuss and identify power limiting defects. Features that remove significant portions of the cell from electrical contact such as micro-cracks are shown to have a larger effect of the performance of the module. Other features such as broken contact fingers, contact forming failures and striation rings do not significantly lower the performance of the module. Thus an understanding of how different features affect the module performance is important in order to correctly interpret the EL results. The intensity of the luminescence emitted is related to the applied voltage and the quantum efficiency of the cell material. The spectrum of the emitted luminescence was modelled and related to the recombination properties of the cell such as surface recombination velocity and minority carrier diffusion length/lifetime. In this study the emitted spectrum was modelled and the effects of recombination properties of the cell on the emitted spectrum were examined. The spectrum of the detected EL was modelled, dependent on the sensitivity of the camera, the transmission of the filters and the emitted photon flux. The integration of short-pass filters into the experimental setup in order to isolate short-wavelength luminescence was discussed. There is a proportional relationship between the intensity of the emitted EL and the local junction voltage. Resistive losses like series and shunt resistances lower the applied voltage and thus affect the EL image. The voltage dependence was assessed by comparing EL images taken at different applied biases. Analysis of the variation in EL intensity with voltage was successful in determining the origin of certain features in an EL image. Certain defects, those that are related to series resistance or shunting are highly voltage dependent. When a feature has little or no dependence on voltage then the defect could be in the laminate layers and not in the cell material. The results of this study allow for in-depth analysis of the defects found in PV modules using the high resolution EL imaging system and the image processing routine. The development of an image processing routine allows the interpretation of the EL image to be done automatically, resulting in a faster and more efficient process. By understanding the defects visible in the EL image, the test is more meaningful and allows the results to be used to predict module performance and potential failures.

The increasing interest in non-fossil fuel based electricity generation has caused a prominent boost for the renewable energy sector, especially the field of Photovoltaics (PV) with one of the main reasons being the decrease in cost of PV electricity generation. However, over the last few years a saturation in the efficiency of solar cells have been reached leading into a renewed search for other means to further reduce the cost of electricity generation from photovoltaic sources. One of the technologies that has attracted a lot of attention is low concentration photovoltaics (LCPV). LCPV investigates an alternative strategy to replace costly semiconductor material with relatively cheap optical materials by developing a Low Concentration Photovoltaic (LCPV) module. A LCPV module is divided into three subsystems, namely, the optical, electrical and thermal subsystem. This study focussed on the design, construction and characterisation of an optical subsystem accompanied by a thorough investigation into the design of an electrical subsystem. A facetted parabolic concentrator using a vertical receiver was modelled and a first prototype was constructed having a geometric concentration factor of 6.00 X. Upon electrical characterisation of this first vertical receiver LCPV prototype a concentration of only 4.53 X (receiver 1) and 4.71 X (receiver 2) was obtained. The first vertical receiver LCPV prototype did not reach the expected concentration factor due to optical losses and misalignment of optical elements. The illumination profile obtained from the reflector element was investigated and an undesirable non-uniform illumination profile was discovered. A second vertical receiver LCPV prototype was constructed in an attempt to improve on the first prototype, this second vertical receiver prototype had a geometrical concentration factor of 5.80 X. The results indicated a much improved illumination profile, yet still containing a number of non-uniformities. The second vertical receiver LCPV module yielded an operational concentration factor of 5.34 X. From the preliminary results obtained it was discovered that under concentrated illumination there was a limitation on the maximum power that could be obtained from the receiver. Upon further investigation it was discovered that this limitation was due to the higher current levels under concentrated illumination accompanied by a high series resistance of the receiver. This lead to the construction of new PV receivers, where this limitation could be minimised. 3 cell, 4 cell, 6 cell and 8 cell string configurations were constructed and used for the electrical characterisation of the prototypes. Due to non-uniformity of the illumination profile obtained from the second LCPV prototype a third vertical receiver LCPV prototype was constructed. This vertical receiver design illustrated more uniformity in the obtained illumination distribution and had a geometrical concentration factor of 4.61 X, although under operation only 4.26 X could be obtained. It is important to note that the geometric concentration factor does not account for reflective losses of the reflective material. One of the main reasons for the difficulty in obtaining a uniform illumination profile with the vertical receiver design is that the facetted reflector element is far away from the PV receiver. This enhances the effect of the slightest misalignment of any of the optical elements. This large distance also increases the effect of lensing from each facet. These factors lead to the consideration of a second design, which would counteract these factors. A horizontal receiver LCPV module design implementing a facetted parabolic reflector was considered to counteract these effects. From a mathematical model a horizontal receiver LCPV prototype was constructed having a geometrical concentration factor 5.3 X. The optical characterisation of the illumination profile showed a much improved illumination profile, which was much more uniform than the previous illumination profiles obtained from the other LCPV prototypes. The uniformity of the illumination profile could be seen in results obtained from the electrical characterisation where the concentrator reached operational concentration factor of 5.01 X. The reliability of the third vertical receiver LCPV prototype and the horizontal receiver LCPV prototype as well as the receivers were investigated by placing each receiver under stressed operational conditions for 60 sun hours. I-V characteristics were obtained after every five sun hours to investigate any signs of degradation. After 60 sun hours none of the receiver displayed any signs of degradation or reduction in electrical performance.

High concentration photovoltaics (HCPV) promise a more efficient, higher power output than traditional photovoltaic modules. This is achieved by concentrating sunlight onto a small 1 cm2 triple junction (CTJ) InGaP/InGaAs/Ge cell by using precision optics. In order to achieve high performance, careful and informed design decisions must be made in the development of a HCPV module . This project investigated the design of a HCPV module and is divided into sections that concentrate on the optical design, thermal dissipation and electrical characterization of a concentration triple junction cell. The first HCPV module (Module I) design was based on the Sandia III Baseline Fresnel module which comprised of a Fresnel lens and truncated reflective secondary as the optical elements. The parameters of the CTJ cell in Module I increased with increased concentration. This included the short circuit current, open circuit voltage, power and efficiency. The best performance achieved was at 336 times operational concentration which produced 10.3 W per cell, a cell efficiency of 38.4 percent, and module efficiency of 24.2 percent Investigation of the optical subsystem revealed that the optics played a large role in the operation of the CTJ cell. Characterization of the optical elements showed a transmission loss of 15 percent of concentrated sunlight for the irradiance of which 66 percent of the loss occurred in wavelength region where the InGaP subcell is active. Characterization of the optical subsystem indicated regions of non-uniform irradiance and spectral intensity across the CTJ cell surface. The optical subsystem caused the InGaP subcell of the series monolithic connected CTJ cell to be current limiting. This was confirmed by the CTJ cell having the same short circuit current as the InGaP subcell. The performance of the CTJ cell decreased with an increase in operational temperature. A form of thermal dissipation was needed as 168 times more heat needs to be dissipated when compared to a flat plate photovoltaic module. The thermal dissipation was achieved by passive means with a heat sink which reduced the operational temperature of the CTJ cell from 50 oC to 21 oC above ambient. Cell damage was noted in Module I due to bubbles in the encapsulation epoxy bursting from a high, non-uniform intensity distribution. The development of the second module (Module II) employed a pre-monitoring criteria that characterized the CTJ cells and eliminated faulty cells from the system. These criteria included visual inspection of the cell, electroluminescence and one sun current-voltage (I-V) characteristic curves. Module II was designed as separate units which comprised of a Fresnel lens, refractive secondary, CTJ cell and heatsink. The optimal configuration between the two modules were compared. The CTJ cells in module II showed no form of degradation in the I-V characteristics and in the detected defects. The units under thermal and optical stress showed a progressive degradation. A feature in the I-V curve at V > Vmax was noted for the thermally stressed unit. This feature in the I-V curve may be attributed to the breakdown of the Ge subcell in the CTJ cell. Based on the results obtained from the two experimental HCPV modules, recommendations for an optimal HCPV module were made.

No description available.

Single crystal silicon solar cells for use under high concentration sunlight presently exhibit the highest conversion efficiencies. The following paper represents further work done to improve the efficiency of crystalline silicon solar cells through improved design. Design features and processing to address the loss mechanisms encountered in silicon solar cells are discussed. An improved solar cell structure has resulted from this work along with a practical processing sequence. Experiments were performed to show the practicality of pattern formation on the walls of the V-groove structures using conventional photolithography and masking techniques. Also, new beam processing techniques are discussed to improve processing.

Dust accumulation on photovoltaic (PV) modules and its effect on their performance are of high concern for regions with a high rate of dust, low frequency and intensity of rain. In this thesis, the effect of dust on PV modules is investigated with respect to dust concentration and spectral transmittance. The measured spectral transmittance of the dust sample shows spectral attenuation effect that varies at different wavelengths. This effect is explained by the particle size distribution of the dust samples: At shorter wavelengths more light is scattered due to the effect of the smaller particles. This effect has a major impact on the PV module as it affects PV technologies with a wider band-gap more than those of a narrower band-gap. The effect of dust is accumulative, i.e. PV module performance is reduced by increasing deposition over time or until it's cleared manually or by rain. The tilt angle of the PV installation plays a major role in the amount of dust accumulated on the devices, where higher tilt angles result in decreased dust concentrations. This effect is demonstrated in outdoor measurements where tilted modules had lower losses in daily as well as total array yield. It is also shown that tilted modules benefit from precipitation more than horizontal modules. However over the exposure period the modules did not show any clear aging effect caused specifically from dust accumulation or exhibit any seasonal variation. Different tilt angles can produce varying non-uniform dust patterns on the device surface. This effect and its pattern over long and short periods of exposure are investigated by means of spatial three dimensional modelling. The simulations compare two dust accumulation patterns that represent a short exposure to a single dusty day (one day) and a long exposure of dust (3 months). Out of the two patterns, the long exposure patterns showed higher losses of 19.4% in comparison to 14.8% for the short exposure. The simulation also showed that dust accumulation that promotes high concentration of dust at the bottom of the PV modules where it covers a full cell has a high risk of triggering hot spots and thus risks permanent module damage. A dust correction model for energy prediction is developed. The model takes into consideration dust concentration, spectral attenuation effect of dust, PV technology, and various meteorological variables. The modified spectral transmittances of the dust were incorporated into the model in the form of pre-measured data. This means in this work samples collected in Kuwait were measured and used to generate the input. The model is compared against the outdoor measured data and a good agreement between measurements iv and simulations is demonstrated. Using this model two procedures were developed. The first evaluates the uncertainties associated with dust over long periods of time. The second is to find the optimised cleaning schedule and frequency of cleaning based on acceptable yield loss margins over the simulated period of time. The optimisation of the cleaning schedule showed that for Kuwait setting the daily energy losses in PV modules at less than 10% will set the cost of cleaning higher than the cost of energy lost due to dust.

A study of fabricating the first electrospun polymer-fiber solar cell with MEHPPV is presented. Motivation for the work and a brief history of solar cell is given. Limiting factors to improvement of polymer solar cell efficiency are illustrated. Electrospinning is introduced as a technique that may increase polymer solar cell efficiency, and a list of advantages in the technique applied to solar cell is discussed. Results of electrospun polymer-fiber solar cell, absorption, and its device parameter diagnosis through an equivalent circuit analysis are presented.

M. Tech. (Department of Electronic Engineering, Faculty of Engineering and Technology) -- Vaal University of Technology / Einstein said, ‘‘the release of energy has not created a new problem, but has made more urgent the necessity of solving an existing one’’. This dissertation presents a method of optimising the available output power from a photovoltaic (PV) panel through empirical testing as this will enable a higher yield of solar energy thereby reducing dependence on traditional energy sources such as fossil fuels. The proposed study intends using existing equations of latitude, mathematical models and simulation packages in combination with the experimental data to analyse the optimum tilt and orientation angles for PV panels. This will assist in identifying ways to improve the installation of PV panels for optimum output power in the Vaal Triangle. Photovoltaic panels are semiconductor devices that convert incident direct beam radiation to electrical energy and the panel is composed of several unitary cells connected in series and/or in parallel. The optimisation process involves the empirical testing of the entire system with the use of existing equations of latitude as suggested by literature for PV installation in the southern hemisphere, power conditioning devices (such as an DC-DC converter, solar charger with MPPT) in order to validate results as well as the correlation of empirical results with a simulation package. The first objective was to have an overview of the types of PV panels that exist; this was done in order to be able to make a right choice of PV panel to be used in this research. A concise literature review was carried to enable this research to have a background of existing information in the areas of optimisation of power from PV panels. The next objective was to carry out a pilot study, this was done to form the foundation for the main study. A data-logging interface circuit (DLIC) was incorporated in the system for some reasons presented in subsequent chapters of this dissertation. At the end of this study data were taken over a two year period, the data were analysed and conclusions were drawn and some recommendation in optimising available output power from a PV panel are suggested. / Vaal University of Technology, Telkom South Africa Ltd, TFMC Pty LTD, M-TEC and THRIP