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Investigation of potential induced degradation as a performance limiting defect in photovoltaic modules

  • Authors: Kwembur, Isaac Morko
  • Date: 2020
  • Subjects: Photovoltaic cells
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: http://hdl.handle.net/10948/48423 , vital:40875
  • Description: Potential Induced Degradation (PID) impacts negatively on photovoltaic (PV) module durability because it significantly affects the output of PV modules and systems. Unless detected at infancy, PID progression can be catastrophic. This study involved systematic PID stressing of PV modules using a custom-built environmental chamber that can achieve suitable environmental conditions, viz., of the 35 °C ± 1 °C and relative humidity of 75 % ± 5 %. The first part of this work was to induce PID using three approaches: climate chamber testing, inducing PID using a conductive aluminium plate on the surface of the module without touching the frame and a localised PID induction on one cell in a module. The second part is to detect induced PID using Electroluminescence (EL) images taken at current corresponding to 10% Isc, EL histograms analysis and Voc ratio taken at 1000 W/m2 to 200 W/m2 . The third part is to study module regeneration after PID shunting degradation in two ways, viz., forced reverse polarization and natural recovery. The PID detection tools used in this work are well known module characterization techniques such as EL imaging, Infrared imaging, and light and dark current-voltage measurements. These characterisation tools are used in combination to detect defects such as optical losses, cracks, breakage, electric circuit degradation and PID. Under normal testing PID was detected and in some cases, modules were able to recover, while for advanced stage PID regeneration or PID reversal was difficult. This thesis focuses on PID detection at infancy using three approaches; EL imaging at current corresponding to 10% of Isc. Light and dark current – voltage measurements (L-IV & D-IV) and open circuit voltage (Voc) ratios at low irradiance. The early detection procedures are essential in reversing the degradation caused by PID which is reversible. The time taken to reverse the PID degradation will depend on the extent of the degradation. If detected early, it will take a short period of time to completely reverse lost power. Infrared thermography is a non-contact characteristic tool that can be deployed in large scale plants using drones to detect the presence of PID in PV plants. Module performance and device parameters extracted from the L-IV curves on a module before and after PID stress, such as Pmpp, Voc, Isc Fill Factor (FF), shunt resistance (Rsh) and series resistance (Rs) and ideality (n) are sensitive to PID shunting. Voc and Rsh drop significantly with the onset of PID, while Rs increases. The decrease in Voc and Rsh is due to heavy shunting on the module resulting in increased carrier recombination, while the increase in Rs is due to increased shunting paths leading to decreased photocurrent. When substantial degradation on a module occurs Pmpp, FF and n will drop and at very advanced stage of PID degradation Isc may drop excessively.
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  • Date Issued: 2020

Correlation of photovoltaics plant performance metrics

  • Authors: Vumbugwa, Monphias
  • Date: 2018
  • Subjects: Photovoltaic cells , Perfomance -- Evaluation , Thin films
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: http://hdl.handle.net/10948/45657 , vital:38924
  • Description: The generation of electrical energy using Photovoltaic (PV) technology has increased globally with the decrease in the cost of PV systems and the rise in electrical power demand. In South Africa, the support by the government in implementing the Renewable Energy Independent Power Producer Procurement Programme (REIPPPP) has seen a growth in PV system deployment and investment in roof and ground mounted, stand alone and grid connected PV plants. This rapid growth shows that the PV industry is becoming highly competitive as there is a shift to low carbon emissions and it is anticipated to be the most affordable source of electricity. Hence, there is need to develop maintenance and fault diagnosis expertise and capabilities in the PV industry, which can in turn improve the dependability, productiveness and lifespan of PV systems. Solar PV modules directly receive and convert solar irradiance into electricity and may not generate the expected optimum energy due to abnormalities which arise when they are exposed to harsh unfavorable environmental conditions in the field. Thermal Infrared (TIR) imaging is widely used as a fault diagnosis tool in operating PV modules and mostly in large PV power plants. Therefore, there is need to research the interpretation of the observed thermal signatures and the impact that the anomalies have on electrical output of the system so as to improve the PV maintenance systems. This research focuses on identifying performance limiting defects using an Infra-Red (I-R) camera, mounted on an Unmanned Aerial Vehicle (UAV), to understand the effect of thermal signatures on current-voltage (I-V) characteristics of PV module strings. Aerial TIR imaging using a UAV can rapidly identify abnormalities in operational PV modules strings as hotspots. Any deviation of the string I-V curve, from the expected, indicates a problem with one or more PV modules in the string. However, locating the faulty module involves measuring I-V parameters of the individual modules in a string, which is not feasible in large PV power plants. Therefore, there is a need to estimate the power loss associated with the thermal signatures in PV module strings. Visual inspection may help in identifying the exact cause of some hotspots, while other hotspots need special characterization techniques, such as Electroluminescence (EL) and UV Fluorescence (UV-F), which can indicate if a solar cell is cracked or has weak busbars or contact finger connections.
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  • Date Issued: 2018

On the characterization of solar cells using advanced imaging techniques

  • Authors: Dix-Peek, Ross Michael
  • Date: 2018
  • Subjects: Photovoltaic cells
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: http://hdl.handle.net/10948/17944 , vital:28544
  • Description: Photovoltaic (PV) cells are devices capable of producing electricity - in particular, from the abundant resource of sunlight. Solar energy (from PV cells) provides a sustainable alternative to fossil fuel energy sources such as coal and oil. PV cells are typically strung in series in PV modules to generate the current and voltage required for commercial use. However, PV cell performance can be limited by defects and degradation. Under operational conditions due to mismatch and shading, individual cells within a PV module can be forced to operate in their reverse bias regime. Depending on the severity of the reverse bias and the defects present in the cell, the longevity of the cell and/or the module can be affected. Reverse bias (assuming bypass diodes are absent) can result in localised heating that can affect the encapsulant polymer’s longevity as well as degrade the cell’s performance over time. However, under more severe reverse bias, the cell could fail, drastically affecting the performance of the module. PV cells can be characterised using various opto-electronic non-destructive techniques, this provides a set of powerful tools which allow the application of multiple such techniques to the same sample. Furthermore, this allows for an in-depth study of the device. Dark Current-Voltage (I-V) measurements, Electroluminescence (EL), Infrared (IR) thermography, Light Beam Induced Current (LBIC) measurements, and the associated techniques are all examples of such tools and are used within this study. An experimental setup was developed to perform dark I-V measurements, EL imaging, IR thermography and LBIC measurements. Part of the development of the experimental setup was the design of an enclosure in which to perform all the measurements. The enclosure minimised internal reflection, and isolated the experiment from electromagnetic radiation. Due to the complex mathematical model applied to the I-V curve, an Evolutionary Algorithm was used to determine optimal parameter values for the equation. More specifically, a Genetic Algorithm was used in the Parameter Optimisation (or Extraction) of the dark I-V parameters based upon the two-diode model for PV cells. The resulting parameters give an indication of the material and device quality. However, to determine the spatial distribution of the defects that effect the I-V response of the device, various imaging techniques were utilised. LBIC is a technique that uses a focussed light beam to raster scan across the surface of a PV cell. The local photo-induced current/voltage can then be measured and compiled into a response map. LBIC was used to determine the local current response across the device. The intensity distribution of EL signal is related to the local junction voltage and the local quantum efficiency. EL intensity imaging with a Si CCD camera was used to determine the spatial distribution of features visible both in the forward bias and in the reverse bias. The experimental setup utilised had a micron scale resolution. A voltage dependent approach was utilised to further characterise features observed. In forward bias, the local junction varies across the device due to parasitic resistances such as series and shunt resistance. At higher forward bias conditions (in the vicinity of and higher than maximum power voltage), series resistance becomes a limiting factor. Therefore, utilising a voltage dependent approach allows for the determination of a series resistance map from voltage dependent EL images. In reverse bias, localised radiative processes can be imaged. These radiative processes are related to defects in the device, such as Al stains, FeSi2 needles and avalanche breakdown. The processes are related to highly localised current flow; this causes localised heating which degrades the device. The voltage dependent Reverse Bias EL (ReBEL) imaging was also used to determine the local breakdown voltage of radiative reverse features. Dark IR thermography is a technique used in the identification of high current sites that leads to localised Joule heating, particularly in reverse bias. In this study, thermography was used to identify breakdown sites and shunts. The results of this study allow for an in-depth analysis of defects found in multi-crystalline Si PV cells using the opto-electronic techniques mentioned above. The multi-pronged approach allowed from a comparison of the various opto-electronic techniques, as well as a more in-depth characterisation of the defects than if only one technique was used.
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  • Date Issued: 2018

On the characterisation of diffused light and optical elements in high concentrator photovoltaic modules

  • Authors: Schultz, Ross Dane
  • Date: 2015
  • Subjects: Photovoltaic cells , Solar concentrators , Optical materials
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: http://hdl.handle.net/10948/5170 , vital:20817
  • Description: High Concentrated Photovoltaics (H-CPV) promise a more efficient, higher power output than traditional photovoltaic modules. This is achieved by concentrating sunlight onto a small triple junction (CTJ) InGaP/InGaAs/Ge cell (ranging from 3.14 mm2 to 1 cm2) by using precision optical systems. These systems utilise non-imaging optics to concentrate and distribute the incident solar flux uniformly onto the CTJ device receiver to achieve maximum performance and power output from an H-CPV module. However, the performance of the device can be reduced due to the partial or complete absorption of a range of wavelengths present in the solar spectrum by the optical materials that are used for concentration. An investigation to determine the current density topographies of each subcell in a CTJ cell by multiple raster scans of an optical fibre receiver of a spectrometer in the plane of the aperture of the secondary’s optical element was conducted. Results showed that the physical properties of the optical elements’ material absorbed different amounts of the spectral content with respect to the subcell photosensitive wavelength regions. The facet properties of the primary optical Fresnel lens showed that the more rounded the Fresnel facets were, the lower the concentration of sunlight incident onto the CTJ cell. The increase in facet numbers showed an increase in scattering of the incident sunlight and chromatic aberrations. Chromatic aberration created by the refractive optics showed a variation in the amount of concentration on each individual subcell as well as the difference in intensity profiles across for the different subcells. Based on these results and the development of new multi-junction devices by industry, the performance of a four and six-junction device with the optical materials was investigated by simulations. The simulations showed that the careful integration of an additional subcell in a multi-junction device could rectify current mismatch between the subcells in the device. Based on the simulations, the best performing multi-junction cell was identified as the four-junction device that showed a cell and module efficiency under operation of 42.5 % and 35.5 %, respectively. Additionally, based on the performance results observed from the H-CPV module, the development of an HCPV module that would attempt to harness the incident tracked diffuse sunlight available to a concentrator photovoltaic (CPV) module for additional energy yield was undertaken. The part of the study comprised of measurements of the solar source, design of a prototype Hybrid High Concentrator Photovoltaic (HH-CPV) module. Results showed that power generation from the H-CPV system was highly dependent on the DNI levels and fluctuates greatly with variation in the DNI. The irradiance levels within the diffuse regions of the H-CPV module showed that the baseplate and vertical sides had an average irradiance range of 140-450 and 50-225 W.m-2, respectively. Irradiance topographic raster scans revealed that the baseplate and vertical sides had a relatively uniform intensity distribution and was identified as favourable sites for diffuse cell population. Simulations of various PV technologies showed the most suitable technology for the placement within the cavity of the HH-CPV module. The developed HH-CPV module was finalized with the utilization of CIS modules to harness the diffuse irradiance. During a 3 month power monitoring of the HH-CPV system, it was determined that the major power generation for the HH-CPV module come from the CPV component, while the CIS modules showed a minor power contribution. The total energy yield for the monitoring period was 45.99, 3.89 and 1.76 kW.h for the CPV, four-vertical sides and baseplate components, respectively. The increase in energy yield of the HH-CPV module when compared to the standard H-CPV module was determined to be 12.35 % for the monitoring period. The incorporation of the CIS modules into the H-CPV module to create the HH-CPV module did increase the energy yield of the module during high DNI conditions and did offset the almost zero power generation during low DNI conditions.
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  • Date Issued: 2015

Characterization of cell mismatch in photovoltaic modules using electroluminescence and associated electro-optic techniques

  • Authors: Crozier, Jacqueline Louise
  • Date: 2012
  • Subjects: Photovoltaic cells , Solar cells
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10545 , http://hdl.handle.net/10948/d1015059
  • Description: Solar cells allow the energy from the sun to be converted into electrical energy; this makes solar energy much more environmentally friendly than fossil fuel energy sources. These 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 the PV module is limited by the performance of the individual cells. Cell mismatch occurs when some cells are damaged or shaded and produce lower current output than the other cells in the series connected string. The cell mismatch lowers the module performance and can result in further damage as the weak cells are reverse biased and dissipate heat. Bypass diodes can be connected into the module to increase the module current output and prevent further damage. Since cell mismatch results in a significant decrease in the performance of deployed modules it is important to fully understand and characterise its effect on PV modules. PV modules can be characterised using various techniques, each providing important information about the performance of the module. Most commonly the current-voltage (I-V) characteristic curve of a module is 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. In addition to this the shape of the curve allows device parameters like series and shunt resistances to be determined using parameter extraction algorithms like Particle Swarm Optimisation (PSO). The extracted parameters can be entered into the diode equation to model the I-V curve of the module. The I-V characteristic of the module can also be used to identify poor current producing cells in the module by using the worst-case cell determination method. In this technique a cell is shaded and the greater the drop in current in the whole module the better the current production of the shaded cell. The photoresponse of cells in a module can be determined by the Large-area Light Beam Induced Current (LA-LBIC) technique which involves scanning a module with a laser beam and recording the current generated. Electroluminescence (EL) is emitted by a forward biased PV module and is used to identify defects in cell material. Defects such as cracks and broken fingers can be detected as well as material features such as grain boundaries. These techniques are used to in conjunction to characterise the modules used in this study. The modules investigated in this study each exhibit cell mismatch resulting from different causes. Each module is characterised using a combination of characterisation techniques which allows the effect of cell mismatch be investigated. EL imaging enabled cracks and defects, invisible to the naked eye, to be detected allowing the reduced performance observed in I-V curves to be explained. It was seen that the cracked cells have a significant effect on the current produced by a string, while the effect of delaminated areas is less severe. Hot spots are observed on weak cells indicating they are in reverse bias conditions and will degrade further with time. PSO parameter extraction from I-V curves revealed that the effect of module degradation of device parameters like series and shunt resistances. A module with cracked cells and degradation of the antireflective coating has low shunt resistance indicating current losses due to shunting. Similar shunting is observed in a module with delamination and moisture ingress. The extracted parameters are used to simulate the I-V curves of modules with reasonable fit. The fit could be improved around the “knee” of the I-V curve by improving the methods of parameter extraction. This study has shown the effects of cell mismatch on the performance and I-V curves of the PV modules. The different causes of cell mismatch are discussed and modules with different cell configuration and damage are characterised. The characterisation techniques used on each module provide information about the photoresponse, current generation, material properties and cell defects. A comprehensive understanding of these techniques allows the cell mismatch in the modules to be fully characterized.
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  • Date Issued: 2012

On the design of concentrator photovoltaic modules

  • Authors: Schultz, Ross Dane
  • Date: 2012
  • Subjects: Photovoltaic cells -- Design and construction , Photovoltaic cells
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10546 , http://hdl.handle.net/10948/d1015766 , Photovoltaic cells -- Design and construction , Photovoltaic cells
  • Description: 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.
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  • Date Issued: 2012

On the optical and electrical design of low concentrator photovoltaic modules

  • Authors: Benecke, Mario Andrew
  • Date: 2012
  • Subjects: Photovoltaic cells -- Design and construction , Photovoltaic cells
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10543 , http://hdl.handle.net/10948/d1013102
  • Description: 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.
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  • Date Issued: 2012

On the thermal and electrical properties of low concentrator photovoltaic systems

  • Authors: Gerber, Jacques Dewald
  • Date: 2012
  • Subjects: Photovoltaic power systems , Photovoltaic cells
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10561 , http://hdl.handle.net/10948/d1021219
  • Description: Low concentrator photovoltaic systems are capable of increasing the power produced by conventional silicon photovoltaic cells, thus effectively lowering the cost per kWh. However, power losses associated with resistance and temperature have limited the large scale implementation of this technology. In this study, the optical-,electrical- and thermal sub-systems of a low concentrator photovoltaic system are theoretically and experimentally evaluated with the aim of minimizing the power losses associated with series resistance and temperature. A 7-facet reflector system, with an effective concentration ratio of 4.7, is used to focus irradiance along a string of series connected poly-crystalline photovoltaic cells. I-V characteristics of 4-, 6- and 8-cell photovoltaic receivers are measured under 1-sun and 4.83-sun conditions. Under concentration, the 8-cell photovoltaic receiver produced 23 percent more power than the 4-cell photovoltaic receiver, which suggests that the effect of series resistance can be minimized if smaller, lower current photovoltaic cells are used. A thermal model, which may be used to predict operating temperatures of a low concentrator photovoltaic system, is experimentally evaluated within a thermally insulated enclosure. The temperatures predicted by the thermal model are generally within 5 percent of the experimental temperatures. The high operating temperatures associated with the low concentrator photovoltaic system are significantly reduced by the addition of aluminium heat sink. In addition, the results of a thermal stress test indicated that these high operating temperatures do not degrade the photovoltaic cells used in this study. The results of this study suggest that the power output of low concentrator photovoltaic systems can be maximized by decreasing the size of the photovoltaic cells and including an appropriate heat sink to aid convective cooling.
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  • Date Issued: 2012

On the design and monitoring of photovoltaic systems for rural homes

  • Authors: Williams, Nathaniel John
  • Date: 2011
  • Subjects: Photovoltaic cells , Dwellings -- Power supply
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10516 , http://hdl.handle.net/10948/1308 , Photovoltaic cells , Dwellings -- Power supply
  • Description: It is estimated that 1.6 billion people today live without access to electricity. Most of these people live in remote rural areas in developing countries. One economic solution to this problem is the deployment of small domestic photovoltaic (PV) systems called solar home systems (SHS). In order to improve the performance and reduce the life cycle cost of these systems, accurate monitoring data of real SHSs is required. To this end, two SHSs typical of those found in the field were designed and installed, one in a rural area of the Eastern Cape of South Africa and the other in the laboratory. Monitoring systems were designed to record energy ows in the system and important environmental parameters. A novel technique was developed to correct for measurement errors occurring during the utilization of pulse width modulation charge control techniques. These errors were found to be as large as 47.6 percent. Simulations show that correction techniques produce measurement errors that are up to 20 times smaller than uncorrected values, depending upon the operating conditions. As a tool to aid in the analysis of monitoring data, a PV performance model was developed. The model, used to predict the maximum power point (MPP) power of a PV array, was able to predict MPP energy production to within 0.2 percent over the course of three days. Monitoring data from the laboratory system shows that the largest sources of energy loss are charge control, module under performance relative to manufacturer specifications and operation of the PV array away from MPP. These accounted for losses of approximately 18-27 percent, 15 percent and 8-11 percent of rated PV energy under standard test conditions, respectively. Energy consumed by loads on the systems was less than 50 percent of rated PV energy for both the remote and laboratory systems. Performance ratios (PR) for the laboratory system ranged from 0.38 to 0.49 for the three monitoring periods. The remote system produced a PR of 0.46. In both systems the PV arrays appear to have been oversized. This was due to overestimation of the energy requirements of the loads on the systems. In the laboratory system, the loads consisting of three compact fluorescent lamps and one incandescent lamp, were used to simulate a typical SHS load pro le and collectively consumed only 85 percent of their rated power. The 8 predicted load profile for the remote system proved to be signi cantly overestimated. The results of the monitoring project demonstrate the importance of acquiring an accurate estimation of the energy demand from loads on the system. Overestimations result in over-sized arrays and energy lost to charge control while under-sized systems risk damaging system batteries and load shedding. Significant under-performance of the PV module used in the laboratory system, underlines the importance of measuring module IV curves and verifying manufacturer specifications before system deployment. It was also found that signi cant PV array performance gains could be obtained by the use of maximum power point tracking charge controllers. Increased PV array performance leads to smaller arrays and reduced system cost.
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  • Date Issued: 2011

Spatially resolved opto-electric measurements of photovoltaic materials and devices

  • Authors: Thantsha, Nicolas Matome
  • Date: 2010
  • Subjects: Photovoltaic cells , Photovoltaic power systems , Photovoltaic power generation
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:10520 , http://hdl.handle.net/10948/1123 , Photovoltaic cells , Photovoltaic power systems , Photovoltaic power generation
  • Description: The objective of this study is to characterize and analyse defects in solar cell devices. Materials used to fabricate solar cells are not defects free and therefore, there is a need to investigate defects in cells. To investigate this, a topographical technique was developed and employed which uses a non-destructive methodology to analyse solar cells. A system was built which uses a technique based on a laser beam induced current (LBIC). LBIC technique involves focusing light on to a surface of a solar cell device in order to create a photo-generated current that can be measured in the external circuit for analyses. The advantage of this technique is that it allows parameter extraction. Parameters that can be extracted include short-circuit current, carrier lifetime and also the external and internal quantum efficiency of a solar cell. In this thesis, LBIC measurements in the form of picture maps are used to indicate the distribution of the localized beam induced current within solar cells. Areas with low minority carrier lifetime in solar cells are made visible by LBIC mapping. Surface reflection intensity measurements of cells can also be mapped using the LBIC system developed in this study. The system is also capable of mapping photo-generated current of a cell below and above room temperature. This thesis also presents an assessment procedure capable of assessing the device and performance parameters with reference to I-V measurements. The dark and illuminated I-V characteristics of solar cells were investigated. The illuminated I-V characteristics of solar cells were obtained using a defocused laser beam. Dark I-V measurements were performed by applying voltage across the cell in the dark and measuring a current through it. The device parameters which describe the behaviour of I-V characteristic were extracted from the I-V data using Particle Swarm Optimization (PSO) method based on a one-and two-diode solar cell models. Solar cells of different technologies were analysed, namely, single-crystalline (c-Si) and multicrystalline (mc-Si) silicon, Edge-defined Film-fed Growth Si (EFG-Si) and Cu(In,Ga)(Se,S)2 (CIGSS) thin film based cells. The LBIC results illustrated the effect of surface reflection features and material defects in the solar cell investigated. IQE at a wavelength of 660 nm were measured on these cells and the results in general emphasised the importance of correcting optical losses, i.e. reflection loss, when characterizing different types of defects. The agreement between the IQE measurements and I-V characteristics of a cell showed that the differences in crystal grains influence the performance of a mc-Si cell. The temperature-dependence of I-V characteristics of a CIGSS solar cell was investigated. The results showed that, for this material, the photo response is reduced at elevated temperatures. In addition to LBIC using a laser beam, solar spectral radiation was employed to obtained device performance parameters. The results emphasised the effect of grain boundaries as a recombination centres for photo-generated hole-pairs. Lastly, mesa diode characterizations of solar cells were investigated. Mesa diodes are achieved by etching down a solar cell so that the plateau regions are formed. Mesa diodes expose the p-n junction, and therefore mesa diode analysis provides a better way of determining and revealing the fundamental current conduction mechanism at the junction. Mesa diodes avoid possible edge effects. This study showed that mesa diodes can be used to characterize spatial non-uniformities in solar cells. The results obtained in this study indicate that LBIC is a useful tool for defect characterization in solar cells. Also LBIC complements other characterization techniques such as I-V characterization.
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  • Date Issued: 2010

Investigation of device and performance parameters of photovoltaic devices

  • Authors: Macabebe, Erees Queen Barrido
  • Date: 2009
  • Subjects: Photovoltaic cells , Solar cells , Photovoltaic power systems , Photovoltaic power generation
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:10538 , http://hdl.handle.net/10948/1003 , http://hdl.handle.net/10948/d1012890 , Photovoltaic cells , Solar cells , Photovoltaic power systems , Photovoltaic power generation
  • Description: In order to investigate the influence of parasitic resistances, saturation current and diode ideality factor on the performance of photovoltaic devices, parameter extraction routines employing the standard iteration (SI) method and the particle swarm optimization (PSO) method were developed to extract the series resistance, shunt resistance, saturation current and ideality factor from the I-V characteristics of solar cells and PV modules. The well-known one- and two-diode models were used to describe the behavior of the I-V curve and the parameters of the models were determined by approximation and iteration techniques. The SI and the PSO extraction programmes were used to assess the suitability of the one- and the two-diode solar cell models in describing the I-V characteristics of mono- and multicrystalline silicon solar cells, CISS- and CIGSS-based solar cells. This exercise revealed that the two-diode model provides more information regarding the different processes involved in solar cell operation. Between the two methods developed, the PSO method is faster, yielded fitted curves with lower standard deviation of residuals and, therefore, was the preferred extraction method. The PSO method was then used to extract the device parameters of CISS-based solar cells with the CISS layer selenized under different selenization process conditions and CIGSS-based solar cells with varying i-ZnO layer thickness. For the CISS-based solar cells, the detrimental effect of parasitic resistances on device performance increased when the temperature and duration of the selenization process was increased. For the CIGSS-based devices, photogeneration improved with increasing i-ZnO layer thickness. At high forward bias, bulk recombination and/or tunneling-assisted recombination were the dominant processes affecting the I-V characteristics of the devices. v Lastly, device and performance parameters of mono-, multicrystalline silicon and CIS modules derived from I-V characteristics obtained under dark and illuminated conditions were analyzed considering the effects of temperature on the performance of the devices. Results showed that the effects of parasitic resistances are greater under illumination and, under outdoor conditions, the values further declined due to increasing temperature. The saturation current and ideality factor also increased under outdoor conditions which suggest increased recombination and, coupled with the adverse effects of parasitic resistances, these factors result in lower FF and lower maximum power point. Analysis performed on crystalline silicon and thin film devices utilized in this study revealed that parameter extraction from I-V characteristics of photovoltaic devices and, in particular, the implementation of PSO in solar cell device parameter extraction developed in this work is a useful characterization technique.
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  • Date Issued: 2009

On the evaluation of spectral effects on photovoltaic modules performance parameters and hotspots in solar cells

  • Authors: Simon, Michael
  • Date: 2009
  • Subjects: Photovoltaic cells , Energy dissipation , Electric power production , Photovoltaic power generation , Solar energy , Spectral energy distribution
  • Language: English
  • Type: Thesis , Doctoral , PhD (Physics)
  • Identifier: vital:11593 , http://hdl.handle.net/10353/257 , Photovoltaic cells , Energy dissipation , Electric power production , Photovoltaic power generation , Solar energy , Spectral energy distribution
  • Description: The performance of photovoltaic (PV) modules in terms of their ability to convert incident photon to electrical energy (efficiency) depends mostly on the spectral distribution of incident radiation from the sun. The incident spectrum finally perceived by the module depends strongly on the composition of the medium in which it has traveled. The composition of the earth’s atmosphere, which includes, amongst others, water vapour, gases such as carbon dioxide and oxygen, absorbs or scatters some of the sunlight. The incident solar spectrum is also modified by the diffuse aspect of radiation from the sky which strongly depends on aerosol concentration, cloudiness and local reflection of the earth’s surface. Although it is well known that the changes in outdoor spectrum affect device performance, little work has been conducted to support this theory. This is probably due to lack of spectral data or in certain instances where data is available, little knowledge of interpreting that data. The outdoor spectral data that one obtains in the field does not come clearly for just simple interpretation. Different analytical interpretation procedures have been proposed, all trying to explain and quantify the spectral influence on PV devices. In this study an assessment methodology for evaluating the effects of outdoor spectra on device performance parameters during the course of the day, seasons and or cloudy cover has been developed. The methodology consists of developing a device dependant concept, Weighted Useful Fraction (WUF) using the outdoor measured spectral data. For measuring PV module’s performance parameters, a current-voltage (I-V) tester was developed in order to monitor the performance of six different module technologies. The Gaussian distribution was used to interpret the data. For hot-spot analysis, different techniques were used, which include Infrared thermographic technique for identifying the hot-spots in the solar cells, SEM and EDX techniques. The AES technique was also used in order to identify other elements at hot-spots sites that could not be detected by the EDX technique. iii Results obtained indicate that multicrystalline modules performance is affected by the changes in the outdoor spectrum during summer or winter seasons. The modules prefer a spectrum characterized by WUF = 0.809 during summer season. This spectrum corresponds to AM 2.19 which is different from AM 1.5 used for device ratings. In winter, the mc-Si module’s WUF (0.7125) peaks at 13h00 at a value corresponding to AM 1.83. Although these devices have a wider wavelength range, they respond differently in real outdoor environment. Results for mono – Si module showed that the device performs best at WUF = 0.6457 which corresponds to AM 1.83 during summer season, while it operates optimally under a winter spectrum indicated by WUF of 0.5691 (AM2.58). The seasonal changes resulted in the shift in WUF during day time corresponding to the “preferred” spectrum. This shift indicates that these devices should be rated using AM values that correspond to the WUF values under which the device operates optimally. For poly-Si, it was also observed the WUF values are lower than the other two crystalline-Si counterparts. The pc-Si was observed to prefer a lower AM value indicated by WUF = 0.5813 during winter season while for summer it prefers a spectrum characterized by WUF = 0.5541 at AM 3.36. The performance of the single junction a-Si module degraded by 67 percent after an initial outdoor exposure of 16 kWh/m² while the HIT module did not exhibit the initial degradation regardless of their similarities in material composition. It was established that the WUF before degradation peaks at 15h00 at a value of 0.7130 corresponding to AM 4.50 while the WUF after degradation “prefers” the spectrum (WUF = 0.6578) experienced at 15h30 corresponding to AM value of 5.57. Comparing the before and after degradation scenarios of a-Si:H, it was observed that the device spends less time under the red spectrum which implies that the device “prefers” a full spectrum to operate optimally. The degradation of a-Si:H device revealed that the device spectral response was also shifted by a 7.7 percent after degradation. A higher percentage difference (61.8 percent) for spectral range for the HIT module is observed, but with no effects on device parameters. Seasonal changes (summer/winter) resulted in the outdoor spectrum of CuInSe2 to vary by WUF = 1.5 percent, which resulted in the decrease in Isc. This was ascertained by iv analyzing the percentage change in WUF and evaluating the corresponding change in Isc. The analysis showed that there was a large percentage difference of the module’s Isc as the outdoor spectrum changed during the course of the day. This confirmed that the 17 percent decrease in Isc was due to a WUF of 1.5 percent. In mc-Si solar cells used in this study, it was found that elemental composition across the entire solar cell was not homogenously distributed resulting in high concentration of transition metals which were detected at hot spot areas. The presence of transition metals causes hot-spot formation in crystalline solar cells. Although several transition elements exist at hot-spot regions, the presence of oxygen, carbon, iron and platinum was detected in high concentrations. From this study, it is highly recommended that transition elements and oxygen must be minimized so as to increase the life expectancy of these devices and improve overall systems reliability
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  • Date Issued: 2009

On the Processing of InAsSb/GaSb photodiodes for infrared detection

  • Authors: Odendaal, Vicky
  • Date: 2008
  • Subjects: Gallium arsenide semiconductors , Photovoltaic cells , Infrared detectors , Gas-detectors
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10523 , http://hdl.handle.net/10948/980 , Gallium arsenide semiconductors , Photovoltaic cells , Infrared detectors , Gas-detectors
  • Description: The objective of this dissertation is the development of the necessary processing steps needed to manufacture infrared photodiodes on InAs1-xSbx material. Preliminary surface preparation steps were performed on both InAs and InSb material, thus covering both possible extremes of the antimony mole fraction. The first experiments endeavoured to characterise the effect of several possible etchants with regards to etch rate, repeatability, limitations for photolithographic patterning and the resultant surface roughness. The etchants investigated include a lactic acid based etchant, a sulphuric acid based etchant, an acetic acid based etchant, an ammonium based etchant, a hydrochloric acid based etchant as well as an organic rinse procedure. These cleaning and etching steps were evaluated at several temperatures. Measurements were performed on an Alpha Step stylus profiler as well as an atomic force microscope. Metal-insulator-semiconductor capacitor devices were manufactured, on both InAs and InSb material, in order to investigate the effects of the above-mentioned etchants combined with surface passivation techniques in terms of surface state densities. Capacitance-versus-bias voltage measurements were done to determine the resultant surface state densities and to compare these to the surface state density of an untreated reference sample. The surface passivation techniques included KOH, Na2S as well as (NH4)2S anodisation. Auger electron spectroscopy measurements were done on InAs and InSb material in order to examine possible surface contamination due to the etchants as well as combinations of these etching and anodisation procedures. The extent of surface coverage by contaminants as well as by the intrinsic elements was measured. The results of the cleaning and etching as well as the surface passivation studies were used to manufacture photovoltaic infrared diodes on an MOCVD (metal oxide chemical vapour deposition) grown p-InAs0.91Sb0.09/i- InAs0.91Sb0.09/n-GaSb structure. Current-versus-voltage and electro-optical measurements were performed on the these diodes in order to evaluate the effect of sulphuric acid based etching combined with KOH, Na2S or (NH4)2S anodisation on the detector performance. The results of surface passivated structures were compared to those of an unpassivated reference detector.
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  • Date Issued: 2008

On the characterization of photovoltaic devices for concentrator purposes

  • Authors: Vorster, Frederick Jacobus
  • Date: 2007
  • Subjects: Photovoltaic cells , Image processing , Solar cells
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:10530 , http://hdl.handle.net/10948/639 , Photovoltaic cells , Image processing , Solar cells
  • Description: This study originated from an evaluation of the performance of a commercially available high concentration point focus concentrator PV system. The effect of module design flaws was studied by using current-voltage (I-V) curves obtained from each module in the array. The position of reverse bias steps revealed the severity of mismatch in a string of series-connected cells. By understanding the effects of the various types of mismatch, power losses and damage to the solar cells resulting from hot spot formation can be minimized and several recommendations for improving the basic performance of similar systems were made. Concern over the extent and type of defect failure of the concentrator photovoltaic (CPV) cells prompted an investigation into the use of a light beam induced current (LBIC) technique to investigate the spatial distribution of defects. An overview of current and developing LBIC techniques revealed that the original standard LBIC techniques have found widespread application, and that far-reaching and important developments of the technique have taken place over the years. These developments are driven by natural progression as well as the availability of newly developed advanced measurement equipment. Several techniques such as Lock-in hermography and the use of infrared cameras have developed as complementary techniques to advanced LBIC techniques. As an accurate contactless evaluation tool that is able to image spatially distributed defects in cell material, the basis of this method seemed promising for the evaluation of concentrator cells.
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  • Date Issued: 2007

On the characterisation of copper indium diselenide based photovoltaic devices

  • Authors: Thantsha, Nicolas Matome
  • Date: 2006
  • Subjects: Photovoltaic cells
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10536 , http://hdl.handle.net/10948/443 , Photovoltaic cells
  • Description: Photovoltaic (PV) modules based on thin film systems of CuInSe2 (CIS) and its alloys on low cost substrates are promising candidates to meet the long term efficiency, reliability and manufacturing cost goals. The attention to the CIS solar cell technology is because of the high absorption coefficient of the solar cell absorber layer. Solar cells and PV modules are conventionally assessed by measuring the currentvoltage characteristic of the device. This thesis presents an assessment procedure developed capable of assessing the device parameters with reference to I-V measurements. This thesis then characterizes the performance of the CIS based solar cells and modules in conjunction with other PV modules of different technologies such as crystalline Silicon modules by analyzing the light and dark I-V measurements of the devices. The light and dark I-V characteristics of PV devices were investigated and device parameters were extracted from the I-V data. The extraction and interpretation of these device parameters has a variety of important applications. It has been proven that the device parameters can be used for quality control during production and to provide insights into the operation of the PV devices, thereby improving the efficiency of the devices. The assessment comprises light I-V measurements at standard test conditions (STC), irradiance dependence measurements, parasitic series and shunt resistances measurements and the dark I-V measurements of the PV devices. The PV modules assessed comprise different technologies, namely, thin film based modules (CIS and a-Si) and multicrystalline Si and Edged-defined Film-fed Growth Si (EFG-Si). The dark I-V measurements results showed that the EFG-Si module has acceptable shunt (900 W) and series (0.4 W) resistances, thereby leading to the higher power output depicted from the light I-V measurements. The low quality cells of a-Si module were so low that the fill factor was the smallest (43%). In addition, the dark I-V measurements results revealed that CIS modules are less dependent to temperature at high voltages.
  • Full Text:
  • Date Issued: 2006
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