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.

Energy is of concern for governments and economies all over the world. As conventional methods of energy production are facing the prospect of depleting fossil fuel reserves, economies are facing energy risks. With this tension, various threats arise in terms of energy supply security. A shift from intensive fossil fuel consumption to alternative energy consumption combined with the calculated use of fossil fuels needs to be implemented. Using the energy radiated from the sun and converted to electricity through photovoltaic energy conversion is one of the alternative and renewable sources to address the limited fossil fuel dilemma. South Africa receives an abundance of sunlight irradiance, but limited knowledge of the implementation and possible energy yield of photovoltaic energy production in South Africa is available. Photovoltaic energy yield knowledge is vital in applications for farms, rural areas and remote transmitting devices where the construction of electricity grids are not cost effective. In this study various meteorological and energy parameters about photovoltaics were captured in Port Elizabeth (South Africa) and analyzed, with data being recorded every few seconds. A model for mean daily photovoltaic power output was developed and the relationships between the independent variables analyzed. A model was developed that can forecast mean daily photovoltaic power output using only temperature derived variables and time. The mean daily photovoltaic power model can then easily be used to forecast daily photovoltaic energy output using the number of sunlight seconds in a given day.

A MicroGrid is typically a small power system, which consists of several microsources and energy storage units, providing heat and electricity to local loads. The MicroGrid has the capability to island and operate autonomously from the main utility network. MicroGrids potentially enable a greater integration of small-scale renewable energy sources. The objective of this thesis is to develop a single-phase microsource interface with energy storage unit embedded into the interface. An integrated energy storage unit avoids the necessity of a separate stand-alone energy storage unit in the MicroGrid. Thus the 'plug-and-play' functionality of the MicroGrid can be improved. A collection of power electronic converter based microsources with storage units connected to form a MicroGrid is a complex system. Development of such simple controllers, which decouple the effect of sub-unit while achieving the desired 'plug-and-play' capabilities, is a complex but important challenge. A photovoltaic panel was considered as the microsource and a battery bank was used as the energy storage unit. The proposed microsource interface consists of three power electronic converters. A photovoltaic panel is connected to a unidirectional dc-dc converter and its output is connected to the input of the single-phase inverter which can be connected to the MicroGrid. Energy storage is connected to the dc-link,which is the input of the single-phase inverter, through a bi-directional dc-dc converter. A simulation model of a photovoltaic panel was developed in the EMTDC/PSCAD software. The limitations of the model and a method to reduce these limitations are discussed. For the experimental validation of the proposed system, an adjustable-voltage-regulator hardware photovoltaic emulator was designed. The characteristic curves of the hardware emulator are compared with those obtained from the simulation model. A controller was designed for the unidirectional dc-dc converter to keep the output voltage of the photovoltaic panel at a given reference. Similarly the controller of the bi-directional dc-dc converter was designed to keep the dc-link voltage approximately constant. The behaviour of the dc-dc converters, which are connected to microsource and energy storage unit, around the steady state and worst-case scenarios was analysed, simulated and experimentally validated. Simulation and experimental results are compared. A current controller, based on instantaneous measured current, was implemented. This was designed to achieve good dynamic stiffness and command tracking properties. Furthermore a smooth grid connection method with the aid of the current controller is presented. The ability of the inverter to control the active and reactive power output was also analysed and verified with the aid of the simulation model and experimental set-up. Results corresponding to current controller, grid connection and power control are presented. The integrated system was simulated in EMTDC/PSCAD. The system response to the variations in the microsource and inverter output power variations was analysed. A smooth start-up method is shown which reduces the inrush current. Simulation results corresponding to different case studies and start-up transient are also included.

Two basic types of photovoltaic (PV) array simulator have been designed and tested. The first involves the use of a pilot panel and variable light source. It is implemented with analogue circuits. A stability analysis based on Popov's method is presented for this simulator with resistance-inductance (R-L) loads. In the second, characteristic array curves are stored in the memory of a microprocessor-based simulator. The design of both simulators is based on the transfer function method. By using the computing facility available, a stability study for the Type I simulator and some dynamic simulations are carried out. Both simulators are capable of driving a special load, namely, an experimental solar pumping system. The experimental results for both types of' simulator are satisfactory in terms of steady state precision and dynamic behaviour when used with this load. Compared with previously-reported PV array simulator designs [6,7,8,9,18], the two simulators described here have the following distinctive features: 1. A new method of sample curve generation for the Type II simulator results in relatively short sampling period and small memory size. 2. The sample curves of the type II simulator are based directly on the real PV array to be simulated. They are more accurate than the sample curves in references [6,7,9]. 3. Different loads (R, R-L and an experimental solar pumping system) have been considered in the design and have been tested in laboratory. 4. A stability analysis and some dynamic simulations are presented for the type I simulator. An analysis of this type has not been reported in previous studies [6,7,8,9,18]. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Solar energy has come to the forefront as a scalable and largely underutilized renewable energy resource. The current cost of solar electricity, namely from photovoltaics, along with other logistics factors, has prevented the widespread adaptation of the technology. A key determinant of efficiency and cost for a solar cell is the current collector grid. This work presents the Collimated Aerosol Beam Direct Write (CAB-DW) system as a non-contact printing method that can achieve current collector grid finger widths of less than 10 μm which are amenable to decreasing both resistive and optical losses. The ability to produce high aspect ratio grid fingers, and deposit optimized grid structures on high efficiency SHJ solar cells using silver nanoparticle inks is also demonstrated. A decrease in shadowing and via profile modification of the grid fingers is presented, along with a study of aging and degradation of electrical properties within silver nanoparticle inks.

archives@tulane.edu / As global energy consumption continues to grow, new paths towards renewable energy generation are needed to reduce environmental impact and allow for more zero-net energy development. This includes not only electricity generation but also energy required for thermal applications. This dissertation explores three different technologies to generate electricity and high temperature heat simultaneously by using an actively tracked parabolic dish concentrator (2.72 m2) and an all-in-one hybrid receiver. This hybrid receiver usually consists of two key components, a PV module assembled with multijunction solar cells based on III-V materials, and a thermal receiver that transfers absorbed solar energy into a working fluid for a variety of commercial and industrial process heating applications. A key goal of this work is to use spectrum splitting and other design innovations to operate PV cells at much lower temperatures than the thermal receiver output temperatures. PV cooling is critical for PV modules to sustain high energy conversion efficiencies and to work for longer duration under concentrated light. A key distinction in different designs reported here is how the PV cells are cooled, either “transmissive microfluidic cooling”, “transmissive direct fluid cooling”, and “non-transmissive microfluidic cooling”. All three technologies show good performance for both efficient PV cooling (< 120°C) and high system energy conversion efficiency (> 80%). This dissertation is divided into four key chapters. Chapter 2 discusses spectrum splitting CPV with transmissive microfluidic cooling, focusing on the optical performance of the PV modules. By applying a transfer matrix-style approach, the cumulative transmission through the entire PV module is calculated: these results are verified experimentally. By doing so, the power collected by the PV cells and thermal receiver can be predicted. Chapter 3 explores a spectrum splitting hybrid receiver design using a cheaper and more straightforward cooling method that flows silicone oil across PV cells to extract their waste heat and to eliminate the use of sapphire for cost reduction. The cooling performance is verified by outdoor tests and the system efficiencies are discussed under different solar concentration. Chapter 4 investigates another hybrid receiver design that utilizes waste heat from high efficiency PV cells to preheat the working fluid in the thermal receiver instead of dumping the energy to surroundings as in the previous two methods. This design allows both the cells and the thermal receiver to be illuminated with concentrated sunlight simultaneously without the need for spectrum splitting. The electrical and thermal performance are tested both in the lab and outdoors. Chapter 5 discusses a proposed way to enhance the transmission of the spectrum splitting III-V solar cells used in Chapters 2 and 3. Epitaxial lift-off is used to remove the III-V cell substrate and to fabricate highly infrared-transmissive, spectrum-splitting thin-film solar cells. In summary, we explore the power collection performance, including optical, electrical, and thermal aspects, for these hybrid solar receiver technologies, enabling their use in a number of promising applications. / 1 / Yaping Ji

Organic photovoltaic (OPV) devices utilizing organic (carbon-based) semiconductors have maintained research interest due to their potential for inexpensive, non-toxic, flexible, and lightweight solar modules. Numerous organic polymers and small molecules have been investigated for OPV applications, however a focus on maximizing the power conversion efficiency (PCE) of lab-scale devices has generated many novel active materials that are too complex to be realistically synthesized on a commercial scale. It has become apparent that developing low-cost, scalable, and stable active materials is crucial for the commercialization of OPV devices. Metal phthalocyanines (MPcs) are a well-known family of molecules with established scale up chemistry from their use as colorants and have demonstrated strong performance as low-cost semiconductors in organic electronic devices. However, their potential in solution-processed OPV devices has not been fully realized. In this thesis, a series of materials based on silicon phthalocyanine (SiPc) and tin phthalocyanine (SnPc) were synthesized and characterized. Novel molecular designs and OPV device architectures were investigated to further establish the use MPcs as low-cost active materials and to probe new applications. Specifically, the chemical and physical differences of structurally analogous soluble SiPc and SnPc derivatives were examined for the first time. The ability of a SiPc derivative to act as a thermal crosslinker to stabilize active layer morphology while simultaneously contributing to photocurrent generation was also proven. SiPc derivatives were then studied as electron acceptors paired with P3HT and PBDB-T donor polymers, achieving a PCE up to 4.3 %. The results herein establish new potential roles for group 14 MPcs in OPV devices while also demonstrating their synthetic simplicity and versatility. This work also serves as a basis for the wealth of chemical functionalization which remains available for continued optimization of these materials.

This is the first thorough evaluation of the viability and appropriateness of photovoltaic (PV) water pumping in South Africa. It is a case study of the operation of a PV water pumping system installed in a rural community vegetable garden in KwaZulu. The system comprised a 574 WP array, DC power maximizer, DC motor and a Mono (positive displacement) pump. The pump delivered an average of 15 m3 I day over a static head of 12.5 metres for a Standard Solar Day of 5 kWh/m2/d. Three facets were considered: technical, economic, and social. · For the technical evaluation the operation of the whole system as well as that of each component under various conditions were monitored in the field using a data logger. The economic evaluation compared the Life Cycle Costs of PV water pumping with those of diesel, petrol, and electric pumps. The social evaluation was based on three sets of interviews over a period of five years ranging from before the introduction of the pump to four years afterwards. The following are the most important conclusions. Technical: the system Daily Energy Efficiency was 2.22%. This is low in comparison with values given in Halcrow's authoritative report of 2.35% for their average systems and 3.28% for their best systems. The reason for this was the low efficiency of the Mono Pump: 39% in comparison with 41.5% for Halcrow's average systems and 59% for their best. This was because the head of 12.5 metres at Sondela was not ideal for the Mono Pump which is designed for 45 metres. All the low-cost PV pumping systems available in South Africa, however, use positive displacement pumps and are thus inefficient at low heads. But because PV pumps are more competitive economically at low heads and low flow rates, it is important that an efficient pump for these applications is designed. Submersible centrifugal pumps should be considered. · Economics: the applicability of various assumptions to developing areas has been thoroughly evaluated. This has laid U1e groundwork for a accurate computer program which would accurately compare the Life Cycle Costs of PV, diesel, petrol and electric pumps under a range of conditions. Connecting to the grid has many advantages and should be considered first. However, the costs of the normal tariff are affected strongly by the site and this option is out of the question for more remote sites. PV pumps are at the moment competitive with diesel pumps at only low hydraulic heads (around 40 m4/day). However, if a PV pump which was efficient at low heads were designed and if the path of the sun were physically tracked, then PV pumps could possibly be competitive up to hydraulic heads of 1400 m4/day. Social: the study showed that installing pumps in community vegetable gardens can almost double the productivity of the gardeners' time. The gardeners interviewed indicated that, because of the many advantages of PV pumps, they would prefer them to diesel pumps if their amortized costs were up to twice those of the diesel pump., But few, if any, community gardens would be able to raise the capital required for a PV pump. For this reason, a scheme similar to that just introduced by ESKOM could make a crucial difference to the marketability of PV pumps: ESKOM will buy and maintain the pump recovering the costs from the user at a fixed monthly rate stipulated before installation. This scheme obviates the two major barriers to the sale of PV pumps: I) high initial cost and ii) the risk of damage or loss of expensive equipment due to floods, theft, or vandalism.

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