Browsing by Subject "Photovoltaic"
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Item Analysis and Design of Smart PV Module(2012-12-10) Mazumdar, PoornimaThis thesis explores the design of a smart photovoltaic (PV) module- a PV module in which PV cells in close proximity are electrically grouped to form a pixel and are connected to dc-dc converter blocks which reside embedded in the back pane of the module. An auto-connected flyback converter topology processing less than full power is used to provide high gain and perform maximum power point tracking (MPPT). These dc-dc converters interface with cascaded H-bridge inverter modules operating on feed forward control for dc-link voltage ripple rejection. By means of feed forward control, a significant reduction in dc link capacitance is achieved by enduring higher dc link ripple voltages. The dc link electrolytic capacitors are replaced with film capacitors thus offering an improvement in the reliability of the smart PV module. The proposed configuration is capable of producing 120V/ 240V AC voltage. The PV module now becomes a smart AC module by virtue of embedded intelligence to selectively actuate the individual dc-dc converters and control the output AC voltages directly, thus becoming a true plug and power energy system. Such a concept is ideal for curved surfaces such as building integrated PV (BIPV) system applications where gradients of insolation and temperature cause not only variations from PV module-to-PV module but from group-to-group of cells within the module itself. A detailed analysis along with simulation and experimental results confirm the feasibility of the proposed system.Item Analysis of classical root-finding methods applied to digital maximum power point tracking for photovoltaic energy generation(2011-08) Chun, Seunghyun; Kwasinski, Alexis; Grady, William; Driga, Mircea; Hallock, Gary; Byoun, JaesooThis dissertation examines the application of various classical root finding methods to digital maximum power point tracking (DMPPT). An overview of root finding methods such as the Newton Raphson Method (NRM), Secant Method (SM), Bisection Method (BSM), Regula Falsi Method (RFM) and a proposed Modified Regula Falsi Method (MRFM) applied to photovoltaic (PV) applications is presented. These methods are compared among themselves. Some of their features are also compared with other commonly used maximum power point (MPP) tracking methods. Issues found when implementing these root finding methods based on continuous variables in a digital domain are explored. Some of these discussed issues include numerical stability, digital implementation of differential operators, and quantization error. Convergence speed is also explored. The analysis is used to provide practical insights into the design of a DMPPT based on classical root finding algorithms. A new DMPPT based on a MRFM is proposed and used as the basis for the discussion. It is shown that this proposed method is faster than the other discussed methods that ensure convergence to the MPP. The discussion is approached from a practical perspective and also includes theoretical analysis to support the observations. Extensive simulation and experimental results with hardware prototypes verify the analysis.Item An analysis of the current costs and future prospects of solar photovoltaic electricity(2009-12) Wong, Alexander Tung-Qiang; Eaton, David J.; Spence, David B.The solar photovoltaic industry has many barriers to overcome before it can become a technically and economically competitive generation source including (1) lowering true generation costs, (2) decreasing reliance from government subsidies, and (3) developing a suitable energy storage solution. Current unsubsidized costs of electricity from solar photovoltaic sources range from 24.0 to 58.3¢/kWh. Subsidies bring the generation costs down to as low as 11¢/kWh, competitive with the average retail price of electricity in certain parts of the country. Current subsidy policies used to encourage technology development may generate more profits rather than research and innovation. The most optimistic predictions for solar photovoltaics include a convergence of a steep and prolonged rise in the cost of fossil-fuel based generation with a deep and prolonged decrease in the cost of photovoltaic generation by 2019. Deviation from optimal conditions will prolong the delay the crossover until at least 2021 and possibly beyond 2030. The development of a solution to store excess electricity when the sun is available during the day for use at night is necessary for photovoltaic electricity to become a dominant generation source.Item The application of light trapping structures and of InGaAs/GaAs quantum wells and quantum dots to improving the performance of single-junction GaAs solar cells(2012-05) McPheeters, Claiborne Ott; Yu, Edward T.; Alu, Andrea; Bank, Seth R.; Chen, Ray T.; Zhang, John X.High efficiency photovoltaic solar cells are expected to continue to be important for a variety of terrestrial and space power applications. Solar cells made of optically thick materials often cannot meet the cost, efficiency, or physical requirements for specialized applications and, increasingly, for traditional applications. This dissertation investigates improving the performance of single-junction GaAs solar cells by incorporating InGaAs/GaAs quantum wells and quantum dots to increase their spectral response bandwidth, and by incorporating structures that confine light in the devices to improve their absorption of it. InGaAs/GaAs quantum dots-in-wells extend the response of GaAs homojunction devices to wavelengths >1200 nm. Nanoparticles that are randomly deposited on the top of optically thick devices scatter light into waveguide modes of the device structures, increasing their absorption of electromagnetic energy and improving their short-circuit current by up to 16%. Multiply periodic diffractive structures have been optimized using rigorous software algorithms and fabricated on the back sides of thin film quantum dot-in-well solar cells, improving their spectral response at wavelengths 850 nm to 1200 nm, where only the quantum dot-in-well structures absorb light, by factors of up to 10. The improvement results from coupling of diffracted light to waveguide modes of the thin film device structure, and from Fabry-Perot interference effects. Simulations of absorption in these device structures corroborate the measured results and indicate that quantum well solar cells of ~2 µm in thickness, and which are equipped with optimized backside gratings, can achieve 1 Sun Airmass 0 short-circuit current densities of up to ~5 mA/cm2 (15%) greater than GaAs homojunction devices, and of up to >2 mA/cm2 (7%) greater than quantum well devices, with planar back reflectors. A combination of Fabry-Perot interference and diffraction into waveguide modes of the thin devices is shown to dominate the simulated device response spectra. Simulations also demonstrate the importance of low-loss metals for realizing optimal light trapping structures. Such device geometries are promising for reducing the cost of high efficiency solar cells that may be suitable for a variety of traditional and emerging applications.Item Colloidal nanocrystal assemblies : self-organization, properties, and applications in photovoltaics(2011-12) Goodfellow, Brian William; Korgel, Brian Allan, 1969-; Chelikowsky, James R; Dodabalapur, Ananth; Ekerdt, John G; Vanden Bout, David AColloidal nanocrystal assemblies offer an attractive opportunity for designer metamaterials. The ability to permute chemical composition, size, shape, and arrangement of nanocrystals leads to an astounding number of unique materials properties that find use in an extensive array of applications---ranging from solar cells to medicine. However, to take full advantage of these materials in useful applications, the nature of their assembly and their behavior under external stimuli must be well understood. Additionally, the assembly of colloidal nanocrystals into thin films provides a promising pathway to the solution-processing of inorganic materials that are prohibitively too expensive and/or difficult to deposit by conventional methods. Nanocrystal superlattices (NCSLs) of sterically stabilized nanocrystals were assembled by slow evaporation of colloidal dispersions on various substrates. Detailed analysis of the NCSL structures was carried out using transmission and scanning electron microscopy (TEM and SEM) and small-angle x-ray scattering (SAXS). Body-centered cubic (bcc) NCSLs, in particular, were studied in detail and ligand packing frustration was proposed as a significant driving force for their assembly. The behavior of NCSLs was also studied by SAXS under mild heating and solvent vapor exposure revealing several remarkable order-order, order-disorder, and amorphous-crystalline structural transitions. Colloidal Cu(In [subscript 1-x] Ga [subscript x])Se₂ (CIGS) nanocrystals were synthesized by arrested precipitation and formulated into inks. These inks were spray deposited into thin films under ambient conditions to serve as the active light absorbing material in printed low-cost photovoltaic (PV) devices. These devices, which were fabricated without the need for high temperature processes, have achieved power conversion efficiencies above 3 % under AM1.5 illumination. While the efficiencies of these devices are still too low for commercial viability, this work does provide a proof of concept that reasonable efficient solar cells can be created with a low-cost printable process using nanocrystal inks. Since high temperatures are not used to form the light-absorbing layer, nanocrystal-based solar cells were built on flexible light weight plastic substrates. The main obstacle to achieving high power conversation efficiencies was found to be the ability to extract the photo induced charge carriers. Nanocrystal films suffer from poor transport that leads to high recombination rates in thicker films. To date, the best efficiencies have been achieved with thin light absorber layers that only absorb a fraction of the incident light.Item Design of vibrational and solar energy harvesting systems for powering wireless sensor networks in bridge structural health monitoring applications(2014-12) Adams, Jacob Allan; Crawford, Richard H.Structural health monitoring systems provide a promising route to real-time data for analyzing the current state of large structures. In the wake of two high-profile bridge collapses due to an aging highway infrastructure, the interest in implementing such systems into fracture-critical and structurally deficient bridges is greater now than at any point in history. Traditionally, these technologies have not been cost-effective as bridges lack existing wiring architecture and the addition of this is cost prohibitive. Modern wireless sensor networks (WSN) now present a viable alternative to traditional networking; however, these systems must incorporate localized power sources capable of decade-long operation with minimal maintenance. To this end, this thesis explores the development of two energy harvesting systems capable of long-term bridge deployment with minimal maintenance. First, an electromagnetic, linear, vibrational energy harvester is explored that utilizes the excitations from passing traffic to induce motion in a translating permanent magnet mass. This motion is then converted to electrical energy using Faraday’s law of induction. This thesis presents a review of vibrational energy harvesting literature before detailing the process of designing, simulating, prototyping, and testing a selected design. Included is an analysis of the effects of frequency, excitation amplitude, load, and damping on the power production potential of the harvester. Second, a solar energy harvester using photovoltaic (PV) panels is explored for powering the critical gateway component of the WSN responsible for data aggregation. As solar energy harvesting is a more mature technology, this thesis focuses on the methodologies for properly sizing a solar harvesting system and experimentally validating the selected design. Fabrication of the prototype system was completed and field testing was performed in Austin, TX. The results validate the selected system’s ability to power the necessary 14 W DC load with a 0° panel azimuth angle (facing direct south) and 45° tilt.Item EtherLux, a low power wireless display(2009-12) Hocker, Andrew Edward; Aziz, Adnan; McDermott, MarkReal time information is essential in many businesses and as a method to inform employees and consumers, so that they can make informed decisions. In offices, warehouse and stores it can be advantageous to have tens to hundreds of smaller displays to deliver a variety of information. This paper details the design, implementation and testing of a wireless low power solar powered display system as a solution to deliver real time information. The system uses an Organic LCD to maintain an image for years on no power and uses very little power to update and refresh the display. The system uses off- the-shelf components to achieve multiple updates per day and, with the right lighting conditions, can perform up to one refresh per minute. The system is entirely powered by incandescent light, has a built in radio, and utilizes capacitors to store charge and deliver power, removing the need for rechargeable batteries. The wireless signal works at 2.4GHz and uses the low power 802.15.4 protocol to send and receive data at a range of 75 feet. It has no observable issue operating in environments with 2.4GHz wireless signals, such as 802.11g. The whole system can be built for under $75.00, and takes up an area of 6" x 8" including the photovoltaic cells.Item Innovation in the U.S. solar industry : a review of patent activity in solar photovoltaic inverters and mounting systems(2012-12) Metteauer, Maureen O’Donnell; Rai, Varun; Gamkhar, ShamaThis report examined patent activity in two areas of the solar photovoltaic industry — inverters and mounting systems — and suggests that market creation policies contributed to innovation within these solar supply chain component areas. Relying on patent counts, the report found that patent activity for inverters and mounting systems increased substantially between 2005 and 2012. Drawing on economic research and innovation theory, the report asserts that market creation policies explain how the solar industry shifted its innovation focus toward improving downstream technology in the solar “balance of system,” creating opportunity for endogenous growth within the industry.Item Maximum power point tracking using ripple correlation control with an interleaved SEPIC converter for photovoltaic applications(2010-05) Maddur Chandrash, Harsha Kumar; Kwasinski, Alexis; Grady, William M.This thesis examines the use of ripple correlation control as a maximum power point tracking algorithm with an interleaved SEPIC converter for use with a solar array. The suitability of existing topologies for use with photovoltaic applications and the tradeoffs involved are discussed. The advantages of interleaving in converters are examined and the benefits it provides to photovoltaic applications are discussed. An interleaved SEPIC converter operated in interleaved mode with a photovoltaic array is studied. The operation of ripple correlation control as a maximum power point tracking technique applied to the interleaved SEPIC converter is examined and simulations with results are presented.Item Methods development and measurements for understanding morphological effects on electronic and optical properties in solution processable photovoltaic materials(2012-12) Ostrowski, David Paul; Vanden Bout, David A.; Rossky, Peter J; Holliday, Bradley J; Korgel, Brian A; Dodabalapur, Ananth JThe effects of morphology on electronic and optical properties in solution processable photovoltaic (PV) materials have been studied through two different approaches. One approach, scanning photocurrent (PC) and photoluminescence (PL) microscopy, involved mapping PC generation and PL in functional PV devices on the length scale of around 250-500 nm. Additionally, local diode characteristics were studied from regions of interest in the PV through local voltage-dependent photocurrent (LVPC) measurements. In a PV made from a Copper Indium Gallium Selenide (CIGS) nanocrystal (NC) "ink", two morphological features were found to cause the spatial heterogeneity in PC generation. Cadmium Sulfide (CdS) aggregates lowered PC generation by blocking incident light to the photoactive layer, and cracks in the CIGS-NC film enhanced PC generation through improved charge carrier extraction. LVPC measurements showed all regions to have similar diode characteristics with the main difference being the PC generated at zero bias voltage. For another PV made from a donor/acceptor blend of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,Nphenyl- 1,4-phenylenediamine (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole)(F8BT), two incident laser wavelengths were used to selectively illuminate only one or both polymers. The results showed that when F8BT is illuminated, the PFB-rich regions produced the most PC and when both polymers are illuminated (but mostly PFB), the F8BT-rich regions produce the most PC; showing PC generation is more affective when less absorber material is present in the morphology. The other approach to study morphological effects on PV properties was to fabricate particles that mimicked morphological variations known to occur in solution-processable PVs. Through solution processing of an oligothiophene molecule, a range of weakly coupled H-aggregate particles were made. These particles, identifiable by shape, were shown to have a varying degree of energetic disorder (as gauged by the 0-0 vibronic band intensity in the emission spectrum), despite all particles showing a similarly high degree of molecular order from fluorescence dichroism (FD) measurements. A trend was observed correlating a decrease in energetic disorder with an increase in the local contact potential (LCP) difference as measured with Kelvin probe force microscopy (KPFM). The LCP difference was found to range by 70 mV between particles of moderate to low energetic disorder.Item Photovoltaic devices based on Cu(In1-xGax)Se2 nanocrystal inks(2011-08) Akhavan, Vahid Atar; Korgel, Brian Allan, 1969-; Ekerdt, John G.; Mullins, Charles B.; Ellison, Christopher J.; Dodabalapur, AnanthThin film copper indium gallium selenide (CIGS) solar cells have exhibited single junction power conversion efficiencies above 20% and have been commercialized. The large scale production of CIGS solar cells, however, is hampered by the relatively high cost and poor stoichiometric control of coevaporating tertiary and quaternary semiconductors in high vacuum. To reduce the overall cost of production, CIGS nanocrystals with predetermined stoichiometry and crystal phase were synthesized in solution. Colloidal nanocrystals of CIGS provide a novel route for production of electronic devices. Colloidal nanocrystals combine the well understood device physics of inorganic crystalline semiconductors with the solution processability of amorphous organic semiconductors. This approach reduces the overall cost of CIGS manufacturing and can be used to fabricate solar cells on flexible and light-weight plastic substrates. As deposited CIGS nanocrystal solar cells were fabricated by ambient spray-deposition. Devices with efficiencies of 3.1% under AM1.5 illumination were fabricated. Examining the external and internal quantum efficiency spectrums of the devices reveal that in nanocrystal devices only the space charge region is actively contributing to the extracted photocurrent. The device efficiency of the as-deposited nanocrystal films is presently limited by the small crystalline grains (≈ 15 nm) in the absorber layer and the relatively large interparticle spacing due to the organic capping ligands on the nanocrystal surfaces. Small grains and large interparticle spacing limits high density extraction of electrons and holes from the nanocrystal film. A Mott-Schottky estimation of the space charge region reveals that only 50 nm depth of the nanocrystalline absorber is effectively contributing to the photogenerated current. One strategy to improve charge collection involves increased space charge region for extraction by vertical stacking of diodes. A much longer absorption path for the photons exists in the space charge region with the stacked devices, increasing the probability that the incident radiation is absorbed and then extracted. This method enables an increase in the collected short circuit current. The overall device efficiency, however, suffers with the increased series resistance and shunt conductance of the device. Growth of nanocrystal grains was deemed necessary to achieve power conversion efficiencies comparable to vapor deposited CIGS films. Simple thermal treatment of the nanocrystal layers did not contribute to the growth of the crystalline grain size. At the same time, because of the loss of selenium and increased trap density in the absorber layer, there was a measurable decrease in device efficiency with thermal processing. For increased grain size, the thermal treatment of the absorber layer took place in presence of compensating amounts of selenium vapor. The process of selenization, as it is called, took place at 500°C in a graphite box and led to an increase of the grain size from 15 nm to several microns in diameter. Devices with the increased grain size yielded efficiencies up to 5.1% under AM1.5 radiation. Mott-Schottky analysis of the selenized films revealed a reduction in doping density and a comparable increase in the space-charge region depth with the increased grain size. The increased collection combined with the much higher carrier mobility in the larger grains led to achieved Jsc values greater than 20 mA/cm2. Light beam induced current microscopy (LBIC) maps of the devices with selenized absorber layers revealed significant heterogeneity in photogenerated current. Distribution of current hotspots in the film corresponded with highly selenized regions of the absorber films. In an effort to improve the overall device efficiency, improvements in the selenization process are necessary. It was determined that the selenization procedure is dependent on the selenization temperature and processing environment. Meanwhile, the reactor geometry and nanocrystal inks composition played important roles in determining selenized film morphology and the resulting device efficiency. Further work is necessary to optimize all the parameters to improve device efficiency even further.Item Scanning probe microscopy study of thin film solar cells(2014-08) Li, Huan, Ph. D.; Shih, Chih-Kang; de Lozanne, Alejandro; Markert, John; Sitz, Greg; Tutuc, EmanuelThin film solar cells, such as CdTe, CuIn [subscript x] Ga [subscript 1-x] Se₂ (CIGS), Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe), have been intensively studied for their unique features and excellent prospect of mass production in industry. The p-n junction is the most critical part of the thin film solar cell and greatly influences the performance. In this thesis work, the p-n junctions and the device layers of multiple kinds of thin film solar cells have been studied by using scanning probe microscopy based techniques. The scanning spreading resistance microscopy (SSRM) has been developed on the cross-section of CdTe solar cells to study the resistance and carrier concentration distribution in different layers of the device. The CdTe sample was cleaved and milled with the argon ion beam to get a flat cross-section. The multiple device layers of the device were identified by the resistance mapping. A high-resistance region around the junction on the CdTe side due to carrier depletion was measured. With the AFM laser illumination, the resistance in the deep depletion region dropped and the resistance across the entire CdTe layer became relatively uniform due to domination of photo-excited carriers. With carriers injected by applying a forward-bias voltage to the working device, the resistance in the deep depletion region decreased and the region moved toward the CdS/CdTe interface. These observed trends and observations are consistent with device physics. We also measured the surface potential and the electric field across the junction using scanning Kelvin probe force microscopy (SKPFM) in the cross-section of the standard CIGS, ZnS(O,OH)/CIGS and the standard CZTSe devices. Both the heterojunction and homojunction situations of the three solar cells were simulated using the PC1D software. The simulation results were compared with the experimental results to analyze the properties of the junction. The comparison results provided the possible ranges of the thickness and carrier concentration of n-CIGS/n-CZTSe layer.Item Single-Phase Inverter and Rectifier for High-Reliability Applications(2014-05-01) Harb, SouhibWith the depletion of fossil fuels and skyrocketed levels of CO_(2) in our atmosphere, Renewable Energy Resources, generated from natural, sustained, clean, and domestic resources, have caught the eye in recent years of both the industries and governments worldwide. In addition to finding these energy resources, new technologies are being sought to improve the efficiency of consuming the generated energy. Power Electronics is the key technology for both generation and the efficient consumption of energy. The recent trend in power electronics is to integrate the electronics into the source (Photovoltaic (PV)) or the load (light). For PV and outdoor lighting applications, this imposes a harsh, wide-range operating environment on the power electronics. Thus, the reliability of power electronics converters becomes a very crucial issue. It is required that the power electronics, used in such environments, have reliability indices, such as lifetime, which match with the source or load one. This eliminates the reoccurring cost of power electronics replacement. Relatively high efficiencies have been reported in the literature, and standards have been developed to measure it. However, the reliability aspect has not received the same level of scrutiny. In this study, two main aspects have been investigated: (1) A new methodology to evaluate the integrated power electronics that becomes more involved task; and (2) new topology and control schemes, for the single-phase DC/AC and AC/DC converters, which will improve the reliability. The proposed methodology has been applied for different PV Module-Integrated-Inverter (MII) that employs different power decoupling techniques. The results showed that the decoupling capacitor is the limiting lifetime component in all the studied topologies. Moreover, topologies use film capacitor instead of electrolytic capacitor showed an order of magnitude improvement in the lifetime. This clearly suggests that replacing the electrolytic capacitor by a high-reliability film capacitor will enhance the reliability of the PV MII. In the second part of this study, the ripple-port concept is applied for the single-phase DC/AC inverter and AC/DC rectifier, which allows for the usage of the minimum required decoupling capacitance. In conclusion, film capacitor can be used, which led to the improvement of the overall reliability and lifetime.Item Structural optimization for a photovoltaic vehicle(2011-05) Ford, Bennett Alan 1984-; O'Connor, James ThomasPhotovoltaic vehicles are designed to harness solar energy and use it for self-propulsion. In order to collect sufficient energy to propel a passenger, a relatively large photovoltaic array is required. Controlling the loads imparted by the array and the body that supports it, while protecting the passenger and minimizing vehicle weight, presents a unique set of design challenges. Weight considerations and geometric constraints often lead system designers toward unconventional structural solutions. This report details analytical and experimental processes aimed at proving the concept of integrating aluminum space-frame elements with composite panels. Finite element analysis is used to simulate load conditions, and results are compared with empirical test data.Item Synthesis and characterization of semiconductor thin films for photoelectrochemical energy conversion(2012-08) Hahn, Nathan Taylor; Mullins, Charles Buddie; Bard, Allen J.; Hwang, Gyeong S.; Stevenson, Keith J.; Johnston, Keith P.The field of solar energy conversion has experienced resurgence in recent years due to mounting concerns related to fossil fuel consumption. The sheer quantity of available solar energy and corresponding opportunity for technological improvement has motivated extensive study of novel light-absorbing semiconductors for solar energy conversion. Often, these studies have focused on new ways of synthesizing and altering thin film semiconductor materials with unique compositions and morphologies in order to optimize them for higher conversion efficiencies. In this dissertation, we discuss the synthesis and electrochemical characterization of a variety of candidate semiconductor materials exhibiting promising characteristics for photoelectrochemical solar energy conversion. Three specific methods of thin film deposition are detailed. The first is a physical vapor deposition technique used to independently tune the morphology and composition of hematite (α-Fe2O3) based materials. Because of hematite’s poor electronic properties, these modifications were able to significantly improve its performance as a photoanode for water oxidation. The second technique is electrodeposition, which was employed to deposit the novel ternary metal oxide, CuBi2O4. The study of these films, along with those prepared by physical vapor deposition, provided insight into the factors limiting the ability of this photo-active material to function as a photocathode for hydrogen evolution from water. The third technique is chemical spray pyrolysis, which was employed to deposit and optimize films of the bismuth chalco-halides BiOI and BiSI. These studies were used to obtain previously unknown properties of these materials relevant to their utilization in photoelectrochemical cells. The manipulation of deposition temperature had significant effects on these properties and dictated the films’ overall photoconversion performance.