Browsing by Subject "Solar"

Now showing 1 - 14 of 14

Assembly and characterization of quantum-dot solar cells
(2009-09) Leschkies, Kurtis Siegfried
Environmentally clean renewable energy resources such as solar energy have gained significant attention due to a continual increase in worldwide energy demand. A variety of technologies have been developed to harness solar energy. For example, photovoltaic (or solar) cells based on silicon wafers can convert solar energy directly into electricity with high efficiency, however they are expensive to manufacture, and thus unattractive for widespread use. As the need for low-cost, solar-derived energy becomes more dire, strategies are underway to identify materials and photovoltaic device architectures that are inexpensive yet efficient compared to traditional silicon solar cells. Nanotechnology enables novel approaches to solar-to-electric energy conversion that may provide both high efficiencies and simpler manufacturing methods. For example, nanometer-size semiconductor crystallites, or semiconductor quantum dots (QDs), can be used as photoactive materials in solar cells to potentially achieve a maximum theoretical power conversion efficiency which exceeds that of current mainstay solar technology at a much lower cost. However, the novel concepts of quantum dot solar cells and their energy conversion designs are still very much in their infancy, as a general understanding of their assembly and operation is limited. This thesis introduces various innovative and novel solar cell architectures based on semiconductor QDs and provides a fundamental understanding of the operating principles that govern the performance of these solar cells. Such effort may lead to the advancement of current nanotechnology-based solar power technologies and perhaps new initiatives in nextgeneration solar energy conversion devices. We assemble QD-based solar cells by depositing photoactive QDs directly onto thin ZnO films or ZnO nanowires. In one scheme, we combine CdSe QDs and singlecrystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell (QDSSC). An array of ZnO nanowires was grown vertically from a fluorine-doped-tinoxide conducting substrate and decorated with an ensemble of CdSe QDs, capped with mercaptopropionic acid. When illuminated with visible light, the CdSe QDs absorb photons and inject electrons into the ZnO nanowires. The morphology of the nanowires then provided these photoinjected electrons with a direct and efficient electrical pathway to the photoanode. When using a liquid electrolyte as the hole transport medium, our quantum-dot-sensitized nanowire solar cells exhibited short-circuit current densities up to 2.1 mA/cm2 and open-circuit voltages between 0.6–0.65 V when illuminated with 100 mW/cm2 of simulated AM1.5 light. Our QDSSCs also demonstrated internal quantum efficiencies as high as 50–60%, comparable to those reported for dye-sensitized solar cells made using similar nanowires. We found that the overall power conversion efficiency of these QDSSCs is largely limited by the surface area of the nanowires available for QD adsorption. Unfortunately, the QDs used to make these devices corrode in the presence of the liquid electrolyte and QDSSC performance degrades after several hours. Consequently, further improvements on the efficiency and stability of these QDSSCs required development of an optimal hole transport medium and a transition away from the liquid electrolyte. Towards improving the reliability of semiconductor QDs in solar cells, we developed a new type of all-solid-based solar cell based on heterojunctions between PbSe QDs and thin ZnO films. We found that the photovoltage obtained in these devices depends on QD size and increases linearly with the QD effective bandgap energy. Thus, these solar cells resemble traditional photovoltaic devices based on a semiconductor– semiconductor heterojunction but with the important difference that the bandgap energy of one of the semiconductors, and consequently the cell’s photovoltage, can be varied by changing the size of the QDs. Under simulated 100 mW/cm2 AM1.5 illumination, these QD-based solar cells exhibit short-circuit current densities as high as 15 mA/cm2 and open-circuit voltages up to 0.45 V, larger than that achieved with solar cells based on junctions between PbSe QDs and metal films. Moreover, we found that incident-photonto- current-conversion efficiency in these solar cells can be increased by replacing the ZnO films with a vertically-oriented array of single crystal ZnO nanowires, separated by distances comparable to the exciton diffusion length, and infiltrating this array with colloidal PbSe QDs. In this scheme, photogenerated excitons can encounter a donor– acceptor junction before they recombine. Thus, we were able to construct solar cells with thick QD absorber layers that were still capable of efficiently extracting charge despite short exciton or charge carrier diffusion lengths. When illuminated with the AM1.5 spectrum, these nanowire-based quantum-dot solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin film ZnO devices constructed with the same amount of QDs. Supporting experiments using field-effect transistors made from the PbSe QDs as well as the sensitivity of these transistors to nitrogen and oxygen gas show that the solar cells described above are unlikely to be operating like traditional p–n heterojunction solar cells. All data, including significant improvements in both photocurrent and power conversion efficiency with increasing nanowire length, suggest that these photovoltaic devices operate as excitonic solar cells.
Characterization of pi-conjugated polymers for transistor and photovoltaic applications
(2012-12) Paulsen, Bryan D.
pi-Conjugated polymers represent a unique class of optoelectronic materials. Being polymers, they are solution processable and inherently "soft" materials. This makes them attractive candidates for the production of roll-to-roll printed electronic devices on flexible substrates. The optical and electronic properties of pi-conjugated polymers are synthetically tunable allowing material sets to be tailored to specific applications. Two of the most heavily researched applications are the thin film transistor, the building block of electronic circuits, and the bulk heterojunction solar cell, which holds great potential as a renewable energy source. Key to developing commercially feasible pi-conjugated polymer devices is a thorough understanding of the electronic structure and charge transport behavior of these materials in relationship with polymer structure. Here this structure property relationship has been investigated through electrical and electrochemical means in concert with a variety of other characterization techniques and device test beds. The tunability of polymer optical band gap and frontier molecular orbital energy level was investigated in systems of vinyl incorporating statistical copolymers. Energy levels and band gaps are crucial parameters in developing efficient photovoltaic devices, with control of these parameters being highly desirable. Additionally, charge transport and density of electronic states were investigated in pi-conjugated polymers at extremely high electrochemically induced charge density. Finally, the effects of molecular weight on pi-conjugated polymer optical properties, energy levels, charge transport, morphology, and photovoltaic device performance was examined.
Computational and experimental studies of dye sensitized solar cells
(2013-09) Vatassery, Rajan Nayar
The dye-sensitized solar cell (DSSC) has been studied by observing charge transfer from an organic terthiophene dye into a CdS nanoparticle. Using NMR and UV-Vis, we find characteristics of dye aggregation and a concomitant reduction in the electron transfer efficiency as measured by ultrafast transient absorption (TA) spectroscopy. Specifically, the NMR and UV-Vis spectra of the dye molecules indicate aggregates are readily formed at high surface loading, or roughly a 20:1 dye:nanoparticle ratio. Upon analysis by TA spectroscopy the same samples show a dominant S₁ state quenching process separate from the expected intersystem crossing and electron transfer (ET) S₁ quenching pathways. We propose that the dominant process is concentration-quenching because it only appears at high surface coverage where aggregates are detected spectroscopically; at lower surface coverage (ratios of dye:nanoparticle of roughly 1:1) the ET mechanism is the dominant pathway for S₁ reduction and the parasitic concentration-quenching pathway is not observed. We therefore suggest that planar oligothiophene dyes should be modified to frustrate packing on the surface in an effort to avoid concentration quenching losses, or that dye loading be considered when creating a DSSC from planar dye molecules. Classical molecular dynamics (MD) simulations are also presented to corroborate the experimental picture described above. These simulations show that dyes aggregate in a variety of orientations, and that dye molecules are stabilized by these aggregation events even in the presence of explicit solvent. The ability of the dye molecules to pack more densely than is found experimentally shows that the surface of the CdS nanoparticle is likely undersaturated. In this situation, dye molecules can be either uniformly distributed around the surface of the nanoparticle, or they can be concentrated in islands on certain crystallographic faces, leaving other faces unoccupied. The experimental signs of aggregation support the latter.
Design and characterization of a concentrating solar simulator.
(2012-08) Krueger, Katherine R.
A concentrating solar simulator is a laboratory-scale tool that is useful in the development of processes to generate solar fuels. Such a device, which produces a concentrated radiative output replicating that of a solar dish, has been designed, built, and characterized at the University of Minnesota to facilitate the testing of prototype solar receivers and reactors. The concentrating solar simulator consists of seven commonly-focused radiation units, each consisting of a xenon arc lamp close-coupled to a reflector in the shape of an ellipsoid of revolution. A systematic design procedure has been developed as part of this work, which involves determining the location and orientation of each of the radiation units by requiring that the target focal points of all reflectors coincide. A set of unique geometric relations have been developed that ensure this requirement on a general scale and provide the framework for further designs by allowing the specification of detailed practical requirements dealing with the space available and manufacturability concerns. After the location and orientation of each of the lamp-reflector modules is established, the shape of the reflector is optimized with the use of a Monte Carlo ray tracing model. The shape of the ellipsoidal reflector is varied by the eccentricity, and sensitivity analyses are carried out to determine the effect of the reflector specular error and effective arc size and shape on the resulting flux distribution and magnitude. The completed facility consists of a dual enclosure specially designed to protect the researchers and the simulator, the array of lamps and reflectors, and the electrical systems necessary to power and control the lamps. The radiative output of the solar simulator constitutes the energy input to the prototype solar receivers and reactors, and therefore must be well-characterized. The output has been measured with respect to its spatial and temporal variations by using an optical technique in which a CCD camera views radiation reflected from a water-cooled Lambertian target through neutral density filters and a lens. The image recorded by the camera is calibrated such that the recorded grayscales correspond to measured values of incident radiative flux, as measured by a circular foil heat flux gage that has been calibrated in-house. Using this method, it was determined that the solar simulator can output up to 9.2 0.4 kW of thermal power to a focal area 60 mm in diameter, corresponding to an average flux of 3240±390 kW m-2. The peak flux, as averaged over a 10 mm diameter focal area, is 7300±890 kW m-2. The UMN solar simulator facility is an excellent tool for testing prototype solar receivers and reactors on a laboratory scale.
The design of a calorimeter to measure concentrated solar flux
(2013-04) Sefkow, Elizabeth Anne Bennett
A water-cooled, cavity calorimeter was designed to accurately measure concentrated solar thermal power produced by the University of Minnesota's solar simulator. The cavity is comprised of copper tubing bent into spiral and helical coils for the base and cylindrical walls, respectively. Insulation surrounds the cavity to reduce heat transfer to the ambient, and a water- cooled aperture cover is positioned at the open end of the cavity. The calorimeter measures the heat gain of water flowing through the system as radiant energy is passed through the aperture. Chilled water flows through the tubing, and the energy incident on the cavity surface is conducted through the wall and convected to the flowing water. The energy increase in the water can be observed by an increase in fluid temperature. A Monte Carlo ray tracing method is used to predict the incident flux distribution and corresponding power on the surfaces of the cavity. These values are used to estimate the thermal losses of the system, and it is found that they account for less that 1% of the total power passed through the aperture. The overall uncertainty of the calorimeter is found by summing the measured uncertainty and the estimated heat loss and is found to be ±2.5% for 9.2 kW of power output and ±3.4% for 3 kW.
Electron transport and recombination in nanowire dye-sensitized solar cells.
(2010-02) Enache-Pommer, Emil
The dye-sensitized solar cell (DSSC) is a promising low cost photovoltaic device. A typical DSSC consists of a porous film made out of TiO2 nanoparticles, a monolayer of dye adsorbed on the TiO2 surface and a liquid electrolyte. The electrolyte fills the pores of the nanoparticle film forming a semiconductor-dye-electrolyte interface with large surface area. During illumination of the cell, the dye molecules inject electrons into the TiO2 nanoparticles. The injected electrons diffuse through the nanoparticle network by hopping from particle to particle until they are collected at a transparent conductive oxide (TCO) anode. Meanwhile, the charged dye molecules are reduced through an electrochemical reaction with a reductant in the electrolyte. The oxidized ionic species diffuse to the counter electrode and are reduced by electrons that have been collected at the anode and have traveled through the load to complete the circuit. Currently, dye-sensitized solar cells have reached efficiencies above 11 %, but further improvement is limited by electrons recombining with the electrolyte during their transport through the semiconductor nanoparticle network. Nanowire DSSCs have been recently introduced and have the potential to overcome the limitations of nanoparticle DSSCs, since the electron percolation through the nanoparticle network is replaced by a direct electron pathway from the point of injection to the TCO. Understanding the electron transport and recombination mechanisms in nanowire DSSCs is one of the key steps to improving DSSC efficiency. Towards this end polycrystalline TiO2, single-crystalline TiO2 and single crystalline ZnO nanowire DSSCs were fabricated and analyzed using current-voltage characteristics, optical measurements, and transient perturbation techniques such as intensity modulated photocurrent spectroscopy, photocurrent decay and open-circuit photovoltage decay. For single-crystal ZnO nanowire DSSCs, the measured electron transport time constants are independent of light intensity but change with nanowire length, seeding method and annealing time. Even if the measured transients are limited by the RC time constant of the solar cell, using the measured time constants as an upper limit for the actual electron transport time leads to the conclusion that the electron transport rate in ZnO nanowires is at least two orders of magnitude faster than the recombination rate. This indicates that the charge collection efficiency in ZnO nanowire DSSCs is nearly 100 %. These results show that films can be made out of 100 μm long ZnO nanowires while maintaining efficient charge collection. For DSSCs based on polycrystalline anatase TiO2 nanowires, the electron transport times show a power-law dependence on illumination intensity similar to that reported for TiO2 nanoparticle DSSCs. The magnitude of the electron transport times is also comparable to that of nanoparticle DSSCs, indicating that electron trapping and detrapping determine transport times for polycrystalline TiO2 nanowire DSSCs. Surprisingly, even for single-crystal rutile TiO2 nanowire DSSCs, the electron transport rate is on the order of the electron transport rate in nanoparticle-based DSSCs and not as fast as would be expected. Electron transport is slow and light intensity dependent indicating that trapping and detrapping, most likely in surface traps, still play an important role in electron transport even in single-crystal rutile TiO2 nanowires.
Energy Migration in Organic Thin Films—From Excitons to Polarons
(2016-04) Mullenbach, Tyler
The rise of organic photovoltaic devices (OPVs) and organic light-emitting devices has generated interest in the physics governing exciton and polaron dynamics in thin films. Energy transfer has been well studied in dilute solutions, but there are emergent properties in thin films and greater complications due to complex morphologies which must be better understood. Despite the intense interest in energy transport in thin films, experimental limitations have slowed discoveries. Here, a new perspective of OPV operation is presented where photovoltage, instead of photocurrent, plays the fundamental role. By exploiting this new vantage point the first method of measuring the diffusion length (LD) of dark (non-luminescent) excitons is developed, a novel photodetector is invented, and the ability to watch exciton arrival, in real-time, at the donor-acceptor heterojunction is presented. Using an enhanced understanding of exciton migration in thin films, paradigms for enhancing LD by molecular modifications are discovered, and the first exciton gate is experimentally and theoretically demonstrated. Generation of polarons from exciton dissociation represents a second phase of energy migration in OPVs that remains understudied. Current approaches are capable of measuring the rate of charge carrier recombination only at open-circuit. To enable a better understanding of polaron dynamics in thin films, two new approaches are presented which are capable of measuring both the charge carrier recombination and transit rates at any OPV operating voltage. These techniques pave the way for a more complete understanding of charge carrier kinetics in molecular thin films.
Optical properties of Iridium(III) cyclometalates:excited state interaction with small molecules and dynamics of light-harvesting materials.
(2012-08) Schwartz, Kyle Robert
The research presented in this thesis concerns the use and understanding of luminescent Ir(III) cyclometalates. Areas of research involve the design, synthesis, and characterization of novel luminescent Ir(III) cyclometalates, including photophysical investigation of their phosphorescent excited states using steady-state and time resolved absorption/luminescence spectroscopies. This broad research description may be further separated into two subareas: study of excited state interaction with small molecules and excited-state dynamics of metal-organic light harvesting dyads. The first chapter of this thesis examines the interaction of Ir(III) cyclometalates with the small molecule carbon dioxide (CO2). It has been the goal of investigators to develop methods for direct optical detection of CO2. This has been difficult as CO2 is considered chemically inert and there are few luminescent probes directly sensitive to CO2. Most optical detection schemes previously developed for CO2 use indirect detection methods, which rely upon measuring changes in pH brought about by hydrolysis of CO2. Research efforts to design a reliable method for the direct optical detection of CO2 were accomplished through development of a system where hydrazine, a simple amino ligand, when coupled into the coordination sphere of an Ir(III) cyclometalate reacts with CO2. The result of this reaction provides a significant shift in the luminescence λmax of the phosphorescent probe, a previously unobserved optical response for the direct detection of CO2. The second chapter investigates phosphorescent excited states and their ability to function as triplet sensitizers for the generation of singlet oxygen (1O2) and luminescent probes for molecular oxygen (O2) concentration. Interaction of phosphorescent excited states with O2 results in energy transfer from the luminescent probe to O2, quenching the phosphorescent excited state. Energy transfer also generates the reactive oxygen species (ROS) 1O2. We have used this duality to develop an analytical methodology to follow the serendipitously discovered photoreactivity of 1O2 with common organic solvent dimethyl sulfoxide (DMSO) using the luminescence profile of Ir(III) and Ru(II) phosphors. Reaction of the triplet sensitized 1O2 with a photooxygenation substrate results in the consumption of O2 from the system and an increase in the observed luminescence intensity. Detailed kinetic investigations of the luminescence recovery and O2 depletion were preformed on air-saturated closed cell systems. Determinations of the quantum efficiencies for the photooxygenation system were performed and differences in choice of triplet sensitizer discussed. Study of 1O2 reactivity with substrates of biological and environmental relevance using this methodology should provide an additional tool to understand better oxidative damage induced by 1O2 within these systems. In chapter three a detailed study involving the design, synthesis, and characterization of the electrochemical and phototophysical properties of Ir(III) cyclometalates with pendant terthiophenes as secondary organic chromophores is presented. The interplay of the excited states between each chromophore represents an interesting photoredox active system for energy-to-light or light-to-energy devices. Greater knowledge of the primary photophysical events within these complexes will provide a better understanding of how energy moves in these hybrid systems after light absorption, leading to increased device efficiency.
Rooftop Solar Photovoltaics: The Untapped Potential of Commercial and Industrial Buildings
(Hubert H. Humphrey School of Public Affairs, 2014-05-14) Risse, William; Rivera, Lloyd; Miller, Caroline; Evans, David; Appleby, Elizabeth
Solar Development of Farmland in Minnesota: Mapping scenarios based on ten metrics at the nexus of solar and agriculture
(2020-05-18) Ingulsrud, Alex
The growing use of farmland for solar is an emerging energy policy issue. Solar has opportunity costs to farming, but pollinator friendly solar provides valuable ecosystem services for cleaning up the land use footprint of farming. Policymakers in Minnesota have addressed the issue of site design with pollinator friendly standards. Site location, however, remains an open question, both in the literature and policy sphere. Policymakers should work with utilities to plan the next generation of transmission infrastructure in where land has greater expected net social benefits of solar. To try find where, I have gathered a GIS dataset of farmland in Minnesota and used ten variables, or metrics, to construct and map scenarios of hypothetical solar development by 2030. Scenarios approximate where solar may be developed, if only one factor mattered.
Solar gasification of biomass: design and characterization of a molten salt gasification reactor
(2013-12) Hathaway, Brandon Jay
The design and implementation of a prototype molten salt solar reactor for gasification of biomass is a significant milestone in the development of a solar gasification process. The reactor developed in this work allows for 3 kWth operation with an average aperture flux of 1530 suns at salt temperatures of 1200 K with pneumatic injection of ground or powdered dry biomass feedstocks directly into the salt melt.Laboratory scale experiments in an electrically heated reactor demonstrate the benefits of molten salt and the data was evaluated to determine the kinetics of pyrolysis and gasification of biomass or carbon in molten salt. In the presence of molten salt overall gas yields are increased by up to 22%; pyrolysis rates double due to improved heat transfer, while carbon gasification rates increase by an order of magnitude. Existing kinetic models for cellulose pyrolysis fit the data well, while carbon gasification in molten salt follows kinetics modeled with a 2/3 order shrinking-grain model with a pre-exponential factor of 1.5*106 min-1 and activation energy of 158 kJ/mol.A reactor concept is developed based around a concentric cylinder geometry with a cavity-style solar receiver immersed within a volume of molten carbonate salt. Concentrated radiation delivered to the cavity is absorbed in the cavity walls and transferred via convection to the salt volume. Feedstock is delivered into the molten salt volume where biomass gasification reactions will be carried out producing the desired product gas. The features of the cavity receiver/reactor concept are optimized based on modeling of the key physical processes. The cavity absorber geometry is optimized according to a parametric survey of radiative exchange using a Monte Carlo ray tracing model, resulting in a cavity design that achieves absorption efficiencies of 80%-90%. A parametric survey coupling the radiative exchange simulations to a CFD model of molten salt natural convection is used to size the annulus containing the molten salt to maximize utilization of absorbed solar energy, resulting in a predicted utilization efficiency of 70%. Finite element analysis was used to finalize the design to achieve acceptable thermal stresses less than 34.5 MPa to avoid material creep.
Solar Jet Hunter: Jet Catalog from HEK Events 2011-2016
(2023-09-25) Musset, Sophie; Sankar, Ramanakumar; Lasko, Kekoa; Jol, Paloma; Glesener, Lindsay; Fleishman, Gregory; Panesar, Navdeep; Zhang, Yixian; Hurlburt, Neal; Fortson, Lucy; Ostlund, Erik; Alnahari, Suhail; Jeunon, Mariana; Kapsiak, Charles; glesener@umn.edu; Glesener, Lindsay; UMN Space Physics Group
This database constitutes the first release of data from the Solar Jet Hunter project. Solar Jet Hunter is a Zooniverse-based citizen science project that has, since 2021, enlisted volunteers from the general public to help identify extreme ultraviolet jets of plasma in the Sun’s corona. These jets release magnetic energy at the Sun and enable streams of plasma and energetic particles to escape to the solar system, but the origins of and mechanisms underlying these jets are still not understood. In Solar Jet Hunter, videos of possible jets are presented to volunteers, who are asked to identify whether a jet is present, and if so, its start time, end time, and base location. Volunteers also box the jet, providing information on its shape and height over time. The results from many volunteers are then aggregated into consensus results for each potential jet in the study. Those results are listed in this data set. The data presented to volunteers is from the Solar Dynamic Observatory / Atmospheric Imaging Assembly instrument, specifically the 304 angstrom filter, and all of the candidate jets were identified as possible jets in the Heliophysics Event Knowledgebase (HEK). The data set included here lists Solar Jet Hunter results from the years 2011 through 2016.
Thermal modeling and design of a solar non-stoichiometric redox reactor with heat recovery
(2013-08) Lapp, Justin L.
A promising new technology for sustainable fuel production is the splitting of water and carbon dioxide by the non-stoichiometric two-step metal oxide redox cycle. Development of oxide materials and reactors to realize the cycle is currently in infancy, with significant room for improvement over previous demonstrations. Research efforts have gone into developing and characterizing reactive metal oxide materials for the cycle, while less literature is devoted to the design and understanding of non-stoichiometric redox reactors. The work presented attempts to close the gap by exploring multiple levels of modeling analysis to determine the important considerations for designing a reactor to perform high efficiency non-stoichiometric redox cycling. Cerium oxide (ceria) is considered as the reactive material for reactors in this work. A reactor for non-stoichiometric redox cycling should allow for continuous use of the solar input and should implement heat recovery. In the first stage of the research, thermodynamic analysis is carried out on a model reactor system to quantify the potential efficiency benefits of heat recovery and to determine the effects of the reduction temperature and sweep gas flow rate. Heat recovery is found to improve reactor efficiency from 4% to 16%. The selection of reduction temperature is important to high efficiency. For many cases the heating of gases is a major source of heat loss, indicating that heat recovery should be applied to the gas flows as well as the solid metal oxide. In the second stage of the research, a reactor is presented which incorporates continuous redox cycling of ceria and heat recovery from the solid ceria by using counter-rotating hollow cylinders of ceria and inert material. Heat transfer modeling is applied to this concept to explore its performance potential and identify the important design factors for effective heat recovery. Energy conservation is applied using a finite volume method with detailed modeling of radiative heat transfer by the Monte Carlo method and the Rosseland diffusion approximation. A simplified model of the rotating cylinders and a more complete model of the full reactor geometry are applied. It is determined that the proposed design can recover over 50% of the heat from the ceria, and provide a temperature differential of 400 K between the reaction steps. Geometric and material parameters are varied in a parametric study to determine which are important forheat recovery. The important parameters for heat recovery and chemical utilization of the material are those which define the heat transport across the ceria cylinder wall. Temperatures, heat transfer rates, heat fluxes, and the chemical state of the material are predicted. Using the heat transfer model results and other analysis, values of thermal design parameters for a prototype reactor are selected as part of an effort leading to a prototype reactor to be built and tested at the University of Minnesota. Heat recovery is found to be a path with great potential for improving the efficiency of solar-driven non-stoichiometric redox cycles. The prototype reactor described has the potential to demonstrate high levels of heat recovery and unprecedented efficiency. However, a careful understanding of the properties of the reactive material and the geometric parameters of the reactor is needed to ensure that heat which is input or removed is effectively transported across the cylinder wall for heat recovery. The models described here account for the important effects and explore the complexity needed to investigate the problem. Primary future improvements to the modeling work will include coupling of heat transfer and fluid mechanics, implementation of chemical rate expressions, and the addition of high-temperature and spectral material properties as they become available.
Zinc aerosol hydrolysis in a transverse jet reactor.
(2011-12) Haltiwanger, Julia Frances
Some of the major challenges---both technical and economic---of the Zn/ZnO two-step thermochemical hydrogen production cycle are investigated in this study. Technically, complete hydrolysis of Zn in the hydrogen production step remains a major barrier to implementation, and much attention has been given to Zn nano-scale reacting aerosols as a solution. Smaller particles favor faster reaction kinetics, and because they can be entrained and reacted in a gas flow, a continuous controllable process is possible. However, success of this continuous process depends on achieving high particle yields and high conversions in the aerosol, neither of which have yet been achieved in laboratory reactors. The ability of a new reactor concept based on transverse jet fluid dynamics to control the flow field and rapidly cool the Zn vapor is investigated. In the transverse jet reactor, evaporated Zn entrained in an Ar carrier gas issues vertically into the horizontal tubular reactor through which cooler H2O and Ar flow. Particles are formed in the presence of steam at ~450 K. The objective of controlling the flow field is to keep Zn away from the walls, thereby reducing particle deposition in the reactor and increasing particle yields on the filter. A computational fluid dynamics (CFD) model indicates that the trajectory of the jet can be controlled so that the majority of the Zn mass is directed down the center of the reactor, not near the reactor walls. Furthermore, the model shows that quench rates of 2x10^4 K/s are achieved and reactants are well mixed. Experimentally, maximum particle yields of 93% of the mass entering the reactor are obtained. Hydrolysis experiments are conducted in the transverse jet reactor at 418 K, 573 K, 603 K, and 713 K to assess the mechanisms of particle growth and hydrolysis. Experiments are conducted with and without steam to assess the effect of the reacting gas on particle morphology. SEM images of particles collected on a filter downstream from the reaction zone indicate that particle growth is dominated by condensation, resulting in hexagonal particles generally with lengths across their hexagonal face of 300 nm to 1micron in experiments with stream, and 1 to 3 micron in experiments without steam. Furthermore, the SEM images indicate that in hydrolysis experiments, ZnO forms on the surface of particles early on, protecting them from re-evaporation. Particle yield on the filter, Y, is defined as the fraction of the total mass entering the reactor that is collected on a filter placed downstream of the reaction zone. Overall conversion, X, is measured by monitoring the H2 content of the effluent gas throughout experiments with a gas chromatograph. Conversion of aerosol particles, Z, is the ZnO content (by mole) of particles collected from the downstream filter; it is measured by x-ray diffractometry with the internal standard calibration method. At all temperatures, particle yield remains high---generally 70 to 80% in hydrolysis experiments---and particle deposition on the walls of the reaction zone is eliminated for temperatures of 573 K and above. However, the conversion in the aerosol is <7% and decreases with reaction zone temperature. The overall conversion ranges from 11% at 418 K to 49% at 713 K. The higher overall conversion than conversion in the aerosol is attributed to heterogeneous Zn vapor hydrolysis. Visual observation proves heterogeneous hydrolysis occurs on the reactor walls; it is inferred that the heterogeneous Zn vapor reaction also occurs on the surface of aerosol particles. In this study, high particle yields are achieved for the first time---an important step forward for the continuous aerosol process. However, complete conversion of the aerosol particles remains a major challenge. In an economic and policy study of the Zn/ZnO cycle, the time frame for economic viability is assessed through the use of experience curves under minimal input, mid-range, and aggressive incentive policy scenarios. For the technology to become cost competitive, incentive policies that lead to early implementation of solar hydrogen plants will be necessary to allow the experience effect to draw down the price. Under such policies, a learning curve analysis suggests that hydrogen produced via the Zn/ZnO cycle could become economically viable between 2032 and 2069, depending on how aggressively the policies encourage the emerging technology. Thus, if the technical challenges are resolved, the Zn/ZnO cycle has the potential to be economically viable by mid-century if incentive policies--such as direct financial support, purchase guarantees, low interest rate loans, and tax breaks--are used to support initial projects.

University Digital Conservancy

Browse by Subject

Browsing by Subject "Solar"