High Efficiency Direct Thermal to Electric Energy Conversion from Radioisotope Decay Using Selective Emitters and Spectrally Tuned Solar Cells PDF Download
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Author: Donald L. Chubb
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Languages : en
Pages : 18
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Author: Donald L. Chubb
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Category :
Languages : en
Pages : 18
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Category : Aeronautics
Languages : en
Pages : 892
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Category : Government publications
Languages : en
Pages :
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Category : Aeronautics
Languages : en
Pages : 898
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Category : Government publications
Languages : en
Pages : 1020
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Category : Science
Languages : en
Pages : 1716
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Category : Government reports announcements & index
Languages : en
Pages : 1372
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Author: Nicholas P. T. Sergeant
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Languages : en
Pages :
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The search for sustainable energy resources has emerged as one of the most significant and universal concerns society will face in the 21st century. Solar energy conversion offers a cost-effective alternative to our traditional greenhouse gas emitting power plants. The most common solar energy conversion processes are solar photovoltaic conversion and solar photothermal conversion. In the first one, solar photons are converted directly into electrical energy, whereas in the latter solar photons are converted into heat. The heat can then be converted into electrical energy by a steam turbine or a thermophotovoltaic cell. In both types of solar energy conversion it is of outmost importance to optimize the absorption process by maximizing the trapping of solar photons inside the photoactive absorber, while minimizing the potential loss mechanisms, such as thermal emission in photothermal conversion or exciton recombination in photovoltaic conversion. In this dissertation coherent light trapping approaches are explored based on wave optics, thus in the regime where light needs to be treated as a wave phenomenon and can be made to interfere or diffract in order to reinforce the electric field in certain regions (spatial tuning) for a desired frequency range (spectral tuning). Photonic design can help spectrally and spatially tune the electric field distribution and consequently the absorption for a specific conversion process, device or application. In chapter 1 we first introduce the physical quantities that will be used throughout this dissertation in greater depth. This chapter covers the optical properties of materials, the physical quantities describing spectral radiance and the emissivity/absorptivity of blackbody and non-blackbody surfaces, as well as the modeling techniques used in the optimization of nanophotonic designs for enhanced absorption or spectral selectivity. In chapter 2 and chapter 3 we focus on spectrally selective absorbers and photon radiators (emitters) for concentrated solar photothermal and thermophotovoltaic applications, respectively. For both applications, spectral tuning of the absorber surface is important. An ideal solar absorber operating at elevated temperature has high absorptivity over the solar spectrum, while suppressing parasitic IR thermal emission from its surface. In chapter 2, we study the use of aperiodic metal-dielectric coatings both on planar as well as on nanostructured substrates, to achieve the desired spectral selectivity over a wide angular range. Based on our modeled results, our optimized aperiodic multilayer stacks have the potential to outperform current commercially available solar thermal coatings. Using a vacuum emissometer, an apparatus that is able to measure the spectral emission of coatings at elevated temperature, we experimentally demonstrate the excellent spectral selectivity of these aperiodic metal-dielectric solar-selective coatings for concentrated solar thermal applications. In chapter 3, we study the use of aperiodic metal-dielectric coatings as photon radiators to achieve high thermophotovoltaic energy conversion at practical temperatures of operation. In chapter 4 we review light trapping strategies for thin-film solar photovoltaic energy conversion. Thin-film solar photovoltaic devices have an enormous potential to reduce the cost of solar electricity. However, because thin photoactive layers are used, optical absorption is incomplete unless light trapping strategies are employed. Since conventional light trapping approaches, based on geometric scattering, are less effective in thin-film devices, coherent light trapping approaches that exploit the wave nature of light are reviewed that have the potential to significantly enhance optical absorption over a broad spectral range. In chapter 5 and chapter 6 we focus on thin-film organic solar cells, a thin-film technology that has great potential to reduce the cost of solar energy conversion by the use of low-cost photoactive materials and the compatibility with high-throughput roll-to-roll manufacturing. In organic photovoltaic cells, the optical absorption is incomplete unless light trapping strategies are employed. As will be shown, it is important to both spectrally and spatially tune the absorption inside the photoactive region to optimize the exciton creation and to minimize parasitic losses. Multilayer metal-dielectric stacks are investigated to enhance absorption efficiency in organic solar cells without sacrificing charge carrier collection efficiency. We have experimentally shown that these multilayer stacks can at the same time serve as a transparent contact as well as enhance photon harvesting, resulting in improved power conversion efficiency. This will be demonstrated on glass substrates in chapter 5, as well as using a roll-to-roll process on flexible substrates in chapter 6.
Author: Daniel Kraemer (Ph. D.)
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Category :
Languages : en
Pages : 289
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Meeting the ever growing global energy demand with mostly fossil fuel based energy technologies is not sustainable, pollutes the environment and is the main cause of climate change threatening our planet as we know it. Solar energy technologies are a promising, sustainable and clean alternative due to the vast abundance of sunlight. Thus far, photovoltaic solar cells and concentrated solar power are considered to be the most promising approaches. Solar cells directly convert sunlight into electricity by photon induced electron-hole pair generation. Concentrated solar power captures the sunlight in form of heat which is then converted to electricity by means of a traditional mechanical power block. In this thesis, we explore solar thermoelectric generators (STEGs) as an alternative way to convert sunlight to electricity. Similar to concentrated solar power STEGs capture the sunlight in form of heat. However, the captured heat is directly converted to electricity by means of a thermoelectric generator. This solid-state direct heat-to-electricity conversion significantly simplifies the system, reduces cost and maintenance and enables transient operation and system scalability without affecting the performance. Therefore, STEGs have the potential to be deployed as small scale solar power converters in remote areas and on rooftops and as large scale concentrated solar power plants. While the concept of solar thermoelectric power conversion has been proposed over a century ago, most successful experimental efforts reported in, the literature have been limited to below 1 % for STEGs without optical concentration and to approximately 3 - 5 % with optical concentration. Theoretical STEG performances as modeled and discussed in this thesis predict significantly higher efficiencies. A detailed STEG model is introduced to theoretically investigate various parasitic losses and how to minimize their effect to obtain highest and most realistic performance predictions. Additionally, a methodology to optimize a photovoltaic-thermoelectric hybrid system based on spectral splitting is introduced. The optimization and performance prediction of a STEG is only accurate if the relevant material properties are known with high accuracy. However, typical spectroscopy techniques to determine the optical properties, namely the solar absorptance and infrared emittance, of a solar absorber have shortcomings which can lead to significant errors. Similarly, typical commercial equipment to measure the properties of thermoelectric materials including the Seebeck coefficient, the electrical resistivity and the thermal conductivity are prone to large errors. Therefore, we introduce in this thesis novel experimental techniques to measure all relevant properties with improved accuracies in particular the techniques to measure the total hemispherical emittance of a surface and a material's thermal conductivity. A record-low total hemispherical emittance of 0.13 at 500 °C is demonstrated for an Yttria-stabilized-Zirconia-based cermet solar absorber with solar absorptance of 0.91 and thermal stability up to 600 °C. Furthermore, a method was developed to directly measure the efficiency of a thermoelectric leg. Using this method a record-high thermoelectric efficiency of 8.5 % is demonstrated at a relatively small temperature difference of 225 °C for a novel MgAgSb-based compound with hot-pressed silver contact pads. By increasing the temperature difference to a material's compatible 275 °C a thermoelectric efficiency of 10 % is achievable which, thus far, has only been achieve at almost twice the temperature difference. The third main contribution of this thesis is the experimental demonstration of solar thermoelectric power conversion. A record-high STEG efficiency of 4.6 % is demonstrated at AM1.5G (1 kW/m 2) conditions which is 7 times higher than previously reported best values. The performance improvement is achieved by using a STEG with nano-structured bulk thermoelectric materials, a spectrally-selective solar absorber and taking advantage of large thermal concentrations under a vacuum. Despite the vacuum environment and the use of a low-temperature spectrally-selective solar absorber the optimal hot-junction operating temperature is limited to approximately 200 °C due to increasing thermal radiation heat loss. In order to substantially increase the operating temperature difference and STEG efficiency, larger incident solar power densities are required. Furthermore, the STEG requires segmented thermoelectric legs and a high-temperature stable solar absorber. The optimized STEGs are fabricated and tested at moderate and high optical solar concentration. Efficiencies of close to 8 % at 38 suns and close to 10 % at 211 suns, measured based on the solar flux at the absorber, are demonstrated for a STEG with a spectrally-selective solar absorber. The maximum demonstrated solar-to-electricity CSTEG efficiency is 7.5 %. Furthermore, the performance of a STEG at moderate optical concentration with a high-temperature stable black paint solar absorber and a directionally-selective solar receiver cavity is demonstrated to be comparable to a STEG with a spectrally-selective surface at similar insolation.
Author: Xiawa Wang
Publisher:
ISBN:
Category :
Languages : en
Pages : 163
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This thesis describes the design, modeling, and measurement results of a radioisotope thermophotovoltaic system (RTPV) using a two-dimensional photonic crystal emitter and low bandgap thermophotovoltaic (TPV) cell to realize spectral control. The RTPV generator aims to use the decay heat released by plutonium-238 fuel to heat up the emitter to incandescence and convert the infrared radiation to electricity in the TPV cell. With spectral control, high energy photons above the cell bandgap ([lambda] 2.25 [mu]m for InGaAsSb cell) are emitted to produce more electrical power while low energy photons ([lambda] 2.25 [mu]m) in far infrared are suppressed to reduce waste heat. We validated a system simulation using the measurements of a prototype system powered by an electrical heater equivalent to one plutonium fuel pellet. The thermal insulation design used multilayer insulation, which was found to be both efficient and chemically compatible with the photonic crystal emitter. We compared the system performance using a photonic crystal emitter to the one using a polished flat tantalum emitter and found that spectral control with the photonic crystal was four times more efficient. Based on the simulation, we further extended the design and performance estimates to real life RTPV generators optimized for both space and terrestrial applications. With the experimentally tested InGaAsSb TPV cell, the system efficiency can potentially reach above 8% with a specific power of 7.5 W/kg. With more advanced InGaAs monolithic-integrated-modules, the system efficiency can reach around 20% with a specific power above 17 W//kg.
