abstract: Solar energy as a limitless source of energy all around the globe has been difficult to harness. This is due to the low direct solar-electric conversion efficiency which has an upper limit set to the Shockley-Queisser limit. Solar thermophotovoltaics (STPV) is a much more efficient solar energy harvesting technology as it has the potential to overcome the Shockley-Queisser limit, by converting the broad-spectrum solar irradiation into narrowband infrared spectrum radiation matched to the PV cell. Despite the potential to surpass the Shockley-Queisser limit, very few experimental results have reported high system-level efficiency. The objective of the thesis is to study the STPV conversion performance with selective metafilm absorber and emitter paired with a commercial GaSb cell at different solar concentrations. Absorber and Emitter metafilm thickness was optimized and fabricated. The optical properties of fabricated metafilms showed good agreement with the theoretically determined properties. The experimental setup was completed and validated by measuring the heat transfer rate across the test setup and comparing it with theoretical calculations. A novel method for maintaining the gap between the emitter and PV cell was developed using glass microspheres. Theoretical calculations show that the use of the glass of microspheres introduces negligible conduction loss across the gap compared to the radiation heat transfer, which is confirmed by experimental heat transfer measurement. This research work will help enhance the fundamental understanding and the development of the high-efficiency solar thermophotovoltaic system. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2020

Thermophotovoltaic (TPV) devices, also known as (nano-TPVs) are energy-conversion systems which generate electric current from thermal radiation energy by a heat source. Although their conversion efficiency is limited in the far field by the Schockley-Queisser limit, in near field the heat flux transferred to a TPV cell can be significantly enchanced due to the contribution of evanescent waves, in particular supporting a surface mode. Unfortunately, spectral mismatch between the emitter and the TPV cell spectrum limits the TPV conversion efficiency. Photons with energy lower than the TPV cell bandgap may not be able to create electron-hole pairs because mobile carriers start diffusing and drifting between conductance and valence band, and try to exceed the upper limit of the band. This destroys the thermal equilibrium of the semiconductor and results in excess heat. Also, for high energy photons, the difference between the photon's energy and the bandgap energy is lost in Joule heating. Thus, quasimonochromatic, narrow-band and coherent emitters at a frequency near the energy bandgap of the converter is an ideal source to achieve high conversion efficiency. Nano-TPV device consisting of tungsten thermal emitter, maintained at 1200K, and the cell made of GaInAsSb are considered; thermal management system is reviewed assuming a constant heat flux boundary due to heat generation by the cell with a fluid temperature fixed at 293K. Tungsten thermal selective emitters are designed, characterized and optimized based on two-dimensional (2D) tungsten PhC by controlling periodic triangular grooves such that channel plasmon polaritons (CPPs) are coupled efficiently into these grooves to excite a localized groove modes which are well-matched to the GaInAsSb cell external quantum efficiency (EQE). The results show that power output and the 2D TE normal efficiency of the system are predicted to be 0.82x10⁴ W/m² and 43.8%, respectively. This leads to a promising device for many different sectors such as military, space and semiconductor industry. / text

Studies of the radiative properties of thin films and near-field radiation transfer in layered structures are important for applications in energy, near-field imaging, coherent thermal emission, and aerospace thermal management. A comprehensive study is performed on the optical constants of dielectric tantalum pentoxide (Ta₂O₅) and hafnium oxide (HfO₂) thin films from visible to the far infrared using spectroscopic methods. These materials have broad applications in metallo-dielectric multilayers, anti-reflection coatings, and coherent emitters based on photonic crystal structures, especially at high temperatures since both materials have melting points above 2000 K. The dielectric functions of HfO₂ and Ta₂O₅ obtained from this work may facilitate future design of devices with these materials. A parametric study of near-field TPV performance using a backside reflecting mirror is also performed. Currently proposed near-field TPV devices have been shown to have increased power throughput compared to their far-field counterparts, but whose conversion efficiencies are lower than desired. This is due to their low quantum efficiency caused by recombination of minority carriers and the waste of sub-bandgap radiation. The efficiency may be improved by adding a gold mirror as well as by reducing the surface recombination velocity, as demonstrated in this thesis. The analysis of the near-field TPV and proposed methods may facilitate the development or high-efficiency energy harvesting devices. Many near-field devices may eventually utilize metallo-dielectric structures which exhibit unique properties such as negative refraction due to their hyperbolic isofrequency contour. These metamaterials are also called indefinite materials because of their ability to support propagating waves with large lateral wavevectors, which can result in enhanced near-field radiative heat transfer. The energy streamlines in such structures are studied for the first time. Energy streamlines illustrate the flow of energy through a structure when the fields are evanescent and energy propagation is not ray like. The energy streamlines through two semi-infinite uniaxially anisotropic effective medium structures, separated by a small vacuum gap, are modeled using the Green’s function. The lateral shift and penetration depth are calculated from the streamlines and shown to be relatively large compared to the vacuum gap dimension. The study of energy streamlines in hyperbolic metamaterials helps understand the near-field energy propagation on a fundamental level.

The work is focused on research and development of a suitable method for creating a selective emitter for the visible and near infrared region so that they are able to work optimally together with silicon photovoltaic cells in a thermophotovoltaic system. The aim of the work was to develop a new method of creating very fine structures outside the current standard, which will increase the emissivity of the base material to meet the needs of a selective emitter for the VID and NIR region. The methods available to us for the creation of structures were chosen, from which we eliminated all unsuitable ones and we introduced the optimal procedure and parameters for their creation for the selected method. In this work, we focused on both ceramic and metallic materials, whose heat resistance and selective properties are key to this work. Part of the development of the emitter structures was also the need for pretreatment of the substrate itself, where great emphasis was placed on the purity of materials and surface roughness. Since ceramic materials cannot achieve a surface roughness so low that the desired structures can be formed, these materials have been purposefully used primarily for the purpose of combining the base material with thin layers of other high temperature material. Their compatibility and suitability were verified in terms of adhesion and subsequent heat resistance. The main material for the formation of fine structures was purposefully chosen tungsten, for which we verified the influence of the formed structure on the emissivity as well as the thermal stability during long-term exposure to high temperatures. The work thus represents not only a new method of creating very fine structures, which are not normally formed in such subtlety, but also opens the way to new possibilities of combining more materials to achieve the required selectivity of the thermophotovoltaic emitter.

Thermal radiation is a fundamental heat transfer process, with certain basic aspects still not fully understood. Furthermore, tailoring its properties has potential to affect a wide range of applications, particularly thermophotovoltaics (TPV) and radiative cooling. TPV converts heat into electricity using thermal radiation to illuminate a photovoltaic diode, with no moving parts. With its realistic efficiency limit up to 50% (heat source at 1200 ^oC), TPV has garnered substantial interest. However, state-of-the-art TPV demonstrations are still well below theoretical limits, because of losses from generating and efficiently converting or recycling thermal radiation. In this thesis, tailored integrated photonic crystal structures are numerically simulated to enhance the efficiency of solar TPV. Next, a high-temperature thin-film Si-based selective absorber and emitter is designed, fabricated and experimentally characterized. It exhibits great potential to open up new applications, as it lends itself to large-scale production with substantial mechanical flexibility and excellent spectral selectivity for extended time periods, even when operating under high operating temperatures (600 ^oC) for up to 6 hours, with partial degradation after 24 hours. To perform this high-temperature characterization, an emittance measurement setup has been built; its performance agrees well with numerical simulations. Second, a unique passive cooling mechanism known as radiative cooling is developed to reduce the operating temperature of the photovoltaic diode. The significant effect of radiative cooling as a complement for an all-passive-cooling TPV system is proposed and numerically analyzed under a range of conditions. Furthermore, an outdoor experiment has been performed to demonstrate the effect of radiative cooling on a concentrating photovoltaic system, which can potentially be applied to the thermal management of a TPV system. In summary, this work paves the way towards the development of reliable, quiet, lightweight, and sustainable TPV and radiatively cooled power sources for outdoor applications.