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Enhancing Thermophotovoltaics via Selective Thermal Emitters and Radiative Thermal ManagementZhiguang Zhou (7908800) 25 November 2019 (has links)
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 <sup>o</sup>C), 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 <sup>o</sup>C) 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.
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Enhancement of Solar Absorbers and Radiative Coolers via Nanostructuring and Improved Reliability and Efficiency of GaN HEMT devicesDavid J. Kortge (5930708) 03 August 2023 (has links)
<p>Management of incoming solar radiation and use of the sky as an ultimate heat sink are technological imperatives as climate change shifts our reliance from fossil fuels to sustainable sources. Selective solar absorbers are a possible route for solar harvesting as they collect the incoming radiation for process heat or space heating. Here, improvement in the performance of selective solar absorbers via photon recycling is investigated using a stepped index rugate filter. The final proposed filter when integrated with a high vacuum selective solar absorber could see an improvment in solar-thermal conversion efficiency from 13% to 30.6%. Then, a frequency selective optical filter is fabricated with uses including improvement of radiative coolers. The measured optical characteristics are compared with simulation data and found to match well.</p>
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<p>The shift to sustainable sources of electricity will require an expansion of the electrical grid. The backbone of the grid for converting high voltage AC to DC, and vice versa, is power electronics. The current state-of-the-art technology is GaN HEMTs, but GaN MISHEMTs are poised to replace them since MISHEMTs reduce the gate leakage current; a deficiency of the GaN HEMT architecture. First, time dependent dielectric breakdown in GaN MISHEMTs is investigated using concurrent electrical and thermoreflectance methods. A susceptibility in the MISHEMT architecture is found and possible solutions are proposed. Then, liquid cooling of GaN HEMT PAs is explored by demonstrating integration of an X-band front end module, printed circuit board, and fluid manifold. The integration shows great promise as two-phase cooling performance improved with increasing power dissipated, while single-phase cooling performance degraded.</p>
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