Thermophotovoltaic (TPV) systems are promising for harnessing solar energy, waste heat, and heat from radioisotope decay or fuel combustion. TPV systems work by heating an emitter that emits light that is converted to electricity. One of the key challenges is designing an emitter that not only preferentially emits light in certain wavelength ranges but also simultaneously satisfies other engineering constraints. To elucidate these engineering constraints, we first provide an overview of the state of the art, by classifying emitters into three categories based on whether they have been used in prototype system demonstrations, fabricated and measured, or simulated. We then present a systematic approach for assessing emitters. This consists of five metrics: optical performance, ability to scale to large areas, stability at high temperatures, ability to integrate into the system, and cost. Using these metrics, we evaluate and discuss the reported results of emitters used in system demonstrations. Although there are many emitters with good optical performance, more studies on their practical attributes are required, especially for those that are not yet used in prototype systems. This framework can serve as a guide for the development of emitters for long-lasting, high-performance TPV systems.
Nanophotonic techniques can enable numerous novel and exciting phenomena. However, in order to make use of these opportunities for many applications of interest (e.g. energy, or displays), one has to have the ability to implement nanophotonic structures over large scales. In this talk, I will present some of our recent theoretical and experimental progress in exploring these opportunities.
We demonstrate designs of dielectric-filled anti-reflection coated (ARC) two-dimensional (2D) metallic photonic
crystals (MPhCs) capable of omnidirectional, polarization insensitive, wavelength selective emission/absorption. Up to
26% improvement in hemispherically averaged emittance/absorptance below the cutoff wavelength is observed for
optimized hafnium oxide filled 2D tantalum (Ta) PhCs over the unfilled 2D Ta PhCs. The optimized designs possess
high hemispherically averaged emittance/absorptance of 0.86 at wavelengths below the cutoff wavelength and low
hemispherically averaged emittance/absorptance of 0.12 at wavelengths above the cutoff wavelength, which is extremely
promising for applications such as thermophotovoltaic energy conversion, solar absorption, and infrared spectroscopy.
A tantalum tungsten solid solution alloy, Ta 3% W, based 2D photonic crystal (PhC) was designed and fabricated for high-temperature energy conversion applications. Ta 3% W presents advantages compared to the non-alloys as it combines the better high-temperature thermomechanical properties of W with the more compliant material properties of Ta, allowing for a direct system integration path of the PhC as selective emitter/absorber into a spectrum of energy conversion systems. Indeed metallic PhCs are promising as high performance selective thermal emitters for thermophotovoltaics (TPV), solar thermal, and solar TPV applications due to the ability to tune their spectral properties and achieve highly selective emission. A 2D PhC was designed to have high spectral selectivity matched to the bandgap of a TPV cell using numerical simulations and fabricated using standard semiconductor processes. The emittance of the Ta 3% WPhC was obtained from near-normal reectance measurements at room temperature before and after annealing at 1200 °C for 24h in vacuum with a protective coating of 40 nm HfO2, showing high selectivity in agreement with simulations. SEM images of the cross section of the PhC prepared by FIB confirm the structural stability of the PhC after anneal, i.e. the coating effectively prevented structural degradation due to surface diffusion. The mechanical and thermal stability of the substrate was characterized as well as the optical properties of the fabricated PhC. To evaluate the performance of the selective emitters, the spectral selectivity and useful emitted power density are calculated as a function of operating temperature. At 1200 °C, the useful emitted irradiance is selectively increased by a factor of 3 using the selective emitter as compared to the non-structured surface. All in all, this paper demonstrates the suitability of 2D PhCs fabricated on polycrystalline Ta-W substrates with an HfO2 coating for TPV applications.
The quest for developing clean, quiet, and portable high energy density, and ultra-compact power sources continues. Although batteries offer a well known solution, limits on the chemistry developed to date constrain the energy density to 0.2 kWh/kg, whereas many hydrocarbon fuels have energy densities closer to 13 kWh/kg. The fundamental challenge remains: how efficiently and robustly can these widely available chemical fuels be converted into electricity in a millimeter to centimeter scale systems? Here we explore two promising technologies for high energy density power generators: thermophotovoltaics (TPV) and thermoelectrics (TE). These heat to electricity conversion processes are appealing because they are fully static leading to quiet and robust operation, allow for multifuel operation due to the ease of generating heat, and offer high power densities. We will present some previous work done in the TPV and TE fields. In addition we will outline the common technological barriers facing both approaches, as well as outline the main differences. Performance for state of the art research generators will be compared as well as projections for future practically achievable systems. A viable TPV or TE power source for a ten watt for one week mission can be built from a <10% efficient device which is achievable with current state of the art technology such as photonic crystals or advanced TE materials.
KEYWORDS: Tantalum, Coating, Solar energy, Photonic crystals, Temperature metrology, Etching, Tungsten, Annealing, Reactive ion etching, System integration
A tantalum tungsten (Ta-W) solid solution alloy, Ta 3% W, based 2D photonic crystal (PhC) was designed and fabricated for high-temperature energy conversion applications. Metallic PhCs are promising as high performance selective thermal emitters for solid-state thermal-to-electricity energy conversion concepts including thermophotovoltaic (TPV) energy conversion, as well as highly selective solar absorbers/emitters for solar thermal and solar TPV applications due to the ability to tune their spectral properties and achieve highly selective emission. The mechanical and thermal stability of the substrate was characterized as well as the optical properties of the fabricated PhC. The Ta 3% W alloy presents advantages compared to the non-alloys as it combines the better high-temperature thermo-mechanical properties of W with the more compliant material properties of Ta, allowing for a direct system integration path of the PhC as selective emitter/absorber into a spectrum of energy conversion systems. Furthermore, the thermo-mechanical properties can be fine-tuned by the W content. A 2D PhC was designed to have high spectral selectivity matched to the bandgap of a TPV cell using numerical simulations and fabricated using standard semiconductor processes. The emittance of the Ta 3% W PhC was obtained from near-normal reflectance measurements at room temperature before and after annealing at 1200°C for 24h in vacuum with a protective coating of 40nm HfO2, showing high selectivity in agreement with simulations. SEM images of the cross section of the PhC prepared by FIB confirm the structural stability of the PhC after anneal, i.e. the coating effectively prevented structural degradation due to surface diffusion.
KEYWORDS: Tantalum, Etching, Solar energy, Photonic crystals, Reactive ion etching, Chromium, Deep reactive ion etching, Solar cells, Black bodies, Lithography
We report highly selective emitters based on high-aspect ratio 2D photonic crystals (PhCs) fabricated on large area (2 inch diameter) polycrystalline tantalum substrates, suitable for high-temperature operation. As an example we present an optimized design for a selective emitter with a cut-off wavelength of 2μm, matched to the bandgap of an InGaAs PV cell, achieving a predicted spectral selectivity of 56.6% at 1200K. We present a fabrication route for these tantalum PhCs, based on standard microfabrication processes including deep reactive ion etch of tantalum by an SF6 based Bosch process, achieving high-aspect ratio cavities (< 8:1). Interference lithography was used to facilitate large area fabrication, maintaining both fabrication precision and uniformity, with a cavity diameter variation of less than 2% across the substrate. The fabricated tantalum PhCs exhibit strong enhancement of the emittance at wavelengths below cut-off wavelength, approaching that of blackbody, and a steep cut-off between high and low emittance spectral regions. Moreover, detailed simulations and numerical modeling show excellent agreement with experimental results. In addition, we propose a surface protective coating, which acts as a thermal barrier coating and diffusion inhibitor, and its conformal fabrication by atomic layer deposition.
This paper explores the optical characteristics of one-dimensional (1D) and two-dimensional (2D) photonic crystals (PhC) as spectral control components for use in thermophotovoltaic (TPV) systems. 1D PhC are used as optical filters while 2D PhC are used as selective thermal emitters. A Si/SiO2 1D PhC is fabricated using low-pressure chemical vapor deposition (LPCVD). The measurement and characterization of this structure is presented. A 2D hexagonal PhC of periodic holes is fabricated using interference litography and reactive ion etching (RIE) process. Our results predict that a TPV system utilizing a 2D PhC selective emitter and 1D Si/SiO2 PhC optical filter promises significant performance improvements over conventional TPV system architectures.
Complex photonic bands and strong anisotropic dispersion characteristics of artificially engineered periodic dielectric structures have been widely investigated. In this paper we explore the self-guiding effect possessed by photonic crystals and the possible applications for integrated photonics. Since this approach does not require a full photonic bandgap, low refractive index materials (i.e. glass or organic polymers) are considered as an alternative with advantages over conventional semiconductor materials. Sensitivity analysis reveals how structural variations influence the performance of this type of photonic crystal based system.
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