Improved Thermal Emitters for Thermophotovoltaic Energy Conversion

Thermophotovoltaic (TPV) energy conversion enables millimeter scale power generation required for portable microelectronics, robotics, etc. In a TPV system, a heat source heats a selective emitter to incandescence, the radiation from which is incident on a low bandgap TPV cell. The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency. The selective emitter requires: thermal stability at high-temperatures for long operational lifetimes, simple and relatively low-cost fabrication, as well as spectrally selective emission over a large uniform area. Generally, the selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters, or can be engineered through microstructuring. Our approach, the 2D photonic crystal fabricated in refractory metals, offers high spectral selectivity and high-temperature stability while being fabricated by standard semiconductor processes. In this work, we present a brief comparison of TPV system efficiencies using these different emitter technologies. We then focus on the design, fabrication, and characterization of our current 2D photonic crystal, which is a square lattice of cylindrical holes fabricated in a refractory metal substrate. The spectral performance and thermal stability of the fabricated photonic crystal thermal emitters are demonstrated and the efficiency gain of our model TPV system is characterized

A Thermophotovoltaic System Using a Photonic Crystal Emitter

The increasing power demands of portable electronics and micro robotics has driven recent interest in millimeter-scale microgenerators. Many technologies (fuel cells, Stirling, thermoelectric, etc.) that potentially enable a portable hydrocarbon microgenerator are under active investigation. Hydrocarbon fuels have specific energies fifty times those of batteries, thus even a relatively inefficient generator can exceed the specific energy of batteries. We proposed, designed, and demonstrated a first-of-a-kind millimeter-scale thermophotovoltaic (TPV) system with a photonic crystal emitter. In a TPV system, combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by photovoltaic cells. Our approach uses a moderate temperature (1000–1200°C) metallic microburner coupled to a high emissivity, high selectivity photonic crystal selective emitter and low bandgap PV cells. This approach is predicted to be capable of up to 30% efficient fuel-to-electricity conversion within a millimeter-scale form factor. We have performed a robust experimental demonstration that validates the theoretical framework and the key system components, and present our results in the context of a TPV microgenerator. Although considerable technological barriers need to be overcome to realize a TPV microgenerator, we predict that 700–900 Wh/kg is possible with the current technology.Micro Autonomous Consortium Systems and Technology (Contract 892730)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (W911NF-13-D- 0001

Photonic Crystal Enabled Thermophotovoltaics for a Portable Microgenerator

This work presents the design and characterization of a first-of-a-kind millimeter- scale thermophotovoltaic (TPV) system using a metallic microburner, photonic crystal emitter, and low-bandgap photovoltaic (PV) cells. In our TPV system, combustion heats the emitter to incandescence and the resulting thermal radiation is converted to electricity by the low bandgap PV cells. Our motivation is to harness the high specific energy of hydrocarbon fuels at the micro- and millimeter-scale in order to meet the increasing power demands of micro robotics and portable electronics. Our experimental demonstration lays the groundwork for developing a TPV microgenerator as a viable battery replacement.United States. Dept. of Energy. Office of Basic Energy Sciences (DE-SC0001299)United States. Dept. of Energy. Office of Basic Energy Sciences (DE-FG02-09ER4657)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (contract W911NF-13-D-0001

Tantalum-tungsten alloy photonic crystals for high-temperature energy conversion systems

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.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract DAAD-19-02-D0002)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-07-D000)United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Grant DE-SC0001299))Austrian Science Fund (J3161-N20

Prototype of radioisotope thermophotovoltaic system using photonic crystal spectral control

This work describes a prototype of a small-size radioisotope thermophotovoltaic (RTPV) system with the two-dimensional metallic photonic crystal emitter and the low bandgap TPV cell. The project demonstrates the simulation and measurement of a system powered by an electrical heat source that mimics the radioisotope fuel pellet. The photonic crystal and the polished Ta3%W substrate are both used as the emitting surfaces to demonstrate the benefits of spectral control. The rest of the system is thermally insulated to increase the overall system efficiency. The photonic crystal emitter demonstrates four times more output power over a flat metal emitter from the 1 cm[superscript 2] TPV cell. With more cell areas, better TPV cells and improved insulation design, the system is expected to reach an efficiency of 7.8%

Nanoimprinted superlattice metallic photonic crystal as ultraselective solar absorber

A two-dimensional superlattice metallic photonic crystal (PhC) and its fabrication by nanoimprint lithography on tantalum substrates are presented. The superior tailoring capacity of the superlattice PhC geometry is used to achieve spectrally selective solar absorption optimized for high-temperature and high-efficiency solar-energy-conversion applications. The scalable fabrication route by nanoimprint lithography allows for a high-throughput and high-resolution replication of this complex pattern over large areas. Despite the high fill factor, the pattern of polygonal cavities is accurately replicated into a resist that hardens under ultraviolet radiation over an area of 10  mm². In this way, cavities of 905 nm and 340 nm width are achieved with a period of 1 μm. After pattern transfer into tantalum via a deep reactive ion-etching process, the achieved cavities are 2.2 μm deep, separated by 85–95 nm wide ridges with vertical sidewalls. The room-temperature reflectance spectra of the fabricated samples show excellent agreement with simulated results, with a high spectral absorptance approaching blackbody absorption in the range from 300 to 1900 nm and a steep cutoff. The calculated solar absorptivity of this superlattice PhC is 96% and its thermal transfer efficiency is 82.8% at an operating temperature of 1500 K and an irradiance of 1000  kW/m².United States. Army Research Office (W911NF-13-D-0001)United States. Department of Energy (DE-SC0001299

Near-field thermal radiation transfer controlled by plasmons in graphene

It is shown that thermally excited plasmon-polariton modes can strongly mediate, enhance and \emph{tune} the near-field radiation transfer between two closely separated graphene sheets. The dependence of near-field heat exchange on doping and electron relaxation time is analyzed in the near infra-red within the framework of fluctuational electrodynamics. The dominant contribution to heat transfer can be controlled to arise from either interband or intraband processes. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement (e.g. $10^2$ to $10^3$ for separations between $0.1\mu m$ to $10nm$) over the Stefan-Boltzmann law, known as the far field limit. Strong, tunable, near-field transfer offers the promise of an externally controllable thermal switch as well as a novel hybrid graphene-graphene thermoelectric/thermophotovoltaic energy conversion platform.Comment: 4 pages, 3 figure

Angular photonic band gap

We present detailed numerical simulations of a class of material systems that strongly discriminate light based primarily on the angle of incidence, over a broad range of frequencies, and independent of the polarization. Unique properties of these systems emerge from exploring photonic crystals whose constituents have an anisotropic dielectric response.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant no. DMR-0819762)United States. Dept. of Energy. Office of Basic Energy Sciences (S3TEC grant no. DE-SC0001299)Massachusetts Institute of Technology. Energy Initiativ

Resonant-cavity enhanced thermal emission

In this paper we present a vertical-cavity enhanced resonant thermal emitter—a highly directional, narrow-band, tunable, partially coherent thermal source. This device enhances thermal emittance of a metallic or any other highly reflective structure to unity near a cavity resonant frequency. The structure consists of a planar metallic surface (e.g., silver, tungsten), a dielectric layer on top of the metal that forms a vertical cavity, followed by a multilayer dielectric stack acting as a partially transparent cavity mirror. The resonant frequency can easily be tuned by changing the cavity thickness (thus shifting resonant emission peak), while the angle at which the maximum emittance appears can be tuned as well by changing the number of dielectric stack layers. The thermal emission exhibits an extremely narrow angular emission lobe, suggesting increased spatial coherence. Furthermore, we show that we can enhance the thermal emission of an arbitrarily low-emittance material, choosing a properly designed thermal cavity, to near unity.MIT/Industry Consortium on Advanced Automotive Electrical/Electronic Components and System

Overcoming the black body limit in plasmonic and graphene near-field thermophotovoltaic systems

Near-field thermophotovoltaic (TPV) systems with carefully tailored emitter-PV properties show large promise for a new temperature range (600 – 1200K) solid state energy conversion, where conventional thermoelectric (TE) devices cannot operate due to high temperatures and far-field TPV schemes suffer from low efficiency and power density. We present a detailed theoretical study of several different implementations of thermal emitters using plasmonic materials and graphene. We find that optimal improvements over the black body limit are achieved for low bandgap semiconductors and properly matched plasmonic frequencies. For a pure plasmonic emitter, theoretically predicted generated power density of 14[W over cm[superscript 2]] and efficiency of 36% can be achieved at 600K (hot-side), for 0.17eV bandgap (InSb). Developing insightful approximations, we argue that large plasmonic losses can, contrary to intuition, be helpful in enhancing the overall near-field transfer. We discuss and quantify the properties of an optimal near-field photovoltaic (PV) diode. In addition, we study plasmons in graphene and show that doping can be used to tune the plasmonic dispersion relation to match the PV cell bangap. In case of graphene, theoretically predicted generated power density of 6(120)[W over cm[superscript 2]] and efficiency of 35(40)% can be achieved at 600(1200)K, for 0.17eV bandgap. With the ability to operate in intermediate temperature range, as well as high efficiency and power density, near-field TPV systems have the potential to complement conventional TE and TPV solid state heat-to-electricity conversion devices