Enas Sakr

High efficiency rare-earth emitter for thermophotovoltaic applications

In this work, we propose a rare-earth-based ceramic thermal emitter design that can boost thermophotovoltaic (TPV) efficiencies significantly without cold-side filters at a temperature of 1573 K (1300 °C). The proposed emitter enhances a naturally occurring rare earth transition using quality-factor matching, with a quarter-wave stack as a highly reflective back mirror, while suppressing parasitic losses via exponential chirping of a multilayer reflector transmitting only at short wavelengths. This allows the emissivity to approach the blackbody limit for wavelengths overlapping with the absorption peak of the rare-earth material, while effectively reducing the losses associated with undesirable long-wavelength emission. We obtain TPV efficiencies of 34% using this layered design, which only requires modest index contrast, making it particularly amenable to fabrication via a wide variety of techniques, including sputtering, spin-coating, and plasma-enhanced chemical vapor deposition.

Solar thermophotovoltaics: reshaping the solar spectrum

Abstract Recently, there has been increasing interest in utilizing solar thermophotovoltaics (STPV) to convert sunlight into electricity, given their potential to exceed the Shockley-Queisser limit. Encouragingly, there have also been several recent demonstrations of improved system-level efficiency as high as 6.2%. In this work, we review prior work in the field, with particular emphasis on the role of several key principles in their experimental operation, performance, and reliability. In particular, for the problem of designing selective solar absorbers, we consider the trade-off between solar absorption and thermal losses, particularly radiative and convective mechanisms. For the selective thermal emitters, we consider the tradeoff between emission at critical wavelengths and parasitic losses. Then for the thermophotovoltaic (TPV) diodes, we consider the trade-off between increasing the potential short-circuit current, and maintaining a reasonable opencircuit voltage. This treatment parallels the historic development of the field, but also connects early insights with recent developments in adjacent fields.With these various components connecting in multiple ways, a system-level end-to-end modeling approach is necessary for a comprehensive understanding and appropriate improvement of STPV systems. This approach will ultimately allow researchers to design STPV systems capable of exceeding recently demonstrated efficiency values.

Thermophotovoltaics with spectral and angular selective doped-oxide thermal emitters

Deliberate control of thermal emission properties using nanophotonics has improved a number of applications including thermophotovoltaics (TPV), radiative cooling and infrared spectroscopy. In this work, we study the effect of simultaneous control of angular and spectral properties of thermal emitters on the efficiencies of TPV systems. While spectral selectivity reduces sub-bandgap losses, angular selectivity is expected to enhance view factors at larger separation distances and hence to provide flexibilities in cooling the photovoltaic converter. We propose a design of an angular and spectral selective thermal emitter based on waveguide perfect absorption phenomena in epsilon-near-zero thin-films. Aluminum-doped Zinc-Oxide is used as an epsilon-near-zero material with a cross-over frequency in the near-infrared. A high contrast grating is designed to restrict the emission in a range of angles around the normal direction, while an integrated filter ensures spectral selectivity to reduce sub-bandgap losses. Theoretical analysis shows an expected relative enhancement of the TPV system efficiency of at least 32% using selective emitters with ideal angular and spectral selectivity at large separation distances compared to a blackbody. This enhancement factor, however, reduces to 3.9% with non-ideal selective emitters. This big reduction of the efficiency is attributed to sub-bandgap losses, off-angular losses and high-temperature dependence of optical constants.

Asymmetric angular-selective thermal emission

Thermal emission from blackbodies and flat metallic surfaces is non-directional, following the Lambert cosine law. However, highly directional thermal emission could be useful for improving the efficiency of a broad range of different applications, including thermophotovoltaics, spectroscopy and infra-red light sources. This is particularly true if strong symmetry breaking could ensure emission only in one particular direction. In this work, we investigate the possibility of tailoring asymmetric thermal emission using structured metasurfaces. These are built from surface grating unit elements that support asymmetric localization of thermal surface plasmon polaritons. The angular dependence of emissivity is studied using a rigorous coupled wave analysis (RCWA) of absorption, plus Kirchhoff’s law of thermal radiation. It is further validated using a direct thermal simulation of emission originating from the metal. Asymmetric angular selectivity with near-blackbody emissivity is demonstrated for different shallow blazed grating structures. We study the effect of changing the period, depth and shape of the grating unit cell on the direction angle, angular spread, and magnitude of coupled radiation mode. In particular, a periodic sawtooth structure with a period of 1.5λ and angle of 8°was shown to create significant asymmetry of at least a factor of 3. Such structures can be considered arbitrary directional sources that can be carefully patterned on metallic surfaces to yield thermal lenses with designed focal lengths, targeted to particular concentration ratios. The benefit of this approach is that it can enhance the view factor between thermal emitters and receivers, without restricting the area ratio or separation distance.

Enhancing selectivity of infrared emitters through quality-factor matching

It has recently been proposed that designing selective emitters with photonic crystals (PhCs) or plasmonic metamaterials can suppress low-energy photon emission, while enhancing higher-energy photon emission. Here, we will consider multiple approaches to designing and fabricating nanophotonic structures concentrating infrared thermal radiation at energies above a critical threshold. These are based on quality factor matching, in which one creates resonant cavities that couple light out at the same rate that the underlying materials emit it. When this quality-factor matching is done properly, emissivities can approach those of a blackbody, but only within a selected range of thermal photon energies. One potential application is for improving the conversion of heat to electricity via a thermophotovoltaic (TPV) system, by using thermal radiation to illuminate a photovoltaic (PV) diode. In this study, realistic simulations of system efficiencies are performed using finite-difference time domain (FDTD) and rigorous coupled wave analysis (RCWA) to capture both thermal radiation and PV diode absorption. We first consider a previously studied 2D molybdenum photonic crystal with a commercially-available silicon PV diode, which can yield TPV efficiencies up to 26.2%. Second, a 1D-periodic samarium-doped glass emitter with a gallium antimonide (GaSb) PV diode is presented, which can yield efficiencies up to 38.5%. Finally, a 2D tungsten photonic crystal with a 1D integrated, chirped filter and the GaSb PV diode can yield efficiencies up to 38.2%; however, the fabrication procedure is expected to be more challenging. The advantages and disadvantages of each strategy will be discussed.

Reconfigurable metasurfaces for dynamic tuning of thermal sources

Metasurfaces have emerged as elegant engineered interfaces capable of controlling optical phases and amplitudes within ultra-flat form factors. Recently, there has been an increasing effort to achieve reconfigurable metasurfaces incorporating various tuning mechanisms, including electrical, optical, mechanical or thermal driving forces. In particular, electronic tuning has previously been shown to provide potential control over virtually a full range of optical phases. However, practical implementation is limited by the maximum doping that can be achieved by applying bias, and by the inherent losses of the constituent materials. In this work, we apply electrically-tuned reconfigurable metasurfaces to achieve dynamically-controlled thermal sources. Kirchhoff’s law of thermal radiation suggests possible active control of spectral and angular properties of radiated heat in carefully designed metasurfaces. This goal can be achieved by coupling optical resonances that imply spectral and angular selectivity, to 2D plasmonic resonances in active structured 2D surfaces. We discuss the potential of different 2D materials, such as graphene, black phosphorus and transition metals dichalcogenides, with respect to their respective optical properties, bandgaps and inherent losses. The ultimate goal is to achieve maximal absorption in a dynamically selected direction at a given wavelength, by exciting surface-confined modes. Enabling active beam steering of coherent thermal sources may provide low-cost alternatives to existing infrared sources for applications such as sensing and thermal management.

Spectral and angular-selective thermal emission from gallium-doped zinc oxide thin film structures

Simultaneously controlling both the spectral and angular emission of thermal photons can qualitatively change the nature of thermal radiation, and offers a great potential to improve a broad range of applications, including infrared light sources and thermophotovoltaic (TPV) conversion of waste heat to electricity. For TPV in particular, frequency-selective emission is necessary for spectral matching with a photovoltaic converter, while directional emission is needed to maximize the fraction of emission reaching the receiver at large separation distances. This can allow the photovoltaics to be moved outside vacuum encapsulation. In this work, we demonstrate both directionally and spectrally-selective thermal emission for p-polarization, using a combination of an epsilon-near-zero (ENZ) thin film backed by a metal reflector, a high contrast grating, and an omnidirectional mirror. Gallium-doped zinc oxide is selected as an ENZ material, with cross-over frequency in the near-infrared. The proposed structure relies on coupling guided modes (instead of plasmonic modes) to the ENZ thin film using the high contrast grating. The angular width is thus controlled by the choice of grating period. Other off-directional modes are then filtered out using the omnidirectional mirror, thus enhancing frequency selectivity. Our emitter design maintains both a high view factor and high frequency selectivity, leading to a factor of 8.85 enhancement over a typical blackbody emitter, through a combination of a 22.26% increase in view factor and a 6.88x enhancement in frequency selectivity. This calculation assumes a PV converter five widths away from the same width emitter in 2D at 1573 K.

Directional Thermal Emitter Simulation

The development of renewable energy sources has attracted increasing interest because of negative externalities associated with fossil fuel use. Thermophotovoltaics is a promising technology, in which a thermal emitter radiates photons which are directly converted into electricity using a photovoltaic diode. However, blackbody emission includes a broad range of wavelengths, but only higher energy photons can be converted into electricity. Thus, tailoring the selectivity of thermal emission is needed to improve the efficiency of TPV. A physics-based simulation tool is needed to understand the fundamental nature of thermal radiation, and the extent to which it may be controlled. The goal is to simulate directional thermal emission that results from coupling of Surface Plasmon Polaritons on lamellar metallic gratings into radiation modes. A rigorous coupled wave analysis in S4 can calculate absorption or emission versus angle. In the absorption mode, light is launched from air, and total absorption is calculated, including specular and diffracted modes. In the emission mode, a number of obliquely incident plane waves at different angles are launched from the metal side to simulate thermal emission. Highly selective emission is observed around angles that satisfy the diffraction law. For one experimental test case, the simulation results have been shown to match the angle and angular width of emission to good accuracy. Hence, our simulation will allow researchers to find an optimum geometric design of the metallic structure at which the emission characteristics best match their particular application. This will help in designing a selective emitter for higher-efficiency TPV.