High temperature thermal emitter and absorber metamaterials

Photonic crystals are periodically nanostructured metamaterials with extraordinary optical properties [1]. The ability to modulate the photonic density of states and hence spontaneous emission rates in photonic crystals opens a wide range of possibilities to design thermal radiation sources. The extraordinary properties of these metamaterials not only allow an enhancement of their emission approaching that of an ideal blackbody radiator, but can be designed to exhibit tailored spectrally and spatially selective behavior. This new class of selective absorbers and emitters is to form the base of next generation high performance energy production and conversion applications like solar thermal applications, photovoltaics and thermophotovoltaics. Solar thermophotovoltaic (TPV) systems have the potential to surpass the theoretical efficiency limit for single- junction solar cells (Shockley-Queisser limit) of 31% without and 41% under full concentration of sunlight by minimizing the losses due to the spectral mismatch of the irradiating photons and the bandgap of the solar cell [2], which is achieved by an intermediate absorber/emitter step. As the operation temperature of this intermediate in solar TPV systems is ideally above 1000 K (e.g. about 1600 K for an absorber/emitter area ratio of 1 and a concentration factor of 100 suns [2]) this poses two crucial requirements on this intermediate absorber/emitter system: first, excellent thermal stability of the involved materials, rendering conventional absorber materials like metal-dielectric composites (cermets) as used in solar thermal applications unsuitable. Further, it is of crucial importance to suppress the emissivity of both absorber and emitter at long wavelengths to avoid fatal losses by waste heat by means of photonic bandgap tailoring. Recently, several approaches for absorber and emitter structures based on metallic metamaterials with high thermal stability have been proposed [3,4]. First experimental demonstrations of emitters based on surface structured tungsten as fabricated by the group of J. Kassakian at MIT have shown promising results [5]. Although these structures featured an enhancement of the emission and absorption at small wavelengths over that of the unstructured base material, an efficient suppression of emission at long wavelengths continues to challenge numerous research groups. Very recently, an improved design aiming at an efficient suppression of long wavelength emission has been proposed by the MIT host group [6], where a 2D metallic photonic crystal emitter is combined with a selective multilayer stack, and an absorber based on a metal-semiconductor tandem structure is improved by additional dielectric coatings, predicted to result in a sharp absorption cut-off. This project encompasses the design, optimization and fabrication of high-temperature metamaterials for solar thermophotovoltaic applications where initial studies will be based on the designs proposed by the hosting group in [6]. The design, whose parameters were found by a global optimization approach, will be adapted for specific high temperature materials and an optimized operating temperature, and a suitable fabrication and implementation scheme for the proposed system will be developed. The experimental demonstration of a high performance, high temperature absorber/emitter compound structure in the course of this project will pave the way towards high efficiency conversion of solar into thermal, electrical and chemical energy and has the potential to lead to novel energy conversion schemes based on high temperature nanoscale photonic materials. [1] J. D. Joannopoulos, "Photonics: Minding the gap," Nature 375, 278 (1995). [2] N. P. Harder and P. Wurfel, "Theoretical limits of thermophotovoltaic solar energy conversion", Semicond. Sci. Technol. 18 (5), S151 (2003). [3] E. Rephaeli and S. Fan, "Absorber and emitter for solar thermophotovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit", Optics Express 17 (17), 15145 (2009). [4] N. P. Sergeant, O. Pincon, M. Agrawal, and P. Peumans, "Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks", Optics Express 17 (25), 22800 (2009). [5] I. Celanovic, N. Jovanovic, and J. Kassakian, "Two-dimensional tungsten photonic crystals as selective thermal emitters", Applied Physics Letters 92 (19), 193101 (2008). [6] P. Bermel, M. Ghebrebrhan, W. Chan, Y. X. Yeng, M. Araghchini, R. Hamam, C. H. Marton, K. F. Jensen, M. Soljacic, J. D. Joannopoulos, S. G. Johnson, and I. Celanovic, "Design and global optimization of high-efficiency thermophotovoltaic systems", Optics Express 18 (103), A314 (2010).

This project studied the use of metallic photonic crystals (PhCs) as thermal radiation sources and absorbers for high temperature energy conversion applications. The high spectral selectivity of the photonic crystal emitters and absorbers demonstrated in this project enables novel solid state energy conversion schemes, including solar-, radioisotope- and combustion thermophotovoltaics (TPV) as well as solarthermochemical energy conversion, with the potential for providing high efficiency, scalable energy conversion solutions.TPV is a thermal to electric energy conversion scheme, whereby photons produced by a thermal emitter drive a suitable low-bandgap photovoltaic (PV) cell. This concept permits direct thermal to electrical energy conversion without any mechanical components and on small device scales, i.e. with high power densities, and is fundamentally limited only by Plancks blackbody law. In a realistic photovoltaic system, the efficiency is limited due to the mismatch of the radiation spectrum and the spectral properties of the PV diode: photons with energies below the bandgap (in the IR) do not contribute to the electrical current, and for each high energy photon (in the UV) the energy in excess of the bandgap is dissipated as waste heat (phonons) and thus lost in the sense of power conversion. The efficiency of any TPV system therefore depends on the careful match of the emitter spectrum to the electronic bandgap and spectral properties of the PV cell, and can be tremendously increased by the use of spectrally selective emitters. In solar thermal as well as solar TPV systems, solar radiation is used as the source of heat and an absorber matched to this high temperature radiation is essential.Therefore, this field is profiting from the outstanding degree of control over the thermal emission properties, i.e. the flow of IR photons, which can be achieved with high-temperature nanoscale photonic materials. The ability to modulate the photonic density of states and hence spontaneous emission rates in photonic crystals opens a wide range of possibilities to design and tailor thermal radiation sources. In addition, the selective emitter has to be suitable for long-term operation at high temperatures (i.e. >800C), since the system efficiency is increasing with operating temperature. In the course of the project, a design approach for high performance selective emitters and absorbers based on 2D photonic crystals on refractory metal substrates was established and a scalable fabrication process using standard semiconductor manufacturing processes for large scale devices was demonstrated. High performance PhC emitters and absorbers with high spectral selectivity were fabricated and their thermal stability was demonstrated at high temperatures (up to 1200C). A selective absorber/emitter pair based on PhCs was designed for a solar TPV system and tested under realistic operating conditions, with unprecedented high measured system efficiency. These results demonstrate the suitability of photonic crystals for high efficiency energy conversion and pave the way for novel high performance energy conversion systems.