N13A-T017 TITLE: Metamaterial Enhanced Thermophotovoltaics

TECHNOLOGY AREAS: Ground/Sea Vehicles, Materials/Processes

ACQUISITION PROGRAM: PM Expeditionary Power Sources; Renewable Sustainable Expedition Power FNC

OBJECTIVE: Using metamaterial technology, develop a coating that, when heated, emits photons over a defined, narrow wavelength range and demonstrate coupling of the photons to a thermophotovoltaic converter.

DESCRIPTION: Thermophotovoltaic (TPV) energy conversion produces electrical power from heat in a simple, solid-state device amenable to fielded operation. Combustion TPV has the potential to achieve power and energy densities greater than 250 W/kg and 1200 Whr/kg, respectively, nearly ten times that of rechargeable batteries. Thus, TPV is an enabling power source for remote systems, such as unmanned vehicles, as well as man-portable power sources. Currently, TPV photon conversion devices suffer from low efficiencies due, in part, to a mismatch between their discrete bandgap and the spectral emission of the heat source (the emitter). Photonic crystals have been shown to enhance conversion efficiency by providing an improved spectral match between emitter and converter, but are limited to collecting light at near normal incidence. Metamaterials are engineered composites exhibiting superior properties not observed in the constituent materials. Recent work has demonstrated high emissivity from metamaterials in the mid-infrared region which may allow their use as efficient, angularly-independent narrow-band emitters to spectrally match thermal or solar radiation to the TPV converter bandgap. Because the TPV converter and emitter are strongly coupled, it is best to develop them as a system. The purpose of this topic is to demonstrate an emitter/TPV converter system that operates in the range of 1100 to 1300K, emits between 1000 and 3000 nm, demonstrates angular insensitivity, and the potential of approaching 30% photon to electron conversion efficiency. The metamaterial based emitter may be made from any combination of constituent materials and be in the form of a coating on a substrate or a stand-alone material. The TPV converter may be an existing technology, such as InGaAs or GaSb, or a novel design. Efforts will include modeling, design, fabrication and testing. The successful proposal will include at least one novel emitter/TPV converter design that holds promise for enabling TPV conversion efficiencies approaching 30%.

PHASE I: Demonstrate the feasibility of designing and fabricating a metamaterial emitter/TPV converter system in which the emitter spectrum can be tailored to match the TPV converter spectral response so that 35% or more of the emitted photons are in-band with respect to the TPV converter and the photon to electron conversion efficiency approaches 30%. Determine the spectral emittance and TPV cell spectral response, and calculate the expected system energy conversion efficiency.

PHASE II: Fabricate and assemble metamaterial/TPV converter systems based on the success of the Phase I work. Develop methods to improve spectral emission selectivity, optical coupling efficiency, and TPV conversion efficiency. Extend Phase I analysis in order to determine tradeoffs between emitter temperature, emitter material, and TPV converter bandgap.

PHASE III: Develop prototype production line for the fabrication of metamaterial emitter and TPV converter and commercialization plans using the knowledge gained during Phases I and II.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Efficient TPV power generators could be used as lightweight, portable power sources.

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