Thermophotovoltaics (TPV) is the conversion of thermal radiation released by a thermal emitter into electricity by means of a photovoltaic cell.
Power generation with a TPV system can be envisaged for almost any process and an absence of moving parts has the advantage of low maintenance costs. Thermal emission scales with temperature to a power of 4, meaning temperature above 1000 °C is required to generate significant power in such systems.
However, the wide spectral width of thermal radiation limits the efficiency of TPV conversion as only part of the spectrum is accepted by the photovoltaic cell. A tight control over the thermal radiation spectrum is required to prevent energy dissipating into the environment. The emitter is structured in such a way as to emit thermal radiation outside the required spectral range.
Current methods use structural resonances to control thermal emission. These require complex lithography and possess intrinsic angle dependent spectral variations.
August’s Paper of the Month by Dyachenko et al., attempts to solve the TPV efficiency problem by designing and using a unique refractory metamaterial as an emitter instead of the traditional structural resonances. This metamaterial is engineered to prevent emission of long wavelength photons through a specifically engineered transition from dielectric to metallic response.
Metamaterials are traditionally made of layers of repeating composite material, designed to have a particular property which is not found naturally. The group designed a new metamaterial made of layers of tungsten and a dielectric material to create a refractory material with unique high temperature stability and selective thermal emission.
Dyachenko et al., conducted several verification experiments to test whether their unique metamaterial matched up to the theoretical expectations. Results indicated a strong absorptivity at short wavelengths and suppression of absorptivity at longer wavelengths, an important parameter for emitters in TPV systems. As the thermal emissivity and absorptivity are equal in reciprocal systems, then the same selective properties are expected for thermal emission. It was also shown that the obtained properties are almost angle independent.
By placing the metamaterial under cycles of extreme thermal stress, they were able to test the thermal stability of the material. The metamaterial was subjected to high temperature annealing experiments in the Linkam TS1500V stage. They chose to use the TS1500 as it allowed measurement of in-situ reflection and emission spectra of the samples using an FTIR spectrometer with a microscope at extreme temperatures up to 1500°C and under vacuum conditions.
The results were encouraging with optical characteristics being stable up to 1000°C supporting its thermal stability for TPV systems.
The refractory metamaterial does not need lithography and can be deposited by alternating magnetron sputtering. The highlight of the work is the support for the thermal stability and spectral properties of the unique metamaterial for TPV systems.
Dyachenko, P. N. et al. Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions. Nat. Commun. 7:11809 doi: 10.1038/ncomms11809 (2016).