  Recently, near-field radiative heat transfer enhancement across nanometer
vacuum gaps has been intensively studied between two hyperbolic metamaterials
(HMMs) due to unlimited wavevectors and high photonic density of state. In this
work, we theoretically analyze the energy conversion performance of a
thermophotovoltaic (TPV) cell made of In0.2Ga0.8Sb when paired with a HMM
emitter composed of tungsten nanowire arrays embedded in Al2O3 host at
nanometer vacuum gaps. Fluctuational electrodynamics integrated with effective
medium theory and anisotropic thin-film optics is used to calculate the
near-field radiative heat transfer. It is found that the spectral radiative
energy is enhanced by the epsilon-near-zero and hyperbolic modes at different
polarizations. As a result, the power output from a semi-infinite TPV cell is
improved by 1.85 times with the nanowire HMM emitter over that with a plain
tungsten emitter at a vacuum gap of 10 nm. Moreover, by using a thin TPV cell
with 10 um thickness, the conversion efficiency can be greatly improved from
19.5% to 31.5% without affecting the power generation, due to the total
internal reflection occurring at the bottom cell interface that minimizes the
sub-bandgap spectral radiative energy. Furthermore, the effects of a TPV cell
and a nanowire emitter with finite thicknesses are also studied. The result
shows that the maximum efficiency of 32.2% is achieved with an optimal cell
thickness of 3 um while the nanowire HMM emitter should be thick enough to be
opaque. The fundamental understanding and insights obtained here will
facilitate the design and application of novel materials in enhancing
near-field TPV energy conversion.
