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Research Papers: Radiative Heat Transfer

A Computational Simulation of Using Tungsten Gratings in Near-Field Thermophotovoltaic Devices

[+] Author and Article Information
J. I. Watjen

George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332

X. L. Liu

George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332;
School of Energy and Power Engineering,
Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China

B. Zhao

George W. Woodruff School of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332

Z. M. Zhang

Fellow ASME
George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332
e-mail: zhuomin.zhang@me.gatech.edu

1Corresponding author.

Presented at the 2016 ASME 5th Micro/Nanoscale Heat & Mass Transfer International Conference. Paper No. MNHMT2016-6632.Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 15, 2016; final manuscript received November 21, 2016; published online February 14, 2017. Assoc. Editor: Chun Yang.

J. Heat Transfer 139(5), 052704 (Feb 14, 2017) (8 pages) Paper No: HT-16-1207; doi: 10.1115/1.4035356 History: Received April 15, 2016; Revised November 21, 2016

Near-field thermophotovoltaic (NFTPV) devices have received much attention lately as an alternative energy harvesting system, whereby a heated emitter exchanges super-Planckian thermal radiation with a photovoltaic (PV) cell to generate electricity. This work describes the use of a grating structure to enhance the power throughput of NFTPV devices, while increasing the energy conversion efficiency by ensuring that a large portion of the radiation entering the PV cell is above the band gap. The device contains a high-temperature tungsten grating that radiates photons to a room-temperature In0.18Ga0.82Sb PV cell through a vacuum gap of several tens of nanometers. Scattering theory is used along with the rigorous coupled-wave analysis (RCWA) to calculate the radiation energy exchange between the grating emitter and the TPV cell. A parametric study is performed by varying the grating depth, period, and ridge width in the range that can be fabricated using available fabrication technologies. It is found that the power output can be increased by 40% while improving the efficiency from 29.9% to 32.0% with a selected grating emitter as compared to the case of a flat tungsten emitter. Reasons for the enhancement are found to be due to the enhanced energy transmission coefficient close to the band gap. This work shows a possible way of improving NFTPV and sheds light on how grating structures interact with thermal radiation at the nanoscale.

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References

Figures

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Fig. 1

Schematic of the NFTPV device showing the coordinate axes, vacuum gap spacing d, and the geometric grating parameters: period P, height H, and ridge width w. The temperatures of the emitter at T1 and receiver at T2 are specified.

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Fig. 2

Contour plots showing the electrical power output and conversion efficiency versus grating period and height: (a) electrical power output in log scale, where Pel is in (W/m2), for the filling ratio corresponding to the highest power output and (b) conversion efficiency for the filling ratio with the highest efficiency. Note that all calculations are at a gap spacing d = 20 nm.

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Fig. 3

(a) Power output and (b) conversion efficiency for the selected grating and planar tungsten emitters. The default parameters of the selected grating are P = 50 nm, H = 500 nm, and f = 0.8.

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Fig. 4

Parametric study for the performance when a single parameter f, P, or H is varied while the others are fixed to the default values of the selected grating. Effects of (a) filling ratio, (b) grating period, and (c) grating height.

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Fig. 5

Contour plots for energy transmission coefficient at ky = 0 for two cases: (a) plain tungsten without grating and (b) tungsten grating with default parameters. The white dashed line represents the light line.

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Fig. 6

Integrated energy transmission coefficient over kx: (a) plain tungsten without grating and (b) tungsten grating with default parameters

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Fig. 7

Spectral energy transmission coefficient for the grating and planar geometries. The band gap corresponding to 8.4 × 1014 rad/s is shown with a dotted vertical line.

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